# Sellmeier

Sellmeier defines a dispersive medium using Sellmeier coefficients.

This object corresponds to tidy3d.Sellmeier (opens new window).

The Sellmeier model is an empirical dispersion model for transparent media. It describes how the refractive index changes with wavelength by adding the contribution of one or more resonances.

The wavelength dependence is:

n(λ)2=1+∑iBiλ2λ2−Ci n(\lambda)^2 = 1 + \sum_i \frac{B_i\lambda^2}{\lambda^2 - C_i}

This maps directly to the GUI coefficients. Each row in coeffs adds one Sellmeier term: B controls the strength of the term, and C is the resonance wavelength squared. The model is usually used away from the resonance wavelengths, where it provides a compact fit for refractive-index dispersion. To learn more about the physical background, see the Sellmeier equation (opens new window).

# Name

name: Optional unique name for the medium.

# Type

type: Medium model used by this material entry.

  • Options: Sellmeier

# Color

color: Display color used for the medium in the GUI.

  • Type: color
  • Default: automatically assigned by the GUI

# General Properties

coeffs: List of Sellmeier coefficient pairs.

  • Type: list of two-value coefficient rows
  • Required field

B: Sellmeier numerator coefficient.

  • Type: float number
  • Unit: unitless
  • Required field

C: Sellmeier resonance wavelength-squared coefficient.

  • Type: float number
  • Unit: um²
  • Constraint: greater than 0
  • Required field

# Advanced

set_from_global_frequency: Toggle whether the frequency range is set from the Global Settings frequency range.

  • Options:
    • True: Use the global frequency range and disable manual frequency range inputs.
    • False: Manually set frequency_range.

frequency_range: Optional range of validity for the medium.

  • Type: two float numbers
  • Unit: Hz, kHz, MHz, GHz, THz, or other available frequency unit
  • Default: None

allow_gain: Toggle whether the medium is allowed to be active.

  • Options: True/False
  • Default: False

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