Circular Dichroism in Low-Cost Plasmonics: 2D Arrays of Nanoholes in Silver

Circular dichroism in a plasmonic array of elliptical nanoholes with square lattice

Chiral properties of plasmonic metasurfaces, especially related to different absorption of left and right circularly polarized light leading to circular dichroism (CD), are a research hot topic in nanophotonics. There is often a need to understand the physical origin of CD for different chiral metasurfaces, and to get guidelines for the design of structures with optimized and robust CD. In this work, we numerically study CD at normal incidence in square arrays of elliptic nanoholes etched in thin metallic layers (Ag, Au, Al) on a glass substrate and tilted with respect to the symmetry axes. Strong CD arises in absorption spectra at the same wavelength region of extraordinary optical transmission, indicating highly resonant coupling between light and surface plasmon polaritons at the metal/glass and metal/air interfaces. We elucidate the physical origin of absorption CD by a careful comparison of optical spectra for different polarizations (linear and circular), with the aid of static and dynamic simulations of local enhancement of the electric field. Furthermore, we optimize the CD as a function of the ellipse parameters (diameters and tilt), the thickness of the metallic layer, and the lattice constant. We find that silver and gold metasurfaces are most useful for CD resonances above 600 nm, while aluminum metasurfaces are convenient for achieving strong CD resonances in the short-wavelength range of the visible regime and in the near UV. The results give a full picture of chiral optical effects at normal incidence in this simple nanohole array, and suggest interesting applications for chiral biomolecules sensing in such plasmonic geometries.

Polarization-sensitive photonic bandgaps in hybrid one-dimensional photonic crystals composed of all-dielectric elliptical metamaterials and isotropic dielectrics

Photonic bandgaps (PBGs) in conventional one-dimensional (1-D) photonic crystals (PhCs) composed of isotropic dielectrics are polarization-insensitive since the optical length within a isotropic dielectric layer is polarization-independent. Herein, we realize polarization-sensitive PBGs in hybrid 1-D PhCs composed of all-dielectric elliptical metamaterials (EMMs) and isotropic dielectrics. Based on the Bragg scattering theory and iso-frequency curve analysis, an analytical model is established to characterize the angle dependence of PBGs under transverse magnetic and transverse electric polarizations. The polarization-dependent property of PBGs can be flexibly controlled by the filling ratio of one of the isotropic dielectrics within all-dielectric EMMs. Assisted by the polarization-sensitive PBGs, high-performance polarization selectivity can be achieved. Our work offers a loss-free platform to achieve polarization-sensitive physical phenomena and optical devices.

Characterization of Chirality in Diffractive Metasurfaces by Photothermal Deflection Technique

Chirality, a lack of mirror symmetry, is present in nature at all scales; at the nanoscale, it governs the biochemical reactions of many molecules, influencing their pharmacology and toxicity. Chiral substances interact with left and right circularly polarized light differently, but this difference is very minor in natural materials. Specially engineered, nanostructured, periodic materials can enhance the chiro-optical effects if the symmetry in their interactions with circular polarization is broken. In the diffraction range of such metasurfaces, the intensity of diffracted orders depends on the chirality of the input beam. In this work, we combine a photothermal deflection experiment with a novel theoretical framework to reconstruct both the thermal and optical behavior of chiro-optical behavior in diffracted beams.

Experiments and simulations of chiro-optical response in lowcost nanohole arrays in silver

2D metasurfaces based on periodic nanoholes in metal have been proposed in various plasmonic platforms. Specifically, their resonant features have led to applications spanning in biosensing. Here we investigate additional degree of freedom in elliptical nanohole arrays with hexagonal geometry: chiro-optical effects. Namely, the in-plane asymmetry and a slightly elliptical shape of nanoholes were previously shown to differently extinct light of opposite handedness, even at normal incidence. We now fully characterize nanoholes in Ag, fabricated by low-cost nanosphere lithography. We first measure the dependence of the transmitted intensity for opposite handedness, in a broad spectral and angle of incidence range. We then resolve the circular polarization degree of the transmitted light when the nanohole array is excited with linear polarization. Finally, we numerically investigate the origin of the chiro-optical effect at the nanoscale. We believe that circular polarization resolving of the transmitted degree could be further adapted as a highly sensitive tool in chiral sensing.

Plasmonic Elliptical Nanohole Arrays for Chiral Absorption and Emission in the Near-Infrared and Visible Range

Chiral plasmonic nanostructures with tunable handedness-dependent absorption in the visible and infrared offer chiro-optical control at the nanoscale. Moreover, coupling them with emitting layers could lead to chiral nanosources, important for nanophotonic circuits. Here, we propose plasmonic elliptical nanohole arrays (ENHA) for circularly dependent near-infrared and visible emission. We first investigate broadband chiral behavior in an Au-ENHA embedded in glass by exciting it with plane waves. We then study the coupling of ENHA with a thin emitting layer embedded in glass; we focus on the emission wavelengths which provided high chirality in plane-wave simulations. Our novel simulation set-up monitors the chirality of the far-field emission by properly averaging a large set of homogeneously distributed, randomly oriented quantum sources. The intrinsic chirality of ENHA influences the circular polarization degree of the emitting layer. Finally, we study the emission dependence on the field distribution at the excitation wavelength. We demonstrate the chiral absorption and emission properties for Au-ENHA emitting in the near-infrared range, and for Ag-ENHA which is excited in green range and emits in the Lumogen Red range. The simple geometry of ENHA can be fabricated with low-cost nanosphere lithography and be covered with emission gel. We thus believe that this design can be of great importance for tunable chiral nanosources.

Opposite Polarity Surface Photovoltage of MoS2 Monolayers on Au Nanodot versus Nanohole Arrays

We prepared MoS₂ monolayers on Au nanodot (ND) and nanohole (NH) arrays. Both these sample arrays exhibited enhanced photoluminescence intensity compared with that of a bare SiO₂/Si substrate. The reflectance spectra of MoS₂/ND and MoS₂/NH had clear features originating from excitation of localized surface plasmon and propagating surface plasmon polaritons. Notably, the surface photovoltages (SPV) of these hybrid plasmonic nanostructures had opposite polarities, indicating negative and positive charging at MoS₂/ND and MoS₂/NH, respectively. Surface potential maps, obtained by Kelvin probe force microscopy, suggested that the potential gradient led to a distinct spatial distribution of photo-generated charges in these two samples under illumination. Furthermore, the local density of photo-generated excitons, as predicted from optical simulations, explained the SPV spectra of MoS₂/ND and MoS₂/NH. We show that the geometric configuration of the plasmonic nanostructures modified the polarity of photo-generated excess charges in MoS₂. These findings point to a useful means of optimizing optoelectronic characteristics and improving the performance of MoS₂-based plasmonic devices.

Increasing the circular dichroism of the planar chiral nanostructure by inducing coupling between the coverage layer and the planar nanostructure

Circular dichroism (CD) has been widely studied in recent decades because of its wide application in biomedical detection. Nanostructures with different heights (NDH) usually increase the transmission CD effect. To achieve such nanostructures, one needs to repeatedly perform the electron-beam lithography (EBL) method twice or more, layer-by-layer, which is a very complicated process. Here, we propose a method to prepare NDH by combining the EBL and oblique angle deposition (OAD) techniques. L-shaped planar silver nanostructures are prepared using EBL and normal electron beam deposition, and the OAD method is then used to partially cover one arm of the L-shaped nanostructure. Numerical simulations reveal that the height difference in the two arms of the L-shaped NDH (LSNDH) causes a difference in the polarization directions of the left- (LCP) and right-circularly polarized (RCP) incident light, thereby, generating CD effects. A 2D material is used to cover the LSNDH to further increase the charge polarization direction differences, which considerably increases the CD effect. These results are useful in simplifying and increasing the convenience of the preparation method of 3D chiral nanostructures. Furthermore, the proposed nanostructure may have potential application in biosensor, such as chiral enantiomer sensors.