Optimal detuning in optically pumped 10 <b> <i>λ</i> </b> ultraviolet vertical-cavity surface-emitting lasers for temperature stability and low threshold

Abstract

In vertical-cavity surface-emitting lasers (VCSELs), the lasing wavelength is defined by the longitudinal cavity mode. The spectral misalignment between the resonance wavelength and the gain peak, known as detuning, is crucial for the device performance. Temperature also influences detuning since the gain peak red shifts faster than the resonance wavelength when the temperature increases. These important effects have not been explored in detail for ultraviolet (UV) VCSELs despite the significant heating that is expected due to high electrical resistance and high thermal impedance. Here, we studied the threshold and detuning dependence in optically pumped AlGaN-based UVB and UVC VCSELs with different cavity lengths operated at different temperatures. The cavity lengths of the VCSELs, fabricated from the same epitaxial material, are varied by post-growth deposition of HfO2 spacer layers with different thicknesses. The results show a strong relation between the threshold and the detuning, where VCSELs have thresholds around 5 MW/cm2 for a nominal (operational) detuning of −2.5 nm (∼−1 nm), and below 1 MW/cm2 when the nominal (operational) detuning is set between 2 and 3 nm (1–3 nm) with a minimum threshold of 0.23 MW/cm2. Additionally, the temperature dependence of the VCSELs' thresholds is investigated and compared by temperature-dependent photoluminescence and an empirical relation. The VCSELs with lower thresholds at room temperature are, on average, 20 times less sensitive to temperature than those with higher thresholds at room temperature, suggesting that VCSELs with a nominal detuning of 2–3 nm are the optimum design choice.