Terahertz Plasmonic Lasers with Distributed-Feedback

Plasmonic lasers generate coherent surface-plasmon-polaritons (SPPs) and couldbe realized at subwavelength dimensions in metallic cavities for applications innanoscale optics. Plasmonic cavities are also utilized for terahertz quantum-cascadelasers (QCLs), which are the brightest available solid-state sources of terahertz radiation(frequency 1 THz - 10 THz, wavelength 300um - 30um). A long standingchallenge for plasmonic lasers that are utilized as nanoscale sources of radiation, istheir poor coupling to the far field radiation. Unlike conventional lasers that couldproduce directional beams, plasmonic lasers have highly divergent radiation patternsdue to their subwavelength apertures.The primary goal of this thesis is to achieve narrow beam patterns for plasmoniclasers. This thesis theoretically and experimentally demonstrates a new techniquefor implementing distributed-feedback (DFB) that is distinct from any other previouslyutilized DFB schemes for semiconductor lasers. The so-termed antenna-feedback scheme leads to single-mode operation in plasmonic lasers, couples the resonantSPP mode to a highly directional far field radiation pattern, and integrateshybrid SPPs in surrounding medium into the operation of the DFB lasers. Experimentally,the antenna-feedback method, which does not require the phase matchingto a well-defined refractive index, is implemented for terahertz QCLs, and single modeterahertz QCLs with beam divergence as small as 4 by 4 degrees are demonstrated,which is the narrowest beam reported for any terahertz QCL to-date. Moreover, incontrast to negligible radiative field in conventional photonic band-edge lasers, inwhich the periodicity follows the integer multiple of half-wavelength inside activemedium, antenna-feedback breaks this integer-limit for the first time and enhancesthe radiative field of lasing mode. A comprehensive analysis of antenna-feedbackscheme is presented, including theory, design, simulation, and experimental results.Terahertz lasers with narrow-beam emission will find applications for integrated aswell as standoff terahertz spectroscopy and sensing. The antenna-feedback schemeis generally applicable to any plasmonic laser with a Fabry-Perot cavity irrespectiveof its operating wavelength, and could bring plasmonic lasers closer to practicalapplications.This thesis also demonstrates a new tuning mechanism for plasmonic lasers withdistributed-feedback, in which refractive-index of the surrounding medium affectsthe resonant-frequency of DFB mode in the same vein as refractive-index of gainmedium inside the cavity. Reversible, continuous, and mode-hop-free tuning ofaround 57 GHz is realized for single-mode narrow-beam terahertz plasmonic QCLs,which emit at 2.8 THz and operate in a liquid-nitrogen cooled dewar. The tuningis based on post-process deposition/etching of a dielectric (Silicon-dioxide) ona QCL chip that has already been soldered and wire-bonded onto a copper mount.This is a considerably larger tuning range compared to previously reported resultsfor single-mode terahertz QCLs with directional far field radiation patterns. Also,the tuning is demonstrated at the much more practical operating temperature of 78 K, whereas all previous tuning results for terahertz QCLs have been reportedat low-temperatures ( ~10 K). The key enabling mechanism for tuning is the newantenna-feedback scheme for plasmonic lasers as described in Chapter 2 and 3, whichleads to generation of hybrid SPP mode propagating outside the cavity of the laserwith a large spatial extent. The effect of dielectric deposition on QCLs' characteristicsis investigated in detail including that on maximum operating temperature, peakoutput power and far field radiation patterns. Single-lobed beam with low divergence(less than 7 degree) is maintained through the tuning range, which is the narrowest divergence for any tunable terahertz QCL to-date. The antenna-feedback scheme isideally suited for modulation of plasmonic lasers due to the sensitive dependence ofspectral and radiative properties of the laser on its surrounding medium.Finally, preliminary work on high-power terahertz QCLs with single-mode operationand single-lobed far field radiation patterns is reported based on surface emittingsecond-order DFB with new photonic designs. Improved outcoupling ofradiation from laser's cavity leads to enhanced output power; 50 mW peak outputpower is realized at 51 K, which is one of the highest reported output powers from single-mode terahertz QCLs. Higher output power is due to broader emissionarea and increased radiative field from grating apertures. Better beam quality withsingle lobe is achieved without any central phase shift in such gratings, which isanother advantage of the demonstrated scheme. A comparative study with conventionalsecond-order DFB terahertz QCLs is presented, where such QCLs have loweroutput power and less desirable radiation patterns with two lobes in the far field.