Candidate
Chao Xu
Title
Toward High Performance Broad-band Frequency Comb Operation of THz Quantum Cascade Lasers
Supervisor
Dayan Ban
Abstract
Recent technological development in Terahertz Quantum Cascade Laser (THz QCL) has pushed this technique further into the practical applications, particularly the spectroscopic field. For this purpose, the operation of frequency combs in THz QCL is of especial interest, for which can provide extreme accuracy and stability to the optical spectra in a broad frequency band. Recently, the frequency comb of THz QCLs has been realized with two separated techniques: employing a broad-gain active region or employing a group velocity dispersion controlled waveguide. However, due to the residual optical dispersion from both the gain medium and the cold waveguide, the comb formation in these reported THz QCLs can only sustain a limited current injection region and the observed comb frequency range is much narrower than the bandwidth of the designed gain medium. To overcome these shortcomings, this thesis is targeting to design a new THz QCL frequency comb device that simultaneously exploits both a broadband gain active region and a group velocity dispersion-compensated waveguide over an octave frequency band of 2-4 THz. In designing a broadband gain active region, two heterogeneous structures are proposed and simulated, with one combining three different bound-to-continuum active regions operating at a temperature of 25 K, and another one consisting four different resonant-phonon active regions operating at the liquid nitrogen temperature (77K) or higher. The simulation results show that both of the two active region designs can provide a broadband and ‘flat-top’ gain profile covering the frequency range of 2-4 THz. In designing a group velocity dispersion-compensated waveguide, the strategies of simulating chirped Distributed Bragg Reflectors (DBRs) for a THz QCL metal-metal waveguide are first explored and the approaches for one-dimensional (1D) and three-dimensional (3D) modeling are established and verified. A novel two-section chirped DBR is proposed, which provides substantially-improved group delay compensation over a broadband octave frequency range from 2 to 4 THz. Two THz QCL structures are grown by using in-house molecule beam epitaxy and THz QCL devices with a metal-metal waveguide are fabricated at local Quantum-Nano-Center clean-room fab. The experimental results demonstrate the lasing operation of a THz QCL with a broadband gain active region design up to a maximum lasing temperature of 111 K. The theoretical and experimental work would ultimately lead to the demonstration of THz QCLs for improved frequency comb operation over a broadband range of 2-4 THz.