David Amans

Dynamical study of bubble expansion following laser ablation in liquids

This work examines the initial growth and collapse stages of bubbles induced by laser ablation in liquids. First, the bubble shape and size are tracked using an ultrafast camera in a shadowgraph imaging setup. The use of an ultrafast camera ensures a high control of the reproducibility, because a thorough measurement of each bubble lifetime is performed. Next, an analytical cavitation-based model is developed to assess the thermodynamic bubble properties. This study demonstrates that the bubble evolution is adiabatic and driven by inertial forces. Surprisingly, it is found that the bubbles consist of significantly more solvent molecules than ablated matter. These results are valuable to the field of nanoparticle synthesis as they provide insight into the mechanics of laser ablation in liquids.

Scalar and vector modulation instabilities induced by vacuum fluctuations in fibers: Numerical study

We study scalar and vector modulation instabilities induced by the vacuum fluctuations in birefringent optical fibers. To this end, stochastic coupled nonlinear Schroedinger equations are derived. The stochastic model is equivalent to the quantum field operators equations and allows for dispersion, nonlinearity, and arbitrary level of birefringence. Numerical integration of the stochastic equations is compared to analytical formulas in the case of scalar modulation instability and nondepleted pump approximation. The effect of classical noise and its competition with vacuum fluctuations for inducing modulation instability is also addressed.

Vector modulation instability induced by vacuum fluctuations in highly birefringent fibers in the anomalous-dispersion regime

We report a detailed experimental study of vector modulation instability in highly birefringent optical fibers in the anomalous-dispersion regime. We prove that the observed instability is mainly induced by vacuum fluctuations. The detuning of the spectral peaks agrees with linear perturbation analysis. The exact shape of the spectrum is well reproduced by numerical integration of stochastic nonlinear Schrödinger equations describing quantum propagation.

Provably secure experimental quantum bit-string generation

Coin tossing is a cryptographic task in which two parties who do not trust each other aim to generate a common random bit. Using classical communication this is impossible, but nontrivial coin tossing is possible using quantum communication. Here we consider the case when the parties do not want to toss a single coin, but many. This is called bit-string generation. We report the experimental generation of strings of coins which are provably more random than achievable using classical communication. The experiment is based on the plug and play scheme developed for quantum cryptography, and therefore well suited for long distance quantum communication.

Atomistic Mechanisms for the Nucleation of Aluminum Oxide Nanoparticles

A predictive model for nanoparticle nucleation has not yet been successfully achieved. Classical nucleation theory fails because the atomistic nature of the seed has to be considered. Indeed, geometrical structure as well as stoichiometry do not always match the bulk values. We present a fully microscopic approach based on a first-principle study of aluminum oxide clusters. We calculated stable structures of AlxOy and their associated thermodynamic properties. From these data, the chemical composition of a gas composed of aluminum and oxygen atoms can be calculated as a function of temperature, pressure, and aluminum to oxygen ratio. We demonstrate the accuracy of this approach in reproducing experimental results obtained with time-resolved spectroscopy of a laser-induced plasma from an Al2O3 target. We thus extended the calculation to lower temperatures, i.e., longer time scales, to propose a scenario of composition gas evolution leading to the first alumina seeds.

Autocorrelation Analysis for the Unbiased Determination of Power-Law Exponents in Single-Quantum-Dot Blinking

We present an unbiased and robust analysis method for power-law blinking statistics in the photoluminescence of single nanoemitters, allowing us to extract both the bright- and dark-state power-law exponents from the emitters intensity autocorrelation functions. As opposed to the widely used threshold method, our technique therefore does not require discriminating the emission levels of bright and dark states in the experimental intensity timetraces. We rely on the simultaneous recording of 450 emission timetraces of single CdSe/CdS core/shell quantum dots at a frame rate of 250 Hz with single photon sensitivity. Under these conditions, our approach can determine ON and OFF power-law exponents with a precision of 3% from a comparison to numerical simulations, even for shot-noise-dominated emission signals with an average intensity below 1 photon per frame and per quantum dot. These capabilities pave the way for the unbiased, threshold-free determination of blinking power-law exponents at the microsecond time scale.

Time-Resolved VUV Excited Luminescence of

Luminescence emission and excitation spectra as well as decay kinetics of Y2O3-Yb nanoparticles elaborated by the polyol mediated synthesis method have been measured at liquid helium temperature under VUV excitation. In nanoparticles the ratio of intensities of Charge Transfer Luminescence (CTL) and exciton excitation peaks differs from that for Y2O3-Yb single crystal, and changes with the particle size variation from 13 to 52 nm. This effect can be connected with the variation of excitation light scattering with particle dimensions. Two types of luminescence decay acceleration are observed. Acceleration of exponential decay time with decrease of nanoparticle size and the appearance of very fast decay component in small nanoparticles that we connected to energy transfer to the surface defects.

\hbox {Y}_{2}\hbox {O}_{3}\hbox {--} \hbox {Yb}

We report on a detailed analytical, numerical and experimental investigation of the modulation instabilities that arise in birefringent silica fibres in the anomalous dispersion regime.

Nanoparticles

Vacuum-fluctuations influence on pulses propagating through birefringent Kerr media is investigated using stochastic nonlinear Schrodinger equations. Computation tools are presented, and numerical results for vector modulation instabilities in the anomalous dispersion regime compared to experiments.