Free-standing plasmonic metal-dielectric-metal bandpass filter with high transmission efficiency

Imprint and transfer fabrication of freestanding plasmonic membranes

This paper reports an imprint and transfer approach for the rapid and inexpensive fabrication of the ultra-thin freestanding plasmonic membrane (FPM) that supports surface plasmon resonances. The imprint and transfer fabrication method involves the soft imprint lithography on an ultrathin polymer film, transfer of the perforated polymer film to a supporting frame, subsequent deposition of gold, and final removal of the polymer film. Without using any sophisticated lithography and etching processes, the imprint and transfer method can produce freestanding gold membranes with 2D arrays of submicrometer-sized holes that support plasmonic modes in the mid-wavelength infrared (mid-IR) range. Two FPM devices with an array constant of 4.0 and 2.5 μm have been simulated, fabricated, and measured for their transmittance characteristics. The fabricated FPMs exhibit surface plasmon polariton Bloch mode and extraordinary optical transmission (EOT) with the enhanced local field around the membrane. The effects of membrane thickness and angle dispersion on the FPM were investigated to show the tuning of EOT modes in IR. Furthermore, we demonstrated the refractometric sensing and enhanced IR absorption of the FPM device for its potential in chemical and biomolecule sensing applications.

Enhancing Spectral Reflection through Controlled Phase Distribution Using Doped Polar-Dielectric Metasurfaces

Controlling the phase distribution of wavefronts using optical metasurfaces has led to interesting optical properties and applications. Here, we explore the control of phase distribution through polar-dielectric metasurfaces composed of doped SiC nanosphere arrays. We investigate the impact of doping concentration on the optical properties of SiC nano-spheres. Our results indicate that increasing the doping of SiC nanoparticles influenced electric dipolar resonances, whereas it did not change the dipolar resonances. Using this concept, we numerically studied the extension of this idea to form metasurface arrays of single, dimer and linear trimers of the doped SiC nano-spheres. Using different doping schemes, we studied the improvement of the reflectivity at frequencies greater than the longitudinal optical phonon frequency.

Plasmonic directional couplers using channel waveguides in random arrays of metal nanoparticles

Plasmonic directional couplers based on channel waveguides embedded in random arrays of metal nanoparticles (NPs) operating in near-infrared are fabricated using electron-beam lithography and investigated experimentally characterizing their performance with leakage-radiation microscopy. The power exchange between coupled waveguides, its spatial period and efficiency, along with the overall power transmission, are determined in the wavelength range from 700 to 800 nm. We introduce a simple coupled-mode approach based on three coupled waveguides. The composite system considers a waveguide consisting of NP-filled stripe with characteristics distinctly different from those of the channel waveguides. Using this model, we describe the performance of investigated composite plasmonic configurations and obtain good qualitative agreement with experimental observations.

Massive Enhancement of Optical Transmission across a Thin Metal Film via Wave Vector Matching in Grating-Coupled Surface Plasmon Resonance

We demonstrate how distinct surface plasmon resonance modes on opposite sides of a metal-coated grating can be coupled across the metal film. This coupling occurs by matching the resonance conditions on each side of the grating by tuning the refractive index directly adjacent to the metal film. In the first example, we deposited a high refractive index layer of tin oxide on top of the grating to red-shift the front side surface plasmon until it coupled with the backside surface plasmon across a semitransparent ∼45 nm thin silver grating. By shifting the resonance condition of the nearby surface plasmon, this high refractive index coating creates an effective matching of wave vectors across the metal film, allowing them to couple and enhance the optical response. A massive increase in the magnitude of enhanced transmission is observed, increasing from a 6-fold transmission enhancement through a bare silver grating to a near 100-fold enhancement after deposition of a tin oxide layer of appropriate thickness (∼310 nm). This optical transmission enhancement is then probed through computational modeling and by experiments with liquids of various refractive index values. The matched system shows an increased amplitude sensitivity with respect to refractive index changes and a waveguide like behavior within the tin oxide film. As an alternative configuration, we also demonstrate coupling the front and back-side plasmon modes by using a lower refractive index substrate in order to blue-shift the back-side surface plasmon. Coupling between the two plasmon modes is then demonstrated by introducing aqueous solutions of various refractive index values. Under the proper conditions, this matched system also shows a substantial enhancement in transmission. This technique of wave vector matching provides a route to substantially increasing the plasmon enhanced optical transmission through metal gratings, which has potential application in improved plasmonic sensing, spectroscopy, and plasmon-based optical devices.

A High-Efficiency Multispectral Filter Based on Plasmonic Hybridization between Two Cascaded Ultrathin Nanogratings

Overcoming the disadvantages of low transmission and broad peak bandwidth of previously reported plasmonic color filters, a high-efficiency multispectral plasmonic color filter is theoretically proposed with two cascaded ultrathin metallic nanogratings separated by two heterogeneous dielectric layers, and its optical properties are theoretically investigated using the finite-difference time-domain method. The transmission spectrum presents three near-unity peak bands accompanied with three near-null dip bands adjacent around them. Both transmission efficiencies of above 90% and ultranarrow peak bandwidth of 20 nm are achieved in the visible regime. The peak band positions can be flexibly tailored by varying the structural parameters. The filter selects the visible color with high signal noise ratio at the peak bands. The outstanding spectral properties of this filter indicate significant improvement for the high-accuracy color filtering and multispectral imaging applications. The simulated near-field electromagnetic distributions suggest that the excitation of the hybrid antisymmetric surface plasmon polariton (SPP) leaky mode and metal-insulator-metal waveguide modes are responsible for the peak transmission bands, while the formation of the hybrid SPP bound modes confined on the bottom nanograting makes the dip transmission bands, all of which are the consequence of the plasmonic hybridization between the two neighboring metallic nanogratings.