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.Comment: 25 pages, 4 figure

Plasmon-enhanced second harmonic sensing

It has been recently suggested that the nonlinear optical processes in plasmonic nanoantennas allow for a substantial boost in the sensitivity of plasmonic sensing platforms. Here we present a sensing device based on an array of non-centrosymmetric plasmonic nanoantennas featuring enhanced second harmonic generation (SHG) integrated in a microfluidic chip. We evaluate its sensitivity both in the linear and nonlinear regime using a figure of merit (FOM = $(\Delta I/I)/\Delta n$) that accounts for the relative change in the measured intensity, \textit{I}, against the variation of the environmental refractive index \textit{n}. While the signal-to-noise ratio achieved in both regimes allows the detection of a minimum refractive index variation $\Delta n_{min} \approx 10^{-3}$, the platform operation in the nonlinear regime features a sensitivity (i.e. the FOM) that is at least 3 times higher than the linear one. Thanks to the surface sensitivity of plasmon-enhanced SHG, our results show that the development of such SHG sensing platforms with sensitivity performances exceeding those of their linear counterparts is within reach.Comment: 19 Pages, 5 Figure

A non-involutory selfduality

We report the effect of the aluminum oxide substrate on the emission of monolithic AlGaAs-on-insulator nonlinear nanoantennas. By coupling nonlinear optical measurements with electron diffraction and microscopy observations, we find that the oxidation-induced stress causes negligible crystal deformation in the AlGaAs nanostructures and only plays a minor role in the polarization state of the harmonic field. This result highlights the reliability of the wet oxidation of thick AlGaAs optical substrates and further confirms the bulk chi(2) origin of second harmonic generation at 1.55 um in these nanoantennas, paving the way for the development of AlGaAs-on-insulator monolithic metasurfaces

Metal–dielectric hybrid nanoantennas for efficient frequency conversion at the anapole mode

Background: Dielectric nanoantennas have recently emerged as an alternative solution to plasmonics for nonlinear light manipulation at the nanoscale, thanks to the magnetic and electric resonances, the strong nonlinearities, and the low ohmic losses characterizing high refractive-index materials in the visible/near-infrared (NIR) region of the spectrum. In this frame, AlGaAs nanoantennas demonstrated to be extremely efficient sources of second harmonic radiation. In particular, the nonlinear polarization of an optical system pumped at the anapole mode can be potentially boosted, due to both the strong dip in the scattering spectrum and the near-field enhancement, which are characteristic of this mode. Plasmonic nanostructures, on the other hand, remain the most promising solution to achieve strong local field confinement, especially in the NIR, where metals such as gold display relatively low losses. Results: We present a nonlinear hybrid antenna based on an AlGaAs nanopillar surrounded by a gold ring, which merges in a single platform the strong field confinement typically produced by plasmonic antennas with the high nonlinearity and low loss characteristics of dielectric nanoantennas. This platform allows enhancing the coupling of light to the nanopillar at coincidence with the anapole mode, hence boosting both second- and third-harmonic generation conversion efficiencies. More than one order of magnitude enhancement factors are measured for both processes with respect to the isolated structure. Conclusion: The present results reveal the possibility to achieve tuneable metamixers and higher resolution in nonlinear sensing and spectroscopy, by means of improved both pump coupling and emission efficiency due to the excitation of the anapole mode enhanced by the plasmonic nanoantenna

Metal–dielectric hybrid nanoantennas for efficient frequency conversion at the anapole mode

Background: Dielectric nanoantennas have recently emerged as an alternative solution to plasmonics for nonlinear light manipulation at the nanoscale, thanks to the magnetic and electric resonances, the strong nonlinearities, and the low ohmic losses characterizing high refractive-index materials in the visible/near-infrared (NIR) region of the spectrum. In this frame, AlGaAs nanoantennas demonstrated to be extremely efficient sources of second harmonic radiation. In particular, the nonlinear polarization of an optical system pumped at the anapole mode can be potentially boosted, due to both the strong dip in the scattering spectrum and the near-field enhancement, which are characteristic of this mode. Plasmonic nanostructures, on the other hand, remain the most promising solution to achieve strong local field confinement, especially in the NIR, where metals such as gold display relatively low losses. Results: We present a nonlinear hybrid antenna based on an AlGaAs nanopillar surrounded by a gold ring, which merges in a single platform the strong field confinement typically produced by plasmonic antennas with the high nonlinearity and low loss characteristics of dielectric nanoantennas. This platform allows enhancing the coupling of light to the nanopillar at coincidence with the anapole mode, hence boosting both second- and third-harmonic generation conversion efficiencies. More than one order of magnitude enhancement factors are measured for both processes with respect to the isolated structure. Conclusion: The present results reveal the possibility to achieve tuneable metamixers and higher resolution in nonlinear sensing and spectroscopy, by means of improved both pump coupling and emission efficiency due to the excitation of the anapole mode enhanced by the plasmonic nanoantenna

Tunable broadband light emission from graphene

Graphene is an ideal material for integrated nonlinear optics thanks to its strong light-matter interaction and large nonlinear optical susceptibility. Graphene has been used in optical modulators, saturable absorbers, nonlinear frequency converters, and broadband light emitters. For the latter application, a key requirement is the ability to control and engineer the emission wavelength and bandwidth, as well as the electronic temperature of graphene. Here, we demonstrate that the emission wavelength of graphene$'$ s broadband hot carrier photoluminescence can be tuned by integration on photonic cavities, while thermal management can be achieved by out-of-plane heat transfer to hexagonal boron nitride. Our results pave the way to graphene-based ultrafast broadband light emitters with tunable emission.Comment: 22 pages, 5 Figure

Imaging spin diffusion in germanium at room temperature

International audienceWe report on the nonlocal detection of optically oriented spins in lightly n-doped germanium at room temperature. Localized spin generation is achieved by scanning a circularly polarized laser beam (λ = 1550 nm) on an array of lithographically defined Pt microstructures. The in-plane oriented spin generated at the edges of such microstructures, placed at different distances from a spin-detection element, allows for a direct imaging of spin diffusion in the semiconductor, leading to a measured spin diffusion length of about 10 μm. Two different spin-detection blocks are employed, consisting of either a magnetic tunnel junction or a platinum stripe where the spin current is converted in an electrical signal by the inverse spin-Hall effect. The second solution represents the realization of a nonlocal spin-injection/detection scheme that is completely free from ferromagnetic functional blocks

The role of segregation in the polarized emission from polyfluorene embedded in a liquid crystal

Polyfluorene (PFO) embedded in a nematic liquid crystal (LC) matrix is investigated. For low PFO weight contents, a homogeneous dispersion is obtained which displays a strong fluorescence anisotropy along the LC director, indicating a significant alignment of the polymeric chains along this direction. Besides, for relatively high PFO weight contents, phase separation takes place. Under these conditions, the sample is composed of micrometer-sized domains, where the two species are in solution, enclosed by segregated polymeric boundaries. By polarized-photoluminescence imaging and spectroscopy, it is found that most of the light emission originates from these boundaries and gets strongly pinned along their orientation. Since boundaries are mainly oriented orthogonal to the LC chains, this morphological alignment results in a system in which the orientation of the polarization emission can be predicted and possibly controlled. Conversely, in the homogeneous sample one can obtain a homogeneous emission polarization by controlling the alignment of the LC. These features are potentially relevant for the development of flexible polarization-sensitive optoelectronic devicesSCOPUS: ar.jinfo:eu-repo/semantics/publishe

Anisotropic optical gain from polyfluorene keto defects induced by a liquid crystalline matrix

info:eu-repo/semantics/nonPublishe

Anisotropic optical gain from polyfluorene keto defects induced by a liquid crystalline matrix

info:eu-repo/semantics/nonPublishe