| The past decade has seen the rapid development of Quantum Cascade (QC) lasers and their promising applications. The progress in growth, design, fabrication and packaging has led to substantial advances in laser power and efficiency performance in the 4-12 mum wavelength regions. However, improving the spectral performance to meet the requirements of various applications remains a challenge. My dissertation explores several new QC laser design concepts to strongly improve the laser spectral capabilities for real applications.;Voltage tunable laser sources are always favored because of their simplicity and fast tuning speed. We investigate the voltage tunablity of several present QC designs and explain why it is difficult to achieve a wide voltage tuning range in QC lasers. Based on this understanding, we demonstrate a QC structure with a 'two-step coupling' injection scheme, and the experimental results show a wide voltage tuning range up to 100 cm-1 at room temperature above laser threshold.;Although voltage tuning of laser spectra has many unique applications, a broadband QC gain medium is more suitable for the widely tunable single mode laser sources as well as broadband mid-infrared amplifiers. We demonstrate broadband QC lasers at 7-9 mum based on "continuum-to-bound" designs, with a gain spectra width up to 250 cm-1 at the threshold electrical field. Then we further improve our design concept and demonstrate a QC design at 4-5 mum based on a "continuum-to-continuum" design, which shows over 400 cm-1 gain bandwidth and enables a tuning range of over 350 cm-1 in an external cavity setup at room temperature. Besides the broad gain spectra, these lasers show the best power and efficiency performance among those reported in literature so far.;We are also expanding the wavelength coverage of QC lasers to very short wavelengths, i.e. below 4 mum, where some important molecular resonances (e.g. the C--H stretch) are located. ZnCdSe/ZnCdMgSe grown on InP substrate is a promising material system for this short wavelength range, because of the large conduction band-offset and absence of inter-valley scattering. By using robust QC designs and improved waveguide structures, we have achieved narrow electroluminescence up to room temperature. |
