Boosting nonlinear frequency conversion in extremely confined volumes remains a crucial task in nano-optics, all-optical signal processing and nanomedicine. Photon-photon interactions at the nanoscale are intrinsically weak and can only be excited using very high light intensities. Optical materials featuring large nonlinearities and low absorption losses offer a valuable solution to circumvent this limitation and effectively generate nonlinear effects in nanoscale volumes. Metal-less nanophotonics has recently raised an increasing interest because the optical response of high-permittivity dielectric nanoparticles exhibits negligible dissipative losses and strong magnetic multipole resonances in the visible and near-IR. We have recently proposed all-dielectric AlGaAs-on-AlOx nanodisks and coupled dimers featuring an intense second harmonic generation (SHG) with conversion efficiency up to 10-^5 upon pumping with an ultrafast Erbium-doped fiber laser centered at 1554 nm (150 fs pulses, 80 MHz repetition rate) at 1.6 GW/cm^2 fluency, which is in excellent agreement with our numerical simulations.
To further enhance the SHG efficiency, we propose a metal-dielectric hybrid nanostructure based on a nanodisk surrounded by a gold plasmonic nanoring. The role of the plasmonic nanoantenna is to efficiently collect the illumination light and convert it to match the anapole mode supported by the AlGaAs nanodisk, to significantly amplify the electric field inside the disk. We theoretically predict and experimentally demonstrate that the SHG conversion efficiency in this nanostructure can be boosted by one order of magnitude with respect to the isolated nanodisk.
The optimization of nonlinear optical processes at the nanoscale is a key challenge in nanoscience. In this framework, plasmon-enhanced nonlinear effects together with the development of innovative nanoantenna designs and hybrid nanostructures are receiving a lot of attention [1-2]. We recently devised a plasmonic nanoantenna working in the near-infrared region of the electromagnetic spectrum, which allows boosting the SHG efficiency. This is achieved by optimizing the nanoantenna geometry to feature (i) a double resonant response at both the excitation and emission wavelengths, (ii) a spatial overlap between the modes involved in the process and (iii) a broken symmetry, to enable dipole-allowed SHG. We found that this nanoantenna behaves like a strongly coherent nanoscale light source, featuring a marked THG along with an intense SHG [3-4].
We find evidence of a SHG-mediated cascaded effect in THG . We have identified a THG polarization behavior that strongly deviates from that of a bulk (3)-mediated effect and unveils a significant contribution coming from the cascaded coherent sum of a SHG photon and a pump photon.
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The last couple of years have witnessed an unprecedented boost in nonlinear nanophotonics, allowed both by constant technological evolution in the fabrication of integrated micro- and nano-structures, and by a deeper understanding of multipolar magnetic and electric response of all-dielectric sub-wavelength nanoparticles. Besides reviewing the recent progress in the hybrid and monolithic semiconductor platforms that are being employed to demonstrate parametric sources on chip, we will focus on high-contrast nonlinear nanophotonic sources in the near-infrared spectral region, where plasmonic devices fall short of broad applicability. To confine photons at the nanoscale by total internal reflection, metal-less devices rely on a high-refractive-index core surrounded by low-index cladding in one or more dimensions, and therefore they typically consist of semiconductor nanostructures that either lie on an oxide substrate or are suspended in air. We will report on our results on χ(2) AlGaAs suspended nanowires  and AlGaAs-on-AlOx nanoantennas , with a perspective view on collective nonlinear effects in 1D and 2D arrays. Finally we will comment on the most sought applications of integrated parametric sources, including optical frequency comb generators and optical parametric oscillators on-chip with sub-milliwatt threshold.
 N. Morais, I. Roland, M. Ravaro, I. Favero, A. Lemaître, S. Wabnitz, M. De Rosa, and G. Leo, SPIE Nanophotonics Australasia 2017, Melbourne 10-13/12/2017.
 V. F. Gili, L. Carletti, A. Locatelli, D. Rocco, M. Finazzi, L. Ghirardini, I. Favero, C. Gomez, A. Lemaître, M. Celebrano, C. De Angelis, and G. Leo, Opt. Express 24, 15965 (2016).
We demonstrate monolithic aluminum gallium arsenide (AlGaAs) optical nanoantennas for enhanced second harmonic generation (SHG) at telecom wavelengths. From measurements on nanocylinders of 400 nm height and varying radius pumped with femtosecond pulses delivered at 1554-nm wavelength, we estimated a peak conversion efficiency exceeding 10<sup>−5</sup>. Our measurements are in excellent agreement with frequency-domain numerical simulations, revealing the microscopic nature of the SHG process in our nanoresonators.
We study nonlinear optical response of nanofabricated gold particles with sub-100-nm spatial resolution by
means of non-linear near-field scanning optical microscopy (NSOM). In our instrument, femtosecond pulses at 800 nm
wavelength are coupled to hollow-pyramid aperture sensors. Such probes show high throughput and preserve pulse
duration and polarization, enabling the achievement of sufficiently high peak power in the near field to perform
nonlinear optics on the nanoscale. We study second-harmonic generation (SHG) from gold nanoparticles of two different
kinds, namely, closely-packed gold triangles and nanoellipsoids. We find a strong dependence of SHG efficiency on the
shape and the fine structure of the nanoparticles. Near-field SHG is therefore a subwavelength resolution probe of local
field enhancements occurring at specific sites of the particles. This work is focused on the dependence of NSOM linear
and nonlinear images on the aperture size and linear polarization direction of light. Our measurements give strong
evidence that SHG is mainly excited by a high field concentration at the rims of the metal NSOM aperture. This
conclusion is supported by the high spatial resolution obtained for SHG even with apertures so large that FW imaging
shows much poorer resolution.
Proc. SPIE. 5927, Plasmonics: Metallic Nanostructures and Their Optical Properties III
KEYWORDS: Gold, Nanostructures, Optical microscopes, Second-harmonic generation, Femtosecond phenomena, Spectroscopy, Near field scanning optical microscopy, Near field, Nonlinear optics, Near field optics
An aperture-type near-field optical microscope based on hollow-pyramid cantilevered probes has been developed and optimized for ultrafast nonlinear nanospectroscopy applications. These probes have many advantages for near-field microscopy such as higher throughput, higher thermal damage threshold, and absence of pulse chirping. The input pulse duration (as short as 30 fs from a mode-locked, stretched cavity 26 MHz Ti:Sapphire oscillator) is maintained beyond the aperture. Such short pulses, combined with the high peak powers available at the output of hollow-pyramid probes, allow experiments of nonlinear microscopy and spectroscopy with higher spatial and temporal resolution as compared to similar experiments based on optical fiber tips. Results on second-harmonic generation by gold nanostructures and BBO nonlinear crystals are reported demonstrating a spatial resolution down to 100 nm with 40 fs pulses. Implications for local femtosecond time-resolved pump-probe spectroscopy are anticipated.