Field enhancement and spectral features of hexagonal necklaces of silver nanoparticles for enhanced nonlinear optical processes

Abstract

The nonlinear properties of hybrid metallic-dielectric systems are attracting great interest due to their potential for the enhancement of frequency conversion processes at nanoscale dimensions. In this work, we theoretically and experimentally address the correlation between the near field distribution of hexagonal plasmonic necklaces of silver nanoparticles formed on the surface of a LiNbO crystal and the second harmonic generation (SHG) produced by this nonlinear crystal in the vicinities of the necklaces. The spectral response of the hexagonal necklaces does not depend on the polarization direction and is characterized by two main modes, the absorptive high-energy mode located in the UV spectral region and the lower energy mode, which is strongly radiant and extends from the visible to the near infrared region. We show that the spatial distribution of the enhanced SHG is consistent with the local field related to the low energy plasmon mode, which spectrally overlaps the fundamental beam. The results are in agreement with the low absorption losses of this mode and the two-photon character of the nonlinear process and provide deeper insight in the connection between the linear and nonlinear optical properties of the hybrid plasmonic-ferroelectric system. The study also highlights the potential of hexagonal necklaces as useful plasmonic platforms for enhanced optical processes at the nanoscale.

Giant Second Harmonic Generation Enhancement by Ag Nanoparticles Compactly Distributed on Hexagonal Arrangements

“…As seen, in both cases a broad resonance with the maximum around λI = 535 nm is observed. In the case of the Ag NPs hexagonal necklaces, the spectrum corresponds to the collective mode sustained by the necklaces as previously reported by the authors [21,37]. In the case of the filled hexagons, the additional spectral band appearing at the low energy side (centered at around λII = 705 nm) can be related to the highly dense Ag NPs distribution decorating the interior of the hexagon.…”

Hybrid Plasmonic–Ferroelectric Architectures for Lasing and SHG Processes at the Nanoscale

“…The far field spectral response of hexagonal necklaces is characterized by two main plasmonic modes: one located in the near UV spectral region, and other broad long wavelength mode that extends from the visible to the NIR region with a dominant radiative character (Figure b). Here, it is worth noting that even when the collective excitation of these modes can be directly correlated with the transverse and longitudinal modes supported by linear chains, the closed nature of the NP arrangements determines the behavior of the plasmonic necklaces with the polarization of the electromagnetic field . For instance, the spectral response of the necklaces does not depend on the polarization direction in agreement with its quasi‐circular character .…”

“…The possibility of assembling 2D plasmonic arrays on top of large areas of functional ferroelectrics is shown in Figure c,d. In this case, by using hexagonal ferroelectric domains as templates, the formation of plasmonic necklaces of Ag NPs is obtained on the polar surface of a LiNbO 3 crystal . The hexagonal necklaces are organized in large spatial regions pointing out the potential of ferroelectric lithography to produce functional plasmonic nanostructures organized into 2D metasurfaces .…”

“…The maximum field enhancement at the gaps is around 100, but for illustrative purposes the plots are saturated at E / E 0 = 10. Adapted with permission . Copyright 2018, Optical Society of America.…”

“…The use of closed arrangements of metallic NPs (closed polygons or necklaces) entails a series of added functions such as polarization insensitive far field response, or light confinement inside the closed arrangements . On the other hand, assembling of functional plasmonic nanostructures into organized 2D superstructures can be at the basis of the next generation of optical devices.…”