Ultra-low threshold polariton lasing at room temperature in a GaN membrane microcavity with a zero-dimensional trap.

Polariton lasers are coherent light sources based on the condensation of exciton-polaritons in semiconductor microcavities, which occurs either in the kinetic or thermodynamic (Bose-Einstein) regime. Besides their fundamental interest, polariton lasers have the potential of extremely low operating thresholds. Here, we demonstrate ultra-low threshold polariton lasing at room temperature, using an all-dielectric, GaN membrane-based microcavity, with a spontaneously-formed zero-dimensional trap. The microcavity is fabricated using an innovative method, which involves photo-electrochemical etching of an InGaN sacrificial layer and allows for the incorporation of optimally-grown GaN active quantum wells inside a cavity with atomically-smooth surfaces. The resulting structure presents near-theoretical Q-factors and pronounced strong-coupling effects, with a record-high Rabi splitting of 64 meV at room-temperature. Polariton lasing is observed at threshold carrier densities 2.5 orders of magnitude lower than the exciton saturation density. Above threshold, angle-resolved emission spectra reveal an ordered pattern in k-space, attributed to polariton condensation at discrete levels of a single confinement site. This confinement mechanism along with the high material and optical quality of the microcavity, accounts for the enhanced performance of our polariton laser, and pave the way for further developments in the area of robust room temperature polaritonic devices.

Predictive value of biochemical marker ADAM-12 at first trimester of pregnancy for hypertension and intrauterine growth restriction.

PURPOSE: Delineate whereas ADAM-12 levels at first trimester of pregnancy may be used as a marker for hypertension-preeclampsia (PE) and intrauterine growth restriction (IUGR). MATERIALS AND METHODS: The present is a case control study. Serum ADAM-12 of women presenting for routine assessment of risk for chromosomal abnormalities at 11+0 to 13+6 weeks of gestation was measured. The study group comprised of 98 pregnancies that subsequently developed pregnancy-induced hypertension (PIH) or PE or small for gestational age fetuses (SGA), and were compared to 100 uncomplicated pregnancies. RESULTS: There was no statistically significant difference of mean log multiple of the expected median (MoM) of ADAM12 between control group and the group that consisted of all women with complicated pregnancy (PE, PIH, and SGA). ADAM-12 levels in women who developed PE during pregnancy were significantly lower than in womien of control group (mean log MoM: 0.109 vs 0.008, p = 0.010). Similarly, ADAM-12 levels in women who developed PE and/or PIH were significantly lower than in women of control group (mean log MoM: 0.066 vs 0.008, p = 0.015). There was no significant difference of ADAM12 levels between controls and pregnancies with SGA fetuses. CONCLUSION: Maternal serum levels of ADAM-12 at the first trimester are significantly lower in women who later develop PE when compared with women with uncomplicated pregnancies.

Strained GaAs/InGaAs Core-Shell Nanowires for Photovoltaic Applications.

We report on the successful growth of strained core-shell GaAs/InGaAs nanowires on Si (111) substrates by molecular beam epitaxy. The as-grown nanowires have a density in the order of 10(8) cm(-2), length between 3 and 3.5 µm, and diameter between 60 and 160 nm, depending on the shell growth duration. By applying a range of characterization techniques, we conclude that the In incorporation in the nanowires is on average significantly smaller than what is nominally expected based on two-dimensional growth calibrations and exhibits a gradient along the nanowire axis. On the other hand, the observation of sharp dot-like emission features in the micro-photoluminescence spectra of single nanowires in the 900-1000-nm spectral range highlights the co-existence of In-rich enclosures with In content locally exceeding 30 %.

Extraction of absorption coefficients from as-grown GaN nanowires on opaque substrates using all-optical method.

We demonstrate a new all-optical method to measure absorption coefficients in any family of as-grown nanowires, provided they are grown on a substrate having considerable difference in permittivity with the nanowire-air matrix. In the case of high crystal quality, strain-free GaN nanowires, grown on Si (111) substrates, the extracted absorption coefficients do not exhibit any enhancement compared to bulk GaN values, unlike relevant claims in the literature. This could be attributed to the relatively small diameters, short heights, and high densities of our nanowire arrays.

A GaAs polariton light-emitting diode operating near room temperature.

The increasing ability to control light-matter interactions at the nanometre scale has improved the performance of semiconductor lasers in the past decade. The ultimate optimization is realized in semiconductor microcavities, in which strong coupling between quantum-well excitons and cavity photons gives rise to hybrid half-light/half-matter polariton quasiparticles. The unique properties of polaritons-such as stimulated scattering, parametric amplification, lasing, condensation and superfluidity-are believed to provide the basis for a new generation of polariton emitters and semiconductor lasers. Until now, polariton lasing and nonlinearities have only been demonstrated in optical experiments, which have shown the potential to reduce lasing thresholds by two orders of magnitude compared to conventional semiconductor lasers. Here we report an experimental realization of an electrically pumped semiconductor polariton light-emitting device, which emits directly from polariton states at a temperature of 235 K. Polariton electroluminescence data reveal characteristic anticrossing between exciton and cavity modes, a clear signature of the strong coupling regime. These findings represent a substantial step towards the realization of ultra-efficient polaritonic devices with unprecedented characteristics.

All-optical spatial light modulator with megahertz modulation rates.

We demonstrate a novel all-optical spatial light modulator capable of megahertz modulation rates. It is based on the quantum-confined Stark effect, but the modulating electric field is entirely photogenerated by strongly asymmetric photocarrier transfer in GaAs/AlAs layers. In a nonoptimized sample, cw optical excitation of approximately 50 W/cm(2) created a 30-kV/cm electric field, inducing a 9-meV exciton red shift at room temperature. Under pulsed excitation the photogenerated electric field can be switched on in a few tens of picoseconds and relaxes in a few hundred nanoseconds, permitting megahertz modulation rates.