We experimentally demonstrate phase encoding in SHG with transparent all-dielectric metasurfaces. While a similar task was previously achieved with plasmonic metasurfaces for THG beam shaping, here we obtain three-order-of-magnitude higher generation efficiency without thermal dissipation.
Nanoscale optical integration is nowadays a strategic technological challenge and the ability of generating and manipulating nonlinear optical processes in sub-wavelength volumes is pivotal to realize efficient sensing probes and photonic sources for the next-generation communication technologies. Yet, confining nonlinear processes below the diffraction limit remains a challenging task because phase-matching is not a viable approach at the nanoscale. The localized fields associated to the resonant modes of plasmonic and dielectric nanoantennas offer a route to enhance and control nonlinear processes in highly confined volumes. In my talk I will discuss two nonlinear platforms based on plasmonic and dielectric nanostructures. The first relies on a broken symmetry antenna design, which brings about an efficient second harmonic generation (SHG). We recently applied this concept to an extended array of non-centrosymmetric nanoantennas for sensing applications. I will also show the evidence of a cascaded second-order process in Third Harmonic Generation (THG) in these nanoantennas.
Recently, dielectric nanoantennas emerged as an alternative to plasmonic nanostructures for nanophotonics applications, thanks to their sharp magnetic and electric Mie resonances along with the low ohmic losses in the visible/near-infrared region of the spectrum. I will present our most recent studies on the nonlinear properties of AlGaAs dielectric nanopillars. The strong localized modes along with the broken symmetry in the crystal structure of AlGaAs allow obtaining more than two orders of magnitude higher SHG efficiency with respect to plasmonic nanoantennas with similar spatial footprint and using the same pump power. I will also discuss a few key strategies we recently adopted to optically switch the SHG in these antennas even on the ultrafast time scale. Finally, I will show how to effectively engineer the sum frequency generation via the Mie resonances in these nanoantennas. These results draw a viable blueprint towards room-temperature all optical logic operation at the nanoscale.
We demonstrate photon-pair generation via spontaneous parametric down-conversion (SPDC) from two types of metasurfaces composed by AlGaAs nanocylinders: 1) monolithically fabricated on a selectively oxidized layer of AlAs epitaxially grown on a GaAs wafer; 2) fabricated by reporting the AlGaAs nanostructures on a transparent wafer via wafer bonding. In these samples, we observed SPDC both in back- and forward-scattering configurations, under excitation with a CW pump around 775 nm and single-photon detection on the signal and idler channels. The Bragg modulation of Mie-resonances enables paraxial SPDC, which demonstrates the potential of all-dielectric metasurfaces for quantum applications like on-axis quantum imaging.
All-dielectric optical metasurfaces consist of 2D arrangements of nanoresonators and are of great importance for shaping polarization, phase and amplitude of both linear and harmonic fields. Here, we demonstrate the generation of second harmonic (SH) with zero-order diffraction from nonlinear AlGaAs metasurfaces with spatial period comparable with a pump wavelength in the near-IR. Upon normal incidence of the pump, we demonstrate paraxial SH light into the zero order. SH polarization is effectively controlled via either the meta-atom shape or the pump polarization, with potential applications for on-axis imaging and free-space communication systems.
We demonstrate that reshaping of electromagnetic fields in hyperbolic metamaterials with plasmonic components can be used to engineer strong tunable bulk second order nonlinear response in plasmonic composites with application in high resolution nonlinear tomography. In order to analyze nonlinear response of an anisotropic plasmonic metamaterial slab we compare numerical and analytical calculations with experimental results. The results open up possibilities to design tuneable frequency-doubling integratable metamaterials with the goal to overcome limitations associated with classical phase matching conditions in thick nonlinear crystals.
Second harmonic generation is one of the fundamental nonlinear optical processes that is at the heart of communication and sensing applications. Due to the underlying crystal symmetry, second harmonic generation in noble metal-nanostructures is dominated by metal/dielectric interfaces with only weak (magneto-dipole and quadrupolar) contributions coming from the bulk of the metal inclusions. Here we demonstrate that, in metamaterials, nonlinear contributions from individual plasmonic inclusions can add up together, resulting in the bulk nonlinear polarization. The resulting nonlinear response can be described in terms of volumetric second harmonic polarizability that relates unit-cell averaged nonlinear polarization to a product of unit-cell averaged fundamental fields. The amplitude of this effective nonlinear polarizability is comparable to that of common nonlinear crystals.
In order to analyze nonlinear response of the plasmonic nanowire arrays we compare experimental results to numerical solutions of Maxwell equations where second harmonic response is calculated using nonlinear hydrodynamic model. Numerical solutions of Maxwell equations are also used to analyze the spatial and spectral distributions of fundamental and nonlinear fields across the composites and, in the end, to guide and validate the development of analytical description of effective second harmonic polarizability. The developed analytical description of the second harmonic generation in plasmonic composites opens new avenues for engineering of nonlinear response.
Plasmonic nanowire metamaterials, arrays of aligned plasmonic nanowires grown inside an insulating substrate, have recently emerged as a flexible platform for engineering refraction, diffraction, and density of photonic states, as well as for applications in bio- and acoustic sensing. Majority of unique optical phenomena associated with nanowire metamaterials have been linked to the collective excitation of cylindrical surface plasmons propagating on individual nanowires. From the effective medium standpoint, this collective excitation can be described as an additional electromagnetic wave, emanating from nonlocal effective permittivity of metamaterial. The electromagnetic fields associated with such mode can are strongly inhomogeneous on the scale of the unit cell.
In this work we analyze the effect of the strong field variation inside nanowire metamaterial on second harmonic generation (SHG). We show that second harmonic generation is strongly enhanced in the frequency region where metamaterial is nonlocal. Overall, the composite is predicted to outperform its homogeneous metal counterparts by several orders of magnitude. Quantitative description of SHG in nanowire medium is developed. The results suggest that bulk second harmonic polarizability emerges as result of collective surface-enhanced SHG by individual components of the composite.
Hyperbolic plasmonic metamaterials provide numerous opportunities for designing unusual linear and nonlinear optical properties. Here we report a full vectorial numerical model to study SHG in a plasmonic nanorod metamaterial slab. Our frequency-domain implementation of the hydrodynamic model of the metal permittivity for conduction electrons provided a full description of the nonlinear susceptibility in a broad spectral range. We show that the modal overlap of fundamental and second-harmonic light in the plasmonic metamaterial slab results in the frequency tuneable enhancement of radiated second-harmonic intensity by up to 2 orders of magnitudes for TM- and TE-polarized fundamental light, compared to a smooth Au film under TM-polarised illumination. A double-resonant condition with both the enhancement of fundamental field and the enhanced scattering of the second-harmonic field can be realised at multiple frequencies due to the mode structure of the metamaterial slab. The nanostructured geometry of the Au nanorod metamaterial provides a larger surface area compared to the centrosymmetric crystal lattice of gold, which is needed for exploiting the intrinsic surface nonlinearity of gold. The numerical model allows us to explain experimental investigations on the spectral behaviour and radiation diagram of the second harmonic signal. In the experiments SHG generated under femtosecond excitation with varying wavelength, polarization, and angle of incidence, was characterized in backward and forward directions. We show that the excitation of plasmonic modes in the array can remarkably enhance the nonlinear response of the system, as predicted by the model. The results open up wide ranging possibilities to design tuneable frequency-doubling metamaterial with the goal to overcome limitations associated with classical phase matching conditions in thick nonlinear crystals.
We designed cylindrical AlGaAs-on-aluminium-oxide all-dielectric nanoantennas with magnetic dipole resonance at near-infrared wavelengths. Our choice of material system offers a few crucial advantages with respect to the silicon-oninsulator platform for operation around 1.55μm wavelength: absence of two-photon absorption, high χ(2) nonlinearity, and the perspective of a monolithic integration with a laser. We analyzed volume second-harmonic generation associated to a magnetic dipole resonance in these nanoantennas, and we predict a conversion efficiency exceeding 10-3 with 1GW/cm2 of pump intensity.
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