Modification of refractive index by a single femtosecond pulse confined inside a bulk of a photorefractive crystal

We demonstrate that the interaction of intense femtosecond pulse with photorefractive crystal at conditions close to the optical-breakdown threshold differs drastically from that of long pulse and cw illumination. Our theoretical estimations show that the high number density of excited electrons modifies the dielectric function leading to the transient negative change in the refractive index, Δn/ n0 ∼- 10-2 that vanishes on nanosecond time scale. Moreover, the high-frequency laser field, two orders of magnitude larger than the field of spontaneous polarization, prevents the stationary charge distribution during the pulse. The diffusion and recombination of charge carriers continues over a nanosecond time scale, after the end of the pulse. The main driving force for the current after the pulse is the field of spontaneous polarization in the ferroelectric medium: the current terminates when the field of charge separation balances this field. We show here that the stationary modification of refractive index according to this model is then independent of the polarization of the pump light beam, in agreement with experiments, and saturates at Δn 10-3 in semiquantitative fit to the experimental data

Evidence of superdense aluminium synthesized by ultrafast microexplosion

At extreme pressures and temperatures, such as those inside planets and stars, common materials form new dense phases with compacted atomic arrangements and unusual physical properties. The synthesis and study of new phases of matter at pressures above 100 GPa and temperatures above 10 4 Kg-warm dense matterg-may reveal the functional details of planet and star interiors, and may lead to materials with extraordinary properties. Many phases have been predicted theoretically that may be realized once appropriate formation conditions are found. Here we report the synthesis of a superdense stable phase of body-centred-cubic aluminium, predicted by first-principles theories to exist at pressures above 380 GPa. The superdense Al phase was synthesized in the non-equilibrium conditions of an ultrafast laser-induced microexplosion confined inside sapphire (α-Al 2O 3). Confined microexplosions offer a strategy to create and recover high-density polymorphs, and a simple method for tabletop study of warm dense matter

Stimulated secondary emission from semiconductor microcavities

We find strong influence of final-state stimulation on the time-resolved light emission dynamics from semiconductor microcavities after pulsed excitation allowing angle-resonant polariton-polariton scattering on the lower-polariton branch. The polariton dynamics can be controlled by injection of final-state polaritons at densities below a polariton saturation density of 5×108cm−2. A bosonic enhancement factor in the dynamics of up to 700 is evaluated

Coherent dynamics of interwell excitons in GaAs/AlxGa1-xAs superlattices

Coherent exciton dynamics in a GaAs/AlxGa1−xAs narrow-miniband superlattice is studied by spectrally resolved transient four-wave mixing. Coherent optical properties of the investigated structure are found to be strongly affected by the existence of two different heavy-hole excitonic states. One of them, the 1s heavy-hole exciton, is almost identical to the same state in noninteracting quantum wells, while the other, the heavy-hole interwell exciton, is composed of an electron and a heavy hole in adjacent wells. The interwell exciton leads to a resonant enhancement in the four-wave mixing spectra and exhibits quantum beats with the 1s heavy-hole exciton. The dephasing of the interwell exciton is one order of magnitude faster than that of the heavy-hole exciton and is mostly due to intensity-independent scattering mechanisms

Spatio-temporal dynamics of quantum-well excitons

We investigate the lateral transport of excitons in ZnSe quantum wells by using time-resolved micro-photoluminescence enhanced by the introduction of a solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps, respectively. Strong deviation from classical diffusion is observed up to 400 ps. This feature is attributed to the hot-exciton effects, consistent with previous experiments under cw excitation. The coupled transport-relaxation process of hot excitons is modelled by Monte Carlo simulation. We prove that two basic assumptions typically accepted in photoluminescence investigations on excitonic transport, namely (i) the classical diffusion model as well as (ii) the equivalence between the temporal and spatial evolution of the exciton population and of the measured photoluminescence, are not valid for low-temperature experiments.Comment: 8 pages, 6 figure

Anodization of nanoporous alumina on impurity-induced hemisphere curved surface of aluminum at room temperature

Nanoporous alumina which was produced by a conventional direct current anodization [DCA] process at low temperatures has received much attention in various applications such as nanomaterial synthesis, sensors, and photonics. In this article, we employed a newly developed hybrid pulse anodization [HPA] method to fabricate the nanoporous alumina on a flat and curved surface of an aluminum [Al] foil at room temperature [RT]. We fabricate the nanopores to grow on a hemisphere curved surface and characterize their behavior along the normal vectors of the hemisphere curve. In a conventional DCA approach, the structures of branched nanopores were grown on a photolithography-and-etched low-curvature curved surface with large interpore distances. However, a high-curvature hemisphere curved surface can be obtained by the HPA technique. Such a curved surface by HPA is intrinsically induced by the high-resistivity impurities in the aluminum foil and leads to branching and bending of nanopore growth via the electric field mechanism rather than the interpore distance in conventional approaches. It is noted that by the HPA technique, the Joule heat during the RT process has been significantly suppressed globally on the material, and nanopores have been grown along the normal vectors of a hemisphere curve. The curvature is much larger than that in other literatures due to different fabrication methods. In theory, the number of nanopores on the hemisphere surface is two times of the conventional flat plane, which is potentially useful for photocatalyst or other applications

Binding of biexcitons in GaAs/AlxGa1-xAs superlattices

Binding of the heavy-hole excitons and biexcitons in GaAs/Al0.3Ga0.7As superlattices is studied using linear and nonlinear optical techniques. High biexciton binding energies characteristic of quasi two-dimensional biexcitons are observed in superlattices with considerable miniband dispersion