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Light scattering by particles

Light scattering by particles refers to the elastic redirection of electromagnetic waves, such as visible light, by small objects suspended in a medium, where the particles' dimensions are comparable to or smaller than the wavelength of the incident radiation.^[1] This interaction arises from the induced oscillations in the particle's electric dipole moment, which reradiates the energy in all directions, altering the light's path, intensity, and polarization.^[2] The phenomenon is quantified by the scattering cross-section, which measures the effective area of the particle responsible for deflecting the light beam.^[1] The theoretical foundation of light scattering by particles is rooted in classical electromagnetism, with key developments including Rayleigh's approximation for particles much smaller than the wavelength (size parameter x = 2\pi a / \lambda \ll 1, where a is the particle radius and \lambda is the wavelength).^[2] In this regime, the scattered intensity is proportional to the fourth power of the frequency, leading to stronger scattering of shorter wavelengths and explaining natural observations like the blue color of the sky.^[2] For larger particles where x \approx 1, Mie theory provides an exact solution for spherical particles by solving Maxwell's equations using vector spherical harmonics, accounting for both scattering and potential absorption.^[1] Beyond basic models, light scattering encompasses more complex scenarios involving nonspherical particles, ensembles, and multiple scattering events, often analyzed using methods like the T-matrix approach for arbitrary shapes or the discrete dipole approximation for irregular forms.^[1] These interactions also involve absorption, where light energy is converted to heat, and emission from thermally excited particles, influencing the overall extinction of the beam.^[1] Polarization effects, described by the Stokes parameters and scattering matrices, play a critical role in characterizing the process, with applications in remote sensing of atmospheric aerosols and planetary atmospheres.^[1] In practical contexts, light scattering by particles is essential for techniques like dynamic light scattering (DLS) to measure particle size distributions in colloids and suspensions, as well as in environmental monitoring of pollutants. It underpins phenomena in optics, such as the formation of rainbows and glories, and informs models in radiative transfer for climate simulations.^[1] Advances in computational tools, including publicly available codes for Mie and T-matrix solutions, enable precise predictions for diverse particle systems.^[1]

Overview

Definition and physical basis

Light scattering by particles refers to the redirection of electromagnetic waves, commonly known as light, upon interaction with particles whose dimensions are much smaller than or comparable to the wavelength of the light. This phenomenon encompasses both elastic scattering, where the scattered light maintains the same wavelength as the incident light with no energy loss to the particle, and inelastic processes such as Raman scattering, though elastic scattering predominates in many applications.^[3] The physical basis of light scattering arises from the interaction between the oscillating electric field of the incident light and the electrons within the particle, which induces oscillating dipoles that serve as secondary sources of radiation. These induced dipoles re-emit electromagnetic waves in all directions, effectively redirecting the original light. This process can be understood through the Huygens-Fresnel principle, which describes the scattered field as the coherent superposition of secondary wavelets emanating from the induced dipoles across the particle's surface or volume.^[3] Particles involved in light scattering can be dielectric (non-absorbing), absorbing, or metallic, and they may be suspended in gases, liquids, or even vacuums. A fundamental dimensionless quantity characterizing the scattering is the size parameter \alpha = \frac{2\pi a}{\lambda}, where a is the particle radius and \lambda is the wavelength of the incident light; this parameter determines the relative scale of the particle to the light wave and influences the scattering regime.^[3]^[4] The scattered electric field \mathbf{E}_s in the far field is related to the incident field \mathbf{E}_i by the general expression

\mathbf{E}_s = \frac{e^{ikr}}{r} f(\theta) \mathbf{E}_i,

where r is the distance from the particle, k = 2\pi / \lambda is the wave number, and f(\theta) is the scattering amplitude, a complex function that depends on the scattering angle \theta and encapsulates the particle's properties.^[3] Light scattering by particles plays a crucial role in atmospheric optics, where it explains phenomena such as the blue color of the sky due to preferential scattering of shorter wavelengths by air molecules and aerosols. In biomedical imaging, it enables non-invasive techniques for probing cellular structures and tissue properties through elastic light interactions. Additionally, in nanotechnology, understanding scattering helps design materials with tailored optical responses, such as in plasmonic nanoparticles for sensing and light manipulation.^[3]^[5]^[6]

Historical development

In the mid-19th century, early experimental investigations into light scattering by particles focused on atmospheric phenomena. In 1869, John Tyndall conducted experiments using a beam of sunlight passed through a tube filled with air containing suspended dust particles, observing that the scattered light appeared blue and was polarized, which he attributed to scattering by these small particles and linked to the blue color of the sky.^[7] Two years later, in 1871, Lord Rayleigh developed a theoretical framework explaining the scattering of light by molecules much smaller than the wavelength, deriving the inverse fourth-power dependence on wavelength that accounts for the predominance of blue light in clear skies. The early 20th century saw significant theoretical advances for scattering by larger particles. In 1908, Gustav Mie published a rigorous solution to Maxwell's equations for the scattering and absorption of electromagnetic waves by a homogeneous sphere of arbitrary size, providing exact expressions for scattering cross-sections and angular distributions. Independently in the same year, Peter Debye derived an equivalent formulation for electromagnetic scattering by spheres, emphasizing radiation pressure and confirming Mie's results through a series expansion approach.^[8] Mid-century developments integrated these theories into broader applications, particularly in atmospheric and colloidal science. The Rayleigh-Gans approximation, developed in the early 20th century by Rayleigh and Gans for large, optically soft particles where phase shifts are small, enabling simplified calculations for non-spherical and inhomogeneous scatterers, was further advanced by Hendrik van de Hulst in his 1957 book, Light Scattering by Small Particles.^[9] Van de Hulst's seminal 1957 book, Light Scattering by Small Particles, synthesized these and other theories, providing a comprehensive reference that bridged exact solutions like Mie theory with approximations and surveyed applications to planetary atmospheres and aerosols.^[10] In 1969, Milton Kerker's book The Scattering of Light and Other Electromagnetic Radiation compiled and expanded upon these foundations, emphasizing experimental validations and extensions to multiple scattering in complex media.^[11] From the 1980s onward, computational advancements facilitated practical implementations and extensions beyond spherical particles. In 1983, Craig Bohren and Donald Huffman published Absorption and Scattering of Light by Small Particles, which included widely adopted FORTRAN code for efficient numerical evaluation of Mie theory, enabling rapid simulations for atmospheric and material science applications. During the 1990s, significant progress occurred in methods for non-spherical particles, including refinements to the T-matrix approach for axisymmetric shapes and the discrete dipole approximation for arbitrary geometries, allowing accurate modeling of ice crystals, dust, and biological aerosols in climate and remote sensing studies.^[12]

Theoretical foundations

Electromagnetic scattering principles

The theoretical foundation of light scattering by particles rests on Maxwell's equations, which govern the behavior of electromagnetic fields in the presence of matter. For time-harmonic fields with angular frequency \omega, assuming non-magnetic media where the permeability \mu = \mu_0 is constant and uniform, the equations simplify to the vector Helmholtz forms:

\nabla \times \mathbf{E} = i \omega \mu_0 \mathbf{H}, \quad \nabla \times \mathbf{H} = -i \omega \epsilon_0 \epsilon(\mathbf{r}) \mathbf{E},

where k_0 = \omega \sqrt{\mu_0 \epsilon_0} is the vacuum wavenumber, \epsilon(\mathbf{r}) is the position-dependent relative permittivity, \mathbf{E} is the electric field, and \mathbf{H} is the magnetic field. These equations must be solved subject to boundary conditions at the interfaces between the particle and the surrounding medium, ensuring continuity of the tangential components of \mathbf{E} and \mathbf{H}, as well as the normal components of \mathbf{D} = \epsilon_0 \epsilon \mathbf{E} and \mathbf{B} = \mu_0 \mathbf{H}. This framework captures the interaction of electromagnetic waves with dielectric or absorbing particles without approximations to particle geometry.^[13] In the scattering formulation, an incident plane wave \mathbf{E}_i = \mathbf{E}_0 e^{i k_0 z} (assuming propagation along the z-axis) illuminates the particle, and the total field is expressed as \mathbf{E} = \mathbf{E}_i + \mathbf{E}_s, where \mathbf{E}_s is the scattered field. The scattered field \mathbf{E}_s satisfies the Sommerfeld radiation condition at infinity, \lim_{r \to \infty} r \left( \frac{\partial \mathbf{E}_s}{\partial r} - i k_0 \mathbf{E}_s \right) = 0, ensuring outgoing spherical waves.^[14] For dielectric particles embedded in a non-absorbing host medium, the problem can be recast as a volume integral equation using the Lippmann-Schwinger formalism. The total electric field inside the particle volume V obeys

\mathbf{E}(\mathbf{r}) = \mathbf{E}_i(\mathbf{r}) + k_0^2 \int_V \mathbf{G}(\mathbf{r}, \mathbf{r}') \chi(\mathbf{r}') \mathbf{E}(\mathbf{r}') \, dV',

where \mathbf{G} is the dyadic Green's function for the background medium, representing the field due to a point dipole source, and the susceptibility \chi(\mathbf{r}) = \epsilon(\mathbf{r}) - 1 accounts for the particle's contrast. This integral equation, derived directly from Maxwell's equations via the Green's theorem, allows numerical solution for arbitrary shapes by discretizing the volume and solving for the induced field iteratively or via matrix inversion.^[14] Surface integral methods offer an alternative by reformulating the problem on the particle's boundary \partial V, avoiding volume discretization for efficiency with smooth surfaces. These approaches, such as the boundary element method, express the scattered field in terms of equivalent electric \mathbf{J} and magnetic \mathbf{M} surface currents on \partial V, satisfying electric field integral equations (EFIE) or magnetic field integral equations (MFIE):

\mathbf{E}_s(\mathbf{r}) = i \omega \mu_0 \int_{\partial V} \mathbf{G}(\mathbf{r}, \mathbf{r}') \cdot \mathbf{M}(\mathbf{r}') \, ds' - \frac{i}{k_0 \eta_0} \nabla \times \int_{\partial V} \mathbf{G}(\mathbf{r}, \mathbf{r}') \cdot \mathbf{J}(\mathbf{r}') \, ds',

with \eta_0 = \sqrt{\mu_0 / \epsilon_0} the free-space impedance.^[15] The currents are determined by enforcing boundary conditions, leading to a system of integral equations solved via Galerkin or collocation techniques.^[16] This method is particularly suited for impenetrable or high-contrast scatterers, as it reduces the dimensionality of the problem. Polarization effects are inherently vectorial in electromagnetic scattering, distinguishing it from scalar wave approximations valid only for unpolarized or isotropic cases. The full treatment requires solving for the vector fields, yielding polarization-dependent scattered intensities described by the Stokes parameters I, Q, U, V, which fully characterize the partial polarization state.^[14] For instance, the scattering matrix relates the incident and scattered Stokes vectors, capturing phenomena like linear depolarization or circular polarization conversion, essential for analyzing non-spherical or oriented particles.^[10] Scalar treatments, while simpler for total intensity, neglect these effects and are limited to Rayleigh-like regimes or averaged unpolarized light.

Key scattering parameters

In light scattering by particles, key parameters characterize the extent and nature of the interaction between electromagnetic waves and scatterers, enabling quantitative descriptions of phenomena such as attenuation, angular redistribution, and polarization changes. These parameters are derived from fundamental solutions to Maxwell's equations and are essential for interpreting scattering data across various regimes, from small dielectric particles to complex aggregates. Standard references establish these quantities as the scattering cross-section for total redirected power, the differential cross-section for angular details, the extinction cross-section linking removal to forward scattering, the phase function for directional preferences, and the Mueller matrix for polarization transformations.^[13] The scattering cross-section \sigma_s quantifies the total power scattered out of the incident beam per unit incident intensity, effectively representing the particle's "scattering area." It is defined as the integral of the differential scattering cross-section over all solid angles:

\sigma_s = \int_{4\pi} \frac{d\sigma}{d\Omega} \, d\Omega,

with units of m², analogous to geometric cross-sections but accounting for wave interference effects. This parameter directly measures how much light is redistributed by the particle without absorption.^[13] The differential scattering cross-section \frac{d\sigma}{d\Omega} provides the angular distribution of scattered power, specifying the fraction scattered into a particular direction. It relates the far-field scattered intensity I_s to the incident intensity I_i at distance r via

I_s = \frac{I_i}{r^2} \frac{d\sigma}{d\Omega},

allowing reconstruction of \sigma_s through integration; variations in \frac{d\sigma}{d\Omega} reveal interference patterns, such as forward lobes in larger particles.^[13] The extinction cross-section \sigma_\text{ext} encompasses both scattering and absorption, defined as \sigma_\text{ext} = \sigma_s + \sigma_a, where \sigma_a is the absorption cross-section representing energy dissipated as heat. A fundamental relation, the optical theorem, connects \sigma_\text{ext} to the imaginary part of the forward scattering amplitude f(0):

\sigma_\text{ext} = \frac{4\pi}{k} \operatorname{Im}[f(0)],

with k = 2\pi / \lambda the wave number; this theorem underscores that total beam attenuation is tied to forward scattering interference, holding for any scatterer in classical electromagnetism.^[13] The phase function P(\theta) describes the normalized probability density of scattering into angle \theta relative to the incident direction, normalized such that

\int_{4\pi} P(\theta) \, d\Omega = 4\pi.

It captures the angular pattern, often peaking forward for particles comparable to the wavelength; the asymmetry parameter g = \langle \cos \theta \rangle = \frac{1}{4\pi} \int P(\theta) \cos \theta \, d\Omega quantifies this, with g > 0 indicating forward preference (e.g., g \approx 0.8 for cloud droplets) and g = 0 for symmetric cases. In the Rayleigh regime for small particles, P(\theta) is nearly isotropic.^[13] For partially polarized light, the Mueller matrix \mathbf{M} is a 4×4 real matrix that linearly transforms the incident Stokes vector \mathbf{S}_i = (I, Q, U, V)^T to the scattered Stokes vector \mathbf{S}_s = \mathbf{M} \mathbf{S}_i, fully encoding changes in intensity, linear polarization, and circular polarization due to scattering. Elements of \mathbf{M} relate to the differential cross-section for unpolarized light (M_{11}) and depolarization effects (e.g., M_{12}, M_{34}), with symmetry properties like M_{ij} = M_{ji} for rotationally invariant scatterers; this formalism extends scalar theories to vectorial wave behavior.^[12]^[13]

Analytical approximation methods

Rayleigh scattering regime

The Rayleigh scattering regime describes the interaction of electromagnetic waves with particles whose characteristic dimensions are much smaller than the wavelength of the incident light, specifically when the size parameter α = 2πa/λ ≪ 1, where a is the particle radius and λ is the wavelength, often valid for α < 0.3 or a ≪ λ/10. In this quasi-static approximation, higher-order multipole contributions are negligible, and the particle responds primarily as an induced electric dipole to the uniform incident field, neglecting retardation effects across the particle volume. This regime is foundational for understanding scattering in dilute gases, aerosols, and molecular atmospheres, as originally formulated by Lord Rayleigh in his analysis of skylight polarization and color. The derivation begins with the incident electric field E_i inducing a dipole moment p = α_p E_i in the particle, where α_p is the complex polarizability. For a homogeneous dielectric sphere with relative permittivity ε, the polarizability is given by α_p = 4π ε_0 a³ (ε - 1)/(ε + 2), derived from the electrostatic solution for a sphere in a uniform field, extended to the optical domain under the assumption of non-magnetic materials (μ = 1) and negligible absorption (real ε). The induced dipole radiates a scattered field akin to that of a Hertzian dipole, with the far-field scattered electric field proportional to the second time derivative of p. Integrating the radiated power over all directions yields the total scattering cross-section σ_s = (8π/3) k⁴ |α_p|² / (4π ε_0)², where k = 2π/λ is the wavenumber; this expression simplifies to σ_s ∝ a⁶ / λ⁴ | (ε - 1)/(ε + 2) |², highlighting the strong inverse fourth-power wavelength dependence that preferentially scatters shorter (blue) wavelengths, as observed in the clear daytime sky.^[17] The differential scattering cross-section exhibits angular dependence due to the dipole radiation pattern. For unpolarized incident light, dσ/dΩ ∝ |α_p|² k⁴ (1 + cos²θ)/2, where θ is the scattering angle relative to the incident direction, showing forward-backward symmetry and maximum intensity perpendicular to the incident propagation (θ = 90°). This polarization-sensitive pattern arises from the vector nature of the dipole oscillation, with linearly polarized incident light producing scattered light polarized parallel to the incident polarization in the forward and backward directions. For small non-absorbing spheres, the scattered light is partially polarized, with the degree of polarization increasing toward the sides.^[17] This approximation has limitations: it assumes electrically small particles where phase shifts are minimal and fails for absorbing materials (complex ε), where imaginary components alter the polarizability and introduce asymmetry, or when α > 0.3, necessitating higher multipole terms as in Mie theory for spheres. Extensions like the Rayleigh-Gans approximation address cases of larger but optically soft particles (|ε - 1| ≪ 1), maintaining the dipole-like response but incorporating phase variations across the particle.

Geometric optics regime

The geometric optics regime applies to light scattering by particles where the size parameter \alpha \gg 1, defined as \alpha = 2\pi a / \lambda with a the particle radius and \lambda the wavelength, such that a \gg \lambda and the wave nature of light becomes negligible.^[18] In this limit, scattering is modeled using ray-tracing techniques that treat light as rays propagating in straight lines, governed by Snell's law for refraction at interfaces and Fresnel equations for the amplitude and polarization of reflected and transmitted rays.^[19] This approximation is particularly suitable for large, non-absorbing particles like cloud droplets or ice crystals, where interference effects are minimal.^[20] Ray paths in the geometric optics regime include external reflection from the particle surface, internal refraction and multiple reflections within transparent particles, and edge diffraction around the particle boundary. External reflections contribute to backscattering, while refracted rays undergo internal bounces that can lead to specific angular features such as the glory (near-backward scattering from rays grazing the surface) and rainbows (from rays with one or more internal reflections). Edge diffraction is approximated using Fermat's principle to determine ray deviations at boundaries, supplementing the primary reflection and refraction contributions.^[21] These paths are traced numerically to compute the scattered intensity, accounting for the particle's shape and refractive index.^[22] In this regime, the extinction cross-section approaches approximately $2\pi a^2, reflecting the extinction paradox where the total removed power equals twice the geometric cross-section due to shadowing and diffraction effects. For opaque spheres, the scattering cross-section simplifies to σ_s ≈ 2π a², consisting of contributions from surface reflections and diffraction (each approximately π a²), without transmission. These values highlight the scale of interaction, where diffraction alone accounts for the forward lobe roughly equal to the geometric area.^[23] Angular distributions feature a prominent forward diffraction peak from rays bent around the particle, broad backscattering from external and internal reflections, and sharp peaks at rainbow angles, such as approximately 138° for primary rainbows in water droplets with refractive index near 1.33.^[24]^[25] The glory appears as a backward enhancement from surface-reflected rays.^[25] Limitations of the geometric optics regime include its neglect of wave interference, which underestimates oscillations in the scattering patterns for finite sizes, making it approximate even for \alpha \sim 100 and best for \alpha > 100 in non-absorbing cases. It can be integrated briefly with Mie theory for hybrid models in intermediate regimes to capture partial wave effects.^[26]^[27]

Exact analytical methods for simple geometries

Mie theory for spherical particles

Mie theory provides the exact analytical solution to Maxwell's equations for the scattering of a plane electromagnetic wave by a homogeneous, isotropic sphere of radius a and relative refractive index m = n + ik, where n is the real part and k the imaginary part accounting for absorption.^[28] The formulation involves solving the vector wave equation in spherical coordinates, expanding both the incident and scattered fields in terms of vector spherical harmonics. The incident plane wave is expressed as a sum over these harmonics, and the scattered field is similarly expanded but with coefficients determined by boundary conditions at the sphere's surface.^[29] The scattering coefficients a_n and b_n, which govern the electric and magnetic multipole contributions, are obtained by matching the tangential components of the electric and magnetic fields at the boundary r = a. These are given by

a_n = \frac{m \psi_n(mx) \psi_n'(x) - \psi_n(x) \psi_n'(mx)}{m \psi_n(mx) \xi_n'(x) - \xi_n(x) \psi_n'(mx)},

b_n = \frac{\psi_n(mx) \psi_n'(x) - m \psi_n(x) \psi_n'(mx)}{\psi_n(mx) \xi_n'(x) - m \xi_n(x) \psi_n'(mx)},

where x = ka = 2\pi a / \lambda is the size parameter with k the wavenumber in the surrounding medium, \psi_n(z) = z j_n(z) the Riccati-Bessel function with spherical Bessel function j_n, and \xi_n(z) = z h_n^{(1)}(z) the Riccati-Hankel function of the first kind with h_n^{(1)}; primes denote derivatives with respect to the argument.^[29] For non-absorbing spheres (k=0), m reduces to the real refractive index ratio.^[28] Key parameters include the scattering efficiency Q_s = \sigma_s / (\pi a^2), extinction efficiency Q_{\rm ext} = \sigma_{\rm ext} / (\pi a^2), and absorption efficiency Q_a = \sigma_a / (\pi a^2), where \sigma_s, \sigma_{\rm ext}, and \sigma_a are the respective cross sections. These are computed as

Q_s = \frac{2}{x^2} \sum_{n=1}^\infty (2n+1) (|a_n|^2 + |b_n|^2), \quad Q_{\rm ext} = \frac{2}{x^2} \sum_{n=1}^\infty (2n+1) \operatorname{Re}(a_n + b_n), \quad Q_a = Q_{\rm ext} - Q_s.

^[29] Plots of Q_s and Q_{\rm ext} versus x exhibit characteristic ripple structures due to interference between diffracted and reflected/refracted waves, with oscillations becoming more pronounced for larger x > 10. In the large-particle limit (x \to \infty), Q_{\rm ext} \to 2, known as the extinction paradox, where the extinction cross section approaches twice the geometric area because diffraction contributes equally to shadowing.^[30]^[31] The theory also describes polarization effects through the amplitude functions for perpendicular and parallel polarizations:

S_1(\theta) = \sum_{n=1}^\infty \frac{2n+1}{n(n+1)} \left( a_n \pi_n(\cos\theta) + b_n \tau_n(\cos\theta) \right),

S_2(\theta) = \sum_{n=1}^\infty \frac{2n+1}{n(n+1)} \left( a_n \tau_n(\cos\theta) + b_n \pi_n(\cos\theta) \right),

where \pi_n and \tau_n are angular functions derived from Legendre polynomials.^[29] The scattered intensity is proportional to |S_1|^2 and |S_2|^2, enabling computation of the full phase function and degree of polarization. For small x \ll 1, the n=1 terms dominate, recovering the Rayleigh scattering approximation. Computational implementation of Mie theory requires careful handling of the series summation to avoid numerical overflow or cancellation errors, particularly for large x. Stable algorithms, such as the one developed by Bohren and Huffman, use logarithmic derivatives and recursive relations for the Bessel functions to ensure accuracy across a wide range of parameters, with the number of terms needed approximately x + 4x^{1/3} + 2.^[32]

T-matrix method for axisymmetric particles

The T-matrix method provides an exact semi-analytical approach for computing electromagnetic scattering by particles possessing axial symmetry, such as spheroids, cylinders, and Chebyshev particles, by generalizing the vector spherical wave function expansions used in Mie theory through the application of addition theorems that account for translations and rotations of coordinate systems.^[33] This enables the method to handle deviations from spherical geometry while maintaining analytical tractability for the far-field scattering properties. In the spherical case, the T-matrix reduces to the diagonal form characteristic of Mie theory, confirming its consistency as a limiting case.^[34] The core of the method lies in the transition matrix, or T-matrix, which linearly relates the coefficients of the scattered field expansion to those of the incident field in a basis of vector spherical wave functions. Specifically, if \mathbf{a} and \mathbf{b} denote the column vectors of scattered and incident field coefficients, respectively, then \mathbf{a} = \mathbf{T} \mathbf{b}. The T-matrix is derived from the boundary conditions at the particle surface using the extended boundary condition method, expressed as \mathbf{T} = \mathbf{A}^{-1} \mathbf{B}, where \mathbf{A} and \mathbf{B} are infinite matrices obtained from surface integrals enforcing continuity of the tangential field components.^[33] For axisymmetric particles, the azimuthal symmetry (invariance under rotations about the symmetry axis) renders the T-matrix block-diagonal, with blocks corresponding to each azimuthal mode index m, significantly reducing computational complexity by decoupling the equations for different m.^[34] This formalism finds broad applications in modeling scattering by non-spherical particles, including prolate and oblate spheroids, finite-length cylinders, and aggregates of such particles, where multiple inclusions can be treated via superposition using the T-matrix addition theorems for their relative translations and orientations. It is particularly valuable for atmospheric and oceanic optics, such as simulations of light scattering by deformed raindrops or ice crystals, and extends to ensembles of randomly oriented particles by averaging the T-matrix over orientations.^[35]^[34] Key advantages include its efficiency for particles with size parameters up to approximately 50 (in units of ka, where k is the wavenumber and a the characteristic dimension) and moderate aspect ratios, allowing rapid computation of the full scattering matrix, including the Mueller matrix elements that describe polarization effects. Unlike fully numerical methods, it leverages analytical expansions to achieve high accuracy with fewer resources for symmetric shapes. The method was originally formulated by Waterman in 1971, establishing the symmetry properties and unitarity of the T-matrix for electromagnetic scattering.^[33] Mishchenko extended this in the 1990s, particularly in 1991, by deriving efficient algorithms for randomly oriented axisymmetric particles and providing publicly available Fortran implementations that handle coated and composite structures, facilitating widespread adoption in remote sensing and aerosol research.^[34] Limitations arise for highly aspherical particles with extreme aspect ratios (beyond about 3:1), where ill-conditioning of the matrices \mathbf{A} and \mathbf{B} can lead to numerical instability, necessitating regularization techniques.^[34]

Numerical methods for complex geometries

Discrete dipole approximation

The discrete dipole approximation (DDA) is a volume integral method for computing electromagnetic scattering and absorption by particles of arbitrary shape, size, and material properties, including inhomogeneous, anisotropic, and chiral media. It discretizes the particle's volume into an array of N polarizable points, treated as point dipoles, which respond to the local electric field by developing induced dipole moments. This approach is particularly suited for non-spherical or irregular geometries where analytical methods fail, as it solves Maxwell's equations numerically in the frequency domain without relying on surface discretizations. In the DDA formulation, the particle is represented by N dipoles located at positions \mathbf{r}_j (j = 1, \dots, N), each with polarizability \alpha_j determined by the local material properties. The induced dipole moment at site j is given by \mathbf{p}_j = \alpha_j \mathbf{E}_j, where \mathbf{E}_j is the total electric field at that location. The total field \mathbf{E}_j comprises the incident field \mathbf{E}_{\rm inc,j} and the scattered field from all other dipoles, mediated by the dyadic Green's function \mathbf{G}(\mathbf{r}_j, \mathbf{r}_k, \omega) for free space:

\mathbf{E}_j = \mathbf{E}_{\rm inc,j} + \sum_{k \neq j} \mathbf{G}(\mathbf{r}_j, \mathbf{r}_k, \omega) \cdot \mathbf{p}_k.

This leads to a system of $3N linear equations for the dipole moments \mathbf{P} = (\mathbf{p}_1, \dots, \mathbf{p}_N):

\left( \alpha_j^{-1} \mathbf{I} - \mathbf{A} \right) \mathbf{P} = \mathbf{E}_{\rm inc},

where \mathbf{A} is the $3N \times 3N interaction matrix with elements A_{jk} = -\mathbf{G}(\mathbf{r}_j, \mathbf{r}_k, \omega) for j \neq k and zero diagonal (or a small self-term), and \mathbf{I} is the identity. For large N, direct solvers are impractical due to O(N^3) cost; instead, iterative methods like the conjugate gradient (CG) algorithm are employed, achieving O(N \log N) or better scaling with fast Fourier transform (FFT) acceleration for near-field interactions. The accuracy of DDA depends on the dipole spacing d, which must satisfy d \ll \lambda (where \lambda is the wavelength in the medium) to resolve field variations. Convergence is typically achieved for d \approx \lambda / 20 to \lambda / 10, with relative errors in scattering quantities scaling as O((k d)^2) (where k = 2\pi / \lambda) plus a linear term for larger d, bounded rigorously under conditions like |m| k d < 1 (m is the refractive index).^[36] For small particles in the Rayleigh limit, DDA reduces to the exact polarizability sum, validating its use for clusters. DDA finds wide application in modeling light scattering by irregular particles, such as atmospheric dust, biological cells, or nanoparticles, as well as in chiral media where handedness affects polarization, and near-field optics for subwavelength structures. Open-source implementations like DDSCAT, developed in the 1990s by Draine and Flatau, facilitate these calculations for particles up to sizes \sim 100\lambda with N \sim 10^5–$10^6.^[37] Despite its versatility, DDA is computationally intensive for very large N > 10^6, requiring significant memory and time even with iterative solvers, and becomes ill-conditioned for metallic particles due to near-resonant polarizabilities that amplify numerical errors.

Finite-difference time-domain method

The finite-difference time-domain (FDTD) method is a full-wave numerical technique for simulating electromagnetic wave propagation and scattering by discretizing Maxwell's equations on a spatiotemporal grid, making it particularly suitable for modeling light interactions with complex particle geometries. The approach employs the Yee grid, a staggered lattice where electric field components are positioned at the centers of cell edges and faces, while magnetic field components are offset to cell vertices, enabling a leapfrog time-stepping scheme that alternates updates between electric (E) and magnetic (H) fields.^[38] This discretization approximates spatial derivatives using central finite differences and advances the fields in time, capturing broadband responses from a single pulsed simulation.^[39] The core update equations derive from the curl forms of Maxwell's equations in isotropic media:

\frac{\partial \mathbf{E}}{\partial t} = \frac{1}{\epsilon} \nabla \times \mathbf{H} - \frac{\sigma}{\epsilon} \mathbf{E},

\frac{\partial \mathbf{H}}{\partial t} = -\frac{1}{\mu} \nabla \times \mathbf{E} - \frac{\sigma^*}{\mu} \mathbf{H},

where \epsilon and \mu are the permittivity and permeability, and \sigma and \sigma^* account for electric and magnetic conductivities, respectively. These are discretized on the Yee grid to yield explicit finite-difference expressions, such as for the x-component of E at time step n+1/2:

E_x^{n+1/2}(i+1/2, j, k) = E_x^{n-1/2}(i+1/2, j, k) + \frac{\Delta t}{\epsilon} \left[ \frac{H_z^{n}(i+1/2, j+1/2, k) - H_z^{n}(i+1/2, j-1/2, k)}{\Delta y} - \frac{H_y^{n}(i+1/2, j, k+1/2) - H_y^{n}(i+1/2, j, k-1/2)}{\Delta z} \right] - \frac{\sigma \Delta t}{\epsilon} E_x^{n+1/2}(i+1/2, j, k),

with analogous updates for other components; H fields are updated similarly using the prior E values.^[38] Numerical stability requires adherence to the Courant-Friedrichs-Lewy condition, \Delta t \leq \frac{1}{c \sqrt{1/(\Delta x)^2 + 1/(\Delta y)^2 + 1/(\Delta z)^2}}, which for a uniform cubic grid simplifies to \Delta t < \Delta x / (c \sqrt{3}), where c is the speed of light. For scattering simulations, the total-field/scattered-field (TFSF) formulation introduces an incident plane wave by partitioning the grid into total-field and scattered-field regions separated by a Huygens surface, where the incident field is added to the scattered field across the boundary to enforce continuity. Absorbing boundaries are implemented using perfectly matched layers (PML), which attenuate outgoing waves with minimal reflection by introducing anisotropic, lossy materials that match the impedance of the surrounding medium; originally formulated by Bérenger in 1994, PML has evolved into uniaxial variants for enhanced stability.^[40] This setup allows efficient computation of scattering cross-sections and near-field patterns for particles embedded in absorbing or dispersive media. The FDTD method excels in applications involving light scattering by nanoparticles, where it resolves subwavelength features like plasmonic resonances in gold spheres or rods, yielding extinction efficiencies that match experimental spectra.^[41] It is also widely used for photonic structures, such as metamaterials or dielectric metasurfaces, to predict broadband scattering and time-resolved dynamics like pulse propagation through scattering media. Unlike frequency-domain methods, FDTD naturally handles material anisotropy, nonlinearity, and gain, enabling simulations of active particles or time-varying responses in biological or atmospheric scattering scenarios.^[42] Pioneered by Allen Taflove in the 1970s building on Kane Yee's 1966 discretization, FDTD originated from early work on electromagnetic pulse propagation and has since become a cornerstone for computational electrodynamics.^[43] Modern implementations include open-source tools like MEEP, developed at MIT, which supports parallel computing and scripting for rapid prototyping of particle scattering setups.^[39] However, the method demands significant memory for fine grids—typically requiring resolutions of \lambda/50 or finer near scatterers to capture evanescent fields accurately—limiting its feasibility for very large or highly detailed structures without adaptive meshing.^[42] Validation studies often compare FDTD results for spherical particles against Mie theory, confirming agreement in scattering efficiencies to within 1% for sizes up to 10\lambda.

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### Summary of Light Scattering by Particles (Lecture 34)
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None
Below is a merged summary of "Light Scattering by Small Particles" (Chapters 1-3, primarily based on Bohren & Huffman, 1998) that consolidates all information from the provided segments into a single, comprehensive response. To retain maximum detail and manage the complexity of the data, I will use a structured table format in CSV style for key recurring elements (e.g., definitions, physical basis, particle types, etc.), followed by a narrative summary for additional context and details that don’t fit neatly into a table. This approach ensures all information is preserved while maintaining clarity and density.
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