We report on a direct experimental observation of dynamic localization (DL) of light in sinusoidallycurved Lithium-Niobate waveguide arrays which provides the optical analog of DL for electrons in periodic potentials subjected to ac electric fields as originally proposed by Dunlap and Kenkre [D.H. Dunlap and V.M. Kenkre, Phys. Rev. B 34, 3625 (1986)]. The theoretical condition for DL in a sinusoidal field is experimentally demonstrated.PACS numbers: 42.82. Et, 63.20.Pw, 42.25.Bs The quantum motion of an electron in a periodic potential subjected to an external field has provided since a long time a paradigmatic model to study fascinating and rather universal coherent dynamical phenomena. These include the long-predicted Bloch oscillations (BO) for dc fields [1], i.e. an oscillatory motion of the wave packet related to the existence of a Wannier-Stark ladder energy spectrum, and the more recently-predicted dynamic localization (DL) for ac fields [2], in which a localized particle periodically returns to its initial state following the periodic change of the field. In recent years, BO have been experimentally observed in a wide variety of systems including semiconductor superlattices [3], atoms in accelerated optical lattices [4], and optical waveguide arrays with a transverse refractive index gradient [5,6,7]. DL is a phenomenon similar to BO which occurs when the electron is subjected to an ac field. The condition for DL, as originally predicted by Dunlap and Kenkre [2] in the nearest-neighbor tight-binding (NNTB) approximation and for a sinusoidal driving field E(t) = F sin(ωt), is that J 0 (Γ) = 0, where Γ = eaF/ ω and a is the lattice period. DL has been shown to be related to the collapse of the quasienergy minibands [8], and the general conditions for DL beyond the NNTB approximation and for generalized ac fields have been identified [9]; DL under the action of both ac and dc fields has been also studied [10], and the influence of excitonic and many-body effects on DL in semiconductor superlattices has been considered (see, e

We describe a novel approach for the fabrication of optical waveguides by focused lowrepetition-rate femtosecond laser pulses. This approach overcomes the main limitation of the technique, i.e., the strong asymmetry of the waveguide profile. By use of an astigmatic beam and suitably controlling both beam waist and focal position in tangential and sagittal planes, it is possible to shape the focal volume in such a way as to obtain waveguides with a circular transverse profile and of the desired size. This technique is applied to the fabrication of active waveguides in Er:Yb-doped glass substrates. The waveguides are single mode at 1.5 m and exhibit propagation losses of 0.25 dB/cm and an internal gain of 1.4 dB at 1534 nm.

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.

Phase-locked single-cycle transients with frequency components between 1 and 60THz and peak fields of up to 12MV/cm are generated as the idler wave of a parametric amplifier. To achieve broadband conversion in GaSe nonlinear crystals, we match the group velocities of signal and idler components. The influence of group-velocity dispersion is minimized by long-wavelength pumping at 1.18mum. Free-space electro-optic sampling monitors the multiterahertz waveforms with direct field resolution.

We study by femtosecond pump-probe microscopy the transient plasmonic response of individual gold nanoantennas fabricated by electron-beam lithography on a glass substrate. By exploiting the capability of the fabrication technique to control geometrical parameters at the nanoscale, we tuned the plasmonic resonance in a broad wavelength range, from the visible to the infrared. Numerical simulations based on a three-temperature model (3TM) for the electrons and lattice dynamics, combined with * To whom correspondence should be addressed full-wave numerical analysis and semiclassical theory of optical transitions in the solid state, are compared with the measurements on a single gold nanoantenna probed at different wavelengths. The agreement between the experiment and the prediction of the 3TM turns out to be comparable to that achievable with the more sophisticated Boltzmann equation formalism. We also investigate the influence of the plasmon detuning with respect to the pump and probe wavelengths on the nonlinear optical response using different nanoantennas. Quantitative comparison of the experimental data with the theoretical model also provides a disentanglement of the different contributions to the optical nonlinearity of gold giving rise to the complex features observed in the transient optical response. Our study provides a complete analysis of the physical mechanisms dominating the nonlinear plasmon dynamics of an individual nanoobject taking place on a few ps time scale.

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.

Spontaneous Raman (SR) microscopy allows label-free chemically specific imaging based on the vibrational response of molecules; however, due to the low Raman scattering cross section, it is intrinsically slow. Coherent Raman scattering (CRS) techniques, by coherently exciting all the vibrational oscillators in the focal volume, increase signal levels by several orders of magnitude. In its single-frequency version, CRS microscopy has reached very high imaging speeds, up to the video rate; however, it provides an information which is not sufficient to distinguish spectrally overlapped chemical species within complex heterogeneous systems, such as cells and tissues. Broadband CRS combines the acquisition speed of CRS with the information content of SR, but it is technically very demanding. This paper reviews the current state of the art in broadband CRS microscopy, both in the coherent anti-Stokes Raman scattering (CARS) and the stimulated Raman scattering (SRS) versions. Different technical solutions for broadband CARS and SRS, working both in the frequency and in the time domains, are compared and their merits and drawbacks assessed.2

A highly simplified architecture for stimulated-Raman-scattering microscopy is demonstrated, where multiple tunable narrowband picosecond pulses are generated by spectral compression of femtosecond pulses emitted by a single compact Er-fiber oscillator. The system provides high sensitivity (2x10(-7)) and spectral resolution (sub-15 cm(-1)), and it offers an unprecedented flexibility for multicolor imaging.