Optical gain observation on silicon nanocrystals embedded in silicon nitride under femtosecond pumping

We report the observation of positive optical gain in silicon nanocrystals (Si-nc) embedded in silicon nitride measured by the variable stripe length technique. We evidence the onset of stimulated emission and report gain coefficients up to 52 cm(-1) at the highest excitation power (6.5 W/cm(2)). Photoluminescence dynamics presents two distinct recombination lifetimes in the nanosecond and the microsecond ranges. This was interpreted in terms of fast carrier trapping in nitrogen-induced localized states in the Si-nc surface and subsequent slow radiative recombination, suggesting that carrier trapping in radiative surface states plays a crucial role in the optical gain mechanism of Si-nc. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3607276

Madurese Seafarers. Prahus, Timber and Illegality on the Margins of the Indonesian State, Kurt Stenross

Dans l’archipel indonésien, l’une des singularités de Madura est d’être une île aride et déshéritée dans un arc d’îles prospères. Les Madourais, quant à eux, constituent le troisième groupe ethnique du pays, estimé en l’absence de statistiques par ethnie à environ 13 millions d’individus dont seulement 2,5 millions sur l’île. Cette île, si proche et pourtant si différente de Java, hormis quelques études linguistiques (Kiliaan, Hendricks conduites vers 1830) a longtemps été ignorée par la comm..

Broadband UV-Vis vibrational coherence spectrometer based on a hollow fiber compressor

doi: 10.1063/1.4962699</p

Experimental investigation of excitonic spin relaxation dynamics in GaN

By performing nondegenerate pump- probe experiments, we study the relaxation dynamics of spin- polarized A and B excitons in wurtzite epitaxial GaN. By analyzing the differential reflectivity spectra (Delta R/R) of the two circularly polarized probe components, we are able to identify each spin relaxation channel ( electron, light- and heavy- hole spin flips ) separately and to extract characteristic times of the different spin relaxation processes. In addition, spectral oscillatory features are observed for negative delays. They show a rapid rise determined by the fast dephasing time T-2 of the excitonic transitions. We show that the high density of dislocations increases the spin relaxation of electrons and holes through the Elliot - Yafet mechanism and makes the exciton dephasing time very short. The measured heavy- hole relaxation time, which is not extremely short compared to the electron relaxation time, can be related to the band structure, in which the degeneracy between different spin valence bands is partially lifted

Time-Resolved Down-Conversion of 2-Aminopurine in a DNA Hairpin

Femtosecond fluorescence anisotropy decay measured by type II difference frequency generation provides new insight into the local structural dynamics of ΔP(-)PBS fragments of the HIV- 1 DNA primary binding sequence, labeled with 2-aminopurine

Ultrafast Site-Specific Fluorescence Quenching of 2-Aminopurine in a DNA Hairpin Studied by Femtosecond Down-Conversion

The Delta P(-)PBS analog of the DNA primary binding sequence (PBS) of the HIV-1 genome labeled at different positions by 2-aminopurine (2-AP) is investigated by a novel femtosecond fluorescence down-conversion experiment with 0.3-ps time resolution. The high signal-to-noise ratio of the fluorescence kinetics makes it possible to reveal four distinct decay times ranging from 0.8 ps to 2-3 ns for all the three labeling positions. This suggests the existence of at least four different quenching conformations of 2-AP with its nearest neighbors, and underscores the structural heterogeneity of the loop region of Delta P(-)PBS. Sub-5-ps components are found and attributed to stacking interactions of 2-AP with the flanking guanine (G) side chains, consistent with the NMR structure of Delta P(-)PBS. The observation of a significant increase of their total amplitude when 2-AP is positioned close to the rigid 3'-half of the G-rich stem gives further support to this assignment. Only a minor portion of conformations involves slow nanosecond collisional quenching

Four-wave mixing in quantum wells using femtosecond pulses with Laguerre-Gauss modes

International audienceWe demonstrate theoretically and experimentally that four-wave mixing processes obey phase-matching conditions that determine not only the conservation of the photon energy and k-momentum but also the orbital angular momentum of light. We report on time-resolved four-wave mixing experiments performed on a CdTe/CdZnTe quantum well in both noncollinear and collinear configurations with Laguerre-Gauss beams. They demonstrate that the polarization wave which is induced in the material keeps memory of the excitation pulse orbital momentum. We show that in the collinear configuration, the large angular acceptance opens up new horizons for improving the spatial resolution in time-resolved experiments

Excited-State Proton Transfer in Oxyluciferin and Its Analogues

Abstract: One of the most characterized bioluminescent reactions involves the firefly luciferase that catalyzes the oxidation of the luciferin producing oxyluciferin in its first excited state. While relaxing to the ground state, oxyluciferin emits visible light with an emission maximum that can vary from green to red. Oxyluciferin exists under six different chemical forms resulting from a keto/enol tautomerization and the deprotonation of the phenol or enol moieties. The optical properties of each chemical form have been recently characterized by the investigations of a variety of oxyluciferin derivatives, indicating unresolved excited-state proton transfer (ESPT) reactions. In this work, femtosecond pump-probe spectroscopy and picosecond-resolved fluorescence are used to investigate the picosecond kinetics of the ESPT reactions and demonstrate the excited state keto to enol conversion of oxyluciferin and its derivatives in aqueous buffer as a function of pH. A comprehensive photophysical scheme is provided describing the complex luminescence pathways of oxyluciferin in protic solution

Dark-level trapping, lateral confinement, and built-in electric field contributions to the carrier dynamics in c-plane GaN/AlN quantum dots emitting in the UV range

c-plane GaN/AlN quantum dots (QDs) are promising zero-dimensional quantum nanostructures that exhibit single photon emission properties up to room temperature and even above. In this context, it is of prime interest to gain a deeper insight into the recombination dynamics of photogenerated electron-hole pairs captured by such dots. Hence, in this work, we study the time-resolved photoluminescence (PL) properties in the low injection regime and at cryogenic temperatures of c-plane GaN/AlN QD ensembles emitting above the bulk GaN bandgap in order to properly understand the nature of the recombination channels behind the observed non-exponential decay time profiles. Such decays reveal the existence of a relaxation channel competing with the radiative recombination one. It is thus observed that for the former process the dynamics is independent of the dot height, which is attributed to a reversible nonradiative transfer that could be mediated by a spin-flip process to a dark-level state. The radiative recombination process is recognizable thanks to the characteristic dependence of its lifetime with the emission energy, which is well accounted for by the built-in electric field inherent to quantum nanostructures grown along the c axis and the variations in the lateral confinement at play in such QDs. Those conclusions are drawn from the analysis of the time evolution of the PL spectra by means of a simple analytical model that enables to exclude any screening of the built-in electric field