Phase-Resolved Imaging of Exciton Polaritons

In this PhD thesis, new imaging techniques have been developed in order to explore the physics of semiconductor microcavities. In these structures, composite bosons called exciton polaritons are the result of strong coupling between the cavity mode and quantum well excitons. A spectroscopic imaging technique has been developed to image the eigenstates of polaritons confined in the traps of a patterned GaAs microcavity. Polariton probability densities have been reconstructed in three dimensions – two spatial dimensions and energy – allowing to retrieve two-dimensional probability density mappings of the eigenstates. In order to image the wave functions (and not the probability densities only), a phase-resolved imaging setup has been built. Interfering the near field or far field of the polariton emission with a reference laser beam allowed to retrieve the full information (amplitude and phase) of the polariton wave functions. This tool allowed to evidence the effect of trap ellipticity on the confined polariton wave functions. Polariton vortices were also identified as a superposition of eigenmodes of the elliptical traps, and a selective excitation method has been used to optically control the sign and value of the vortex charge. Combining phase-resolved imaging with ultrafast optics allowed to probe the time evolution of coherent superpositions of confined polariton states. In particular, Rabi oscillations between vortex and anti-vortex states have been observed. Eventually, the time and phase resolved imaging tools have been used to explore the physics of quantum fluids. The scattering of polariton wave packets on a structural defect has been studied. Different flow regimes have been identified, and, in particular, quantum turbulence has been observed in the form of quantized vortices nucleating in the wake of the defect. The nucleation conditions have been established in terms of local fluid velocity and density on the obstacle perimeter. The results were successfully reproduced by numerical simulations based on generalized Gross-Pitaevskii equations

Multidimensional coherent optical spectroscopy of semiconductor nanostructures: a review

Multidimensional coherent optical spectroscopy (MDCS) is an elegant and versatile tool to measure the ultrafast nonlinear optical response of materials. Of particular interest for semiconductor nanostructures, MDCS enables the separation of homogeneous and inhomogeneous linewidths, reveals the nature of coupling between resonances, and is able to identify the signatures of many-body interactions. As an extension of transient four-wave mixing (FWM) experiments, MDCS can be implemented in various geometries, in which different strategies can be used to isolate the FWM signal and measure its phase. I review and compare different practical implementations of MDCS experiments adapted to the study of semiconductor materials. The power of MDCS is illustrated by discussing experimental results obtained on semiconductor nanostructures such as quantum dots, quantum wells, microcavities, and layered semiconductors

Selective photoexcitation of exciton-polariton vortices

We resonantly excite exciton-polariton states confined in cylindrical traps. Using a homodyne detection setup, we are able to image the phase and amplitude of the confined polariton states. We evidence the excitation of vortex states, carrying an integer angular orbital momentum m, analogous to the transverse TEM01* "donut" mode of cylindrically symmetric optical resonators. Tuning the excitation conditions allows us to select the charge of the vortex. In this way, the injection of singly charged (m = 1 & m = -1) and doubly charged (m = 2) polariton vortices is shown. This work demonstrates the potential of in-plane confinement coupled with selective excitation for the topological tailoring of polariton wavefunctions

Direct imaging of surface plasmon polariton dispersion in gold and silver thin films

We image the dispersion of surface plasmon polaritons in gold and silver thin films of 30 and 50 nm thickness, using angle-resolved white light spectroscopy in the Kretschmann geometry. Calibrated dispersion curves are obtained over a wavelength range spanning from 550 to 900 nm. We obtain good qualitative agreement with calculated dispersion curves that take into account the thickness of the thin film

Dynamics of dark-soliton formation in a polariton quantum fluid

Polariton fluids have revealed huge potentialities in order to investigate the properties of bosonic fluids at the quantum scale. Among those properties, the opportunity to create dark as well as bright solitons has been demonstrated recently. In the present experiments, we image the formation dynamics of oblique dark solitons. They nucleate in the wake of an engineered attractive potential that perturbs the polariton quantum fluid. Thanks to time and phase measurements, we assess quantitatively the formation process. The formation velocity is observed to increase with increasing distance between the flow injection point and the obstacle which modulates the density distribution of the polariton fluid. We propose an explanation in terms of the increased resistance to the flow and of the conditions for the convective instability of dark solitons. By using an iterative solution of the generalized Gross-Pitaevskii equation, we are able to reproduce qualitatively our experimental results

Phase-resolved imaging of confined exciton-polariton wave functions in elliptical traps

We study the wave functions of exciton polaritons trapped in the elliptical traps of a patterned microcavity. A homodyne detection setup with numerical off-axis filtering allows us to retrieve the amplitude and the phase of the wave functions. Doublet states are observed as the result of the ellipticity of the confinement potential and are successfully compared to even and odd solutions of Mathieu equations. We also show how superpositions of odd and even states can be used to produce "donut" and "eight-shape" states which can be interpreted as polariton vortices

Coherent oscillations between polariton vortex and anti-vortex states in an elliptical resonator

In this experimental work, we optically excite the eigenmodes of an elliptical resonator in a semiconductor micro-cavity. Using a pulsed excitation, we create a superposition of eigenmodes, and image the time evolution of the coherent emission pattern. Oscillations between vortex and anti-vortex states are observed, and remarkably well described within the Poincar sphere representation for an eigenmode containing an orbital angular momentum. A semiconductor quantum well is embedded in the microcavity structure. The system is operated in the strong light matter coupling regime, where the eigenmodes are hybrid half-photonic half-excitonic quasiparticles called exciton polaritons

The Excitation Ladder of Cavity Polaritons

Multidimensional coherent spectroscopy directly unravels multiply excited states that overlap in a linear spectrum. We report multidimensional coherent optical photocurrent spectroscopy in a semiconductor polariton diode and explore the excitation ladder of cavity polaritons. We measure doubly and triply avoided crossings for pairs and triplets of exciton-polaritons, demonstrating the strong coupling between light and dressed doublet and triplet semiconductor excitations. These results demonstrate that multiply excited excitonic states strongly coupled to a microcavity can be described as two coupled quantum-anharmonic ladders

Excitation Ladder of Cavity Polaritons

Multidimensional coherent spectroscopy directly unravels multiply excited states that overlap in a linear spectrum. We report multidimensional coherent optical photocurrent spectroscopy in a semiconductor polariton diode and explore the excitation ladder of cavity polaritons. We measure doubly and triply avoided crossings for pairs and triplets of exciton polaritons, demonstrating the strong coupling between light and dressed doublet and triplet semiconductor excitations. These results demonstrate that multiply excited excitonic states strongly coupled to a microcavity can be described as two coupled quantum-anharmonic ladders