Plasmonic Interactions at Close Proximity in Chiral Geometries: Route toward Broadband Chiroptical Response and Giant Enantiomeric Sensitivity

Abstract

Chiral metamaterials can have diverse technological applications, such as engineering strongly twisted local electromagnetic fields for sensitive detection of chiral molecules, negative indices of refraction, broadband circular polarization devices, and many more. These are commonly achieved by arranging a group of noble-metal nanoparticles in a chiral geometry, which, for example, can be a helix, whose chiroptical response originates in the dynamic electromagnetic interactions between the localized plasmon modes of the individual nanoparticles. A key question relevant to the chiroptical response of such materials is the role of plasmon interactions as the constituent particles are brought closer, which is investigated in this paper through theoretical and experimental studies. The results of our theoretical analysis, when the particles are brought in close proximity are dramatic, showing a large red shift and enhancement of the spectral width and a near-exponential rise in the strength of the chiroptical response. These predictions were further confirmed with experimental studies of gold and silver nanoparticles arranged on a helical template, where the role of particle separation could be investigated in a systematic manner. The "optical chirality" of the electromagnetic fields in the vicinity of the nanoparticles was estimated to be orders of magnitude larger than what could be achieved in all other nanoplasmonic geometries considered so far, implying the suitability of the experimental system for sensitive detection of chiral molecules. ■ INTRODUCTIONStrong light matter interactions in metallic nanoparticles (NPs), especially those made of noble metals such as gold and silver, is at the heart of much ongoing research in nanophotonics. Individual NPs can support collective excitations (plasmons) of the electron plasma at certain wavelengths, known as the localized surface plasmon resonance (LSPR), 1 which forms the basis of bright colors in colloidal solutions of Au and Ag and provides a powerful platform for various sensing, imaging, and therapeutic technologies. 2,3 It is interesting to note that the optical properties of a collection of NPs can be significantly different from isolated particles, an effect that originates in the electromagnetic interactions between the localized plasmon modes. This problem has been considered in one and two dimensions with periodic, 4−7 as well as with random, 8−10 arrays of NPs, and it was found that as the particles are brought closer, the plasmon resonances show distinct broadening, along with significant red or blue shifts in their spectral positions, where the sign of the shift depends on the direction of polarization with respect to the interparticle axes. With recent advances in nanotechnology, it has now been possible to develop wafer scale methods of fabricating 3-D arrays of NPs, 11,12 whose sizes and spacing could be engineered with high precision. Of great current interest is that of a chiral arrangement of NPs 13−16 that has been shown to give rise to strong chi...

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

“…Experimentally, such helical assemblies were realized using dielectric scaffolds, such as twisted fibers and peptides . A different methods, using glancing angle deposition, was adapted by the Ghosh group …”

Tuning the chiro-plasmonic response using high refractive index-dielectric templates

“…[33][34][35][36][37] In this work, we fabricated helical templates of various dielectric materials, onto which metallic NPs of various sizes were coated. Moreover, the magnitude of the asymmetry factor (g), which is defined as the ratio of the differential absorption (DA) of CPL by the nanostructures to the average absorbance (A) of the nanostructures, was shown to be one of the highest 38,39 reported to date in the literature. CD is defined as the difference in absorption for left (A L ) and right (A R ) CPL, given by CD = DA = A L À A R = log(I R /I L ) E 2(I L À I R )/(I L + I R ), where I L and I R are the intensities of the transmitted light for the two CP components.…”

“…From our earlier studies, 39 we know that the CD response was largely enhanced by altering the arrangement of the plasmonic NPs on the helical template, e.g., by changing the distance between the metallic NPs in a specified chiral geometry. From our earlier studies, 39 we know that the CD response was largely enhanced by altering the arrangement of the plasmonic NPs on the helical template, e.g., by changing the distance between the metallic NPs in a specified chiral geometry.…”

Materials and 3D Designs of Helix Nanostructures for Chirality at Optical Frequencies

“…The tailoring of chiro-optical response down to optical frequencies was achieved by chiral arrangement of plasmonic objects, usually nanospheres, analytically [86][87][88] proposed and experimentally realized, employing metal decorated helical scaffolds obtained by self-assembly of gold nanorods on a fiber backbone [31,[89][90][91][92] or peptide nanotubes [93] and exhibiting strong chiro-optical properties in the visible.…”