Vittorio Degiorgio

Dispersive wave generation by solitons in microstructured optical fibers.

We study the nonlinear propagation of femtosecond pulses in the anomalous dispersion region of microstructured fibers, where soliton fission mechanisms play an important role. The experiment shows that the output spectrum contains, besides the infrared supercontinuum, a narrow-band 430-nm peak, carrying about one fourth of the input energy. By combining simulation and experiments, we explore the generation mechanism of the visible peak and describe its properties. The simulation demonstrates that the blue peak is generated only when the input pulse is so strongly compressed that the short-wavelength tail of the spectrum includes the wavelength predicted for the dispersive wave. In agreement with simulation, intensity-autocorrelation measurements show that the duration of the blue pulse is in the picosecond time range, and that, by increasing the input intensity, satellite pulses of lower intensity are generated.

Nonlinear optical properties of semiconductor nanocrystals

This review is devoted to the description of recent experimental results concerning the nonlinear optical properties of semiconductor-doped glasses SDGs with particular emphasis on the regime in which the energy of the incident photon is smaller than the energy gap. A considerable theoretical and experimental effort has been devoted in the last 10years to the fundamental aspects of quantumconfined structures, which have properties somewhat intermediate between the bulk crystals and atoms or molecules. From this point of view, SDGs represent an easily available test system, and optical techniques have been a major diagnostic tool. Luminescence and absorption spectroscopy were extensively used to characterize the electronic states. The experiments aimed at the measurement of the real and imaginary parts of the third-order optical susceptibility of SDGs below the bandgap are described in some detail, and the results obtained with different techniques are compared. Besides the intrinsic fast nonlinearity due ...

Reduced photorefraction in hafnium-doped single-domain and periodically poled lithium niobate crystals

Measurements of photo-induced birefringence n single domain and periodically poled lithium niobate (LN) crystals containing different non-photorefractive impurities are presented. We find that doping by HfO/sub 2/ is very effective in reducing the photorefraction.

Laser-light scattering investigation of the micellar properties of gangliosides

The micellar properties of gangliosides in water solutions were investigated by quasielastic light scattering measurements. GM1 and GD1a gangliosides were isolated from calf brain, purified to more than 99% and dissolved in 0.025 M Tris--HCl buffer (pH 6.8) at 37 degrees C. The average intensity of scattered light and the intensity correlation function were measured by an apparatus including a 5145 A argon laser and a real-time digital correlator. The scattered intensity data allowed the derivation of an upper limit to the critical micelle concentration (c0) and the evaluation of the molecular weight (M) of the micelle. The intensity correlation function gave the diffusion coefficient D, and hence the hydrodynamic radius RH, and also contained information on the polydispersity of the sample. We find co less than 1 x 10(-6) M for both GM1 and GD1a, M = 532000 +/- 50000 and RH = 63.9 +/- 2 A for GM1, and M = 417000 +/- 40000 and RH = 59.5 +/- 2 A for GD1a. The mixture 3:1 of the two gangliosides gave intermediate values for all examined parameters. The presence of cations, within the physiological concentration range, and, in particular of Ca2+, did not influence significantly the values of co and the main features of the micelle.

Numerical modeling and optimization of cascaded CW Raman fiber lasers

The authors solve a model which describes a nested fiber Raman cavity using Bragg reflectors by taking into account all interactions between forward and backward traveling waves. They apply the model to the cases in which a laser source at 1117 nm pumps a two-step Raman cascade leading to a 1240-nm output or a five-step cascade leading to a 1480-nm output. In order to insert into the calculations realistic fiber parameters, they have performed measurements of both Raman gain and linear losses for a few optical fibers. They derive from the numerical analysis the optimum fiber length and output coupling, and calculate, for the optimum configuration, the dependence of the output power on the pump power. The model describes with good accuracy the published experimental results.

Light-scattering study on the micellar properties of a non-ionic surfactant

Abstract Aqueous solutions of Triton X-100 are investigated over a wide range of temperatures and concentrations by Rayleigh scattering of laser light. The measured temperature dependence of the micellar parameters is shown to be consistent with the hypothesis of critical concentration fluctuations.

Bound electronic and free carrier nonlinearities in Silicon nanocrystals at 1550nm

We present a detailed investigation of the different processes responsible for the optical nonlinearities of silicon nanocrystals at 1550 nm. Through z-scan measurements, the bound-electronic and excited carrier contributions to the nonlinear refraction were measured in presence of two-photon absorption. A study of the nonlinear response at different excitation powers has permitted to determine the change in the refractive index per unit of photo-excited carrier density sigma(r) and the value of the real bound-electronic nonlinear refraction n(2be) as a function of the nanocrystals size. Moreover at high excitation power, a saturation of the nonlinear absorption was observed due to band-filling effects.

Enhancement of stability and efficiency of a nonlinear mirror mode-locked Nd:YVO(4) oscillator by an active Q-switch.

The stability and the peak power of a nonlinear-mirror mode-locked Nd:YVO(4) laser are significantly increased by the insertion of an acousto-optic modulator inside the laser cavity. The repetition rate for reliable operation can be varied in the range 35 kHz - 50 kHz. The laser generates the most intense and stable mode-locked pulses of width 9 ps lying underneath a Q-switched envelope of width 110 ns with a Q-switch modulation frequency of 38 kHz. For 10 W of pump power, a 224 times enhancement of peak power over that of cw mode-locking is obtained under reliable Q-switched and mode-locked operation.

WAVELENGTH SHIFTING AND AMPLIFICATION OF OPTICAL PULSES THROUGH CASCADED SECOND-ORDER PROCESSES IN PERIODICALLY POLED LITHIUM NIOBATE

We investigate the efficiency of wavelength shifting (generation of a pulse at the wavelength λp−Δλ from a signal at λp+Δλ under the action of a pump at λp) and parametric amplification through a cascaded second-order process in a periodically poled lithium niobate crystal. Our results show that, in a 19-mm-long sample, at λp=1.8 μm and Δλ as large as 60 nm, wavelength shifting with unit gain can be obtained with a pump intensity of 6 MW/cm2, while amplification by a factor 10 requires 18 MW/cm2.

Critical properties of nonionic micellar solutions

Solutions of the nonionic amphiphile n‐dodecyl octaoxyethylene glycol monoether (C12E8) in D2O and in salty H2O have been investigated by static and dynamic light scattering. It is found that the micellar properties are little influenced by the choice of the solvent, whereas the critical properties (in particular, the critical exponents γ and ν) show a marked solvent effect. The interpretation of the observed critical behavior in terms of first‐order correction‐to‐scaling contributions is discussed.