Rigorous Analysis and Systematical Design of Double-Layer Metal Superlens for Improved Subwavelength Imaging Mediated by Surface Plasmon Polaritons

A double-layer metal superlens was rigorously analyzed and systematically designed to improve subwavelength imaging ability. It was revealed that transmission properties of the imaging system could be accurately interpreted by the five-layer waveguide mode theory-each amplification peak among the spatial frequency range of evanescent waves was associated with a corresponding surface plasmon polariton (SPP) mode of an insulator-metal-insulator-metal-insulator (IMIMI) structure. On the basis of such physical insight, evanescent waves of higher spatial frequency were effectively amplified via increasing propagation constants of symmetrically coupled short-range SPP (s-SRSPP) and antisymmetrically coupled short-range SPP (a-SRSPP), and evanescent waves of lower spatial frequency were appropriately diminished by approaching to cut off symmetrically coupled long-range SPP (s-LRSPP). A flat and broad optical transfer function of the imaging system was then achieved, and improved subwavelength imaging performance was validated by imaging an ideal thin object of two slits with a 20-nm width distanced by a 20-nm spacer, under 193-nm illumination. The resolution limit of the designed imaging system with double-layer superlens was further demonstrated to be at least ~λ/16 for an isolated two-slit object model. This work provided sound theoretical analysis and a systematic design approach of double-layer metal superlens for near-field subwavelength imaging, such as fluorescent micro/nanoscopy or plasmonic nanolithography.

A voltage-controlled silver nanograting device for dynamic modulation of transmitted light based on the surface plasmon polariton effect

An active-controlled plasmonic device is designed and fabricated based on the index-sensitive properties of surface plasmon polaritons (SPPs). We utilize a one-dimensional silver nanograting with a period of 320 nm overlayered with a liquid crystal (LC) layer (50 μm in thickness), to transmit selectively the surface plasmon resonance (SPR) wavelength. This device realizes the active, reversible and continuous control of the transmitted light wavelength by modulating the external voltage signal applied to the LC layer. This voltage-controlled plasmonic filter has a dynamic wavelength modulation range of 17 nm, a fast respond speed of 4.24 ms and a low driving voltage of 1.06 V μm(-1). This study opens up a unique way for the design of tunable nanophotonic devices, such as a micro light sources and switches.

When are surface plasmon polaritons excited in the Kretschmann-Raether configuration?

It is widely believed that the reflection minimum in a Kretschmann-Raether experiment results from direct coupling into surface plasmon polariton modes. Our experimental results provide a surprising discrepancy between the leakage radiation patterns of surface plasmon polaritons (SPPs) launched on a layered gold/germanium film compared to the K-R minimum, clearly challenging this belief. We provide definitive evidence that the reflectance dip in K-R experiments does not correlate with excitation of an SPP mode, but rather corresponds to a particular type of perfectly absorbing (PA) mode. Results from rigorous electrodynamics simulations show that the PA mode can only exist under external driving, whereas the SPP can exist in regions free from direct interaction with the driving field. These simulations show that it is possible to indirectly excite propagating SPPs guided by the reflectance minimum in a K-R experiment, but demonstrate the efficiency can be lower by more than a factor of 3. We find that optimal coupling into the SPP can be guided by the square magnitude of the Fresnel transmission amplitude.

Metallic-nanowire-loaded silicon-on-insulator structures: a route to low-loss plasmon waveguiding on the nanoscale

The simultaneous realization of nanoscale field localization and low transmission loss remains one of the major challenges in nanophotonics. Metal nanowire waveguides can fulfill this goal to a certain extent by confining light within subwavelength space, yet their optical performances are still restricted by the tradeoff between confinement and loss, which results in quite limited propagation distances when their mode sizes are reduced down to the nanometer scale. Here we introduce a class of low-loss guiding schemes by integrating silicon-on-insulator (SOI) waveguides with plasmon nanowire structures. The closely spaced silicon and metal configurations allow efficient light squeezing within the nanometer, low-index silica gaps between them, enabling deep-subwavelength light transmission with low modal attenuation. Optimizations of key structural parameters unravel the wide-range existence of the high-performance hybrid nanowire plasmon mode, which demonstrates improved guiding properties compared to the conventional hybrid and nanowire plasmon polaritons. The excitation strategy of the guided mode and the feasibility of the waveguide for compact photonic integration as well as active components are also discussed to lay the foundation for its practical implementation. The remarkable properties of these metallic-nanowire-loaded SOI waveguides potentially lend themselves to the implementation of high performance nanophotonic components, and open up promising opportunities for a variety of intriguing applications on the nanoscale.

Electrically driven plasmon mediated energy transfer between ZnO microwires and Au nanoparticles

Electrically driven energy transfer between the surface defect states of ZnO quadrilateral microwires (MWs) and localized surface plasmon polaritons has been realized by means of introducing Au nanoparticles (NPs). An electroluminescence device with green emission using ZnO quadrilateral MWs, was fabricated. Once the Au NPs are sputtered on the surfaces of the ZnO MWs, the electroluminescence of the ZnO MWs will shift from green to red. Meanwhile, dual emissions were observed by means of sputtering Au NPs on a single ZnO MW periodically. Due to the Au NPs, electrically driven plasmon mediated energy transfer can achieve the modulation of amplifying, or quenching the surface defect emission. The relevant dynamic process of the surface plasmon mode mediated energy transfer was investigated. This new energy transfer method potentially offers an approach of modification and recombination of the surface defect state excitations of wide bandgap semiconductor materials.