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Photonics

Photonics is the science of light, involving the generation, control, and detection of lightwaves and photons across the electromagnetic spectrum, from gamma rays to radio waves.^[1] It encompasses a broad range of wavelengths, with a particular emphasis on the visible spectrum extending from far infrared to far ultraviolet, and focuses on harnessing the unique properties of light such as its speed, coherence, and ability to carry information.^[1] The field of photonics emerged approximately 50 years ago as an extension of classical optics, drawing on centuries of human study of light phenomena, from ancient observations of rainbows to the 19th-century elucidation of the electromagnetic spectrum by James Clerk Maxwell.^[1] The term "photonics" was coined in analogy to electronics, reflecting the shift toward photon-based technologies following key inventions like the laser in 1960 and the concept of photonic integrated circuits proposed in 1969.^[2] This evolution positioned photonics as a cornerstone of 21st-century innovation, much like electronics defined the previous century, with organizations such as Optica (founded in 1916 as the Optical Society) playing pivotal roles in advancing the discipline.^[3] At its core, photonics relies on key technologies including lasers, light-emitting diodes (LEDs), optical fibers, waveguides, nanophotonics, and metamaterials, which enable precise manipulation of light at scales from macroscopic devices to nanoscale structures.^[1] These components facilitate processes such as emission, transmission, modulation, amplification, and sensing, often integrated into compact systems like photonic chips for high-speed data processing.^[1] Photonics also intersects with quantum technologies, extending into quantum photonics for applications in secure communications and computing.^[4] Photonics drives transformative applications across multiple sectors, including fiber-optic telecommunications that form the backbone of the internet, optical sensors for environmental monitoring and climate change detection, and medical tools for imaging, diagnostics, and therapeutics such as microscopy and laser surgeries.^[1] In consumer electronics, it powers cameras, smartphones, displays, and LED lighting; in energy, it supports solar cells and efficient photovoltaics; and in computing, it enables high-bandwidth data centers and photonic processors.^[1] Broader impacts include bioimaging, manufacturing processes, defense systems, and automotive technologies, underpinning a global market for photonics-enabled products projected to exceed $2.5 trillion in 2024.^[5] Optics and photonics serve as key enabling technologies in these areas, fostering advancements in communications, displays, and industry while addressing challenges like energy efficiency and data throughput.

Definition and Fundamentals

Definition and Scope

Photonics is the physical science and technology concerned with the generation, detection, and manipulation of light in the form of photons, encompassing processes such as emission, transmission, modulation, signal processing, switching, amplification, and sensing.^[6] This field integrates principles from physics, engineering, and materials science to harness photons—massless particles of light—for practical applications, distinguishing it from broader optical studies by its focus on quantum and wave-particle duality aspects of light.^[7] The scope of photonics extends across diverse applications, including high-speed communications via optical fibers, medical diagnostics and therapies such as laser surgery and biophotonic imaging, and advanced computing through photonic integrated circuits that enable faster data processing.^[6] It operates at scales ranging from macroscopic devices like fiber optic networks to nanoscale structures such as photonic crystals and nanowires, allowing for compact integration and enhanced functionality in modern technologies.^[6] A key distinction from electronics lies in photonics' use of photons rather than electrons for information transfer; photons travel at the speed of light with minimal interference and resistive losses, resulting in higher operational speeds, greater bandwidth, and lower energy consumption compared to electron-based systems.^[8] The term "photonics" was coined in 1967 by French physicist Pierre Aigrain to describe this emerging discipline.^[7]

Basic Principles of Light and Photons

The photon serves as the fundamental quantum of electromagnetic radiation, representing the smallest discrete unit of light energy. Its energy E is given by the relation E = h\nu, where h is Planck's constant ($6.626 \times 10^{-34} J·s) and \nu is the frequency of the radiation.^[9]^[10] This quantization arises from the Planck relation, which describes how electromagnetic waves are emitted or absorbed in discrete packets rather than continuously.^[11] Light exhibits wave-particle duality, manifesting both wave-like and particle-like properties depending on the experimental context. As a wave, light demonstrates interference and diffraction, where patterns emerge from the superposition of wavefronts passing through slits or around obstacles, such as in Young's double-slit experiment.^[12] Conversely, its particle nature is evident in the photoelectric effect, where photons eject electrons from a metal surface only if their energy exceeds the material's work function, with the electron kinetic energy given by E_k = h\nu - \phi.^[13] This duality underscores that photons behave as localized energy packets while also propagating as oscillating electromagnetic fields.^[14] In photonics, the relevant portion of the electromagnetic spectrum spans the ultraviolet (UV, ~10–400 nm), visible (~400–700 nm), and infrared (IR, ~700 nm–1 mm) regions, where photon energies range from approximately 3.1–124 eV for UV, 1.77–3.1 eV for visible, and 0.001–1.77 eV for IR.^[15]^[16] These wavelengths are critical because they align with the energy scales of electronic transitions in materials, enabling applications in detection, manipulation, and transmission of light signals.^[17] The spectrum's continuity allows photonic devices to operate across these bands, with visible light often serving as a benchmark for human-perceptible interactions, while IR and UV extend capabilities for thermal imaging and high-energy processes, respectively.^[18] Key interactions between photons and matter include absorption, emission, and scattering, which govern light propagation and energy transfer in photonic systems. Absorption occurs when a photon excites an electron from a lower to a higher energy state, converting the photon's energy into atomic or molecular excitation, provided h\nu matches or exceeds the transition energy.^[19] Emission follows, either spontaneously—where an excited state decays randomly, releasing a photon in an isotropic direction—or through stimulated emission, in which an incoming photon triggers the release of an identical photon with the same phase, direction, and frequency, amplifying coherent light.^[19] Scattering involves photons redirecting upon encountering particles without energy loss (elastic) or with partial loss (inelastic); Rayleigh scattering predominates for particles much smaller than the wavelength, as in atmospheric blue-sky effects, scaling as $1/\lambda^4, while Mie scattering applies to larger particles like aerosols, producing less wavelength-dependent forward scattering.^[19] Electromagnetic wave propagation, essential for photonic waveguiding and transmission, is described by Maxwell's equations in differential form. These include Faraday's law, \nabla \times \mathbf{E} = -\frac{\partial \mathbf{B}}{\partial t}, which links the curl of the electric field to the time-varying magnetic flux, and Ampère's law with Maxwell's correction, \nabla \times \mathbf{H} = \mathbf{J} + \frac{\partial \mathbf{D}}{\partial t}, relating the curl of the magnetic field to current density and the time-varying electric displacement.^[20] In source-free, linear media, these equations yield the wave equation \nabla^2 \mathbf{E} = \mu\epsilon \frac{\partial^2 \mathbf{E}}{\partial t^2}, predicting transverse waves traveling at speed c = 1/\sqrt{\mu\epsilon}. This framework explains light's wavelike behavior, including polarization and refraction, foundational to photonic design.^[20]

Historical Development

Early Foundations

The foundations of photonics trace back to ancient observations of light and vision, where early thinkers laid the groundwork for understanding optical phenomena. Around 300 BCE, the Greek mathematician Euclid authored Optics, a seminal work that treated light as straight lines propagating from sources to the eye, employing geometric principles to explain reflection and refraction. This ray-based model dominated early optics and influenced subsequent studies by focusing on the paths of visual rays rather than the physical nature of light itself.^[21] In the 11th century, the Islamic scholar Ibn al-Haytham (known as Alhazen) advanced these ideas in his Book of Optics, where he experimentally demonstrated the camera obscura, showing that light travels in straight lines from objects through a small aperture to form inverted images on a screen. His rejection of emission theories of vision in favor of intromission—light entering the eye—marked a shift toward empirical investigation and refuted earlier Greek models like those of Euclid and Ptolemy.^[22] The 17th and 18th centuries saw pivotal experiments that began to reveal light's composite nature and wave-like properties. In 1666, Isaac Newton conducted groundbreaking prism experiments, dispersing white sunlight into a spectrum of colors and demonstrating that white light is a heterogeneous mixture of rays with different refrangibilities, challenging prevailing views and establishing the corpuscular theory's dominance for over a century.^[23] Concurrently, in 1678, Christiaan Huygens proposed a wave theory in his Treatise on Light, positing that light propagates as longitudinal waves through an elastic ether, with each point on a wavefront serving as a source of secondary wavelets—a principle that explained refraction and laid the basis for later wave optics.^[24] The 19th century built on these insights with experiments confirming light's wave characteristics and introducing magneto-optical interactions. In 1801, Thomas Young performed the double-slit experiment, observing interference fringes from light passing through two closely spaced slits, providing compelling evidence for the wave theory by showing constructive and destructive interference patterns.^[25] Augustin-Jean Fresnel extended this work through his diffraction theory around 1818, mathematically modeling how light bends around obstacles using Huygens' principle, which accurately predicted fringe patterns and solidified the wave model against corpuscular rivals.^[26] Michael Faraday discovered magneto-optical effects in 1845, observing that a magnetic field rotates the plane of polarization of light passing through certain materials like lead borate glass—a phenomenon now known as the Faraday effect—hinting at deeper connections between light and electromagnetism.^[27] This era culminated in James Clerk Maxwell's 1865 unification of electricity, magnetism, and optics in his Dynamical Theory of the Electromagnetic Field, where he derived equations showing that light is a transverse electromagnetic wave propagating at a finite speed in vacuum, matching the known speed of light and predicting its electric and magnetic components.^[28] Maxwell's framework bridged classical optics with electromagnetism, setting the theoretical stage for photonics by revealing light's dual wave-particle potential, though the particle aspect awaited quantum developments.^[29]

Key Milestones in the 20th Century

The foundations of photonics in the 20th century were laid by breakthroughs in quantum theory, which shifted understanding from classical wave optics to the particle-like nature of light. In 1900, Max Planck introduced the quantum hypothesis to resolve discrepancies in black-body radiation spectra, proposing that electromagnetic energy is exchanged in discrete packets, or quanta, proportional to frequency with energy E = h\nu, where h is Planck's constant.^[30] This concept, detailed in his seminal paper "Zur Theorie des Gesetzes der Energieverteilung im Normalspectrum," marked the birth of quantum mechanics and enabled subsequent photon-based technologies.^[31] Building on Planck's work, Albert Einstein in 1905 explained the photoelectric effect, where light ejects electrons from a metal surface only above a threshold frequency, by treating light as discrete quanta called photons with energy E = h\nu.^[32] This particle model of light, outlined in his paper "Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt," demonstrated that photon energy determines electron kinetic energy, \frac{1}{2}mv^2 = h\nu - \phi, where \phi is the work function.^[33] Einstein's explanation, awarded the 1921 Nobel Prize in Physics, bridged quantum theory with experimental observations and underscored photons as fundamental carriers in photonic systems.^[32] The mid-century invention of the laser revolutionized light generation and control, enabling coherent photon manipulation essential to photonics. In May 1960, Theodore Maiman achieved the first stimulated emission at Hughes Research Laboratories using a ruby crystal pumped by a flashlamp, producing a pulsed output at 694.3 nm with peak power exceeding 10 kW.^[34] Detailed in his Nature paper "Stimulated Optical Radiation in Ruby," this solid-state device confirmed theoretical predictions by Townes and Schawlow, demonstrating optical amplification through population inversion in chromium ions.^[34] Later in December 1960, Ali Javan, William R. Bennett Jr., and Donald R. Herriott at Bell Labs demonstrated the first continuous-wave (CW) laser, a helium-neon (He-Ne) device operating at 1.153 \mum with milliwatt output power.^[35] Their Physical Review Letters report "Population Inversion and Continuous Optical Maser Oscillation in a Gas Discharge Containing a He-Ne Mixture" highlighted gas discharge excitation for stable, low-noise operation, paving the way for semiconductor laser developments.^[35] Advancements in fiber optics addressed light transmission challenges, transforming photonics into a practical communication technology. In 1966, Charles K. Kao and George A. Hockham at Standard Telecommunication Laboratories theorized that silica glass fibers with purity levels reducing attenuation to below 20 dB/km could enable long-haul optical waveguides, attributing losses primarily to material impurities rather than waveguide geometry.^[36] Their Proceedings of the IEE paper "Dielectric-Fibre Surface Waveguides for Optical Frequencies" calculated dispersion and mode propagation, predicting single-mode operation at 1 \mum wavelengths and inspiring global purification efforts; Kao received the 2009 Nobel Prize in Physics for this vision.^[37] Realization came in 1970 when Robert D. Maurer, Donald B. Keck, and Peter C. Schultz at Corning Glass Works fabricated the first low-loss multimode fiber using an inside vapor deposition process, achieving 17 dB/km attenuation at 632.8 nm—four times better than prior records.^[38] Reported in Applied Physics Letters as "Radiation Losses in Glass Optical Waveguides," this breakthrough demonstrated intrinsic material limits near 0.2 dB/km, enabling commercial fiber deployment by the 1980s.^[38] Integrated optics emerged to miniaturize photonic circuits, drawing from semiconductor fabrication techniques. In 1969, Stewart E. Miller at Bell Laboratories proposed the concept of integrated optics in his seminal paper, outlining thin-film waveguides formed by evaporated dielectric layers on substrates for guiding light with projected low losses, enabling compact couplers and modulators.^[39] Experimental demonstrations soon followed, with P. K. Tien and colleagues achieving light propagation in such thin-film structures with losses under 1 dB/cm.^[40] This work, building on early waveguide experiments, introduced hybrid integration for signal processing. By the 1980s, Richard A. Soref advanced silicon-based photonics, predicting electrooptic modulation via carrier injection in silicon waveguides with refractive index shifts up to $10^{-3} at 1.3 \mum.^[41] His IEEE Journal of Quantum Electronics paper "Electrooptical Effects in Silicon," co-authored with Bennett, used Kramers-Kronig relations to model free-carrier dispersion, laying groundwork for CMOS-compatible photonic integrated circuits.^[41] Toward century's end, photonic crystals introduced bandgap engineering for light confinement. In 1987, Eli Yablonovitch proposed periodic dielectric structures to create photonic bandgaps, forbidding photon propagation in specific frequency ranges analogous to electronic bandgaps in semiconductors, thereby inhibiting spontaneous emission for efficient lasers.^[42] His Physical Review Letters article "Inhibited Spontaneous Emission in Solid-State Physics and Electronics" calculated three-dimensional lattices with gaps exceeding 20% of mid-gap frequency, suggesting applications in quantum optics. Independently, Sajeev John theorized strong Anderson localization of photons in disordered dielectric superlattices, where multiple scattering traps light in subwavelength volumes with localization lengths below 0.1\lambda.^[43] Detailed in his Physical Review Letters paper "Strong Localization of Photons in Certain Disordered Dielectric Superlattices," this mechanism promised defect-induced cavities for single-photon sources, spurring research into photonic metamaterials.

Contemporary Advances

The 21st century has witnessed a surge in silicon photonics, driven by the need for high-speed, low-power data transmission in data centers and telecommunications. In 2013, Intel announced its silicon photonics platform, including the first demonstrations of 100G transceivers that integrated lasers, modulators, and detectors on a single silicon chip, paving the way for commercial adoption by leveraging existing CMOS fabrication infrastructure.^[44] By the 2020s, deeper integration with CMOS processes enabled monolithic photonic-electronic circuits, reducing costs and improving performance for applications like high-bandwidth interconnects, with production scaling on 300mm wafers to support terabit-scale networking.^[45]^[46] Photonic technologies have increasingly intersected with artificial intelligence, where light-based computing offers advantages in speed and energy efficiency over electronic counterparts. Lightmatter introduced its first photonic processors in 2020, utilizing silicon photonics for analog matrix operations central to neural networks, achieving up to 10x faster inference with significantly lower power draw compared to GPU-based systems.^[47] Advancements in optical neural networks accelerated from 2023 to 2025, with integrated photonic chips demonstrating sub-nanosecond latencies, parallel processing of complex AI models, and energy efficiencies exceeding 100 TOPS/W, enabling real-time applications in edge computing and large language models.^[48] These developments highlight photonics' role in addressing AI's scaling challenges, such as bandwidth bottlenecks in data centers. Seminal breakthroughs in photonics earned Nobel recognition, underscoring their transformative impact. The 2014 Nobel Prize in Physics was awarded to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura for inventing efficient blue light-emitting diodes, which combined with red and green LEDs to produce white light sources up to 300 lm/W efficient, revolutionizing displays, lighting, and optoelectronics while reducing global energy consumption for illumination by an estimated 25%.^[49] In 2023, Pierre Agostini, Ferenc Krausz, and Anne L'Huillier received the prize for experimental methods generating attosecond pulses of light, allowing unprecedented observation of electron motion in atoms and molecules, with applications in ultrafast photonic control for advanced lasers and quantum technologies.^[50] International initiatives have fueled these advances through coordinated research and commercialization. The European Union's Graphene Flagship, launched in 2013 and spanning a decade to 2023, invested over €1 billion to develop graphene-based photonics, resulting in more than a dozen spin-off companies focused on optical detectors, modulators, and sensors, alongside spearhead projects that integrated 2D materials into high-speed photonic devices for communications and imaging.^[51] In the United States, DARPA's Photonics in the Package for Extreme Scalability (PIPES) program, active in the 2020s, promotes co-packaged optics with electronics to achieve petabit-per-second interconnects, supporting scalable AI and high-performance computing systems.^[52] As of 2025, key updates include progress in room-temperature quantum dots for displays, where self-assembled blue QD-LEDs have achieved external quantum efficiencies over 38%, enabling brighter, more stable next-generation screens with reduced power needs and compatibility with flexible substrates.^[53] Concurrently, scalable photonic quantum computing prototypes have advanced, with modular systems using 35 interconnected photonic chips to perform fault-tolerant operations on tens of qubits, demonstrating feasibility for practical quantum simulations in chemistry and optimization problems.^[54]

Optics and Optoelectronics

Photonics draws its foundational principles from classical optics, where geometric and physical optics provide the essential frameworks for light manipulation and beam control. Geometric optics approximates light as rays propagating in straight lines, enabling the design of optical elements like lenses and mirrors to focus, collimate, or redirect beams through refraction and reflection. These concepts are crucial precursors to photonic systems, as they underpin the passive components used in modern light guiding and imaging. For instance, lenses and mirrors remain integral for initial beam shaping in photonic devices, ensuring efficient light delivery to active elements.^[55] Physical optics extends this by incorporating the wave nature of light, accounting for phenomena such as interference, diffraction, and polarization, which are vital for understanding wave propagation in photonic structures. This wave-based perspective bridges classical optics to photonics by explaining how light interacts with media at scales where ray approximations fail, informing the design of waveguides and interferometers. Together, these classical approaches form the bedrock for photonics, transitioning from macroscopic optical systems to integrated light control at micro- and nanoscales. Optoelectronics represents the hybrid interface between photonics and electronics, focusing on devices that interconvert electrical and optical signals through semiconductor structures. Key examples include light-emitting diodes (LEDs), which generate light via electroluminescence in forward-biased p-n junctions of direct-bandgap materials like gallium arsenide, and photodiodes, which detect light by generating photocurrent in reverse-biased p-n junctions, often using silicon or germanium for absorption. These p-n junctions exploit the bandgap energy of semiconductors to enable efficient electron-photon coupling, forming the core of optoelectronic functionality in photonic circuits.^[56] A critical overlap between optics and optoelectronics in photonics lies in the electro-optic effect, which enables dynamic control of light through electric fields. The Pockels effect, a linear variation in refractive index proportional to the applied field, occurs in non-centrosymmetric materials like lithium niobate and is widely used for high-speed phase and amplitude modulation in waveguides. Complementing this, the Kerr effect provides a quadratic refractive index change, suitable for isotropic media and applications requiring finer field-dependent tuning. These effects facilitate refractive index modulation for beam steering, switching, and signal processing in integrated photonic devices.^[57] In contrast to traditional electronics, which relies on electron charge carriers for information processing, photonics prioritizes photons as the primary information bearers, leveraging their non-interacting nature for low-loss, high-speed transmission over distances. This photon-based paradigm reduces heat generation and crosstalk compared to electron-based systems, though it requires optoelectronic interfaces for input-output conversion. Quantum extensions of these principles further enhance photonic capabilities, but classical optoelectronics remains central to device-level integration.^[58]

Electronics and Materials Science

Photonics integrates with electronics through hybrid approaches that leverage established semiconductor fabrication processes, enabling the co-integration of photonic and electronic components on a single chip. CMOS-compatible photonics utilizes silicon-based platforms to align with mature complementary metal-oxide-semiconductor (CMOS) technology, allowing for cost-effective mass production and seamless interfacing between optical and electrical signals. The silicon-on-insulator (SOI) platform, in particular, provides high-index contrast for compact waveguides and modulators while maintaining compatibility with front-end CMOS processes, facilitating monolithic integration for applications in data centers and telecommunications.^[58]^[59] This hybrid integration mitigates the need for separate packaging, reducing latency and power consumption compared to discrete optoelectronic assemblies. Key materials in photonic devices span semiconductors, dielectrics, and polymers, each tailored to specific functions in light generation, guiding, and manipulation. III-V semiconductors like gallium arsenide (GaAs) and indium phosphide (InP) are essential for active components such as lasers, owing to their direct bandgaps that enable efficient light emission in the near-infrared spectrum.^[60]^[61] Dielectrics, particularly silicon dioxide (SiO2), serve as low-loss cladding for waveguides, providing optical isolation and thermal stability in integrated circuits.^[62] Polymers, valued for their mechanical flexibility and low-cost processing, enable bendable photonic structures suitable for wearable sensors and conformable displays, often incorporating dyes or nanoparticles for waveguiding.^[63]^[64] Bandgap engineering in semiconductors optimizes light emission by adjusting the energy difference between valence and conduction bands through alloying or strain, allowing precise tuning of emission wavelengths for lasers and LEDs.^[65] For instance, quaternary alloys in InP-based structures achieve low-threshold lasing at telecom wavelengths. Plasmonics enhances subwavelength confinement by coupling light to surface plasmons at metal-dielectric interfaces, enabling nanoscale waveguides that surpass diffraction limits while boosting light-matter interactions in sensors and modulators.^[66]^[67]^[68] However, integrating photonics with electronics faces challenges from impedance mismatches, where differing signal characteristics—such as photonic propagation speeds near the speed of light versus electronic carrier velocities—lead to bandwidth limitations and signal distortion at interfaces.^[60]^[69] These issues necessitate advanced packaging techniques like micro-bumps or interposers to ensure efficient electro-optic conversion.^[70]

Emerging Interdisciplinary Areas

Biophotonics represents a key intersection of photonics and biology, leveraging light-tissue interactions to enable advanced imaging and diagnostic techniques. Fundamental to this field is the study of how photons propagate, scatter, absorb, and fluoresce within biological tissues, which exhibit complex optical properties due to their heterogeneous composition of cells, extracellular matrices, and fluids. These interactions form the basis for non-invasive imaging modalities that provide high-resolution views of internal structures without ionizing radiation. A seminal example is optical coherence tomography (OCT), invented in 1991 by Huang et al., which uses low-coherence interferometry to achieve micrometer-scale axial resolution for cross-sectional imaging of tissues like the retina and cardiovascular structures. In the 2020s, artificial intelligence (AI) enhancements have significantly advanced OCT capabilities, with deep learning algorithms improving image segmentation, noise reduction, and automated disease detection in ophthalmic applications.^[71] These AI integrations process vast OCT datasets to enhance diagnostic precision, enabling real-time clinical decision-making and expanding biophotonics' role in personalized medicine.^[72] Photonic computing emerges as another interdisciplinary frontier, integrating photonics with computer science to address limitations in traditional electronic systems. By employing optical logic gates—devices that perform Boolean operations using light beams via nonlinear optical effects—photonic processors enable parallel data handling at speeds up to terahertz, far surpassing electronic counterparts. A primary advantage is overcoming the von Neumann bottleneck, where data shuttling between memory and processing units causes latency and energy inefficiency; photonic architectures integrate computation and storage optically, reducing power consumption to femtojoule levels per operation. Such advances position photonic computing as a viable path for sustainable, high-performance neuromorphic systems inspired by biological neural networks. As of November 2025, China's photonic quantum chip has demonstrated over 1000-fold acceleration for complex AI computing tasks, highlighting rapid progress toward practical photonic processors.^[73] In quantum information science, photonics plays a pivotal role by utilizing photons as qubits due to their low decoherence rates and ease of transmission over long distances. Photonic qubits, encoded in properties like polarization or time-bin, enable quantum operations such as superposition and measurement with high fidelity, leveraging integrated photonic chips for scalable quantum circuits. A critical application is entanglement distribution, where paired photons maintain quantum correlations despite separation, foundational for quantum key distribution and teleportation. Optical fibers facilitate this by guiding entangled photons with minimal loss, supporting multihop networks over hundreds of kilometers. As of 2025, studies propose satellite-based entanglement distribution using airborne reflectors to exceed 1 Hz over 1000 km links with quantum bit error rates below 5%, building on prior demonstrations like the Micius satellite that achieved ~1 Hz over 1200 km. Hybrid satellite-fiber systems further integrate these, demonstrating entanglement swapping between terrestrial nodes and orbital platforms, paving the way for a quantum internet with secure, long-range connectivity.^[74] These developments underscore photonics' centrality in realizing practical quantum technologies.^[75] Photonics also intersects with sustainability efforts, particularly through evolutions in photovoltaics and passive cooling materials that enhance energy efficiency and reduce environmental impact. Photovoltaic advancements harness photonic structures like nanostructures and perovskites to boost light absorption and charge carrier extraction, achieving power conversion efficiencies above 25% in tandem cells while improving stability against degradation. In 2025, mixed-cation perovskite solar cells demonstrated enhanced durability, retaining over 90% efficiency after more than 1000 hours of operation under ambient conditions, through alloying that reduces defect density and minimizes non-radiative recombination. Complementing this, photonic cooling materials exploit radiative heat dissipation to the atmosphere's cold sky window (8-13 μm wavelength), enabling sub-ambient cooling without electricity. Breakthroughs in 2023 introduced scalable polymer-based metamaterials with emissivities near 0.97 in the atmospheric window and reflectivities over 95% in solar spectra, achieving daytime cooling powers of 100 W/m² and reducing building energy use by 20-30%. These materials, often incorporating photonic crystals for selective emission, support global sustainability by curbing air-conditioning demands, projected to lower carbon emissions by billions of kilograms annually.^[76]^[77]

Core Components

Light Sources

Light sources in photonics are essential for generating light that can be manipulated for various applications, categorized primarily into incoherent and coherent types based on the nature of photon emission. Incoherent sources produce light through spontaneous emission, where photons are emitted randomly in phase and direction, resulting in a broad spectrum and low spatial coherence.^[78] These sources are widely used due to their simplicity and efficiency in non-coherent applications. Coherent sources, in contrast, generate light via stimulated emission, yielding narrow linewidths, high directionality, and phase-locked photons suitable for precision photonic devices.^[79] Incoherent light sources rely on spontaneous emission, a process where excited electrons in a material decay to lower energy states, releasing photons without external stimulation. Light-emitting diodes (LEDs) exemplify this, operating through electron-hole recombination in semiconductors like gallium arsenide or gallium nitride, producing broadband emission with typical efficiencies exceeding 50% in modern devices.^[80] LEDs emit incoherent light due to the random timing and direction of spontaneous emission events, making them ideal for illumination and signaling in photonic systems. Another fundamental incoherent source is the blackbody radiator, which emits thermal radiation across a continuous spectrum governed by Planck's law, as seen in incandescent lamps or heated filaments where charged particles accelerate and radiate incoherently at temperatures around 2000–3000 K.^[81] These sources provide versatile, low-coherence illumination but lack the monochromaticity required for advanced photonic manipulations. Coherent light sources, primarily lasers, achieve light amplification through stimulated emission, where an incoming photon triggers the release of identical photons from excited atoms or molecules. The core principle involves creating a population inversion in the gain medium—a material like a gas, crystal, or semiconductor—where more atoms occupy the upper energy level (N₂) than the lower one (N₁), defying thermal equilibrium.^[82] This inversion is maintained by an external pump source, such as electrical discharge, optical excitation, or current injection, which populates the upper level. The gain medium is placed within an optical cavity, typically formed by two mirrors, one partially reflective, to provide feedback and confine the light, enabling resonant amplification of specific wavelengths.^[83] The laser threshold condition requires population inversion, N₂ > N₁, to ensure net stimulated emission exceeds absorption. This arises from the Einstein coefficients for absorption (B₁₂) and stimulated emission (B₂₁ = B₁₂), where the rate of stimulated emission per atom in the upper level is B₂₁ ρ N₂ (ρ is the energy density), and absorption is B₁₂ ρ N₁. For net gain, stimulated emission must dominate: B₂₁ ρ N₂ > B₁₂ ρ N₁, simplifying to N₂ > N₁ since B₂₁ = B₁₂.^[84] This condition ensures the gain coefficient γ = σ ΔN (σ is the cross-section) overcomes cavity losses for lasing. Lasers are classified by gain medium: gas lasers, such as the helium-neon (He-Ne) laser, use a low-pressure gas mixture excited by electrical discharge to produce continuous-wave output at 632.8 nm with low power (milliwatts), valued for its stability in alignment applications.^[85] Solid-state lasers, like the neodymium-doped yttrium aluminum garnet (Nd:YAG), employ a crystalline host doped with rare-earth ions, optically pumped by lamps or diodes to achieve population inversion at 1064 nm, delivering high peak powers up to kilowatts in pulsed modes for material processing.^[82] Semiconductor lasers, including vertical-cavity surface-emitting lasers (VCSELs), integrate the gain medium and cavity monolithically in structures like gallium arsenide, electrically pumped for compact, low-threshold operation at wavelengths from 850 nm to 1550 nm, enabling array integration in photonic circuits.^[86] Significant advances include the quantum cascade laser (QCL), invented in 1994, which uses engineered quantum wells in semiconductors for intersubband transitions, allowing tunable mid-infrared emission (3–20 μm) without bandgap limitations, revolutionizing spectroscopy with room-temperature operation and powers exceeding 1 W.^[87] High-power fiber lasers, such as those using diode-pumped ytterbium-doped fibers, provide high efficiency for industrial cutting and directed energy systems.^[81] These sources are often coupled into transmission structures for photonic propagation.^[81]

Transmission and Guiding Structures

Transmission and guiding structures in photonics are essential components designed to direct and confine light propagation with minimal loss, enabling efficient transport from sources such as lasers or LEDs. These structures exploit principles of wave optics to maintain beam integrity over various distances, ranging from micrometers in integrated devices to kilometers in long-haul systems. Optical fibers and waveguides represent the primary confined-media approaches, while free-space optics provides an unguided alternative for specific applications requiring flexibility in beam path. Optical fibers guide light through total internal reflection (TIR) at the core-cladding interface, where the core has a higher refractive index than the surrounding cladding.^[88] This phenomenon occurs when light strikes the interface at an angle greater than the critical angle, defined by Snell's law: n_1 \sin \theta_1 = n_2 \sin \theta_2, with n_1 > n_2 for the core and cladding, respectively, preventing transmission into the cladding.^[89] Fibers are classified as single-mode or multimode based on core diameter and wavelength; single-mode fibers (typically 8-10 μm core) support one propagation mode for long-distance, low-dispersion transmission, while multimode fibers (50-62.5 μm core) allow multiple modes, suitable for shorter links but prone to modal dispersion.^[88] In silica-based fibers, attenuation is remarkably low, achieving less than 0.2 dB/km at 1550 nm due to optimized material purity and index profiles.^[90] Waveguides extend these principles to planar and integrated formats, confining light in two dimensions for on-chip photonics. Planar waveguides, such as ridge and strip types, are fabricated by etching or depositing materials like silicon or silica on a substrate, with the ridge structure featuring a raised central region for enhanced vertical confinement and the strip providing full etching for stronger overall guiding.^[91] Photonic crystal fibers (PCFs), developed in the 2000s, represent an advanced variant using a periodic array of air holes in silica to create bandgap or modified TIR guidance, enabling air-core propagation that avoids material nonlinearities. Dispersion management in these structures compensates for wavelength-dependent velocity variations, often through tailored index gradients or hybrid fiber segments to minimize pulse broadening in high-bit-rate systems.^[92] Free-space optics employs lenses and mirrors to steer and focus beams without physical confinement, ideal for short-range or adaptive links where alignment is maintained via mechanical or optical feedback. Lenses collimate or converge the beam to control divergence, while mirrors enable precise angular adjustments for pointing accuracy in dynamic environments.^[93] Losses in transmission structures arise from absorption by material impurities, scattering at imperfections, and radiation due to bending, which shifts modes into the cladding via enhanced evanescent coupling. In curved waveguides, bending induces additional leakage as the effective index decreases with radius. The evanescent field in the cladding decays exponentially, characterized by the attenuation coefficient \alpha = 2 \Im(k), where k is the complex wavevector and \Im(k) denotes its imaginary part, ensuring confinement but contributing to losses if perturbed.^[94]

Amplifiers and Modulators

Optical amplifiers and modulators are essential active components in photonic systems, enabling the boosting of weak light signals and the encoding of information onto optical carriers, respectively. These devices facilitate long-distance transmission and high-speed data processing by compensating for losses in guiding structures and imposing temporal variations on light properties such as intensity, phase, or polarization.^[95] Among optical amplifiers, the erbium-doped fiber amplifier (EDFA) stands out as a cornerstone technology, invented in 1987 by researchers including Robert Mears at the University of Southampton. EDFAs operate by doping silica fibers with erbium ions, which are pumped at around 980 nm or 1480 nm to achieve population inversion, providing gain in the 1550 nm telecommunications window through stimulated emission. This all-fiber design integrates seamlessly with transmission media, offering low noise figures below 4 dB and gains up to 40 dB over bandwidths exceeding 30 nm.^[96]^[97] Semiconductor optical amplifiers (SOAs) represent another key class, leveraging the gain from quantum-confined structures in materials like InGaAsP to amplify signals directly in waveguide formats. SOAs provide compact integration with lasers and modulators, achieving fiber-to-fiber gains greater than 20 dB, often in the 25-35 dB range, with saturation output powers up to 20 dBm. However, they exhibit higher noise figures, typically 6-8 dB, due to amplified spontaneous emission.^[95] The fundamental principle of gain in many optical amplifiers, including EDFAs and SOAs, follows the exponential relation G = \exp(gL), where [G](/page/G) is the power gain, g is the material gain coefficient (dependent on inversion and wavelength), and L is the amplifier length. This small-signal approximation holds for unsaturated operation, highlighting how longer interaction lengths or higher inversion levels exponentially enhance amplification.^[98] Distributed amplification principles, such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), exploit nonlinear interactions in optical fibers to provide gain without discrete doping. In SRS, a high-intensity pump beam at a shorter wavelength transfers energy to a Stokes signal via virtual energy levels and optical phonons, yielding broadband gain (up to 100 nm) proportional to pump power; this backward or forward Raman amplification extends reach in fiber spans by 20-50% compared to lumped amplifiers. SBS, conversely, involves acoustic phonons for narrower-band (tens of MHz) forward or backward scattering, offering high gain (up to 30 dB/m) but limited by threshold powers around 1-10 mW due to phase-matching constraints.^[99]^[100] Optical modulators alter light signals using external fields to encode data, with electro-optic types dominating high-speed applications. Lithium niobate (LiNbO3)-based Mach-Zehnder interferometers (MZIs) exploit the Pockels effect, where an applied voltage induces a refractive index change via the electro-optic coefficient r_{33} \approx 30 pm/V, splitting and recombining the beam for phase or amplitude modulation. These devices achieve half-wave voltages below 4 V and bandwidths over 40 GHz, supporting formats like quadrature phase-shift keying for coherent systems.^[101] Acousto-optic modulators (AOMs) operate on the photoelastic effect, where radio-frequency-driven transducers generate sound waves in a crystal (e.g., tellurium dioxide), creating a moving refractive index grating that diffracts light via Bragg scattering. This enables intensity modulation up to 100% at frequencies from kHz to GHz, with deflection angles tunable by acoustic wavelength, though limited by acoustic velocity to rise times around 10-100 ns. AOMs are valued for their robustness in Q-switching lasers or beam steering.^[102] Modulation can target phase for applications like interferometry or amplitude for direct intensity encoding, with hybrid schemes combining both for advanced formats. These modulators enhance signals propagating through fiber waveguides by imprinting data at rates up to tens of Gbps in bulk formats. As of 2025, trends in integrated photonic modulators emphasize silicon or thin-film lithium niobate platforms, achieving data rates exceeding 100 Gbps per channel through reduced footprints (under 1 mm) and low drive voltages below 2 V, driven by demands in data-center interconnects and 6G networks.^[103]

Detection and Sensing

Photodetectors

Photodetectors are essential components in photonic systems, converting incident optical signals into electrical currents through the absorption of photons that generate charge carriers. The fundamental operating principles rely on photoconductivity, where light increases the electrical conductivity of a material by exciting electrons from the valence band to the conduction band, or the photovoltaic effect in junction devices, which separates these carriers to produce a measurable photocurrent. A key performance metric is the quantum efficiency \eta, defined as the ratio of the number of generated charge carriers to the number of incident photons, expressed as \eta = \frac{I_{ph}}{e \cdot \frac{P}{h\nu}}, where I_{ph} is the photocurrent, e is the elementary charge, P is the incident optical power, and h\nu is the photon energy.^[104] This efficiency quantifies the detector's ability to convert photons to electrons, typically ranging from 50% to over 90% in optimized devices. Common types of photodetectors include PIN photodiodes, which feature a p-type/intrinsic/n-type semiconductor structure to minimize capacitance and enable fast response without internal gain; avalanche photodiodes (APDs), which incorporate a high electric field region for impact ionization, providing internal gain factors up to 100 for enhanced sensitivity at low light levels; and photomultiplier tubes (PMTs), vacuum devices that amplify photoelectrons through a series of dynodes via secondary electron emission, achieving gains exceeding $10^6.^[105] Materials selection is critical for wavelength compatibility: silicon (Si) is widely used for visible and near-infrared detection up to about 1100 nm due to its suitable bandgap and low cost, while indium gallium arsenide (InGaAs) excels in the infrared telecom bands around 1550 nm, offering high absorption and compatibility with fiber optics.^[106]^[107] Performance characteristics emphasize speed and sensitivity, with modern photodetectors achieving 3 dB bandwidths exceeding 100 GHz through optimized designs like traveling-wave structures, enabling applications in high-bit-rate communications. Responsivity, the ratio of photocurrent to optical power (often 0.5–1 A/W), and noise considerations are paramount; shot noise arises from the statistical nature of photon and carrier arrival (proportional to \sqrt{I}), while thermal noise stems from resistor fluctuations, limiting the signal-to-noise ratio in low-light scenarios.^[108]^[109] Recent advances include single-photon avalanche diodes (SPADs), which operate in Geiger mode for near-unity quantum efficiency at the single-photon level, finding critical use in quantum information processing and secure communications during the 2020s.^[110] These detectors support extensions to imaging by providing pixelated arrays for spatial resolution, though system-level architectures are beyond basic detection.^[111]

Imaging and Spectroscopy Techniques

Imaging techniques in photonics leverage light-matter interactions to achieve high-resolution visualization of structures, overcoming limitations of traditional microscopy through advanced optical configurations. Confocal microscopy, patented by Marvin Minsky in 1957, employs a pinhole to eliminate out-of-focus light, enabling optical sectioning and three-dimensional reconstruction with improved axial resolution.^[112] Two-photon microscopy, introduced by Denk, Strickler, and Webb in 1990, utilizes nonlinear excitation where fluorophores absorb two near-infrared photons simultaneously, providing intrinsic optical sectioning and deeper tissue penetration due to reduced scattering.^[113] Holography, pioneered by Dennis Gabor in 1948, records the interference pattern of light scattered from an object with a reference beam, reconstructing the full wavefront for three-dimensional imaging; modern digital holography enhances this with computational reconstruction for quantitative phase imaging. These methods often integrate photodetectors, such as avalanche photodiodes or CCD arrays, for signal capture.^[114] Spectroscopy techniques in photonics analyze light absorption, emission, or scattering to probe material properties at molecular scales. Absorption spectroscopy measures the attenuation of light intensity as it passes through a sample, revealing electronic or vibrational transitions based on Beer's law.^[115] Emission spectroscopy, conversely, detects light emitted from excited species, such as in fluorescence, providing insights into energy levels and dynamics.^[115] Raman spectroscopy, discovered by C.V. Raman in 1928, exploits inelastic scattering where photons exchange energy with molecular vibrations, yielding a spectral fingerprint shifted from the incident wavelength.^[116] Fourier transform infrared (FTIR) spectroscopy, conceptualized by Fellgett in 1951, uses an interferometer to encode spectral information in an interferogram, which is then Fourier-transformed to yield the spectrum, offering multiplex advantage for higher signal-to-noise ratios.^[117] Fundamental principles underpin the resolution and spectral fidelity of these techniques. The diffraction limit, formulated by Ernst Abbe in 1873, sets the minimum resolvable distance as d = \frac{\lambda}{2 \mathrm{NA}}, where \lambda is the wavelength and NA is the numerical aperture, constraining optical imaging to scales near the light wavelength.^[118] Fourier transforms play a central role in processing, as in FTIR where the spectrum S(\nu) is obtained via

S(\nu) = \int_{-\infty}^{\infty} I(\delta) e^{-i 2\pi \nu \delta} \, d\delta,

with I(\delta) the interferogram and \delta the optical path difference, enabling efficient reconstruction of high-resolution spectra.^[117] In imaging, Fourier optics decomposes the object into spatial frequencies, filtered by the pupil function to determine resolution limits.^[119] Photonics applications extend these techniques to advanced analytical tools. Hyperspectral imaging combines spatial and spectral resolution to capture hundreds of narrow bands, enabling material identification through unique spectral signatures, as reviewed in recent advancements for environmental and industrial monitoring.^[120] Optical coherence tomography (OCT), developed by Huang et al. in 1991, employs low-coherence interferometry for micron-scale 3D imaging, achieving axial resolutions around 1 \mum via broadband light sources and time-domain or spectral-domain detection. By 2025, AI-enhanced real-time spectroscopy integrates machine learning to process complex datasets, such as disentangling overlapping Raman signals for rapid chemical analysis in photonic platforms.^[121]

Integrated Systems

Photonic Integrated Circuits

Photonic integrated circuits (PICs) enable the monolithic or heterogeneous integration of multiple optical components, such as light sources, modulators, and photodetectors, onto a single chip to achieve compact, high-performance photonic systems. On silicon platforms, heterogeneous integration involves bonding III-V semiconductor materials, like indium phosphide (InP), onto silicon-on-insulator (SOI) substrates to combine the mature CMOS-compatible fabrication of silicon photonics with the light-emitting properties of III-V compounds. Monolithic integration on III-V platforms, such as InP, allows direct epitaxial growth of lasers, amplifiers, and detectors without the need for bonding, offering seamless compatibility for active devices but limited by smaller wafer sizes compared to silicon. These approaches facilitate the creation of fully integrated transceivers that incorporate core components like lasers for emission, electro-optic modulators for signal encoding, and photodetectors for reception, all within a unified structure. Fabrication of PICs primarily relies on photolithography and reactive ion etching to pattern waveguides and devices with sub-micron precision, akin to semiconductor processing techniques. In monolithic III-V integration, molecular beam epitaxy or metal-organic chemical vapor deposition grows layered structures directly on the substrate, followed by lithography-defined etching to form ridge or buried heterostructure waveguides. Heterogeneous integration on silicon contrasts by using wafer bonding or direct epitaxial techniques to transfer III-V layers onto patterned SOI wafers, enabling hybrid assemblies where silicon handles passive elements like waveguides while III-V provides active functionality; this method offers scalability through large silicon wafers but introduces challenges like thermal mismatch and alignment precision. Hybrid integration, often via flip-chip bonding or adhesive attachment, provides flexibility for post-fabrication assembly of disparate materials, though it may increase optical losses compared to fully monolithic schemes. Key architectures in PICs include arrayed waveguide gratings (AWGs) for wavelength multiplexing and ring resonators for filtering and modulation. AWGs consist of an input waveguide splitting light into an array of equally spaced waveguides with incremental length differences, creating phase shifts that disperse wavelengths at the output slab coupler for demultiplexing; they are essential for dense wavelength-division multiplexing (DWDM) in integrated transceivers, achieving low crosstalk and compact footprints on silicon nitride or SOI platforms. Ring resonators, formed by closed-loop waveguides evanescently coupled to bus waveguides, support resonant modes for add-drop filtering or electro-optic modulation via carrier injection or thermal tuning, enabling high-Q operation for narrowband applications in silicon photonics. PICs demonstrate impressive performance metrics, with individual components like modulators or detectors occupying footprints under 1 mm², enabling dense integration for chip-scale systems. Power efficiency has advanced to below 1 pJ/bit for electro-optic modulation in silicon-based links, driven by low-capacitance designs and efficient III-V gain media, which supports high-speed data transmission with minimal energy dissipation. As of 2025, foundry services like those from AIM Photonics provide multi-project wafer runs on silicon and InP platforms, accelerating prototyping and commercialization for scalable datacenter applications through standardized process design kits and high-volume production capabilities.

Photonic Networks and Devices

Photonic networks and devices represent assembled systems that interconnect photonic elements to enable high-capacity data transmission and routing, leveraging principles of light propagation for efficient signal management. These systems scale from board-level integrations to expansive infrastructures supporting global connectivity, often utilizing photonic integrated circuits (PICs) as foundational building blocks for modular assembly.^[122] Key devices within these networks include optical switches and multiplexers, which facilitate dynamic signal rerouting and wavelength management, while hybrid approaches like optoelectronic integrated circuits (OEICs) combine photonic and electronic functionalities to bridge optical and electrical domains.^[123] Optical switches form the core of routing mechanisms in photonic networks, enabling reconfiguration of light paths without electrical conversion. Micro-electromechanical systems (MEMS)-based switches use mechanical mirrors to redirect beams with low insertion loss and high crosstalk isolation, achieving switching times on the order of milliseconds, though they face challenges in miniaturization due to fabrication costs.^[124] Thermo-optic switches, prevalent in silicon photonics, exploit temperature-induced refractive index changes to alter waveguide paths, offering compact integration and power efficiencies below 10 mW per channel for 2×2 configurations, as demonstrated in scalable array designs.^[125] Wavelength-division multiplexing (WDM) multiplexers aggregate multiple optical signals onto a single fiber by combining distinct wavelengths, typically using arrayed waveguide gratings (AWGs) or thin-film filters to achieve channel spacings of 100 GHz or finer, thereby multiplying bandwidth without increasing fiber count.^[126] In access networks, passive optical networks (PONs) distribute signals from a central office to end-users via unpowered splitters, supporting symmetric rates up to 10 Gbps per wavelength in next-generation deployments like NG-PON2, which enhances scalability for fiber-to-the-home (FTTH) architectures.^[127] Metro and core networks handle long-haul transmission over hundreds of kilometers, employing dense WDM (DWDM) systems with up to 80 channels per fiber to transport terabits per second, where reconfigurable optical add-drop multiplexers (ROADMs) enable flexible wavelength provisioning.^[128] All-optical routing minimizes latency by processing signals entirely in the optical domain, using techniques like wavelength-selective switches to bypass optoelectronic conversions, thereby supporting transparent end-to-end connectivity in backbone infrastructures.^[129] Fundamental principles governing these networks include nonlinear effects such as cross-phase modulation (XPM), where the intensity of a pump beam induces a phase shift in a co-propagating signal beam via the Kerr effect, enabling ultrafast all-optical switching with response times below 1 ps in silicon waveguides.^[130] System performance is quantified by bit error rates (BER) typically maintained below 10^{-12}, ensuring reliable data integrity over long distances through forward error correction and optimized modulation formats like quadrature phase-shift keying (QPSK).^[131] These networks operate across scales, from board-level interconnects handling gigabits per second in data centers to the global internet backbone, driven by widespread FTTH and 5G backhaul deployments.^[132] Hybrid optoelectronic integrated circuits (OEICs) integrate lasers, modulators, and detectors with electronic drivers on a single chip, reducing latency and power consumption in transceiver modules, with advancements in III-V on silicon bonding enabling multi-wavelength operation up to 400 Gbps.^[133]

Applications

Telecommunications

Photonics plays a pivotal role in telecommunications by enabling high-speed, long-distance data transmission through fiber-optic systems, which utilize light signals to carry vast amounts of information with minimal loss. Dense wavelength division multiplexing (DWDM) is a core technology in these systems, allowing multiple optical signals at different wavelengths to share a single fiber, typically supporting up to 96 channels in commercial deployments.^[134] Recent advancements have pushed system capacities beyond 400 Tb/s; for instance, in 2024, researchers achieved a record 402 Tb/s transmission over standard single-mode fiber using a 37.6 THz bandwidth, demonstrating the scalability of DWDM for future networks.^[135] Key components in photonic telecommunications include transceivers such as small form-factor pluggable (SFP) and quad SFP (QSFP) modules, which convert electrical signals to optical for transmission and support data rates from 1 Gb/s to 400 Gb/s over distances up to 40 km or more depending on the variant.^[136] Erbium-doped fiber amplifiers (EDFAs) serve as repeaters to boost optical signals, typically spaced every 80 km in long-haul networks to compensate for attenuation without converting to electrical signals.^[137] These components rely on standard single-mode fibers like ITU-T G.652, which define characteristics such as low attenuation (around 0.2 dB/km at 1550 nm) and zero-dispersion wavelength near 1310 nm for efficient long-distance propagation.^[138] Coherent detection techniques, involving phase and polarization modulation with digital signal processing, enable high-spectral-efficiency transmission for 400G and beyond Ethernet standards, achieving rates up to 800 Gb/s over ultra-long distances like 2580 km.^[139]^[140] The evolution of photonic telecommunications has seen dramatic increases in data rates, from 10 Gb/s systems dominant in the 2000s to terabit-per-second capacities in the 2020s, driven by advancements in modulation formats and multiplexing.^[141] For example, single-fiber capacities grew from 2.5 Gb/s in the late 1980s to over 32 Tb/s by 2019, with commercial systems now routinely achieving over 20 Tb/s per fiber pair.^[142]^[143] Submarine cables exemplify this progress; the MAREA transatlantic cable, operational since 2018, provides a design capacity of 200 Tb/s across eight fiber pairs, supporting up to 700 Gb/s per wavelength for intercontinental data traffic.^[144] Photonics offers significant benefits over traditional copper-based systems, including low propagation latency of approximately 5 μs per km in fiber (resulting in about 5 ms one-way for 1000 km) due to the high speed of light signals at roughly two-thirds of vacuum speed.^[145] Additionally, fiber-optic networks achieve 70-80% lower energy consumption per connection compared to copper, primarily because optical transmission requires no active powering along the cable and incurs lower losses over distance, yielding substantial operational savings in large-scale telecom infrastructures.^[146]

Biomedical and Sensing

Photonics plays a pivotal role in biomedical applications, particularly through laser technologies that enable precise surgical interventions. Carbon dioxide (CO2) lasers, operating at a wavelength of 10.6 μm, are widely used for cutting and ablating soft tissues due to their high absorption by water, which minimizes thermal damage to surrounding areas.^[147] In endoscopic procedures, photonic systems integrate optical fibers and imaging modalities to provide minimally invasive visualization of internal organs, enhancing diagnostic accuracy in gastroenterology and pulmonology.^[148] Photodynamic therapy (PDT) leverages photosensitizers activated by specific wavelengths of light to generate reactive oxygen species, selectively destroying cancer cells while sparing healthy tissue, with clinical applications in treating skin, esophageal, and lung cancers.^[149] In sensing applications, fiber Bragg gratings (FBGs) inscribed in optical fibers serve as robust sensors for monitoring strain and temperature in biomedical contexts, such as implantable devices for real-time physiological tracking, by detecting shifts in reflected wavelength caused by mechanical or thermal perturbations.^[150] Plasmonic sensors, exploiting surface plasmons on metallic nanostructures, offer ultrasensitive detection of biomolecules like proteins and DNA through refractive index changes, enabling label-free identification of biomarkers in blood or saliva for disease diagnosis.^[151] Advanced photonic techniques further expand diagnostic capabilities. Fluorescence lifetime imaging (FLIM) measures the decay time of fluorescence emission to map metabolic changes in tissues, providing contrast for early detection of cellular abnormalities without relying on intensity variations.^[152] For environmental monitoring, light detection and ranging (LIDAR) systems employ pulsed lasers to profile atmospheric constituents, such as aerosols and gases, supporting air quality assessment and climate research through remote, high-resolution sensing.^[153] Recent advances as of 2025 highlight the portability and wearability of photonic devices. Wearable photonic sensors utilizing integrated waveguides and spectrometers enable continuous, non-invasive glucose monitoring via sweat or interstitial fluid analysis, achieving detection limits below 1 mM for diabetes management.^[154] Portable optical coherence tomography (OCT) devices, now handheld and battery-operated, deliver micron-scale cross-sectional imaging of skin and ocular tissues in point-of-care settings, facilitating rapid screening in resource-limited environments.^[155] These photonic innovations drive significant impacts in non-invasive diagnostics, allowing early cancer detection with resolutions approaching nanometer scales through techniques like plasmonic enhancement and super-resolution microscopy, which identify precancerous lesions via subtle biomolecular signatures before morphological changes occur.^[156] Such capabilities reduce the need for biopsies, improve patient outcomes, and enable personalized medicine by quantifying disease progression at molecular levels.^[157]

Computing and Energy

Photonics plays a pivotal role in advancing computing by leveraging light for faster data processing and reduced energy consumption. Optical interconnects in data centers replace traditional electrical links with photonic alternatives, enabling higher bandwidth densities—up to terabits per second per channel—and lower power dissipation, which is critical for scaling AI workloads. These interconnects, often based on silicon photonics platforms, mitigate thermal bottlenecks and support massive parallelism in hyperscale environments. For instance, vertical-cavity surface-emitting laser (VCSEL) arrays serve as key devices for parallel processing, allowing simultaneous transmission across multiple channels with low crosstalk and high modulation speeds exceeding 50 Gb/s per channel. This architecture facilitates efficient data routing in multi-rack systems, enhancing overall throughput while consuming significantly less energy than copper-based counterparts.^[158] Photonic accelerators further revolutionize machine learning by performing matrix-vector multiplications optically, achieving ultralow latency and up to 10x speedups over electronic counterparts in 2025 demonstrations. A core principle enabling this is the optical Fourier transform, which computes frequency-domain operations at the speed of light with near-zero energy cost, as realized in integrated photonic chips using phase modulators and diffractive elements. These accelerators excel in convolutional neural networks, where light-based analog processing bypasses digital von Neumann bottlenecks, delivering inferences at gigahertz rates. Recent trends highlight photonic neuromorphic chips, with 2024 prototypes integrating wavelength-division multiplexing and nonlinear optics to mimic synaptic behaviors, promising energy-efficient edge computing for real-time AI tasks.^[159] In renewable energy applications, photonics enhances photovoltaic performance through spectral management and light harvesting innovations. Solar cells incorporating photonic structures, such as nanostructures for light trapping and anti-reflection coatings, have surpassed 25% power conversion efficiency, as seen in tandem perovskite-silicon devices that broaden absorption spectra. Upconversion processes in photovoltaics utilize lanthanide-doped materials to convert sub-bandgap infrared photons into visible ones, potentially increasing the thermodynamic efficiency limit by 30-40% under concentrated sunlight. Complementary devices like luminescent solar concentrators employ waveguide-embedded fluorophores to capture diffuse light over large areas and guide it to edge-mounted cells, achieving optical concentration factors of 10-50x with minimal tracking requirements.^[160] Light-emitting diodes (LEDs) benefited from photonic engineering have reached luminous efficacies exceeding 200 lm/W, enabling widespread adoption in energy-efficient lighting that reduces global electricity demand for illumination by up to 50%. These advancements stem from optimized quantum wells and photonic crystals that enhance extraction efficiency and color rendering. Broader trends in green photonics align with net-zero energy goals, integrating low-loss optical components in solar tracking systems and energy-efficient data processing to cut carbon emissions across sectors, with photonic technologies projected to contribute 10-20% savings in ICT energy use by 2030.^[161]

Research Directions

Nanophotonics

Nanophotonics encompasses the manipulation of light at the nanoscale, where structures comparable to or smaller than the wavelength of light enable phenomena that surpass classical optical limits. This field leverages nanoscale confinement to control electromagnetic waves with subwavelength precision, facilitating applications in sensing, imaging, and integrated optics. Key to nanophotonics are effects arising from near-field interactions, artificial materials engineered for unusual responses, and plasmonic resonances that couple light to electron oscillations. Near-field optics exploits evanescent waves generated close to nanostructures to achieve subwavelength resolution beyond the diffraction limit of far-field microscopy. In conventional optics, the diffraction limit restricts resolution to approximately half the wavelength of light, but near-field techniques, such as scanning near-field optical microscopy (SNOM), probe evanescent fields within tens of nanometers of the sample surface, enabling imaging and spectroscopy at scales down to 10-20 nm. This approach confines light to nanometric domains, allowing for the resolution of features like individual molecules or quantum dots that are otherwise inaccessible. Metamaterials, artificially structured composites, enable exotic light propagation by achieving effective permittivities and permeabilities not found in natural materials. Victor Veselago theoretically predicted in 1968 that substances with simultaneously negative permittivity (ε < 0) and permeability (μ < 0) would exhibit negative refraction, where light bends oppositely to the normal Snell's law, leading to reversed phase and group velocities. This concept remained unrealized until the early 2000s, when David Smith and colleagues demonstrated a composite medium with negative refractive index at microwave frequencies using arrays of split-ring resonators and wire arrays, confirming negative refraction experimentally. Building on this, metamaterials advanced to cloaking devices in the 2010s, with John Pendry's 2006 transformation optics framework enabling designs that guide electromagnetic waves around an object, rendering it invisible to specific wavelengths; subsequent microwave and optical realizations, such as those using layered dielectrics, achieved broadband cloaking over limited angles.^[162] Plasmonics, a cornerstone of nanophotonics, involves surface plasmons—collective electron oscillations at metal-dielectric interfaces—that strongly confine light to nanoscale volumes. Surface plasmon polaritons (SPPs), hybrid light-matter modes, propagate along these interfaces with a dispersion relation given by

k_{\mathrm{spp}} = k_0 \sqrt{\frac{\epsilon_m \epsilon_d}{\epsilon_m + \epsilon_d}},

where k_0 = \omega / c is the free-space wavevector, \epsilon_m is the metal permittivity, and \epsilon_d is the dielectric permittivity; this relation shows that k_{\mathrm{spp}} exceeds k_0, enabling subwavelength guiding with propagation lengths up to tens of micrometers before ohmic losses dominate. Applications of plasmonics include nanoantennas, which are subwavelength metallic nanostructures that efficiently couple far-field light to localized near-field hotspots, enhancing emission or absorption for single-molecule detection. For instance, bowtie-shaped gold nanoantennas have demonstrated directive radiation patterns and field enhancements exceeding 1000, useful in nanoscale optical tweezers and photodetectors. A prominent example is surface-enhanced Raman scattering (SERS), where plasmonic hotspots amplify Raman signals by electromagnetic field intensification, achieving enhancement factors up to $10^{10}, allowing detection of analytes at femtomolar concentrations for chemical sensing and biospectroscopy.^[163] As of 2025, two-dimensional (2D) materials like graphene have emerged for tunable nanophotonic devices, leveraging their gate-voltage-controlled Fermi level to dynamically adjust plasmonic resonances. Graphene supports highly confined SPPs with wavelengths down to 10% of free-space values, enabling electrostatically tunable nanoantennas and modulators operating from terahertz to visible frequencies. Recent integrations, such as graphene-van der Waals heterostructures in photonic chips, have realized broadband absorbers and phase shifters with modulation depths over 90%, paving the way for compact, reconfigurable nanophotonic systems in communications and sensing.^[164]

Quantum Photonics

Quantum photonics harnesses the quantum mechanical properties of light, such as superposition and entanglement, to enable advanced information processing and sensing technologies that surpass classical limits. Central to this field are quantum states of light, including single photons and squeezed states, which exhibit non-classical behaviors essential for secure communication and precision measurements. Single photons serve as indivisible quanta of light, ideal for applications requiring minimal noise, while squeezed states reduce uncertainty in one quadrature of the electromagnetic field below the standard quantum limit at the expense of the conjugate quadrature, enhancing sensitivity in interferometric setups.^[165]^[166] A hallmark demonstration of these properties is the Hong-Ou-Mandel (HOM) interference, where two indistinguishable photons incident on a 50:50 beam splitter bunch into the same output port due to destructive interference in the coincident detection probability, achieving near-perfect visibility for identical photons.^[167] Key devices in quantum photonics include quantum dots as efficient single-photon sources and beam splitters as fundamental gates for photonic quantum circuits. Semiconductor quantum dots, such as those based on InAs/GaAs heterostructures, generate on-demand single photons with high purity (>99%) and indistinguishability (>95%) through radiative recombination of excitons, enabling scalable quantum networks when integrated with photonic structures.^[168] Beam splitters, often realized in waveguides or fiber optics, perform linear optical operations that form the basis of quantum gates, such as controlled-NOT equivalents via measurement-induced nonlinearity, facilitating universal quantum computing with photons.^[169] The degree of two-photon interference in HOM setups is quantified by the visibility formula:

V = 1 - \frac{2 P_{\text{coinc}}}{P_1 P_2}

where P_{\text{coinc}} is the coincidence detection rate at zero delay, and P_1 and P_2 are the individual photon detection rates, with ideal indistinguishability yielding V = 1.^[170] Prominent applications of quantum photonics include quantum key distribution (QKD) and enhanced sensing. The BB84 protocol, proposed by Bennett and Brassard in 1984, uses polarized single photons to securely distribute encryption keys, detecting eavesdropping through quantum uncertainty principles and enabling information-theoretically secure communication. Commercial QKD systems based on BB84 have proliferated in the 2020s, with deployments over fiber networks exceeding 100 km and integration into metropolitan infrastructures for secure data links.^[171] In sensing, quantum photonic techniques employing squeezed states or entangled photons surpass the standard quantum limit (SQL), achieving Heisenberg-limited precision; for instance, single-photon interferometry has demonstrated sub-SQL phase sensitivity for large photon numbers (~30,000), improving applications in gravitational wave detection and biomedicine.^[172] By 2025, advancements in integrated quantum photonic chips have enabled compact, scalable platforms combining quantum dot sources and waveguide circuits on silicon substrates, achieving high-fidelity operations for on-chip quantum processing.^[173] Satellite-based quantum networks, exemplified by extensions of China's Micius mission, have established long-distance QKD links over thousands of kilometers, with new low-Earth orbit satellites launched in 2025 supporting global quantum-secure communication infrastructures.^[174]

Challenges and Future Prospects

One major challenge in advancing photonics lies in achieving scalable integration of photonic components, particularly in high-density systems where 3D stacking increases power density and complicates manufacturing yields.^[60] Thermal management emerges as a critical bottleneck, with high-density integration leading to temperature sensitivities that can cause gain drops of approximately 5 dB per 20 °C rise, necessitating advanced cooling solutions like thermoelectric coolers that inflate power budgets.^[175] Additionally, the cost of III-V materials remains a significant barrier, as heterogeneous integration with silicon for light generation adds complexity and expense, limiting widespread adoption in cost-sensitive applications.^[176] Photonics systems also grapple with losses and efficiency constraints that hinder performance. Coupling losses between fibers and chips often exceed 3 dB in standard configurations, particularly for edge couplers with strict alignment tolerances of ±1 μm, which impacts overall system efficiency.^[177] Quantum efficiency in photodetectors and related devices faces inherent limits, typically constrained by material properties and integration challenges, though recent optimizations have pushed efficiencies toward 98.5% in germanium-based platforms.^[178] Looking ahead, photonic AI hardware holds promise for energy-efficient computing, with electro-photonic hybrids enabling up to 10 times the throughput of traditional GPUs while reducing carbon footprints by orders of magnitude through optical general matrix multiplication operations.^[179] Sustainable manufacturing practices are gaining traction, emphasizing lower embodied carbon in fabrication—potentially 4.1 times less than comparable CMOS processes—and higher yields to minimize e-waste.^[179] Efforts toward global standards, such as those discussed at the Global Photonics Economic Forum, aim to harmonize integration schemes and packaging to accelerate commercialization.^[180] Prospects for exascale optical computing by 2030 include leveraging photonic interconnects to overcome bandwidth bottlenecks in high-performance systems, enabling peta- and exa-scale data processing with reduced latency.^[181] In telecommunications, photonics integration into 6G networks via terahertz bands offers ultra-high data rates but must address propagation losses and spectrum management challenges in these frequencies.^[182] As of 2025, emerging AI-photonics hybrids, such as silicon-organic integrated circuits achieving 110 GHz bandwidths, are bridging computational gaps for real-time inference in data centers.^[183] Photonics also addresses climate applications through optical sensing for monitoring CO₂ levels, air quality, and renewable energy efficiency, contributing to efforts to limit global warming to 1.5°C via sustainable development pathways.^[184]

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Hybrid integrated photonic platforms: opinion
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DWDM Technology Explained: High-Capacity Optical Networking
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Photodynamic Therapy Review: Principles, Photosensitizers ...
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Advances and Applications of Wearable Photonic Sensors
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A review of label-free photonics-based techniques for cancer ...
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Ultra-Low Loss and Large Bandwidth Fiber-to-Chip Edge Coupler for Aluminum Oxide Photonic Platform at UV-Visible Wavelengths
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Light for life - SPIE
Jan 1, 2025 · Photonics has transformative potential to address the UN's 17 sustainabile development goals, and represents a critical technology pathway ...