The terahertz region (1–5 THz) of the electromagnetic spectrum remains one of the least developed spectral regions, despite its proven potential in a number of areas, including security screening, biochemical detection, remote sensing, nondestructive materials evaluation, communications, and astronomy. The reason for slow development of the terahertz systems is lack of both convenient radiation sources and sensitive room-temperature detectors. Continuous-wave terahertz sources are of particular value to the development of terahertz systems as they can provide narrowband emission and therefore may be used as both illumination sources and local oscillators for heterodyne-based terahertz detection. Heterodyne terahertz detectors, employing, e.g., Schottky diodes, provide highly sensitive, simple, and inexpensive way of detecting of terahertz radiation at room temperature. Most applications ideally require at least milliwatt-level terahertz sources. Until this report, of all table-top systems, only gas lasers could provide continuous-wave terahertz radiation with above-milliwatt power levels in the 1–5 THz frequency range at room temperature. However, gas lasers are very bulky and can operate only at a few fixed frequencies. In contrast, the terahertz source presented in this work is reasonably compact (approximately 20 cm x 20 cm with an additional 33 cm x 17 cm x 7 cm pump laser), has relatively low power consumption (250 W), and is tunable. Its operation principle is based n a nonlinear process of difference-frequency generation with terahertz frequency being generated in a nonlinear crystal by two infrared beams. This approach is not a new idea, but the authors have perfected their optical setup to produce continuous-wave power output of over 2 mW at 1.9 THz with a high-quality Gaussian output beam. This is an improvement of almost 3 orders of magnitude in power over previous reports on such systems. The advancement was achieved using intracavity difference-frequency generation scheme in a high-finesse optical resonator with a nonlinear crystal quasi-phase matched for vertical terahertz radiation extraction.
Milliwatt-level power output makes the reported system suitable for a wide range of practical applications. Moreover, the authors indicate the terahertz output power may be further increased to tens of milliwatts and beyond using a number of simple improvements in their optical setup. Several topics are left for further research: terahertz emission linewidth remains to be determined and the tuning range of the current system is currently limited to about 200 GHz. Increasing tuning range, however, appears to be a purely technical issue, and it is very likely that, using frequency stabilization, the terahertz emission linewidth will be sufficiently narrow for heterodyne detection. Overall, the new terahertz source presents a compact and more versatile alternative to terahertz gas lasers and will likely be in high demand for various applications.
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