The optical fingerprint of a material
The dielectric function ε(E) = ε₁ + i ε₂ is just another way of writing the refractive index, since ñ² = ε. It is the quantity electrodynamics cares about.
For our purposes it is the sum of all the optical contributions of a material: a flat background ε∞from high-energy transitions, plus one Lorentzian for every discrete transition we care about. CrSBr's simplified model has a single Lorentzian — the exciton — sitting on top of ε∞ ≈ 7.9.
ε(E) = ε∞ + f / (E₀² − E² − iγE)See it move
Tweak the resonance and watch n and κ deform — the very same plot you saw on the refractive-index page, because they are two faces of the same coin:
ε(E) = ε∞ + f / (E₀² − E² − iγE). The real part of n bumps up before the resonance and dips after it (anomalous dispersion); the absorption κ peaks right at E₀ — same shape as a Lorentzian.
ε(E)= sum of background + one Lorentz term per electronic transition.- In CrSBr a single exciton dominates the part of the spectrum we look at.
- The simulator builds a per-layer
ε(E)on the fly from the phase window.