Extraordinary Transmission Research Articles

Coupling between plasmonic nanohole array and nanorod array: the emerging of a new extraordinary optical transmission mode and epsilon-near-zero property

The compound Ag nanohole structures with an Ag nanorod in the middle of an Ag nanohole are investigated by finite-difference-time-domain calculations. The structure shows strong polarization-dependent optical properties. A new extraordinary optical transmission (EOT) mode is observed when the polarization direction is aligned with the long axis of the nanorods, and its resonance wavelength red-shifts exponentially with the nanorod length. Around this resonance wavelength, the real part of the effective permittivity also becomes zero, demonstrating the epsilon-near-zero (ENZ) property. The new EOT mode is found to be the result of the enhanced local radiation of the nanorods as well as the electromagnetic coupling to the nanoholes to produce the enhanced transmission. The tunable resonance wavelength, EOT, strong near-field enhancement, and ENZ properties make the structure ideal for many potential applications, such as ultrathin optical filter, polarizers, surface-enhanced spectroscopies, etc.

Loss compensated extraordinary transmission in hybridized plasmonic nanocavities

Extraordinary optical transmission has been utilized in many optical applications, but the plasmonic losses hinder their full potential. To obtain enhanced transmission, one of the loss compensation methods is to introduce gain. However, an enhanced transmission or even eliminated absorption does not guarantee plasmonic loss compensation. Here, we reveal the distinction between the transmission enhancement mechanisms in gain-assisted plasmonic arrays. To uncover the underlying mechanisms of the modified transmissions, we calculate the effective electric permittivity by employing a self-consistent gain model. We demonstrate that a large transmission enhancement in a plasmonic system composed of periodic nanocavities and coaxially placed nanoislands, is led by the loss compensation, which manifests itself as narrowing in effective permittivity. In contrast, a slight transmission enhancement in a plasmonic array without the nanoislands arises from the background amplification.

(Invited) Novel Optical Phenomena in Nanoplasmonic Arrays

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

Metasurface homogenization based on contravariant tensor averaging in smooth field approximation

The homogenization of the transverse parameters of metasurfaces is introduced through the concept of covariant permittivity tensor averaging. The proposed scheme is based on a covariant form of Maxwell’s equations written in the matched coordinates system. Therefore, the average characteristics of the periodic structure take into account not only all of the physical boundary conditions, but also the geometrical details of the periodic structure that affect the electromagnetic field propagation. The proposed method is successfully applied to analyze the extraordinary optical transmission through a thick layer subwavelength periodic annular slit array.

Large-area grain-boundary-free copper films for plasmonics

Ultrasmooth single-crystalline metallic thin films provide several key advantages for the fabrication of well-defined and high-resolution plasmonic nanostructures, particularly complex integrated nanocircuits. For this purpose, copper is generally regarded as a poor plasmonic material compared to gold and silver because of its notorious oxidation issues when subjected to air exposure. Here, we report on the use of large-area grain-boundary-free copper films grown epitaxially on sapphire substrates in combination with focused ion beam milling to pattern plasmonic nanostructures with superior quality. The copper surfaces prepared using a single-crystalline copper sputtering target exhibit a very low roughness without any grain boundaries for varying film thicknesses and a strong resistance to oxidation, overcoming the bottleneck in conventional copper film fabrication. Surface plasmon resonance measurements show that improved dielectric constants with higher conductivity and long-term stability can be achieved using the single-crystalline copper films. Plasmonic nanohole arrays patterned from these high-quality films are found to display a stronger field enhancement compared to those made from polycrystalline copper films, thus resulting in an enhanced extraordinary optical transmission performance. This study suggests that our fabrication method is ideally suited for applications in copper-based plasmonic and nanophotonic devices as well as integrated nanocircuits on a large scale.

Extraordinary optical transmission and second harmonic generation in sub–10-nm plasmonic coaxial aperture

Abstract Recent development in nanofabrication technology has enabled the fabrication of plasmonic nanoapertures that can provide strong field concentrations beyond the diffraction limit. Further utilization of plasmonic nanoaperture requires the broadband tuning of the operating wavelength and precise control of aperture geometry. Here, we present a novel plasmonic coaxial aperture that can support resonant extraordinary optical transmission (EOT) with a peak transmittance of ~10% and a wide tuning range over a few hundred nanometers. Because of the shadow deposition process, we could precisely control the gap size of the coaxial aperture down to the sub–10-nm scale. The plasmonic resonance of the SiNx/Au disk at the center of the coaxial aperture efficiently funnels the incident light into the sub–10-nm gap and allows strong electric field confinement for efficient second harmonic generation (SHG), as well as EOT. In addition to the experiment, we theoretically investigated the modal properties of the plasmonic coaxial aperture depending on the structural parameters and correlation between EOT and SHG through finite-difference time-domain simulations. We believe that our plasmonic coaxial apertures, which are readily fabricated by the nanoimprinting process, can be a versatile, practical platform for enhanced light–matter interaction and its nonlinear optical applications.

Unidirectional Extraordinary Sound Transmission with Mode-Selective Resonant Materials

Realizing direction-dependent energy responses greatly benefits the construction of switching and logic devices. This study proposes an archetype for an acoustic resonant-tunneling diode, made of mode-selective resonant metamaterial. Using this material, the authors experimentally demonstrate a broadband, high contrast ratio and single-mode unidirectional sound tunneling. This result is expected to impact control methodology in biomedical ultrasonography, acoustic communication, sound identification, and noise control.

Progress and Perspectives in Point of Care Testing for Urogenital Chlamydia trachomatis Infection: A Review.

Worldwide, genital infection with Chlamydia trachomatis (C. trachomatis) is one of the most common sexually transmitted infections. Most infections are asymptomatic. However, particularly in women, untreated infection with C. trachomatis can lead to complications that include pelvic inflammatory disease, infertility, and tubal ectopic pregnancy. Rapid methods for early and accurate diagnosis for infection with C. trachomatis that can be performed in the clinic would allow for earlier treatment to prevent complications. Traditional laboratory-based tests for C. trachomatis infection include culture, enzyme immunoassay, direct immunofluorescence, nucleic acid hybridization, and nucleic acid amplification tests, which take time but have high diagnostic sensitivity. Novel and rapid diagnostic tests include extraordinary optical transmission (EOT), loop-mediated isothermal amplification (LAMP), recombinase polymerase amplification (RPA), and microwave-accelerated metal-enhanced fluorescence (MAMEF). Although these new tests offer the promise of rapid screening and diagnosis, they may have lower diagnostic sensitivity. This review aims to provide an overview of traditional methods for the diagnosis of urogenital infection with C. trachomatis, the current status of POC testing for urogenital C. trachomatis infection and discusses recent progress and perspectives.

Mechanisms of Fano-resonant biosensing: Mechanical loading of plasmonic oscillators

Distinctively narrow and asymmetric line shape Fano resonances arise due to resonant interactions of sub-radiant and super-radiant modes in plasmonic nanostructures and metamaterials. A number of recent experimental studies have shown unique opportunities provided by highly dispersive Fano resonances in biosensing applications. However, there is limited understanding of Fano resonant optical response to biomolecular accumulation. Here, we introduce a phenomenological model that can precisely describe the intricate nature of the Fano resonances in plasmonic nanohole arrays and provide unambiguous physical insights into biosensing experiments. Using rigorous electromagnetic simulations and experimental measurements as benchmarking tools, we show that the non-trivial contribution of molecular accumulation to Fano resonant plasmonic response can be incorporated as a mechanical loading effect in a coupled-oscillator model. Quite remarkably, our phenomenological approach captures the complex spectral response of the Fano resonance profile and asymmetric linewidth broadening upon molecular accumulation. Furthermore, in strong agreement with our experimental measurements, we show that our parameterized​ model has predictive power in fine tuning the Fano resonant extraordinary light transmission lineshape using structural design parameters without resorting to electromagnetic simulations. Our phenomenological model provides a general analytical method that can be adapted to understand biomolecular detection measurements in different plasmonic and metamaterial systems.

Plasmonic titanium nitride via atomic layer deposition: A low-temperature route

To integrate plasmonic devices into industry, it is essential to develop scalable and CMOS compatible plasmonic materials. In this work, we report high plasmonic quality titanium nitride (TiN) on c-plane sapphire grown by plasma-enhanced atomic layer deposition. TiN with low losses, high metallicity, and a plasma frequency below 500 nm was achieved at temperatures less than 500 °C by exploring the effects of chemisorption time, substrate temperature, and plasma exposure time on the material properties. A reduction in chemisorption time mitigates premature precursor decomposition at TS > 375 °C, and a trade-off between reduced impurity concentration and structural degradation caused by plasma bombardment is achieved for 25 s plasma exposure. 85 nm thick TiN films grown at a substrate temperature of 450 °C, compatible with CMOS processes, with 0.5 s chemisorption time and 25 s plasma exposure exhibited a high plasmonic figure of merit (|ε′/ε′′|) of 2.8 and resistivity of 31μΩcm. As a result of the improved quality, subwavelength apertures were fabricated in the TiN thin films and are shown to exhibit extraordinary transmission.

Giant extraordinary transmission of acoustic waves through a nanowire.

Wave concentration beyond the diffraction limit by transmission through subwavelength structures has proved to be a milestone in high-resolution imaging. Here, we show that a sound wave incident inside a solid over a diameter of 110 nm can be squeezed through a resonant meta-atom consisting of a nanowire with a diameter of 5 nm equal to λ/23, where λ is the incident acoustic wavelength, corresponding to a transmission efficiency of 500 or an energy densification of ~14,000. This remarkable level of extraordinary acoustic transmission is achieved in the absence of ultrasonic attenuation by connecting a tungsten nanowire between two tungsten blocks, the block on the input side being furnished with concentric grooves. We also demonstrate that these "solid organ pipes" exhibit Rayleigh end corrections to their effective longitudinal resonant lengths notably larger than their in-air analogs. Grooves on the output side lead to in-solid directed acoustic beams, important for nanosensing.

Revealing the underlying mechanisms behind TE extraordinary THz transmission

Transmission through seemingly opaque surfaces, so-called extraordinary transmission, provides an exciting platform for strong light–matter interaction, spectroscopy, optical trapping, and color filtering. Much of the effort has been devoted to understanding and exploiting TM extraordinary transmission, while TE anomalous extraordinary transmission has been largely omitted in the literature. This is regrettable from a practical point of view since the stronger dependence of the TE anomalous extraordinary transmission on the array’s substrate provides additional design parameters for exploitation. To provide high-performance and cost-effective applications based on TE anomalous extraordinary transmission, a complete physical insight about the underlying mechanisms of the phenomenon must be first laid down. To this end, resorting to a combined methodology including quasi-optical terahertz (THz) time-domain measurements, full-wave simulations, and method of moments analysis, subwavelength slit arrays under s-polarized illumination are studied here, filling the void in the current literature. We believe this work unequivocally reveals the leaky-wave role of the grounded-dielectric slab mode mediating in TE anomalous extraordinary transmission and provides the necessary framework to design practical high-performance THz components and systems.

Chiral effects in low-cost plasmonic arrays of elliptic nanoholes

In this work we present an approach to induce chiral effects in well-known plasmonic nanohole arrays with triangular unit cell. Arrays with circular nanoholes in metals can exhibit interesting light-matter interaction mechanisms such as surface plasmon polaritons and extraordinary optical transmission. Moving from circular to elliptical shape and tilting the ellipse away from the lattice symmetry lines, a symmetry breaking induces a different absorption of the circularly polarized light of opposite handedness, i.e. circular dichroism. We numerically investigate circular dichroism at normal incidence in elliptic nanoholes in Au in the spectral range 400–1000 nm. We focus on its dependence on the elliptic nanohole tilt, and further proceed with the ellipse radii optimization. The optimized circular dichroism reaches 84%, and we report on the near field absorption distribution at the wavelengths of this maximum value.

Metasurfaces homogenization technique based on the computation of the average value of the contravariant tensors elements

The purpose of this paper is to present a 2D metasurfaces homogenization based on the computation of the average value of the contravariant tensors elements. A simple semi-analytical model for the comprehension of the extraordinary optical transmission (EOT) through a 2D array of periodic subwavelength metasurface is analyzed through the proposed method. In this single mode model, the mono layer of the metasurface film is considered as homogeneous medium. Therefore, the electromagnetic response of this structure to a plane wave excitation is equivalent to that of a slab with homogeneous equivalent properties. In a coordinates system where the coordinate curves xj coincide with the physical discontinuity of the material, the covariant formalism involving contravariant components of density flux and covariant components of the electromagnetic field, allows to efficiently handle the continuity properties of the electromagnetic field. By averaging all these quantities involving local fields over the periods of the unit cell, and assuming that, these macroscopic fields and densities satisfy a constitutive relationship, the electromagnetic parameters such as averaged permittivity and permeability can be defined.

Transition from discrete patches to plasmonic nanohole array by glancing angle deposition on nanosphere monolayers

By combining nanosphere lithography and glancing angle deposition, a morphological transition from disconnected patchy silver (Ag) coated nanosphere particles to a connected Ag nanohole sheet on close-packed nanosphere monolayers has been demonstrated, which significantly changes the optical property of the Ag nanostructure deposited. For different sized nanosphere monolayers, when the vapor incident angle was set to be 55°, the transmission spectra showed complicated features when the Ag deposition thickness was less than 60 nm. When the thickness was large enough (≥60 nm), a distinguished extraordinary optical transmission (EOT) peak was observed. The EOT peak wavelength position is independent of the Ag thickness deposited and is proportional to the nanosphere diameter. The obtained EOT peaks possess a high quality factor and have high transmission values compared to those reported in the literature for similar structures. The Monte Carlo growth simulations demonstrate the morphological transition from the patchy arrays to nanohole arrays while the electromagnetic numerical calculations confirm the change in the optical properties. Such a high quality EOT response could be used for constructing better sensors or developing other plasmonic applications.

The optical properties of a visible light filter integrated on the silicon substrate

There have been many investigations performed for color filtering based on surface plasmon resonances in recent years. However, research results on color filters integrated on the silicon-based photodetectors are infrequently reported. At present, problems including low light transmission, wide full width at half maximum (FWHM), and poor color tunability exist in the light filters integrated on the silicon-based photodetectors. We propose and numerically demonstrate a light filter with the periodic nanostructure and dielectric multi-layers integrated on the silicon substrate. Continuously tunable transmission visible spectra (400∼760 nm) are applied for the proposed filters. Hybrid resonant modes including plasmonic resonances and waveguide modes result in the extraordinary light transmission in the proposed filters. The simulations indicate that the integrated filter has more than 75% transmission and less than 25 nm full width at half maximum (FWHM) of the transmission curve for the destined wavelength. The proposed filter provides a solution for the combination of color filters with the silicon-based photosensors like CMOS photodiodes.

Polarization effects in metallic plates perforated with periodic arrays of subwavelength circular holes

The enhanced transmission phenomenon (ET) discovered by Ebbesen et al has recently been a topic of the intensive research, leading to such a branch of research as fishnet metamaterials. An application of the metamaterials is polarization sensitive devices. Polarization sensitivity for ET frequency selective structures, as for classical ones, is usually associated with the hole shapes being polarization sensitive. This paper discusses a new phenomenon of polarization sensitive extraordinary transmission through a thick screen perforated with polarization independent subwavelength circular holes. The possibility to control the polarization state of the waves transmitted through the screen by manipulation periodical cell dimensions only is shown, while hole shapes are polarization independent. A linear plane wave can be rotated through an arbitrary angle, and converted to a circular or elliptical one, when passing through the screen in the vicinity of Floquet-wave grazing points. Therefore, one can realize a simple and compact polarization device at the expense of reducing bandwidth. The origin of the phenomenon is in a plane wave coupling with different types of symmetry eigenmodes leading to polarization sensitive enhanced transmission. When an incident plane wave couples to orthogonal symmetrical and antisymmetrical eigenmodes with close eigenfrequencies, the screen can behave like a half-wave plate. For the case of several closely spaced eigenmodes jointly contributing to transmitted wave, the effect of enhanced polarization conversion can be observed for the screen to behave like a quarter-wave plate.

Large-Scale Fabrication of Optically Active Plasmonic Arrays via Displacement Talbot Lithography

Periodic nanoapertures fabricated in thin metal films exhibit a range of interesting properties in the presence of electromagnetic waves including phenomena such as extraordinary optical transmission (EOT). Fundamentally, these effects are mediated by plasmons and have been shown to have a vast range of applications, including, colour filtering, chemical sensing, and as components in solar cells. In the majority of cases, the high spatial resolution required for precise fabrication of these structures is limited to direct writing techniques such as Focused Ion Beam (FIB) and Electron beam lithography (EBL), which only cover relatively small, micron-sized, areas. In this article, we describe and demonstrate the fabrication of plasmonically active devices in the visible range using Displacement Talbot Lithography (DTL). This method allows nanometre-resolution photolithography to be performed over very large areas (whole wafers) without any significant degradation in quality. We present experimental results for a range of different structures including periodic, aperiodic and hexagonal configurations in silver films.

A tunable double negative device consisting of a plasma array and a negative-permeability metamaterial

Extraordinary wave transmission is demonstrated through a double-negative composite comprised of a negative-permeability array of double split ring resonators and a negative-permittivity array of plasma discharge tubes at microwave frequencies. A transmission peak emerges in a double-negative band and controlling the electron density inside the plasma tubes dynamically regulates the transmission properties. By performing experiments and theoretical calculations, we verify that the composite permits wave propagation with negative permeability and controllable permittivity, which indicates that a tunable negative-refractive-index device is achieved.

Investigating extraordinary optical transmission and sensing performance through periodic bilayer magneto-plasmonic structure

This paper proposes the phenomenon of extraordinary optical transmission via a magneto-plasmonic nanostructure, which combines magnetic and plasmonic functionalities. The structure includes an active magnetic film magnetized perpendicular to its surface and a plasmonic metal film, perforated with subwavelength circular annular arrays, with a ring placed in the middle of each annular circle. We use the finite element method and the finite-difference time-domain method for simulation of the structure. Numerical analysis shows an improvement in the Faraday rotation and optical transmission, simultaneously, in a magneto-plasmonic structure based on a silver- and bismuth-substituted ferrite garnet. Simultaneous improvement is achieved by coupling the TE and TM waveguide-plasmon modes. The amount of enhancement is adjusted by changing the dimensions, the periodicity of the hole arrays, and the refractive index of the materials filled in the holes. The influence of excitation of the two kinds of plasmon modes and the application of the external magnetic field are used to enhance the optical response. The resulting investigation shows two resonance peaks in the near-infrared range of the Faraday effect spectrum. Because of the strong Faraday rotation coinciding with the dual-band transmission of approximately 90%, the maximum figure of merit can also be obtained. Finally, this structure is investigated as a sensor in different reflective indexes from 1 to 1.5 RIU, and sensitivity of 45.97 nm/RIU was achieved. The potential applications of these nanostructures include, for example, subwavelength optics, optoelectronic devices, biosensing devices, and magneto-optical devices.