Predicting the refractive index of amorphous materials using the Bruggeman effective medium approximation.

Area of Science:

• Materials Science
• Optics
• Astrophysics

Background:

• The Lorentz-Lorenz relationship (molar refractivity/specific refractivity effective medium approximation) has been used to predict amorphous water ice refractive indices from crystalline data.
• Previous models provided reasonable, but not optimal, predictions for amorphous material optical properties.

Purpose of the Study:

• To evaluate the accuracy of the Bruggeman effective medium approximation for predicting the refractive index of amorphous materials, including amorphous ice.
• To compare the predictive power of the Bruggeman model against the Lorentz-Lorenz model.

Main Methods:

• Applied the Bruggeman effective medium approximation to various amorphous materials using their crystalline phase refractive indices.
• Compared model predictions with experimental measurements of refractive indices for amorphous materials.
• Assessed the validity of assumptions regarding constant volume fraction versus preserved molar properties.

Main Results:

• The Bruggeman effective medium approximation demonstrated a closer match to measured refractive indices of amorphous materials compared to the Lorentz-Lorenz model.
• The Bruggeman model accurately predicted the refractive index of amorphous ice.
• Constant volume fraction assumption proved more robust than preserving molar properties across crystalline and amorphous states.

Conclusions:

• The Bruggeman effective medium approximation is a superior model for predicting the refractive index of amorphous materials, including amorphous ice.
• The assumption of a constant volume fraction of scattering centers is more reliable for amorphous materials than assuming preserved molar refractivity.
• Findings have significant implications for astrophysical applications and the general optics of non-crystalline materials.