COMSOL 6.4 - Scattering Domain

Use the feature to model extinction of light by a medium containing a number of small spherical particles. As rays propagate through the selected domains, some energy is either converted into heat within the particles (absorption) or sent in a different direction (scattering). The overall removal of light from the initial beam propagation direction is called extinction.

The list is only shown if the ray intensity or power is solved for. If so, choose either or (the default). If ray intensity or power is not solved for, this feature always behaves as if is selected.

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If is selected, the ray intensity and power will continuously decrease as rays propagate through the selected domains, but the total number or rays will remain the same. If the number density and size of the scattering particles is spatially uniform, the intensity and power will decay exponentially.

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If is selected, rays will have some probability of disappearing as they propagate through the selected domains. If the ray intensity and power are solved for, they will not continuously decrease along the ray trajectories.

Use the option for a reproducible result where nothing is random and the ray variables vary continuously; the option is more of a Monte Carlo approach.

Choose an option from the list. The correct choice depends on the size of the scattering particles in comparison with the wavelength. The individual scattering particles are said to be optically large if the particle diameter is much larger than the wavelength of the radiation. The following options are available:

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(the default) is an approximate solution for optically small particles. The particle must be much smaller than the wavelength both inside and outside the particle, an important consideration for tiny metallic particles.

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is a full solution to the electromagnetic wave equation as a plane wave interacts with a sphere. It is the most accurate solution across a wide range of optical sizes, and the only approach that gives excellent accuracy at intermediate optical sizes, when the wavelength and particle diameter are similar in size. The calculation becomes more time consuming for extremely large particles.

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represents the asymptotic limit of scattering by spheres much larger than the wavelength. The total extinction cross section is double the geometrical cross section of the sphere because the sphere blocks a certain amount of light and also causes diffraction along its perimeter.

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If is selected, enter values or expressions for the dimensionless extinction and scattering efficiency factors.

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If is selected, enter values of expressions for the scattering and absorption cross sections directly.

For all choices of except , enter the Rsc (SI unit: m). The default is 10 nm.

For all choices of , enter the Nsc (SI unit: 1/m3). The default is 1020 m−3.

If or is selected, enter the np,r (dimensionless). The default is 1.4. Also enter the np,i (dimensionless). The default is 10−6. Internally the complex-valued refractive index is defined as n = np,r − inp,i, so positive values here indicate that the scattering particle is absorbing.

If is selected, enter the Qext (dimensionless). The default is 2. Also enter the Qsca (dimensionless). The default is 2. The absorption efficiency Qabs is then the difference between the extinction efficiency and the scattering efficiency.

If is selected, enter the σext (SI unit: m2). The default is 10−15 m2. Also enter the σsca (SI unit: m2). The default is 10−15 m2. The absorption cross section σabs is then the difference between the extinction cross section and the scattering cross section. The scattering efficiency (cross section) should not be greater than the extinction efficiency (cross section).

	Volumetric Scattering Theory

is enabled if the option is selected for the . If checkbox is selected (default), the ray undergoes extinction (absorption and scattering) after a randomly sampled time tsc,defined as:

where r is a random variable uniformly distributed in the interval [0, 1], ν is the collision frequency defined as

Here, σext is the extinction cross-section, Nsc is the number density, and Vg is the group velocity.

Otherwise, the probability p that the ray undergoes extinction in the next time step is computed

where τ is the “timestep” variable.

The setting changes the effect of the extinction on the primary ray:

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If the option is selected (default), the ray is not displayed after extinction.

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If the option is selected, the ray position does not change after extinction.

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If the option is selected, the ray becomes scattered at the designated scattering time. The scattering angle is determined randomly using the selected model from , either the or phase function. For model, specify the g (dimensionless), with a default value of 0.95.

If the or options are selected, allows tracking power dissipated inside the scattering domain:

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If is selected, the total power dissipated due to both scattering and absorption is tracked.