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The development of quantum-cascade lasers (QCLs) at 2.1 THz (λ ≈ 141 µm), which is the longest wavelength QCL to date without the assistance of magnetic fields, is reported. This laser uses a structure based on resonant-phonon depopulation, and a metal-metal waveguide to obtain high modal confinement with low waveguide losses. Lasing was observed up to a heatsink temperature of 72 K in pulsed mode and 40 K in continuous-wave (CW) mode, and 1.2 mW of power was obtained in CW mode at 17 K.
Introduction: Compact, coherent sources of reasonable power are highly desired in the terahertz frequency range (1-10 THz) for spectroscopy, sensing, and imaging applications, and in particular as local oscillators in heterodyne receiver systems. Solid-state electronic sources of radiation are limited by both transit-time and RC highfrequency roll-offs, and have limited output power (sub-milliwatt) above 1 THz [1]. For this reason, efforts are under way to extend the recently developed terahertz quantum cascade lasers (QCLs) [2-4] to the lowest possible frequencies (longest wavelengths), in order to bridge the gap with electronic sources. However, building QCLs at such long wavelengths becomes increasingly challenging since the intersubband energy separations are extremely small (1 THz corresponds to ^o'4meV). It becomes difficult to achieve the selective injection and removal of carriers necessary to obtain an intersubband population inversion, especially as the energy separations become comparable to the subband broadenings. Furthermore, the free-carrier absorption loss scales as l2 and thus increases significantly at low frequencies. The first terahertz QCLs were demonstrated at approximately 4.5 THz using chirped superlattice active region structures and semi-insulating (SI) surface plasmon waveguides [2, 3]. Recently, lasers have been reported at ^2.3THz (l^130mm) in a bound-tocontinuum structure, and ^1.9 THz (l ^ 160 mm) in an intrawell structure that requires a magnetic field [5]. In this Letter, we report the development of a QCL that operates at a frequency of 2.1 THz (l ' 141 mm), based on a resonant-phonon active region structure and that uses a low-loss, high-confinement metal-metal waveguide.
Design and fabrication: The resonant-phonon active region design uses a combination of resonant tunnelling...