Mode Selection And Tailoration Of The Radiation Efficiency In Terahertz Quantum Cascade Lasers

Terahertz wave has the potentials for various applications including but not limited to: astrophysics and atmospheric sensing,biomedicine,high-speed communication,non-destructive evaluation and security screening,promoting the development of terahertz sources towards miniaturization,high radiation-efficiency and low-cost.Terahertz quantum cascade lasers(THz-QCLs)based on semiconductor heterostructure materials,as one of most promising compact terahertz sources,are devoted to improve the output power,beam quality,operation temperature and continuous tuning ranges for practical application.This thesis briefly describes the application of terahertz radiation,several terahertz sources and terahertz detection technologies,systematically introduce the principle and status quo of the THz-QCLs,including the current development trends and the major pressing issues,sums up the theory of several common resonant cavities widely applied in semiconductor lasers and succinctly introduces the corresponding methods of electromagnetic simulation and some key characters of the THz-QCLs.On the basis of the above,this thesis focuses on two core issues,namely the mode selection and the tailoration of the radiation efficiency in single-mode THz-QCL,and carries out systematic theoretical and experimental research.The resonant cavity with an active distributed Bragg reflector for mode selection and the cavity with a grating coupler to tailor the radiation efficiency are proposed respectively based on the metal-metal(MM)waveguide.Based on the sufficient verification of the aforementioned two kinds of cavities,the active distributed Bragg reflector and the grating coupler are integrated into one THz-QCL based on the metal-metal waveguide,which can realize approximately independent control of the mode selection and the tailoration of the radiation efficiency.Consequently,the peak output power of the single-mode THz-QCL in pulsed mode is improved distinctly.Finally,we combine the single-mode THz-QCL with the photoacoustic spectroscopy technology for the trace H2 S gas sensing and obtain sub-ppm detection limit.The main representative innovations are as follows:1)We demonstrated stable single-mode operation from THz-QCL via an active distributed Bragg reflector(ADBR)integrated with the FP cavity.This resonant cavity is based on the MM waveguide with two facets.A series of periodic air slits are formed on the top metallization near one facet,as an ADBR,followed with the absorbing boundary.The other facet is a cleaved facet.The terahertz wave is coupled out via the cleaved facet when oscillating between the cleaved facet and ADBR.When the net gain of the ADBR section is sufficiently high,the reflectivity of the light whose frequency near the band edge modes rises,forming resonance amplification peaks with very narrow band-width(< 15 GHz)in the reflection spectrum,and the total phase shift in one round-trip changes about 2? in one peak,permitting the THz-QCL single-mode operation.Experimentally,the THz-QCLs integrated with ADBR have been actualized and exhibiting single-mode operation.Two-dimensional photonic crystal is used instead of one-dimensional grating as ADBR for stable single-mode operation.The influence of the nonuniformity of the gain on the mode selection is diminished when reducing the width of the band gap.The 3.4-mm-long THz-QCLs integrated with ADBR show stable single-mode operation.This structure breaks through the restriction of the passive DBR on the cavity length of the single-mode THz-QCLs.2)We demonstrated highly efficient power extraction from THz-QCL via a grating coupler(GC).The GC,formed in the top metallization of a Fabry-Perot(FP)THz-QCL with a metal-metal(MM)waveguide,diffracts the THz wave into the free space when it oscillates between the two cleaved facets.Its superiority mainly embodies in:(I)The radiation loss of a THz-GC-QCL can be controlled over a wide range by adjusting the structural parameters of GC.(II)The asymmetric positioning of the GC,which is in one side of the cavity,enables near-unidirectional emission,and the enlarged emission surface reduces the beam divergence.(III)The MM waveguide keeps the optical confinement factor near unity,guaranteeing temperature performance of the laser.The advantages mentioned above have been proven sufficiently in the experiment.Compared with the FP laser based on the semi-insulator single plasmon(SISP)waveguide,the THz-QCL with a grating coupler shows superior performances in terms of output power and operation temperature.Compared with the the FP laser based on MM waveguide,the output power and beam quality of the THz-QCL integrated with the GC are greatly improved at the cost of slightly reducing the temperature performance.3)Based on the research of the aforementioned cavities,we demonstrated a mechanism to approximately independent control on the mode selection and radiation efficiency of a semiconductor laser.The ADBR and the GC are integrated at opposite ends of the top metallization,the former one for selecting lasing mode and the latter one for tailoring the radiation efficiency.Thereby the output power of the single-mode laser has been improved significantly.Experimentally,the peak output power of the THz-QCL integrated with ADBR and GC reached 214 m W at 20 K,which is 26.7 times as that of the conventional single THz-DFB-QCL from the same material.Experimental results confirm that the peak output power is advanced to 366 m W at 20 K and 246 m W at 77 K while utilizing a modified active region design and material.The maximum operation temperature is up to 147 K.4)The single-mode THz-QCL cooled with liquid nitrogen,que the source,was combined with the quartz enhanced photoacoustic spectroscopy(QEPAS)initially realized the hydrogen sulfide(H2S)detection.The peak of the spectral line of the laser can sweep through one of the characteristic absorption peaks of H2S(95.63 cm-1)by controlling the operation temperature and driving current of the THz-QCL.The peak output power of the single-mode THz-QCL is over 150 m W(in pulsed mode at 81 K),the diameter of the focal spot is about 1.0 mm,and the prong spacing of the quartz tuning fork is 1.5 mm.These factors significantly improve the signal-to-noise ratio.A detection sensitivity record for THz-QEPAS of 360 part-per-billion(ppb)in volume was carried out when the system integration time was 10 s.Finally,the thesis proposes some further notions on the following aspects:(I)further improve the beam collimation of the THz-QCLs integrated with ADBR and GC;(II)extend the design of the THz-QCLs integrated with ADBR and GC to continuous working mode and frequency tuning.