Sub-250nm room temperature optical gain from AlGaN materials with strong compositional fluctuations

Compact and portable deep-UV LEDs and laser sources are needed for a number of engineering applications including optical communications, gas sensing, biochemical agent detection, disinfection, biotechnology and medical diagnostics. We investigate the deep-UV optical emission and gain properties of Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N/Al$_{\mathrm{y}}$Ga$_{\mathrm{1-y}}$N multiple quantum wells structure. These structures were grown by molecular-beam epitaxy on 6H-SiC substrates resulting in either homogeneous wells or various degrees of band-structure compositional fluctuations in the form of cluster-like features within the wells. We measured the TE-polarized amplified spontaneous emission in the sample with cluster-like features and quantified the optical absorption/gain coefficients and gain spectra by the Variable Stripe Length (VSL) technique under ultrafast optical pumping. We report blue-shift and narrowing of the emission, VSL traces, gain spectra, polarization studies, and the validity of the Schalow--Townes relation to demonstrate a maximum net modal gain of 120 cm$^{-1}$ at 250 nm in the sample with strong compositional fluctuations. Moreover, we measure a very low gain threshold (15 $\mu$J/cm$^{2})$. On the other hand, we found that samples with homogeneous quantum wells lead to absorption only. In addition, we report gain measurements in graded-index-separate-confined heterostructure (GRINSCH) designed to increase the device optical confinement factor.