Fig. 1. Scanning electron microscope images of (a) a partially buried heterostructure quantum cascade laser (QCL) device and (b) the improved buried heterostructure device in this work. (c) Temperature-dependent mid-IR (MIR) light-current-voltage curves of a 13.7 μm quantum cascade laser. A room temperature emission spectrum is shown in the inset.

Fig. 2. (a) THz nonlinear QCL device with abutted Si lens. (b), (c) Results of 3D COMSOL simulations of the Cherenkov THz power intensity when outcoupled from the device into (b) Si and (c) air. Upper figures of (b) and (c) display the simulated magnetic field (H_x) of the THz output of the device, respectively.

Fig. 3. Performance of a 3 mm long, 16 μm wide nonlinear-QCL with multimode spectra when operated at 288 K. Emission spectra for (a) MIR and (b) THz at various currents. (c) Peak THz output powers versus products of the MIR pump powers for the device with and without the Si lens.

Fig. 4. Measured far-field beam patterns: (a) no-lens device and (b) lens-coupled device. A 2D far-field profile for the lens-coupled device is also shown in the inset.

Fig. 5. (a) THz light output-current-voltage characteristics of the 16 μm wide and 3 mm long QCL device at different heat-sink temperatures when measured in pulsed mode (pulse width of 250 ns and repetition rate of 100 kHz). (b) THz spectra at different temperatures.

Fig. 6. (a) Schematic of Si lens-coupled EC system setup used in the experiments. (b) Light output-current characteristics of the MIR and THz emissions from the THz EC system with a nonlinear QCL when operating at 420 GHz. The THz emission spectrum measured at a current density of 5.0  kA/cm2 is also shown in the inset. (c) MIR emission spectra and power outputs from the two MIR pumps for the EC nonlinear-QCL system described in (a) at different EC diffraction grating positions. The MIR powers for ωDFB (black squares) and ωEC (red circles) pumps are shown as a function of the ωEC pump wavenumber. The dashed line represents the transmission spectrum of the MIR short-pass filter. (d) THz emission spectra of the EC nonlinear-QCL system at a current density of 5.0  kA/cm2. The THz peak power (blue diamonds and right axis) and the MIR-to-THz conversion efficiency (red-white circles and left axis) are also plotted as a function of the THz frequency.

Fig. 7. Far-field THz emission profile of the EC lens-coupled QCL device described in Fig. 6: (a) vertical and (b) horizontal. The vertical angle θ is defined relative to the direction normal to the laser facet, as shown in the right inset of (a).