M. C. Larciprete

Accessing the optical limiting properties of metallo-dielectric photonic band gap structures

The optical limiting properties of a one-dimensional, transparent metallodielectric photonic band gap structure are studied. Due to light confinement in the structure that enhances the nonlinear response of the layers, a nonlinear transmission dependence on the incident light intensity is found. Experimental results are reported for a four period sample where the single period consists of ZnO and Ag layers 109 and 17 nm thick, respectively. The structure was designed to exhibit a transmission resonance at 532 nm. Under the action of a Q-switched frequency doubled Nd:yttrium–aluminum–garnet laser, a decrease in transmission of approximately 50% is obtained for a maximum incident light intensity of 2u2002GW/cm2. These results are explained in terms of a dynamic change of the absorption coefficient due to the enhancement of the two-photon absorption process. These results suggest that the structure is suitable for optical limiting applications in the visible range.

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.

Pre-fractal multilayer structure for polarization- insensitive temporally and spatially coherent thermal emitter

We investigate electromagnetic wave propagation through one-dimensional stacks arranged as truncated pre-fractal Cantor multilayer. Taking into account materials dispersion as well as real absorptive losses, we studied the spectral and spatial emissivity in both on-axis and off-axis direction. The typical cavity mode resonances associated to the pre-fractal structure are exploited to design a polarization-insensitive infrared emitter pertaining both temporal and spatial coherence.

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.

Realization and characterization of tetra(tert-butyl) zinc phtalocyanine poly(methyl methacrylate) films for optical limiting applications

We investigated optical limiting effects in films of tetra(tert-butyl) zinc phtalocyanine dissolved into poly(methyl methacrylate) within a 0.15% maximum weight ratio. Spectroscopic analysis was performed on the realized films in order to retrieve the corresponding linear optical absorption coefficient, whose interpolation according to a model including a set of three Lorentzian oscillators enabled the evaluation of the degrees of monomer and dimer concentrations. Thereafter, nonlinear transmission measurements were carried out at two different wavelengths, 532 and 634 nm. All the investigated films showed a nonlinear optical response, which was found to increase as the resonant condition was approached.

Polarization insensitive infrared absorbing behaviour of one-dimensional multilayer stack: a fractal approach

The control and tailoring of infrared absorbance/emittance is a crucial task for all those applications involving thermal radiation management and detection. We theoretically investigated the peculiar absorbing/emitting behaviour of pre-fractal Cantor multilayers, in order to design a polarization-insensitive multilayer stack absorbing over a wide angular lobe in the mid wavelength infrared range (8-10 μm). Using transfer matrix method, we explored the spectral properties arising from both the material and the geometrical dispersion. We considered several combinations of the constituent materials: SiO2 was combined with TiO2 and Si, respectively.

Semiconductor-metal phase transition of vanadium dioxide nanostructures on silicon substrate: Applications for thermal control of spacecraft

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 (MIR) region by analyzing the transmittance and the reflectance measurements, and the calculated emissivity. 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, together with a strong hysteresis phenomenon, both useful to design tunable thermal devices to be applied for the thermal control of spacecraft. We have also applied the photothermal radiometry in order to study the changes in the modulated emissivity induced by laser. Experimental results show how the use of these techniques represent a good tool for a quantitative measurement of the optothermal properties of vanadium dioxide based structures.

Photothermal induced variations in the optical transmission of a photonic band gap structure

We consider an application of the photothermal effect for the realization of an optical limiter with a photonic band gap (PBG) structure. By means of a pump–probe setup, we investigate the optical transmission of 12 periods of a ZnSe/MgF2 PBG structure under different heating conditions. The investigated spectral range (600–700 nm) corresponds to a band edge in the transmission spectrum. The induced temperature increase is responsible for both thermal expansion of the layers and refractive index changes, thus redshifting the transmission curve by several nanometers. The initial transmission of the probe beam is restored after pump laser removal, thus indicating the reversible nature of the observed changes. The maximum transmission variation we obtained is 40% of signal at 660 nm. A theoretical reconstruction of the transmission spectra was calculated for different temperatures in order to evaluate the temperature increase ΔT corresponding to the observed shift. In order to have a confirmation, ΔT has als...

Tuning thermal emission in metamaterials composed by oriented polar inclusions

We investigated a metamaterial composed by silicon carbide (SiC) subwavelength oriented wires, onto silicon substrate in the mid- to long- infrared range. A simple but versatile method was developed and implemented, combining homogenization techniques with the transfer matrix method for birefringent layered materials to model an effective medium layer where different inclusions content (filling factor) as well as different shape and orientation of inclusions (depolarization factors) are taken into account. The typical spectral features associated to oriented inclusions are thus exploited to design a selective emissivity feature, pertaining sharp resonances at infrared wavelengths. Taming and tuning the strength and the position of the phonon resonance of polar materials allows the design of versatile optical elements and infrared filters, in order to design an effective medium with specifically tuned emissive properties.

Spatial and spectral control of infrared thermal emission in VO2-based composite nanoantennas (Conference Presentation)

The possibility to control the infrared (IR) absorption and thermal emission on subwavelength scales has attracted large interest in the recent years thanks to the opportunities granted by nanostructured metamaterials. For example highly frequency selective and directional thermal emitters/absorbers have been proposed for a large variety of applications ranging from sensing and security to cooling and energy harvesting [1]. In order to introduce a control of the emissivity as a function of the temperature thermochromic and phase change materials have been considered. In particular Vanadium dioxide (VO2) has become a widely-studied material for applications such as metamaterials, smart windows and supercapacitors, thanks to its strong optical transmittance changes at the IR and THz regions and huge resistance jump [2]. The control mechanism is achieved by taking advantage of the metal-insulator phase transition of VO2 at its critical temperature (~68u2009°C). nnWe numerically show the control of spatial and spectral features of the far field thermal emission pattern of nanoantenna arrays, composed of alternating Gold and VO2 rods, as a function of the temperature. In this work we performed a numerical study by modifying a previously developed model [3] based on the fluctuational electrodynamics approach and on the discretization of the resulting volume integral equation to calculate relative emissivity and spatial emission pattern of nanoparticle ensembles smaller than the thermal wavelength lam=hc/kBT [4].nnThe drastic changes of the VO2 refractive index across its metal-insulator phase transition produce strong differences in the behavior of the overall system by creating or destroying evanescent wave coupling between different elements of the nanoantenna. The study of IR thermal nano-emitters is crucial for the realization of coherent thermal nano-sources in the mid and far IR for sensing applications and thermal management as well as thermal logic gates on the nanoscale.nnReferences:nn[1] I. E. Khodasevych, L. Wang, A. Mitchell, and G. Rosengarten, Micro- and nano- structured surfaces for selective solar absorption, Adv. Opt. Mat. 3, 852 (2015).n[2] Liu, M. et al. Phase transition in bulk single crystals and thin films of VO2 by nanoscale infrared spectroscopy and imaging. Phys. Rev. B 91, 245155 (2015).n[3] M. Centini, A. Benedetti, M. C. Larciprete, A. Belardini, R. Li Voti, M. Bertolotti, C. Sibila, “Midinfrared thermal emission properties of finite arrays of gold dipole nanoantennas” Phys. Rev. B 92(20) 205411 (2015)n[4] C. Wuttke and A. Rauschenbeutel, “Thermalization via Heat Radiation of an Individual Object Thinner than the Thermal Wavelength”, Phys. Rev. Lett. 111, 024301 (2013)