Gian Piero Banfi

Threshold evaluations for an x-ray laser

Abstract Photoelectric pumping of inner shells of intermidiate Z materials is shown to be efficient for inverting the population of radiative transitions around 1 keV. Non-radiative processes are shown to be essential for achieving a stationary inversion. Reported intense X-ray bursts from plasmas generated by laser radiation suggest the use of a Nd laser as exciting source. Threshold evaluations show that a mirrorless X-ray laser is possible by use of nowadays available Nd lasers.

High power, subpicosecond, 750-1770 nm tunable pulses from travelling wave parametric generator

Abstract Transform limited subpicosecond pulses have been obtained by parametric conversion of the second harmonic of a short pulse generated by a Nd:glass laser system. The arrangement consists of a KDP followed by a BBO respectively with type II and type I phase-matching, pumped in a travelling wave configuration without any spectral selecting element between them. It provides tunability from 750 to 1770 nm with pulses as short as 0.6 ps. Pulse energies above 50 μJ, with a total conversion efficiency of 13%, have been achieved.

Total second harmonic emission from a laser produced plasma

Abstract The total and specular light emitted at 2ω by a laser produced plasma (τ = 30 ps, λ = 1 μm) have been measured versus intensity (10 12 -10 14 W/cm 2 ), polarization and angle of incidence of the laser beam. The possibility for second harmonic to arise from resonant absorption, through a rippled critical surface, or from parametric decay instability is discussed.

Cascading of second-order processes in a planar Ti-indiffused LiNbO3 waveguide: application to frequency shifting

Abstract We studied the cascading of two second order processes and its use for frequency conversion (generation of a pulse at frequency ω p — Δω from a signal at ω p + Δω under the action of a pump at ω p ) in a planar Ti-indiffused lithium niobate waveguide. Consistently with the predictions of a numerical model based on the fabrication parameters of the waveguide and the substrate refractive index, we find that the TE 1 mode best serves as the intermediate sum-frequency field for the TM 0 signals and pump. With a propagation length of 17 mm, pulses of 20 ps time duration and λ ≈ 1.1 μm, we observed wavelength conversion with 4% efficiency when the energy of the pump pulse per unit width was about 20 μJ/m.

Picosecond nonlinear optical characterization of GaAs at 1.064 μm

Abstract We report on degenerate four-wave mixing results of differently doped GaAs samples, obtained with 30 ps pulses at λ=1.064 μm. From the value of the effective third order nonlinear susceptibility measured at low fluences we derive the refracting index change per carrier. At higher fluences, two-photon absorption becomes dominant, and the behavior of the reflectivity is accounted for by a numerical model.

Optical study of the transition of doped potassium tantalate

Abstract We report on recent findings obtained through Raman scattering, second harmonic generation and linear birefringence on the polar phase of doped KTaO3. At low temperature, the polar coherence length ranges from a few nanometers to 20 nm for 1.6% and 6% Li doping, respectively and shows no criticality. Comparable figures are found for the case of 12% Na doping. The evolution of low energy Raman spectra and the behaviour under a poling field provides further evidence of phenomena which limit the growth and cooperative ordering of polar microregions.

Ultrashort-pulse investigation of the propagation properties of the LP 11 mode in 1.55-μm communication fibers

The coupling of an ultrashort laser pulse into a single-mode optical communication fiber gives rise to two propagating pulses as a result of the excitation of two guided modes, the fundamental, LP(01) , and the leaky, LP(11) . Such a phenomenon provides a new approach to the study of the propagation properties of the LP(11) mode. An experiment with tunable 110-fs pulses at a wavelength near 1550 nm is described. Information about the group velocity, the polarization-rotation length, the attenuation coefficient, and the cutoff wavelength of the LP(11) mode is obtained in a simple and direct way for various fibers.

Bandwidth and beam quality of barium borate parametric oscillator synchronously pumped by an active-passive modelocked Nd:YAG laser

Abstract We report on the generation of tunable pulses in the near IR through a BBO parametric oscillator pumped by the second harmonic of a conventional active-passive modelocked Nd:YAG laser. Different arrangements will be compared. Without spectral selective elements peak pump conversions of 75% are observed. With a grating, we show that it is possible to achieve frequency widths of the IR pulses comparable to those of the pump itself. The factor M 2 , through which we characterize the spatial quality of the beam, is 1.7 just above threshold, and it does not exceed 2.5 even when the depletion of the pump pulses is larger than 30%.

Characterization of ferroelectric micro-regions in KTaO3:Na

Abstract Low-frequency Raman scattering in the mixed perovskite KTaO3 (Na 12%) is shown to give information on the correlation length of the local order parameter which is in agreement with that obtained by combined measurements of linear birefringence and second harmonic generation. Accordingly, the behavior of this system is interpreted in terms of a frustrated structural transition, characterized by freezing of the polar domain dimension (300A) in the low temperature region.

Correlation of Polar Order in K1-xNaxTaO3

Second-harmonic light generation and birefringence data are reported for KTaO3:Na. They are evaluated in terms of an inhomogeneous lattice polarization. The correlation of polar orientation is shown to extend to hundreds of A in the paraelectric phase of KTaO3:Na. A sharp increase of the correlation length for 15% Na doping to 1500 A is associated to the polar transition at 12 K. From the low-temperature value of the correlation length, conclusions are drawn about the nature of the polar phase.