Progress toward 3–5μm strained quantum cascade laser design using quasi-phased matched non-linear conversion on GaAs (111)B

Typical infrared quantum cascade laser (QCL) designs focus on the high-mid and far-infrared regions due to the small intersubband spacing achievable using near lattice matched systems — GaAs/AlGaAs, GaSb/AlAb, GaInAs/GaInSb/InP, etc. Requirements for shorter wavelengths, 3–30μm, force designers to choose laser designs using band-to-band transitions (Type I). Using these designs, it is difficult to achieve high-efficiency allowing for room temperature continuous wave operation due to inherent losses associated with Auger recombination. One potential solution to this problem is to intentionally incorporate non-lattice matched (strained) materials into the laser design [1]. In the case of GaAs based QCLs, incorporation of indium into the well regions increases the electron affinity, thereby increasing the achievable barrier height and allowing for larger intersubband transitions. Traditional designs using the (100) surface are limited to only small incorporations of indium, only a few percent, due to defect formation arising from the large strain. By using a novel surface-index, such as (111), researchers, including our group, have shown that it is feasible to produce two-dimensional binary InAs on GaAs (111) [2]. This opens up a range of design possibilities as well as other useful phenomena such as a built-in field from the piezoelectric effect [3]. When compared growth on the (100) surface using traditional molecular beam epitaxy (MBE), the growth parameter window on (111) is significantly narrower making optical quality growth very challenging when incorporating vastly different material systems. Using statistical experimental design practices we have found that optimal growth regions for GaAs/AlGaAs on (111)B exist for a substrate temperature T s = 570–600 °C and a BEP ratio (V:III) of 15–20:1. Continued work is in progress to determine the optimal range for two dimensional binary InAs, but current experiments indicate that the material system is highly sensitive to base surface morphology due to high adatom mobility. Suppression of InAs's high adatom mobility is necessary and can be achieved by lowering of T s and high BEP ratio. Successful designs which incorporate both (Al)GaAs and InGaAs materials need to balance the amount of indium incorporation with feasibility of high-quality material growth. Simulations of multi-quantum well regions with high-indium incorporation in the well show potential for intersubband transitions greater than 400 meV. However, this is larger than the bandgap of InAs raising the possibility for band-to-band photon reabsorption. With this in mind our focus has been on the development of designs which focus on using a non-linear quasi-phase matching conversion technique [4] to produce a second harmonic 3–5μm beam from an initial 6–10μm output from the QCL active region. The non-linear conversion is achieved through a periodic inversion of the GaAs (111) to GaAs (111) with a specific period (coherence) length. Simulations of the non-linear conversion process show promising second harmonic generation within 10+ periods and a relatively low efficiency decrease if the initial beam is slightly higher or lower in wavelength than expected. Final QCL designs will bring the three design elements of growth parameter optimization, high-indium content QCL design, and efficient non-linear optical conversion together to produce a highly-efficient 3–5μm hybrid QCL, see Fig. 1.