S. De Silvestri

Millijoule-level phase-stabilized few-optical-cycle infrared parametric source.

Ultrabroadband self-phase-stabilized near-IR pulses have been generated by difference-frequency generation of a filament broadened supercontinuum followed by two-stage optical parametric amplification. Pulses with energy up to 1.2 mJ and duration down to 17 fs are demonstrated. These characteristics make such a source suited as a driver for high-order harmonic generation and isolated attosecond pulse production.

Femtosecond micromachining of symmetric waveguides at 1.5 µm by astigmatic beam focusing

We report on a new spatial beam-shaping approach for fabrication of waveguides with a circular transverse profile by femtosecond laser pulses, using an astigmatic beam and controlling both beam waist and focal position in the tangential and sagittal planes. We apply this technique to write single-mode active waveguides at 1.5microm in Er:Yb-doped glass substrates. The experimental results are well described by a simple nonlinear absorption model.

Few-optical-cycle pulses tunable from the visible to the mid-infrared by optical parametric amplifiers

Ultrafast optical parametric amplifiers (OPAs) can provide, under suitable conditions, ultra-broad gain bandwidths and can thus be used as effective tools for the generation of widely tunable few-optical-cycle light pulses. In this paper we review recent work on the development of ultra-broadband OPAs and experimentally demonstrate pulses with durations approaching the single-cycle limit and almost continuous tunability from the visible to the mid-IR.

The role of cavity dispersion in CW mode-locked dye lasers

The contribution to dispersion of a few components of a mode-locked dye laser cavity has been evaluated. The chirp produced by the self-phase modulation within the saturable absorber has also been evaluated and compared with that resulting from the dispersive components. Cavity mirrors and blocks of transparent materials (glass or quartz) of a length exceeding 1 mm give the largest dispersion. The effect of self-phase modulation, at least for high saturation of the absorber, seems to play a comparable role. The dispersive contributions should influence the mode-locking behavior for pulses of about 100 fs or shorter.

Cluster effects in high-order harmonics generated by ultrashort light pulses

High-order harmonic generation in argon driven by 25-fs-light pulses is investigated from the gaseous to the cluster regime. The harmonic cutoff observed in presence of clusters shows a considerable extension with respect to the gaseous phase. Harmonic spectra are investigated as a function of cluster size, showing the existence of an optimal cluster dimension, which maximizes the harmonic photon yield.

Two-optical-cycle pulses in the mid-infrared from an optical parametric amplifier

Ultrabroadband mid IR pulses with energy as high as 2 microJ and tunability from 2 to 5 microm are generated as the idler beam of an 800 nm pumped optical parametric amplifier in periodically poled stoichiometric lithium tantalate. After bulk compression in a Ge plate and frequency-resolved-opticle-gating characterization, a pulse duration as low as 25 fs was measured, corresponding to two optical cycles of the 3.6 microm carrier wavelength.

Full temporal resolution of the two-step photoinduced energy–electron transfer in a fullerene–oligothiophene–fullerene triad using sub-10 fs pump–probe spectroscopy

Abstract The intramolecular photoinduced energy and electron transfer in a fullerene–oligothiophene–fullerene triad with nine thiophene units (C60–9T–C60) has been investigated with sub-10 fs pump–probe spectroscopy in solvents of different polarity. Photoexcitation of the oligothiophene moiety results in an ultrafast singlet-energy transfer reaction to create the fullerene singlet-excited state with a time constant of 95 fs, irrespective of the polarity of the medium. In a polar solvent, the ultrafast singlet-energy transfer is followed by intramolecular electron transfer from the oligothiophene to the fullerene with a time constant of 10 ps, resulting in a charge-separated state (CSS) with a lifetime of 80 ps.

Efficient continuum generation exceeding 200 eV by intense ultrashort two-color driver

A temporal gating on the high-order harmonic emission process is achieved using an intense 20 fs, 1.45 microm pulse (IR) in combination with an intense 13 fs, 800 nm pulse [visible (VIS)]. Exploiting this two-color gating scheme, a coherent continuous emission extending up to 160 eV using Ar gas and 200 eV using Ne gas is efficiently generated. The IR pulse contributes to significantly extending the harmonic emission to higher photon energies, whereas the VIS pulse improves the conversion efficiency of the process. These results indicate the possibility to produce bright attosecond pulses approaching the soft X spectral region.

Few-optical-cycle pulses in the near-infrared from a noncollinear optical parametric amplifier.

We extend the concept of broadband phase matching in a noncollinear optical parametric amplifier (NOPA) to the near-IR. In an 800 nm pumped NOPA using periodically poled stoichiometric lithium tantalate, we amplify a spectrum spanning the 1.1-1.7 microm range and corresponding to two optical cycles of the carrier wavelength. A limited portion of the spectrum is compressed by a prism pair down to 16 fs.

Toward a terawatt-scale sub-10-fs laser technology

Powerful techniques for spectral broadening and ultrabroad-band dispersion control, which allow compression of high-energy femtosecond pulses to a duration of a few optical cycles, are analyzed. Spectral broadening in a gas-filled hollow fiber and compression by chirped mirrors with high-energy 20-fs input pulses are presented. Using 1-mJ seed pulses we have demonstrated the generation of 0.5-mJ 5-fs pulses at 0.8-/spl mu/m and 1-kHz repetition rate. General design criteria to scale the compression technique toward the terawatt level are presented.