THz absorption spectroscopy with a broadly tunable, single-mode quantum-cascade laser

In this work, we demonstrate a broadband THz laser absorption spectroscopy system based on a widely tunable single-mode source and two room-temperature detectors. The device, a quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL), is continuously tunable across any 35-45 GHz window within its 2.54-2.84 THz bandwidth and emits up to 200 µW CW in a high-quality Gaussian beam. Continuous and coarse tuning are both accomplished by actuating the piezoelectric stage which supports the output coupler. The system is capable of both stepped-scan operation for improved sensitivity and resolution, and of Hz-rate, time-resolved operation for real-time sensing. We present results from measurements of hydrogen sulfide under different gas cell conditions and discuss the interplay between the system’s sensitivity, resolution, scan range and scan rate, which are all coupled together by the slew rate of the piezoelectric stage.