arXiv subjects

Michael Scalora

Publications and source records attributed to Michael Scalora.

At least 37 records · Page 2Linked to original sources

arXiv2017

Surface Plasmon Excitation of Second Harmonic light: Emission and Absorption

We aim to clarify the role that absorption plays in nonlinear optical processes in a variety of metallic nanostructures and show how it relates to emission and conversion efficiency. We define a figure of merit that establishes the structure's ability to either favor or impede second harmonic generation. Our findings suggest that, despite the best efforts embarked upon to enhance local fields and light coupling via plasmon excitation, nearly always the absorbed harmonic energy far surpasses the harmonic energy emitted in the far field. Qualitative and quantitative understanding of absorption processes is crucial in the evaluation of practical designs of plasmonic nanostructures for the purpose of frequency mixing.

physics.optics↗

arXiv2015

Unlocking the full potential of wave-matter nonlinear coupling in the epsilon-near-zero regime

In recent years, unconventional metamaterial properties have triggered a revolution of electromagnetic research which has unveiled novel scenarios of wave-matter interaction. A very small dielectric permittivity is a leading example of such unusual features, since it produces an exotic static-like regime where the electromagnetic field is spatially slowly-varying over a physically large region. The so-called epsilon-near-zero metamaterials thus offer an ideal platform where to manipulate the inner details of the "stretched" field. Here we theoretically prove that a standard nonlinearity is able to operate such a manipulation to the point that even a thin slab produces a dramatic nonlinear pulse transformation, if the dielectric permittivity is very small within the field bandwidth. The predicted non-resonant releasing of full nonlinear coupling produced by the epsilon-near-zero condition does not resort to any field enhancement mechanisms and opens novel routes to exploiting matter nonlinearity for steering the radiation by means of ultra-compact structures.

physics.optics↗

arXiv2015

One-dimensional chirality: strong optical activity in epsilon-near-zero metamaterials

We suggest that electromagnetic chirality, generally displayed by 3D or 2D complex chiral structures, can occur in 1D patterned composites whose components are achiral. This feature is highly unexpected in a 1D system which is geometrically achiral since its mirror image can always be superposed onto it by a 180 deg rotation. We analytically evaluate from first principles the bi-anisotropic response of multilayered metamaterials and we show that the chiral tensor is not vanishing if the system is geometrically one-dimensional chiral, i.e. its mirror image can not be superposed onto it by using translations without resorting to rotations. As a signature of 1D chirality, we show that 1D chiral metamaterials support optical activity and we prove that this phenomenon undergoes a dramatic non-resonant enhancement in the epsilon-near-zero regime where the magneto-electric coupling can become dominant in the constitutive relations.

physics.optics↗

arXiv2015

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.

physics.optics↗

arXiv2014

Fano collective resonance as complex mode in a two dimensional planar metasurface of plasmonic nanoparticles

Fano resonances are features in transmissivity/reflectivity/absorption that owe their origin to the interaction between a bright resonance and a dark (i.e., sub-radiant) narrower resonance, and may emerge in the optical properties of planar two-dimensional (2D) periodic arrays (metasurfaces) of plasmonic nanoparticles. In this Letter, we provide a thorough assessment of their nature for the general case of normal and oblique plane wave incidence, highlighting when a Fano resonance is affected by the mutual coupling in an array and its capability to support free modal solutions. We analyze the representative case of a metasurface of plasmonic nanoshells at ultraviolet frequencies and compute its absorption under TE- and TM-polarized, oblique plane-wave incidence. In particular, we find that plasmonic metasurfaces display two distinct types of resonances observable as absorption peaks: one is related to the Mie, dipolar resonance of each nanoparticle; the other is due to the forced excitation of free modes with small attenuation constant, usually found at oblique incidence. The latter is thus an array-induced collective Fano resonance. This realization opens up to manifold flexible designs at optical frequencies mixing individual and collective resonances. We explain the physical origin of such Fano resonances using the modal analysis, which allows to calculate the free modes with complex wavenumber supported by the metasurface. We define equivalent array dipolar polarizabilities that are directly related to the absorption physics at oblique incidence and show a direct dependence between array modal phase and attenuation constant and Fano resonances. We thus provide a more complete picture of Fano resonances that may lead to the design of filters, energy-harvesting devices, photodetectors, and sensors at ultraviolet frequencies.

physics.optics↗

arXiv2014

Nonlocal and Quantum Tunneling Contributions to Harmonic Generation in Nanostructures: Electron Cloud Screening Effects

Our theoretical examination of second and third harmonic generation from metal-based nanostructures predicts that nonlocal and quantum tunneling phenomena can significantly exceed expectations based solely on local, classical electromagnetism. Mindful that the diameter of typical transition metal atoms is approximately 3Å, we adopt a theoretical model that treats nanometer-size features and/or sub-nanometer size gaps or spacers by taking into account: (i) the limits imposed by atomic size to fulfill the requirements of continuum electrodynamics; (ii) spillage of the nearly-free electron cloud into the surrounding vacuum; and (iii) the increased probability of quantum tunneling as objects are placed in close proximity. Our approach also includes the treatment of bound charges, which add crucial, dynamical components to the dielectric constant that are neglected in the conventional hydrodynamic model, especially in the visible and UV ranges, where interband transitions are important. The model attempts to inject into the classical electrodynamic picture a simple, perhaps more realistic description of the metal surface by incorporating a thin patina of free-electrons that screens an internal, polarizable medium.

physics.optics↗

arXiv2013

Dramatic Enhancement of Third-Harmonic Generation in Plasmonic Nanostructures via Nonlocal Effects

Classical nonlocality in conducting nanostructures has been shown to dramatically alter the linear optical response, by placing a fundamental limit on the maximum field enhancement that can be achieved. This limit directly extends to all nonlinear processes, which depend on field amplitudes. A study of third-harmonic generation in metal film-coupled nanowires reveals that for sub-nanometer vacuum gaps the nonlocality enhances the effective nonlinearity by four orders of magnitude as the field penetrates deeper inside the metal than that predicted assuming a purely local electronic response.

cond-mat.mes-hall↗

arXiv2013

Quantum Conductivity for Metal-Insulator-Metal Nanostructures

We present a methodology based on quantum mechanics for assigning quantum conductivity when an ac field is applied across a variable gap between two plasmonic nanoparticles with an insulator sandwiched between them. The quantum tunneling effect is portrayed by a set of quantum conductivity coefficients describing the linear ac conductivity responding at the frequency of the applied field and nonlinear coefficients that modulate the field amplitude at the fundamental frequency and its harmonics. The quantum conductivity, determined with no fit parameters, has both frequency and gap dependence that can be applied to determine the nonlinear quantum effects of strong applied electromagnetic fields even when the system is composed of dissimilar metal nanostructures. Our methodology compares well to results on quantum tunneling effects reported in the literature and it is simple to extend it to a number of systems with different metals and different insulators between them.

physics.optics↗

arXiv2013

Second harmonic double resonance cones in dispersive hyperbolic metamaterials

We study the formation of second harmonic double-resonance cones in hyperbolic metamaterials. An electric dipole on the surface of the structure induces second harmonic light to propagate into two distinct volume plasmon-polariton channels: A signal that propagates within its own peculiar resonance cone; and a phase-locked signal that is trapped under the pump's resonance cone. Metamaterial dispersion and birefringence induce a large angular divergence between the two volume plasmon-polaritons, making these structures ideal for subwavelength second and higher harmonic imaging microscopy.

physics.optics↗

arXiv2013

Polariton excitation in epsilon-near-zero slabs: transient trapping of slow light

We numerically investigate the propagation of a spatially localized and quasi-monochromatic electromagnetic pulse through a slab with Lorentz dielectric response in the epsilon-near-zero regime, where the real part of the permittivity vanishes at the pulse carrier frequency. We show that the pulse is able to excite a set of virtual polariton modes supported by the slab, the excitation undergoing a generally slow damping due to absorption and radiation leakage. Our numerical and analytical approaches indicate that in its transient dynamics the electromagnetic field displays the very same enhancement of the field component perpendicular to the slab, as in the monochromatic regime. The transient trapping is inherently accompanied by a significantly reduced group velocity ensuing from the small dielectric permittivity, thus providing a novel platform for achieving control and manipulation of slow light.

physics.optics↗

arXiv2013

Low-damping epsilon-near-zero slabs: nonlinear and nonlocal optical properties

We investigate second harmonic generation, low-threshold multistability, all-optical switching, and inherently nonlocal effects due to the free-electron gas pressure in an epsilon-near-zero (ENZ) metamaterial slab made of cylindrical, plasmonic nanoshells illuminated by TM-polarized light. Damping compensation in the ENZ frequency region, achieved by using gain medium inside the shells' dielectric cores, enhances the nonlinear properties. Reflection is inhibited and the electric field component normal to the slab interface is enhanced near the effective pseudo-Brewster angle, where the effective ε-near-zero condition triggers a non-resonant, impedance-matching phenomenon. We show that the slab displays a strong effective, spatial nonlocality associated with leaky modes that are mediated by the compensation of damping. The presence of these leaky modes then induces further spectral and angular conditions where the local fields are enhanced, thus opening new windows of opportunity for the enhancement of nonlinear optical processes.

physics.optics↗

arXiv2012

Electric Field Enhancement in ε-near-zero Slabs under TM-Polarized Oblique Incidence

We investigate local field enhancement phenomena in subwavelength, ε-near-zero (ENZ) slabs that do not exploit Fabry-Pérot resonances. In particular, we study the linear response of engineered metamaterial slabs of finite thickness based on plasmonic nanoshells that show an ENZ band in the visible range, and naturally occurring materials (e.g., SiO2) that also display ENZ properties, under oblique, TM-polarized plane wave incidence. We then introduce active gain material in engineered metamaterial slabs that adds peculiar spectral and angular features to transmission, reflection, and absorption properties, and leads to a further local field enhancement. These findings are supported by two theoretical studies: First, a simple interface between two semi-infinite media, namely free space and a generic ENZ medium; then, an ENZ slab of finite thickness, with the aim of understanding the system's behavior when varying the ENZ properties as well as the incident angle. For either case we report three distinct physical conditions for which we explain spectral and angular features that might result in strong field enhancement. The gain-assisted metamaterial implementation has the potential of triggering and enhancing low-threshold nonlinear phenomena thanks to the large local fields found at specific frequency and angular bands.

physics.optics↗

arXiv2012

Gain assisted harmonic generation in near-zero permittivity metamaterials made of plasmonic nanoshells

We investigate enhanced harmonic generation processes in gain-assisted, near-zero permittivity metamaterials composed of spherical plasmonic nanoshells. We report the presence of narrow-band features in transmission, reflection and absorption induced by the presence of an active material inside the core of the nanoshells. The damping-compensation mechanism used to achieve the near-zero effective permittivity condition also induces a significant increase in field localization and strength and, consequently, enhancement of linear absorption. When only metal nonlinearities are considered, second and third harmonic generation efficiencies obtained by probing the structure in the vicinity of the near-zero permittivity condition approach values as high as for irradiance value as low as . These results clearly demonstrate that a relatively straightforward path now exists to the development of exotic and extreme nonlinear optical phenomena in the KW/cm2 range

physics.optics↗

arXiv2012

Origin of second-harmonic generation enhancement in optical split-ring resonators

We present a study of the second-order nonlinear optical properties of metal-based metamaterials. A hydrodynamic model for electronic response is used, in which nonlinear surface contributions are expressed in terms of the bulk polarization. The model is in good agreement with published experimental results, and clarifies the mechanisms contributing to the nonlinear response. In particular, we show that the reported enhancement of second-harmonic in split-ring resonator based media is driven by the electric rather than the magnetic properties of the structure.

physics.optics↗

arXiv2011

Experimental demonstration of a novel bio-sensing platform via plasmonic band gap formation in gold nano-patch arrays

We discuss the possibility of implementing a novel bio-sensing platform based on the observation of the shift of the leaky surface plasmon mode that occurs at the edge of the plasmonic band gap of metal gratings when an analyte is deposited on top of the metallic structure. We provide experimental proof of the sensing capabilities of a two-dimensional array of gold nano-patches by observing color variations in the diffracted field when the air overlayer is replaced with a small quantity of Isopropyl Alcohol (IPA). Effects of rounded corners and surface imperfections are also discussed. Finally, we also report proof of changes in color intensities as a function of the air/filling ratio of the structure and discuss their relation with the diffracted spectra.

physics.optics↗

arXiv2011

Harmonic generation and energy transport in dielectric and semiconductors at visible and UV wavelengths: the case of GaP

We study inhibition of absorption, transparency, energy and momentum transport of the inhomogeneous component of harmonic pulses in dielectrics and semiconductors, at visible and UV wavelengths, focusing on materials like GaP. In these spectral regions GaP is characterized by large absorption, metallic behavior or a combination of both. We show that phase locking causes the generated inhomogeneous signals to propagate through a bulk metallic medium without being absorbed, that is occurs even in centrosymmetric materials via the magnetic Lorentz force, and that the transport of energy and momentum is quite peculiar and seemingly anomalous. These results make it clear that there are new opportunities in ultrafast nonlinear optics and nano-plasmonics in new wavelength ranges.

physics.optics↗

arXiv2009

Surface spontaneous parametric down-conversion

Surface spontaneous parametric down-conversion is predicted as a consequence of continuity requirements for electric- and magnetic-field amplitudes at a discontinuity of chi2 nonlinearity. A generalization of the usual two-photon spectral amplitude is suggested to describe this effect. Examples of nonlinear layered structures and periodically-poled nonlinear crystals show that surface contributions to spontaneous down-conversion can be important.

quant-ph↗

arXiv2009

Transmission function properties for multi-layered structures: Application to super-resolution

We discuss the properties of the transmission function in the k-space for a generic multi-layered structure. In particular we analytically demonstrate that a transmission greater than one in the evanescent spectrum (amplification of the evanescent modes) can be directly linked to the guided modes supported by the structure. Moreover we show that the slope of the phase of the transmission function in the propagating spectrum is inversely proportional to the ability of the structure to compensate the diffraction of the propagating modes. We apply these findings to discuss several examples where super-resolution is achieved thanks to the simultaneous availability of the amplification of the evanescent modes and the diffraction compensation of the propagating modes.

physics.optics↗