Optical properties of inhomogeneous metallic hydrogen plasmas

TL;DR

This paper studies how inhomogeneities in metallic hydrogen plasmas affect optical measurements, providing a new calculation method and analyzing experimental data to estimate the plasma's conductivity.

Contribution

It introduces a technique to account for density inhomogeneities in optical property calculations of metallic hydrogen, improving interpretation of experimental results.

Findings

Inhomogeneity significantly affects reflectance and transmittance calculations.

The proposed method yields a conductivity estimate of approximately 2100 (Ω cm)$^{-1}$ for metallic hydrogen.

The analysis refines understanding of hydrogen's transition to a metallic state under high pressure.

Abstract

We investigate the optical properties of hydrogen as it undergoes a transition from the insulating molecular to the metallic atomic phase, when heated by a pulsed laser at megabar pressures in a diamond anvil cell. Most current experiments attempt to observe this transition by detecting a change in the optical reflectance and/or transmittance. Theoretical models for this change are based on the dielectric function calculated for bulk, homogeneous slabs of material. Experimentally, one expects a hydrogen plasma density that varies on a length scale not substantially smaller than the wave length of the probing light. We show that taking this inhomogeneity into account can lead to significant corrections in the reflectance and transmittance. We present a technique to calculate the optical properties of systems with a smoothly varying density of charge carriers, determine the optical response for metallic hydrogen in the diamond anvil cell experiment and contrast this with the standard results. Analyzing recent experimental results we obtain $\sigma^{Drude}_{DC}=(2.1 \pm 1.3) \times 10^3$ ($\Omega$ cm)$^{-1}$ for the conductivity of metallic hydrogen at 170 GPa and 1250 K.