Antireflection TiO x Coating with Plasmonic Metal Nanoparticles for Silicon Solar Cells

It is known that the light scattering from the metal particles deposited on the surfaces of cells can be used for increasing light trapping in the solar cells. In this work, plasmonic structures are composite materials that consisted of silver nanoparticles embedded in dielectric films of TiO _x —used as cell antireflection coating. The films are deposited by sol–gel method using spin-on technique. Microstructure of prepared samples is analyzed by SEM observation. Good homogenity and particles density was obtained by this simple, cheap, and short time-demanding method. We demonstrate that due to light scattering by metal particles, the plasmonic-ARC layer is more effective than TiO _x layer without Ag nanoparticles. Implementation of nanoparticles on bare cell surface was carried out too. The influence of the plasmonic structures on the silicon solar cells parameters is presented as well. We announce about 5 % additional growth in short circuit current for cells with nanoparticles.     

Design of Plasmonic Comb-Like Filters Using Loop-Based Resonators

A subwavelength plasmonic comb-like filter is proposed by using dual symmetric slot cavities which are placed between two parallel metal–insulator–metal (MIM) structure waveguides. The structure can be considered as a resonance loop which consists of slot cavity resonators and MIM waveguide resonators. The reflective wavelength range and channel spacing are determined by the lengths of slot cavities and MIM waveguides, respectively. Three, four, and five reflective channels with high reflection are achieved in a small wavelength range. Higher channel count can be available by increasing the length or the real part of effective index of MIM waveguides. Such a device can find applications in various optical systems such as wavelength demultiplexing components.     

Metal-Dielectric-Graphene Hybrid Heterostructures with Enhanced Surface Plasmon Resonance Sensitivity Based on Amplitude and Phase Measurements

Metal-dielectric-graphene hybrid heterostructures based on oxides Al₂O₃, HfO₂, and ZrO₂ as well as on complementary metal–oxide–semiconductor compatible dielectric Si₃N₄ covering plasmonic metals Cu and Ag have been fabricated and studied. We show that the characteristics of these heterostructures are important for surface plasmon resonance biosensing (such as minimum reflectivity, sharp phase changes, resonance full width at half minimum and resonance sensitivity to refractive index unit (RIU) changes) can be significantly improved by adding dielectric/graphene layers. We demonstrate maximum plasmon resonance spectral sensitivity of more than 30,000 nm/RIU for Cu/Al₂O₃ (ZrO₂, Si₃N₄), Ag/Si₃N₄ bilayers and Cu/dielectric/graphene three-layers for near-infrared wavelengths. The sensitivities of the fabricated heterostructures were?~?5–8 times higher than those of bare Cu or Ag thin films. We also found that the width of the plasmon resonance reflectivity curves can be reduced by adding dielectric/graphene layers. An unexpected blueshift of the plasmon resonance spectral position was observed after covering noble metals with high-index dielectric/graphene heterostructures. We suggest that the observed blueshift and a large enhancement of surface plasmon resonance sensitivity in metal-dielectric-graphene hybrid heterostructures are produced by stationary surface dipoles which generate a strong electric field concentrated at the very thin top dielectric/graphene layer.

     

Plasmonic Photonic Bloch Oscillations in Composite Metal–Insulator–Metal Waveguide Structure

This study proposes the time-evolved plasmonic photonic Bloch oscillations (PBOs) in a composite metal–insulator–metal (CMIM) waveguide structure. This device contains two kinds of MIM waveguide with different thickness of the insulator gaps. The time-resolved plasmonic PBO motion in this CMIM waveguide can be observed by introducing a linearly graded dielectric material. The ray trajectory results from the Hamiltonian optics are consistent with the finite-difference time-domain simulation results.     

Multiple-Wavelength Focusing and Demultiplexing Plasmonic Lens Based on Asymmetric Nanoslit Arrays

A multiple-wavelength focusing and demultiplexing plasmonic lens based on asymmetric nanoslit arrays is designed. The nanoslit arrays are perforated in a gold film and act as metal–insulator–metal plasmonic waveguides. By manipulating the widths of the slit arrays, the plasmonic lens can concentrate two incident plane wave beams to two separated focal points corresponding to their wavelengths. The full wave simulation is performed to verify the designed lens. This work provides a way to design more compact and integrated wavelength-division multiplexing plasmonic devices for nanophotonic communication and spectral imaging.     

Plasmonic Y-splitters of High Wavelength Resolution Based on Strongly Coupled-Resonator Effects

Based on the strongly coupled-resonator effects, a high wavelength-resolution plasmonic Y-splitter, consisting of a Y-branch metal–insulator–metal waveguide with a baffle in each channel, is numerically investigated using the finite element method. Due to the coupling of different resonators (with nearly equal bandwidths) in the Y-splitter, sharp and asymmetric transmission spectra occur. This greatly increases the wavelength resolution of the Y-splitter to be Δλ?≈?15 nm, which is significantly narrower than the bandwidth of the single resonator (Δλ _FWHM?≈?110 nm). An analytic model based on the scattering matrix theory is provided to describe and explain this phenomenon.     

Near-infrared Plasmonic Far-field Nanofocusing Effects with Elongated Depth of Focus based on Hybrid Au–Dielectric–Ag Subwavelength Structures

In this paper, we propose a new far-field nanofocusing lens with elongated depth of focus (DOF) under near-infrared (NIR) wavelength. The surface plasmons can be excited by using the hybrid metal–insulator–metal (MIM) subwavelength structure under the NIR wavelength. The constructive interference of surface plasmons launched by the subwavelength MIM structure can form a nanoscale focus that is modulated by the novel metal grating from the near field to the far field. The numerical simulations demonstrated that a nanoscale focal spot (in plane focal area 0.177λ ²) with elongated DOF (3.358λ) and long focal length (5.084λ) can be realized with reasonably designing parameters of the lens. By controlling the positions of the inner radii of each slit ring and the grating width, the focal length, focal spot, and DOF can be tuned easily. This design method, which can obtain the nanoscale focal spot and micron DOF in far field under NIR illumination, paved the road for utilizing the NIR plasmonic lens in superresolution optical microscopic imaging, optical trapping, biosensing, and complex wavefront/beam shaper.     

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.     

Ultrafast Nano-scale Optical Switching in a Plasmonic Interferometer with Enhanced Tunability

An all-optical switch based on plasmonic metal–insulator–metal (MIM) waveguides and the Mach–Zehnder (MZ) interferometer is designed. In order to realize an all-optical and active switch, a nonlinear material with intensity-dependent refractive index is introduced in one arm. Other than studying a typical MZ structure, we also investigate the asymmetric case where unequal thicknesses and distances for MZ arms are proposed. The finite element method (FEM) with a refined triangle mesh is employed for simulations. Results for ON and OFF states are provided with or without employing the pump field. Investigation of the geometrical dispersion reveals tunability of the structure for specific frequencies in the terahertz region. Finally, we show that introducing asymmetric arms provides better tunability in the designed ultrafast nano-scale switch and suggests its potential applications in integrated optical circuits.

     

Search of Extremely Sensitive Near-Infrared Plasmonic Interfaces: A Theoretical Study

The sensitivity of the wavelength position of localized surface plasmon resonance (LSPR) in metal nanostructures to local changes in the refractive index has been widely used for label-free detection strategies. Tuning the optical properties of the nanostructures from the visible to the infrared region is expected to have a drastic effect on the refractive index sensitivity. Here, we theoretically investigate the optical response of a newly designed plasmonic interface to changes in the bulk refractive index by the finite difference time domain method. It consists of a structured interface, where the planar interface is superposed with dielectric pillars 30 nm in height and 125 nm in length with a separation distance of 15 nm. The pillars are covered with U-shaped gold nanostructures of 50 nm in height, 125 nm in length, and 5 nm of gold base thickness. The whole structure is finally covered with a 5-nm thick dielectric layer of n ₂?=?2.63. This plasmonic structure shows bulk refractive index sensitivities up to 1750 nm/RIU (RIU : refractive index unit) in the near infrared (λ?=?2621 nm). The enhanced sensitivity is a consequence of the extremely enhanced electrical field between the gold nanopillars of the plasmonic interface.     

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 μm thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm², which can be increased up to 17-18 mA/cm² (~25% higher) after application of a simple diffuse back reflector made of a white paint.To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23°C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF₂, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF₂, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm², up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.

Introduction

Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) ¹ and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) ². For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector ³, silicon plasma texturing ⁴ or high refractive index nanoparticle reflector ⁵ have been suggested.Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect ⁶. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment ^6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested ⁸. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles ⁹. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance ¹⁰.The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF₂ or SiO₂, from the rear reflector to avoid mechanical and chemical damage ⁷. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric ⁶. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.     

Design and Optimization of Open-cladded Plasmonic Waveguides for CMOS Integration on Si3N4 Platform

Herein, we present a design analysis and optimization of open-cladded plasmonic waveguides on a Si₃N₄ photonic waveguide platform targeting CMOS-compatible manufacturing. For this purpose, two design approaches have been followed aiming to efficiently transfer light from the hosting photonic platform to the plasmonic waveguide and vice versa: (i) an in-plane, end-fire coupling configuration based on a thin-film plasmonic structure and (ii) an out-of-plane directional coupling scheme based on a hybrid slot waveguide. A comprehensive numerical study has been conducted, initially deploying gold as the reference metal material for validating the numerical models with already published experimental results, and then aluminum and copper have been investigated for CMOS manufacturing revealing similar performance. To further enhance coupling efficiency from the photonic to the plasmonic part, implementation of plasmonic tapering schemes was examined. After thorough investigation, plasmo-photonic structures with coupling losses per single interface in the order of 1 dB or even in the sub-dB level are proposed, which additionally exhibit increased tolerance to deviations of critical geometrical parameters and enable CMOS-compatible manufacturing.

     

Enhanced Optical Absorption and Spectral Photocurrent in a-Si:H by Single- and Double-Layer Silver Plasmonic Interfaces

Single and double plasmonic interfaces consisting of silver nanoparticles embedded in media with different dielectric constants including SiO₂, SiN_x, and Al:ZnO have been fabricated by a self-assembled dewetting technique and integrated to amorphous silicon films. Single plasmonic interfaces exhibit plasmonic resonances whose frequency is red-shifted with increasing particle size and with the thickness of a dielectric spacer layer. Double plasmonic interfaces consisting of two different particle sizes exhibit resonances consisting of double minima in the transmittance spectra. The optical extinction of a-Si:H deposited on these interfaces is broadened into the red indicating higher absorption and/or scattering at wavelengths higher than those typically absorbed by a-Si:H without plasmonic interfaces. While the photocurrent shows an overall decrease for the samples with the interfaces, significant enhancement of photocurrent is observed near the low-energy edge of the bandgap (600–700 nm). These results correlate well with the broadened extinction spectra of the interfaces and are interpreted in terms of enhanced absorption in that region.     

Influence of SiO<Subscript>2</Subscript> Spacer Layer Thickness on Performance of Plasmonic Textured Silicon Solar Cell

We investigated the effect of SiO₂ spacer layer thickness between the textured silicon surface and silver nanoparticles (Ag NPs) on solar cell performance using quantum efficiency analysis. Separation of Ag NPs from high index silicon with SiO₂ layer led to modified absorption and scattering cross-sections due to graded refractive index medium. The forward scattering from Ag NPs is very sensitive to SiO₂ layer thickness in plasmonic silicon cell performance due to the evanescent character of generated near-fields around the NPs. With the optimized ～30–40 nm SiO₂ spacer layer, we observed an enhancement of solar cell efficiency from ～8.7 to ～10 %, which is due to the photocurrent enhancement in the off-resonance surface plasmon region. We also estimated minority carrier diffusion lengths (L _eff) from internal quantum efficiency data, which are also sensitive to SiO₂ spacer layer thickness. We observed that the L _eff values are enhanced from ～356 to ～420 μm after placing Ag NPs on ～40 nm spacer layer due to improved forward (angular) scattering of light from the Ag NPs into silicon.     

β-Diketiminato 3d-metal compounds: Synthesis, characterization and catalytic behavior towards ethylene

The lithium β-diketiminate (1c, [Li{N(2,6-ⁱPr₂C₆H₃)C(Ph)CHC(^tBu)NH}]₂ represented as (LiL)₂) reacted with 3d-metal (II) chlorides to afford the corresponding compounds (2-7). All metal compounds were fully characterized by elemental, spectroscopic analyses and the single-crystal X-ray diffraction. The coordination geometries around the metals are shown to be tetrahedral within the trinuclear Co₂Li compound (2), planar in ML₂ (M = Co, 3), pseudo-tetrahedral conformation in the ML₂ with M as Mn (4), Fe (5) or Zn (6), and square planar in the dinickel compound (7). Indicated by the trimetallic Co₂Li compound 2, a six-membered ring is constructed of three metal atoms and three bridged chlorides as a twisted conformation. An inversion center is present in the centroid of the Ni₂Cl₂ four-membered ring within compound 7. The plausible mechanism of forming ML₂ was proposed through the chloro-bridged multinuclear compounds on the basis of isolated intermediates of trinuclear (2) and dinuclearic (7) compounds. Upon treatment with methylaluminoxane (MAO), the nickel compound 7 possessed good activity towards ethylene oligomerization, whereas the other metal compounds showed moderate activities towards ethylene polymerization.     

Reconfigurable Plasmonic Logic Gates

Reconfigurable one-, two-, and three-bit plasmonic logic gate configurations have been proposed, which work by covering a straight slot waveguide with materials with tunable dielectric constants, such as graphene. By encoding the logic states in the values of dielectric constants as opposed to different waveguides, the plasmon excitation problems are minimized and the simplified logic gate configurations could be easily implemented.

     

Enhancement of Second Harmonic Generation in Metal-Insulator-Metal Plasmonic Waveguides

Nonlinear effects such as second harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, the generation of second harmonic in metal-insulator-metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. This study considered two different structures as plasmonic waveguides for the generation of second harmonic, and analysis was performed using the finite-difference time-domain method. Besides, the structure has grating on both sides for more coupling between photons and plasmons. The wavelength duration of grating per unit length (number of grooves) was optimized to reach the highest second harmonic generation. To perform this optimization, the wavelength of operation (λ = 458 nm) was considered. It was shown that field enhancement in symmetric MIM waveguides can result in the enhancement of SHG magnitude when compared to literature values. Also, asymmetric devices result in more than two orders of magnitude enhancement in SHG, as compared to the symmetric structure. It has been shown that the electric field of the second harmonic depends on the thickness of the crystal (insulator). Hence, its thickness was optimized to achieve the highest electric field.     

The influence of inductive ligand electronics on the metrical parameters and redox potentials of a series of manganese(II) compounds

In order to assess the changes in the redox activity of a metal ion that result from inductive effects, three electronically modified derivatives of the ligand, N-benzyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L^H), have been prepared: N-(4-nitro)benzyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L^NO2), N-(4-chloro)benzyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L^Cl), and N-(4-methoxy)benzyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L^OMe). Due to the lack of a fully conjugated π-system between the 4-benzyl substituent and the N-donors, the electronic perturbation should influence a bound metal ion’s redox properties through primarily inductive pathways. The organic ligands react with MnCl₂ to form mononuclear complexes with the general formula [Mn(L^R)Cl₂]. The parent ligand, L^H, and its three derivatives each coordinate Mn(II) ions in a cis-α conformation, with the amine N-donors installed trans to the Mn-Cl bonds. Despite its distance from the metal ion, the electron-donating or - withdrawing group has a notable impact on both the metrical parameters of the Mn(II) compounds and the Mn(III/II) reduction potential. A single inductive perturbation can vary the reduction potential by as much as 50 mV.     

Theoretical Investigation of an Interferometer-type Plasmonic Biosensor Using a Metal-insulator-silicon Waveguide

This work proposes and investigates theoretically a biosensor that is an integrated plasmonic Mach–Zehnder interferometer. The biosensor consists of three sections. The first and third sections are input and output dielectric waveguides whose core is a silicon film. The second section is a combination of a surface plasmon polariton waveguide and a metal-insulator-silicon waveguide, which are separated by a thick gold film. The former and the latter function as sensing and reference arms, respectively. The latter supports a mode whose fields are highly enhanced in a thin insulator, silicon nitride film, and it has relatively small propagation loss. It is shown that the biosensor has insertion loss lower than 2 dB, and that it is very compact since the length of its second section for sensing is shorter than 6 μm. In addition, it is discussed that it can be easily implemented by using simple fabrication processes. Analyzed are the characteristics of sensing a refractive index change of liquid covering the biosensor. Despite its compactness, they are similar to those of previous surface plasmon interferometers. Also, its characteristics as a DNA sensor are analyzed. The analysis demonstrates that the biosensor can detect sensitively target single-stranded DNAs whose total weight is smaller than 10 fg.