# Mid-Wave Infrared Polarization Combiner Based on Reflective Metasurface

## TL;DR

This paper introduces a new mid-wave infrared polarization beam combiner using a reflective metasurface that efficiently combines laser beams with orthogonal polarizations.

## Contribution

The novel contribution is a metasurface-based polarization beam combiner for mid-wave infrared with high efficiency and low wavefront distortion.

## Key findings

- The metasurface achieved an overall beam combining efficiency of 88.9%.
- The combined beam had M2 factors of 1.21 and 1.14 along the fast and slow axes.
- The design showed negligible wavefront distortion and is a promising alternative to traditional bulk optics.

## Abstract

Polarization beam combining (PBC) is an important technology for enhancing laser brightness. The conventional bulk polarization beam combiners are Brewster plates and birefringent polarization prisms. However, in the mid- and long-wave infrared range, the beam combining performance is limited by the transmission and birefringent coefficient of the available materials. In this paper, a polarization beam combiner based on a reflection metasurface was proposed. The phases of incident beams with orthogonal linear polarizations were individually manipulated by the side lengths of the rectangular silicon pillar. A metasurface polarization beam combiner operating band was designed and fabricated. When the two beams at 4.6 μm with orthogonal linear polarizations were incident on the metasurface at angles of −13.3° and 13.3°, respectively, they were reflected in the 0°-direction. The overall beam combining efficiency was 88.9%. When both of the quantum cascade lasers used in the experiments were in the fundamental transverse Gaussian mode, the measured beam quality factors M2 of the combined beam were 1.21 and 1.14 along the fast and slow axes, respectively. Both simulation and experimental results demonstrated that the proposed metasurface was an efficient polarization beam combiner with negligible wavefront distortion. It is a promising alternative to traditional bulk optics for the mid- and long-wave infrared.