III-nitride based microcavities

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.

Polariton polarization-sensitive phenomena in planar semiconductor microcavities

Timothy Chi Hin Liew

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.

2010

High-temperature ultrafast polariton parametric amplification in semiconductor microcavities

Cristiano Ciuti, Benoît Marie Joseph Deveaud, Stefan Kundermann, Michele Saba

Cavity polaritons, the elementary optical excitations of semiconductor microcavities, may be understood as a superposition of excitons and cavity photons(1). Owing to their composite nature, these bosonic particles have a distinct optical response, at the same time very fast and highly nonlinear. Very efficient light amplification due to polariton-polariton parametric scattering has recently been reported in semiconductor microcavities at liquid-helium temperatures(2-11). Here we demonstrate polariton parametric amplification up to 120 K in GaAlAs-based microcavities and up to 220 K in CdTe-based microcavities. We show that the cut-off temperature for the amplification is ultimately determined by the binding energy of the exciton. A 5-mum-thick planar microcavity can amplify a weak light pulse more than 5,000 times. The effective gain coefficient of an equivalent homogeneous medium would be 10(7) cm(-1). The subpicosecond duration and high efficiency of the amplification could be exploited for high-repetition all-optical microscopic switches and amplifiers. 10(5) polaritons occupy the same quantum state during the amplification, realizing a dynamical condensate of strongly interacting bosons which can be studied at high temperature.

2001

Towards a room-temperature polariton amplifier

Cristiano Ciuti, Benoît Marie Joseph Deveaud, Stefan Kundermann, Michele Saba

Microcavity exciton polaritons, the fundamental optical excitations of semiconductor microcavities with quantum wells inside, have been proposed as promising candidates for observing stimulated scattering, condensation and other phenomena related to the bosonic nature of excitons. Having a light mass, quantum degeneracy of polaritons can be reached at low densities and high temperatures. But the radiative time of polaritons is very short (in the picosecond range) and usually prevents an efficient thermalization and cooling of the excited cloud of polaritons. A 'coherently driven condensate', not corresponding to a thermal equilibrium, but featuring multiple occupation of single-particle states, can however be created by an external laser source resonantly exciting polaritons. Under this condition, stimulated parametric scattering of polaritons can provide huge optical gain on a weak probe pulse shined on the sample. In this work we demonstrate that this phenomenon can survive at temperatures close to room temperature and could be achieved in the future even above this limit. Clever sample designs favour the thermal robustness of polariton parametric amplification, but from the experimental data it turns out that the parameter that ultimately limits the highest temperature for polariton parametric scattering is the exciton binding energy.

2003