Optical Transmission Through Multilayered Ultra-Thin Metal Gratings

Optical transmission properties of multilayered ultra-thin metal gratings are numerically studied. The transmission spectrum has a broad stop-band with extremely low transmittance compared to that of a single-layer one for TM polarization. The stop-band is shown to be formed by multiple-interference tunneling and various plasmon resonance processes in ultra-thin-metal and dielectric multilayers. That is on the transmission background of non-apertured metal/dielectric multilayer structures that have low transmission in the long-wavelength range due to destructive multiple-interference tunneling, the transmission is further suppressed in the stop-band by plasmon resonances in the top metal/dielectric layers, e.g., the anti-symmetric bound surface plasmon mode in the ultra-thin metal layer and the gap surface plasmon mode in the metal-sandwiched dielectric layer. High transmission beyond the stop-band is due to coupled gap surface plasmon mode in the entire multilayer structures. Applications of the optical properties of the multilayered ultra-thin metal gratings are suggested for optical filtering (wavelength or polarization selective).     

Tunable Plasmon-Induced Transparency of Double Continuous Metal Films Sandwiched with a Plasmonic Array

Making a continuous metal film with near-unity transparency has received more and more attention in recent years because of its potential applications for various optoelectronic devices. Here, we theoretically show that a high tunable plasmon-induced transparency metal film structure can be performed by double continuous metal films inserted with a two-dimensional hexagonal lattice array of plasmonic nanopariticles. The proposed structure shows near-unity anti-reflection and intensively enhanced transmission via the cooperative effects of strong resonant near-field light input and output coupling by the plasmonic array and the excitation of surface electromagnetic waves of the metal films. The optical response can be efficiently mediated by varying the sizes of nanoparticles and the separated distance between the metal array and the metal films. With the merits of high transparency, sub-wavelength sizes and wholly retained metal characteristics including high conductivity via using the pure metallic materials, the structure proposed here suggests various potential applications in optoelectronic integrated circuits.     

Short-Range Surface Plasmon Polaritons for Extraordinary Low Transmission Through Ultra-Thin Metal Films with Nanopatterns

We provide both experimental and theoretical investigation on extraordinary low transmission through one-dimensional nanoslit and two-dimensional nanohole arrays on ultra-thin metal films. Unambiguous proofs demonstrate that short-range surface plasmon polaritons play a key role leading to this novel phenomenon, which could be useful for creating new polarization filters and other integrated plasmonic components.     

Measurements of Light Transmission Through Single Melanophores

A photometrical method has been developed that allows assessment of subcellular pigment migration in melanophores of the fish cockoo wrasse (Labrus ossifagus L.). The pigment migration was studied with local light spot transmission measurements. Depending on where the light beam is placed on the melanophores it is possible to study events within an area of approximately 75 μm². Measuring pigment translocation in different parts of a melanophore gives new possibilities to study how cell membrane receptor-mediated signals are spread within a single cell, which will increase our understanding of how receptor activating drugs exert their cellular effect. The technique can be used in pharmacological and biophysical studies and in biosensors, pharmaceutical screens, environmental detectors, etc. The method clearly has the ability to study local and small changes in light transmission due to displacement of melanophore pigment granules. Since one melanophore on the tip of an optical fibre would be enough to obtain a measurable effect, the presented technique provides the basis for future development of biosensors small enough for in vivo applications, e.g., to monitor the catecholamine levels of circulating blood.     

Tunable Absorption of Light via Localized Plasmon Resonances on a Metal Surface with Interspaced Ultra-thin Metal Gratings

We studied optical reflection properties of complex metal (Ag) surfaces with close-interspaced ultra-thin metal gratings. Prominent reflection minima were observed corresponding to enhanced absorption of light. Our analysis convinced us that the period-dependent mode is ascribed to Bloch-wave-like resonances of surface plasmon waves at the overall effective metal surfaces, and the ridge-width-dependent mode to Fabry-Pérot-like resonances of localized surface plasmon waves in micro-/nanocavities defined by regions of the grating ridges. The latter resonance mode is shown highly tunable with variation of the grating ridge width. Such structures may be applied in spectrum resolvable photovoltaic devices, bio-sensing, and studying optical properties of cavity-coupled molecules or functional nanomaterials.     

Optical Bifacial Transmission by Asymmetric Charge-Oscillation-Induced Light Transmission Through a Plasmonic Structure

From first-principles computation, we reveal that optical bifacial transmission can be induced within an asymmetric metallic subwavelength structure. This phenomenon can be explained by a concrete picture in which the intensity of the driving forces for surface plasmon or charge wave is asymmetric for the two incident directions. Two distinguished different numerical methods, finite difference time domain (FDTD), and rigorous coupled wave analysis (RCWA) are utilized to verify that optical bifacial transmission can exist for linear plasmonic metamaterial. Previous results are also reviewed to confirm the physical meaning of optical bifacial transmission for a planar linear metamaterial. The incident light can provide direct driving forces for surface plasmon in one direction. While in the opposite direction, forces provided by the light diffraction are quite feeble. With the asymmetric driving forces, the excitation, propagation, and light-charge conversion of surface plasmon give the rise of bifacial charge-oscillation-induced transmission. In periodic a structure, the excitation of surface plasmon polariton can lead to the spoof vanish of such phenomenon. The transmissions for two incident directions get the same in macroscopic while the bifacial still exists in microscale.     

Recovery of Lost Photons in Plasmon-Mediated Transmission Through Continuous Metal Film

The plasmon-mediated transmission across a subwavelength-undulated, nanometer-thick continuous metal film embedded in a polymer exhibits an, as yet, unexplained loss, reducing the resonant transmission to half its theoretical value, precluding practical applications of this interesting spatially resolved polarization- and wavelength-selective effect. Slow chemical dissolution of the surrounding polymer while measuring the transmission spectrum under floating conditions reveals near complete recovery of the missing photons. The excess losses originate in nanoclusters of overheated polymer spots at the metal interface giving rise to index and geometrical nonuniformities where the mediating plasmon modes have their field maximum.     

Suppressed Optical Transmission Through an Array of Ultrathin Interlayer Metal Slits

In contrast to the enhanced peak transmission in a subwavelength metal hole array structure (Ebbesen et al., Nature 391:667–669, 1998), here we theoretically investigate the spectral transmission through an array of identical metal slits with ultrathin interlayers and surprisingly find the depressed optical transmission for both infinite and finite array case. Notably, in the latter system, the narrowband dip transmission is evidently produced with the accompaniment of selective field enhancement and phase jumping across the structure. Analyses suggest that this phenomenon is intrinsically related to the penetrant coupling of intracavity surface plasmon polaritons together with the slit termination effect.     

Light Funneling Profile During Enhanced Transmission Through a Subwavelength Metallic Slit

Plasmonics - The funneling profile of enhanced light transmission through a subwavelength slit in a perfect electric conductor is studied with finite-difference time-domain simulation. From the...     

Plasmon-Enhanced Optical Transmission at Multiple Wavelengths Through an Asymmetric Corrugated Thin Silver Film

Micro- and nanometallic structures that exhibit extraordinary optical transmission (EOT) have attracted much attention for their potential applications in photonic devices. However, most existing reports have only discussed EOT at one specific wavelength, which limits its use in multi-wavelength applications. Here, we experimentally demonstrate EOT at multiple wavelengths through an asymmetric corrugated thin silver film due to simultaneous excitation of multiple plasmonic resonances at both interfaces. A unique method that applies single-pulse nanosecond laser interference lithography is introduced to produce the silver grating, which shows high quality over large area. At oblique incidence, each EOT peak is observed to split into two peaks oppositely shifted towards higher and lower frequencies. At some specific angles of incidence, overlap of these shifted peaks gives rise to distorted transmission spectra. Our method may find applications involving multiple wavelengths such as multi-wavelength bandpass filters, anti-Stokes Raman scattering spectroscopy, enhanced four-wave mixing, and so on.     

Engineering Multiphase Metal Halide Perovskites Thin Films for Stable and Efficient Solar Cells

The intrinsic instability of lead halide perovskite semiconductors in an ambient atmosphere is one of the most critical issues that impedes perovskite solar cell commercialization. To overcome it, the use of bulky organic spacers has emerged as a promising solution. The resulting perovskite thin films present complex morphologies, difficult to predict, which can directly affect the device efficiency. Here, by combining in‐depth morphological and spectroscopic characterization, it is shown that both the ionic size and the relative concentration of the organic cation, drive the integration of bulky organic cations into the crystal unit cell and the thin film, inducing different perovskite phases and different vertical distribution, then causing a significant change in the final thin film morphology. Based on these studies, a fine‐engineered perovskite is constructed by employing two different large cations, namely, ethyl ammonium and butyl ammonium. The first one takes part in the 3D perovskite phase formation, the second one works as a surface modifier by forming a passivating layer on top of the thin film. Together they lead to improved photovoltaic performance and device stability when tested in air under continuous illumination. These findings propose a general approach to achieve reliability in perovskite‐based optoelectronic devices.     

Optimization of Dielectric-Coated Silver Nanoparticle Films for Plasmonic-Enhanced Light Trapping in Thin Film Silicon Solar Cells

Surface plasmonic-enhanced light trapping from metal nanoparticles is a promising way of increasing the light absorption in the active silicon layer and, therefore, the photocurrent of the silicon solar cells. In this paper, we applied silver nanoparticles on the rear side of polycrystalline silicon thin film solar cell and systematically studied the dielectric environment effect on the absorption and short-circuit current density (Jsc) of the device. Three different dielectric layers, magnesium fluoride (MgF₂, n?=?1.4), tantalum pentoxide (Ta₂O₅, n?=?2.2), and titanium dioxide (TiO₂, n?=?2.6), were investigated. Experimentally, we found that higher refractive index dielectric coatings results in a redshift of the main plasmonic extinction peak and higher modes were excited within the spectral region that is of interest in our thin film solar cell application. The optical characterization shows that nanoparticles coated with highest refractive index dielectric TiO₂ provides highest absorption enhancement 75.6 %; however, from the external quantum efficiency characterization, highest short-circuit current density Jsc enhancement of 45.8 % was achieved by coating the nanoparticles with lower refractive index MgF₂. We also further optimize the thickness of MgF₂ and a final 50.2 % Jsc enhancement was achieved with a 210-nm MgF₂ coating and a back reflector.     

Tailoring Absorption in Metal Gratings with Resonant Ultrathin Bridges

We present a theoretical analysis of the effects of short range surface plasmon polariton excitation on subwavelength bridges in metal gratings. We show that localized resonances in thin metal bridges placed within the slit of a free-standing silver grating dramatically modify transmission spectra and boost absorption regardless of the periodicity of the grating. Additionally, the interference of multiple localized resonances makes it possible to tailor the absorption properties of ultrathin gratings, regardless of the apertures’ geometrical size. This tunable, narrow band, enhanced–absorption mechanism triggered by resonant, short-range surface plasmon polaritons may also enhance nonlinear optical processes like harmonic generation, in view of the large third-order susceptibility of metals.     

Determination of the Optical Thickness of sub 10-nm Thin Metal Films by SPR Experiments

We discuss the experimental data of surface plasmon resonance (SPR) occurring at the interface between air and single and bimetallic thin layers of Au and Ag prepared on glass substrates. The bilayer configuration allowed for the measurements of the optical constants of metallic films that are ultra thin; e.g., below 10 nm of thickness since SPR modes on such thin films in a single-layer configuration are shallow. We also discuss the effect of film thickness on SPR coupling. Thickness and refractive index of the films were determined by matching experimental SPR curves to the theoretical ones. Thickness and roughness of the films were also measured by atomic force microscopy. The results obtained by experimental measurements are in good agreement with AFM analysis.     

Bidirectional Edge Asymmetric Light Transmission in Metal/Dielectric Device Based on Asymmetric Diffraction

Asymmetric light transmission (ALT) or optical diode-like nanodevices have attracted many research interests in recent years for its rosy potential application in all optical computing and information systems. In this work, we propose and numerically demonstrate a bidirectional edge asymmetric light transmission (BE-ALT) device, which is composed by the easy-processing metal/dielectric cylinders arranged periodically on glass substrate. The ALT effect in the proposed BE-ALT device shows a saltation at one critical wavelength, i.e., the asymmetric subtraction owns different signs for the wavelength larger and smaller than the critical wavelength. The asymmetric subtraction designed in this work changes dramatically from − 60% to + 80% at around 600 nm, which can be effectively manipulated by applying different structure parameters. The underlying physical mechanism has been investigated systematically, including the asymmetric diffraction effect, localized surface plasmonic resonance (LSPR), and the waveguide mode (WGM). Our designed BE-ALT device provides a new choice for the practical applications of ALT effect.

     

Theoretical and Experimental Investigation of Enhanced Transmission Through Periodic Metal Nanoslits for Sensing in Water Environment

Experimental and theoretical study of sensors based on enhanced transmission through periodic metal nanoslits is presented. Our approach consists of the design of one-dimensional nanoslits array and its application in sensing for water quality control. Rigorous coupled waves analysis was used for the design and fit to the experimental data. Two types of surface plasmon resonance excitations are shown to be possible, one at the upper grating–analyte interface and one at the lower grating–substrate interface. This latter resonance is shown to be affected by the multiple interference or cavity-type effects. Those structures were fabricated by deposition of the metal layer and electron beam lithography of the nanostructure. We found that Ag-based periodic array exhibits the highest sensitivity to refractive index variations. Sensitivity enhancement was measured by ethanol concentrations in water. Stability of the Ag-based sensor was improved by covering the grating with less than 15 nm polymethyl methacrylate capping layer without deterioration of the sensitivity.     

Resonant TE Transmission Through a Continuous Metal Film: Perspectives for Low-Loss Plasmonic Elements

Whereas resonant transverse magnetic transmission across an undulated continuous metal film is achieved with the mediation of plasmon modes excited by the undulation, it is shown here that transverse electric (TE) resonant transmission through a continuous metal film can also be achieved with the mediation of the second-order TE₁ mode of a dielectric slab waveguide having the metal film sandwiched at its middle. The demonstration is made by using the materials currently used in the domain of optical security and counterfeit deterrence: ZnS is shown to possibly be a lossless interface/adhesion layer between a polymer and a noble metal for plasmonic resonant elements.     

High Plasmonic Resonant Reflection and Transmission at Continuous Metal Films on Undulated Photosensitive Polymer

Whereas noble metal films deposited directly onto an undulated photosensitive polymer exhibit plasmon-mediated resonant transmission of free-space waves accompanied by excess losses of approximately 40 %, adequate hard baking of the photosensitive polymer, together with pre- and post-deposition of nanometer-thick ZnS or MgF₂ dielectric interfacial layers, restores excellent propagation conditions for the long-range plasmon mode, reducing resonant transmission excess losses to 10 %, and creating the conditions for the experimental evidence of the long-sought plasmonic anomalous reflection phenomenon.