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 . 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 . The control mechanism is achieved by taking advantage of the metal-insulator phase transition of VO2 at its critical temperature (~68 °C).
We 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  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 .
The 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.
 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).
 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).
 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)
 C. Wuttke and A. Rauschenbeutel, “Thermalization via Heat Radiation of an Individual Object Thinner than the Thermal Wavelength”, Phys. Rev. Lett. 111, 024301 (2013)
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.
Gold nanowires in general demonstrate very interesting plasmonic properties. Here, by applying the generalized Snell’s law introduced by F. Capasso in 2011, we study how the resonant behavior of the nanowires and their geometrical feature such as the radius of curvature can produce a bent in the propagation direction of a transmitted light beam. The measurements that were performed at a wavelength larger than the nanopatterned features reveal information on the meatusurface morphology.
We studied the infrared properties randomly oriented silver nanowires films deposited onto different substrates. The investigated nanowires have cross-sectional diameters included between few to hundreds nanometers, while their lengths span from some microns to some tenths microns. Several films of silver nanowires were realized and the infrared emission of the obtained films was characterized in the long infrared range, i.e. 8-12 microns, by observing their temperature evolution under heating regime (maximum applied temperature ~90°) with a focal plane array (FPA) infrared camera. Under heating conditions, the apparent temperature of the silver nanowires films qualitatively follows the trend of the corresponding heating source temperature, while the absolute value keeps always somewhat lower. The experimental results show that the choice of metal filling factor may affect the resulting infrared emission and suggest that these coatings are suitable for infrared signature reduction applications.
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.
We present a study on the design, growth and optical characterization of a GaN/AlGaN microcavity for the enhancement
of second order non linear effects. The proposed system exploits the high second order nonlinear optical response of
GaN due to the non centrosymmetric crystalline structure of this material. It consists of a GaN cavity embedded between
two GaN/AlGaN Distributed Bragg Reflectors designed for a reference mode coincident with a second harmonic field
generated in the near UV region (~ 400 nm). Critical issues for this target are the crystalline quality of the material,
together with sharp and abrupt interfaces among the multi-stacked layers. A detailed investigation on the growth
evolution of GaN and AlGaN epilayers in such a configuration is reported, with the aim to obtain high quality factor in
the desiderated spectral range. Non linear second harmonic generation experiments have been performed and the results
were compared with bulk GaN sample, highlighting the effect of the microcavity on the non linear optical response of
In this paper we present a reliable process to fabricate GaN/AlGaN one dimensional photonic crystal (1D-PhC)
microcavities with nonlinear optical properties. We used a heterostructure with a GaN layer embedded between two
AlGaN/GaN Distributed Bragg Reflectors on sapphire substrate, designed to generate a λ= 800 nm frequency downconverted
signal (χ<sup>(2)</sup> effect) from an incident pump signal at λ= 400 nm. The heterostructure was epitaxially grown by
metal organic chemical vapour deposition (MOCVD) and integrates a properly designed 1D-PhC grating, which
amplifies the signal by exploiting the double effect of cavity resonance and non linear GaN enhancement. The integrated
1D-PhC microcavity was fabricate combing a high resolution e-beam writing with a deep etching technique. For the
pattern transfer we used ~ 170 nm layer Cr metal etch mask obtained by means of high quality lift-off technique based
on the use of bi-layer resist (PMMA/MMA). At the same time, plasma conditions have been optimized in order to
achieve deeply etched structures (depth over 1 micron) with a good verticality of the sidewalls (very close to 90°).
Gratings with well controlled sizes (periods of 150 nm, 230 nm and 400 nm respectively) were achieved after the pattern
is transferred to the GaN/AlGaN heterostructure.
We present experimental results on noncollinear second harmonic generation from III-V nitrides
structures, discussing the collinear and noncollinear configuration as a function of polarization
state of both fundamental and generated beams .
In this work we investigated the nonlinear optical properits of a one-dimensional, tranparent metallo-dielectric photonic band gap structure. Specifically, we consider multilayer samples having a nonlinear material as a component. We chose ZnO due to its transparency in the visible region and to its well known optical nonlinearity. By means of dual ion beam sputtering technique we realized a four period sample where the single period consists of ZnO and Ag layers. Sample structure was opportunely designed in order to achieve high nonlinear optical response. A Q-switched frequency doubled Nd:YAG laser has been used in order to investigate the transmissive properties of the sample under high light intensities. A transmission decrease of approximately 50% was found for a maximum incident light intensity of 2 GW/cm<sup>2</sup>.