Large-area polycrystalline $\alpha$-MoO3 thin films for IR photonics

TL;DR

This paper reports the fabrication and IR characterization of large-area polycrystalline alpha-MoO3 films supporting surface phonon polaritons with high Q-factors, enabling scalable mid-IR photonic devices for sensing and filtering.

Contribution

It introduces a scalable pulsed laser deposition method for large-area alpha-MoO3 films with high-Q SPhP resonances in the mid-IR range, suitable for practical photonic applications.

Findings

Achieved high-Q (up to 53) SPhP resonances in large-area alpha-MoO3 films.

Demonstrated polarization-dependent reflection tuning with a broad dynamic range.

Observed near-perfect absorption at 972 cm$^{-1}$ over broad angles.

Abstract

In recent years, excitation of surface phonon polaritons (SPhPs) in van der Waals materials received wide attention from the nanophotonics community. Alpha-phase Molybdenum trioxide ($\alpha$-MoO3), a naturally occurring biaxial hyperbolic crystal, emerged as a promising polaritonic material due to its ability to support SPhPs for three orthogonal directions at different wavelength bands (range 10-20 $\mu$m). Here, we report on the fabrication and IR characterization of large-area (over 1 cm$^2$ size) $\alpha$-MoO3 polycrystalline films deposited on fused silica substrates by pulsed laser deposition. Single alpha-phase MoO3 films exhibiting a polarization-dependent reflection peak at 1006 cm$^{-1}$ with a resonance Q-factor as high as 53 were achieved. Reflection can be tuned via changing incident polarization with a dynamic range of $\Delta$R=0.3 at 45 deg. incidence angle. We also report a polarization-independent almost perfect absorption condition (R<0.01) at 972 cm$^{-1}$ which is preserved for a broad angle of incidence. The development of a low-cost polaritonic platform with high-Q resonances in the mid-infrared (mid-IR) range is crucial for a wide number of functionalities including sensors, filters, thermal emitters, and label-free biochemical sensing devices. In this framework our findings appear extremely promising for the further development of lithography-free, scalable films, for efficient and large-scale devices operating in the free space, using far-field detection setups.