Frequency tunable terahertz quantum cascade lasers

Terahertz (THz) quantum cascade lasers (QCLs) are compact solid–state sources of coheren radiation operating in the far–infrared (FIR) range of the electromagnetic spectrum. THz QCL ridge waveguides are typically Fabry–Pérot (FP) cavities and exhibit characteristic multiple–mode emission. However, widely tunable single–mode (SM) THz QCLs are ideally suited to many THz-sensing applications, such as trace gas detection, atmospheric observations and security screening. Tunable THz QCLs are also highly desirable for techniques like heterodyne mixing and self–mixing interferometry. SM emission from THz QCLs has been demonstrated using distributed feedback (DFB) cavities, photonic lattices (PLs) and photonic crystals (PhCs). Tunable THz QCLs have also been demonstrated using various techniques, such as external coupled mirrors, variation of growth parameters, deposition of nitrogen gas and dielectric materials, and aperiodic PLs. In this study, SM emission from THz QCLs is obtained from PLs patterned with electron beam lithography (EBL). This lithography based processing has the advantage of integrated device processing. Spectral performance of the PLs was simulated using finite element modelling (FEM) and coupled mode theory. A frequency stopband centred at the characteristic Bragg frequency was computed with emission predicted outside this stopband. Spectral emission of experimentally fabricated devices was observed outside the stopband, as was predicted from simulations. The design of the THz QCLs with PL was modified to investigate frequency tenability by depleting carriers under the PL metallised sections using a three–section device. A bulk Drude model was used to simulate the variation of refractive index as a function of carrier concentration. The PLs were deposited such that they form a Schottky junction and a thin depletion layer at the active material interface. The PLs were driven with an independent external electrical connection. An electrical model was designed, which explained the experimentally observed behaviour. This electrical model was used to calculate the depletion layer and the redistribution of carriers under the PL. The resulting variation in the refractive index was computed using FEM. A 15–20 GHz shift in the Bragg frequency was predicted using the Drude model. The frequency stopband was also predicted to reduce from ~90 GHz to ~77 GHz with carrier depletion and a ~5–6 GHz shift in the stopband band edge was predicted. In an exemplar device, a tuning of 15 GHz was observed. A change in spectral power density (SPD) amongst modes was observed in all other devices. A different approach towards the realisation of a frequency tunable THz QCLs was adopted based on two–coupled cavities. This design was based on a Vernier selection rule and promised a wide band tuning from a small refractive index perturbation. One of the two cavities formed the lasing section, while the other formed a tuning section. A thermal tuning mechanism based on a localised Joule heating was used to tune frequency of the coupled–cavity. THz QCLs with coupled–cavities were modelled using transfer matrices and a bulk thermal model. Two devices were designed to exhibit a blue shift in frequency when the shorter of the coupled–cavities acted as a tuning element. The frequency spacing of the devices were ~15 and ~25 GHz respectively. The devices were also optimised such that a reversal in tuning direction is observed by swapping the functions of the lasing and tuning cavities. A monotonic discrete frequency hopping with a blue shift of ~50 and ~85 GHz was observed from the two devices. A red shift in frequency was also observed as the lasing and tuning cavities were swapped. Additionally, since the tuning element is isolated from the lasing section, the power emission of the lasing section was unaffected by the tuning current. The coupled cavity designs were further optimised to disrupt the monotonic frequency hopping to obtain a quasi–continuous frequency tuning. Unlike, the discrete tuning design, this detuned design required variation of current at the lasing and tuning cavities simultaneously, along with a variation in heat sink temperature. Spectral behaviour was modelled using the same transfer matrices, bulk thermal mode and coupled mode theory. Closely spaced discrete tuning over a range of ~67 and 100 GHz was observed from two devices, with continuous tuning of ~5 GHz observed at certain dominant modes. Continuous tuning was also investigated using coupled–cavities with an integrated PL. A continuous tuning of ~3 GHz was observed from experimental devices. Unlike the detuned coupled cavity devices, the power emission from these devices were unaffected by the tuning current. However, these devices are limited by a low tuning range.