Band-structure and gain-cavity tuning of 2.4-µm GaSb buried tunnel junction VCSELs

Abstract

One promising approach to cover the spectral range between 2 and 3 µm, where several pollutant gases such as CO, CO 2 , CH 4 and NH 3 have strong absorption lines, is the development of GaSb-based inter-band lasers. Low-cost, continuous-wave GaSb-based vertical cavity surface emitting lasers (VCSELs) operating at ∼2.4 µm up to 50°C have been demonstrated recently [1]. In this work we have used high pressure techniques to investigate ways to improve their performance and extend their working temperature range. Since the band-gap and energy of the gain peak (E p ) increase with pressure at 0.126 meV/MPa [2] at constant temperature, when applied to edge emitting lasers (EEL) we can use pressure to determine the radiative and non-radiative recombination processes occurring [3]. In VCSELs, the pressure also tunes E p relative to the cavity mode energy E cm , which has a much weaker pressure dependence. Figure 1 shows the pressure dependence of the threshold current, I th , of the VCSEL at three different temperatures (−10°C, 20°C and 30°C) and of a reference EEL with the same active region (I th in this case normalised to its value at atmospheric pressure). The decrease in I th with increasing pressure in the EEL indicates that Auger recombination is dominant at room temperature in this materials system and explains the temperature sensitivity of these EELs [3]. In the VCSEL the pressure dependence of I th is much more complicated. At −10°C, pressure moves E p above E cm and the detuning effect dominates I th , which therefore increases. At the higher temperatures the decreasing Auger recombination initially dominates. Detailed analysis will be given, but one can immediately note that at ∼130 MPa I th is lower at 20°C and is almost stable with temperature. Therefore we predict that either increasing the band gap or increasing the operating wavelength will allow an improved temperature performance of these GaSb-based VCSELs.