Octupolar Plasmonic Meta-Molecules for Nonlinear Chiral Watermarking at Subwavelength Scale

Abstract

Nonlinear chiroptical effects of precisely designed chiral plasmonic nanomaterials can be much stronger than such effects observed in the linear regime. We take advantage of this property to demonstrate the use of circularly polarized second-harmonic generation microscopy towards the efficient read-out of a microscopic pattern encoded by an array of triangular gold nanoprisms forming arrangements of adequate chiral symmetry. Strong chiroptical effects are observed in the backscattered second-harmonic generation intensity, enabling clear distinction of the laterally arranged enantiomers, down to nearly 1 µm resolution with an overall intensity contrast of about 40 % (second-harmonic generation circular dichroism of 20 %). Numerical simulations show a noticeable change in the spatial distribution of plasmonic hot spots within the individual nanostructures under excitation by circularly polarized light of different handedness. This leads to rather weak chiroptical effect in the linear backscattering (theoretical circular dichroism not exceeding 3 %), in contrast with the much more significant change of the second-harmonic generation in the far-field (secondharmonic generation circular dichroism from 16 up to 37%). These results open the possibility of designing deeply subwavelength chiral nanostructures for encoding microscopic "watermarks", which cannot be easily accessed by linear optical methods, moreover requiring a nonlinear microscopy set-up for reading out the encoded pattern. Graphic Over the past two decades, research devoted to the second-harmonic generation (SHG) from plasmonic nanostructures evolved from the investigation of hyper-Rayleigh scattering in colloidal suspensions of metal nanoparticles [1-3] towards the measurements of SHG from individual plasmonic nano-objects [4][5][6]. Far-field SHG radiation pattern has been demonstrated as a highly sensitive probe of the nanoscale optical field distribution [7], allowing for the investigation of multipolar plasmon modes [8][9][10], the analysis of surface plasmon polariton (SPP)-mediated interactions [11,12], refractive index sensing [13] and precise characterization of the nanostructure geometry [14][15][16]. In parallel, advanced theoretical tools for SHG modelling were developed, based on numerical methods such as finite elements to solve Maxwell equations and yield the scattered SHG radiation [17][18][19]. Among the geometrical properties that can be addressed by SHG microscopy, chirality stands-out as one of the most interesting probes of light-matter interactions, especially in the context of chiral plasmonic meta-materials which have been reported to exhibit giant optical activity in the linear regime [20,21]. Valev et al. have thus reported extensive work on chirality in SHG from specially patterned superchiral meta-surfaces [7,[22][23][24]. The location of plasmonic hot spots and their influence on the chiroptical response have been thoroughly addressed [22,23], leading to reports of nonlinear chiroptical effects reaching magnitudes as high as 5...

Chirality and Chiroptical Effects in Metal Nanostructures: Fundamentals and Current Trends

“…(Right) Calculated electric field distribution of one of the meta‐molecule enantiomers under LCP (L) and RCP (R) illumination. Reproduced with permission . Copyright 2015, American Chemical Society.…”

Giant Nonlinear Optical Activity of Achiral Origin in Planar Metasurfaces with Quadratic and Cubic Nonlinearities

“…It should be pointed out that even seemingly 2D chiral materials, such as monolayers of chiral thin-fi lm nanostructures are intrinsically 3D chiral as recently demonstrated in the linear [ 24 ] and in the nonlinear optical regimes. [ 23 ] In such nanostructures, chirality arises from the presence of a substrate on one side of the thin-fi lms and not on the other.…”

“…[12][13][14][15] In particular, nonlinear optical CD of metamaterial and plasmonic nanostructures have been demonstrated. [16][17][18][19][20][21][22][23] However, the observed nonlinear optical CD in most experiments arises either from the chirality in 3D nanostructures, or from the extrinsic contribution in the case of 2D structures when the fundamental beam is incident at oblique angles. It should be pointed out that even seemingly 2D chiral materials, such as monolayers of chiral thin-fi lm nanostructures are intrinsically 3D chiral as recently demonstrated in the linear [ 24 ] and in the nonlinear optical regimes.…”

Controlling third harmonic generation with gammadion-shaped chiral metamaterials

“…15 Recently, there is also a growing interest in nonlinear optical signals generated from chiral plasmonic nanostructures, which provide a promising platform to study chiral-selective nonlinear optical interactions. [16][17][18][19][20][21][22] For example, asymmetric SHG from chiral G-shaped gold nanostructures upon a reverse of the handedness of the structures was presented. 16 The interplay of nonlinearity and circular dichroism (CD) was highlighted from the observation of strong cross-phase modulation and four-wave mixing in a type of varactor-loaded nonlinear chiral metamaterial.…”