Novel approaches for robust polaritonics

Abstract

The possibility of having low-threshold, inversion-less lasers, makinguse of the macroscopic occupation, of the low density of states, at thebottom of the lower polariton branch, has intensified polariton researchin the last two decades. State of the art devices based on this admixedquasiparticle have already been realized using GaAs and CdTe active layers,although the accomplishment of room temperature lasers has beenlimited by their relatively weak exciton binding energy. The high excitonbinding energy and oscillator strength, as well as the advantageous relaxationdynamics of wide bandgap semiconductors, such as GaN, are wellsuited for room temperature polariton operation. The up to date demonstrationsof GaN based polariton lasers have used as the active layer bulkGaN, GaN quantum wells (QW’s), and GaN nanowires. In the latterapproach, individual nanowires are positioned in a microcavity showingremarkable polariton characteristics, but questions remain on the scalabilityof the approach, as well as on how to turn these nanowire-basedstructures into real electrically-injected devices. The former two casesare technologically viable, but are currently limited by the relatively poorquality of the active region, due to the structural disorder introduced bythe bottom GaN based Distributed Bragg Reflector (DBR) mirrors.In this thesis, a very straightforward processing technique is used toetch away an InGaN sacrificial layer, using photo-electrochemical (PEC)etching, creating ultra-smooth membranes containing GaN/AlGaN QW’s,which are then embedded between high quality dielectric DBR mirrors,on which polaritonic studies are performed. The GaN membrane or the active region is carefully engineered, ensuring superior optical properties,both prior to and after etching. At room temperature, the QW emission isstate of the art, with a linewidth of ~ 28meV, and a corresponding lifetimeof ~ 275ps. The PEC lateral etching parameters are optimised in sucha way, that the rms roughness of the membranes, measured by AtomicForce Microscopy (AFM), is as small as 0.65nm, very close to epitaxialquality. A temperature dependent study on the full-microcavity structure,unveils the strong coupling regime, exhibiting a robust Rabi splittingas large as 64meV at room temperature. The non-linear propertiesare examined, under non-resonant quasi-continuous excitation, with polaritonlasing demonstrated at an ultra-low, average threshold of ~ 4.5W/ cm2(~ 594μJ / cm2), the lowest ever reported for a 2D GaN basedsystem, accompanied by a spectacular condensation pattern in k-space.The latter is attributed to a site-specific polariton trapping mechanism,where polaritons accumulate in discrete levels within the trapping potential,helping to escalate the polariton density locally. This, along with thehigh optical quality of the all-dielectric microcavity (Q-factor ~ 1770), explainsthe obtained ultra-low threshold. It should be noted that the useof ultra-smooth GaN membranes in microcavities is fully compatible withthe realisation of electrically injected GaN polariton devices.In the direction of obtaining even more robust polaritonic devices, thebasic optical properties of high quality, strain free, GaN nanowires arestudied. However, to make the most out of this novel system, the absorptioncoefficients are extracted from as-grown GaN nanowires, on silicon<111>substrates, developing an all-optical method, analysing merelythe reflectivity spectra, which is demonstrated for the first time. It shouldbe noted that the absorption coefficients (directly proportional to oscillator strengths) corresponding to the excitons, provide a glance into theappropriateness of the respective GaN nanowire system, as optimal candidatesfor hefty polaritonics. However, the nanowires studied here, failedto shown an enhancement of absorption, which can be mainly attributedto the nanowire dimensions. The new method demonstrated here, can beextended to any family of nanowires, provided they are grown on a substratehaving considerable difference in permittivity with the nanowire-airmatrix.