ABSTRACT
C Gmachl, H M Ng, D L Sivco, J D Heber, R Colombelli, A Soibel, F Capasso and A Y Cho Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, USA
Abstract. We review two recent developments in the field of quantum cascade lasers. First, early studies of the GaN/AlGaN material system demonstrate the suitability of the latter for the communications wave-length range. Intersubband absorption measurements at wavelengths as short as X > 1.35 pm are reported in multiple coupled quantum wells. An electron scattering time of few 100 fs between the subbands is obtained. Secondly, in InP-based quantum cascade lasers, the recent development of lasers with heterogeneous cascades, i.e. lasers with stacks of many intentionally dissimilar active regions interleaved with injectors is leading to novel concepts for multi-wavelength operation. We report broadband optical gain from 5 to 8 pm and Fabry-Perot laser action covering all wavelengths from 6 to 8 pm simultaneously in a single device. 1
1. Introduction to GaN-based intersubband structures Optical devices, predominantly lasers and photo-detectors, based on intersubband (IS) transitions are well established in the mid-infrared range of the electromagnetic spectrum. Recently, they face a rising interest also in other wavelength ranges. The intrinsically ultrafast electron dynamics associated with IS-transitions encourages research particularly for the communications wavelength range (A ~ 1.55 pm) for modulators, wavelength converters, and sources. Few material systems, however, provide a sufficiently large band-discontinuity for the quantum wells (QWs) to accommodate the large subband spacing needed for such short wavelengths. Here, we present measurements of IS-transitions in the communications wavelength range using GaN/AlGaN heterostructures; i.e. multiple single and coupled QWs. While the comparatively large electron mass as well as the large phonon energy inherent to the GaN-based materials are advantageous to achieve ultrafast carrier dynamics, the material system also comes with considerable challenges: Large intrinsic in-built electric fields complicate the bandstructure design and reduce the usable band-offset, the lack of general availability of a native substrate contributing in part to the piezo-electric field also affects the material quality and defect density, high AIN mole fraction barriers and thin QWs are needed to achieve short wavelength IS-resonances, and finally, the high electron mass also only allows for ultra-narrow barriers to still facilitate electron tunnelling. Nevertheless, despite the challenges, several groups including ours have recently reported IS-transitions in GaN-based QWs [1-6].
