Low-Frequency Terahertz Quantum Cascade Laser Based On Intra-Cavity Nonlinear Mixing

Terahertz quantum cascade laser sources based on intracavity difference frequency generation are currently the only electrically-pumped monolithic semiconductor light sources operating at room temperature in the 1–6 THz spectral range. These devices demonstrated drastic improvements in performance in the past several years. Recent efforts in the wavefunction engineering using an active region design based on a dual-upper-state concept led to a significant enhancement of the optical nonlinearity of the active region for efficient terahertz generation.1 Our terahertz quantum cascade laser sources packaged in butterfly modules exhibit the power of >0.3 mW, with octave spanning comb-like terahertz emission spectra. In this presentation, we discuss low frequency generation from terahertz quantum cascade laser sources based on intra-cavity nonlinear frequency mixing. In order to obtain higher nonlinear susceptibility in low frequency region, we design a long wavelength dual-upper-state active region in which transition dipole moments are increased.2 A fabricated device with distributed feedback grating demonstrates a THz peak output power of $40\ \mu \mathrm{W}$ at room temperature, with multi-mode THz emission at a frequency of 1.4 THz. Besides, a device produces THz output power of $> 250\ \mu \mathrm{W}$ at 110 K, which is higher output power, compared to low-frequency (1.2-1.6 THz) THz-QCLs at liquid helium temperature (10 K). Furthermore, we have recently expanded their operation frequency ranges to sub-THz. This is the lowest frequency (longest wavelength) electrically pumped monolithic semiconductor laser source.