Development of Quantum Cascade Lasers with Novel Active Regions and Integrated Nano-Antennas

Quantum Cascade Laser (QCL), invented in 1994, has led to path-breaking improvements in room-temperature operation in mid and long wave infrared, and has been used in gas and chemical sensing, bio-imaging, free-space communications and many other military applications. One of the major operational drawbacks of standard QCL is added phonon relaxation in the injector region leading to generation of excess heat. The first part of my thesis focuses on developing a novel injectorless QCL (I-QCL) which circumvents this problem. The fabricated laser was both electrically and optically tested and compared with two types of standard QCLs---one developed in our laboratory and another provided by MIT Lincoln Laboratory. Voltage defect is a key parameter used to quantify excess heat generated in a QCL. We were able to measure a record low voltage defect of ∼ 57 meV at 77 K using the I-QCL we have developed. The effect of injectors on thermal performance of QCL was further analyzed through time-resolved spectral analysis. Next, we focused on developing a composite material based plasmonic antenna integrated QCL. The device was capable of squeezing the optical mode to ∼ 100 nm which is 60 times smaller than the operating wavelength (∼ 6 um). Such mode confinement can overcome the primary drawback in a mid-IR bio-sensor where the dimensional mismatch between long wavelengths (order of microns) and tiny probed molecules (∼ few nanometers) makes probe-particle interaction strength extremely weak. An apertureless near-field scanning optical microscope (a-NSOM) was built to measure the antenna near-field characteristic. We further worked on measuring the optical force generated near the antenna "hotspot" due to high electric field gradient. We then worked on understanding the coupling between antenna plasmonic modes and the laser cavity mode. This unusual coupling has been explained based on optical feedback effect. The final part of my research focused on delivering the bio-molecules on the 'hot-spot' using different methods like Microfluidics, Dip-pen nanolithography etc. Through the design of a novel injectorless QCL and integration of plasmonic antenna on laser facet, we developed a compact device which has potential and significant applications in high sensitive, infrared bio-sensing.