Coherent Fano resonances in a plasmonic nanocluster enhance optical four-wave mixing

Abstract

Plasmonic nanoclusters, an ordered assembly of coupled metallic nanoparticles, support unique spectral features known as Fano resonances due to the coupling between their subradiant and superradiant plasmon modes. Within the Fano resonance, absorption is significantly enhanced, giving rise to highly localized, intense near fields with the potential to enhance nonlinear optical processes. Here, we report a structure supporting the coherent oscillation of two distinct Fano resonances within an individual plasmonic nanocluster. We show how this coherence enhances the optical four-wave mixing process in comparison with other doubleresonant plasmonic clusters that lack this property. A model that explains the observed four-wave mixing features is proposed, which is generally applicable to any third-order process in plasmonic nanostructures. With a larger effective susceptibility χ (3) relative to existing nonlinear optical materials, this coherent double-resonant nanocluster offers a strategy for designing high-performance thirdorder nonlinear optical media.nanostructured materials | nonlinear optics | phase matching T raditionally, nonlinear optical phenomena have relied on crystalline media that combine material susceptibilities and phase matching to optimize nonlinear optical processes. It has recently been shown that certain plasmonic nanostructures can produce an enhanced nonlinear response when excited at their resonant frequency (1, 2). Phase-matching requirements (3-5) for nonlinear optics in macroscopic media are usually optimally fulfilled at nanoscale dimensions [sinc 2 (Δk z/2) ∼1 for small z, where z is the propagation distance through the medium]. For plasmonic nanostructures, the most important property for the enhancement of nonlinear properties is their increased local fields at resonance, which can provide larger effective susceptibilities than their intrinsic material susceptibility.In the third-order nonlinear process of four-wave mixing (FWM), two external fields E 0 (ω 1 ) and E 0 (ω 2 ) are simultaneously incident on the nanostructure, inducing local fields E(ω 1 ) and E(ω 2 ); absorbing two ω 2 and one ω 1 photons and emitting a photon at ω FWM = 2ω 2 − ω 1 (Fig. 1A) (3). The electromagnetic FWM enhancement G FWM = jE(ω 2 )/E 0 (ω 2 )j 4 · jE(ω 1 )/E 0 (ω 1 )j 2 thus depends on the field enhancements at the input frequencies. Fanoresonant structures can exhibit very large local field enhancements (6, 7), making these structures prime candidates for nonlinear frequency generation. Although previous studies of nonlinear plasmonics used nanostructures with a single dipolar resonance (8, 9), in a multiinput process such as FWM, the conversion efficiency is expected to be further enhanced if the plasmon modes of the nanostructure are resonant with both input frequencies (10).In this study, we demonstrate highly efficient FWM from a plasmonic nanocluster that supports two distinct Fano resonances (FRs) (6,7,(11)(12)(13)(14). When excited by a coherent source, the two spatially coherent FRs oscillate...

Coherent anti-Stokes Raman scattering with single-molecule sensitivity using a plasmonic Fano resonance

“…These simultaneous resonances in the same spatial location result in a 'mixed frequency coherent mode' as previously described on more complex plasmonic clusters 22 .…”

“…Plasmonic Fano resonances by their nature as coupled subradiant and superradiant modes can provide much larger field enhancements than bright modes [22][23][24][25][26] . The substrate we use is a quadrumer, that is, four coupled nanodisks (Fig.…”

“…3). The focused laser spot was B1 mm in diameter, where both incident beams behave like normal-incident plane waves on the quadrumer surface 22 . The CARS emission was collected by an objective in transmission and analysed by a spectrometer.…”

“…3b) from p-MA molecules (B10 3 molecules bound to the Au surfaces of the probe region of the quadrumer structure) consists of three sharp anti-Stokes features on top of a broadband background. The background has two main contributions: a dominant twophoton luminescence signal 29,30 , as well as a much weaker FWM signal 22 , both due to the Au nanostructure ( Supplementary Fig. 5 and Supplementary Note 1).…”

Ultrasmooth, Highly Spherical Monocrystalline Gold Particles for Precision Plasmonics

“…6 Furthermore, the high monodispersity of the particles we demonstrate should facilitate the self-assembly of nanoparticle clusters with uniform linear or non-linear optical resonances, which could in turn be used to fabricate optical metafluids 7 or to investigate enhanced optical four-wave mixing. 37 Narrow size distributions are required not only to control the spectral features, but also the morphology and yield of clusters in certain assembly schemes. 38 A similar cyclic process of anisotropic growth followed by isotropic etching could be used to design spherical crystals of other metals, which might find uses as building blocks for optical sensors and circuits, 18,19 probes for biomedical applications, 17,39 nucleation sites for nanobubble generation, 40,41 and components for other applications in which smoothness, thermal stability, and uniformity of the optical response are critical.…”