Giant Circular Dichroism at Visible Frequencies Enabled by Plasmonic Ramp-Shaped Nanostructures

Abstract

Plasmonic nanostructures with spatial symmetry breaking have a variety of applications, from enhancing the enantioselective detection of chiral molecules to creating photonics devices such as circular polarizers. Compared to their molecular counterparts, engineered nanostructures exhibit orders of magnitude larger circular dichroism (CD) at optical frequencies. Although 3D nanostructures such as nanohelices have been reported with high CD at mid-IR frequencies, such high CDs have not yet been achieved at visible frequencies with decent efficiencies. Here, we propose a planar array of plasmonic ramp-shaped nanostructures with an azimuthally gradient depth that exhibits a giant CD and dissymmetry factor at visible frequencies. The structure is fabricated on a gold-coated glass slide using focused ion beam (FIB) with gradient intensity to induce the required gradient depth, hence, breaking symmetry. Optical experimental characterization in the reflection spectrum shows a CD up to 64% and a dissymmetry factor up to 1.13 at 678 nm, in a good agreement with numerical simulations. We envision our proposed structure together with the suggested fabrication method to inspire the design of novel optical devices such as nanoscale circular polarizers and a host of chiral molecules to improve enantioselectivity in the pharmaceutical industry.

Chiral Plasmonics and Their Potential for Point-of-Care Biosensing Applications

“…Despite the advantages of this type of chiral plasmonic structure, setting nanoparticles at customized regions for high chiral interactions constitutes a major hurdle for practical biosensing applications. A recent approach to circumvent this latter issue is the use of plasmonic chiral metasurfaces [46][47][48][49][50][51][52][53][54], where several 2D plasmonic arrays with symmetry broken along the third dimension have been developed. These metasurfaces have their chiral plasmonic near-fields directly accessible to nearby molecules on the surface.…”

“…Figure 2a shows the schematics of a ramp-shaped plasmonic nanostructure. Metasurfaces comprised of a 2D periodic array of these gradient depth unit-cells were produced in Reference [49] using the focused ion beam (FIB) method. Plasmonic resonances were characterized for these nanostructures under LCP and RCP incident light, with high dissymmetries in the reflection spectra at the incident wavelengths λ = 605 nm and λ = 655 nm.…”

Spin-preserving chiral photonic crystal mirror

“…For this to occur, according to a long-held notion, the structure has to be composed of complicated three-dimensional chiral elements, such as helices or, alternatively, made of multi-layer patterns carrying structural chirality. 1,7 To date, several demonstrations of plasmon-assisted intrinsic chiroptical responses in metastructures composed of subwavelength array of 3D chiral shapes 7,[13][14][15] or multilayer patterns of mirrorsymmetry-broken structures [16][17][18][19][20][21] have been reported. Although such structures may exhibit strong and wide-band chiaroptical response, their fabrication is not compatible with 2D patterning techniques.…”

Tunable atom-trapping based on a plasmonic chiral metamaterial

“…Rajaei et al experimentally reported giant circular dichroism (CD) with ramp-shaped plasmonic nanostructures [34]. In the above-mentioned works [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], we find that all designs are based on structure-surface-confined field effects to realize a variety of applications while ignoring spatial effects such as neutral atom trapping, an application that requires a noncontact field to be implemented.…”

“…However, the chiral responses are very weak and typically require oblique incidence. In order to obtain a large chiral response in the case of normal incidence, various novel chiral structures have been designed [31][32][33][34]. For instance, Cui et al demonstrated a giant chiral optical response from a twisted-arc metamaterial [31].…”