Tunable mid-infrared quantum cascade lasers

• Main Author: Meng, Bo
• Other Authors: Wang Qijie
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/62926
• Institution: Nanyang Technological University

The tremendous developments of quantum cascade lasers (QCLs) have proven themselves as powerful tools for spectroscopy, homeland security, thermal imaging, and free-space communication applications. Compact broadly tunable QCLs in both mid-infrared (3-24 μm) and terahertz (1.2-5 THz) spectrum regions are especially important for high sensitivity spectroscopy due to their intrinsic narrow linewidth, high output power, robustness, and versatile emission wavelength designability. To achieve single-mode emission from QCLs, a number of schemes have been employed to exploit the broad gain spectrum of QCLs, e.g. external cavity QCLs (EC-QCLs), distributed feedback QCLs (DFB-QCLs), DFB QCL array, and sampled grating QCLs (SG-QCLs) etc. However, the above approaches either suffer from complex fabrication processes, e. g. InP regrowth and e-beam lithography, or reduced robustness due to the mechanically moving parts. Monolithic single-mode QCLs with broad tuning range and simple fabrication process through photolithography are thus necessary for both real-life applications and laboratory researches. Therefore, the main objective of this thesis is to develop novel tunable single-mode QCLs with easy fabrication and high performances. In this thesis, first, we have proposed and experimentally demonstrated compact tunable single-mode QCLs based on slot waveguide tunable structure at wavelength of ~10 μm. The slot-QCLs demonstrates a tuning range of 77 cm-1, which corresponds to ~7.8 % of relative tuning, while maintaining ~20 dB side mode suppression ratio (SMSR) within the whole tuning range. Compared with DFB-QCLs, the broader tuning range together with significantly simplified fabrication process makes slot-QCLs better candidates for high resolution spectroscopy. To further increase the wavelength modulation speed, we have also proposed and investigated tunable single-mode mid-infrared quantum cascade lasers based on surface acoustic-wave (SAW) modulation mechanism. The air-waveguide and surface plasmon waveguide structures with two-section active regions were proposed, together with Zinc Oxide (ZnO) thin film deposited on top of these devices to enhance the piezoelectricity of the materials. Coupling coefficients of ~2.5 cm-1 were calculated for both waveguide structures, showing the possibility of achieving tunable single-mode emission by using SAW modulation. Furthermore, to improve the single-mode QCLs performances in terms of power and modulation bandwidth, we have studied the high modulation bandwidth injection-locked single-mode QCLs. Mid-infrared QCLs with wavelength of 4.6 μm and 9 μm were investigated for comparison. Enhanced modulation Bandwidths of ~30 GHz and ~70 GHz were obtained for 4.6 μm and 9 μm, respectively, under a 5-dB optical injection ratio, showing threefold increases of modulation bandwidth for both wavelengths. These injection-locked QCLs are expected to be important components in mid-infrared free space communications.