G. Leahu

Optimization of thermochromic VO2 based structures with tunable thermal emissivity

In this paper, we design and simulate VO2/metal multilayers to obtain a large tunability of the thermal emissivity of infrared (IR) filters in the typical mid wave IR window of many infrared cameras. The multilayer structure is optimized to realise a low emissivity filter at high temperatures useful for military purposes. The values of tunability found for VO2/metal multilayers are larger than the value for a single thick layer of VO2.

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...

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.

Anomalous optical switching and thermal hysteresis during semiconductor-metal phase transition of VO2 films on Si substrate

We present a detailed infrared study of the semiconductor-to-metal transition (SMT) in a vanadium dioxide (VO2) film deposited on silicon wafer. The VO2 phase transition is studied in the mid-infrared region by analyzing the transmittance and the reflectance measurements, and the calculated emissivity. The reflectance has been measured in two configurations: from the side of the VO2 film and from that of Si wafer. The results show a strong asymmetry between the emissivity in the two configurations, and the fact that the emissivity dynamic range from the silicon side is twice as large than that from the VO2 side. The temperature behaviour of the emissivity during the SMT put into evidence the phenomenon of the anomalous absorption in VO2, which has been explained by applying the Maxwell Garnett effective medium approximation theory.

Thermally induced transmission variations in ZnSe/MgF2 photonic band gap structures

We investigate thermally induced transmission variations in a 3.6-μm-thick ZnSe/MgF2 photonic band gap structure by means of a pump–probe setup, in the 600–700 nm range, under cw pump conditions. An induced temperature increase is responsible for the thermal expansion of the layers, as well as changes in the index of refraction. As a result, the band gap structure is redshifted by several nanometers. The initial transmission of the probe beam was restored following the removal of the pump laser, thus indicating the reversible nature of the process.

Optical response of multilayer thermochromic VO2-based structures

We considered the emissivity properties of VO2 thin films as a function of temperature through several simulations, considering different substrates and multilayer structures. Formulating the concept of emissivity tunability is introduced and we found that a large difference in emissivity, below and above the transition temperature, can be obtained which may be used for the design of medium-wave infrared (3 to 5 μm) filters. Specifically, we optimized a multilayer structure, to function as a low-emissivity filter, at high temperature for the reduction of infrared signature. The values of emissivity changes, found for a VO2/metal multilayer, are larger than the value of a single layer of VO2.

Light scattering from a rough metal surface: theory and experiment

There is still great interest in the determination of microtopographic properties of rough metallic surfaces from light scattering measurements. According to Beckmann-Kirchhoff theory a clear relationship is established between the in-plane angular scattered light intensity and the statistical properties of the surface. We discuss one way to invert this relationship, and we introduce a new iterative procedure to retrieve the height autocorrelation function even for a very rough metallic surface (rms surface roughness of the same order of the optical wavelength). The procedure is eventually applied to the experimental data of a known metallic surface for validation.

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.

Blue second harmonic generation from aluminum nitride films deposited onto silicon by sputtering technique

We studied the second order optical nonlinearity of aluminum nitride films grown by sputtering onto silicon substrates. The crystalline properties of the films were investigated by x-ray diffraction measurements. Preliminary linear optical characterization of the films was carried out by spectrophotometric optical reflectance measurements at different incidence angles; thus the dispersion laws for both ordinary and extraordinary refractive indices were retrieved. Finally, second harmonic generation measurements in reflection mode were performed at a fixed angle from a fundamental beam provided by a picosecond Ti:sapphire laser system at λ=800nm. In the experiments a high blue light conversion efficiency was found for samples 1.5 and 2μm thick, and the second order nonlinear coefficient d33=11±1pm∕V was found.