C. de Lisio

Transmittivity measurements by means of squeezed vacuum light

A method for measuring the transmittivity of optical samples by using squeezed vacuum radiation is illustrated. A squeezed vacuum field generated by a below threshold optical parametric oscillator is propagated through a non-dispersive medium and detected by a homodyne apparatus. The variance of the detected quadrature is used for measuring the transmittivity. With this method the number of photons passing through the sample during the measurement interval is drastically reduced. The results of some tests are reported.

Diffractionless beams and their use for harmonic generation

Abstract Use of diffraction-free beams to generate low-order harmonics in xenon jets has been investigated. Nearly diffractionless beams have been generated from a Tixa0:xa0sapphire, 100xa0fs, laser system by focussing ring-type radiation, which has been produced in two different ways: either, by inserting an annular slit into the incident Gaussian beam, or by placing a doublet of axicons in a vertex-to-vertex configuration before the focussing lens. The results obtained with the first scheme are discussed and interpreted in terms of a simple perturbative model of the harmonic generation (HG) process. The first measurements performed with the doublet of axicons are also reported and discussed.

Probing interfacial properties by optical second-harmonic generation

Abstract Optical second-harmonic generation from surfaces has recently proven to be a very effective and versatile probe for surface studies. It allows, among other things, direct probing of molecular adsorption on a solid substrate from a liquid. In this paper we review the most important features of this new technique. We discuss the possibility of applying it to the study of adsorption of ionic surfactants at solid–liquid interfaces, commonly used in a great number of scientific and technological fields including industrial applications. Moreover, we describe the preliminary results of an experiment currently in progress.

Influence of the medium length on high-order harmonic generation

We study high-order harmonic generation using a 110 fs Ti:sapphire laser loosely focused into a variable-length gas cell filled with neon or argon at 5 mbar pressure. The harmonic intensity is recorded as a function of the medium length, varying between 2 and 21 mm. Several cases are examined, the 17th and the 29th harmonic in argon, and the 29th and 51st harmonic in neon, at the same intensity 4 10 14 Wc m 2 . We find that the length which maximizes the harmonic yield varies from 10 mm to more than 20 mm. We discuss the different effects affecting the photon yield of the high-order harmonics.

Ultrafast quasiparticle relaxation dynamics in high quality epitaxial FeSe0.5Te0.5 thin films

Femtosecond spectroscopy has been used to investigate the quasiparticle relaxation times in a nearly optimally doped chalcogenide superconducting FeSe0.5Te0.5 thin film. It is characterized by an enhancement of the critical temperature Tcxa0=xa019.1xa0K by strain. Experimental results indicate the existence of a temperature dependence of the quasiparticle recombination time τR in the superconducting state where an energy gap Δ(T) is present. A fitting procedure based on a BCS-like assumption for τR gives Δ0xa0=xa04.1xa0±xa00.4xa0meV for the superconducting energy gap extrapolated at Txa0=xa00, which corresponds to 2Δ0/kBTcxa0=xa04.9xa0±xa00.5. Accordingly, an estimation of the characteristic phonon energy and the electron–phonon coupling constant λ is reported and discussed. The obtained value λxa0≈xa00.6 is in good agreement with other similar determinations obtained by different techniques and numerical approaches. The small value of λ confirms that a phonon-mediated coupling cannot be the only mechanism leading to the formation of the superconducting state in FeSeTe films.

Measurement of Porous Silicon Dielectric Constant by VUV Laser Harmonic Radiation

By combining the use of laser harmonic radiation as vacuum ultraviolet (VUV) light source with that of a standard NIR–VIS–UV spectrometer, the reflectance of porous silicon over a wide energy spectral range from 1 to 16 eV was measured. Reflected intensity modulation due to interference effects arising from the finite thickness of the porous layer in the visible range was also taken into account and reduced by fringe fitting. Porous silicon dielectric constant was then deduced from reflectance measurements by Kramers-Kronig analysis. Data are found to be in good agreement with those reported in literature, thus showing that laser harmonics represent a new alternative and suitable VUV source for optical characterisation of materials.

Ultrafast Photoresponse of Superconductor/Ferromagnet Nano-Layered Hybrids

Interactions of superconducting nanostructures with external optical radiation represent a very interesting topic in both the framework of nonequilibrium physics and photodetector applications. Heterogeneous nano-layers, such as proximized superconductor/normal metal and superconductor/ferromagnet (S/F) bilayers, are very promising since they exhibit the ultrafast Cooper-pair and quasiparticle dynamics. We have characterized Nb/NiCu, NbN/NiCu, YBaCuO/Au/NiCu, and YBaCuO/LaSrMnO proximized S/F nano-bilayers, using time-resolved, all-optical, femtosecond pump-probe spectroscopy measurements down to 4 K. Our experiments demonstrate that these bilayers are very promising for novel, ultrafast photodetection and spintronics device applications. Moreover, our time-resolved studies of the carrier dynamics in oxide-based S/F structures open the way to novel basic-physics investigations of nonequilibrium effects in correlated systems.

Fabrication and optical properties of ultrathin ferromagnet/superconductor metallic bilayers

Ferromagnet/superconductor (F/S) bilayers composed of ultrathin metallic films demonstrate new proximity effects and show a peculiar behavior of the superconductor-order parameter. In terms of applications, F films provide additional anchoring of the S film vortex lattice, resulting in increased critical current. By changing composition and the thickness ratio of the F/S bilayer, one can modify the quasiparticle dynamics through an effective enhancement or suppression of the quasiparticle lifetime under nonequilibrium conditions. The latter effect should lead to custom-designed superconducting detectors, with the sensitivity and photoresponse speed tailored to a given application. We present fabrication details of nanometer-thickness NiCu/Nb bilayers of different F/S ratios. The NiCu/Nb structures have been implemented to fabricate Josephson junctions and tested using femtosecond optical spectroscopy. The time-resolved photoresponse experiments allowed us to measure the quasiparticle relaxation dynamics in our bilayers. In addition, the photoresponse temperature dependence studies revealed the spatial evolution of the superconductor-order parameter across the bilayer. Comparison between the bilayer data and the single-material data is also presented.

Application of VUV laser harmonic radiation to the measurement of porous silicon dielectric function

Abstract Vacuum ultra-violet (VUV) laser harmonics have been generated in a noble gas jet, which, combined with a standard spectrophotometer, have allowed measurements of the reflectance of porous silicon over a wide energy spectral range from 1 to 16xa0eV. Porous silicon dielectric function was, then, deduced from reflectance measurements by Kramers–Kronig analysis. Data are found to be in good agreement with those reported in literature, thus showing that laser harmonics represent a new, alternative, and suitable VUV source for optical characterisation of materials such as semiconductors and thin films.

Distortion of the laser beam induced by the electron plasma in harmonic generation experiments

The authors analyse the distortion of the pump laser beam propagating through the electron plasma produced by non-resonant MPI ionization. For calculating the aberration eikonal they use the complex eikonal technique developed by Choudhary and Felson (1973, 1974). In particular, they show that this eikonal can be expressed as an integral of the refractive index profile. Successively, they use the eikonal for calculating the amplitudes of the different Gauss-Laguerre modes of the qth harmonic generated by the laser pump beam. Finally, they apply this formalism to the study of the seventh harmonic produced in the non-resonant 11-photon ionization of Xe by a Nd-YAG laser.