(Invited) Novel Optical Phenomena in Nanoplasmonic Arrays
Authors: Biswas Rana , Peer Akshit 
Published: May 2020

Extraordinary optical transmission (EOT) through subwavelength-scale hole arrays in metal films, discovered nearly 20 years ago by Ebbesen and co-workers, has spurred intense effort in nano-plasmonics and nano-optics. To date virtually all the measurements in this field have involved intricate hole arrays in metal films fabricated using complex lithography and etching. This results result in structures less than a millimeter square, which necessitates a complex setup of an optical microscope tied to a spectrophotometer for measurements. We describe a far simpler system that exhibits the EOT, that does not require any lithography, and is scalable to large areas, and generate intense optical fields that serve as a platform for novel luminescent phenomena. We experimentally and theoretically demonstrate that a continuous gold film on a periodically textured substrate of nanocup arrays, surprisingly exhibits extraordinary optical transmission, even though no holes were etched in the film.
To fabricate nanocup arrays we utilized a polycarbonate master with a periodic array of nanocups (of pitch 750 nm) and replicated the inverse of that pattern to a poly-dimethyl-siloxane (PDMS) elastomer. The PDMS elastomer was then used to nanoimprint the nanocup array on a polystyrene film. A thin non-conformal gold film was sputter-deposited on the polystyrene with angle-directed deposition. Although the gold film was continuous, its thickness varied spatially, with the film being thinnest at the bottom of the nanocup. Optical measurements [1] on the gold-coated nanocup array revealed an extraordinary transmission peak at a wavelength just smaller than the period, with an enhancement of ~2.5 compared to the classically expected value, even though there were no holes in the film. Scattering matrix simulations model well the transmission and reflectance measurements when an ultrathin gold layer (~5 nm), smaller than the skin depth, is retained at the bottom of the nanocups. Electric field intensities are enhanced by >100 within the nanocup. The ultrathin gold layer causes transmission through it with an enhancement of the field intensity by ~40.
These arrays are a novel platform for nanoscale luminescent phenomena. Quantum dots embedded within the nanocup arrays show [2] a radiative decay rate that is increased by as much as a factor of 20, from the interaction of the strong optical field with the luminescent quantum dot. In an emerging bio-marker application we demonstrate [3] slowed release rate from medicinal coatings on these nanocups – a feature that could be useful to many emerging drug therapies. Novel metasurfaces of nanocup and nanocone arrays offer a novel platform for nanoplasmonics, nanophotonics, control of light matter interactions, and surface charges [4]. Such nano-arrays hence provide a novel platform for luminescent nano-materials.
We acknowledge the financial support from the National Science Foundation through grants CMMI-1265844 and CMMI-170648.
1. A. Peer and R. Biswas, Nanoscale 8, 4657 - 4666 (2016).
2. A. Peer, Z. Hu, A. Singh, J. A. Hollingsworth, R. Biswas, and H. Htoon, Small 13, 1700660 (2017).
3. A. Peer, R. Dhakal, R. Biswas, J. Kim, Nanoscale Patterning of Biopolymers for functional bio-surfaces and controlled drug release, Nanoscale 8, 18654–18664 (2016).
4. Q. Li, A. Peer, I. Cho, R. Biswas, J. Kim, Replica molding-based nanopatterning of tribocharge on elastomer with application to electrohydrodynamic nanolithography, Nature Communications 9, 974 (2018).

Figure 1