A. Belardini

Tailored second harmonic generation from self-organized metal nano-wires arrays

Here we report the second harmonic emission properties of self-organized gold nanowires arrays supported on dielectric substrates with a sub-wavelength periodic pattern. The peculiar morphology of the nanowires, which are locally tilted with respect to the average plane of the substrate, allows to generate maximum second harmonic signal at normal incidence with a polarization direction driven by the orientation of the wires (perpendicular to the wires). The generation efficiency was increased by tailoring the growth process in order to tune the metal plasmon resonance close to the pump field frequency and also by increasing the local tilt of the nanowires.

Measurement of the electro-optic properties of poled polymers at λ = 1.55 μm by means of sandwich structures with zinc oxide transparent electrode

We report on the measurement of the electro-optic properties of poled polymers at λ=1.55 μm via the Teng and Man technique. Measurements of the electro-optic coefficient obtained for two different sandwich structures, using either indium tin oxide (ITO) or aluminum doped zinc oxide (ZnO:Al) semitransparent electrodes, are compared. The experimental results show that the use of ITO electrodes can lead to a largely wrong evaluation of the electro-optic coefficient r33, with respect to that obtained when using ZnO:Al electrodes, whose plasma resonance is shifted to longer wavelengths. Results on the disperse red 1/methyl-metacrylate based sidechain benchmark system are reported.

Infrared properties of randomly oriented silver nanowires

We experimentally investigated the infrared properties of a set of randomly oriented silver nanowires films deposited onto glass substrate. Infrared emission of the obtained films was characterized in the long infrared range, i.e., 8–12 μm, by observing their temperature evolution under heating regime with a focal plane array infrared camera as well as a thermocouple. The obtained experimental results showed that the infrared emission from a mesh composed of silver nanowires might be tailored by opportunely assessing preparation condition, such as the metal filling factor. From the theoretical point of view, the real and imaginary part of the electrical permittivity components were retrieved from the calculations of effective permittivities of in-plane randomly oriented metallic wires, thus giving the refractive index and extinction coefficients for the four different silver nanowires meshes. Due to the correspondence between emissivity and absorbance, the experimental results are interpreted with the rec...

Optical limiting behavior of zinc phthalocyanines in polymeric matrix

Phthalocyanines are promising molecules which present very attractive properties for nonlinear optical applications of both second and third orders. Different solutions of zinc phthalocyanine in toluene at increasing concentration were investigated by spectroscopic analysis in order to get the best conditions for the realization of films in poly(methyl methacrylate). Some films of zinc phthalocyanine in polymeric matrix were realized and their nonlinear optical behavior was characterized. The ratio by weight of the compound to the host polymer was varied between 0.004% and 0.15%. In this range all samples show a low degree of aggregation. The nonlinear optical behavior of the films was evaluated by measuring nonlinear transmission. The measurements of nonlinear transmission were carried out at 532nm using a nanosecond Nd:YAG laser followed by an optical parametric oscillator, and a nonlinear response was observed for the two highest concentrations (0.083% and 0.15%). Finally, the nonlinear absorption coef...

Second order nonlinear optical properties of zinc oxide films deposited by low temperature dual ion beam sputtering

We investigated second order optical nonlinearity of zinc oxide thin films, grown on glass substrates by the dual ion beam sputtering technique under different deposition conditions. Linear optical characterization of the films was carried out by spectrophotometric optical transmittance and reflectance measurements, giving the complex refractive index dispersion. Resistivity of the films was determined using the four-point probe sheet resistance method. Second harmonic generation measurements were performed by means of the Maker fringes technique where the fundamental beam was originated by nanosecond laser at λ=1064nm. We found a relatively high nonlinear optical response, and evidence of a dependence of the nonlinear coefficient on the deposition parameters for each sample. Moreover, the crystalline properties of the films were investigated by x-ray diffraction measurements and correlation with second order nonlinearity were analyzed. Finally, we investigated the influence of the oxygen flow rate during...

Chiral light intrinsically couples to extrinsic/pseudo-chiral metasurfaces made of tilted gold nanowires

Extrinsic or pseudo-chiral (meta)surfaces have an achiral structure, yet they can give rise to circular dichroism when the experiment itself becomes chiral. Although these surfaces are known to yield differences in reflected and transmitted circularly polarized light, the exact mechanism of the interaction has never been directly demonstrated. Here we present a comprehensive linear and nonlinear optical investigation of a metasurface composed of tilted gold nanowires. In the linear regime, we directly demonstrate the selective absorption of circularly polarised light depending on the orientation of the metasurface. In the nonlinear regime, we demonstrate for the first time how second harmonic generation circular dichroism in such extrinsic/pseudo-chiral materials can be understood in terms of effective nonlinear susceptibility tensor elements that switch sign depending on the orientation of the metasurface. By providing fundamental understanding of the chiroptical interactions in achiral metasurfaces, our work opens up new perspectives for the optimisation of their properties.

Second-harmonic generation from metallodielectric multilayer photonic-band-gap structures

We experimentally and theoretically investigate the second order nonlinear optical response of metallo-dielectric multilayer structures composed of Ag and Ta2O5 layers, deposited by magnetron sputtering. Second harmonic generation measurements were performed in reflection mode as a function of incidence angle, using femtosecond pulses originating from a Ti:Sapphire laser system tuned at 800 nm. The dependence of the generated signal was investigated as a function of pump intensity and polarization state. Our experimental results show that the conversion efficiency from a periodic metallo-dielectric sample may be enhanced by at least a factor of 30 with respect to the conversion efficiency from a single metal layer, thanks in part to the increased number of active surfaces, pump field localization and penetration inside the metal layers. The conversion efficiency maximum shifts from 70 degrees for the single silver layer down to approximately 55 degrees for the stack. The experimental results are found to be in good agreement with calculations based on coupled Maxwell-Drude oscillators under the action of a nonlinear Lorentz force term.

Complete spatial and temporal locking in phase-mismatched second-harmonic generation

We experimentally demonstrate simultaneous phase and group velocity locking of fundamental and generated second harmonic pulses in Lithium Niobate, under conditions of material phase mismatch. In phase-mismatched, pulsed second harmonic generation in addition to a reflected signal two forward-propagating pulses are also generated at the interface between a linear and a second order nonlinear material: the first pulse results from the solution of the homogeneous wave equation, and propagates at the group velocity expected from material dispersion; the second pulse is the solution of the inhomogeneous wave equation, is phase-locked and trapped by the pump pulse, and follows the pump trajectory. At normal incidence, the normal and phase locked pulses simply trail each other. At oblique incidence, the consequences can be quite dramatic. The homogeneous pulse refracts as predicted by material dispersion and Snells law, yielding at least two spatially separate second harmonic spots at the mediums exit. We thus report the first experimental results showing that, at oblique incidence, fundamental and phase-locked second harmonic pulses travel with the same group velocity and follow the same trajectory. This is direct evidence that, at least up to first order, the effective dispersion of the phase-locked pulse is similar to the dispersion of the pump pulse.

Mapping the nonlinear optical susceptibility by noncollinear second-harmonic generation.

We present a method, based on noncollinear second-harmonic generation, to evaluate the nonzero elements of the nonlinear optical susceptibility. At a fixed incidence angle, the generated signal is investigated by varying the polarization state of both fundamental beams. The resulting polarization charts allows us to verify if Kleinmans symmetry rules can be applied to a given material or to retrieve the absolute value of the nonlinear optical tensor terms, from a reference measurement. Experimental measurements obtained from gallium nitride layers are reported. The proposed method does not require an angular scan and thus is useful when the generated signal is strongly affected by sample rotation.

Femtosecond pulses chirping compensation by using one-dimensional compact multiple-defect photonic crystals

Transmission properties of ultrashort pulses propagating through one-dimensional photonic crystals with multiple defects (cavities) were experimentally investigated. Coupling between multiple cavities is responsible for a wide resonance inside the photonic crystal gap suitable to distortion free propagation of 70fs pulses at 800nm. Geometry-induced anomalous dispersion across the resonance allows a compensation of 15% chirp preinduced on Fourier transform limited 70fs pulses.