Strong influence of hole shape on extraordinary transmission through periodic arrays of subwavelength holes

Non-polarized and ultra-narrow band filter in MIR based on multilayer metasurface

We propose an ultra narrow-band filter in the mid infrared region (MIR) using artificial metamaterials (AMM), which is suitable for the design of on-chip photonic spectrometers. 2-D rectangular holes with a cross-like layerout are adopted to enhance the filter's efficiency and precision. Considering the penetration depth of electromagnetic (EM) waves in the metal film, we opt for multi-layer films composed of metal layers and dielectric layers, instead of a single metal layer, to improve the structure's performance in the MIR. This multilayer configuration significantly enhances the efficiency and precision of the AMM structures in the MIR. The transmission peak, with a full width at half maximum (FWHM) of 30 nm, can be achieved and tuned in the wavelength range from 3.0 μm to 10.0 μm by changing the periods of the unit cell (enlarging the unit cell from 3.0 to 10.0 μm). The proposed AMM structures, with tunable narrow band transmittance in MIR, exhibit promising potential in the fabrication of narrow band photonic detectors and on-chip spectrometers.

Circular dichroism in a plasmonic array of elliptical nanoholes with square lattice

Chiral properties of plasmonic metasurfaces, especially related to different absorption of left and right circularly polarized light leading to circular dichroism (CD), are a research hot topic in nanophotonics. There is often a need to understand the physical origin of CD for different chiral metasurfaces, and to get guidelines for the design of structures with optimized and robust CD. In this work, we numerically study CD at normal incidence in square arrays of elliptic nanoholes etched in thin metallic layers (Ag, Au, Al) on a glass substrate and tilted with respect to the symmetry axes. Strong CD arises in absorption spectra at the same wavelength region of extraordinary optical transmission, indicating highly resonant coupling between light and surface plasmon polaritons at the metal/glass and metal/air interfaces. We elucidate the physical origin of absorption CD by a careful comparison of optical spectra for different polarizations (linear and circular), with the aid of static and dynamic simulations of local enhancement of the electric field. Furthermore, we optimize the CD as a function of the ellipse parameters (diameters and tilt), the thickness of the metallic layer, and the lattice constant. We find that silver and gold metasurfaces are most useful for CD resonances above 600 nm, while aluminum metasurfaces are convenient for achieving strong CD resonances in the short-wavelength range of the visible regime and in the near UV. The results give a full picture of chiral optical effects at normal incidence in this simple nanohole array, and suggest interesting applications for chiral biomolecules sensing in such plasmonic geometries.

Electrically switchable structural colors based on liquid-crystal-overlaid aluminum anisotropic nanoaperture arrays

Actively tunable or reconfigurable structural colors are highly promising in future development for high resolution imaging and displaying applications. To this end, we demonstrate switchable structural colors covering the entire visible range by integrating aluminum nanoaperture arrays with nematic liquid crystals. The geometrically anisotropic design of the nanoapertures provides strong polarization-dependent coloration. By overlaying a nematic liquid crystal layer, we further demonstrate switchable ability of the structural colors by either changing the polarization of the incident light or applying an external voltage. The switchable structural colors have a fast response time of 28 ms at a driving voltage of 6.5 V. Furthermore, colorful patterns are demonstrated by coding the colors with various dimensions of nanoaperture arrays with dual switching modes. Our proposed technique in this work provides a dual-mode switchable structural colors, which is highly promising for polarimetric displays, imaging sensors, and visual cryptography.

Freestanding and Permeable Nanoporous Gold Membranes for Surface-Enhanced Raman Scattering

Surface-enhanced Raman spectroscopy (SERS) demands reliable, high-enhancement substrates in order to be used in different fields of application. Here we introduce freestanding porous gold membranes (PAuM) as easy-to-produce, scalable, mechanically stable, and effective SERS substrates. We fabricate large-scale sub-30 nm thick PAuM that form freestanding membranes with varying morphologies depending on the nominal gold thickness. These PAuM are mechanically stable for pressures up to more than 3 bar and exhibit surface-enhanced Raman scattering with local enhancement factors from 10⁴ to 10⁵, which we demonstrate by wavelength-dependent and spatially resolved Raman measurements using graphene as a local Raman probe. Numerical simulations reveal that the enhancement arises from individual, nanoscale pores in the membrane acting as optical slot antennas. Our PAuM are mechanically stable, provide robust SERS enhancement for excitation power densities up to 10⁶ W cm^-2, and may find use as a building block in SERS-based sensing applications.

Metasurface-empowered spectral and spatial light modulation for disruptive holographic displays

The holographic display, one of the most realistic ways to reconstruct optical images in three-dimensional (3D) space, has gained a lot of attention as a next-generation display platform for providing deeper immersive experiences to users. So far, diffractive optical elements (DOEs) and spatial light modulators (SLMs) have been used to generate holographic images by modulating electromagnetic waves at each pixel. However, such architectures suffer from limitations in terms of having a resolution of only a few microns and the bulkiness of the entire optical system. In this review, we describe novel metasurfaces-based nanophotonic platforms that have shown exceptional control of electromagnetic waves at the subwavelength scale as promising candidates to overcome existing restrictions, while realizing flat optical devices. After introducing the fundamentals of metasurfaces in terms of spatial and spectral wavefront modulation, we present a variety of multiplexing approaches for high-capacity and full-color metaholograms exploiting the multiple properties of light as an information carrier. We then review tunable metaholograms using active materials modulated by several external stimuli. Afterward, we discuss the integration of metasurfaces with other optical elements required for future 3D display platforms in augmented/virtual reality (AR/VR) displays such as lenses, beam splitters, diffusers, and eye-tracking sensors. Finally, we address the challenges of conventional nanofabrication methods and introduce scalable preparation techniques that can be applied to metasurface-based nanophotonic technologies towards commercially and ergonomically viable future holographic displays.

A merged lattice metal nanohole array based dual-mode plasmonic laser with an ultra-low threshold

Plasmonic lasers offer great potential for cutting-edge, disruptive applications. However, they suffer from a high loss in metals, lack of spatial coherence in the near field, and divergent far-field emission. The challenges are even more significant for a plasmonic laser emitting more than one wavelength mode. The design complexity required for creating multiple modes often limits avenues for minimizing losses and converging far-field emission patterns. This work exploits plasmonic resonances at the junction of a merged lattice metal nanohole array (NHA) and a one-dimensional photonic crystal to achieve dual-mode lasing. The merged lattice NHA is designed by concentrically combining two simple NHAs with different periodicities to create pseudo randomness, leading to enhanced localization and confinement of light in multiple wavelength modes. The proposed structure notably produces intense dual-mode lasing at an ultra-low threshold compared to recent state-of-the-art plasmonic laser demonstrations. The wavelengths of the lasing modes and the separation between them can be tuned over a broad range by changing the structural parameters. The proposed laser also creates a highly directional far-field pattern with a divergence angle of only <0.35°.

Fabrication of Antireflection Micro/Nanostructures on the Surface of Aluminum Alloy by Femtosecond Laser

Designed micro-nano structures on the surface of aluminum alloy provide excellent light trapping properties that can be used extensively in thermal photovoltaics, sensors, etc. However, the fabrication of high-performance antireflective micro-nano structures on aluminum alloy is challenging because aluminum has shallow intrinsic losses and weak absorption. A two-step strategy is proposed for fabricating broadband antireflection structures by superimposing nanostructures onto microscale structures. By optimizing the processing parameters of femtosecond laser, the average reflectances of 2.6% within the visible spectral region (400–800 nm) and 5.14% within the Vis-NIR spectral region (400–2500 nm) are obtained.

Giant Extra-Ordinary Near Infrared Transmission from Seemingly Opaque Plasmonic Metasurface: Sensing Applications

In the present study, we report giant extra-ordinary transmission of near infrared (NIR) light, more than 90%, through a seemingly opaque plasmonic metasurface, which consists of two metal nano-slits arrays (MNSAs) with alternate opening arrangements. By using perfect coupling of the plasmonic modes formed between the sharp edges of the upper and lower MNSAs of silver, a giant, wavelength selective transmission could be obtained. The study is accompanied by optimization of electromagnetic (EM) field coupling for different interlayer spacings and lateral overlap between the two MNSAs to understand their significance in light transmission through the metasurface. The interlayer spacing between the MNSAs works as the transmitting channel for light. The optimization of performance with different fill factors and plasmonic metals was performed as well. Because of the excitation of extended surface plasmons (ESPs) generated at both the MNSAs, the metasurface can be used for refractive index (RI) sensing as one of its applications by using a transparent and flexible polymer, such as polydimethylsiloxane (PDMS), as substrate. The maximum sensitivity which could be achieved for the optimal configuration of the metasurface was 1435.71 nm/RIU, with a figure of merit (FOM) of 80 RIU^-1 for 90.45% optical transmission of light for the refractive index variation of analyte medium from 1.33 to 1.38 RIU. The present study strengthens the concept of light funneling through subwavelength structures due to plasmons, which are responsible for light transmission through this seemingly opaque metasurface and finds use in highly sensitive, flexible, and cost-effective EOT-based sensors.

Electrically Switchable, Polarization-Sensitive Encryption Based on Aluminum Nanoaperture Arrays Integrated with Polymer-Dispersed Liquid Crystals

Metasurface-based structural coloration is a promising enabling technology for advanced optical encryption with a high-security level. Herein, we propose a paradigm of electrically switchable, polarization-sensitive optical encryption based on designed metasurfaces integrated with polymer-dispersed liquid crystals. The metasurfaces consist of anisotropic and isotropic aluminum nanoaperture arrays. Optical images can be encrypted by elaborately arranging anisotropic and isotropic nanoapertures based on their polarization-dependent plasmonic resonance characteristics. We demonstrate high-quality encrypted images and QR codes with electrically switchable, polarization-sensitive properties based on PDLC-integrated aluminum nanoaperture arrays. The proposed technique can be applied to many fields including high-security optical encryption, security tags, anticounterfeiting, multichannel imaging, and dynamic displays.

Electrically tunable terahertz switch based on superconducting subwavelength hole arrays

We experimentally demonstrate an electrically tunable superconducting device capable of switching the extraordinary terahertz (THz) transmission. The planar device consists of subwavelength hole arrays with real-time control capability. The maximum transmission coefficient at 0.33 THz is 0.98 and decreases to 0.17 when the applied voltage only increases to 1.3 V. A relative intensity modulation of 82.7% is observed, making this device an efficient THz switch. Additionally, this device exhibits good narrow-bandpass characteristics within 2 THz, which can be used as a frequency-selective component. This study offers an ideal tuning method and delivers a promising approach for designing active and miniaturized devices in THz cryogenic systems.

A Thermopile Device with Sub-Wavelength Hole Arrays by CMOS-MEMS Technology

A thermopile device with sub-wavelength hole array (SHA) is numerically and experimentally investigated. The infrared absorbance (IRA) effect of SHAs in active area of the thermopile device is clearly analyzed by the finite-difference time-domain (FDTD) method. The prototypes are manufactured by the 0.35 μm 2P4M complementary metal-oxide-semiconductor micro-electro-mechanical-systems (CMOS-MEMS) process in Taiwan semiconductor manufacturing company (TSMC). The measurement results of those prototypes are similar to their simulation results. Based on the simulation technology, more sub-wavelength hole structural effects for IRA of such thermopile device are discussed. It is found from simulation results that the results of SHAs arranged in a hexagonal shape are significantly better than the results of SHAs arranged in a square and the infrared absorption efficiencies (IAEs) of specific asymmetric rectangle and elliptical hole structure arrays are higher than the relatively symmetric square and circular hole structure arrays. The overall best results are respectively up to 3.532 and 3.573 times higher than that without sub-wavelength structure at the target temperature of 60 °C when the minimum structure line width limit of the process is ignored. Obviously, the IRA can be enhanced when the SHAs are considered in active area of the thermopile device and the structural optimization of the SHAs is absolutely necessary.

Nearly perfect transmission of unpolarized infrared radiation through a one-dimensional metal grating embedded in a monolithic high-contrast grating

We demonstrate a conceptually simple polarization-independent mechanism for nearly perfect infrared light transmission through a subwavelength one-dimensional metal grating implemented in the grooves of a deep-subwavelength monolithic high-contrast grating (metalMHCG). We provide theoretical background explaining the transmission mechanism, which eliminates Fresnel reflection as well as significantly reduces metal absorption and the reflection of transverse electric and transverse magnetic light polarizations. Careful design of a metalMHCG implemented at the interface between the regions of high refractive index contrast enables the coincidence of high transmission conditions for both light polarizations, enabling up to 97% transmission of polarization-independent infrared radiation. Our analysis shows excellent electrical properties of the metalMHCG as evidenced by sheet resistance of 2 ΩSq^-1 facilitating straightforward horizontal electron transport and vertical injection of the current into the semiconductor substrate on which the electrode is implemented.

Quantum teleportation mediated by surface plasmon polariton

Surface plasmon polaritons (SPPs) are collective excitations of free electrons propagating along a metal-dielectric interface. Although some basic quantum properties of SPPs, such as the preservation of entanglement, the wave-particle duality of a single plasmon, the quantum interference of two plasmons, and the verification of entanglement generation, have been shown, more advanced quantum information protocols have yet to be demonstrated with SPPs. Here, we experimentally realize quantum state teleportation between single photons and SPPs. To achieve this, we use polarization-entangled photon pairs, coherent photon–plasmon–photon conversion on a metallic subwavelength hole array, complete Bell-state measurements and an active feed-forward technique. The results of both quantum state and quantum process tomography confirm the quantum nature of the SPP mediated teleportation. An average state fidelity of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.889\pm 0.004$$\end{document}0.889±0.004 and a process fidelity of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.820\pm 0.005$$\end{document}0.820±0.005, which are well above the classical limit, are achieved. Our work shows that SPPs may be useful for realizing complex quantum protocols in a photonic-plasmonic hybrid quantum network.

Enhanced extraordinary optical transmission and refractive-index sensing sensitivity in tapered plasmonic nanohole arrays

The phenomenon of extraordinary optical transmission (EOT) caused by light through metallic nanohole arrays has attracted significant attention due to its potential applications for monolithic color filters and ultrasensitive label-free biosensing. However, the EOT spectra of these nanohole arrays have multiple resonance peaks that are spectrally close to each other due to the multiple resonance modes generated by different media on the upper and lower surfaces of metal. In addition, owing to the absorption loss of metal and the scattering of holes, the EOT resonance peaks have low transmission coefficient for practical applications. In this work, utilizing a tapered nanohole arrays structure which is stacked by multiple cylindrical holes with the same depth but different radii, we show that tapered nanohole arrays can effectively suppress the excitation of multiple resonance peaks, and a single EOT peak emerges in the transmission spectrum and simultaneously exhibits significantly enhanced transmission (∼7 times) and narrow linewidth (∼15 nm). The enhanced EOT of tapered nanohole arrays can be also found in other wavelength regions and plasmonic materials. Benefiting from isolated transmission peak, high transmission efficiency and extremely narrow linewidth, a highly sensitive plasmonic nanosensor with sensitivity of 1580 nm/RIU and figure of merit of 105 can be attained. We believe that the tapered nanohole structure would enable applications for ultrasensitive sensors, switches and efficient filters.

Periodic Metallo-Dielectric Structures: Electromagnetic Absorption and its Related Developed Temperatures

Multi-layer, metallo-dielectric structures (screens) have long been employed as electromagnetic band filters, either in transmission or in reflection modes. Here we study the radiation energy not transmitted or reflected by these structures (trapped radiation, which is denoted-absorption). The trapped radiation leads to hot surfaces. In these bi-layer screens, the top (front) screen is made of metallic hole-array and the bottom (back) screen is made of metallic disk-array. The gap between them is filled with an array of dielectric spheres. The spheres are embedded in a dielectric host material, which is made of either a heat-insulating (air, polyimide) or heat-conducting (MgO) layer. Electromagnetic intensity trapping of 97% is obtained when a 0.15 micron gap is filled with MgO and Si spheres, which are treated as pure dielectrics (namely, with no added absorption loss). Envisioned applications are anti-fogging surfaces, electromagnetic shields, and energy harvesting structures.

Plasmonics for Biosensing

Techniques based on plasmonic resonance can provide label-free, signal enhanced, and real-time sensing means for bioparticles and bioprocesses at the molecular level. With the development in nanofabrication and material science, plasmonics based on synthesized nanoparticles and manufactured nano-patterns in thin films have been prosperously explored. In this short review, resonance modes, materials, and hybrid functions by simultaneously using electrical conductivity for plasmonic biosensing techniques are exclusively reviewed for designs containing nanovoids in thin films. This type of plasmonic biosensors provide prominent potential to achieve integrated lab-on-a-chip which is capable of transporting and detecting minute of multiple bio-analytes with extremely high sensitivity, selectivity, multi-channel and dynamic monitoring for the next generation of point-of-care devices.

A Photonic Switch Based on a Hybrid Combination of Metallic Nanoholes and Phase-change Vanadium Dioxide

A photonic switch is an integral part of optical telecommunication systems. A plasmonic bandpass filter integrated with materials exhibiting phase transition can be used as a thermally reconfigurable optical switch. This paper presents the design and demonstration of a broadband photonic switch based on an aluminium nanohole array on quartz utilising the semiconductor-to-metal phase transition of vanadium dioxide. The fabricated switch shows an operating range over 650 nm around the optical communication C, L, and U band with maximum 20%, 23% and 26% transmission difference in switching in the C band, L band, and U band, respectively. The extinction ratio is around 5 dB in the entire operation range. This architecture is a precursor for developing micron-size photonic switches and ultra-compact modulators for thin film photonics.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

Extraordinary optical transmission through a rectangular hole filled with extreme uniaxial metamaterials

This Letter presents a theory of extraordinary optical transmission (EOT) through a rectangular hole filled with the extreme uniaxial metamaterials with infinite longitudinal components of permittivity (ϵ_z) and permeability (μ_z). We demonstrate theoretically and numerically that a number of high-order transverse electromagnetic (TEM) modes can be supported by such a structure, and that, more interestingly, their normalized transmittance can be remarkably enhanced due to the Fabry-Perot resonance effect. A set of illustrative examples has been provided to demonstrate that such an EOT property could be explored for the purpose of subwavelength-resolution imaging.

Angle resolved transmission through metal hole gratings

We present the first angle resolved measurements of extraordinary optical transmission (EOT) through hole array gratings in a gold film. Varying the lattice spacing of the arrays and looking at higher diffraction orders, we retrieve the angular emission pattern of the constituent holes with better signal to noise ratio than with single-hole experiments. We present a method to determine separately the angular dependence of the direct and resonant contribution to EOT by using the spectral features of the diffraction orders together with an established model. The comparison of our results with the known angular transmission of a single hole in a metal film yields a good agreement for s-polarized light. Deviations are found for illumination with p-polarized light and we address the discrepancy with Coupled Mode Model calculations and Finite Difference Time Domain simulations. These measured deviations are currently not fully understood.

High refractive index metamaterials using corrugated metallic slots

We report on a method for realizing high refractive index metamaterials using corrugated metallic slot structures at terahertz frequencies. The effective refractive index and peak index frequency can be controlled by varying the width of the air gap in the corrugated slot arrays. The phenomenon occurs because of the secondary resonance effect due to the fundamental inductive-capacitive resonance, which generates a red-shift of the fundamental resonance determined by twice the length of the corrugated metallic slots. In addition, multiple gaps in the corrugated slots act as plasmonic hotspots which have the properties of three-dimensional subwavelength confinement due to extremely strong enhancement of the terahertz waves. The versatile characteristics of the structures may have many potential applications in designing compact optical devices incorporating various functionalities and in developing highly sensitive spectroscopic/imaging systems.

Plasmonic control of extraordinary optical transmission in the infrared regime

We demonstrate that the spectral location of extraordinary optical transmission (EOT) resonances in metallic arrays of rectangular holes can be plasmonically tuned in the near and mid-infrared ranges. The experiments have been performed on patterned gold films. We focus on a subset of localized resonances occurring close to the cut-off wavelength of the holes, λ _c. Metals are usually regarded as perfect electric conductors in the infrared regime, with an EOT cut-off resonance found around λ _c = 2 L for rectangular holes (L being the long edge). For real metals, the penetration of the electromagnetic fields is simply seen as effectively enlarging L. However, by changing the hole short edge, we have found that λ _c varies due to the excitation of gap surface plasmon polaritons. Finite-element calculations confirm that in these high aspect ratio rectangles with short edges two important aspects have to be taken into account in order to explain the experiments: the finite conductivity of the metal and the excitation of gap-surface plasmons inside the nanoholes.

Continuously Tunable, Polarization Controlled, Colour Palette Produced from Nanoscale Plasmonic Pixels

Colour filters based on nano-apertures in thin metallic films have been widely studied due to their extraordinary optical transmission and small size. These properties make them prime candidates for use in high-resolution colour displays and high accuracy bio-sensors. The inclusion of polarization sensitive plasmonic features in such devices allow additional control over the electromagnetic field distribution, critical for investigations of polarization induced phenomena. Here we demonstrate that cross-shaped nano-apertures can be used for polarization controlled color tuning in the visible range and apply fundamental theoretical models to interpret key features of the transmitted spectrum. Full color transmission was achieved by fine-tuning the periodicity of the apertures, whilst keeping the geometry of individual apertures constant. We demonstrate this effect for both transverse electric and magnetic fields. Furthermore we have been able to demonstrate the same polarization sensitivity even for nano-size, sub-wavelength sets of arrays, which is paramount for ultra-high resolution compact colour displays.

Oscillatory penetration of near-fields in plasmonic excitation at metal-dielectric interfaces

The electric field immediately below an illuminated metal-film that is perforated with a hole array on a dielectric consists of direct transmission and scattering of the incident light through the holes and evanescent near-field from plasmonic excitations. Depending on the size and shape of the hole apertures, it exhibits an oscillatory decay in the propagation direction. This unusual field penetration is explained by the interference between these contributions, and is experimentally confirmed through an aperture which is engineered with four arms stretched out from a simple circle to manipulate a specific plasmonic excitation available in the metal film. A numerical simulation quantitatively supports the experiment. This fundamental characteristic will impact plasmonics with the near-fields designed by aperture engineering for practical applications.

Plasmonic metasurface for simultaneous detection of polarization and spectrum

We present a new plasmonic metasurface for simultaneous detection of polarization and spectrum of incident light. The demonstrated metasurface is a rationally designed cluster of artificial atoms that are engineered to exhibit polarization and wavelength-selective optical transmission. The fundamental building block of this structure is periodically coupled subwavelength aperture arrays with different orientations and lattice constants. When integrated with pixelated photodetectors, the metasurface can be used to measure the polarization and spectral information of an optical input. In this Letter, simultaneous detection of the polarization and spectrum of polarized light was experimentally demonstrated by analyzing the transmitted intensity distribution through the metasurface. The demonstrated metasurface offers great potential for many applications, such as polarimetric multispectral imaging and polarization-division multiplexing in optical communications.

Extraordinary optical transmission in nanopatterned ultrathin metal films without holes

We experimentally and theoretically demonstrate that a continuous gold film on a periodically textured substrate exhibits extraordinary optical transmission, even though no holes were etched in the film. Our film synthesis started by nanoimprinting a periodic array of nanocups with a period of ∼750 nm on a polystyrene film over a glass substrate. A thin non-conformal gold film was sputter-deposited on the polystyrene by angle-directed deposition. The gold film was continuous with spatial thickness variation, the film being thinnest at the bottom of the nanocup. Measurements 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. 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, and ∼40 in the ultrathin gold layer causing transmission through it. We show a wavelength red-shift of ∼30 nm in the extraordinary transmission peak when the nanocups are coated with a thin film of a few nanometers, which can be utilized for biosensing. The continuous corrugated metal films are far simpler structures to observe extraordinary transmission, circumventing the difficult process of etching the metal film. Such continuous metal films with ultrathin regions are simple platforms for non-linear optics, plasmonics, and biological and chemical sensing.

Absorption-induced transparency metamaterials in the terahertz regime

Contrary to what might be expected, when an organic dye is sputtered onto an opaque holey metal film, transmission bands can be observed at the absorption energies of the molecules. This phenomenon, known as absorption-induced transparency, is aided by a strong modification of the propagation properties of light inside the holes when filled by the molecules. Despite having been initially observed in metallic structures in the optical regime, new routes for investigation and applications at different spectral regimes can be devised. Here, to illustrate the potential use of absorption-induced transparency at terahertz, a method for molecular detection is presented and supported by a theoretical analysis.

Tunable Plasmonic Nanohole Arrays Actuated by a Thermoresponsive Hydrogel Cushion

New plasmonic structure with actively tunable optical characteristics based on thermoresponsive hydrogel is reported. It consists of a thin, template-stripped Au film with arrays of nanoholes that is tethered to a transparent support by a cross-linked poly(N-isopropylacrylamide) (pNIPAAm)-based polymer network. Upon a contact of the porous Au surface with an aqueous environment, a rapid flow of water through the pores enables swelling and collapsing of the underlying pNIPAAm network. The swelling and collapsing could be triggered by small temperature changes around the lower critical solution temperature (LCST) of the hydrogel. The process is reversible, and it is associated with strong refractive index changes of Δn ∼ 0.1, which characteristically alters the spectrum of surface plasmon modes supported by the porous Au film. This approach can offer new attractive means for optical biosensors with flow-through architecture and actively tunable plasmonic transmission optical filters.

Polarization-independent plasmonic subtractive color filtering in ultrathin Ag nanodisks with high transmission

We demonstrate a TE/TM polarization-independent plasmonic subtractive color filtering scheme employing ultrathin two-dimensional Ag nanodisks. These TE/TM polarization-independent subtractive color filters exhibit small feature sizes (below 200 nm) and high transmission up to 70% in the visible spectral region, superior to previously reported plasmonic color filters. Simulated optical transmission spectra and colors are in good agreement with experimental results. The color-filtering behaviors strongly depend on thickness and period of nanodisks. Underlying mechanisms are also discussed in detail.

Broadband chirality-coded meta-aperture for photon-spin resolving

The behaviour of light transmitted through an individual subwavelength aperture becomes counterintuitive in the presence of surrounding ‘decoration', a phenomenon known as the extraordinary optical transmission. Despite being polarization-sensitive, such an individual nano-aperture, however, often cannot differentiate between the two distinct spin-states of photons because of the loss of photon information on light-aperture interaction. This creates a ‘blind-spot' for the aperture with respect to the helicity of chiral light. Here we report the development of a subwavelength aperture embedded with metasurfaces dubbed a ‘meta-aperture', which breaks this spin degeneracy. By exploiting the phase-shaping capabilities of metasurfaces, we are able to create specific meta-apertures in which the pair of circularly polarized light spin-states produces opposite transmission spectra over a broad spectral range. The concept incorporating metasurfaces with nano-apertures provides a venue for exploring new physics on spin-aperture interaction and potentially has a broad range of applications in spin-optoelectronics and chiral sensing.

Nano-apertures cannot distinguish between distinct spin-states of photons because of information loss upon light-aperture interaction. Here, Du et al. report a subwavelength aperture integrated with metasurfaces which breaks spin degeneracy and produces opposite transmission spectra over a broad spectral range.

Antenna-coupled photon emission from hexagonal boron nitride tunnel junctions

The ultrafast conversion of electrical signals to optical signals at the nanoscale is of fundamental interest for data processing, telecommunication and optical interconnects. However, the modulation bandwidths of semiconductor light-emitting diodes are limited by the spontaneous recombination rate of electron-hole pairs, and the footprint of electrically driven ultrafast lasers is too large for practical on-chip integration. A metal-insulator-metal tunnel junction approaches the ultimate size limit of electronic devices and its operating speed is fundamentally limited only by the tunnelling time. Here, we study the conversion of electrons (localized in vertical gold-hexagonal boron nitride-gold tunnel junctions) to free-space photons, mediated by resonant slot antennas. Optical antennas efficiently bridge the size mismatch between nanoscale volumes and far-field radiation and strongly enhance the electron-photon conversion efficiency. We achieve polarized, directional and resonantly enhanced light emission from inelastic electron tunnelling and establish a novel platform for studying the interaction of electrons with strongly localized electromagnetic fields.

Hybrid Metal-Semiconductor Nanostructure for Ultrahigh Optical Absorption and Low Electrical Resistance at Optoelectronic Interfaces

Engineered optoelectronic surfaces must control both the flow of light and the flow of electrons at an interface; however, nanostructures for photon and electron management have typically been studied and optimized separately. In this work, we unify these concepts in a new hybrid metal-semiconductor surface that offers both strong light absorption and high electrical conductivity. We use metal-assisted chemical etching to nanostructure the surface of a silicon wafer, creating an array of silicon nanopillars protruding through holes in a gold film. When coated with a silicon nitride anti-reflection layer, we observe broad-band absorption of up to 97% in this structure, which is remarkable considering that metal covers 60% of the top surface. We use optical simulations to show that Mie-like resonances in the nanopillars funnel light around the metal layer and into the substrate, rendering the metal nearly transparent to the incoming light. Our results show that, across a wide parameter space, hybrid metal-semiconductor surfaces with absorption above 90% and sheet resistance below 20 Ω/□ are realizable, suggesting a new paradigm wherein transparent electrodes and photon management textures are designed and fabricated together to create high-performance optoelectronic interfaces.

Spatial optical crosstalk in CMOS image sensors integrated with plasmonic color filters

Imaging resolution of complementary metal oxide semiconductor (CMOS) image sensor (CIS) keeps increasing to approximately 7k × 4k. As a result, the pixel size shrinks down to sub-2μm, which greatly increases the spatial optical crosstalk. Recently, plasmonic color filter was proposed as an alternative to conventional colorant pigmented ones. However, there is little work on its size effect and the spatial optical crosstalk in a model of CIS. By numerical simulation, we investigate the size effect of nanocross array plasmonic color filters and analyze the spatial optical crosstalk of each pixel in a Bayer array of a CIS with a pixel size of 1μm. It is found that the small pixel size deteriorates the filtering performance of nanocross color filters and induces substantial spatial color crosstalk. By integrating the plasmonic filters in the low Metal layer in standard CMOS process, the crosstalk reduces significantly, which is compatible to pigmented filters in a state-of-the-art backside illumination CIS.

Polarization-selective optical transmission through a plasmonic metasurface

We present the design, fabrication, and experimental characterization of a nanoslit-based metasurface that offers polarization-selective optical transmission for advanced imaging applications. The metasurface consists of an array of meta-atoms, constructed with two orthogonally coupled subwavelength apertures. Highly enhanced optical transmission was achieved by selective excitation of surface plasmon waves on the metasurface. By rotating the orientation of the linearly polarized incident beam, switching of enhanced optical transmission bands through the metasurface was experimentally demonstrated. This demonstration is a significant step towards developing advanced multispectral imaging devices.

Rational design of metallic nanocavities for resonantly enhanced four-wave mixing

Optimizing the shape of nanostructures and nano-antennas for specific optical properties has evolved to be a very fruitful activity. With modern fabrication tools a large variety of possibilities is available for shaping both nanoparticles and nanocavities; in particular nanocavities in thin metal films have emerged as attractive candidates for new metamaterials and strong linear and nonlinear optical systems. Here we rationally design metallic nanocavities to boost their Four-Wave Mixing response by resonating the optical plasmonic resonances with the incoming and generated beams. The linear and nonlinear optical responses as well as the propagation of the electric fields inside the cavities are derived from the solution of Maxwell's equations by using the 3D finite-differences time domain method. The observed conversion-efficiency of near-infrared to visible light equals or surpasses that of BBO of equivalent thickness. Implications to further optimization for efficient and broadband ultrathin nonlinear optical materials are discussed.

Color generation via subwavelength plasmonic nanostructures

Recent developments in color filtering and display technologies have focused predominantly on high resolution, color vibrancy, high efficiency, and slim dimensions. To achieve these goals, metallic nanostructures have attracted extensive research interest due to their abilities to manipulate the properties of light through surface plasmon resonances. In this paper, we review recent representative developments in plasmonic color engineering at the nanoscale using subwavelength nanostructures, demonstrating their great potential in high-resolution and high-fidelity color rendering, spectral filtering applications, holography, three-dimensional stereoscopic imaging, etc.

Surface-mode model of the extraordinary optical transmission without plasmons

We propose a microscopic surface-mode model for the extraordinary optical transmission (EOT) through subwavelength metallic slit array covered with a thin dielectric layer under illumination of transverse-electric (TE) polarization. Remarkably, surface plasmon polarizations (SPPs) do not exist for this polarization. It is commonly believed that the waveguide mode in the dielectric layer plays a role similar to that of the SPP in classical EOT. To check the intuitive belief, we derive a surface-mode model by considering the multiple scattering process of the fundamental waveguide mode and neglecting all other residual field in the thin dielectric layer. The model captures the main feature of EOT and provides a phase-matching condition to predict the transmission resonance. Quantitative comparison between fully-vectorial calculations and model predictions shows that besides the fundamental waveguide mode, other residual field in the thin dielectric layer also contributes to the EOT without SPP.

Nanostructure arrays in free-space: optical properties and applications

Dielectric and metallic gratings have been studied for more than a century. Nevertheless, novel optical phenomena and fabrication techniques have emerged recently and have opened new perspectives for applications in the visible and infrared domains. Here, we review the design rules and the resonant mechanisms that can lead to very efficient light-matter interactions in sub-wavelength nanostructure arrays. We emphasize the role of symmetries and free-space coupling of resonant structures. We present the different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures. We discuss the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays.

Efficient multiband and broadband cross polarization converters based on slotted L-shaped nanoantennas

In this paper, we presented highly efficient reflective cross polarization converters based on metamaterials operating in the infrared regime, which are composed of a dielectric spacer sandwiched between slotted L-shaped metallic nanoantennas and a ground plane. The proposed polarization converters can convert a linearly polarized wave to its cross polarized wave with high polarization conversion ratio (> 0.95) over multiple / broad frequency bands. The resulting multi-band and broadband operations are induced by the localized mode hybridizations between the slot and the original metallic nanoantenna. Furthermore, the performance of the proposed converters under different incident angles is also explored. It is found that the first broad band (or the first two resonant frequencies) of the proposed broadband (or multi-band) converters appears to be independent of the incident angle (up to 47°).

Asymmetric half-cone/nanohole array films with structural and directional reshaping of extraordinary optical transmission

Structured films with periodic arrays of nanoholes covered by half-cone shells are fabricated via a simple and efficient colloidal lithography method. The designed films show strong polarization dependence in optical transmission. By decreasing the height of half-cone shells the peak shifts and this shift varies strongly for different orthogonal polarizations. Furthermore, the three-dimensional (3D) asymmetric arrays exhibit a pronounced increase in the transmission intensity by changing the direction of the incident light from the half-cone shell (shelter) side to the empty side. Special surface plasmon resonances excited by the unique 3D asymmetric structure are responsible for these novel properties, and the experimental results are in good agreement with numerical simulations. The nanostructured films in this work will be useful for metallic nanophotonic elements in many applications, including surface plasmon enhanced optical sensing and ultrafast optical switching, as well as versatile substrates for surface enhanced Raman spectroscopy, anisotropic wettability and other potential uses.

Monopole resonators in planar plasmonic metamaterials

We first present a new phenomenon: the quarter-wavelength resonance of an electromagnetic field in planar plasmonic metamaterials consisting of asymmetrically coupled air-slot arrays, which is essential for a monopole resonator. The anti-nodal electric field intensity of the quarter-wavelength fundamental mode is formed by strong charge concentrations at the sharp metallic edges of the crossing position of the air-slots, and the nodal point of the electric field intensity naturally occurs at the other end of the air-slot. By tuning the structural asymmetry, the quarter-wavelength resonances were successfully split from the half-wavelength resonance, experimentally and numerically.

Omnidirectional light absorption of disordered nano-hole structure inspired from Papilio ulysses

Butterflies routinely produce nanostructured surfaces with useful properties. Here, we report a disordered nano-hole structure with ridges inspired by Papilio ulysses that produce omnidirectional light absorption compared with the common ordered structure. The result shows that the omnidirectional light absorption is affected by polarization, the incident angle, and the wavelength. Using the finite-difference time-domain (FDTD) method, the stable omnidirectional light absorption is achieved in the structure inspired from the Papilio ulysses over a wide incident angle range and with various wavelengths. This explains some of the mysteries of the structure of the Papilio ulysses butterfly. These conclusions can guide the design of omnidirectional absorption materials.

Nonreciprocal optical transmission through a single conical air hole in an Ag film

In this paper, we propose a simple metal micro-nano structure having the character of nonreciprocal optical zero-order transmission. The structure is a single conical air hole (CAH) in an Ag film whose optical absorption with geometric asymmetry breaks the time reversal symmetry of the electromagnetic field. By comparing the transmissions of Ag CAH with those of ideal conductor (IC) CAH, three effects of Ag CAH, including diffraction, Fabry-Perot-like (FPL) resonance and localized surface plasmon (LSP) resonance, are analyzed and discussed. Under optimized conditions, we find that the ratio of forward transmission to backward one can be larger than 9 at a proper wavelength in visible range. This kind of Ag CAH is expected to have the potential served as all-optical diode.

Origin of shape resonance in second-harmonic generation from metallic nanohole arrays

Second-harmonic generation (SHG) from periodic arrays of subwavelength rectangular air hole with various aspect ratios perforated in gold thin films can get resonantly enhanced for some specific geometric shapes. Here we clarify the physical origin of this shape resonance effect. A nonlinear coupled-mode theory is set up to solve energy conversion from fundamental wave (FW) mode to second-harmonic wave (SHW) mode within the nanoscale air hole. It reveals that several physical mechanisms, including the FW mode excitation amplitude, FW-SHW modal spatial overlap, FW-SHW mode phase mismatch, and SHW mode attenuation, are all geometric shape sensitive and altogether act to induce the SHG shape resonance effect. The theory agrees well with experimental observations and provides an accurate and complete explanation for the long-emphasized but elusive shape effect. The study may stimulate deeper insights to visualize general nonlinear nanophotonic processes and pave the way to engineering high-efficiency nonlinear nanophotonic devices.

Subwavelength plasmonic color printing protected for ambient use

We demonstrate plasmonic color printing with subwavelength resolution using circular gap-plasmon resonators (GPRs) arranged in 340 nm period arrays of square unit cells and fabricated with single-step electron-beam lithography. We develop a printing procedure resulting in correct single-pixel color reproduction, high color uniformity of colored areas, and high reproduction fidelity. Furthermore, we demonstrate that, due to inherent stability of GPRs with respect to surfactants, the fabricated color print can be protected with a transparent dielectric overlay for ambient use without destroying its coloring. Using finite-element simulations, we uncover the physical mechanisms responsible for color printing with GPR arrays and suggest the appropriate design procedure minimizing the influence of the protection layer.

Polarization anisotropic transmission through metallic Sierpinski-Carpet aperture array

Extraordinary optical transmission through rectangular Sierpinski -Carpet aperture array on an Ag film has been observed. Attributed to the fractal-featured rectangle array, it exhibits polarization dependence and dual-band transmission simultaneously. In addition, the incident angle invariance transmission displays within a certain angle range, which is quite different from ordinary rectangles. This report provides a way to achieve the polarization-manipulated multi-band transmission in infrared region.