We report on the realization of polariton quantum boxes in a semiconductor microcavity under strong coupling regime. The quantum boxes consist of mesas, etched on the top of the spacer of a microcavity, that confine the cavity photon. For mesas with sizes of the order of a few microns in width and nanometers in depth, we observe quantization of the polariton modes in several states, caused by the lateral confinement. We evidence the strong exciton-photon coupling regime through a typical anticrossing curve for each quantized level. Moreover, the growth technique permits one to obtain high-quality samples, and opens the way for the conception of new optoelectronic devices. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2172409͔Confining semiconductor structures allows the study of various fundamental effects, ranging from the Purcell effect to the full quantum confinement. Such confinement is also used for applications in many fields, from optoelectronics to quantum information. Previous works have focused on different aspects: on the one hand, on the matter part, with the confinement of the excitonic resonances in quantum wells, quantum wires, and quantum dots. On the other hand, environment for the electromagnetic field has been modified by optical confinement in different types of cavities. Additionally since the middle of the 90s, low dimensional devices have been realized in the strong coupling regime. 1 Confinement can enhance the interactions, modify the real and imaginary parts of the resonance's energy, or open access to new interaction processes. It is also often considered as a possible way to obtain a condensed phase of bosons in semiconductors, 2 but so far the fermionic nature of excitons has always become dominant upon increasing density. In this sense, polaritons are of great interest as, despite their excitonic content, they have a very small effective mass in comparison to the exciton ͑thanks to their photonic component͒, which theoretically increases their temperature of condensation ͑above 0.1 K͒. 3 The peculiar trap shape of the lower microcavity polariton dispersion curve has motivated several relaxation experiments towards the bottom of this "trap" 4,5 but no clear evidence for the formation of spontaneous coherence formation has been given yet.Zero-dimensional ͑0D͒ Polariton confinement can be achieved either through their excitonic or through their photonic component. Recently, evidence for 0D polaritons has been given with single quantum dots in micropillars, 6 photonic nanocavities, 7 or microdisks, 8 and for a large number of excitations in micropillar structures. 9-11 Here we consider a novel system under strong coupling regime, where 0D confinement is achieved through the photonic part of polaritons in high Q cavities. Our original structure contains polariton quantum boxes, constituted by mesas in the spacer layer of a semiconductor microcavity, allowing to keep the strong coupling regime. Each mesa, by acting on the two degrees of freedom of the photonic component of the two...

We demonstrate three-dimensional spatial confinement of exciton-polaritons in a semiconductor microcavity. Polaritons are confined within a micron-sized region of slightly larger cavity thickness, called mesa, through lateral trapping of their photon component. This results in a shallow potential well that allows the simultaneous existence of extended states above the barrier. Photoluminescence spectra were measured as a function of either the emission angle or the position on the sample. Striking signatures of confined states of lower and upper polaritons, together with the corresponding extended states at higher energy, were found. In particular, the confined states appear only within the mesa region, and are characterized by a discrete energy spectrum and a broad angular pattern. A theoretical model of polariton states, based on a realistic description of the confined photon modes, supports our observations.

We present a novel semiconductor structure in which 0D polaritons coexist with 2D microcavity polaritons. Spatial trapping of the 2D microcavity polaritons results from the confinement of their photonic part in a potential well, consisting of an adjustable thickness variation of the spacer layer. This original technique allows to create polaritonic boxes of any size and shape. Strong coupling regime is evidenced by the typical energy level anticrossing, in real space and in momentum space, and supported by a theoretical model.Semiconductor heterostructures allow the analysis of light-matter interaction in nanoscopic or mesoscopic systems, in which properties of both uncoupled light and matter oscillators can be strongly engineered. Beyond the motivation of the realization of optoelectronic devices, monitoring of light by matter and vice versa has already led to a wide range of new fascinating physics. Of particular interest is the strong coupling regime, where the dressed states basis becomes the relevant one to describe and understand the coupled system. Historic experiments of quantum electrodynamics have been realized using a few atoms in the strong coupling regime with a few cavity photons [1,2]. In strong analogy, three groups have recently reported the successful achievement of strong coupling regime for one single Quantum Dot (QD) with a high-quality factor (Q) cavity mode [3][4][5]. In these systems the small number of electronic excitations opens the way to the realization of solid state devices for quantum information operation.Research on solid state systems has also been motivated, during the last 50 years, by the possibility to achieve Bose-Einstein condensation (BEC) at high temperature. This effect involves a huge number of particles, that massively accumulate into a single quantum state below a critical temperature, thus displaying macroscopical quantum properties. To reach this goal, the most promising candidates are microcavity polaritons, eigenstates of semiconductor microcavities in strong coupling regime [6]. These quasiparticles have the great advantage over excitons to exhibit a very light effective mass. Their bosonic behavior at low density has been demonstrated already by final state stimulation in parametric scattering process [7], and by direct evidence of quantum degeneracy [8]. Rather high temperatures have been reached for such effects [9]. However, polariton BEC has not been demonstrated yet. A more favorable situation for BEC should be obtained by confining the polaritons within a small volume, as suggested by

PACS 63.20. Ls, 73.20.Àr, 78.55.Cr, 78.67.Lt Luminescence spectra of V-groove GaAs/AlGaAs quantum wires are investigated at different temperatures by spatially resolved photoluminescence spectroscopy using a low temperature scanning near-field optical microscope (SNOM). Raising the sample temperature, the evolution of the photoluminescence spectra evidences that excitons are not completely relaxed in the deepest minima of the disordered potential. Simulations of exciton relaxation in a disordered 1D potential show that this behaviour is related to a reduction of the phonon scattering rates for the lowest lying states, which causes a bottleneck effect for exciton relaxation. These results suggest that the observed level repulsion effect in quantum wires is due to light emission from both fundamental and excited states localized in the same potential minima. 1 Introduction A recurrent difficulty in the growth of semiconductor nanostructures, nowadays, concerns the quality of the interfaces. Quantum well (QW) and quantum wire (QWR) interfaces, although of very high quality, are rough at the atomic scale and just in few cases interfaces of exceptional quality extend over mesoscopic distances [1][2][3]. Interface roughness leads to fluctuations of the confining electric potential and thus creates localized states, which play a key role in the optical and transport properties of low-dimensional semiconductor structures [4,5].A concept tightly related to localization in a quantum system is level repulsion: states localized in the same spatial area must show an energy splitting [6]. Recently this effect has been evidenced on excitonic levels in disordered QWs [7] and in QWRs [8] by measuring near-field photoluminescence (PL) spectra. However the interpretation of the experimental results has remained controversial [9]. In general the equivalence between measured PL spectra and computed absorption spectra was assumed, neglecting the impact of exciton relaxation processes through coupling with the phonon bath. As a result, until now, a direct comparison between experiment and theory has not been possible.We present near-field measurements which show that, locally and at low temperatures, no thermal equilibrium is established within the excitonic population in a stationary state. Furthermore we show, by simulating exciton relaxation through coupling with acoustic longitudinal phonons, that in a disordered 1D system disorder reduces the coupling efficiency to phonons. This introduces a competition

Subject classification: 73.21.Hb; 78.55.Cr; 81.05.Ea; S7.12 GaAs/GaAlAs quantum wires grown by modulated flow rate metalorganic chemical vapour deposition were investigated by spatially resolved photoluminescence spectroscopy using a scanning near-field optical microscope. It was found that the wires decompose into a series of regions that emit luminescence of varying intensity. The spectra of these regions feature several narrow emission lines, which means that there is a series of more or less localised exciton states inside each region. It is expected that these exciton states are very close to each other and are correlated, which leads to level repulsion. The mean autocorrelation function taken from a series of near-field spectra clearly reveals this level repulsion, which amounts to roughly 2 meV.Introduction One of the most serious problems of the growth of semiconductor nanostructures nowadays concerns the quality of the interfaces. The interfaces of quantum wells (QWs) and quantum wires (QWRs) are in general rough, and in only few cases interfaces of exceptional quality extend over macroscopic distances [1,2]. Interface roughness strongly affects the electronic properties of nanostructures. It causes fluctuations of the confining electric potential, which in many cases can lead to localisation of the exciton (X) states. A tool for obtaining valuable information on the electronic properties of semiconductor nanostructures is the (scanning) near-field optical microscope (SNOM). It permits one to perform photoluminescence (PL) spectroscopy with a spatial resolution of the order of 100 nm [3,4]. An important fraction of the light emitted from such a small near-field (NF) spot might come from spatially and energetically correlated X states, and this could also affect the spatially resolved optical spectra. The information extracted from such spectra permits one to draw conclusions concerning the energetic correlation, the wave function overlap, and the localisation to which the X states confined in imperfect nanostructures are exposed.

V groove GaAs/AlGaAs quantum wires are investigated by spatially resolved photoluminescence spectroscopy using a low-temperature scanning near-field optical microscope. The spectra along the wires feature several sharp emission lines, which are understood as the emission from exciton states localized in inhomogeneities of the confining potential. It is expected that these exciton states spatially overlap and that their energies are correlated, which leads to level repulsion. The statistical analysis of the spectra in terms of autocorrelation functions clearly reveals this effect. In order to model photoluminescence rather than absorption spectra, we perform detailed simulations of exciton relaxation and luminescence kinetics in a disordered one-dimensional system, taking into account the realistic structure of the facets at the bottom of the V groove in our disorder model. Combining near-field measurements and numerical simulations we show that disorder prevents the full relaxation of excitons towards the local ground states in the dips of the disordered potential. As a consequence, luminescence from excited states is observed. We identify in the spatial and energy correlation between localized states the origin of the observed level repulsion and discuss the role played by the two facets at the bottom of the V groove in this mechanism. This analysis also highlights the relevance of the broad photoluminescence background observed in this and in analogous experiments. We propose as explanation for the origin of this broad background the strong exciton-acoustic phonon coupling that results in phonon sidebands in the spectrum of each localized state.

We present a novel semiconductor structure in which 0D polaritons coexist with 2D microcavity polaritons. Spatial trapping of the 2D microcavity polaritons results from the confinement of their photonic part in a potential well, consisting of an adjustable thickness variation of the spacer layer. This original technique allows to create polaritonic boxes of any size and shape. Strong coupling regime is evidenced by the typical energy level anticrossing, in real space and in momentum space, and supported by a theoretical model.Semiconductor heterostructures allow the analysis of light-matter interaction in nanoscopic or mesoscopic systems, in which properties of both uncoupled light and matter oscillators can be strongly engineered. Beyond the motivation of the realization of optoelectronic devices, monitoring of light by matter and vice versa has already led to a wide range of new fascinating physics. Of particular interest is the strong coupling regime, where the dressed states basis becomes the relevant one to describe and understand the coupled system. Historic experiments of quantum electrodynamics have been realized using a few atoms in the strong coupling regime with a few cavity photons [1,2]. In strong analogy, three groups have recently reported the successful achievement of strong coupling regime for one single Quantum Dot (QD) with a high-quality factor (Q) cavity mode [3][4][5]. In these systems the small number of electronic excitations opens the way to the realization of solid state devices for quantum information operation.Research on solid state systems has also been motivated, during the last 50 years, by the possibility to achieve Bose-Einstein condensation (BEC) at high temperature. This effect involves a huge number of particles, that massively accumulate into a single quantum state below a critical temperature, thus displaying macroscopical quantum properties. To reach this goal, the most promising candidates are microcavity polaritons, eigenstates of semiconductor microcavities in strong coupling regime [6]. These quasiparticles have the great advantage over excitons to exhibit a very light effective mass. Their bosonic behavior at low density has been demonstrated already by final state stimulation in parametric scattering process [7], and by direct evidence of quantum degeneracy [8]. Rather high temperatures have been reached for such effects [9]. However, polariton BEC has not been demonstrated yet. A more favorable situation for BEC should be obtained by confining the polaritons within a small volume, as suggested by

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