Terahertz-frequency quantum cascade lasers (THz QCLs) have numerous potential applications as 1-5 THz radiation sources in space science, biomedical and industrial sensing scenarios. However, the key obstacles to their wide-scale adoption outside laboratory environments have included their poor far-field beam quality and the lack of mechanically robust schemes that allow integration of QCLs with THz waveguides, mixers and other system components. A block integration scheme is presented, in which a continuous-wave ∼3.4 THz double-metal QCL is bonded into a precision-machined rectangular waveguide within a copper heat-sink block. This highly reproducible approach provides a single-lobed far-field beam profile with a divergence of ≲20°, and with no significant degradation in threshold current or in the range of operating temperatures.Introduction: Terahertz-frequency quantum cascade lasers (THz QCLs) [1] are compact sources of coherent radiation in the 1-5 THz band, with peak output powers of up to 1 W [2]. Numerous potential applications exist, including their use as local oscillators for satellite-borne astronomy and atmospheric science instrumentation [3] and as radiation sources for industrial inspection, security and biomedical imaging [4]. However, widespread commercialisation of THz QCLs has not yet been realised. One significant issue is the lack of a mechanically robust and reproducible scheme for integrating THz QCLs with external waveguides and mixers. This typically leads to large and fragile arrangements of discrete optical components, although several elegant yet complex semiconductor integration techniques have been proposed (e.g. [5]). QCLs in double-metal waveguides yield the best thermal performance, and are well-suited to 'near-field' radiation coupling (e.g. into external waveguides). However, their poor 'far-field' beam quality and wide divergence [6] lead to poor coupling into external freespace optical components. Previous beam optimisation techniques have been employed in either device patterning approaches [7,8] or assemblies of antennas or lenses [9, 10]. Although a far-field beam divergence of &20 W is achievable with these techniques, they are relatively complex and their reproducibility and mechanical robustness have not been demonstrated.We report a new packaging and waveguide-integration scheme, in which a 3.4 THz double-metal QCL is ribbon bonded to a direct current (dc) stripline within a copper (Cu) heat-sink enclosure containing a rectangular cross-section metallic waveguide. This approach makes use of highly reproducible mechanical microfabrication techniques previously developed to support the construction of waveguide-integrated THz-frequency mixers. Since the QCL cavity itself is not modified, this approach has a very low impact on the threshold current and operating temperature range of the device, while yielding a beam divergence of <20°, comparable to the previous techniques discussed above.

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Antenna-pattern measurements obtained from a double-metal supra-terahertz-frequency (supra-THz) quantum cascade laser (QCL) are presented. The QCL is mounted within a mechanically micro-machined waveguide cavity containing dual diagonal feedhorns. Operating in continuous-wave mode at 3.5 THz, and at an ambient temperature of ~60 K, QCL emission has been directed via the feedhorns to a supra-THz detector mounted on a multi-axis linear scanner. Comparison of simulated and measured far-field antenna patterns shows an excellent degree of correlation between beamwidth (full-width-half-maximum) and sidelobe content and a very substantial improvement when compared with unmounted devices. Additionally, a single output has been used to successfully illuminate and demonstrate an optical breadboard arrangement associated with a future supra-THz Earth observation space-borne payload. Our novel device has therefore provided a valuable demonstration of the effectiveness of supra-THz diagonal feedhorns and QCL devices for future space-borne ultra-high-frequency Earth-observing heterodyne radiometers.

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