Browsing by Subject "Difference frequency generation"

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    Terahertz difference frequency generation quantum cascade lasers with improved terahertz out-coupling efficiency
    (2020-03-27) Kim, Jae Hyun; Belkin, Mikhail A.; Chen, Ray T; Bank, Seth R; Wasserman, Daniel M; Belyanin, Alexey
    The terahertz (THz) quantum cascade lasers based on intra-cavity difference frequency generation (DFG) in mid-infrared (mid-IR) quantum cascade lasers (QCLs), so-called THz DFG-QCLs, are the only electrically pumped, compact, and mass-producible THz semiconductor sources operating at room temperature. The Cherenkov phase matching scheme enables efficient out-coupling of THz radiation from the active region of the THz DFG-QCLs. However, the use of semi-insulating (S.I.) InP substrate and lateral current injection limit further improvements. This dissertation focuses on out-coupling methods for THz DFG-QCLs to overcome the limitations of Cherenkov THz DFG-QCLs. An application of Ⅲ-Ⅴ-on-silicon hybrid semiconductor laser concept to Cherenkov THz DFG-QCLs improves THz out-coupling efficiency owing to the smaller refractive index and THz absorption of float-zone high-resistivity silicon (FZ HR Si) substrate used in these devices than S.I. InP substrate. The transfer-printing method using a SU-8 adhesion layer is introduced to replace the original S.I. InP substrate with FZ HR Si substrate. These devices showed 5 times higher out-coupling efficiency than the reference Cherenkov THz DFG-QCLs on S.I. InP substrate at 3.5 THz. The use of wire grid polarizer pattern (WGP) instead of highly doped semiconductor current injection layer in the Cherenkov THz DFG-QCLs reduces the resistance of the devices with minimized output power decrease. Our proof of concept device with WGP sandwiched by SU-8 adhesive layer used for bonding FZ HR Si substrate and the waveguide of THz DFG-QCL demonstrated significant decrease of applied voltage while showing 1.8 times higher THz power output than the reference Cherenkov THz DFG-QCL devices at 3 THz. The double-metal waveguide THz DFG-QCL devices with surface grating out-coupler on top of the waveguide emitting the THz radiation in forward direction were also demonstrated in this dissertation. The surface grating out-coupler enables THz extraction from the whole length of the waveguide, therefore, efficient out-coupling can be achieved. Our device represents at least a factor of 2 improvement in the performance compared to the best Cherenkov DFG-QCLs operating below 2 THz