arXiv subjects

Costantino De Angelis

Publications and source records attributed to Costantino De Angelis.

28 records · Page 2Linked to original sources

arXiv2020

Silicon metasurfaces for third harmonic geometric phase manipulation and multiplexed holography

Nonlinear wavefront control is a crucial requirement in realizing nonlinear optical applications with metasurfaces. Numerous aspects of nonlinear frequency conversion and wavefront control have been demonstrated for plasmonic metasurfaces. However, several disadvantages limit their applicability in nonlinear nanophotonics, including high dissipative loss and low optical damage threshold. In contrast, it has been shown that metasurfaces made of high-index dielectrics can provide strong nonlinear responses. Regardless of the recent progress in nonlinear optical processes using all-dielectric nanostructures and metasurfaces, much less advancement has been made in realizing a full wavefront control directly with the generation process. Here, we demonstrate the nonlinear wavefront control for the third-harmonic generation with a silicon metasurface. We use a Pancharatnam-Berry phase approach to encode phase gradients and holographic images on nanostructured silicon metasurfaces. We experimentally demonstrate the polarization-dependent wavefront control and the reconstruction of an encoded hologram at the third-harmonic wavelength with high fidelity. Further, we show that holographic multiplexing is possible by utilizing the polarization states of the third harmonic generation. Our approach eases design and fabrication processes and paves the way to an easy to use toolbox for nonlinear optical wavefront control with all-dielectric metasurfaces.

physics.optics↗

arXiv2019

Evidence for cascaded third harmonic generation in non-centrosymmetric gold nanoantennas

The optimization of nonlinear optical processes at the nanoscale is a crucial step for the development of nanoscale photon sources for quantum-optical networks. The development of innovative plasmonic nanoantenna designs and hybrid nanostructures to enhance optical nonlinearities in very small volumes represents one of the most promising routes. In such systems, the upconversion of photons can be achieved with high efficiencies via third-order processes, such as third harmonic generation (THG), thanks to the resonantly-enhanced volume currents. Conversely, second-order processes, such as second harmonic generation (SHG), are often inhibited by the symmetry of metal lattices and of common nanoantenna geometries. SHG and THG processes in plasmonic nanostructures are generally treated independently, since they both represent a small perturbation in the light-matter interaction mechanisms. In this work, we demonstrate that this paradigm does not hold in general, by providing evidence of a cascaded process in THG, which is fueled by SHG and sizably contributes to the overall yield. We address this mechanism by unveiling an anomalous fingerprint in the polarization state of the nonlinear emission from non-centrosymmetric gold nanoantennas and point out that such cascaded processes may also appear for structures that exhibit only moderate SHG yields - signifying its general relevance in plasmon-enhanced nonlinear optics. The presence of this peculiar mechanism in THG from plasmonic nanoantennas at telecommunication wavelengths allows gaining further insight on the physics of plasmon-enhanced nonlinear optical processes. This could be crucial in the realization of nanoscale elements for photon conversion and manipulation operating at room-temperature.

cond-mat.mes-hall↗

arXiv2019

Spontaneous photon-pair generation at the nanoscale

Optical nanoantennas have shown a great capacity for efficient extraction of photons from the near to the far-field, enabling directional emission from nanoscale single-photon sources. However, their potential for the generation and extraction of multi-photon quantum states remains unexplored. Here we demonstrate experimentally the nanoscale generation of two-photon quantum states at telecommunication wavelengths based on spontaneous parametric down-conversion in an optical nanoantenna. The antenna is a crystalline AlGaAs nanocylinder, possessing Mie-type resonances at both the pump and the bi-photon wavelengths and when excited by a pump beam generates photonpairs with a rate of 35 Hz. Normalized to the pump energy stored by the nanoantenna, this rate corresponds to 1.4 GHz/Wm, being one order of magnitude higher than conventional on-chip or bulk photon-pair sources. Our experiments open the way for multiplexing several antennas for coherent generation of multi-photon quantum states with complex spatial-mode entanglement and applications in free-space quantum communications and sensing.

quant-ph↗

arXiv2018

Giant nonlinear response at the nanoscale driven by bound states in the continuum

Being motivated by the recent prediction of high-$Q$ supercavity modes in subwavelength dielectric resonators, we study the second-harmonic generation from isolated subwavelength AlGaAs nanoantennas pumped by a structured light. We reveal that nonlinear effects at the nanoscale can be enhanced dramatically provided the resonator parameters are tuned to the regime of the bound state in the continuum. We predict a record-high conversion efficiency for nanoscale resonators that exceeds by two orders of magnitude the conversion efficiency observed at the conditions of magnetic dipole Mie resonance, thus opening the way for highly-efficient nonlinear metadevices.

physics.optics↗

arXiv2017

Nonlinear anisotropic dielectric metasurfaces for ultrafast nanophotonics

We report on the broadband transient optical response from anisotropic nanobrick amorphous silicon particles, exhibiting Mie-type resonances. A quantitative model is developed to identify and disentangle the three physical processes that govern the ultrafast changes of the nanobrick optical properties, namely two-photon absorption, free-carrier relaxation, and lattice heating. We reveal a set of operating windows where ultrafast all-optical modulation of transmission is achieved with full return to zero in 20 ps. This is made possible due to the interplay between the competing nonlinear processes and despite the slow (nanosecond) internal lattice dynamics. The observed ultrafast switching behavior can be independently engineered for both or- thogonal polarizations using the large anisotropy of nanobricks thus allowing ultrafast anisotropy control. Our results categorically ascertain the potential of all-dielectric resonant nanophotonics as a platform for ultrafast optical devices, and reveal the pos- sibility for ultrafast polarization-multiplexed displays and polarization rotators.

physics.optics↗

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↗

arXiv2014

Mode-matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation

Boosting nonlinear frequency conversion in extremely confined volumes remains a key challenge in nano-optics, nanomedicine, photocatalysis, and background-free biosensing. To this aim, field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase-matching at the nanoscale. Second harmonic generation (SHG) is, however, strongly quenched by the high degree of symmetry in plasmonic materials at the atomic scale and in nanoantenna designs. Here, we devise a plasmonic nanoantenna lacking axial symmetry, which exhibits spatial and frequency mode overlap at both the excitation and the SHG wavelengths. The effective combination of these features in a single device allows obtaining unprecedented SHG conversion efficiency. Our results shed new light on the optimization of SHG at the nanoscale, paving the way to new classes of nanoscale coherent light sources and molecular sensing devices based on nonlinear plasmonic platforms.

cond-mat.mes-hall↗

arXiv2010

Velocity-locked solitary waves in quadratic media

We demonstrate experimentally the existence of three-wave resonant interaction solitary triplets in quadratic media. Stable velocity-locked bright-dark-bright spatial solitary triplets, determined by the balance between the energy exchange rates and the velocity mismatch between the interacting waves, are excited in a KTP crystal.

physics.optics↗

arXiv2010

Nonlinear envelope equation for broadband optical pulses in quadratic media

We derive a nonlinear envelope equation to describe the propagation of broadband optical pulses in second order nonlinear materials. The equation is first order in the propagation coordinate and is valid for arbitrarily wide pulse bandwidth. Our approach goes beyond the usual coupled wave description of $χ^{(2)}$ phenomena and provides an accurate modelling of the evolution of ultra-broadband pulses also when the separation into different coupled frequency components is not possible or not profitable.

physics.optics↗

arXiv2008

Subwavelength Diffraction Management

We study light propagation in nanoscale periodic structures composed of dielectric and metal in the visible range. We demonstrate that diffraction can be tailored both in magnitude and in sign by varying the geometric features of the waveguides. Diffraction management on a subwavelength scale is demonstrated by numerical solution of Maxwell equations in frequency domain.

physics.optics↗