Enhanced third harmonic generation from the epsilon-near-zero modes of ultrathin films

We experimentally demonstrate efficient third harmonic generation from an indium tin oxide (ITO) nanofilm (lambda/42 thick) on a glass substrate for a pump wavelength of 1.4 um. A conversion efficiency of 3.3x10^-6 is achieved by exploiting the field enhancement properties of the epsilon-near-zero (ENZ) mode with an enhancement factor of 200. This nanoscale frequency conversion method is applicable to other plasmonic materials and reststrahlen materials in proximity of the longitudinal optical phonon frequencies.Comment: 13 pages, 5 figure

Second and Third Harmonic Generation in Metal-Based Nanostructures

We present a new theoretical approach to the study of second and third harmonic generation from metallic nanostructures and nanocavities filled with a nonlinear material, in the ultrashort pulse regime. We model the metal as a two-component medium, using the hydrodynamic model to describe free electrons, and Lorentz oscillators to account for core electron contributions to both the linear dielectric constant and to harmonic generation. The active nonlinear medium that may fill a metallic nanocavity, or be positioned between metallic layers in a stack, is also modeled using Lorentz oscillators and surface phenomena due to symmetry breaking are taken into account. We study the effects of incident TE- and TM-polarized fields and show that a simple re-examination of the basic equations reveals additional exploitable dynamical features of nonlinear frequency conversion in plasmonic nanostructures.Comment: 33 pages, including 11 figures and 74 references; corrected affiliations and some typo

Sub-wavelength diffraction-free imaging with low-loss metal-dielectric multilayers

We demonstrate numerically the diffraction-free propagation of sub-wavelength sized optical beams through simple elements built of metal-dielectric multilayers. The proposed metamaterial consists of silver and a high refractive index dielectric, and is designed using the effective medium theory as strongly anisotropic and impedance matched to air. Further it is characterised with the transfer matrix method, and investigated with FDTD. The diffraction-free behaviour is verified by the analysis of FWHM of PSF in the function of the number of periods. Small reflections, small attenuation, and reduced Fabry Perot resonances make it a flexible diffraction-free material for arbitrarily shaped optical planar elements with sizes of the order of one wavelength.Comment: 5 pages, 4 figure

Singularity-driven Second and Third Harmonic Generation in a {\epsilon}-near-zero nanolayer

We show a new path to {\epsilon}~0 materials without resorting to metal-based metamaterial composites. A medium that can be modeled using Lorentz oscillators usually displays {\epsilon}=0 crossing points, e.g. {\epsilon}=0 at {\lambda}~7{\mu}m and 20{\mu}m for SiO2 and CaF2, respectively. We show that a Lorentz medium yields a singularity-driven enhancement of the electric field followed by dramatic lowering of thresholds for a plethora of nonlinear optical phenomena. We illustrate the remarkable enhancement of second and third harmonic generation in a layer of {\epsilon}~0 material 20nm thick, and discuss the role of nonlinear surface sources

Optical limiting sensor based on multilayer optimization of Ag/VO2 phase changing material

An optical limiting sensor working in the infrared was developed to address the need for eye and sensor protection against laser threats. Metallic and dielectric photonic resonators (thin-film multilayers) incorporating phase-change-materials (PCM) like VO2 were simulated and experimentally realized, with optimization of the deposition procedure by RF magnetron sputtering at low temperature. For the first time, the silver is placed between the substrate and the VO2 , thus improving the device limiting performances. By maximizing the difference of transmittance between the ‘ON’ and the ‘OFF’ states at the standard light wavelength for telecom applications (1550 nm), we calculated optimum thickness for VO2 and Ag films. The deposited thin films were characterized by SEM and Raman spectroscopy, and VO2 transition temperature was investigated by measuring resistance changes. As a proof of concept of the device working principle, we calculated transmittance drop of 70% when the sensor is exposed to a laser pulse excitation (20ps, 500 MW/cm^2 ). Our results pave the way for multilayer with optical limiting properties

Second harmonic generation from metallo-dielectric 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

Enhancement and Suppression of Transmission in 3-D Nanoslits Arrays with 1-and 2-D Periodicities

ABSTRACT We investigate the transmission properties of arrays of three-dimensional (3-D) gold patches having one-and twodimensional (1-and 2-D) periodicities, and describe the interaction of cavity and surface plasmon modes. We vary the main geometrical parameters to assess similarities and emphasize differences between 1-D and 2-D periodic patterns. We analyze the spectral response as a function of incident angle and polarization to corroborate our findings. We will also consider form and air filling factors of the grating to assess our ability to control the transmission spectrum. In particular, we observe strong inhibition of the transmission when the impinging wave-vector parallel to the surface of the metal matches the surface plasmon wave-vector of the unperturbed air-gold interface when added to the grating lattice wave-vector. This phenomenon favors the opening of a plasmonic band gap, featuring the suppression of transmission and simultaneous coupling to back-radiation (reflections) of the unperturbed surface plasmon. High-Q, resonating modes occur at the edges of the forbidden band, boosting the energy transfer across the grating thus providing enhanced transmission and broadside directivity at the exit side of the grating

Specialized astrocytes mediate glutamatergic gliotransmission in the CNS

Multimodal astrocyte–neuron communications govern brain circuitry assembly and function1. For example, through rapid glutamate release, astrocytes can control excitability, plasticity and synchronous activity2,3 of synaptic networks, while also contributing to their dysregulation in neuropsychiatric conditions4–7. For astrocytes to communicate through fast focal glutamate release, they should possess an apparatus for Ca2+-dependent exocytosis similar to neurons8–10. However, the existence of this mechanism has been questioned11–13 owing to inconsistent data14–17 and a lack of direct supporting evidence. Here we revisited the astrocyte glutamate exocytosis hypothesis by considering the emerging molecular heterogeneity of astrocytes18–21 and using molecular, bioinformatic and imaging approaches, together with cell-specific genetic tools that interfere with glutamate exocytosis in vivo. By analysing existing single-cell RNA-sequencing databases and our patch-seq data, we identified nine molecularly distinct clusters of hippocampal astrocytes, among which we found a notable subpopulation that selectively expressed synaptic-like glutamate-release machinery and localized to discrete hippocampal sites. Using GluSnFR-based glutamate imaging22 in situ and in vivo, we identified a corresponding astrocyte subgroup that responds reliably to astrocyte-selective stimulations with subsecond glutamate release events at spatially precise hotspots, which were suppressed by astrocyte-targeted deletion of vesicular glutamate transporter 1 (VGLUT1). Furthermore, deletion of this transporter or its isoform VGLUT2 revealed specific contributions of glutamatergic astrocytes in cortico-hippocampal and nigrostriatal circuits during normal behaviour and pathological processes. By uncovering this atypical subpopulation of specialized astrocytes in the adult brain, we provide insights into the complex roles of astrocytes in central nervous system (CNS) physiology and diseases, and identify a potential therapeutic target

Optical Properties of Ordered Plasmonic Nanostructures in Linear and Nonlinear Regimes

ABSTRACT In this paper we review our recent advances in subwavelength plasmonic structures in linear and nonlinear regimes. We show several examples of subwavelength plasmonic devices in different configurations and schemes. We emphasize how these devices can be exploited for many applications, such as sensing, spectroscopy, photovoltaics and optical communications. Finally we will illustrate some challenges that are still open and need to be fulfilled. Keywords: plasmonics, plasmonic resonance, scattering, field enhancement, thin film, nonlinear harmonic generation. INTRODUCTION The propagation of Surface Plasmon Polaritons (SPPs) has gained great interest in the last decades since it has been demonstrated that their excitation on metal gratings or metal nanostructures can be usefully exploited to solve several problems relative to many applications, such as Raman spectroscopy, sensing, data storage, light emission and amplification as well as absorption enhancement for energy harvesting purpose