Chromatic Plasmonic Polarizer-Based Synapse for All-Optical Convolutional Neural Network

Emerging memory devices have been demonstrated as artificial synapses for neural networks. However, the process of rewriting these synapses is often inefficient, in terms of hardware and energy usage. Herein, we present a novel surface plasmon resonance polarizer-based all-optical synapse for realizing convolutional filters and optical convolutional neural networks. The synaptic device comprises nanoscale crossed gold arrays with varying vertical and horizontal arms that respond strongly to the incident light's polarization angle. The presented synapse in an optical convolutional neural network achieved excellent performance in four different convolutional results for classifying the Modified National Institute of Standards and Technology (MNIST) handwritten digit data set. After training on 1,000 images, the network achieved a classification accuracy of over 98% when tested on a separate set of 10,000 images. This presents a promising approach for designing artificial neural networks with efficient hardware and energy consumption, low cost, and scalable fabrication.

Surface-acoustic-wave-driven graphene plasmonic sensor for fingerprinting ultrathin biolayers down to the monolayer limit

Surface plasmon polaritons in graphene can enhance the performance of mid-infrared spectroscopy, which is key for the study of both the composition and the conformation of organic molecules via their vibrational resonances. In this paper, a plasmonic biosensor using a graphene-based van der Waals heterostructure on a piezoelectric substrate is theoretically demonstrated, where far-field light is coupled to surface plasmon-phonon polaritons (SPPPs) through a surface acoustic wave (SAW). The SAW creates an electrically-controlled virtual diffraction grating, suppressing the need for patterning the 2D materials, that limits the polariton lifetime, and enabling differential measurement schemes, which increase the signal-to-noise ratio and allow a quick commutation between reference and sample signals. A transfer matrix method has been used for simulating the SPPPs propagating in the system, which are electrically tuned to interact with the vibrational resonances of the analytes. Furthermore, the analysis of the sensor response with a coupled oscillators model has proven its capability of fingerprinting ultrathin biolayers, even when the interaction is too weak to induce a Fano interference pattern, with a sensitivity down to the monolayer limit, as tested with a protein bilayer or a peptide monolayer. The proposed device paves the way for the development of advanced SAW-assisted lab-on-chip systems combining the existing SAW-mediated physical sensing and microfluidic functionalities with the chemical fingerprinting capability of this novel SAW-driven plasmonic approach.

Highly Localized Surface Plasmon Nanolasers via Strong Coupling

Surface plasmons have robust and strong confinement to the light field which is beneficial for the light-matter interaction. Surface plasmon amplification by stimulated emission of radiation (SPACER) has the potential to be integrated on the semiconductor chip as a compact coherent light source, which can play an important role in further extension of Moore's law. In this study, we demonstrate the localized surface plasmon lasing at room temperature in the communication band using metallic nanoholes as the plasmonic nanocavity and InP nanowires as the gain medium. Optimizing laser performance has been demonstrated by coupling between two metallic nanoholes which adds another degree of freedom for manipulating the lasing properties. Our plasmonic nanolasers exhibit lower power consumption, smaller mode volumes, and higher spontaneous emission coupling factors due to enhanced light-matter interactions, which are very promising in the applications of high-density sensing and photonic integrated circuits.

High-Speed Plasmonic Electro-Optic Beam Deflectors

Highly integrated active nanophotonics addressing both device footprint and operation speed demands is a key enabling technology for the next generation optical networks. Plasmonic systems have proven to be a serious contender to alleviate current performance limitations in electro-optic devices. Here, we demonstrate a plasmonic optical phased array (OPA) consisting of two 10 μm long plasmonic phase shifters, utilized to control the far-field radiation pattern of two subwavelength-separated emitters for aliasing-free beam steering with an angular range of ±5° and flat frequency response up to 18 GHz (with the potential bandwidth of 1.2 THz). Extreme optical and electrostatic field confinement with great spatial overlap results in high phase modulation efficiency (V_πL = 0.24 Vcm). The demonstrated approach of using plasmonic lithium niobate technology for optical beam manipulation offers inertia-free, robust, ultracompact, and high-speed beam steering.

A THz graphene metasurface for polarization selective virus sensing

We propose a novel method to exploit chirality of highly sensitive graphene plasmonic metasurfaces to characterize complex refractive indexes (RI) of viruses by detecting the polarization state of the reflected electric fields in the THz spectrum. A dispersive graphene metasurface is designed to produce chiral surface currents to couple linearly polarized incident fields to circularly polarized reflected fields. The metasurface sensing sensitivity is the result of surface plasmon currents that flow in a chiral fashion with strong intensity due to the underlying geometrical resonance. Consequently, unique polarization states are observed in the far-field with the ellipticity values that change rapidly with the analyte's RI. The determination of bimolecular RI is treated as an inverse problem in which the polarization states of the virus is compared with a pre-calculated calibration model that is obtained by full-wave electromagnetic simulations. We demonstrate the polarization selective sensing method by RI discrimination of three different types of Avian Influenza (AI) viruses including H1N1, H5N2, and H9N2 is possible. Since the proposed virus characterization method only requires determination of the polarization ellipses including its orientation at monochromatic frequency, the required instrumentation is simpler compared to traditional spectroscopic methods which need a broadband frequency scan.

Strong Light Confinement in Metal-Coated Si Nanopillars: Interplay of Plasmonic Effects and Geometric Resonance

We investigated the influence of metal coating on the optical characteristics of Si nanopillar (NP) arrays with and without thin metal layers coated on the sample surface. The reflection dips of the metal-coated arrays were much broader and more pronounced than those of the bare arrays. The coated metal layers consisted of two parts-the metal disks on the Si NP top and the holey metal backreflectors on the Si substrate. The Mie-like geometrical resonance in the NPs, the localized surface plasmons in the metal disks, and the propagation of surface plasmon polariton along the backreflector/substrate interface could contribute to the reflection spectra. Finite-difference time-domain simulation results showed that the interplay of the plasmonic effects and the geometric resonance gave rise to significantly enhanced light confinement and consequent local absorption in the metal-Si hybrid nanostructures.

Electrostrictive Mechanism of Nanostructure Formation at Solid Surfaces Irradiated by Femtosecond Laser Pulses

The significance of the mechanical pressure of light in creation of laser-induced periodic surface structures (LIPSSs) is investigated. Distributions of the electrically induced normal pressure and tangential stress at the illuminated solid surface, as well as the field of volume electrostrictive forces, are calculated taking into account surface plasmon polariton (SPP) excitation. Based on these calculations, we predict surface destruction and structure formation due to inelastic deformations during single femtosecond pulses. The calculated fields of the electromagnetic forces are found to agree well with the experimental ripple structures. We thus conclude that the electrostrictive forces can explain the origin of the periodic ripple structures.

Plasmonic Probe With Circular Nano-Moat for far-Field Free Nanofocusing

In this work, a metallic probe with a sharp tip and two half-circular nanostructures on its base is introduced and investigated. The proposed design aims at improving the detection performance of a probe for scattering scanning near-field optical microscopy in terms of enhanced signal-to-noise ratio. Under the premise of processing feasibility, the structure of the probe is designed and optimized with three-dimensional finite-difference time-domain method. And then the performance and optical property of the probe are theoretically investigated and experimentally demonstrated using a scanning near-field optical microscope with aperture probe. It is indicated that a tightly confined optical field with significantly reduced far-field background can be achieved at the tip apex of the probe.

Extending the Frequency Range of Surface Plasmon Polariton Mode with Meta-Material

The frequency range that surface plasmon polariton (SPP) mode exists is mainly limited by the metal material. With high permittivity dielectrics above metal surface, the SPP mode at high frequency has extremely large loss or can be cutoff, which limits the potential applications of SPP in the field of optical interconnection, active SPP devices and so on. To extend the frequency range of SPP mode, the surface mode guided by metal/dielectric multilayers meta-material has been studied based on the theory of electromagnetic field. It is demonstrated that surface mode not only could be supported by the meta-material but also extends the frequency to where conventional metal SPP cannot exist. Meanwhile, the characteristics of this surface mode, such as dispersion relation, frequency range, propagation loss and skin depth in meta-material and dielectrics, are also studied. It is indicated that, by varying the structure parameters, the meta-material guided SPP mode presents its advantages and flexibility over traditional metal one.

Color-tunable mixed photoluminescence emission from Alq3 organic layer in metal-Alq3-metal surface plasmon structure

This work reports the color-tunable mixed photoluminescence (PL) emission from an Alq3 organic layer in an Au-Alq3-Au plasmonic structure through the combination of organic fluorescence emission and another form of emission that is enabled by the surface plasmons in the plasmonic structure. The emission wavelength of the latter depends on the Alq3 thickness and can be tuned within the Alq3 fluorescent spectra. Therefore, a two-color broadband, color-tunable mixed PL structure was obtained. Obvious changes in the Commission Internationale d'Eclairage (CIE) coordinates and the corresponding emission colors of Au-Alq3-Au samples clearly varied with the Alq3 thickness (90, 130, and 156 nm).

Spectral and mode properties of surface plasmon polariton waveguides studied by near-field excitation and leakage-mode radiation measurement

We present a method to couple surface plasmon polariton (SPP) guiding mode into dielectric-loaded SPP waveguide (DLSPPW) devices with spectral and mode selectivity. The method combined a transmission-mode near-field spectroscopy to excite the SPP mode and a leakage radiation optical microscope for direct visualization. By using a near-field fiber tip, incident photons with different wavelengths were converted into SPPs at the metal/dielectric interface. Real-time SPP radiation images were taken through leakage radiation images. The wavelength-dependent propagation lengths for silver- and gold-based DLSPPWs were measured and compared. It confirms that silver-based SPP has a propagation length longer than a gold-based one by 1.25, 1.38, and 1.52 times for red, green, and blue photons. The resonant coupling as a function of wavelength in dual DLSPPWs was measured. The coupling lengths measured from leakage radiation images were in good agreement with finite-difference time domain simulations. In addition, the propagation profile due to multi-SPP modes interference was studied by changing position of the fiber tip. In a multimode DLSPPW, SPP was split into two branches with a gap of 2.237 μm when the tip was at the center of the waveguide. It became a zigzag profile when the SPP was excited at the corner of the waveguide.