Title: Near-field and far-field engineering of semiconductor lasers
Description: Plasmonics focuses on the interaction between light and metallic films or nanostructures. In the last two decades, intensive research efforts were devoted to exploring the extremely broad applications of plasmonics. My research combines the versatility of plasmonics with active light sources, i.e., quantum cascade lasers (QCLs). This thesis focuses on the application of plasmonics in near-field and far-field engineering of semiconductor lasers, specifically, subwavelength focusing in the near-field, and laser beam collimation and polarization control. The first chapter of this thesis lays out fundamental materials necessary for understanding the following chapters. Systematic simulation and experimental results are presented in Chapter 2 to demonstrate that the integration of a suitably designed one dimensional or two dimensional plasmonic structures on the facet of QCLs can reduce the beam divergence by more than one order of magnitude. The devices with optimized collimators preserve a high output power, comparable to that of the unpatterned lasers. Chapter 3 demonstrates that the polarization state of the output of semiconductor lasers can be controlled by defining plasmonic structures on the laser facet. An integrated plasmonic polarizer can project the polarization of a semiconductor laser onto other directions. By patterning a facet with two orthogonal grating-aperture structures, a QCL can produce emission consisting of a superposition of a linearly and right-circularly polarized light, a first step towards a circularly-polarized laser.

Chapter 4 presents experimental work on the coupled-rod antennas and the bowtie antennas patterned on the facet of QCLs. Both designs can provide an optical field confinement on the order of λ/50 and with peak intensity on the order of 1 GW/cm 2 in the antenna gap. The bowtie devices are more advanced due to better confinement of light into a single spot. Chapter 5 and 6 discuss two side research topics. Chapter 5 investigates four-wave mixing interaction between the transverse modes of quantum cascade lasers originating from the strong optical nonlinearity of the gain transition. The last chapter discusses that the near-field transfer of radiation energy from an optically excited atomic dipole to a metallic film in the vicinity can greatly enhance the decay rate of the dipole in a well-controllable and predictable way.