Photo-acoustic detection of chirality in metal-polystyrene metasurfaces

Abstract

Nowadays nanophotonics aims towards low-cost, chip-scale devices that can tailor electromagnetic properties, one of which is the control of the circular polarization at the nanoscale, important for novel optical devices. Here we show that nanosphere lithography, combined with tilted metal deposition, can provide novel metasurfaces with chiral properties.We apply the photo-acoustic technique to characterize the circular dichroism at 633 nm of polystyrene nanospheres covered by three different metals: Au-and Cr-covered samples show extrinsic chiral behavior, while the Ag-covered sample shows circular dichroism at normal incidence, characteristic for intrinsic chirality. As the experimental data are in good agreement with numerical predictions, we believe that such design can be optimized to get efficient circularly polarized detection at the nanoscale. THE MANUSCRIPTChirality, a lack of the mirror symmetry of an object 1 , is an important property of some of the building blocks of our world: many molecules, amino-acids, DNA, sugars, drugs are chiral. Two mirror images of the same object differently interact with circularly polarized light of the opposite handedness, while having other measurable properties equal. In particular, chirality can affect the absorption and/or phase velocity of circularly polarized light, therefore it is possible to measure a difference in absorption directly related to the molecules' chirality. This measurement is known as Circular Dichroism (CD). At the nanoscale, when the nanostructures are comparable or smaller than the light wavelength, and organized periodically, they form a metasurface; generally, if the symmetry of the metasurface is broken, a chiral behavior is expected 2 . Chiral metasurfaces can manipulate electromagnetic fields and enhance the interaction with chiral molecules, important for chiral sensing 3 . On the other hand, they can control the polarization state of the optical field, or emit circularly polarized light, thus leading to applications in optical and quantum communications 4 . Geometric features of intrinsically chiral metasurfaces (the nanostructure in the unit cell is usually helix or gammadion-like) can be complicated to fabricate and implement at the nanoscale. This problem can be solved by a proper experimental set-up following the rule that the impinging light wavevector, the average surface normal, and the sample direction must be nonplanar. Such chiral behavior is called extrinsic chirality as it is governed by both experimental set-up and the a) Electronic mail:

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

“…It is important to numerically investigate the origin of the CD behavior. The electromagnetic field behavior can be visualized as a 3D absorption density ρ abs in the unit cell [43]. In Figure 4, the extinction maximum wavelength is investigated, i.e., 693 nm.…”

“…The Scanning Electron Microscopy (SEM) image of this sample is shown in Figure 1a. In [43] we characterized the chiral behavior of similar samples by means of photo-acoustic spectroscopy; moreover, we calculated the CD signal for the same geometry without PNSs with Ag semishells, and noticed CD improvement. This is due to the asymmetric shadow formed on the substrate during the oblique and tilted Ag deposition.…”

“…Starting from a self-assembled 2D metasurface of polystyrene nanospheres (PNSs), many high quality plasmonic nanostructures were fabricated [9,[38][39][40][41][42]. Recently, we showed that the NSL approach combined with tilted metal deposition can lead to extrinsic chirality in hybrid polystyrene-metal metasurfaces [43], while authors in [44] used NSL to obtain giant CD in plasmonic nanocrescents. In [43] we noted that in the case of an Ag-polystyrene metasurface, a small in-plane tilt leads to a measurable CD, even at normal incidence.…”

“…Recently, we showed that the NSL approach combined with tilted metal deposition can lead to extrinsic chirality in hybrid polystyrene-metal metasurfaces [43], while authors in [44] used NSL to obtain giant CD in plasmonic nanocrescents. In [43] we noted that in the case of an Ag-polystyrene metasurface, a small in-plane tilt leads to a measurable CD, even at normal incidence. We further proposed PNS removal for the CD investigation in titled elliptical nanoholes.…”

“Hot” in Plasmonics: Temperature‐Related Concepts and Applications of Metal Nanostructures

“…These measurements usually serve to derive the intensity of absorbed light. Recently, Belardini et al employed photoacoustic techniques to directly measure the intensity of absorbed light in gold nanowires, chiral platinum‐gallium‐carbon nanohelices, in self‐assembled GaAs nanowires and in chiral metal‐polystyrene metasurfaces . Upon absorption in the samples, light causes local heating, which is transferred to the small volume of air surrounding the sample, creating a pressure wave.…”