Polariton polarization-sensitive phenomena in planar semiconductor microcavities

From a theoretical point of view, we discuss a variety of phenomena linked to the spin and polarization degree of freedom of exciton-polaritons in semiconductor microcavities. We start with linear optical effects including the optical spin Hall effect, formation of polarization vortices and ballistic propagation of polarized exciton-polaritons. Next, the interplay between spin-dependent dynamics and Bose condensation in the 2D system of microcavity polaritons is addressed. Theoretically, this many-body system of interacting particles is described by the spinor Gross-Pitaevskii equations. These equations provide a description of the time evolution of polarized polariton fields under different conditions of optical excitation as well as an understanding of the phenomena of superfluidity, multistability and hysteresis via renormalization of the dispersion of elementary excitations. The comprehension of polarization-sensitive dynamics can be made through the introduction of several effective fields of different nature acting on the polariton pseudospin. The theory of parametric scattering of exciton-polaritons is presented, using the second quantization formalism. It is found that the combination of nonlinearity and various mechanisms of spin reorientation leads to self-organization and the formation of polarized patterns such as polarization crosses, vortices and rings. The manipulation of polariton spins can lead to various applications in signal processing, including the construction of optical logic gates and spin memory elements; the creation of spin currents; and the control of polarized signal propagation in the microcavity plane. The concept of polariton neurons is discussed in this connection.

Physics of Polariton Condensates in GaN-based Planar Microcavities

Jacques Levrat

Since its prediction in 1996 by Imamoğlu and coworkers, the use of a non-equilibrium polariton condensate to produce an intense coherent light source referred to as a polariton laser has attracted a lot of interest in the whole physics community as it should allow the realization of ultralow threshold coherent light-emitting devices due to the release of the Bernard-Duraffourg condition. Excitons-polaritons, admixed particles resulting from the strong coupling between a cavity photon and an exciton, are the eigenmodes of a strongly coupled microcavity and exhibit a very light effective mass at the center of the Brillouin zone (105 times lighter that a free electron) inherited from the cavity photon. In the present work, we are interested in III-nitride based microcavities embedding GaN quantum wells in the active region. Thanks to the stability of the excitons at room temperature in this system and a large oscillator strength, polariton condensation has been observed up to 340K under optical excitation, paving the way toward the realization of the first electrically injected polariton laser. The goal of the present study is to provide a detailed analysis of the system properties accounting for nitride specificities, to describe the mechanisms leading to the formation of polariton condensates and to give the key elements for the optimization of devices relying on polariton nonlinearities. For this purpose, a Fourier-imaging setup allowing for the simultaneous monitoring of real space and far-field energy dispersions was carefully designed to operate in the UV spectral range in order to probe the sample emission at various temperatures. The first main result of this thesis is the establishment of the complete polariton phase diagram of our multiple quantum well-based GaN microcavity, which provides a comprehensive tool to favor or inhibit the condensation threshold by adjusting the microcavity parameters. The condensation is shown to be governed either by the kinetics or by the thermodynamics depending on the strength of the interactions. As polaritons are half-light, half-matter particles, the mechanisms leading to the nonlinear threshold are totally different from those of a conventional semiconductor laser. In particular, the possibility to tune the interactions in the system by changing the photonic fraction of the polaritons or the lattice temperature allows discriminating between different relaxation regimes. Then the spin of the polariton condensate is discussed. It is shown that the dimensionality of the system plays a major role in the polarization state of the emitted light. In particular above threshold, for a bulk microcavity, the polarization is randomly oriented whereas for a GaN multiple quantum well based microcavity, the polarization is pinned by the system anisotropy originating from the static disorder. With increasing pumping power, a depinning of the polarization is observed resulting in a progressive decrease in the polarization degree of the emitted light. These two results are well accounted for by a stochastic model of the condensate formation taking into account the in-plane anisotropy caused by the stationary photonic disorder, the self-induced Larmor precession of the condensate pseudospin and the interplay between energy and polarization relaxation rates. In the last part of this work, the case of nonpolar m-plane GaN based microcavities is addressed. In these structures, the optical axis lies in the plane of the cavity leading to a twofold anisotropy: the birefringence is responsible for the anisotropy of the cavity mode and the distribution of the exciton oscillator strength causes different coupling constants between light and matter along the two orthogonal directions. In such structures different selection rules and optical constants for light polarization perpendicular and parallel to the optical axis can lead to the coexistence of weak and strong coupling regimes with a transition to nonlinear emissions.

EPFL2012

III-nitride based microcavities

Gabriel Christmann

This PhD thesis work describes the study of III-nitride based planar microcavities operating in the strong coupling regime at room temperature. The aim is to use the nonlinear emission properties of such samples in realistic devices. Furthermore these studies allow investigating macroscopic quantum phenomena at room temperature. Strongly coupled semiconductor microcavities are characterized by the non-perturbative coupling between the photon associated to the cavity mode and the semiconductor first excited state, namely the exciton. The quasiparticle resulting from this interaction is called cavity polariton and exhibits a peculiar in plane wave vector (k‖) dispersion. This results from the strong effective mass difference between the cavity photon and the exciton. Consequently, polaritons are subject to trapping in reciprocal space around k‖ = 0. Polaritons are thus characterized by a light effective mass in this trap. Theoretically, this point is favorable for the observation of macroscopic coherent polariton phases at low densities and high temperatures, in particular at room temperature. So far such phenomena have been mainly studied in III-arsenides and II-VI semiconductors. The main outcome is the recent demonstration of polariton Bose-Einstein condensation at cryogenic temperatures. This last point is the main limitation of III-arsenide semiconductors: the strong coupling regime is hardly maintained at room temperatures. This prevents the observation of such macroscopic coherent phases of polaritons in such conditions. In this context, the use of these materials for devices based on such physical phenomena appears strongly compromised. Indeed a potential use would be the realization of low threshold compact coherent light sources, and for such applications, an operation at cryogenic temperatures is not realistic. In this context, the use of III-nitride semiconductors is a very promising approach. Indeed, thanks to better intrinsic material parameters, the strong coupling regime is kept at room temperature. It has thus been observed in microcavities whose quality is much poorer than their III-arsenide or II-VI counterparts. Using high quality planar microcavities is however a prerequisite to observe these nonlinear effects. The aim of this work is to study such samples that can now be obtained thanks to the use of Bragg mirrors made of AlInN layers lattice matched to GaN. The strong coupling regime has been demonstrated at room temperature on several samples grown using this approach through angle resolved photoluminescence and reflectivity measurements. Furthermore, a strong nonlinear emission is observed at room temperature under nonresonant optical excitation for two different samples. This emission exhibits a strong spectral narrowing indicating the onset of an increased temporal coherence. These results are fully consistent with a room temperature polariton lasing picture. With the recent demonstration of the electrical injection of polaritons in III-arsenides, this latter result indicates that the realization of a coherent light source based on cavity polaritons and operating at room temperature is now realistic. In addition to this first major result, numerous properties of this nonlinear emission have been investigated. Thus, some preliminary evidences indicating that the transition is driven by thermodynamics have been obtained. Then some intriguing properties related to the macroscopic polarization buildup have been observed. One of the samples exhibiting this nonlinear emission is indeed characterized by the spontaneous buildup of a random polarization vector for each experimental realization. This result is interpreted in terms of spontaneous symmetry breaking at the transition, which is a fundamental property of phase transition theory. These latter results pave the way to the observation of room temperature Bose-Einstein condensation in the solid state.

EPFL2009

Coherence properties of microcavity polaritons

Stefan Kundermann

This thesis presents the coherence properties of polaritons in semiconductor microcavities. Semiconductor microcavities are microstructures in which the exciton ground state of a semiconductor quantum well is coupled to a photonic mode of a microresonator. The strong coupling mixes the character of excitons and photons, giving rise to the lower and upper polariton branches, quasiparticles with an unusual energetic dispersion relation due to the extreme mass difference between exciton and photon. Particularly special is the dispersion of the lower polariton, which forms a dip in the 2-dimensional k-space 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. Polaritons can be resonantly excited at desired points on the polariton dispersion by shining on the microcavity laser light at the appropriate angle and wavelength. Polaritons can interact and scatter pairwise with each other conserving energy and in plane momentum k, a process similar to parametric scattering of photons in a nonlinear crystal. One polariton from a pump reservoir scatters down to the signal state at k = 0 (corresponding to normal incidence) and a second takes away the excess energy and momentum of the first and scatters up to the idler position ({kP, kP} → {0, 2kP }). This process can be stimulated by a small amount of signal polaritons injected with a probe laser beam at normal incidence. Here the coherence properties of the polariton parametric scattering have been investigated using spectroscopy techniques sensitive to the optical phase, for example coherent control with phase-locked femtosecond probe pulses. Just above the threshold for the stimulated parametric scattering, the parametric amplification process is given by the linear superposition of the individual amplification processes of each probe pulse. The emission of signal, pump, and idler can be controlled by tuning the relative phase of the 150fs-long probe pulses, which are separated by a few picoseconds in time. Experiments are presented that deal with the real-time dynamics of the parametric scattering in the spontaneous and the stimulated regime. It is shown, that in the spontaneous regime the scattering is started by a small amount of polaritons which have relaxed to the band bottom by emitting phonons. In the regime where polariton scattering is stimulated by an external probe, the rise of the signal intensity is delayed with respect to the arrival time of both pump and probe, a feature that can be attributed to the complex phase-matching mechanism for the parametric scattering. In the second part of the thesis, the spontaneous build up of a macroscopic coherence in a CdTe microcavity under non-resonant laser excitation is analysed. The build up of a long-range spatial coherence easily exceeding the thermal wavelength of the polaritons is shown. This is the hallmark of Bose-Einstein condensation and the proof of a macroscopic wavefunction. Experimental data on the statistical distribution of the polaritons in time, the polarisation of the non-linear emission, and the quantum transition from a thermal to a coherent state1 confirm that Bose-Einstein condensation of microcavity polaritons has been observed. We regard these observations as the first bullet-proof evidence for spontaneous Bose-Einstein condensation in a solid state system, a phenomenon that has been the subject to many investigations and controversies during the past four decades. ------------------------------ 1 The data about the statistical distribution, the polarisation, and the transition from a thermal to a coherent state is by courtesy of Jacek Kasprzak of the University of Grenoble.

EPFL2006

Coherence properties of microcavity polaritons

Stefan Kundermann

EPFL2006

Physics of Polariton Condensates in GaN-based Planar Microcavities

Jacques Levrat

EPFL2012

III-nitride based microcavities

Gabriel Christmann

EPFL2009

Polariton polarization-sensitive phenomena in planar semiconductor microcavities

Physics of Polariton Condensates in GaN-based Planar Microcavities

III-nitride based microcavities

Coherence properties of microcavity polaritons

L'onde de polarisation est

Coherence properties of microcavity polaritons

Physics of Polariton Condensates in GaN-based Planar Microcavities

III-nitride based microcavities