Fig. 1. Comparison of conventional p-th order DFB and antenna-feedback scheme for THz QCLs. (a) Principle of conventional DFB that can be implemented in the waveguide of THz QCLs;(b) phase mismatch of surface plasma THz fields between adjacent transmission apertures of conventional DFB;(c) discontinuous single-sided plasmas in surrounding medium owing to destructive interference; (d) principle of antenna-feedback scheme for terahertz QCLs; (e) antenna-feedback leads to a fixed surface plasma THz field p

Fig. 2. Optical image and scanning electron microscope image of THz QCL, and THz QCL spectrum of pulsed mode at 78 K when silicon-dioxide is deposited on surface of THz QCL[27]

Fig. 3. Output power of representative single-mode THz QCL based on double-metal waveguide

Fig. 4. Comparison of hybrid second- and fourth-order DFB and second-order DFB for terahertz QCLs. Periodic DFB structure is fabricated on the top metallic layer. Mode-spectrum for a 1.4 mm DFB grating (period is 27 μm and slit-width is ~3 μm) is computed with finite-element modeling method. Red and blue lines represent surface radiative losses for various resonant modes with second-order DFB and hybrid second- and fourth-order DFB, respectively. Electric field profiles for lower and upper band-edge mod

Fig. 5. Experimental results of hybrid second- and fourth-order DFB THz QCL. (a) Scanning electron microscope image of fabricated THz QCL with hybrid second- and fourth-order DFB; (b) current-voltage (I-V) curve at 62 K and light intensity-current (L-I) curve at different operating temperatures measured in pulsed mode of operation. Insert is spectrum under different voltage at 62 K. Size of THz QCL is 10 μm×200 μm×1.5 mm, and grating period Λ is 28 μm; (c) far-field radiation pattern (optical intensity)