J. L. Staehli

Bose-Einstein condensation of exciton polaritons

Phase transitions to quantum condensed phases—such as Bose–Einstein condensation (BEC), superfluidity, and superconductivity—have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place. Promising candidate systems are semiconductor microcavities, in which photons are confined and strongly coupled to electronic excitations, leading to the creation of exciton polaritons. These bosonic quasi-particles are 109 times lighter than rubidium atoms, thus theoretically permitting BEC to occur at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for BEC of polaritons. Above a critical density, we observe massive occupation of the ground state developing from a polariton gas at thermal equilibrium at 19 K, an increase of temporal coherence, and the build-up of long-range spatial coherence and linear polarization, all of which indicate the spontaneous onset of a macroscopic quantum phase.

High-temperature ultrafast polariton parametric amplification in semiconductor microcavities

Cavity polaritons, the elementary optical excitations of semiconductor microcavities, may be understood as a superposition of excitons and cavity photons. 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. 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-µm-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 107 cm-1. The subpicosecond duration and high efficiency of the amplification could be exploited for high-repetition all-optical microscopic switches and amplifiers. 105 polaritons occupy the same quantum state during the amplification, realizing a dynamical condensate of strongly interacting bosons which can be studied at high temperature.

Polariton quantum boxes in semiconductor microcavities

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.

Engineering the spatial confinement of exciton-polaritons in semiconductors

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.

Growth of Gainas(P) and Gainasp/Gainas Mqw Structures by Cbe

We discuss the growth of lattice matched GaInAs, GaInAsP and GaInAsP/GaInAs multiple quantum well structures for high speed laser applications. Optical cavities, including five 90 angstrom GaInAs quantum wells with GaInAsP barriers and waveguides showed a 4 K photoluminescence line width of 8.7 meV. Transmission electron microscopy and X-ray diffraction confirm the good control over the growth of these structures.

Optical study on ultrathin InAs/InP single quantum wells

SummaryOur interest is centred on the very thin layers consisting of only one or a few monolayers of InAs. The optical transition energies, measured by photoluminescence spectroscopy, are compared with theoretical calculations obtained in envelope function approximation and through an empirical tight-binding method. This comparison yields values for the not well-known valence band offset at the InAs/InP interface, and the luminescence lines observed at different energies could be assigned to layers between one and 13 monolayers thick.

Level repulsion of localised excitons observed in near-field photoluminescence spectra

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.

Blue light-emission from a nanostructured organic polymer semiconductor

We report on the enhancement of the photoluminescence (PL) intensity from a nanostructured organic material composed of nanosized aggregates of poly(para-phenylene) (PPP) in a polystyrene insulating matrix. In addition, the peak of the PL emission spectrum from these nanostructured organic semiconductors is blue-shifted from that of unstructured PPP. Furthermore, these PL characteristics correlate with the presence of an unusual sub-gap absorption feature that is present only in the nanostructured PPP. These results suggest the departure of the optical properties of random nanostructured organic polymers from their unstructured counterparts.

Growth of GaInAs by chemical beam epitaxy

Keywords: CRYSTALLOGRAPHIC PROPERTIES ; X-RAY ; INP ; INGAAS Note: Ecole polytech fed lausanne,inst appl phys,dept phys,ch-1015 lausanne,switzerland. Carlin, jf, ecole polytech fed lausanne,inst micro & optoelectr,dept phys,ch-1015 lausanne,switzerland.ISI Document Delivery No.: EY072Times Cited: 10Cited Reference Count: 11Cited References:ASONEN H, 1990, J CRYST GROWTH, V105, P101BASS SJ, 1986, J CRYST GROWTH, V79, P378BASSIGNANA IC, 1989, J APPL PHYS, V65, P4299FERRARI C, 1988, J APPL PHYS, V63, P2628GOETZ KH, 1983, J APPL PHYS, V54, P4543HALLIWELL MAG, 1984, J CRYST GROWTH, V68, P523KAWAGUCHI Y, 1986, 18TH C SOL STAT DEV, P619MACRANDER AT, 1987, J ELECTROCHEM SOC, V134, P1248MATSUI Y, 1985, J VAC SCI TECHNOL B, V3, P528RAZEGHI M, 1989, MOCVD CHALLENGE, V1TSANG WT, 1986, APPL PHYS LETT, V49, P170 Reference LOEQ-ARTICLE-1991-012doi:10.1016/0022-0248(91)90609-9 Record created on 2007-08-31, modified on 2017-05-12

Ultra stable tuning fork sensor for low-temperature near-field spectroscopy

We report on a distance control system for low-temperature scanning near-field optical microscopy, based on quartz tuning fork as shear force sensor. By means of a particular tuning fork-optical fiber configuration, the sensor is electrically dithered by an applied alternate voltage, without any supplementary driving piezo, as done so far. The sensitivity in the approach direction is 0.2nm, and quality factors up to 2850 have been reached. No electronic components are needed close to the sensor, allowing to employ it in a liquid He environment. The system is extremely compact and allows for several hours of stability at 5 K.