P. Di Trapani

Traveling-wave parametric generation of widely tunable, highly coherent femtosecond light pulses

We report on the generation of ultrashort tunable pulses with a cavityless traveling-wave scheme consisting of a parametric superfluorescence seed source and a parametric amplifier. We show that the traveling-wave approach, with its advantages of simplicity and direct generation of tunable energetic single pulses, can be used in the femtosecond regime, and to this end we discuss the performances that were obtained with pump pulses of ≈1-ps and 200-fs duration at wavelengths of 0.53 and 0.6 μm, respectively. Of particular interest is the β-barium borate-based traveling-wave parametric generator (type-II phase matching), since it offers the possibility of generating nearly transform-limited pulses that are continuously tunable within a wide spectral range to as high as 3 μm in the IR. With a diffraction-limited pump at 0.53 μm. we obtained tunable pulses in a 1.2× diffraction-limited beam, which could be focused, with an f/20 optics lens, to an intensity of 1013 GW/cm2. A temperature-tuned lithium triborate-based femtosecond parametric generator, with its smaller group-velocity dispersion and absence of walk-off, can operate at a pump energy of as low as 30 μJ in a 200-fs pulse.

Spontaneously Generated X-Shaped Light Bullets

We observe the formation of an intense optical wave packet fully localized in all dimensions, i.e., both longitudinally (in time) and in the transverse plane, with an extension of a few tens of fsec and microns, respectively. Our measurements show that the self-trapped wave is an X-shaped light bullet spontaneously generated from a standard laser wave packet via the nonlinear material response (i.e., second-harmonic generation), which extend the soliton concept to a new realm, where the main hump coexists with conical tails which reflect the symmetry of linear dispersion relationship.

Detection of Sub-Shot-Noise Spatial Correlation in High-Gain Parametric Down Conversion

Using a 1 GW, 1 ps pump laser pulse in high-gain parametric down conversion allows us to detect sub-shot-noise spatial quantum correlation with up to 100 photoelectrons per mode by means of a high efficiency charge coupled device. The statistics is performed in single shot over independent spatial replica of the system. Evident quantum correlations were observed between symmetrical signal and idler spatial areas in the far field. In accordance with the predictions of numerical calculations, the observed transition from the quantum to the classical regime is interpreted as a consequence of the narrowing of the down-converted beams in the very high-gain regime.

Matching of group velocities in three-wave parametric interaction with femtosecond pulses and application to traveling-wave generators

The use of suitable noncollinear phase-matching configurations in type-I (e–oo) parametric interaction greatly improves the group-velocity matching among pump, signal, and idler pulses. A numerical model, well supported by the experiments, shows that the compensation of group-velocity mismatch can be achieved over the entire tuning range for pulses as short as few tens of femtoseconds. A general feature of the noncollinear interaction, the front tilt of the generated pulses, is experimentally investigated for a LiIO3 traveling-wave generator. A novel β-barium borate traveling-wave parametric converter pumped with 130-fs blue pulses from a frequency-doubled amplified Ti:sapphire system provides broad tunability in the visible with pulse durations as low as 90 fs and with a single-pass conversion efficiency as large as 40%.

Matching of group velocities by spatial walk-off in collinear three-wave interaction with tilted pulses

We demonstrate that, for tilted pulses, the contribution to group velocity that is due to spatial walk-off can compensate for the group-velocity mismatch in three-wave interactions. With 100-fs, 0.4-Zmicrom pump pulses in beta-barium borate, we obtained efficient parametric generation of collinear superfluorescence tunable in the range from 0.456 to 3.25 microm.

Self-diffraction through cascaded second-order frequency-mixing effects in β-barium borate

The large effective third-order nonlinearity produced by cascaded second-order processes is evidenced by the spatial self-diffraction of two beams interacting in a β-barium borate crystal. In this configuration, both the real and imaginary parts of the induced third-order susceptibility play a role, and the effect reaches its maximum around the phase-matching condition.

Spatio-temporal reshaping and X Wave dynamics in optical filaments.

Focus Serial: Frontiers of Nonlinear Optics We investigate ultrashort laser pulse filamentation within the framework of spontaneous X Wave formation. After a brief overview of the filamentation process we study the case of an intense filament co-propagating with a weaker seed pulse. The filament is shown to induce strong Cross-Phase-Modulation (XPM) effects on the weak seed pulse: driven by the pump, the seed pulse undergoes pulse splitting with the daughter pulses slaved to their pump counterparts. They undergo strong spatio-temporal reshaping and are transformed into XWaves traveling at the same group velocities as the pump split-off pulses. In the presence of a gain mechanism such as Four-Wave-Mixing or Stimulated Raman Scattering, energy is then transferred from the pump filament leading to amplification of the seed X Wave and formation of a temporally compressed intensity peak.

Self-reconstruction of light filaments.

By observing how a light filament generated in water reconstructs itself after hitting a beam stopper in the presence and in the absence of a nonlinear medium, we describe the occurrence of an important linear contribution to reconstruction that is associated with the conical nature of the wave. A possible scenario by which conical wave components are generated inside the medium by the distributed stopper or reflector created by nonlinear losses or plasma is presented.

Highly efficient four-wave parametric amplification in transparent bulk Kerr medium

We report on highly efficient four-wave optical parametric amplification in a water cell pumped by an elliptically shaped, ultrashort pulsed laser beam under non-collinear phase-matching configuration. Energy conversion from pump to parametric waves as high as 25 % is obtained owing to the achievement of 1-dimensional spatial-soliton regime, which guarantees high intensity over a large interaction length and ensures high beam quality.

Effective lensing effects in parametric frequency conversion

We show that, in the high wave-vector-mismatch (cascading) limit, the well-known paraxial description of parametric frequency conversion in quadratic media entails effective lensing effects, which can have a self-focusing or a self-defocusing nature, critically depending on the mismatch sign, the selected wave, and the launching condition (second-harmonic generation or downconversion). Numerical and experimental evidence of this behavior is reported.