Optically Pumped AlGaN Double Heterostructure Deep-UV Laser by Molecular Beam Homoepitaxy: Mirror Imperfections and Cavity Loss

TL;DR

This paper reports the first optically pumped deep-UV laser structures grown on AlN, analyzing mirror imperfections' impact on cavity loss, crucial for developing electrically injected deep-UV laser diodes.

Contribution

It introduces a new growth method for deep-UV laser structures and derives an expression quantifying mirror imperfections' effect on cavity loss.

Findings

Mirror imperfections cause superlinear increase in optical loss.

Loss scales with inverse wavelength squared of the lasing mode.

Device processing must optimize mirror quality for deep-UV lasers.

Abstract

We demonstrate the first optically pumped sub-300 nm UV laser structures grown by plasma-assisted molecular beam epitaxy on single-crystal bulk AlN. The edge-emitting laser structures fabricated with the AlN/AlGaN heterostructures exhibit multi-mode emission with peak gain at ~284 nm. Having the goal of electrically injected, continuous wave deep-UV AlGaN laser diodes in mind, with its intrinsic material challenges of achieving sufficient optical gain, the optical cavity loss of a laser diode should be minimized. We derive an expression to quantify the effect of mirror imperfections, including slant and surface roughness on the optical mirror loss of a Fabry-P\'erot cavity. It is found that the optical imperfection loss is a superlinear function of the RMS roughness and slant angle of the facets, and also scales as the inverse wavelength squared of the principal lasing mode. This highlights the importance of device processing optimization as Fabry-P\'erot cavities couple to lower wavelengths.