Effects of ambient air pressure on surface structures produced by ultrashort laser pulse irradiation.

We report an experimental analysis addressing striking effects of residual air ambient pressure, from atmospheric conditions (103 mbar) to high vacuum (10-4 mbar), on the surface structures induced on a silicon target by direct femtosecond laser irradiation. We observe an interesting direct impact of the ambient pressure on the period and depth of the generated ripples as well as on the formation of microgrooves. Moreover, a significant correlation is observed between the ripples' period and depth. The change of pressure is accompanied by a variation of the degree of nanoparticle coverage, which is eventually recognized as an important factor for the development of the final surface structures. These results shed light on the intriguing mechanisms underlying the formation of the various surface textures, also evidencing that the ambient pressure can act as an effective parameter to tailor some characteristic features of the processed surface.

Laser ablation of silicon induced by a femtosecond optical vortex beam.

We investigate laser ablation of crystalline silicon induced by a femtosecond optical vortex beam, addressing how beam properties can be obtained by analyzing the ablation crater. The morphology of the surface structures formed in the annular crater surface allows direct visualization of the beam polarization, while analysis of the crater size provides beam spot parameters. We also determine the diverse threshold fluences for the formation of various complex microstructures generated within the annular laser spot on the silicon sample. Our analysis indicates an incubation behavior of the threshold fluence as a function of the number of laser pulses, independent of the optical vortex polarization, in weak focusing conditions.

Stepwise behaviour of magnetization temperature dependence in iron nanoparticle assembled films.

An unusual stepwise behaviour is reported in the temperature dependence of the zero field cooled magnetization in iron nanoparticle dense films produced by ultra-short pulsed laser deposition assisted by irradiation of nanoparticles with a nanosecond UV laser pulse, appropriately delayed, during their flight from the target to the substrate. This behaviour, induced by the particle system's morphology, characterized by clusters of tightly coupled nanoparticles as well as by some voids between them, is ascribed to the competition between Zeeman energy density, intracluster anisotropy energy density and intercluster exchange energy density. A phenomenological model and Monte Carlo simulations are reported, which support the proposed interpretation.

Generation of high energy, 30 fs pulses at 527 nm by hollow-fiber compression technique.

The compression of 300-fs-long, chirp-free laser pulses at 527 nm down to 30 fs is reported. The laser pulses, originated from a frequency-doubled, mode-locked Nd:glass laser, were compressed by a 0.7-m-long, 150-microm-bore-diameter, argon-filled hollow fiber, and a pair of SF10 prisms with a final energy of 160 microJ. These are the shortest, high energy pulses ever produced by direct pulse compression at the central wavelength of 527 nm. The spectral broadening of the pulses propagating inside the hollow fiber was experimentally examined for various filling-gas pressures and input pulse energies. The spectral width of the pulses was broadened up to 25 nm, and 27 nm for argon- and krypton-filled hollow fiber, respectively, at a gas pressure lower than 2 bar. The physical limitations of the hollow-fiber pulse compression technique applied in the visible range are also studied.

Hollow-fiber compression of visible, 200 fs laser pulses to 40 fs pulse duration.

We demonstrate the use of a very simple, compact, and versatile method, based on the hollow-fiber compression technique, to shorten the temporal length of visible laser pulses of 100-300 fs to pulse durations shorter than approximately 50 fs. In particular, 200 fs, frequency-doubled, Nd:glass laser pulses (527 nm) were spectrally broadened to final bandwidths as large as 25 nm by nonlinear propagation through an Ar-filled hollow fiber. A compact, dispersive, prism-pair compressor was then used to produce as short as 40 fs, 150 microJ pulses. A very satisfactory agreement between numerical simulations and measurements is found.

High-brightness high-order harmonic generation by truncated bessel beams in the sub-10-fs regime.

Low-divergence, high-brightness harmonic emission has been generated by using a fundamental beam with a truncated Bessel intensity profile. Such a beam is directly obtained by using the hollow-fiber compression technique, which indeed allows one to optimize both temporal and spatial characteristics of the high-order harmonic generation process. This is particularly important for the applications of radiation, where extreme temporal resolution and high brightness are required.

Spectral features and modeling of high-order harmonics generated by sub-10-fs pulses

Harmonic radiation generated in a neon gas jet by sub-10-fs laser pulses was investigated both experimentally and theoretically. The spectral profile of the harmonics with respect to the order, their intensity and relative spectral shifts were measured as a function of the position of the gas jet. The results point out spectral features typical of the quasi-single-cycle excitation regime. A nonadiabatic three-dimensional numerical model was developed, which provides harmonic spectra in remarkable agreement with the experiments.

Mode size and time duration fluctuations in a picosecond Nd:YAG laser.

A new technique is successfully used to analyze in real time the pulse-to-pulse fluctuations of mode size and time duration in a picosecond Nd:YAG laser. In particular we show that the pulse length (30 psec) of our active-passive mode-locked Nd:YAG laser is stable to within 10% when the cavityis perfectly tuned and the saturable absorber is fresh. This technique is experimentally shown to be effective and reliable for real-time analysis of the stability of ultrashort laser pulses undera broad range of experimental conditions.

Intrinsic optical transistor action in homeotropically aligned nematic crystal films.

We report the observation of intrinsic optical transistor action in a homeotropically aligned nematic liquid-crystal film, resulting from the large nonlinearity associated with the optical Freedericksz transition in the nematic sample. No external feedback is employed.