Tunable cw thz generation from an all-quantum-dot-based system

Summary form only given. The development of a compact, continuous wave (CW), room-temperature, broadly-tunable terahertz (THz) laser source is of great interest for a number of applications ranging from biomedical imaging and indoor communication to spectroscopy, security and defence. Among the usually used laser sources, direct quantum cascade lasers (QCLs) and semiconductor mixers of mid-IR QCLs are regarded as most popular sources offering the highest-to-date wall-plug efficiency. However, such lasers require a cryogenic cooling and suffer from the drawbacks in terms of the production complexity, the lack of broad tunability and the limitation of the generated frequencies range to above 2 THz. On the other hand, THz emitters based on photomixing in semiconductor photoconductive antennae are very promising for the development of practical, compact, sufficiently powerful and reasonably-cheap, room-temperature THz sources. In this respect, quantum-dot (QD) semiconductor materials, InAs/GaAs QDs in particular, are of great interest for the use in both the pump laser and the photoconductive antenna for the development of a compact, room-temperature, widely-tunable THz laser source. Such QD-based lasers can offer broad wavelength coverage [1] together with the ability to generate two tunable longitudinal modes simultaneously [2], and InAs/GaAs QD-based antennae are capable of being pumped at high optical intensities of more than 1W [3]. Furthermore, the use of similar InAs/GaAs QD structures in both the pump laser and the antenna is beneficial for the generation of efficient THz radiation, particularly for the pump of the antenna in the vicinity of the QD excited states [3].In this work, a dual-wavelength high-power InAs/GaAs QD laser diode in the double-grating quasi-Littrow configuration (Fig.1), operating in the 1150nm - 1301nm wavelength region and producing the output with tunable difference frequencies between 0.28THz and 30THz (corresponding to the wavelength difference ranging from 1.4 nm to 151 nm), was used as a pump source to achieve tunable THz generation in an InAs/GaAs QD-based coplanar stripline antenna with a photoconductive gap of 50μm. The QD laser chip, similar to the reported in [4], in the double-grating external cavity demonstrated a maximum output power of 0.28W for the simultaneous dual-wavelength operation at 1230.7nm and 1232.1nm (corresponding to 0.28THz difference frequency). In order to obtain efficient THz generation, the laser was set to the wavelengths in the vicinity of the first QD excited state and the dual-colour laser operation was stabilised to have both laser modes generated with similar intensities. The laser output was coupled onto the antenna [3] by an AR-coated aspheric lens with a pump-spot width of about 40 μm, thus covering the most of the antenna's photoconductive gap, and centred closer to one of the electrodes for the enhanced operation.