{\it Ab--initio} finite temperature excitons

The coupling with the lattice vibrations is shown to drastically modify the state--of--the--art picture of the excitonic states based on a frozen atom approximation. The zero--point vibrations renormalize the bare energies and optical strengths. Excitons acquire a non--radiative lifetime that decreases with increasing temperature. The optical brightness and efficiency turn out to be strongly temperature dependent such as to induce bright to dark (and vice versa) transitions. The finite temperature experimental optical absorption spectra of bulk Si and hexagonal BN are successfully explained without using any external parameter.Comment: 4 pages, 2 figure

Ultra--fast carriers relaxation in bulk silicon following photo--excitation with a short and polarized laser pulse

A novel approach based on the merging of the out--of--equilibrium Green's function method with the ab-initio, Density--Functional--Theory is used to describe the ultra--fast carriers relaxation in Silicon. The results are compared with recent two photon photo--emission measurements. We show that the interpretation of the carrier relaxation in terms of L -> X inter--valley scattering is not correct. The ultra--fast dynamics measured experimentally is, instead, due to the scattering between degenerate $L$ states that is activated by the non symmetric population of the conduction bands induced by the laser field. This ultra--fast relaxation is, then, entirely due to the specific experimental setup and it can be interpreted by introducing a novel definition of the quasi--particle lifetimes in an out--of--equilibrium context.Comment: 4 page, 2 figure

Velocity fluctuations in cooling granular gases

We study the formation and the dynamics of correlations in the velocity field for 1D and 2D cooling granular gases with the assumption of negligible density fluctuations (``Homogeneous Velocity-correlated Cooling State'', HVCS). It is shown that the predictions of mean field models fail when velocity fluctuations become important. The study of correlations is done by means of molecular dynamics and introducing an Inelastic Lattice Maxwell Models. This lattice model is able to reproduce all the properties of the Homogeneous Cooling State and several features of the HVCS. Moreover it allows very precise measurements of structure functions and other crucial statistical indicators. The study suggests that both the 1D and the 2D dynamics of the velocity field are compatible with a diffusive dynamics at large scale with a more complex behavior at small scale. In 2D the issue of scale separation, which is of interest in the context of kinetic theories, is addressed.Comment: 24 pages, 16 figures, conference proceedin

Strong Raman-induced non-instantaneous soliton interactions in gas-filled photonic crystal fibers

We have developed an analytical model based on the perturbation theory in order to study the optical propagation of two successive intense solitons in hollow-core photonic crystal fibers filled with Raman-active gases. Based on the time delay between the two solitons, we have found that the trailing soliton dynamics can experience unusual nonlinear phenomena such as spectral and temporal soliton oscillations and transport towards the leading soliton. The overall dynamics can lead to a spatiotemporal modulation of the refractive index with a uniform temporal period and a uniform or chirped spatial period

Clausius relation for active particles: what can we learn from fluctuations?

Many kinds of active particles, such as bacteria or active colloids, move in a thermostatted fluid by means of self-propulsion. Energy injected by such a non-equilibrium force is eventually dissipated as heat in the thermostat. Since thermal fluctuations are much faster and weaker than self-propulsion forces, they are often neglected, blurring the identification of dissipated heat in theoretical models. For the same reason, some freedom - or arbitrariness - appears when defining entropy production. Recently three different recipes to define heat and entropy production have been proposed for the same model where the role of self-propulsion is played by a Gaussian coloured noise. Here we compare and discuss the relation between such proposals and their physical meaning. One of these proposals takes into account the heat exchanged with a non-equilibrium active bath: such an "active heat" satisfies the original Clausius relation and can be experimentally verified.Comment: 10 pages, submitted to Entropy journal for the special issue "Thermodynamics and Statistical Mechanics of Small Systems" (see http://www.mdpi.com/journal/entropy/special_issues/small_systems

Self-frequency blue-shift of dissipative solitons in silicon based waveguides

We analyze the dynamics of dissipative solitons in silicon on insulator waveguides embedded in a gain medium. The optical propagation is modeled through a cubic Ginzburg-Landau equation for the field envelope coupled with an ordinary differential equation accounting for the generation of free carriers owing to two-photon absorption. Our numerical simulations clearly indicate that dissipative solitons accelerate due to the carrier-induced index change and experience a considerable blue-shift, which is mainly hampered by the gain dispersion of the active material. Numerical results are fully explained by analytical predictions based on soliton perturbation theory