Towards nanoprinting with metals on graphene.

Graphene and carbon nanotubes are envisaged as suitable materials for the fabrication of the new generation of nanoelectronics. The controlled patterning of such nanostructures with metal nanoparticles is conditioned by the transfer between a recipient and the surface to pattern. Electromigration under the impact of an applied voltage stands at the base of printing discrete digits at the nanoscale. Here we report the use of carbon nanotubes as nanoreservoirs for iron nanoparticles transfer on few-layer graphene. An initial Joule-induced annealing is required to ensure the control of the mass transfer with the nanotube acting as a 'pen' for the writing process. By applying a voltage, the tube filled with metal nanoparticles can deposit metal on the surface of the graphene sheet at precise locations. The reverse transfer of nanoparticles from the graphene surface to the nanotube when changing the voltage polarity opens the way for error corrections.

Wavelet analysis of short light pulses.

We demonstrate the possible use of the Wigner-Ville transformation for describing ultrashort light pulses in both time and frequency and give support to the intuitive pictures used by experimentalists in the field. Two cases are considered: the effect of linear dispersion and self phase modulation. Various characteristics can be extracted from the Wigner-Ville distribution, e.g., time broadening, power spectrum broadening and reshaping, instantaneous frequency time variation.

Wave group decomposition of ultrashort light pulses.

We present a new method for analyzing the propagation of short optical pulses through a dispersive medium. The idea is to decompose the pulse Fourier spectrum into an infinite set of elementary wave groups, each propagating with its own group velocity. Such a method may be generalized to the case of nonlinear self-phase-modulation and points out the asymmetry observed in the white-light continua generated in the femtosecond regime.

Energy transport studies using spatially resolved luminescence.

A method is described in which a small and well-defined area on the face of a crystal is illuminated by a focused laser beam and the light emitted from adjoining areas is spatially analyzed. Our experimental studies, done on nitrogen-doped III-V semiconductors, show that spatial distribution of a Raman spectral line properly monitors the illumination spot geometry as well as the instrumental response. It is shown that the zero phonon line associated to the nitrogen-trapped exciton and the corresponding phonon replica exhibit different spatial distributions. The effect of temperature on the spatial distribution of light emitted both by a zero phonon process and its phonon replica is examined. These results are discussed in terms of possible implications for the energy transfer process.

Femtosecond measurements of the time of flight of photons in a three-dimensional photonic crystal.

We report on experimental measurements of the propagation behavior of short optical pulses in a three-dimensional photonic crystal in the visible spectrum. The propagation delay of 70 fs light pulses transmitted through a sample of a fcc synthetic opal at frequencies lying in a photonic stop band was measured directly using a time-of-flight technique. Taking into account spectral reshaping of the transmitted pulses as well as the residual frequency chirp of the incoming pulses, it is found that the pulses significantly slow down at the photonic band edges.

Femtosecond white-light continuum pulses.

We obtain gigawatt white-light continuum pulses that permit spectroscopic measurements with a time resolution of 80 fsec. These pulses extend continuously from 0.19 to 1.6 microm and have time sweeps as small as 10 fsec/1000 A. We find temporal, spatial, and spectral properties that are consistent with self-phase modulation having a prominent role in generation of the continuum.

Amplification of femtosecond optical pulses using a double confocal resonator.

We experimentally examine a class of multipass amplifiers based on an open double confocal resonator. A preamplifier provides a gain of 800. An intermediate-stage amplifier produces pulse energies of 2 microJ for only 1.4 W of pump power at a repetition rate of 10 kHz. A third structure, which includes a four-prism sequence, provides on each pass adjustable forms of the four basic pulse-shaping mechanisms of saturable gain, saturable absorption, group-velocity dispersion, and self-phase modulation, as well as spectral filtering. This latter structure also reversibly lengthens the pulse duration in the gain medium from 50 fsec to over 450 fsec.