Evidence of Cascaded Third-Harmonic Generation in Noncentrosymmetric Gold Nanoantennas

Abstract

The optimization of nonlinear optical processes at the nanoscale is a crucial step for the integration of complex functionalities into compact photonic devices and metasurfaces. In such systems, photon upconversion 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 typically represent small perturbations in the light-matter interaction mechanisms. In this work, we demonstrate that this paradigm does not hold for plasmon-enhanced nonlinear optics, by providing evidence of a sum frequency generation process seeded by SHG, which sizably contributes to the overall THG yield.We address this mechanism by unveiling a characteristic fingerprint in the polarization state of the THG emission from non-centrosymmetric gold nanoantennas, which directly reflects the asymmetric distribution of second harmonic fields within the structure and does not depend on the model one employ to describe photon upconversion. We suggest that such cascaded processes may also appear for structures that exhibit only moderate SHG yields. 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.

Harmonic generation at the nanoscale

“…Given the remarkably high efficiency attained at the nanoscale, individual plasmonic nanoantennas were recently exploited also for ultrafast pulse characterization [Extermann et al (2008), Accanto et al (2014), Gennaro et al (2018)]. In perspective, the possibility of finely tuning the geometry of plasmonic nanoantennas using modern lithographic techniques would allow one to further enhance nonlinear processes at the nanoscale through cascaded nonlinear processes [Celebrano et al (2019)] and envision their employment also as nanoscale parametric amplifiers [Zhang et al (2016)].…”

Emission Manipulation by DNA Origami‐Assisted Plasmonic Nanoantennas

“…[20] Plasmonic nanoantennas can be used to generate hot electrons [21] or current [22] and find applications in photochemistry, [23] energy harvesting devices, [24] and photon detectors. [25] Field enhancement in the hotspot near nanoparticle surfaces is used to enhance nonlinear effects [26] such as second harmonic, [27,28] third harmonic signal, [29] coherent anti-Stokes Raman scattering, [30] and four-wave mixing. [31] Likewise, if a plasmonic nanoantenna is combined with a gain material and their modes overlap spatially and spectrally, plasmonic lasing can be achieved.…”

Multiorder Nonlinear Mixing in Metal Oxide Nanoparticles

“…Some of these signals exerted on micro-and nano-structures have been previously observed individually [6][7][8] or in combinations of two or three of them. [9][10][11][12][13][14][15] Very recently, Liu et al reported the simultaneous emission of harmonics and wave-mixing signals from a 3 µm diameter region of a GaAs-based dielectric metasurface optimized for 1.24 and 1.57 µm excitation using a kHz source. 16 Here, using a dual output laser developed for microscopy and sub-micron particles, we demonstrate a similar multi-order phase-coherent response spanning from fifth harmonic generation (FiHG) of a 1300 nm pulse (260 nm) to four-wave mixing (4WM) leading to emission at 2061 nm.…”