The structural and optical characteristics of InP-based compressively strained InGaAs quantum wells have been significantly improved by using gas source molecular beam epitaxy grown InAs/Ino.53Ga0.47As digital alloy triangular well layers and tensile Ino.53Ga0.47As/InAiGaAs digital alloy barrier layers. The x-ray diffraction and transmission electron microscope characterisations indicate that the digital alloy structures present favourable lattice quality. Photo- luminescence （PL） and electroluminescence （EL） measurements show that the use of digital alloy barriers offers better optical characteristics than that of conventional random alloy barriers. A significantly improved PL signal of around 2.1μm at 300 K and an EL signal of around 1.95μm at 100 K have been obtained.
Surface-emitting distributed feedback quantum-cascade lasers operating at λ≈7.8 μm are demonstrated. The metal-covered second-order grating is shallow-etched into the surface of a thin InGaAs contact and cladding layer. This forms a hybrid waveguide and used to achieve relatively low waveguide losses and high coupling strengths. The devices exhibit stable single-mode operation from 90 to 130 K with a side mode suppression ratio above 20 dB. A slope efficiency of 194 mW/A is obtained at 90 K, which is twice higher than that of the Fabry-Perot counterpart.
An InP-based one-dimensional photonic crystal quantum cascade laser is realized. With photo lithography instead of electron beam lithography and using inductively coupled plasma etching, four-period air-semiconductor couples are defined as Bragg reflectors at one end of the resonator. The spectral measurement at 80K shows the quasi-continuous-wave operation with the wavelength of 5.36μm for a 22gm-wide and 2mm-long epilayer-up bonded device.
A 7.8-μm surface emitting second-order distributed feedback quantum cascade laser （DFB QCL） structure with metallized surface grating is studied. The modal property of this structure is described by utilizing coupled-mode theory where the coupling coefficients are derived from exact Floquet-Bloch solutions of infinite periodic structure. Based on this theory, the influence of waveguide structure and grating topography as well as device length on the laser performance is numerically investigated. The optimized surface emitting second-order DFB QCL structure design exhibits a high surface outcoupling efficiency of 22% and a low threshold gain of 10 cm-1. Using a π phase-shift in the centre of the grating, a high-quality single-lobe far-field radiation pattern is obtained.
