Terahertz-frequency quantum cascade lasers (THz QCLs) are compact solid-state sources of coherent radiation in the 1–5 THz region of the electromagnetic spectrum [1]. THz QCLs typically exhibit multiple longitudinal modes characteristic of Fabry–Pérot cavities. However, widely-tunable (single-mode) THz QCLs would be ideally suited to many THz- applications, such as atmospheric observations [2], and security screening [3]. Here we demonstrate discrete Vernier tuning using a simple two-section coupled-cavity geometry comprising of a ‘lasing section’, which is electrically driven above the lasing threshold, and a ‘tuning section’, which is driven below threshold. Our THz QCLs, based on a bound-to- continuum design [4], were processed into 150-μm-wide single–metal waveguides with lengths 4.5–4.8 mm. Devices were etched after packaging using a focused ion beam milling system to sculpt a14-μm–wide and 12-μm-deep air gap to form the two-section cavity [Fig. 1 (a)]. Devices were cooled in a continuous-flow helium cryostat and emission spectra measured using a Fourier-transform infrared spectrometer. The tuning section of the laser was heated below threshold using a train of 10-μs-long current pulses at a repetition rate of 8.21 kHz. The lasing section was driven with a single 500-ns-long pulses above threshold. Both the pulse trains were trigerred using a 600-Hz reference frequency. Discrete tuning with a blue shift in frequency was observed over bandwidths of 50 and 85 GHz from two devices with mode spacing of 15 GHz and 30 GHz respectively [Fig. 1 (b, c)]. A red shift in frequency over 30 GHz was also observed in device 2 by simply swapping the function of the lasing and tuning sections [Fig. 1 (c) Inset]. Negligible degradation in output power was observed with tuning current. R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and F. Rossi, "Terahertz semiconductor-heterostructure laser," Nature 417, 156–159 (2002). P. H. Siegel, "Terahertz technology," Microw. Theory Tech. IEEE Trans. On 50, 910 –928 (2002). A. G. Davies, A. D. Burnett, W. Fan, E. H. Linfield, and J. E. Cunningham, "Terahertz spectroscopy of explosives and drugs," Mater. Today 11, 18 – 26 (2008). S. Barbieri, J. Alton, H. E. Beere, J. Fowler, E. H. Linfield, and D. A. Ritchie, "2.9 THz quantum cascade lasers operating up to 70 K in continuous wave," Appl. Phys. Lett. 85, 1674–1676 (2004)
