Wave propagation in highly inhomogeneous thin films : exactly solvable models

We present an exact treatment of wave propagation in some inhomogeneous thin films with highly space-dependent dielectric constant. It is based on a space transformation which replaces the physical space by the optical path. In the new space, the dispersion equation is that of a normal progressive wave. We will show that the dispersion properties of such films are plasma- or waveguide-like, the characteristic frequency being determined by the spatial characteristics of the dielectric constant's variations only. The theory is scalable, so that it can be applied in any wavelength range : optical, IR, radiofrequency, etc. depending only on the characteristic space scales. Several applications will be presented, concerning the reflection properties of such films (broadband anti-reflection, or dichroic coatings) or to the propagation and transmission through the film. We will show that depending on the type of space dependence, an incident wave can either propagate or tunnel through such films. We will investigate the behaviour of the light group-velocity and tunneling time inside or through such films. Though we can reproduce the phase-time saturation corresponding to the Hartman effect, analysis of the group velocity in the tunneling case shows no sign of superluminal propagation. A strong frequency dependence can be obtained in some situations, which allows to anticipate a strong reshaping of brodband laser pulses

Exactly solvable models : a solution to different problems of laser matter interaction

With the increasing use of ultrashort laser pulses and nanoscale-materials, one is regularly confronted to situations in which the properties of the media supporting propagation are not varying slowly with time (or space). Hence, the usual WKB-type approximations fail, and one has to resort to numerical treatments of the problems, with a considerable loss in our insight into the physics of laser-matter interaction. We will present a new approach which allows a fully analytical solution of such problems, based on a transformation of the propagation equations into a new space where phase accumulation is linear with either time or space, which greatly simplifies their treatment. Though this method is restricted to some special models of the time or space varying dielectric constant, those are however general enough to encompass practically all experimental situations. It allows to introduce the concept of "non-stationarity induced" (or "inhomogeneity induced") dispersion. We will analyse the problem of reflection and propagation in two types of media whose dielectric constant vary rapidly at either the laser period or the laser wavelength scale. Extension of such techniques to the case of arbitrarily high non linearities will be considered too

Mécanismes fondamentaux de l'ablation laser femtoseconde en "flux intermédiaire"

In French - Acte Ecole thématique "Femto2004", Porquerolles, Sept. 2004This chapter aims at identifying what is specific to femtosecond laser ablation. It reviews the essential basic processes which contribute to femtosecond laser ablation of various materials : energy absorption by the material's electrons, electronic relaxation processes involving either electron-electron or electron-lattice interactions. A number of widely used models are briefly discussed, as well as some pending questions

OPTICS OF INSTANTANEOUS WAVES

This review is devoted to the transient optical phenomena, displayed both in the spatiotemporal dynamics of ultrashort single - cycle wave pulses in free space and dispersive dielectrics as well as to interaction of light with non-stationary media. The interplay of diffractive and dispersive phenomena, including the coupled processes of amplitude and phase reshaping, spectral variations, polarity reversal for different types of light pulses, is examined in frequency and time domains. Reflection - refraction effects on the interfaces of media with time-dependent dielectric susceptibility are considered by means of exact analytical solutions of Maxwell equations for these media. The non - stationarity - induced dispersion is shown to provide a dynamical regime of reflectivity of non-stationary media, depending upon both instantaneous dielectric susceptibility and its temporal derivative; the relevant generalization of Fresnel formulae is presente

Polarization - dependent tunneling of light in gradient optics

Reflection-refraction properties of photonic barriers, formed by dielectric gradient nanofilms, for inclined incidence of both S- and P-polarized electromagnetic (EM) waves are examined by means of exactly solvable models. We present generalized Fresnel formulae, describing the influence of the non-local dispersion on reflectance and transmittance of single- and double-layer gradient photonic barriers for S- and P- waves and arbitrary angles of incidence. The non-local dispersion of such layers, arising due to a concave spatial profile of dielectric susceptibility across the plane film, is shown to result in a peculiar heterogeneity - induced optical anisotropy, providing the propagation of S(P) waves in tunneling (travelling) regimes. The results obtained indicate the possibility of narrow-band non-attenuated tunneling (complete transmittance) of oblique S- waves through such heterogeneous barriers, and the existence of spectral areas characterized by strong reflection of P-waves and deep contrast between transmitted S- and P-waves. The scalability of obtained exact analytical solutions of Maxwell equations into the different spectral ranges is discussed and the application potential of these phenomena for miniaturized polarizers and filters is demonstrated

New pico- and femtosecond laser based sources: from the Infra-Red to the XUV, Journal of Telecommunications and Information Technology, 2000, nr 1-2

In this paper, we review a number of recentdevelopments concerning laser or laser-based sources generatingpico- or subpicosecond pulses of light at wavelengthsnow ranging from the mid Infra-Red (50 mm) to the XUV(11.9 nm). Those include the new generation of „Chirp PulseAmplification” laser systems, Free Electron Lasers operatingin the IR or visible range, and a number of laser drivensources covering the XUV range, generally based on the highintensity irradiation of solid or gaseous targets. The mainperspectives, applications and essential issues concerning suchsources are discussed

Luminescence of Aluminoborosilicate Glasses Doped with Gd3+ Ions

International audienceThe twophoton absorption that leads to the ultraviolet upconversion luminescence in the SiO2–Al2O3–B2O3–Na2O3–Zr2O : Gd3+ glass has been investigated. The inference has been made that no photon cascade emission takes place under excitation by monochromatic light corresponding to the maximum of the absorption band of the Cd3+ ion (204 nm). The mechanisms of concentration quenching and energy transfer between Cd3+ ions and optically active defects of the aluminoborosilicate glass have been discussed

SPATIALLY PERIODIC STRUCTURES, UNDER FEMTOSECOND PULSED EXCITATION OF CRYSTALS

Measuring the luminescence intensity of specially prepared irradiation defects induced in crystals, we observe that the longitudinal structure of quasi-interferences induced by two orthogonally polarized femtosecond pulses propagating together with different velocities is insensitive to the spatial broadening due to velocity dispersion in the crystals. On the contrary, it does depend on the pulse duration when it is changed by varying the spectral width of the radiation. It thus allows a direct measurement of the coherence time of such pulses. Stability of the axial selectivity is a good sign, taking away a number of serious limitations concerning possible applications

Reflectionless Tunnelling of Light in Gradient Optics

We analyse the optical (or microwave) tunnelling properties of electromagnetic waves passing through thin films presenting a specific index profile providing a cut-off frequency, when they are used below this frequency. We show that contrary to the usual case of a square index profile, where tunnelling is accompanied with a strong attenuation of the wave due to reflection, such films present the possibility of a reflectionless tunnelling, where the incoming intensity is totally transmitted