Deep-subwavelength Phase Retarders at Mid-Infrared Frequencies with van der Waals Flakes

TL;DR

This paper demonstrates ultra-thin, efficient mid-infrared phase retarders using van der Waals flakes of $ extalpha$-MoO$_3$, enabling miniaturized polarization control at these frequencies.

Contribution

It introduces a novel approach of using exfoliated low-dimensional crystals for mid-IR phase retardation, achieving significant polarization rotation with sub-wavelength thickness.

Findings

Achieved 90° polarization rotation within 1 micrometer of $ extalpha$-MoO$_3$ flakes.

Reported conversion ratios above 50% in reflection and transmission modes.

Demonstrated wavelength tunability of several micrometers.

Abstract

Phase retardation is a cornerstone of modern optics, yet, at mid-infrared (mid-IR) frequencies, it remains a major challenge due to the scarcity of simultaneously transparent and birefringent crystals. Most materials resonantly absorb due to lattice vibrations occurring at mid-IR frequencies, and natural birefringence is weak, calling for hundreds of microns to millimeters-thick phase retarders for sufficient polarization rotation. We demonstrate mid-IR phase retardation with flakes of $\alpha$-molybdenum trioxide ($\alpha$-MoO$_3$) that are more than ten times thinner than the operational wavelength, achieving 90 degrees polarization rotation within one micrometer of material. We report conversion ratios above 50% in reflection and transmission mode, and wavelength tunability by several micrometers. Our results showcase that exfoliated flakes of low-dimensional crystals can serve as a platform for mid-IR miniaturized integrated polarization control.