Metallic Nanoslit Research Articles

Borophene-assisted selective enhancement of transmission through metallic nanoslits in telecommunication waveband

Two-dimensional materials show the ability to manipulate light at extreme subwavelength scales, opening up a new opportunity toward next-generation efficient integrated optical modulator. Herein, we theoretically designed a borophene-based modulator for the effective modulation of transmission through silver nanoslits in telecommunication waveband. Simulations reveal that by simply covering borophene monolayer on silver nanoslits, the transmission of the device can be selectively enhanced due to the excitation of magnetic polaritons. Maximum transmission of larger than 90% can be achieved for both crystal directions of borophene. By further dynamically adjusting the electron density of borophene, the transmission can be continuously modulated, of which the resonant wavelengths can be well predicted by the equivalent inductor-capacitor circuit model. High modulation efficiency of 88.42% can be achieved at 1550 nm. In addition, the device exhibits excellent robustness to incident angles. The designed borophene-based device offers a promising way to achieve the effective transmission modulation.

Metasurfaces for generating higher-order Poincaré beams by polarization-selective focusing and overall elimination of co-polarization components.

Focused higher-order Poincaré (HOP) beams are of particular interest because they facilitate understanding the exotic properties of structured light and their applications in classical physics and quantum information. However, generating focused HOP beams using metasurfaces is challenging. In this study, we proposed a metasurface design comprising two sets of metal nanoslits for generating coaxially focused HOP beams. The nanoslits were interleaved on equispaced alternating rings. The initial rings started at the two adjacent Fresnel zones to provide opposite propagation phases for overall elimination of the co-polarization components. With the designed hyperbolic and helical profiles of the geometric phases, the two vortices of the opposite cross-circular-polarizations were formed and selectively focused, realizing HOP beams of improved quality. Simulations and experimental results demonstrated the feasibility of the proposed metasurface design. This study is of significance in the integration of miniaturized optical devices and enriches the application areas of metasurfaces.

High performance extra-ordinary optical transmission based self-referenced plasmonic metagrating sensor in the NIR communication band

Simulation of an extra-ordinary optical transmission based self-referenced, flexible plasmonic metagrating has been reported. The metagrating was optimized to work as a refractive index (RI) sensor with high figure of merit (FOM) for near infra-red (NIR) communication band. The metagrating consists of two metal nanoslit arrays (MNSAs) in a manner that the open portion (groove) of the upper MNSA overlaps with the closed portion (pit) of the lower MNSA and vice versa. The metagrating structure was optimized to support dual plasmonic modes; one of them being sensing mode and the other, self-referenced. Transmission efficiency of 57%, the sensitivity of 1147 nm RIU−1, and FOM of 271/RIU were achieved for the analyte RI range 1.30–1.38. This design of metagrating possesses a stronger coupling of electromagnetic (EM) fields between the constituent MNSAs, which results in higher (almost double) transmission efficiency and FOM as compared to trivial MNSAs. Control simulations were performed to understand the role of various parameters on self-referencing operation, to evaluate the fabrication tolerances, and to estimate the performance at various ambient temperatures. The present study will be useful in development of flexible, low-cost, yet performance-enhanced metagrating sensors, which could easily be integrated on the tip of optical fibers working in the NIR communication window.

Virtual lattice resonance of a single nanoresonator in a metal nanoslit

We study the transmission through a subwavelength metallic slit loaded with a single nanoresonator. To gain physical insight into the problem a theoretical model combining the dipole approximation and image theory is developed. The model shows that the coupling between the single nanoresonator and the slit's cavity modes serves as a localized analog to an infinite nanoresonator array. This virtual array supports a surface image-lattice resonance due to the coherent self-scattering of the single nanoresonator. Thereby, it may lead to the ability to mimic many recently reported intriguing physical phenomena of real surface-lattice resonances in nanoresonator arrays, by a single-subwavelength system. We specifically show that it leads to enhanced light-matter interaction, and to the appearance of an extraordinary transmission window. The theoretical results are in good agreement with full-wave numerical simulations.

Arbitrary manipulations of focused higher-order Poincaré beams by a Fresnel zone metasurface with alternate binary geometric and propagation phases

The manipulation of high-quality vector beams (VBs) with metasurfaces is an important topic and has potential for classical and quantum applications. In this paper, we propose a Fresnel zone (FZ) metasurface with metallic nanoslits arranged on FZs, which sets alternate binary geometric and propagation phases to cancel the incident spin component and focus the converted spin component (CSC). The rotation designs of nanoslits transform the incident polarization state on the conventional Poincaré sphere to VBs on the higher-order Poincaré (HOP) sphere. The two orbital angular momentum states of the CSCs were manipulated, and the focused HOP beams were generated. The experimental results demonstrate the broadband generation of arbitrarily focused HOP beams of high quality under the illumination of the red (632.8 nm), green (532 nm), and blue (473 nm) light. This work will be of significance for the applications of VBs in different areas, such as precision metrology, optical micromanipulation, and quantum information.

Plasmonic metasurfaces manipulating the two spin components from spin-orbit interactions of light with lattice field generations.

Artificial nanostructures in metasurfaces induce strong spin-orbit interactions (SOIs), by which incident circularly polarized light can be transformed into two opposite spin components. The component with an opposite helicity to the incident light acquires a geometric phase and is used to realize the versatile functions of the metasurfaces; however, the other component, with an identical helicity, is often neglected as a diffused background. Here, by simultaneously manipulating the two spin components originating from the SOI in plasmonic metasurfaces, independent wavefields in the primary and converted spin channels are achieved; the wavefield in the primary channel is controlled by tailoring the dynamic phase, and that in the converted channel is regulated by designing the Pancharatnam-Berry phase in concurrence with the dynamic phase. The scheme is realized by generating optical lattice fields with different topologies in two spin channels, with the metasurfaces composed of metal nanoslits within six round-apertures mimicking the multi-beam interference. This study demonstrates independent optical fields in a dual-spin channel based on the SOI effect in the metasurface, which provides a higher polarization degree of freedom to modify optical properties at the subwavelength scale.

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.

Impact of Fabrication and Operation Errors on Super-Focusing Performance of a Nanoslit-Based Metalens

Metasurfaces with optical manipulation at subwavelength resolution show promises for developing ultrathin and flat optical components, attracting great interest from the optical scientific community. In our recent work, a metalens has demonstrated a special capability of quasi-far-field super-resolution focusing, which comprised a metallic nanoslit array with the incidence of a transverse-electric (TE) polarized light. In this paper, in order to guide practical fabrication and operation of a device, we perform a study on the metalens to analyze the impact of many imperfections on the super-resolution focusing capability. We take fabrication and operation errors into account, including errors in nanoslit width, metal film thickness, operating wavelength, polarization and incident angle of incident light. Numerical results illustrate that the sensitivity of the metalens focusing performance to each error is different. To be specific, the focusing performance of the metalens is considerably susceptible to the error in the incidence angle. Therefore, we not only need to fabricate the metalens device precisely, but also need to ensure that the working conditions agree well with the design, so as to achieve the desired focusing performance. Our research offers a valuable guide for the realization of the super-resolution focusing technology in practice.

Strong confinement-induced nonlinear terahertz response in semiconductor nanostructures revealed by Monte Carlo calculations

Nonlinear terahertz conductivity spectra of charges confined in semiconductor nanostructures were calculated using a semiclassical Monte Carlo method. The confinement-induced nonlinear response per charge carrier is much stronger than the intrinsic nonlinearity of common bulk semiconductors and more than 20 times stronger than in graphene, which has been considered as a material with one of the highest terahertz nonlinearities. Moderate intensities of the terahertz radiation are thus sufficient to achieve efficient frequency mixing or high-harmonics generation. Enclosing the nanostructures into metallic nanoslits concentrates the electric field into the semiconductor and thus easily provides nonlinear terahertz signal strength comparable to the linear one.

Narrow-band, low-sideband plasmonic filter of asymmetric bi-layer metallic nanoslit arrays.

We propose a narrow-band plasmonic filter with low sidebands in the VIS-NIR regime, consisting of two closely spaced, optically thin layers of asymmetric metallic nanoslit arrays that have equal periods but different slit widths. Based on numerical simulations, we clarify that the filtering characteristics in the transmission spectrum is mainly due to intercoupled local plasmon resonance (LPR) modes in the top- and bottom-layer nanoslits and in-plane waveguiding surface plasmon resonance (SPR) modes bound to the top and bottom metal structure layers respectively. The intercoupled LPR modes boost the transmission in a way that the adjoining nanoslits in the top and bottom metal layers act as optical antennas efficiently receiving and emitting light via intermediate plasmon modes, while the in-plane SPR modes at neighboring wavelengths suppress the transmission, so as to shape the passband peak. It's important that asymmetry of the nanoslits helps to improve quality factor of the intercoupled LPR mode and thus to reduce the passband width. Also, asymmetry of the bi-layer metallic nanoslit arrays helps to suppress the sidebands that are relevant to the higher-order in-plane SPR modes at shorter wavelengths. In the spectrum at longer wavelengths, non-resonant transmission of light is suppressed by increased total thickness of the structured metal layers. Furthermore, a two-dimensional version of the filter structure is presented, demonstrating similar filtering characteristics that can be optimally used for arbitrarily polarized or unpolarized light.

Optical Spin Hall Effect in Closed Elliptical Plasmonic Nanoslit with Noncircular Symmetry.

We investigated the optical spin Hall effect (OSHE) of the light field from a closed elliptical metallic curvilinear nanoslit instead of the usual truncated curvilinear nanoslit. By making use of the characteristic bright spots in the light field formed by the noncircular symmetry of the elliptical slit and by introducing a method to separate the incident spin component (ISC) and converted spin component (CSC) of the output field, the OSHE manifested in the spot shifts in the CSC was more clearly observable and easily measurable. The slope of the elliptical slit, which was inverse along the principal axes, provided a geometric phase gradient to yield the opposite shifts of the characteristic spots in centrosymmetry, with a double shift achieved between the spots. Regarding the mechanism of this phenomenon, the flip of the spin angular momentum (SAM) of CSC gave rise to an extrinsic orbital angular momentum corresponding to the shifts of the wavelet profiles of slit elements in the same rotational direction to satisfy the conservation law. The analytical calculation and simulation of finite-difference time domain were performed for both the slit element and the whole slit ellipse, and the evolutions of the spot shifts as well as the underlying OSHE with the parameters of the ellipse were achieved. Experimental demonstrations were conducted and had consistent results. This study could be of great significance for subjects related to the applications of the OSHE.

Numerical Study on Enhanced Line Focusing via Buried Metallic Nanowire Assisted Binary Plate.

Line focusing, which collects light into a line rather than a single point, has an advantage on variable fields such as machining and imaging. The 1-dimensional metallic zone plate is one of the candidates for line focusing, which is ultra-thin and simple to fabricate. Metallic nano-slits can replace the metal blocked region to increase the efficiency, however, the efficiency and stability are still low. Therefore, this paper proposes a structure with an additional dielectric layer to protect the metallic nano-slit layer—a buried metallic wire structure—and verify the idea based on numerical simulations. Two structures are proposed. In terms of stability, a flat surface structure is proposed and a corrugated surface structure with a consistent thickness with the nano-slit is proposed which has low fabrication difficulty. The optimization of the buried wire structure and performance after applying the buried wire structure to the dual-line focusing plate is calculated by numerical simulation. Finally, it was shown that the electric field intensity was 2.13 times greater.

Vortex Beam Detection Based on Plasmonic in Plane Zone-Plate

A plasmonic in-plane zone-plate (IPZP) is analytically proposed and numerically analyzed. The proposed scheme consists of several discretely metallic nano-slits, which couple incident vortices into SPP and form focal spots. Wave front information of incident beam is reflected in the field distribution along “focal plane” of IPZP, and therefore, the proposed structure can be used for topological charge detection. Besides, an orthogonal slit array IPZP achieving simultaneous spin and orbital angular momentum detection is elaborated. IPZP structure has relatively high fabrication tolerance and the parameters can be adjusted to fit various operating wavelength. The proposed structure is expected to find potential applications in orbital angular momentum communication and high-dimensional quantum entanglement.

Dual dielectric cap gold nanoslits array optical resonance filter with large figure-of-merit.

In this work, we investigate a gold nanoslits array optical transmission filter with dual dielectric cap layers on top of the metal nanoslits. By integrating a low index of refraction dielectric layer between a high index of refraction dielectric cap layer and the gold nanoslits, a narrow spectral linewidth optical filter with a transmission peak far away from the Rayleigh anomaly wavelength is shown. Furthermore, we propose a figure-of-merit as the ratio of the spectral distance between a transmission peak and the Rayleigh anomaly over the spectral linewidth to characterize the performance of gold nanoslits optical filters. It is shown that dual dielectric cap gold nanoslits array optical filters have significantly larger figure-of-merits than that of traditional single dielectric cap gold nanoslits array optical filters.

Metasurface with metallic nanoantennas and graphene nanoslits for sensing of protein monolayers and sub-monolayers.

Biomolecule sensing plays an important role in both fundamental biological studies and medical diagnostic applications. Infrared (IR) spectroscopy presents opportunities for sensing biomolecules as it allows their fingerprints to be determined by directly measuring their absorption spectra. However, the detection of biomolecules at low concentrations is difficult with conventional IR spectroscopy due to signal-to-noise considerations. This has led to recent interest on the use of nanostructured surfaces to boost the signals from biomolecules in a method termed surface enhanced infrared spectroscopy. So far, efforts have largely involved the use of metallic nanoantennas (which produce large field enhancement) or graphene nanostructures (which produce strong field confinement and provide electrical tunability). Here, we propose a nanostructured surface that combines the large field enhancement of metallic nanoantennas with the strong field confinement and electrical tunability of graphene plasmons. Our device consists of an array of plasmonic nanoantennas and graphene nanoslits on a resonant substrate. We perform systematic electromagnetic simulations to quantify the sensing performance of the proposed device and show that it outperforms designs in which only plasmons from metallic nanoantennas or plasmons from graphene are utilized. These investigations consider the model system of a representative protein-goat anti-mouse immunoglobulin G (IgG) - in monolayer or sub-monolayer form. Our findings provide guidance for future biosensors for the sensitive quantification and identification of biomolecules.

Extra-narrowband metallic filters with an ultrathin single-layer metallic grating**Project supported by the National Key Research and Development Program of China (Grant Nos. 2018YFA0704401, 2017YFF0206103, and 2016YFA0203500), the National Natural Science Foundation of China (Grant Nos. 61922002, 91850103, 11674014, 61475005, 11527901, 11525414, and 91850111), and the Beijing Natural Science Foundation, China (Grant No. Z180015).

Narrowband and high-transmission optical filters are extensively used in color display technology, optical information processing, and high-sensitive sensing. Because of large ohmic losses in metallic nanostructures, metallic filters usually exhibit low transmittances and broad bandwidths. By employing both strong field enhancements in metallic nano-slits and the Wood’s anomaly in a periodic metallic grating, an extra-narrowband and high-transmission metallic filter is numerically predicted in an ultrathin single-layer metallic grating. Simulation results show that the Wood’s anomaly in the ultrathin (thickness H = 60 nm) single-layer metallic grating results in large field enhancements in the substrate and low losses in the metallic grating. As a result, the transmission bandwidth (transmittance T > 60%) at λ = 1200 nm is as small as ΔλFWHM = 1.6 nm, which is smaller than 4% of that in the previous thin dielectric and metallic filters. The corresponding quality factor is as high as Q = λ/ΔλFWHM = 750, which is 40 times greater than that in the previous reports. Moreover, the thickness of our metallic filter (H = 60 nm) is smaller than 40% of that in the previous reports, and its maximum transmittance can reach up to 80%. In experiments, a narrowband metallic filter with a bandwidth of about ΔλFWHM = 10 nm, which is smaller than 25% of that in the previous metallic filters, is demonstrated.

Ultrasensitive detection of saccharides using terahertz sensor based on metallic nano-slits

Unambiguous identification of trace amounts of biochemical molecules in a complex background using terahertz spectroscopy is extremely challenging owing to the extremely small absorption cross sections of these molecules in the terahertz regime. Herein, we numerically propose a terahertz nonresonant nano-slits structure that serves as a powerful sensor. The structure exhibits strongly enhanced electric field in the slits (five orders of magnitude), as well as high transmittance over an extra-wide frequency range that covers the characteristic frequencies of most molecules. Fingerprint features of lactose and maltose are clearly detected using this slits structure, indicating that this structure can be used to identify different saccharides without changing its geometrical parameters. The absorption signal strengths of lactose and maltose with a thickness of 200 nm are strongly enhanced by factors of 52.5 and 33.4, respectively. This structure is very sensitive to thin thickness and is suitable for the detection of trace sample, and the lactose thickness can be predicted on the basis of absorption signal strength when the thickness is less than 250 nm. The detection of a mixture of lactose and maltose indicates that this structure can also achieve multi-sensing which is very difficult to realize by using the resonant structures.

A Compound Phase-Modulated Beam Splitter to Distinguish Both Spin and Orbital Angular Momentum

Based on a phase-modulated metallic nanoslit array, an angular momentum (AM) beam splitter has been demonstrated to distinguish both spin and orbital components carried by the light beam. With such a device, the AM modes could be coupled into nondiffracting surface plasmonic beams propagating toward different directions. According to the experimental results, the extinction ratio for spin AM beam splitting is larger than 10 and the spatial interval of adjacent orbital AM modes (with a topological charge interval of 2) is more than 1.1 μm. We believe that such a device would have great potential to achieve a highly compact photonic integrated circuit with the plasmonic beam.

Brightening and Guiding Single‐Photon Emission by Plasmonic Waveguide–Slit Structures on a Metallic Substrate

AbstractBy designing a plasmonic waveguide–slit structure (a nanoslit etched in a silver nanowire) on a silver substrate, an ultrahigh Purcell factor and ultralarge figure of merit (FOM) are numerically predicted. Because of the large field enhancement (>150 times the incident field) and the ultrasmall optical volume (V ≈ 2 × 10−5λ3) of the resonant mode in the metallic nanoslit, the simulations show that the Purcell factor in the system can reach up to FP = 1.68 × 105, which is more than ten times the maximum Purcell factor in previous work (by placing metallic nanoparticles on a metal surface with a nanogap). Because of the utilization of a silver substrate rather than the common dielectric substrate, the mode cutoff of the surface plasmon polariton (SPP) waveguide mode is completely eliminated, which provides a large selection range of the nanowire radii to support the resonant mode in the nanoslit. Moreover, the SPP propagation length is significantly increased by more than 30 times. As a result, an ultralarge FOM of 1.40 × 107 is obtained, which is more than 80 times the maximum FOM in previous work where the metallic nanowire is placed on or surrounded by dielectric materials.

A Novel Theory for the Scattering of P-Polarized Hermite-Gaussian Electromagnetic Beams by a Double Metallic Nano-Slit

We present a rigorous theory for oblique incident Hermite-Gaussian beams, diffracted by two optical nano-slits of width l and separation d, in a thick metallic screen for the case of polarization TM (P). The far field spectra as a function of several opto-geometrical parameters, wavelength λ , slit width l, separation d , incidence angle θi  and Hermite order m  is analyzed. In the vectorial diffraction region given when λ /l >0.2, where l is the incident wavelength and as a function of the separation between slits d; we have numerically analyzed: the far field spectra, the energy diffracted along the incident beam direction Ei , and the validity of an approximate diffraction (scalar) property, namely Ei= Ntao/lambda .