Partial synchronisation of stochastic oscillators through hydrodynamic coupling

Holographic optical tweezers are used to construct a static bistable optical potential energy landscape where a Brownian particle experiences restoring forces from two nearby optical traps and undergoes thermally activated transitions between the two energy minima. Hydrodynamic coupling between two such systems results in their partial synchronisation. This is interpreted as an emergence of higher mobility pathways, along which it is easier to overcome barriers to structural rearrangement.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let

Exciton-photon hybridisation in ZnSe based microcavities

This thesis presents the design, fabrication and experimental analysis of ZnSe based microcavities. Semiconductor microcavities are micro-structures in which the exciton ground state of a semiconductor is coupled to a photonic mode of an optical cavity. The strong light matter coupling mixes the character of excitons and photons, giving rise to the lower and upper cavity polaritons, quasiparticles with an unusual dispersion due to the extreme mass contrast between the composite exciton and photon. In particular, the dispersion of the lower polariton forms a dip around the lowest energy state with zero in-plane momentum. In this dip, which can be seen as a trap in momentum space, the polaritons are efficiently isolated from dephasing mechanisms involving phonons. The features of these quasiparticles promise a variety of applications, for instance lasing without inversion and micro-optical parametric amplifiers, and an environment to study fundamental physics, such as Bose-Einstein condensation in the solid state. By overcoming the longstanding fabrication problems of ZnSe-based microcavities, the enlarged exciton binding energy in combination with the use of highly reflective dielectric mirrors makes this material system ideally suited to the realisation of polariton-based devices operating at room temperature. An epitaxial liftoff technology is developed that relies on the high etch selectively between the ZnSe heterostructure and a novel II-VI release layer, MgS. Three hybrid microcavities are fabricated with the liftoff technique and spectroscopically characterised. Angle resolved transmission experiments reveal strong hybridization of the ZnSe/Zn0:9Cd0:1Se quantum well excitons and cavity photons in a fixed microcavity. A completely length tunable microcavity is presented and shown to exhibit similar dispersion as for the fixed microcavity, with the addition of evidencing the cavity polariton bottleneck effect. The nonlinear optical features are discussed. Photoluminescence data is presented that evidences the first observation of the build up of cavity polaritons at the edge of the momentum space trap in the lower polariton branch, the bottleneck effect, in a ZnSe based microcavity. Finally, lasing at room temperature in the blue spectral region is presented for a metal/dielectric hybrid microcavity

Dislocation controlled formation and kinetics of grain boundary loops in two-dimensional crystals

Significance Twisting a circular patch inside a large crystal does not always lead to a bicrystal. This is due to the topological properties of the resulting loop-shaped grain boundaries and the defects they are made of. By deforming a crystalline monolayer of colloidal particles using optical tweezers, we find that the product of the angle of rotation and the size of the patch needs to exceed a universal value that depends only on the lattice type for a grain boundary loop to form, while otherwise the deformation is elastically restored. We measure this universal value from the shrinkage kinetics of grain boundary loops and relate it to the complex dislocation structure and reactions, which we directly visualize in our experiments.</jats:p

Core-shell particles for simultaneous 3D imaging and optical tweezing in dense colloidal materials

A new colloidal system which consists of core-shell "probe" particles embedded in an optically transparent "host" particle suspension is developed. This system enables simultaneous fast confocal imaging and optical tweezing in dense 3D colloidal materials

A tunable microcavity

We present a generic microcavity platform for cavity experiments on optically active nanostructures, such as quantum dots, nanocrystals, color centers, and carbon nanotubes. The cavity is of the Fabry-Perot type with a planar back mirror and a miniature concave top mirror with radius of curvature similar to 100 mu m. Optical access is achieved by free beam coupling, allowing good mode-matching to the cavity mode. The cavity has a high Q-factor, reasonably small mode volume, open access, spatial and spectral tunability, and operates at cryogenic temperatures. Spectral and spatial tuning of the Purcell effect (weak coupling regime) on a single InGaAs quantum dot is demonstrated. (C) 2011 American Institute of Physics. [doi:10.1063/1.3632057

Exciton-photon hybridisation in ZnSe based microcavities

This thesis presents the design, fabrication and experimental analysis of ZnSe based microcavities. Semiconductor microcavities are micro-structures in which the exciton ground state of a semiconductor is coupled to a photonic mode of an optical cavity. The strong light matter coupling mixes the character of excitons and photons, giving rise to the lower and upper cavity polaritons, quasiparticles with an unusual dispersion due to the extreme mass contrast between the composite exciton and photon. In particular, the dispersion of the lower polariton forms a dip around the lowest energy state with zero in-plane momentum. In this dip, which can be seen as a trap in momentum space, the polaritons are efficiently isolated from dephasing mechanisms involving phonons. The features of these quasiparticles promise a variety of applications, for instance lasing without inversion and micro-optical parametric amplifiers, and an environment to study fundamental physics, such as Bose-Einstein condensation in the solid state. By overcoming the longstanding fabrication problems of ZnSe-based microcavities, the enlarged exciton binding energy in combination with the use of highly reflective dielectric mirrors makes this material system ideally suited to the realisation of polariton-based devices operating at room temperature. An epitaxial liftoff technology is developed that relies on the high etch selectively between the ZnSe heterostructure and a novel II-VI release layer, MgS. Three hybrid microcavities are fabricated with the liftoff technique and spectroscopically characterised. Angle resolved transmission experiments reveal strong hybridization of the ZnSe/Zn0:9Cd0:1Se quantum well excitons and cavity photons in a fixed microcavity. A completely length tunable microcavity is presented and shown to exhibit similar dispersion as for the fixed microcavity, with the addition of evidencing the cavity polariton bottleneck effect. The nonlinear optical features are discussed. Photoluminescence data is presented that evidences the first observation of the build up of cavity polaritons at the edge of the momentum space trap in the lower polariton branch, the bottleneck effect, in a ZnSe based microcavity. Finally, lasing at room temperature in the blue spectral region is presented for a metal/dielectric hybrid microcavity.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Shrinkage mechanisms of grain boundary loops in two-dimensional colloidal crystals

We discuss the various mechanisms involved in the spontaneous shrinkage of circular grain boundaries in two-dimensional colloidal crystals. We provide experimental evidence that these grain boundary loops shrink owing to three intermittent mechanisms proposed for atomic materials, namely purely curvature-driven migration, coupled grain boundary migration, and grain boundary sliding. Throughout shrinkage, the product of the radius and misorientation of the grain boundary loop remains higher than a fundamental limit resulting from the specific dislocation structure of grain boundary loops, except for the very last stage where the loop character is lost. Despite its complexity, this process can be effectively described by a single kinetic coefficient, allowing for a simplified description of grain boundary loop kinetics

Determination of the etching mechanism in MgS and ZnMgSSe epitaxial lift-off layers

During epitaxial lift-off of II-VI semiconductors a sacrificial layer of MgS is dissolved by acid. Here we show that the etching speed of this process-varies inversely as the square root of the layer thickness, following a model developed previously for III-V lift-off where the rate limiting step in both cases is transport of insoluble product gases from the etching layer. We also propose a model to explain why sacrificial layer etching fails when strong cohesive forces resist the lifting of the epilayer. This occurs when the sacrificial layer is too thin or when it contains more than a critical amount of an insoluble component, cohension arising from dispersion forces or chains of insoluble atoms, respectively