COMSOL 6.2 - Medium Properties

Medium Properties

Use the node to specify the refractive index of the medium. An instance of this feature is created by default, including all of the selected domains for the physics interface. For the region outside the geometry and for any domains not included in the physics interface selection, the refractive index is instead controlled by the in the physics interface section.

Medium Properties

Use the settings in this section to specify how the real part of the refractive index is defined. If the ray intensity or power is computed, then you can also specify the imaginary part of the refractive index, which is used to describe absorption within the medium.

Refractive Index of Domains

Choose an option from the list:

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If (the default) is selected, the can be taken , or it can be entered directly. The default value is 1. This index is considered absolute, that is, relative to vacuum.

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If is selected, the can be taken , or it can be entered directly. The default value is 1. Then enter the Tref,rel (SI unit: K, default 293.15 K) and the Pref,rel (SI unit: Pa, default 0). The given refractive index is understood to be relative to air; that is, the absolute refractive index is the product of the specified value with the refractive index of air at the reference temperature and pressure.

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If is selected, choose an option from the list. The following options are available:

, and

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If is selected, the dispersion model in each domain is automatically deduced from the nodes and their selections. This allows different optical dispersion models to be used in different domains in the geometry, using only a single node.

For example, suppose that a model contains two lenses consisting of two different glasses, and that the first glass is defined using coefficients whereas the second glass is defined using coefficients. Then selecting will automatically express the refractive index using the equation in the first lens and the (polynomial) equation in the second. An alternative way to use different optical dispersion models in different domains is to use more than one instance of the node and select the dispersion models manually.

For more information on each optical dispersion model, see Table 3-3 in Theory for the Geometrical Optics Interface, Optical Dispersion Models section.

The coefficients for each of these dispersion models are taken by default. Alternatively, coefficients may be entered.


	For the built-in optical dispersion models, the wavelength is always assumed to be in units of microns (μm). For example, in the Schott (polynomial) model, the coefficients A0, A1, A2, A3, and so on have units of 1, μm, μm2, μm3, and so on. If another source were to provide these coefficients using nanometers instead of microns, then some manual conversion would be required.

Similar to the option , most of the built-in optical dispersion models define the refractive index relative to air, and therefore they all require a reference temperature and pressure to be specified. The only exception is the , in which the refractive index is assumed to be absolute, such that n = 1 is the refractive index of an ideal vacuum. To use any other optical dispersion model to define an absolute index, set the reference pressure to zero.

Using the model also disables the settings for selecting a (see the following subsection).

The conversion from relative to absolute refractive index is made using the Edlén model (Ref. 2) for the refractive index of air. See Optical Dispersion Models for further details.

Optical Attenuation Model

By default the list is not shown. However, it will be shown if you select any option except from the list in the physics interface settings. This is because the speed of light in a medium only depends on the real part of the refractive index, whereas the imaginary part affects the attenuation of ray intensity or power if they are solved for.

The standard used by the Geometrical Optics interface is to treat absorbing media as having complex-valued refractive indices of the form n - iκ, where n and κ are real dimensionless numbers and κ > 0. The case κ < 0 would constitute a gain medium.

The list has the following options:

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For , specify the value of κ directly. This can be taken or ; if the default is 0. The Geometrical Optics interface only recognizes isotropic κ where the absorption rate does not depend on ray polarization.

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For , specify the value of the α (SI unit: 1/m) which is related to κ by α = 4πκ/λ0. The default is 1 1/m.

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For , enter a value or expression for the internal transmittance of a 2 mm thick sample of the material, neglecting Fresnel losses. Internal transmittances are dimensionless and are denoted τi,d, where d is replaced with the sample thickness, for example, τi,2. The default is 0.9. The relationship between τi,d and κ is κ = -(λ0 log τi,d)/(4πd).

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The remaining options are internal transmittances at other values of the sample thickness. Options for 5 mm, 10 mm, and 25 mm samples are available. Otherwise the settings are the same as the option.

Conversions Between Optical Attenuation Models

The material properties for different optical attenuation models are used to define the corresponding properties for other attenuation models automatically. For example, if you select and try to take the value of κ , but the selected material only has internal transmittance data, then the value of the internal transmittance data is automatically converted to a value of κ based on the wavelength.

If the selected material has internal transmittance data for two or more sample thicknesses, the data for the greatest thickness is automatically used unless otherwise specified. This default behavior is intended to give the best possible numerical precision when the internal transmittance values are close to 1, implying weak absorption. However, it may be preferable to use internal transmittance data for a smaller sample thickness in IR or UV applications when most of the energy is absorbed.

How to Automatically Detect Optical Dispersion Models

The option is unique because it allows a single node to detect different optical dispersion models from a large number of different materials having different domain selections. For example, the materials used in the Petzval Lens tutorial are shown in Figure 3-2 below. To automatically detect the optical dispersion models of the loaded glasses, take the following steps:

Add materials to the model using the Optical Material Library. Most of the glasses in this material library use an optical dispersion model to define the refractive index. As the materials are added, the coefficients used by the optical dispersion model are automatically loaded.

Locate the default node.

From the list, select . If the glasses loaded in step 1 also provide thermo-optic coefficients, it is important to specify an accurate value of the as well.

Figure 3-2: Workflow for automatically detecting optical dispersion models from the glasses in an optical prescription.

Thermo-Optic Dispersion Models

A temperature-dependent offset in the Refractive index may be specified using a thermo-optic dispersion model. Two options are possible:

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(the default): no offset will be applied.

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: this is the only built-in thermo-optic dispersion model available. The coefficients used to compute a temperature dependent offset in the refractive index can either be taken (the default), or . A reference temperature is also required. This may also be taken (the default), or .

For further details, see Thermo-Optic Dispersion Models.