Metallic Nanoslit Arrays Research Articles

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.

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.

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.

Tunable Plasmonic Absorber Using a Nanoslit Array Patterned on a Ge2Sb2Te5 -Inserted Fabry–Pérot Resonator

A wavelength-selective tunable plasmonic absorber is required for various applications, such as thermal imaging sensors, active biosensors, and switching devices in integrated optics. Using Ge2Sb2Te5 (GST), which is a representative phase change material showing a large change in optical characteristics, we propose a tunable plasmonic absorber that consists of a metal nanoslit array patterned on a GST-inserted dielectric layer. Based on systematic optimization, the proposed structure achieved a nearly zero reflectance for both states of the inserted GST film with a very large resonance shift after the phase transition. Moreover, the resonance wavelength of the proposed absorber could be flexibly tuned within a broad near-infrared range by tuning the geometric conditions of the nanoslit array and the thicknesses of the dielectric–GST–dielectric multilayer.

High-efficiency refractive index sensor based on the metallic nanoslit arrays with gain-assisted materials

AbstractWe have designed and investigated a three-band refractive index (RI) sensor in the range of 550–900 nm based on the metal nanoslit array with gain-assisted materials. The underlying mechanism of the three-band and enhanced characteristics of the metal nanoslit array with gain-assisted materials, have also been investigated theoretically and numerically. Three resonant peaks in transmission spectra are deemed to be in different plasmonic resonant modes in the metal nanoslit array, which leads to different responses for the plasmonic sensor. By embedding the structure into the CYTOP with proper gain-assisted materials, the sensing performances can be greatly enhanced due to a dramatic amplification of the extraordinary optical transmission (EOT) resonance by the gain medium. When the gain values reach their corresponding thresholds for the three plasmonic modes, the ultrahigh sensitivities in three bands can be obtained, and especially for the second resonant wavelength (λ2), the FOM=128.1 and FOM* = 39100 can be attained at the gain threshold of k =0.011. Due to these unique features, the designing scheme of the proposed gain-assisted nanoslit sensor could provide a powerful approach to optimize the performance of EOT-based sensors and offer an excellent platform for biological sensing.

Plasmonic Responses in Metal Nanoslit Array Fabricated by Interference Lithography

Gap tunable gold nanoslit arrays were fabricated by interference lithography and investigated numerically to understand the impact of fabrication errors on plasmonic responses. To fabricate the gap tunable gold nanoslit arrays, photoresist nanoslit arrays on quartz substrate were first formed by laser interference, and then converted to gold nanoslit array on glass substrate by perpendicular gold deposition and photoresist lift-off. Because the photoresist nanoslit has a sinusoidal profile due to the laser light interference lithography, different photoresist development time from 20[Formula: see text]s to 30[Formula: see text]s can tune the photoresist width from 100[Formula: see text]nm to 70[Formula: see text]nm, thus allows the gap-width-tuned metallic nanoslits to be attained accordingly. The optical properties of the fabricated gold nanoslit arrays were investigated experimentally and theoretically by studying the absorption in the transmission spectra. Within the wavelength range of 400[Formula: see text]nm to 860[Formula: see text]nm, the nanoslit in air has two prominent absorption peaks at 500[Formula: see text]nm and 670[Formula: see text]nm. It is found that a simulation model with gold nanoslit fabrication errors such as size variation, chromium adhesive layer and gold residue in nanoslit gaps considered can better match the simulation peaks with the experiments. The simulation of the gold nanoslit array in air indicates that the 500[Formula: see text]nm peak includes the interband transition and surface plasmon polariton (SPP) at air-gold surface, and the other peak at 670[Formula: see text]nm is SPP at glass side. The two SPP peaks are both sensitive to the refractive index of surrounding solution, with sensitivities of the two peaks demonstrated to be 267[Formula: see text]nm/RIU and 111[Formula: see text]nm/RIU in experiments, and 462[Formula: see text]nm/RIU and 180[Formula: see text]nm/RIU by simulation. The lower sensitivity detected by experiments might be due to some air bubbles in the flow cell reducing the effective refractive index around the nanoslit. The shorter wavelength SPP mode is 2.4 (in experiments) or 2.6 times (by simulation) more sensitive than the long wavelength SPP mode because its plasmonic field concentrates on water-gold surface. The plasmonic responses we simulated with fabrication errors explained our experimental investigations, and deepened our understanding on the application of the gold nanoslit array for refractive index-based biosensing.

Extraordinary transmission of light enhanced by exciting hybrid states of Tamm and surface plasmon polaritions in a single nano-slit

Extraordinary optical transmission (EOT) through a metallic nano-slit or nano-slit arrays has become an efficient method to manipulate the light on a subwavelength scale. While a variety of nano-devices based on surface plasmon polaritons (SPPs) could be an ideal candidate for the next-generation ultra-compact integrated photonic circuits, this EOT phenomenon is also generally attributed to the excitation of SPPs in the nano-slit. Thus, due to its being compact in structure and amenable to integrate with other nano-devices, single nano-slit can be implemented to construct an optical source in the nano-device based on SPPs. However, the transmission through an isolated nano-slit is too low to be practically used. The main reason is that the excitation efficiency of SPPs in the nano-slit is not high enough. In fact, one of the key issues is how to enhance the excitation efficiency in a nano-slit. In this paper, a novel method and the related structure are proposed to effectively enhance the EOT in a single nano-slit by improving the excitation efficiency of SPPs. This structure is made up of a silver film on a distributed Bragg reflector (DBR), where a single nano-slit is imbedded in the silver film. Under the illumination of a TM polarized light from the DBR side of this structure, the Tamm plasmon polaritons (TPPs) at the interface between the silver film and the DBR and the SPPs in the nano-slit can be excited simultaneously. The TPP is another surface mode, which describes how an electromagnetic field is localized at the boundary of silver film and the DBR. In this structure, coupling between the TPPs and the SPPs leads to the appearance of a TPP-SPP hybrid state. When the wave-vectors between the TPP and the SPP modes are matched, due to the local field enhancement of the TPP mode, the excitation efficiency of SPPs can be improved significantly. Furthermore, utilizing the quasi Fabry-Pérot (F-P) resonance in the nano-slit, where a single nano-slit can be regarded as an F-P cavity with two open ends, a high light transmission through the single nano-slit can be achieved. In the present paper, the transmission properties of the “DBR-silver nano-slit” structure are analyzed with the finite element method and the transfer matrix method. After optimizing the structure parameters, with a thickness of the silver film of 100 nm and a width of the nano-slit of 11 nm, the light transmission through the single nano-slit in this structure can be increased by about 16 times, in comparison with the light transmission through a single nano-slit in a silver film on the TiO2 substrate (without DBR). This method of enhancing the light transmission through a single nano-slit by exciting TPPs mode and utilizing its local field enhancement property, has potential applications in the polariton lasers, the nano-scale photonic integration, the near-field imaging and sensing, and other relevant areas.

Resonances of hybridized bound plasmon modes in optically-thin metallic nanoslit arrays for narrow-band transmissive filtering

Abstract We present extremely narrow-band transmission of light through optically-thin metallic nanoslit arrays on substrates coated with high/low-index dielectric layers for high-finesse wavelength filtering. The transmission peak is shown resulting from resonances of various hybrid plasmon modes, formed by symmetric or antisymmetric coupling of the symmetric and anti-symmetric bound surface plasmon modes at the ultrathin metal layer with the optical waveguide mode in the dielectric layers. Band-width of the peak can be down to a few nanometers, with no side peaks comparable in transmittance. For the purpose, all structure components have critical roles, clarified with numerical simulations.

Spatial and spectral selective characteristics of the plasmonic sensing using metallic nanoslit arrays

A novel spatial and spectral selective plasmonic sensing based on the metal nanoslit arrays has been proposed and investigated theoretically, which shows a high performance in the multiplexing biomolecular detections. By properly tuning the geometric parameters of metal nanoslit arrays, the enhanced optical fields at different regions can be obtained selectively due to the excitation of SPP, cavity mode (CM), and their coupling effects. Simulation results show that the resonances of the metal nanoslit arrays at different spatial locations and different wavelengths can be achieved simultaneously. A relative bigger red-shift of 57nm can be realized when a layer of biomolecular film is adsorbing at the slit walls, and the corresponding total intensity difference will be enhanced near 10 times compared to that at the top surface. In addition, when a BSA protein monolayer is adsorbing at slit walls with different slit widths, the corresponding wavelength shifts can reach to more than 80nm by modulating the widths of the slit. The simulated results demonstrate that our designed metal nanoslit arrays can serve as a portable, low-cost biosensing with a high spatial and spectral selective performance.

Active control of beams by metallic nanoslit array lens with movable dielectric substrate

We proposed a plasmonic structure consisting of uniform metallic nanoslits with a movable dielectric substrate for subwavelength beam manipulation. In this structure, the phase of incident light can be modulated beforehand by the substrate before bumping the metal slits; therefore, the shape of transmitted light from nanoslits can be actively controlled by changing the substrate profile. According to this idea, plasmonic lenses with different functions, such as focusing light to two points, focus length variation, and beam deflection, are designed and investigated numerically. The proposed structure has a unique advantage of multifunction and reduces the difficulty in fabrication.

Unified theory of surface-plasmonic enhancement and extinction of light transmission through metallic nanoslit arrays.

Metallic nanostructures are of immense scientific interest owing to unexpectedly strong interaction with light in deep subwavelength scales. Resonant excitations of surface and cavity plasmonic modes mediate strong light localization in nanoscale objects. Nevertheless, the role of surface plasmon-polaritons (SPP) in light transmission through a simple one-dimensional system with metallic nanoslits has been the subject of longstanding debates. Here, we propose a unified theory that consistently explains the controversial effects of SPPs in metallic nanoslit arrays. We show that the SPPs excited on the entrance and exit interfaces induce near-total internal reflection and abrupt phase change of the slit-guided mode. These fundamental effects quantitatively describe positive and negative effects of SPP excitation in a self-consistent manner. Importantly, the theory shows excellent agreement with rigorous numerical calculations while providing profound physical insight into the properties of nanoplasmonic systems.

Spatially Selective Plasmonic Sensing Using Metallic Nanoslit Arrays

Label-free biosensing based on extraordinary optical transmission (EOT) through metallic nanoaperture arrays is a highly promising application of nanoplasmonics. The optical properties of these nanostructures, however, are complex due to the coupling between propagating and localized plasmon resonances, and some important features of the sensing mechanism have not been fully exploited. In this paper, in contrast to most previous studies that focused on the optimization of sensor response to bulk refractive index changes, we investigate the sensor response upon biomolecule bindings at different sensor positions inside or outside the nanoapertures. By properly tuning the geometric parameters of a gold nanoslit array, we show that the enhanced optical field in this EOT-based sensor can be spatially tailored to increase its interaction volume with the binding target biomolecules and improve the sensor performance. The results presented deepen the current understanding of the EOT-based sensor properties and open up new opportunities to further optimize their sensing performance.

Transmission of TE-polarized light through metallic nanoslit arrays assisted by a quasi surface wave

Optically thick metallic nanoslit arrays are opaque to TE-polarized light, in contrast to enhanced transmission of TM-polarized light. Here, we numerically show that, by introducing an ultrathin high-index dielectric coating on the metal surfaces, a quasi surface wave can be excited at the metasurfaces to enhance the transmission of TE-polarized light. The quasi surface wave is shown to behave like surface plasmon waves, and enhance the transmission in similar mechanisms as surface plasmon waves do for TM-polarized light. In this work, we suggest a way of manipulating TE-polarized light in metallic subwavelength structures.

Transition from a spectrum filter to a polarizer in a metallic nano-slit array

The transition from a spectrum filter (resonant transmission) to a polarizer (broadband transmission) for TM polarized light is observed in a metallic nano-slit array as period is decreased. A theoretical model is developed and shows that the spectrum filter behavior is caused by the coupled slit/grating resonance. With decreasing period, the slit resonance is decoupled from the grating resonance, which then dominates the transmission spectrum and broadens the transmission peak. With further reducing period, the slit resonance diminishes and the peak spectrum transforms to a broadband transmission. This effect is the basis for the operation of wire grid polarizers. The transition is explained by the change of the impedance to the incoming wave.

Beam Manipulation by Metallic Nanoslit Arrays with Perpendicular Cuts inside Slits

Beam manipulation by metallic nanoslit arrays with perpendicular cuts inside the slits was investigated numerically. The simulated results performed by the finite element method (FEM) show that perpendicular cuts with different heights can modulate phase retardation of the transmitted light through the slits. With the proper distribution of cut height, a focused beam is achieved in our metallic nanostructure with four-time amplitude at the focus point and half focal length compared to a slit array without cuts inside. By using asymmetric distribution of height amplitude, a beam deflection around 6° can also be realized in our design.

Narrow-band optical transmission of metallic nanoslit arrays

Metallic nanoslit arrays usually demonstrate wide transmission bands for transverse-magnetic-polarized incidence light. Here, we show that by introducing multi-dielectric layers underneath the metallic structure layer on the substrate, a narrow peak is formed, whose bandwidth can be down to a few nanometers. Three types of resonance modes in the region under the metal layer are identified responsible for the formation of the peak, i.e., a two-dimensional cavity resonance mode, which supports optical transmission, and two in-plane hybrid surface plasmon resonance modes locating on both sides of the peak that suppresses the transmission. Such structures can be applied in advanced photonic devices.

Analytic Theory of the Resonance Properties of Metallic Nanoslit Arrays

By applying formulation based on time reversibility, we provide the analytic theory of resonance properties of metallic nanoslit arrays. We model lossy resonant systems in which a resonance is induced by a single quasi-bound mode (QBM). It is consistent with the Fano resonance theory of quantum interference of auto-ionizing atoms and captures the essential characteristics of dissipative resonant systems. We show that time-reversibility requirements lead to analytical solutions for the resonant transmission and the associated nonreciprocal absorption in terms of a minimal number of independent basic parameters that include partial decay probabilities of resonant pathways and the non-resonant transmission amplitude. With a clear view of interfering electromagnetic field configurations and the associated absorbing processes, the theory reveals the essential physics of resonant optical transmission. In particular, the enhanced transmission peak is given by the product of partial decay probabilities and is independent of the non-resonant light wave amplitude. In a highly asymmetric coupling regime, the excitation of the QBM leads to anti-resonant extinction of the transmission, indicating a negative role of the QBM. The parity of the QBM determines occurrence of red or blue tails in the spectral profile. Absorbance measurements yield direct determination of the partial decay probabilities by which the main features of the resonant transmission are quantitatively explained. Thus, these basic parameters can be directly established experimentally. Full numerical calculations of the transmission spectra are in complete quantitative agreement with our analytical formulation for optical transmission mediated by both slit cavity modes and plasmonic modes.

Metallic Planar Lens With Binary Nanoscale Slits

We propose a binary plasmonic lens based on metallic nanoslit array with different fillings. The phase range of π is achieved by changing the refractive indices of the filling materials in the slits. These index-modulated slits are demonstrated to have unique advantages in phase delay compared with the width-modulated ones. The proposed binary structures can keep the advantage in phase modulation and reduce the difficulty in fabrication. Lenses with different functions, such as focusing light to one or two points or correcting oblique incident light, are designed by a simulated annealing algorithm and investigated by the finite-difference time-domain method.

Plasmonic Nanoslit Array Enhanced Metal–Semiconductor–Metal Optical Detectors

Metallic nanoslit arrays integrated on germanium metal-semiconductor-metal photodetectors show many folds of absorption enhancement for transverse-magnetic polarization in the telecommunication C-band. Such high enhancement is attributed to resonant interference of surface plasmon modes at the metal-semiconductor interface. Horizontal surface plasmon modes were reported earlier to inhibit photodetector performance. We computationally show, however, that horizontal modes enhance the efficiency of surface devices despite reducing transmitted light in the far field.

Theory and experiments of Bragg cavity modes in passive and active metallic nanoslit array devices

Metallic nanoslit arrays exhibit several unique, surprising, and useful properties, such as resonant enhanced transmission and resonant local field enhancements. Here we present both a theoretical study of these static properties, as well as experiments showing the utilization of these features combined with active optical media. We develop an approximated, simple closed-form model for predicting and explaining the general emergence of enhanced transmission resonances through metallic gratings, in various configurations and polarizations. This model is based on an effective index approximation and it unifies in a simple way the underlying mechanism of all forms of enhanced transmission in such structures as emerging from standing wave resonances of the different diffraction orders of periodic structures. The known excitation of surface plasmon polaritons or slit cavity modes emerges as a limiting case of a more general condition. We also use this understanding of the resonant behavior of nanoslit arrays to design and fabricate such structures with embedded nanocrystal quantum dots, and show beaming of nonclassical light to a narrow angle of less than 4 deg, as well as an enhancement of the two-photon upconversion fluorescence process by a factor of ∼400.