Metallic Nanostrips Research Articles

Novel Highly Efficient Buried Gratings for Selective Coupling of SPP Waves onto Single Interfaces

Diffraction gratings have always been used to effectively couple optical radiation within integrated waveguides. This is also valid for plasmonic structures that support Surface Plasmon Polariton (SPP) waves. Traditional gratings usually excite SPP waves at the interface where they are located or, for thin metal nanostrips, at both interfaces. But reducing the thickness of the metal layer in the presence of a grating has the handicap of increasing the tunnelling of light towards the substrate, which means higher losses and reduced coupling efficiency. In this paper, we design and optimize novel gratings buried within the metallic thin films for selective coupling of SPP waves onto individual interfaces. Compared with traditional superficial gratings, the novel buried ones demonstrate higher efficiency and much lower residual tunnelling of light through the coupling structures.

Ultra-broadband interconnection between two SPP nanostrips by a photorefractive soliton waveguide.

We propose a very efficient approach to interconnect together two metallic nanostrips supporting the propagation of surface plasmon polariton (SPP) waves by fabricating a photorefractive soliton guide. By designing a multilayer geometry for plasmon systems, it is possible to control the diffraction of light at the end of the metallic nanostrip, reducing its angular dispersion and directing it towards the second nanostrip. Between the two, a photorefractive crystal allows the self-confinement of light, creating a waveguide that can be used both by the light that wrote it and by other wavelengths sent as signals. These signals can be recoupled in the form of SPP waves in the second nanostrip with an efficiency of about 40% for a broad band of wavelengths.

Fabrication of large-area metal and semiconductor nanobridge arrays using nanotransfer printing and UV lithography

A new method for the fabrication of large-area metal and semiconductor nanobridge arrays is proposed. The method combines the possibilities offered by the nanotransfer printing technique and by UV lithography. The essence of the method consists in the formation of an array of metal or semiconductor nanostrips “imprinted” into a smooth layer of resist and in subsequent exposure of the resist to UV radiation through an additional mask defining the regions where the suspended nanostrip bridges will be formed during subsequent development of the resist layer. Large-area arrays of Au and Ti/Au nanobridge, 100 nm wide, 25–35 nm thick and 1–4 μm long, with their ends embedded into the SU-8 resist layer, were fabricated. SiGe nanobridge arrays of 18 nm thick and 1 μm long, with their ends embedded into the Shipley 1813 resist layer were fabricated. The proposed method is simple and low-cost, and it does not require using expensive reactive plasma-etching facilities and can be implemented both on solid and polymer substrates on the areas at the excess of 150 cm2.

Realization of tuneable ultrabroadband interconnection for solitonic-plasmonic synapsis by exploiting epsilon near zero conducting oxides

This research introduces a novel highly efficient method to interconnect two metallic nanostrips that support the propagation of surface plasmon polariton (SPP) waves exploiting photorefractive soliton guide. The intricate design of the multilayer geometry enables light control diffraction at the metallic nanostrip’s end and reduces its angular dispersion. Moreover, the system’s on/off state can be switched by exploiting the epsilon near zero properties of the indium tin oxide (ITO) layer. The photorefractive crystal positioned between the two plasmonic waveguides enables the self-confinement of light, generating a waveguide that can be utilized by both the writing light and other wavelengths transmitted as signals. The resulting SPP waves can be efficiently recoupled in the second nanostrip, with an efficiency of around 40% across a broad range of wavelengths. This cutting-edge approach paves the way for significant advancements in the field of nanophotonics and provides a fundamental framework for the development of new, highly efficient optical interconnects in nanoscale systems. The findings of this study have implications for a wide range of applications, including nanoscale sensing, optical computing, and data communication.

Micro and nanostrips in spintronics: How to keep them cool

This Tutorial explores the problem of Joule heating on metallic micro or nanostrips, still one of the most popular geometries in modern spintronics. Many of the effects that result from the interaction of a spin polarized current and the local magnetization require a sizeable current density. This implies, quite often, an unignorable Joule heating. Despite the few articles devoted to some aspects of Joule heating, there is still disinformation and many misconceptions on this topic, which is key for the correct interpretation of the scientific results. In this Tutorial, we highlight the material parameters that are important to keep the temperature of the strip under control and those that give only a marginal advantage. In the vast majority of papers, at least one of these parameters is missing. We also focus on some misconceptions, such as the belief that performing the measurement on a cryostat rules Joule heating out. In fact, for a fixed current density, measuring in a cryostat decreases the temperature but not enough to justify the use of such a costly measuring setup. At the practical level, we put forward a 1D model to calculate, in few seconds, if Joule heating is present and if it should be taken into account when interpreting the results. Finally, and importantly, we describe a simple fabrication route to enhance the dissipation of heat in the strip considerably. This fabrication strategy is more effective at keeping the temperature under control than performing the experiment at cryogenic temperatures.

Shaping Metallic Nanolattices: Design by Microcontact Printing from Wrinkled Stamps.

A method for the fabrication of well-defined metallic nanostructures is presented here in a simple and straightforward fashion. As an alternative to lithographic techniques, this routine employs microcontact printing utilizing wrinkled stamps, which are prepared from polydimethylsiloxane (PDMS), and includes the formation of hydrophobic stripe patterns on a substrate via the transfer of oligomeric PDMS. Subsequent backfilling of the interspaces between these stripes with a hydroxyl-functional poly(2-vinyl pyridine) then provides the basic pattern for the deposition of citrate-stabilized gold nanoparticles promoted by electrostatic interaction. The resulting metallic nanostripes can be further customized by peeling off particles in a second microcontact printing step, which employs poly(ethylene imine) surface-decorated wrinkled stamps, to form nanolattices. Due to the independent adjustability of the period dimensions of the wrinkled stamps and stamp orientation with respect to the substrate, particle arrays on the (sub)micro-scale with various kinds of geometries are accessible in a straightforward fashion. This work provides an alternative, cost-effective, and scalable surface-patterning technique to fabricate nanolattice structures applicable to multiple types of functional nanoparticles. Being a top-down method, this process could be readily implemented into, e.g., the fabrication of optical and sensing devices on a large scale.

Experimental Demonstration of an Extreme Subwavelength Nanomagnetic Acoustic Antenna Actuated by Spin–Orbit Torque from a Heavy Metal Nanostrip

AbstractA novel on‐chip extreme subwavelength “acoustic antenna” whose radiation efficiency is ≈50 times larger than the theoretical limit for a resonantly driven antenna is demonstrated. The antenna is composed of magnetostrictive nanomagnets deposited on a piezoelectric substrate. The nanomagnets are partially in contact with a heavy metal (Pt) nanostrip. Passage of alternating current through the nanostrip exerts alternating spin–orbit torque on the nanomagnets and periodically rotates their magnetizations. During the rotation, the magnetostrictive nanomagnets expand and contract, thereby setting up alternating tensile and compressive strain in the piezoelectric substrate underneath. This leads to the generation of a surface acoustic wave in the substrate and makes the nanomagnet assembly act as an acoustic antenna. The measured radiation efficiency of this acoustic antenna at the detected frequency is ≈1%, while the wavelength to antenna dimension ratio is ≈67:1. For a standard antenna driven at acoustic resonance, the efficiency would have been limited to ≈(1/67)2 = 0.02%. It became possible to beat that limit (by ≈50 times) via actuating the antenna not at acoustic resonance, but by using a completely different mechanism involving spin–orbit torque originating from the giant spin Hall effect in Pt.

Geometrical optimization of nanostrips for surface plasmon excitation: an analytical approach.

We give a simple tool for the optimization of the dimensions of a metallic nanostrip illuminated at a given wavelength under normal incidence, to get a maximum of the electromagnetic field amplitude in the nanostrip. We propose an analytical formula that gives the widths and heights of the series of nanostrips that produce field enhancement. The validity of the analytical formula is checked by using the finite element method. This design of a nanostrip could be useful for sensors and thermally active components.

Highly Efficient Colored Perovskite Solar Cells Integrated with Ultrathin Subwavelength Plasmonic Nanoresonators

Highly efficient colored perovskite solar cells that exploit localized surface plasmon resonances in ultrathin subwavelength plasmonic nanoresonators are demonstrated. Localized resonances in ultrathin metal nano-strip optical resonators consisting of an array of metallic subwavelength nanowires on a transparent substrate, fabricated by using low-cost nanoimprint lithography over a large area, lead to a sharp peak in a reflection spectrum for distinctive color generation with angle-insensitive property up to 60°, and simultaneously transmit the complementary spectrum of visible light that can be efficiently harvested by the perovskite solar cells for electric power generation. The plasmonic color filter-integrated perovskite solar cells provide 10.12%, 8.17% and 7.72% of power conversion efficiencies with capabilities of creating vivid reflective red, green and blue colors. The scheme described in this work could be applied to a variety of applications such as power-generating decorations, tandem cells, power-saving wearable devices and energy-efficient reflective display technologies.

Fano resonances and electromagnetically induced transparency in silicon waveguides loaded with plasmonic nanoresonators

The fundamental electric dipolar resonance of metallic nanostrips placed on top of a dielectric waveguide can be excited via evanescent wave coupling, thus giving rise to broad dips in the transmission spectrum of the waveguide. Here we show via numerical simulations that narrower and steeper Fano-like resonances can be obtained by asymmetrically coupling in the near field a larger nanostrip—supporting an electric quadrupole in the frequency regime of interest—to the original, shorter nanostrip. Under certain conditions, the spectral response corresponding to the electromagnetically induced transparency phenomenon is observed. We suggest that this hybrid plasmonic–photonic approach could be especially relevant for sensing or all-optical switching applications in a photonic integrated platform such as silicon photonics.

Polarization conversion based on plasmonic phase control by an ultra-thin metallic nano-strips

Ultra-thin metallic nano-strips (thinner than skin depth) can lead to anomalous reflection for a transverse magnetic (TM) incidence of some wave-lengths, due to the phase modulation of localized surface plasmon resonance. Based on the principle above, we proposed a method of polarization modulation using ultra-thin metallic nano-strips. When irradiating nano-strips vertically by light with a given polarized angle, we can utilize the phase difference of the TM transmission and transverse electric (TE) transmission near anomalous reflection region to modulate transmission polarization. We have designed and fabricated the ultra-thin metallic nano-strips with the function of quarter-wave plate, the attained transmission Stokes parameter S3 is 0.95. The nano-strips is easy to design and fabricate, also compatible with other optics devices, hence has the potential applications in integrated optics field.

Enhanced four-wave mixing with nonlinear plasmonic metasurfaces.

Plasmonic metasurfaces provide an effective way to increase the efficiency of several nonlinear processes while maintaining nanoscale dimensions. In this work, nonlinear metasurfaces based on film-coupled silver nanostripes loaded with Kerr nonlinear material are proposed to achieve efficient four-wave mixing (FWM). Highly localized plasmon resonances are formed in the nanogap between the metallic film and nanostripes. The local electric field is dramatically enhanced in this subwavelength nanoregion. These properties combined with the relaxed phase matching condition due to the ultrathin area lead to a giant FWM efficiency, which is enhanced by nineteen orders of magnitude compared to a bare silver screen. In addition, efficient visible and low-THz sources can be constructed based on the proposed nonlinear metasurfaces. The FWM generated coherent wave has a directional radiation pattern and its output power is relatively insensitive to the incident angles of the excitation sources. This radiated power can be further enhanced by increasing the excitation power. The dielectric nonlinear material placed in the nanogap is mainly responsible for the ultrastrong FWM response. Compact and efficient wave mixers and optical sources spanning different frequency ranges are envisioned to be designed based on the proposed nonlinear metasurface designs.

Leaky domino-modes in regular arrays of substantially thick metal nanostrips

In previous works, an efficient light trapping performed by arrays of metal nanoantennas whose building block was a slightly tapered (trapezoidal) substantially thick nanostrip was revealed. This light trapping implied a broad spectrum of solar light concentrated in a subwavelength depth of the semiconductor substrate. This is a very advantageous feature allowing our structure to enhance thin-film solar cells. However, the physics of the broadband resonant absorption in the substrate was not investigated. In the present paper, we show that our arrays support so-called leaky domino-modes, responsible for such the light trapping. These modes are multipole oscillations of the array of substantially thick nanostrips. In this work we have thoroughly studied these leaky modes relating them to resonances of high-order multipole moments and to broadband light-trapping effect.

Scattering of light waves by finite metal nanostrip gratings: Nystrom-Type method and resonance effects

Efficient and rapidly convergent numerical algorithm for the simulation of the scattering of light waves by a finite gratings consisting of thin (thinner than the wavelength in the free space) metal nanostrips is presented. The model is based on the utilization of generalized boundary conditions (GBC), which allow one to exclude from consideration the field inside each strip and to reduce the two-dimensional boundary problem to one-dimensional systems of singular/hypersingular integral equations (IE). The obtained IE are solved numerically using the Nystrom-type method and the quadrature formulas of interpolation type, that provides guarantee convergence and controlled accuracy. The article presents the results of characteristics calculations for optical scattering and absorption by the gratings, which consist of silver nanostrips, as dependences on the width and on the thickness of the strips, and on the grating period. The nature of resonance phenomena has been investigated, namely the article presents the analysis of intensive optical scaterring and absorption in the case of excitation of plasmonic modes (plasmons) and of grating modes, which are induced by the periodicity.

Microwave magnetic dynamics in highly conducting magnetic nanostructures

We performed low-noise broadband microstrip ferromagnetic resonance (FMR) measurements of the resonant modes of an array of metallic ferromagnetic nanostripes. In addition to a strong signal of the fundamental mode, we observed up to five weak-amplitude peaks in the field-resolved FMR traces, depending on the frequency. These higher-order absorption peaks have been theoretically identified as due to resonant excitation of odd and even standing spin waves across the direction of confinement in array plane (i.e., across the stripe width). The theory we developed suggests that the odd modes become excited in the spatially uniform microwave field of the FMR setup due to the large conductivity of metals. This promotes excitation of large-amplitude eddy currents in the sample by the incident microwave magnetic field and ultimately results in excitation of these modes. Following this theory, we found that the eddy current contribution is present only for patterned materials and when the microwave magnetic field is incident on one surface of sample surface, as it is in the case of a microstrip FMR.

Resistive coupling of localized plasmon resonances in metallic nanostripes through a graphene layer

We study a hybrid graphene plasmonic device that consists of a gold nanostripe array combined with graphene. We have measured significant changes in the optical properties of the nanostripe array after the transfer of monolayer or bilayer graphene on the top of it. We suggest that this modification is partly due to the resistive coupling of localized plasmon resonances through the highly mobile electrons of the graphene layer. Our results open up new possibilities for designing active optical elements to exploit the high sensitivity of graphene carrier-density to bias voltage for ultra-fast light modulation. This is especially interesting as the plasmonic resonances of metal nanostructures can lie in the visible, NIR and UV; important areas for many applications.

Improved extraordinary optical transmission though single nano-slit by nano-defocusing

Abstract In this work, by using the finite difference time domain (FDTD) method, we have numerically studied the transmission properties of a composite structure consisting of a de-focusing tapered slit and nano-strip. For a fixed wavelength, the dependence of transmission efficiency of light on the structure parameters is demonstrated in detail. we have found that the extraordinary optical transmission through a single tapered metallic nano-slit is enhanced by placing a metallic nano-strip over it, in which tapered slit with large taper angle is used for de-focusing of light and be avoid of Fabry–Perot resonance in straight metallic nano-slit. It is shown that a suitable tapered slit assisted by nano-cavity-antenna can enhance the transmission by about 50% (compared with that in straight slit case). Furthermore, the parameters of the tapered slit (the taper angle and the taper length) hardly change the resonance in the horizontal nano-cavity. The transmission spectrum of the tapered slit can also be tuned by adjusting the taper angle. Such striking characteristic tapered slit has promising applications in designing optical nano-scale device.

Coupling light into and out from the surface plasmon polaritons of a nanometer-thin metal film with a metal nanostrip

Scattering of light by a metal nanostrip placed near a nanometer-thin metal film in a homogeneous dielectric environment is considered. For a plane wave incident on the geometry from outside we study the scattering cross sections governing the excitation of symmetric and antisymmetric surface plasmon polariton (SPP) waves and out-of-plane (OUP) propagating waves. For a symmetric (long-range) or antisymmetric (short-range) SPP propagating along the metal film and being incident on the nanostrip, we study the fraction of power coupled into forward and backward propagating symmetric and antisymmetric SPP waves, and into OUP propagating waves, respectively. Resonance peaks in the scattering cross sections, or dips in the relative power going into all scattering channels due to resonant absorption, are related to standing-wave resonances of counterpropagating gap SPP waves in the gap between the metal nanostrip and the metal film. The resonances are studied with respect to the width of the metal nanostrip and the size of the gap between the metal film and nanostrip. For a 15-nm-thick film it is found that coupling of light in and out of the symmetric SPP is weak compared with coupling to and from OUP and antisymmetric SPP waves. Improving the in and out coupling of power in the symmetric SPP waves with thicker films (50 nm) and larger scattering objects is considered.

Double-Layered Metal Nano-Strip Antennas for Sensing Applications

A coupled plasmonic system based on double-layered metal nano-strips for sensing applications is investigated by means of mode analysis and two-dimensional finite-difference time-domain simulations. The nano-strips act as optical antennas through constructive interference of short-range surface plasmon polaritons, thus increasing their scattering cross-section and optical field enhancement. Near-field modulation by optical trapped metal nanoparticles (NPs) is also demonstrated. Our results reveal that the device exhibits a refractive index sensitivity of ∼200 nm/RIU, and a maximum surface-enhanced Raman scattering (SERS) factor of 109–1010 from metal NPs trapped in the near-field region. The proposed device shows reasonable figure-of-merit and is ready for integration with common optofluidic biosensors.

Inverse Optimization of Plasmonic and Antireflective Grating in Thin Film PV Cells

This work addresses inverse optimization of three dimensional front and back surface texture grating specifications, for the purpose of shaping the absorptivity spectrum of silicon thin film cells in targeted ways. Periodic plasmonic gratings with dimensions comparable or less than the incident light wavelength are known to enhance light absorption. We consider surface patterning of amorphous silicon (a-Si) thin films using front and/or back metallic nanostrips and ITO coatings, and show that wideband enhancement in unpolarized absorptivity spectrum can be achieved when back reflectors are used. The overall short circuit current enhancement using such structures is significant and can be as high as 97%. For TM-polarized wave it can be even higher as reported in previous work. In this work however, we focus on the optimization for the more realistic unpolarized radiation which is of significantly higher complexity. In addition, optimization is done with respect to two objective functions independently: spectral absorptivity and gain-bandwidth product of the absorptivity spectrum.