N. A. Papadogiannis

Direct observation of attosecond light bunching.

Temporal probing of a number of fundamental dynamical processes requires intense pulses at femtosecond or even attosecond (1 as = 10-18 s) timescales. A frequency ‘comb’ of extreme-ultraviolet odd harmonics can easily be generated in the interaction of subpicosecond laser pulses with rare gases: if the spectral components within this comb possess an appropriate phase relationship to one another, their Fourier synthesis results in an attosecond pulse train. Laser pulses spanning many optical cycles have been used for the production of such light bunching, but in the limit of few-cycle pulses the same process produces isolated attosecond bursts. If these bursts are intense enough to induce a nonlinear process in a target system, they can be used for subfemtosecond pump–probe studies of ultrafast processes. To date, all methods for the quantitative investigation of attosecond light localization and ultrafast dynamics rely on modelling of the cross-correlation process between the extreme-ultraviolet pulses and the fundamental laser field used in their generation. Here we report the direct determination of the temporal characteristics of pulses in the subfemtosecond regime, by measuring the second-order autocorrelation trace of a train of attosecond pulses. The method exhibits distinct capabilities for the characterization and utilization of attosecond pulses for a host of applications in attoscience.

Photon statistics of laserlike emission from polymeric scattering gain media.

The coherent properties of the temporally and spectrally narrowed emission of laser-induced fluorescence of organic dyes hosted inside artificial scattering matrices (random lasers) were investigated. The excitation source was a frequency-doubled 200-fs pulsed laser emitting at 400 nm. Spectral and temporal features were simultaneously recorded with a spectrograph and a streak camera operating in photon-counting mode. Photon-number distributions were thus created. The temporal coherence of the laserlike emission above and below the excitation energy threshold was investigated from the photon-number distribution that was obtained.

Random lasing following two-photon excitation of highly scattering gain media

We present experimental evidence of laser emission following two-photon excitation of dye-infiltrated random gain media with optical properties similar to biological tissue. The excitation was performed with femtosecond laser pulses at 800 nm and the emission (at 480 nm) was recorded with a spectrograph streak camera system. The coherent properties of the random lasing emission were also investigated by performing single photon counting. The applications of coherent laser light that can be emitted deep inside a random medium are far reaching, particularly for imaging and therapeutic purposes.

Ultrashort laser-induced electron photoemission: a method to characterize metallic photocathodes

The photoemission properties of metallic photocathodes are studied by a time-resolved single-photon laser technique in the femtosecond regime. Experimental results obtained for different metals (Au, Cu, W, Al, Fe) are interpreted by a theoretical model, taking into account electron-electron and electron-phonon dynamics during the illumination by ultrashort, high-intensity ultraviolet laser pulses. This model points out the physical process of ultrashort photoemission from metals and the correlation of microscopic transient effects with macroscopic steady-state characteristics.

Electron relaxation phenomena on a copper surface via nonlinear ultrashort single-photon photoelectric emission

Time-resolved single-photon photoelectric emission measurements from polycrystalline copper, using an autocorrelation system of 450 fs duration at 248 nm laser pulses, are reported. This experimental technique makes possible the direct measurement of the electron energy relaxation time at the surface as well as the evaluation of the electron - phonon scattering time in metallic samples. A nonlinear increase of the electron photocurrent versus the laser intensity is observed. This nonlinearity is attributed to the non-thermal equilibrium between the electron gas and the lattice produced by the subpicosecond laser pulses and is related to the dynamics of energy relaxation between the electrons and the lattice. The influence of the non-thermal electrons on the photoemission and their thermalization time is discussed. Our experimental results may be explained by a model based on the Fowler - Dubridge equation for photocurrent and on thermal-balance equations between electron and lattice subsystems.

Nonlinear enhancement of the efficiency of the second harmonic radiation produced by ultrashort laser pulses on a gold surface

Abstract An unexpected nonlinear enhancement in the efficiency of the second harmonic generation as a function of the fundamental laser intensity is observed, when a gold surface is illuminated by few tens of GW/cm 2 subpicosecond pulses of an excimer-pumped dye laser at grazing incidence. The evidence of this effect is tested by time resolved autocorrelation measurements. This enhancement is attributed to the thermal nonequilibrium, between the electron gas and lattice subsystems, produced by the subpicosecond laser pulses on the metallic surface.

Second-order autocorrelation measurements of attosecond XUV pulse trains

The unequivocal demonstration of attosecond XUV pulse train generation based on the synthesis of a comb of harmonics of an infrared femtosecond laser pulse is inevitably linked to a reliable method of pulse characterization in this temporal regime. The most commonly used method to date relies on a cross-correlation technique between the fundamental laser frequency and the XUV light from which a measurement of the relative phases of the consecutive harmonics is obtained and the attosecond temporal structure in a train of XUV pulses is inferred. Here we report on an alternative method that renders possible the direct observation of a train of attosecond pulses in form of a second-order autocorrelation trace. The method is an extension in the XUV spectral range of the well-established technique commonly used for the characterization of femtosecond laser pulses. Besides being the first direct visualization of a periodic attosecond structure, the approach supplies quantitative information on the overall pulse duration including the influence of the macroscopic conditions of generation.

Observation of the inversion of second and third harmonic generation efficiencies on a gold surface in the femtosecond regime

We present measurements of second and third harmonic generation efficiencies of gold, induced by p-polarized 290 fs Ti:sapphire laser pulses at 800 nm, in a broad intensity regime up to the plasma threshold. Inversion of second and third harmonic generation conversion efficiencies is successfully demonstrated for laser intensities below the limit of breakdown of perturbation theory. Non-linear enhancement in the efficiencies of harmonics, caused by the thermal non-equilibrium between the electron gas and lattice, as well as intensity saturation phenomena, are also observed and discussed.

Temporal characterization of short-pulse third-harmonic generation in an atomic gas by a transmission-grating Michelson interferometer

By use of a transmission-grating-based Michelson interferometer, second-order interferometric as well as intensity autocorrelation traces of the third harmonic of a Ti:sapphire 50-fs laser beam produced in Ar have been measured. The duration of the harmonic is found to be that expected from lowest-order perturbation theory. At this wavelength, the performance of the interferometer with respect to pulse-front distortion and dispersion is found to be satisfactory. This result is a first step toward the use of the interferometer for the temporal characterization of higher harmonics or harmonic superposition forming attosecond pulse trains.

Three dimensional transient behavior of thin films surface under pulsed laser excitation

The three dimensional spatiotemporal response of thin metal films surfaces excited by nanosecond laser pulses is investigated in both the thermoelastic and the ablation regimes. An experimental laser whole-field interferometric technique allows for the direct monitoring of the dynamic deformation of a macroscopic area on the surface with ultrahigh lateral resolution. A specially developed three dimension finite element model simulates the laser-surface interaction, predicts the experimentally obtained results, and computes key parameters of matters thermomechanical response. This method provides a powerful instrument for spatiotemporal behavior of thin-film surfaces under extreme conditions demanded for innovative applications.