Tunable mid-infrared quantum cascade lasers
Date of Issue2015
School of Electrical and Electronic Engineering
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.
DRNTU::Engineering::Electrical and electronic engineering