Near-field enhancement of optical second harmonic generation in hybrid gold–lithium niobate nanostructures

Abstract

Nanophotonics research has focused recently on the ability of nonlinear optical processes to mediate and transform optical signals in a myriad of novel devices, including optical modulators, transducers, color filters, photodetectors, photon sources, and ultrafast optical switches. The inherent weakness of optical nonlinearities at smaller scales has, however, hindered the realization of efficient miniaturized devices, and strategies for enhancing both device efficiencies and synthesis throughput via nanoengineering remain limited. Here, we demonstrate a novel mechanism by which second harmonic generation, a prototypical nonlinear optical phenomenon, from individual lithium niobate particles can be significantly enhanced through nonradiative coupling to the localized surface plasmon resonances of embedded gold nanoparticles. A joint experimental and theoretical investigation of single mesoporous lithium niobate particles coated with a dispersed layer of ~10 nm diameter gold nanoparticles shows that a ~32-fold enhancement of second harmonic generation can be achieved without introducing finely tailored radiative nanoantennas to mediate photon transfer to or from the nonlinear material. This work highlights the limitations of current strategies for enhancing nonlinear optical phenomena and proposes a route through which a new class of subwavelength nonlinear optical platforms can be designed to maximize nonlinear efficiencies through near-field energy exchange.

Black Gold Plasmon Response of a Raspberry Shell Grown on Lithium Niobate Nonlinear Nanoparticles

“…SH enhancements are typically observed when the frequency ω of the incoming photons matches resonances in the linear spectrum at ω or 2ω. This has been demonstrated for many plasmonic or hybrid nanostructures ,− and Mie dielectric NPs , and can be explained by an increase of either the local field E ω or the radiated harmonic far-field E 2ω . The following expression (eq ) can be used to qualitatively interpret the SHG spectra response of any nanostructure I 2 ω ∝ ( ∫ E loc 2 false( ω , r false) d V ) 2 ( ∫ E loc 2 false( 2 ω , r false) d V ) …”

“…Several hypotheses were proposed to explain the mechanism, leading to enhanced SHG emission: coupling effects between Au NPs lead to an energy transfer back to the LN or super-radiant plasmon scattering due to the collective interaction of Au NPs upon light excitation. 14 Noteworthily, a high SHG signal at 400 nm was also observed from NPs consisting of a centrosymmetric 300 nm silica core surrounded by an incomplete Au shell, 50 evidencing a rough surface similar to the raspberry-like morphology of the LN@Au NPs. The presence of plasmonic "hot spots" leading to very large field factor enhancement then results in these high SHG signals from the Au surface.…”

“…Recently, Ali et al demonstrated a 32-fold SHG signal enhancement for LN microspheres with Au NPs inserted into their mesoporous structure. Several hypotheses were proposed to explain the mechanism, leading to enhanced SHG emission: coupling effects between Au NPs lead to an energy transfer back to the LN or super-radiant plasmon scattering due to the collective interaction of Au NPs upon light excitation . Noteworthily, a high SHG signal at 400 nm was also observed from NPs consisting of a centrosymmetric 300 nm silica core surrounded by an incomplete Au shell, evidencing a rough surface similar to the raspberry-like morphology of the LN@Au NPs.…”

“…Due to its noncentrosymmetric crystalline structure, LN, which is a mixed-metal oxide exhibiting piezoelectricity, electro-optics, and second-order nonlinear optical properties at the nanoscale, is utilized for both multimodal imaging and phototriggered release of chemotherapeutics. − Combining the intrinsic nonlinear optical properties of noncentrosymmetric oxide materials with plasmonic resonances is an effective way to enhance several nonlinear processes, as already demonstrated for mesoporous LN microspheres, for LN bulk crystals combined with various Au (Au) nanostructures, , and for another dielectric core composed of barium titanate (BaTiO 3 ). − Note that, in the bulk form, hybrid plasmonic waveguides are able to produce giant second harmonic (SH) signals, thanks to the influence of plasmonic resonances on the confined modes. , The encapsulation of LN NPs in a raspberry-like Au nanoshell by a wet chemistry approach has several noticeable advantages. First, it avoids the difficulties related to the nanofabrication of precisely positioned nanostructures and provides a scalable elaboration route . Second, the rough Au surface offers a high density of localized “hot spots” or “disordered nanoantennas”, which may provide an increased SERS effect, a better far-field coupling with long-lived plasmonic modes, a tunable plasmon band over a large range of wavelengths, and high absorption cross sections.…”

“…First, it avoids the difficulties related to the nanofabrication of precisely positioned nanostructures and provides a scalable elaboration route. 14 Second, the rough Au surface offers a high density of localized "hot spots" or "disordered nanoantennas", which may provide an increased SERS effect, 22 a better far-field coupling with long-lived plasmonic modes, 23 a tunable plasmon band over a large range of wavelengths, and high absorption cross sections. This core−shell nanoraspberry structure is thus expected to strongly influence the nonlinear optical properties of LN, such as its wavelength-dependent second harmonic generation (SHG), first hyperpolarizability, and depolarization ratio.…”