Design and analysis of ultra-broadband gain medium and waveguide structure for terahertz emission

Abstract

Quantum cascade laser (QCL) is a promising candidate for comparatively less developed terahertz spectral range. Design-dependent emission frequency and sequential pumping make QCL suitable for Hetero-active region operation, which can emit multiple frequency at the same time. From this multiple frequency emitting QCL, using various tuning techniques, we can achieve a broadband terahertz emission from a single QCL cavity. GaAs based multiple frequency emitting QCL has already been reported, but GaN based QCLs provide significant advantages over traditional GaAs based QCLs due to their higher LO phonon energy. We propose a hetero cavity terahertz GaN based QCL emitting around 1.5 THz, 4 THz, and 7 THz. Using bias tuning, we have achieved an ultra-broadband emission covering a range of 1–10 THz. Current mismatch is an inherent problem in the hetero-structure devices. We calculate the effect of current mismatching in hetero-active structures. We find that both higher and lower current injection results in lower population inversion and hence lower gain, and mismatch of more than 10% results in gain less than typical loss in the cavity. Waveguide design is of significant importance in terahertz range due to the confinement problem of longer wavelength. We design and optimize the waveguide for the QCL using double metal configuration to support the broadband emission of 1–10 THz.