Hansik Yun

Plasmonic cavity-apertures as dynamic pixels for the simultaneous control of colour and intensity

Despite steady technological progress, displays are still subject to inherent limitations in resolution improvement and pixel miniaturization because a series of colours is generally expressed by a combination of at least three primary colour pixels. Here we propose a structure comprising a metal cavity and a nanoaperture, which we refer to as a cavity-aperture, to simultaneously control the colour and intensity of transmitted light in a single pixel. The metal cavity constructs plasmonic standing waves to organize the spatial distribution of amplitudes according to wavelength, and the nanoaperture permits light with a specific wavelength and amplitude to pass through it, depending on the nanoaperatures relative position in the cavity and the polarization state of the incident light. Therefore, the cavity-aperture has the potential to function as a dynamic colour pixel. This design method may be helpful in developing various photonic devices, such as micro-imaging systems and multiplexed sensors.

Near-field focus steering along arbitrary trajectory via multi-lined distributed nanoslits

The modulation of near-field signals has recently attracted considerable interest because of demands for the development of nano-scale optical devices that are capable of overcoming the diffraction limit of light. In this paper, we propose a new type of tuneable plasmonic lens that permits the foci of surface plasmon polariton (SPP) signals to be continuously steered by adjusting the input polarization state. The proposed structure consists of multi-lined nanoslit arrays, in which each array is tilted at a different angle to provide polarization sensitivity and the nanoslit size is adjusted to balance the relative amplitudes of the excited SPPs from each line. The nanoslits of each line are designed to focus SPPs at different positions; hence, the SPP focal length can be tuned by modifying the incident polarization state. Unlike in previously reported studies, our method enables plasmonic foci to be continuously varied with a smooth change in the incident linear polarization state. The proposed structures provide a novel degree of freedom in the multiplexing of near fields. Such characteristics are expected to enable the realization of active SPP modulation that can be applied in near-field imaging, optical tweezing systems, and integrated nano-devices.

Polarization-Independent Plasmon-Induced Transparency in a Symmetric Metamaterial

We propose a novel mechanism for generating electromagnetically induced transparency (EIT) in a symmetric metamaterial. The proposed metamaterial consists of double nanorod pairs, perpendicular to each other in the form of a 2-by-2 matrix. This metamaterial shows a polarization-independent EIT-like effect without breaking its symmetry. Since conventional plasmonic EIT metamaterials induce transparency in the transparent window through the coupling between the localized surface plasmon resonance modes, most of existing structures have asymmetric geometry. This limitation means the EIT-like effect can occur only under a particular linearly polarized incident light. The proposed structure, however, has a simple geometry and polarization independence, so it has the potential to be used in high sensitivity sensors and to slow light in the visible spectral range.

Plasmonics in Nanoslit for Manipulation of Light

A nanoslit is capable of performing versatile functions in nanophotonic applications. In this invited paper, we discuss the physics and manipulation methods of surface plasmon excitation such as directional switching at a nanoslit. Furthermore, enhancing the light intensity passing through a nanoslit by employing embedded metallic nanoislands is experimentally presented and numerically analyzed.

Broadband ultrathin circular polarizer at visible and near-infrared wavelengths using a non-resonant characteristic in helically stacked nano-gratings

Modern imaging and spectroscopy systems require to implement diverse functionalities with thin thickness and wide wavelength ranges. In order to meet this demand, polarization-resolved imaging has been widely investigated with integrated circular polarizers. However, the circular polarizers which operate at the entire visible wavelengths and have a thickness of several tens of nanometers have not been developed yet. Here, a circular polarizer, operating at the entire visible wavelength range, is demonstrated using helically stacked aluminum nano-grating layers. High extinction ratio and broad operation bandwidth are simultaneously achieved by using non-resonant anisotropic characteristics of the nano-grating. It is theoretically verified that the averaged extinction ratio becomes up to 8 over the entire visible wavelength range while having a thickness of 390 nm. Also, the feasibility of the proposed structure and circular polarization selectivity at the visible wavelength range are experimentally verified. It is expected that the proposed structure will lead to extreme miniaturization of a circular polarizer and contribute greatly to the development of mobile/wearable imaging systems such as virtual reality and augmented reality displays.

Site-selective synthesis of silver nanoparticles in pre-patterned trenches and their localized surface plasmon resonances

A method for depositing silver nanoparticles in a pre-patterned trench by site-selective synthesis is described. In the trench patterns with various shapes, silver nanoparticles can be selectively nucleated and grown only on polyvinylpyrrolidone (PVP) domains by attraction (or repulsion) between silver ions and the hydrophilic PVP island domains in a silica matrix of the trench (or the hydrophobic fluorosilane layer). Regarding the silver nanoparticles in the trench, localized surface plasmon resonance (LSPR) could be excited by obliquely incident light, reradiating the enhanced electromagnetic field in the far- and near-fields. Even in the case of a large angle incidence in total internal reflection (TIR), the patterned silver nanoparticle clusters underwent strong scattering with a high intensity, due to the LSPR effect.

Intermediate plasmonic characteristics in a quasi-continuous metallic monolayer.

There has been a significant interest on plasmonics in a metallic structure with very narrow gaps for studies of nanophotonics. However, little attention has been paid to the behavior of surface plasmons (SPs) in quasi-continuous metallic structures. This study observes and analyzes intermediate characteristics between propagating SPs (PSPs) and localized SPs (LSPs) in a quasi-continuous metallic monolayer of core-shell nanocubes. We reveal that, in a very narrow region of few-nanometer gaps among the nanocubes, the intrinsic energy bands of PSPs and LSPs intersect each other to generate two hybrid bands and an anti-crossing. Using a self-assembly method instead of the lithographic techniques which have several limitations as of now, we materialize the quasi-continuous metallic layer with plenty of nano-gaps that exhibit intermediate plasmonic characteristics. The intermediate plasmonic characteristics observed in this study will lead to interesting subjects, such as band engineering and slow SPs, in nanophotonics.

Switchable beaming from a nanoslit with metallic gratings controlled by the phase difference between incident beams

We propose a switching method for optical beaming generated from a metal slit surrounded by surface gratings. The principle of the method is based on the interference of diffracted surface plasmon polaritons from the gratings which are controlled by the relative phases of two oblique incident beams that are illuminated on the metal slit. By adjusting the relative position of the interference pattern of the incident beams with respect to the metal slit, beaming from the proposed structure can be switched from the on- to the off-mode by virtue of the change in the symmetry of the generated surface plasmon polaritons. An experimental demonstration of the method is presented in which an electrically controlled interferometric configuration is used.

Hybrid states of propagating and localized surface plasmons at silver core/silica shell nanocubes on a thin silver layer.

Hybrid characteristics of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs) appear at a combined structure of a thin silver (Ag) layer and silver core/silica shell nanocubes (AgNC@SiO(2)s) in the Kretschmann configuration, because the resonant condition of PSPs on the thin Ag layer is significantly modified by LSPs of the AgNC@SiO(2)s. We investigate theoretically and experimentally that due to the hybrid property, the slope and position of the minimum reflectance band can be controlled on a graph of incident angle versus wavelength of reflected light, by changing structural parameters. The hybrid properties of PSPs and LSPs have a potential to simultaneously detect surface plasmon resonance signals and fluorescence images.

Active directional switching of surface plasmon polaritons using a phase transition material

Active switching of near-field directivity, which is an essential functionality for compact integrated photonics and small optoelectronic elements, has been challenging due to small modulation depth and complicated fabrication methods for devices including active optical materials. Here, we theoretically and experimentally realize a nanoscale active directional switching of surface plasmon polaritons (SPPs) using a phase transition material for the first time. The SPP switching device with noticeable distinction is demonstrated based on the phase transition of vanadium dioxide (VO2) at the telecom wavelength. As the insulator-to-metal phase transition (IMT) of VO2 induces the large change of VO2 permittivity at telecom wavelengths, the plasmonic response of a nanoantenna made of VO2 can be largely tuned by external thermal stimuli. The VO2-insulator-metal (VIM) nanoantenna and its periodic array, the VIM metagrating, are suggested as optical switches. The directional power distinction ratio is designed to change from 8.13:1 to 1:10.56 by the IMT and it is experimentally verified that the ratio changes from 3.725:1 to 1:3.132 as the VIM metagratings are heated up to 90 °C. With an electro-thermally controllable configuration and an optimized resonant design, we expect potential applications of the active switching mechanism for integrable active plasmonic elements and reconfigurable imaging.