2D Subwavelength Research Articles

Hollow‐Out Patterning Ultrathin Acoustic Metasurfaces for Multifunctionalities Using Soft fiber/Rigid Bead Networks

AbstractAcoustic metasurfaces that can manipulate and control sound waves at 2D subwavelength scales open new avenues to unusual applications, such as asymmetric transmission, super‐resolution imaging, and particle manipulation. However, the long‐standing goals of pushing frontier metamaterials research into real practice are still severely constrained by cumbersome configuration, large acoustic loss, and rigid structure of the existing metamaterials. An ultrathin metasurface (10–300 µm in thickness, up to ≈λ/650, λ the wavelength) that is capable of imparting sound wave with a nontrivial phase shift with high transmittance (>80%) in the range of 5–30 kHz is fabricated here. The metasurface is comprised of a porous network of soft polymer fiber/rigid beads that are physically equivalent to crosslinked spring‐mass resonators. Moreover, the traditional paper‐cutting art to carve the ultrathin metasurface into hollow‐out patterns is incorporated, resulting in a variety of remarkable functions, including acoustic vortex, focusing, and super‐resolution. The hollow‐out patterning approach innovates the traditional one‐step metadevice fabrication process into two separated steps: 1) fabrication of ultrathin metasurfaces; 2) hollow‐out patterning of metasurfaces. The strategy opens an avenue to mass production of acoustic metadevices, shedding light on the applications of the metamaterials in acoustic cloaking, acoustic positioning, and particle manipulation.

Evanescent waveguide mode induced surface plasmonic wave excitation in subwavelength hole array enhanced quantum dot infrared photodetectors

In this paper, we present the analysis of the evanescent waveguide modes in a 2D subwavelength hole array (2DSHA) plasmonic structure and their relationship with the excited surface plasmonic resonance (SPR). It is found that when the metal film thickness is larger than 0.0625 µm, the excited SPRs have a linear dependence on the evanescent waveguide modes, whereas when the metal film is thin, the linear relationship does not hold, primarily due to the coupling of the SPRs on the asymmetric interfaces of the metal film. The linear dependence is also verified by curve-fitting the Fano transmission profiles of the 2DSHA structures. 2DSHA enhanced quantum dot infrared photodetectors with various metal film thicknesses were fabricated and tested. The measured plasmonic enhancement ratios agree well with the analysis.

Random Coherent Perfect Absorption with 2D Atomic Materials Mediated by Anderson Localization

Tailoring the spatiotemporal confinement of light with two-dimensional (2D) atomic materials is critical to harnessing their unique and attractive optoelectronic properties. Here, we report on the interplay between the localization of light within a disordered medium and the extreme material and loss localization afforded by deeply subwavelength (∼λ0/5000) single-layer 2D atomic materials. Structures in the regime of Anderson localization are found to support a condition termed “random coherent perfect absorption” (RCPA). This yields an array of optical effects that are unachievable in conventional lossy random media, including >99.9% absorption with Q-factors ranging from ∼102 to 106, Fano-resonance behavior in 1D, coexistence of RCPA and extraordinary transmission, and angle-selective conditions supporting the coalescence of all random modes toward perfect absorption.

Implementing Quantum Search Algorithm with Metamaterials.

Metamaterials, artificially structured electromagnetic (EM) materials, have enabled the realization of many unconventional EM properties not found in nature, such as negative refractive index, magnetic response, invisibility cloaking, and so on. Based on these man-made materials with novel EM properties, various devices are designed and realized. However, quantum analog devices based on metamaterials have not been achieved so far. Here, metamaterials are designed and printed to perform quantum search algorithm. The structures, comprising of an array of 2D subwavelength air holes with different radii perforated on the dielectric layer, are fabricated using a 3D-printing technique. When an incident wave enters in the designed metamaterials, the profile of beam wavefront is processed iteratively as it propagates through the metamaterial periodically. After ≈N roundtrips, precisely the same as the efficiency of quantum search algorithm, searched items will be found with the incident wave all focusing on the marked positions. Such a metamaterial-based quantum searching simulator may lead to remarkable achievements in wave-based signal processors.

Chromium plasmonic polarizer for high intensity light

In this work, we investigate a thin-film polarizer for a high intensity of the electromagnetic (EM) beam based on Cr nano wire arrays. Commonly used thin-film polarizing components are very sensitive for high power of EM waves and can be easily damaged by focused beams. The solution to this problem could be the thin-film polarizer based on metallic subwavelengths structures. This type of optical element has huge resistance comparing to typical thin-film polarizers. However, designing such an optical element for proper wavelength of EM wave and transmissions is not easy task. In this paper we present numerical as well as experimental results for specially designed chromium thin-film polarizer for wavelength 532nm Full Text: PDF ReferencesW. Zhou, K. Li, C. Song, P. Hao, M. Chi, M. Yu and Y. Wu, "Polarization-independent and omnidirectional nearly perfect absorber with ultra-thin 2D subwavelength metal grating in the visible region", Opt. Express 23, 11 (2015). CrossRef W. L. Barnes, A . Dereux, and T. W. Ebbesen, "Surface plasmon subwavelength optics", Nature 424, 824-830 (2003). CrossRef C. Lee, E. Sim, D. Kim, "Blazed wire-grid polarizer for plasmon-enhanced polarization extinction: design and analysis", Opt. Express 25, 7 (2017). CrossRef A. Lehmuskero, Metallic thin film structures and polarization shaping gratings (University of Eastern Finland 2010).Y. Leroux, J. C. Lacroix, C. Fave, V. Stockhausen, N. Felidj, J. Grandm, A. Hohenau, J. R. Krenn, "Active plasmonic devices with anisotropic optical response: a step toward active polarizer", Nano Lett. 5, 9 (2009). CrossRef R. T. Perkins, D. P. Hansen, E. W. Gardner, J. M. Thorne, A. A. Robbins, Broadband wire grid polarizer for the visible spectrum, US 6122103 (2000). DirectLink D. M. Sullivan, Electromagnetic simulation using the FDTD method, New York: IEEE Press Series (2000). CrossRef J. P. Berenger, Perfectly Matched Layer (PML) for Computational Electromagnetics, Morgan & Claypool Publishers (2007). CrossRef Yu, W., and R. Mittra, "A conformal FDTD software package modeling antennas and microstrip circuit components", IEEE Antennas Propagat. Magazine 42, 28 (2000) . CrossRef L. W. Bos, D. W. Lynch, "Optical Properties of Antiferromagnetic Chromium and Dilute Cr-Mn and Cr-Re Alloys", Phys. Rev. Sect. B, 2, 4267 (1970). CrossRef

Decoupling co-existing surface plasmon polariton (SPP) modes in a nanowire plasmonic waveguide for quantitative mode analysis

Knowledge of surface plasmon polariton (SPP) modes in one-dimensional (1D) metallic nanostructures is essential for the development of subwavelength optical devices such as photonic circuits, integrated light sources, and photodetectors. Despite many efforts to characterize the propagation parameters of these subwavelength 1D plasmonic waveguides, such as Ag nanowires, large discrepancies exist among available reports owing to their sensitivity to the relative weights of co-existing SPP modes and the lack of a method of decoupling these modes and analyzing them separately. In this work, we develop an interference method to distinguish different SPP modes that are simultaneously excited in a Ag nanowire waveguide and measure their propagation parameters separately. By extracting information from the propagation-distancedependent intensity oscillations of the scattered light from the nanowire tip, the effective refractive indices, propagation lengths, and relative mode weights of co-existing SPP modes supported by the nanowire are derived from a mode interference model. These parameters depend strongly on the nanowire diameter and excitation wavelength. In particular, we demonstrate the possibility of selective excitation of different SPP modes by varying the nanowire diameter. This new mode analysis technique provides unique insights into the development and optimization of SPP-based applications. Open image in new window

Fluorescent Nanowire Ring Illumination for Wide-Field Far-Field Subdiffraction Imaging

Here we demonstrate an active method which pioneers in utilizing a combination of a spatial frequency shift and a Stokes frequency shift to enable wide-field far-field subdiffraction imaging. A fluorescent nanowire ring acts as a localized source and is combined with a film waveguide to produce omnidirectional illuminating evanescent waves. Benefitting from the high wave vector of illumination, the high spatial frequencies of an object can be shifted to the passband of a conventional imaging system, contributing subwavelength spatial information to the far-field image. A structure featuring 70-nm-wide slots spaced 70nm apart has been resolved at a wavelength of 520nm with a 0.85 numerical aperture standard objective based on this method. The versatility of this approach has been demonstrated by imaging integrated chips, Blu-ray DVDs, biological cells, and various subwavelength 2D patterns, with a viewing area of up to 1000 μm^{2}, which is one order of magnitude larger than the previous far-field and full-field nanoscopy methods. This new resolving technique is label-free, is conveniently integrated with conventional microscopes, and can potentially become an important tool in cellular biology, the on-chip industry, as well as other fields requiring wide-field nanoscale visualization.

一维增透亚波长光栅的研究

一维增透亚波长光栅的研究

Merging plasmonics and metamaterials by two-dimensional subwavelength structures

Functional plasmonic 2D subwavelength structures are reviewed in a new framework of modes exchanging between freely propagating and bound waves.

Polarization-driven self-organization of silver nanoparticles in 1D and 2D subwavelength gratings for plasmonic photocatalysis

One of the main challenges in plasmonics is to conceive large-scale, low-cost techniques suitable for the fabrication of metal nanoparticle patterns showing precise spatial organization. Here, we introduce a simple method based on continuous-wave laser illumination to induce the self-organization of silver nanoparticles within high-index thin films. We show that highly regular and homogeneous nanoparticle gratings can be produced on large areas using laser-controlled self-organization processes. This very versatile technique can provide 1D and 2D patterns at a subwavelength scale with tunable features. It does not need any stabilization or expensive devices, such as those required by optical or electron lithography, and is rapid to implement. Accurate in-plane and in-depth characterizations provide valuable information to explain the mechanisms that lead to pattern formation and especially how 2D self-organization can fall into place with successive laser scans. The regular and homogeneous 2D self-organization of metallic NPs with a single laser scan is also reported for the first time in this article. As the reported nanostructures are embedded in porous TiO2, we also theoretically explore the interesting potential of organization on the photocatalytic activity of Ag-NP-containing TiO2 porous films, which is one of the most promising materials for self-cleaning or remediation applications. Realistic electromagnetic simulations demonstrate that the periodic organization of silver nanoparticles can increase the light intensity within the film more than ten times that produced with randomly distributed nanoparticles, leading as expected to enhanced photocatalytic efficiency.

Lithographically Defined, Room Temperature Low Threshold Subwavelength Red-Emitting Hybrid Plasmonic Lasers.

Hybrid plasmonic lasers provide deep subwavelength optical confinement, strongly enhanced light-matter interaction and together with nanoscale footprint promise new applications in optical communication, biosensing, and photolithography. The subwavelength hybrid plasmonic lasers reported so far often use bottom-up grown nanowires, nanorods, and nanosquares, making it difficult to integrate these devices into industry-relevant high density plasmonic circuits. Here, we report the first experimental demonstration of AlGaInP based, red-emitting hybrid plasmonic lasers at room temperature using lithography based fabrication processes. Resonant cavities with deep subwavelength 2D and 3D mode confinement of λ2/56 and λ3/199, respectively, are demonstrated. A range of cavity geometries (waveguides, rings, squares, and disks) show very low lasing thresholds of 0.6-1.8 mJ/cm2 with wide gain bandwidth (610 nm-685 nm), which are attributed to the heterogeneous geometry of the gain material, the optimized etching technique, and the strong overlap of the gain material with the plasmonic modes. Most importantly, we establish the connection between mode confinements and enhanced absorption and stimulated emission, which plays critical roles in maintaining low lasing thresholds at extremely small hybrid plasmonic cavities. Our results pave the way for the further integration of dense arrays of hybrid plasmonic lasers with optical and electronic technology platforms.

Leaky-Wave Radiations by Modulating Surface Impedance on Subwavelength Corrugated Metal Structures.

One-dimensional (1D) subwavelength corrugated metal structures has been described to support spoof surface plasmon polaritons (SPPs). Here we demonstrate that a periodically modulated 1D subwavelength corrugated metal structure can convert spoof SPPs to propagating waves. The structure is fed at the center through a slit with a connected waveguide on the input side. The subwavelength corrugated metal structure on the output surface is regarded as metasurface and modulated periodically to realize the leaky-wave radiation at the broadside. The surface impedance of the corrugated metal structure is modulated by using cosine function and triangle-wave function, respectively, to reach the radiation effect. Full wave simulations and measuremental results are presented to validate the proposed design.

Surface plasmon polariton scattering by subwavelength silicon wires.

Surface plasmon polariton scattering from 2D subwavelength silicon wires is investigated using the finite-difference time-domain method. It is shown that coupling an incident surface plasmon polariton to intercavity modes of the particle can dramatically change transmitted fields and plasmon-induced forces. In particular, both transmission and optical forces are highly sensitive to the particle size that is related to the excitation of whispering gallery modes or standing wave modes depending on the particle shape and size. These features might have potential sensing applications.

Tunable dark modes in one-dimensional "diatomic" dielectric gratings.

Recently researchers have demonstrated ultra high quality factor (Q) resonances in one-dimensional (1D) dielectric gratings. Here we theoretically investigate a new class of subwavelength 1D gratings, namely "diatomic" gratings with two nonequivalent subcells in one period, and utilize their intrinsic dark modes to achieve robust ultra high Q resonances. Such "diatomic" gratings provide extra design flexibility, and enable high Q resonators using thinner geometry with smaller filling factors compared to conventional designs like the high contrast gratings (HCGs). More importantly, we show that these high Q resonances can be efficiently tuned in situ, making the design appealing in various applications including optical sensing, filtering and displays.

Polarization-independent and omnidirectional nearly perfect absorber with ultra-thin 2D subwavelength metal grating in the visible region.

A polarization-independent and omnidirectional nearly perfect absorber in the visible region has been proposed. The absorber is two-layer structure consisting of a subwavelength metal grating layer embedded in the high refractive index and lossless dielectric layer on the metal substrate. Extraordinary optical absorption with absorption peaks of over 99% can be achieved over the whole visible region for both TM and TE polarization. This absorption is attributed to cavity mode (CM) resonance caused by the coupled surface plasmon polaritons (SPP). Through adjusting the grating thickness, the absorption peak can be tuned linearly, which is highly advantageous to design various absorbers. Furthermore, the absorbance retains ultra-high over a wide angular range of incidence for both TM and TE polarization. This nearly perfect absorber offers great potential in the refractive index (RI) sensors, integrated photodetectors, solar cells and so on.

Properties of two-dimensional resonant reflectors with zero-contrast gratings.

The spectral properties of high-reflection mirrors imbued with two-dimensional (2D) subwavelength periodicity are investigated. The reflectors are designed in a silicon-on-glass film that is partially etched to implement a zero-contrast interface between the grating pillars and the sublayer, thereby annulling the local reflections and phase changes associated with hard interfaces. This approach has shown impressive results for 1D polarized reflectors; here we strive to discover analogous wideband unpolarized reflectors. Using particle swarm optimization, we report wideband unpolarized reflectors in the 1.4-2.0μm wavelength band. A 2D reflector with square pillars exhibits 99% reflectance across a bandwidth exceeding 350nm and possesses tolerance against angular deviations. The complementary structure with rectangular periodic voids achieves a bandwidth of 370nm. A comparable, optimized 2D high-contrast grating reflector with grating pillars residing directly on the substrate yields a 99% bandwidth of 240nm.

Uniform distribution of Ag particles upon imprinted polymer grating for Raman signal enhancement

Surface Enhanced Raman Scattering (SERS) is gaining popularity among analytical methods in biosciences and sensor technology since it provides high specificity, non-destructiveness, and the unique fingerprint spectra of the molecules. Historically, glass has been the primary choice as a substrate for SERS, but polymers are attractive due to their plasticity, ease of handling, and their low cost. Herein, the performance of cyclo olefin polymer (COP) as a substrate with 1D subwavelength modulations combined with silver nanoparticles is studied for SERS measurements. These 1D grating structures on polymer are fabricated by hot embossing method followed by deposition of silver nanoparticles (AgNPs) using the drop-casting method. Spatial variations of the substrate surface have been reduced by providing a consistent distribution of hot-spots. We present an analysis of the surface uniformity related to the distribution of Ag particles. We achieve around 8-fold Raman signal enhancements with improved reproducibility in comparison to smooth, unmodulated surfaces with AgNPs. This method of fabrication of SERS substrates is simple and inexpensive compared to the thermal evaporation method (TEM) of metallic layer deposition. It also helps to control the tarnishing effect on metallic surfaces due to silver deposition prior to Raman measurements. This kind of polymer gratings combined with AgNPs have potential applications in medical, biological and chemical sensing, where Raman signal enhancement with high reproducibility is required.

Waveguide-mode interference lithography technique for high contrast subwavelength structures in the visible region.

We explore possibilities of waveguide-mode interference lithography (WMIL) technique for high contrast subwavelength structures in the visible region. Selecting an appropriate waveguide-mode, we demonstrate high contrast resist mask patterns for the first time. TM1 mode in the waveguide is shown to be useful for providing a three-dimensional structure whose cross section is checkerboard pattern. Applying our WMIL technique, we demonstrate 1D, 2D and 3D subwavelength resist patterns that are widely used for the fabrication of metamteterials in the visible region. In addition to the resist patterns, we demonstrate a resonance at 1.9 eV for a split tube structure experimentally.

Design study of nanograting-based surface plasmon resonance biosensor in the near-infrared wavelength

A key issue with surface plasmon resonance (SPR) biosensors, which are the focus of many researchers, is improving their sensitivity to detect lower amounts of analyte in a solution. Most SPR developments have focused on the grating-based sensitivity-enhancement approach. In addition to sensitivity, a substantial enhancement of other sensor characteristics such as resolution and signal-to-noise ratio (SNR) is desired for designing a practical sensor. So, in this paper, the characteristics of surface plasmon polaritons sustained by 1D subwavelength metallic gratings on a thin metal slab (under the Krestchmann configuration) have been investigated numerically for the analyte-ligand interactions detection. Effects of different structural parameters, such as grating period, grating depth, metal film thickness, and fill factor have been evaluated on the sensor sensitivity as well as resolution and SNR. Numerical results indicate that the sensor working in the near-infrared wavelength has a better performance than that in the visible one. The result of numerical investigation has been used to design an optimized sensor with the best figure of merit.

Two-dimensional atom localization via two standing-wave fields in a four-level atomic system

We propose a scheme for the two-dimensional (2D) localization of an atom in a four-level Y-type atomic system. By applying two orthogonal standing-wave fields, the atoms can be localized at some special positions, leading to the formation of sub-wavelength 2D periodic spatial distributions. The localization peak position and number as well as the conditional position probability can be controlled by the intensities and detunings of optical fields.