Plasmonic nanostructure design for efficient light coupling into solar cells

Plasmonic thin film InP/graphene-based Schottky-junction solar cell using nanorods

Herein, the design and simulation of graphene/InP thin film solar cells with a novel periodic array of nanorods and plasmonic back-reflectors of the nano-semi sphere was proposed. In this structure, a single-layer of the graphene sheet was placed on the vertical nanorods of InP to form a Schottky junction. The electromagnetic field was determined using solving three-dimensional Maxwell's equations discretized by the finite difference method (FDM). The enhancement of light trapping in the absorbing layer was illustrated, thereby increasing the short circuit current to a maximum value of 31.57 mA/cm² with nanorods having a radius of 400 nm, height of 1250 nm, and nano-semi sphere radius of 50 nm, under a solar irradiation of AM1.5G. The maximum ultimate efficiency was determined to be 45.8% for an angle of incidence of 60°. This structure has shown a very good light trapping ability when graphene and ITO layers were used at the top and as a back-reflector in the proposed photonic crystal structure of the InP nanorods. Thence, this structure improves the short-circuit current density and the ultimate efficiency of 12% and 2.7%, respectively, in comparison with the InP-nanowire solar cells.

Nanoscale imaging of photocurrent enhancement by resonator array photovoltaic coatings

Nanoscale surface patterning commonly used to increase absorption of solar cells can adversely impact the open-circuit voltage due to increased surface area and recombination. Here, we demonstrate absorptivity and photocurrent enhancement using silicon dioxide (SiO₂) nanosphere arrays on a gallium arsenide (GaAs) solar cell that do not require direct surface patterning. Due to the combined effects of thin-film interference and whispering gallery-like resonances within nanosphere arrays, there is more than 20% enhancement in both absorptivity and photocurrent. To determine the effect of the resonance coupling between nanospheres, we perform a scanning photocurrent microscopy based on a near-field scanning optical microscopy measurement and find a substantial local photocurrent enhancement. The nanosphere-based antireflection coating (ARC), made by the Meyer rod rolling technique, is a scalable and a room-temperature process; and, can replace the conventional thin-film-based ARCs requiring expensive high-temperature vacuum deposition.

Recent Advances of Plasmonic Organic Solar Cells: Photophysical Investigations

The surface plasmon resonance (SPR) of metallic nanomaterials, such as gold (Au) and silver (Ag), has been extensively exploited to improve the optical absorption, the charge carrier transport, and the ultimate device performances in organic photovoltaic cells (OPV). With the incorporation of diverse metallic nanostructures in active layers, buffer layers, electrodes, or between adjacent layers of OPVs, multiple plasmonic mechanisms may occur and need to be distinguished to better understand plasmonic enhancement. Steady-state photophysics is a powerful tool for unraveling the plasmonic nature and revealing plasmonic mechanisms such as the localized surface plasmon resonance (LSPR), the propagating plasmon-polariton (SPP), and the plasmon-gap mode. Furthermore, the charge transfer dynamics in the organic semiconductor materials can be elucidated from the transient photophysical investigations. In this review article, the basics of the plasmonic mechanisms and the related metallic nanostructures are briefly introduced. We then outline the recent advances of the plasmonic applications in OPVs emphasizing the linkage between the photophysical properties, the nanometallic geometries, and the photovoltaic performance of the OPV devices.

Stabilisation of small mono- and bimetallic gold–silver nanoparticles using calix[8]arene derivatives

We report a facile approach to synthesise small Au–Ag alloyed nanoparticles using a new cali[8]arene derivative as a stabiliser.

Bright off-axis directional emission with plasmonic corrugations

In this work, a new plasmonic bulls-eye structure is introduced to efficiently harvest the emitted light from diamond nitrogen vacancy (NV) centers. We show that the presence of a simple metal sub-layer underneath of a conventional bulls-eye antenna, separated by a dielectric layer, results in the spontaneous emission enhancement and increment in out-coupled light intensity. High Purcell factor is accessible in such a structure, which consequently boosts efficiency of the radiated light intensity from the structure. The structure shows considerable enhancement in far-field intensity, about three times higher than that of a one-side corrugated (conventional) optimized structure. In addition, we study for the first time asymmetric structures to steer emitted beams in two-axis. Our results show that spatial off-axial steering over a cone is approachable by introducing optimal asymmetries to grooves and ridges of the structure. The steered light retains a level of intensity even higher than conventional symmetric structures. A high value of directivity of 16 for off-axis steering is reported.

Momentum Distribution of Electrons Emitted from Resonantly Excited Individual Gold Nanorods

Electron emission by femtosecond laser pulses from individual Au nanorods is studied with a time-of-flight momentum resolving photoemission electron microscope (ToF k-PEEM). The Au nanorods adhere to a transparent indium-tin oxide substrate, allowing for illumination from the rear side at normal incidence. Localized plasmon polaritons are resonantly excited at 800 nm with 100 fs long pulses. The momentum distribution of emitted electrons reveals two distinct emission mechanisms: a coherent multiphoton photoemission process from the optically heated electron gas leads to an isotropic emission distribution. In contrast, an additional emission process resulting from the optical field enhancement at both ends of the nanorod leads to a strongly directional emission parallel to the nanorod's long axis. The relative intensity of both contributions can be controlled by the peak intensity of the incident light.

Flexible Semiconductor Technologies with Nanoholes-Provided High Areal Coverages and Their Application in Plasmonic-Enhanced Thin Film Photovoltaics

Mechanical flexibility and advanced light management have gained great attentions in designing high performance, flexible thin film photovoltaics for the realization of building-integrated optoelectronic devices and portable energy sources. This study develops a soft thermal nanoimprint process for fabricating nanostructure decorated substrates integrated with amorphous silicon solar cells. Amorphous silicon (a-Si:H) solar cells have been constructed on nanoholes array textured polyimide (PI) substrates. It has been demonstrated that the nanostructures not only are beneficial to the mechanical flexibility improvement but also contribute to sunlight harvesting enhancement. The a-Si:H solar cells constructed on such nanopatterned substrates possess broadband-enhanced light absorption, high quantum efficiency and desirable power conversion efficiency (PCE) and still experience minimal PCE loss even bending around 180°. The PCE performance without antireflection coatings increases to 7.70% and it improves 40% compared with the planar devices. Although the advantages and feasibility of the schemes are demonstrated only in the application of a-Si:H solar cells, the ideas are able to extend to applications of other thin film photovoltaics and semiconductor devices.

Transparent Electrochemical Gratings from a Patterned Bistable Silver Mirror

Silver mirror patterns were formed reversibly on a polystyrene (PS)-patterned electrode to produce gratings through the electrochemical reduction of silver ions. The electrochemical gratings exhibited high transparency (T > 95%), similar to a see-through window, by matching the refractive index of the grating pattern with the surrounding medium. The gratings switch to a diffractive state upon the formation of a mirror pattern (T < 5%) with a high diffraction efficiency up to 40%, providing reversible diffractive gratings. The diffraction state was maintained in the voltage-off state (V-off) for 40 min, which demonstrated bistable reversible electrochemical grating (BREG) behavior. By carefully combining the BREGs through period matching, dual-color switching was achieved within the full color region, which exhibited three distinct optical switching states between -2.5, 0, and +2.5 V. The wide range of light tenability using the metallic BREGs developed herein enabled IR modulation, NIR light reflection, and on-demand heat transfer.

Dendritic optical antennas: scattering properties and fluorescence enhancement

With the development of nanotechnologies, researchers have brought the concept of antenna to the optical regime for manipulation of nano-scaled light matter interactions. Most optical nanoantennas optimize optical function, but are not electrically connected. In order to realize functions that require electrical addressing, optical nanoantennas that are electrically continuous are desirable. In this article, we study the optical response of a type of electrically connected nanoantennas, which we propose to call “dendritic” antennas. While they are connected, they follow similar antenna hybridization trends to unconnected plasmon phased array antennas. The optical resonances supported by this type of nanoantennas are mapped both experimentally and theoretically to unravel their optical response. Photoluminescence measurements indicate a potential Purcell enhancement of more than a factor of 58.

Synergetic SERS Enhancement in a Metal-Like/Metal Double-Shell Structure for Sensitive and Stable Application

Because of either thermal/chemical instability or high optical loss in noble metal nanostructures, searching for alternative plasmonic materials is becoming more and more urgent, considering the practical biosensing applications under various extreme conditions. In this work, titanium nitride (TiN), a low-loss metal-like material with both excellent thermal and excellent chemical stabilities, was proposed to composite with Ag hollow nanosphere (HNS) nanostructures as an effective surface-enhanced Raman scattering (SERS) substrate to realize both highly sensitive and highly stable molecular detection. Because of the multiple-mode local surface plasmon resonance around the spherical composite nanospheres and the "gap effect" derived from the ultrasmall nanogaps within the precisely controlled plasmonic arrays, an intensively enhanced local field was successfully induced on this SERS substrate. Combined with the unique charge transferring process between Ag and TiN, a synergistically enhanced SERS sensitivity involving both physical and chemical mechanisms was achieved. Especially, with the isolation of TiN, a time-durable Raman detection on these TiN-Ag HNS arrays was accomplished, showing great potential for practical applications.

Measurement of Nanoplasmonic Field Enhancement with Ultrafast Photoemission

Probing nanooptical near-fields is a major challenge in plasmonics. Here, we demonstrate an experimental method utilizing ultrafast photoemission from plasmonic nanostructures that is capable of probing the maximum nanoplasmonic field enhancement in any metallic surface environment. Directly measured field enhancement values for various samples are in good agreement with detailed finite-difference time-domain simulations. These results establish ultrafast plasmonic photoelectrons as versatile probes for nanoplasmonic near-fields.

Order of magnitude enhancement of monolayer MoS2 photoluminescence due to near-field energy influx from nanocrystal films

Two-dimensional transition metal dichalcogenides (TMDCs) like MoS2 are promising candidates for various optoelectronic applications. The typical photoluminescence (PL) of monolayer MoS2 is however known to suffer very low quantum yields. We demonstrate a 10-fold increase of MoS2 excitonic PL enabled by nonradiative energy transfer (NRET) from adjacent nanocrystal quantum dot (NQD) films. The understanding of this effect is facilitated by our application of transient absorption (TA) spectroscopy to monitor the energy influx into the monolayer MoS2 in the process of ET from photoexcited CdSe/ZnS nanocrystals. In contrast to PL spectroscopy, TA can detect even non-emissive excitons, and we register an order of magnitude enhancement of the MoS2 excitonic TA signatures in hybrids with NQDs. The appearance of ET-induced nanosecond-scale kinetics in TA features is consistent with PL dynamics of energy-accepting MoS2 and PL quenching data of the energy-donating NQDs. The observed enhancement is attributed to the reduction of recombination losses for excitons gradually transferred into MoS2 under quasi-resonant conditions as compared with their direct photoproduction. The TA and PL data clearly illustrate the efficacy of MoS2 and likely other TMDC materials as energy acceptors and the possibility of their practical utilization in NRET-coupled hybrid nanostructures.

Photovoltaic Devices: Opto-Electro-Thermal Physics and Modeling

An opto-electro-thermal simulation of solar cells (SCs) is presented by addressing optoelectronic and thermodynamic responses simultaneously. The photocurrent losses due to carrier recombinations and the intrinsic heat generation (thermalization/Joule/Peltier/recombination heat) and dissipation (convective/radiative cooling) processes in the SCs are investigated quantitatively.

Rapidly fabricating large-scale plasmonic silver nanosphere arrays with sub-20 nm gap on Si-pyramids by inverted annealing for highly sensitive SERS detection

An inverted annealing method is developed to fabricate rapidly plasmonic silver nanosphere arrays with sub-20 nm gaps for highly sensitive SERS detection.

A low-cost, high-efficiency light absorption structure inspired by the Papilio ulysses butterfly

The nano-hole array structure in the black scales of the butterfly can be viewed as a natural solar collector.

Controllable photoinduced scattering and optimized light emission intensity in Nd3+ doped (Pb,La)(Zr,Ti)O3 perovskite ceramics

Controllable photoinduced scatterers were investigated in Nd³⁺-doped lead lanthanum zirconate titanate (PLZT) perovskite ceramics, the total number of which will increase dramatically with the induction of light intensity.

Enhanced light-harvesting capability for silicon single-nanowire solar cells coupled with metallic cavity

Single-nanowire solar cells (SNSCs) are attracting increasing interest due to their unique optical antenna effect beneficial for achieving higher light-trapping capability. However, for conventional circular-cross-sectional SNSCs, the light-trapping performance is still far from the expectation. Here we demonstrate that integrating a silicon single nanowire into a metallic slit can dramatically enhance the absorption efficiency over almost the whole spectral band due to strengthened optical antenna effect. Especially, it is found that by using finite-size metallic blocks to form a nanoscale metallic cavity, the light-trapping performance of the SNSCs can be further improved. Through examining the detailed optical spectral response, electric field distribution, and cavity dispersion characteristics, the metallic-coupled SNSC system is optimized and the underlying physics are provided. Simulation results indicate that the photocurrent density of the SNSCs coupled with the designed metallic cavity can be enhanced by 44.4% than that of the conventional bare SNSCs.

Plasmonic Solar Cells: From Rational Design to Mechanism Overview

Plasmonic effects have been proposed as a solution to overcome the limited light absorption in thin-film photovoltaic devices, and various types of plasmonic solar cells have been developed. This review provides a comprehensive overview of the state-of-the-art progress on the design and fabrication of plasmonic solar cells and their enhancement mechanism. The working principle is first addressed in terms of the combined effects of plasmon decay, scattering, near-field enhancement, and plasmonic energy transfer, including direct hot electron transfer and resonant energy transfer. Then, we summarize recent developments for various types of plasmonic solar cells based on silicon, dye-sensitized, organic photovoltaic, and other types of solar cells, including quantum dot and perovskite variants. We also address several issues regarding the limitations of plasmonic nanostructures, including their electrical, chemical, and physical stability, charge recombination, narrowband absorption, and high cost. Next, we propose a few potentially useful approaches that can improve the performance of plasmonic cells, such as the inclusion of graphene plasmonics, plasmon-upconversion coupling, and coupling between fluorescence resonance energy transfer and plasmon resonance energy transfer. This review is concluded with remarks on future prospects for plasmonic solar cell use.

A Simple Optical Model Well Explains Plasmonic-Nanoparticle-Enhanced Spectral Photocurrent in Optically Thin Solar Cells

A simple optical model for photocurrent enhancement by plasmonic metal nanoparticles atop solar cells has been developed. Our model deals with the absorption, reflection, and scattering of incident sunlight as well as radiation efficiencies on metallic nanoparticles. Our calculation results satisfactorily reproduce a series of experimental spectral data for optically thin GaAs solar cells with Ag and Al nanoparticles of various dimensions, demonstrating the validity of our modeling approach. Our model is likely to be a powerful tool for investigations of surface plasmon-enhanced thin-film solar cells.

Monitoring interfacial lectin binding with nanomolar sensitivity using a plasmon field effect transistor

By immobilizing glycopolymers onto the surface of the recently developed plasmonic field effect transistor (FET), the recognition between lectins and surface-immobilized glycopolymers can be detected over a wide dynamic range (10^-10 to 10^-4 M) in an environment that resembles the glycocalyx. The binding to the sensor surface by various lectins was tested, and the selectivities and relative binding affinity trends observed in solution were maintained on the sensor surface, and the significantly higher avidities are attributed to cluster-glycoside effects that occur on the surface. The combination of polymer surface chemistry and optoelectronic output in this device architecture produces amongst the highest reported detection sensitivity for ConA. This work demonstrates the benefits that arise from combining emerging device architectures and soft-matter systems to create cutting edge nanotechnologies that lend themselves to fundamental biological studies and integration into point-of-use diagnostics and sensors.

Large Absorption Enhancement in Ultrathin Solar Cells Patterned by Metallic Nanocavity Arrays

A new type of light trapping structure utilizing ring-shaped metallic nanocavity arrays is proposed for the absorption enhancement in ultrathin solar cells with few photonic waveguide modes. Dozens of times of broadband absorption enhancement in the spectral range of 700 to 1100 nm is demonstrated in an ultrathin Si₃N₄/c-Si/Ag prototype solar cell by means of finite-difference time-domain (FDTD) simulation, and this dramatic absorption enhancement can be attributed to the excitation of plasmonic cavity modes in these nanocavity arrays. The cavity modes optimally compensate for the lack of resonances in the longer wavelength range for ultrathin solar cells, and eventually a maximum J_sc enhancement factor of 2.15 is achieved under AM 1.5G solar illumination. This study opens a new perspective for light management in thin film solar cells and other optoelectronic devices.

Multi-type particle layer improved light trapping for photovoltaic applications

This work discusses regular particle arrays as nanostructured front layers for possible application in photovoltaic devices yielding strongly increased forward scattering. I used a rigorous plane-wave method to investigate multi-type particle layers combining different radii and configurations. The absorbance was enhanced compared to the bare Si wafer and I demonstrated on mixing particles a broadband boost in the absorbance within the homogeneous wafer region, excluding parasitic absorption in the particle layer. I studied the efficiency enhancement for varying geometries. Multi-type layers made of Si disks with two different radii achieved up to 33% (24%) and with four different radii up to 40% (30%) improvement in the short circuit current and integrated absorbance, respectively, without yet standard anti-reflection coatings. Broadband efficiency enhancement for metal multi-type layers was not observed because they show strong parasitic absorption and boost the absorbance only in narrow wavelength regions.

Near-Unity Absorption in van der Waals Semiconductors for Ultrathin Optoelectronics

We demonstrate near-unity, broadband absorbing optoelectronic devices using sub-15 nm thick transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as van der Waals semiconductor active layers. Specifically, we report that near-unity light absorption is possible in extremely thin (<15 nm) van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We further fabricate Schottky junction devices using these highly absorbing heterostructures and characterize their optoelectronic performance. Our work addresses one of the key criteria to enable TMDCs as potential candidates to achieve high optoelectronic efficiency.

Aspect-ratio driven evolution of high-order resonant modes and near-field distributions in localized surface phonon polariton nanostructures

Polar dielectrics have garnered much attention as an alternative to plasmonic metals in the mid- to long-wave infrared spectral regime due to their low optical losses. As such, nanoscale resonators composed of these materials demonstrate figures of merit beyond those achievable in plasmonic equivalents. However, until now, only low-order, phonon-mediated, localized polariton resonances, known as surface phonon polaritons (SPhPs), have been observed in polar dielectric optical resonators. In the present work, we investigate the excitation of 16 distinct high-order, multipolar, localized surface phonon polariton resonances that are optically excited in rectangular pillars etched into a semi-insulating silicon carbide substrate. By elongating a single pillar axis we are able to significantly modify the far- and near-field properties of localized SPhP resonances, opening the door to realizing narrow-band infrared sources with tailored radiation patterns. Such control of the near-field behavior of resonances can also impact surface enhanced infrared optical sensing, which is mediated by polarization selection rules, as well as the morphology and strength of resonator hot spots. Furthermore, through the careful choice of polar dielectric material, these results can also serve as the guiding principles for the generalized design of optical devices that operate from the mid- to far-infrared.

Plasmon resonance energy transfer and plexcitonic solar cell

Plasmon-mediated energy transfer is highly desirable in photo-electronic nanodevices, but the direct injection efficiency of "hot electrons" in plasmonic photo-detectors and plasmon-sensitized solar cells (plasmon-SSCs) is poor. On another front, Fano resonance induced by strong plasmon-exciton coupling provides an efficient channel of coherent energy transfer from metallic plasmons to molecular excitons, and organic dye molecules have a much better injection efficiency in exciton-SSCs than "hot electrons". Here, we investigate enhanced light-harvesting of chlorophyll-a molecules strongly coupled to Au nanostructured films via Fano resonance. The enhanced local field and plasmon resonance energy transfer are experimentally revealed by monitoring the ultrafast dynamical processes of the plexcitons and the photocurrent flows of the assembled plexciton-SSCs. By tuning the Fano factor and anti-resonance wavelengths, we find that the local field is largely enhanced and the efficiency of plexciton-SSCs consisting of ultrathin TiO2 films is significantly improved. Most strikingly, the output power of the plexciton-SSCs is much larger than the sum of those of the individual plasmon- and exciton-SSCs. Our observations provide a practical approach to monitor energy and electron transfer in plasmon-exciton hybrids at a strong coupling regime and also offer a new strategy to design photovoltaic nanodevices.

Nano/Micro-Manufacturing of Bioinspired Materials: a Review of Methods to Mimic Natural Structures

Through billions of years of evolution and natural selection, biological systems have developed strategies to achieve advantageous unification between structure and bulk properties. The discovery of these fascinating properties and phenomena has triggered increasing interest in identifying characteristics of biological materials, through modern characterization and modeling techniques. In an effort to produce better engineered materials, scientists and engineers have developed new methods and approaches to construct artificial advanced materials that resemble natural architecture and function. A brief review of typical naturally occurring materials is presented here, with a focus on chemical composition, nano-structure, and architecture. The critical mechanisms underlying their properties are summarized, with a particular emphasis on the role of material architecture. A review of recent progress on the nano/micro-manufacturing of bio-inspired hybrid materials is then presented in detail. In this case, the focus is on nacre and bone-inspired structural materials, petals and gecko foot-inspired adhesive films, lotus and mosquito eye inspired superhydrophobic materials, brittlestar and Morpho butterfly-inspired photonic structured coatings. Finally, some applications, current challenges and future directions with regard to manufacturing bio-inspired hybrid materials are provided.

Wavelength-tunable perfect absorber based on guided-mode resonances

We numerically investigate the triple-band perfect absorption in a metal-insulator-metal structure. The absorption peak from the TE-polarized guided-mode resonance (GMR) is highly sensitive to the incident angle. Thus a wavelength-tunable perfect absorber (PA) based on the TE GMR is proposed for the first time. By the precise control of the incident angle, the ∼5  nm narrowband perfect absorber can be modulated linearly about 3 nm/° in our structure. For single frequency light, the intensity tunability of the absorption between 6.2%-99.27% is realized only by changing the incident angle of 5°. The further study focused on TM polarization confirms the possibility to realize a polarization-independent wavelength-tunable PA. Such a PA possesses potential for applications in absorption filter, thermal emitter, surface-enhanced Raman scattering, biosensing, and nonlinear optics.

Probing plasmons in three dimensions by combining complementary spectroscopies in a scanning transmission electron microscope

The nanoscale optical response of surface plasmons in three-dimensional metallic nanostructures plays an important role in many nanotechnology applications, where precise spatial and spectral characteristics of plasmonic elements control device performance. Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) within a scanning transmission electron microscope have proven to be valuable tools for studying plasmonics at the nanoscale. Each technique has been used separately, producing three-dimensional reconstructions through tomography, often aided by simulations for complete characterization. Here we demonstrate that the complementary nature of the two techniques, namely that EELS probes beam-induced electronic excitations while CL probes radiative decay, allows us to directly obtain a spatially- and spectrally-resolved picture of the plasmonic characteristics of nanostructures in three dimensions. The approach enables nanoparticle-by-nanoparticle plasmonic analysis in three dimensions to aid in the design of diverse nanoplasmonic applications.

Anisotropic Nanoantenna-Based Magnetoplasmonic Crystals for Highly Enhanced and Tunable Magneto-Optical Activity

We present a novel concept of a magnetically tunable plasmonic crystal based on the excitation of Fano lattice surface modes in periodic arrays of magnetic and optically anisotropic nanoantennas. We show how coherent diffractive far-field coupling between elliptical nickel nanoantennas is governed by the two in-plane, orthogonal and spectrally detuned plasmonic responses of the individual building block, one directly induced by the incident radiation and the other induced by the application of an external magnetic field. The consequent excitation of magnetic field-induced Fano lattice surface modes leads to highly tunable and amplified magneto-optical effects as compared to a continuous film or metasurfaces made of disordered noninteracting magnetoplasmonic anisotropic nanoantennas. The concepts presented here can be exploited to design novel magnetoplasmonic sensors based on coupled localized plasmonic resonances, and nanoscale metamaterials for precise control and magnetically driven tunability of light polarization states.

Plasmonic Color-Graded Nanosystems with Achromatic Subwavelength Architectures for Light Filtering and Advanced SERS Detection

Plasmonic color-graded systems are devices featuring a spatially variable plasmonic response over their surface. They are widely used as nanoscale color filters; their typical size is small enough to allow integration with miniaturized electronic circuits, paving the way to realize novel nanophotonic devices. Currently, most plasmonic color-graded systems are intrinsically discrete because their chromatic response exploits the tailored plasmon resonance of microarchitectures characterized by different size or geometry for each target color. Here, we report the realization of multifunctional plasmon-graded devices where continuously graded chromatic response is achieved by smoothly tuning the composition of the resonator material while simultaneously maintaining an achromatic nanoscale geometry. The result is a new class of versatile materials: we show their application as plasmonic filters with a potential pixel size smaller than half of the exciting wavelength but also as multiplexed surface-enhanced Raman spectroscopy (SERS) substrates. Many more implementations, such as photovoltaic efficiency boosters or color routers, await and will benefit from the low fabrication cost and intrinsic plasmonic flexibility of the presented systems.

Ultrathin tandem-plasmonic photovoltaic structures for synergistically enhanced light absorption

We have proposed and simulated a tandem ultra-thin silicon solar cell, in which each layer is integrated with metal nanostructures, using the FDTD method.

Modular, polymer-directed nanoparticle assembly for fabricating metamaterials

We achieve the fabrication of plasmonic meta-atoms by utilizing a novel, modular approach to nanoparticle self-assembly that utilizes polymer templating to control meta-atom size and geometry. Ag nanocubes are deposited and embedded into a polymer thin-film, where the polymer embedding depth is used to dictate which nanocube faces are available for further nanocrystal binding. Horizontal and vertical nanocube dimers were successfully fabricated with remarkably high yield using a bifunctional molecular linker to bind a second nanocube. Surface plasmon coupling can be readily tuned by varying the size, shape, and orientation of the second nanoparticle. We show that meta-atoms can be fabricated to exhibit angle- and polarization-dependent optical properties. This scalable technique for meta-atom assembly can be used to fabricate large-area metasurfaces for polarization- and phase-sensitive applications, such as optical sensing.

Enabling High Efficiency Nanoplasmonics with Novel Nanoantenna Architectures

Surface plasmon polaritons (SPPs) are propagating excitations that arise from coupling of light with collective electron oscillations. Characterized by high field intensity and nanometric dimensions, SPPs fashion rapid expansion of interest from fundamental and applicative perspectives. However, high metallic losses at optical frequencies still make nanoplasmonics impractical when high absolute efficiency is paramount, with major challenge is efficient plasmon generation in deep nanoscale. Here we introduce the Plantenna, the first reported nanodevice with the potential of addressing these limitations utilizing novel plasmonic architecture. The Plantenna has simple 2D structure, ultracompact dimensions and is fabricated on Silicon chip for future CMOS integration. We design the Plantenna to feed channel (20 nm × 20 nm) nanoplasmonic waveguides, achieving 52% coupling efficiency with Plantenna dimensions of λ³/17,000. We theoretically and experimentally show that the Plantenna enormously outperforms dipole couplers, achieving 28 dB higher efficiency with broad polarization diversity and huge local field enhancement. Our findings confirm the Plantenna as enabling device for high efficiency plasmonic technologies such as quantum nanoplasmonics, molecular strong coupling and plasmon nanolasers.

Angular dependence of light trapping in nanophotonic thin-film solar cells

The angular dependence of light-trapping in nanophotonic thin-film solar cells is inherent due to the wavelength-scale dimensions of the periodic nanopatterns. In this paper, we experimentally investigate the dependence of light coupling to waveguide modes for light trapping in a-Si:H solar cells deposited on nanopatterned back contacts. First, we accurately determine the spectral positions of individual waveguide modes in thin-film solar cells in external quantum efficiency and absorptance. Second, we demonstrate the strong angular dependence of this spectral position for our solar cells. Third, a moderate level of disorder is introduced to the initially periodic nanopattern of the back contacts. As a result, the angular dependence is reduced. Last, we experimentally compare this dependence on the angle of incidence for randomly textured, 2D periodically nanopatterned and 2D disordered back contacts in external quantum efficiency and short-circuit current density.

Enhanced Magneto-Optical Edge Excitation in Nanoscale Magnetic Disks

We report unexpected enhancements of the magneto-optical effect in ferromagnetic Permalloy disks of diameter D<400 nm. The effect becomes increasingly pronounced for smaller D, reaching more than a 100% enhancement for D=100 nm samples. By means of experiments and simulations, the origin of this effect is identified as a nanoscale ring-shaped region at the disk edges, in which the magneto-optically induced electric polarization is enhanced. This leads to a modification of the electromagnetic near fields and causes the enhanced magneto-optical excitation, independent from any optical resonance.

Mechanistic Understanding of the Plasmonic Enhancement for Solar Water Splitting

H2 generation by solar water splitting is one of the most promising solutions to meet the increasing energy demands of the fast developing society. However, the efficiency of solar-water-splitting systems is still too low for practical applications, which requires further enhancement via different strategies such as doping, construction of heterojunctions, morphology control, and optimization of the crystal structure. Recently, integration of plasmonic metals to semiconductor photocatalysts has been proved to be an effective way to improve their photocatalytic activities. Thus, in-depth understanding of the enhancement mechanisms is of great importance for better utilization of the plasmonic effect. This review describes the relevant mechanisms from three aspects, including: i) light absorption and scattering; ii) hot-electron injection and iii) plasmon-induced resonance energy transfer (PIRET). Perspectives are also proposed to trigger further innovative thinking on plasmonic-enhanced solar water splitting.

Light trapping in randomly arranged silicon nanorocket arrays for photovoltaic applications

Realization of broadband optical absorption enhancement in thin film c-Si solar cells is essential for improving energy conversion efficiency and reducing cost. Here, we demonstrate the fabrication of randomly arranged silicon nanorocket (SiNR) arrays as a new light trapping structure design for thin film silicon solar cells. The optical absorption of the randomly arranged SiNR arrays is investigated via finite-difference-frequency-domain (FDTD) simulation. Our calculations reveal that the light trapping structures facilitate the coupling of incident sunlight into the resonant modes and lead to significant photon absorption enhancement across a wide solar spectrum, resulting in ultimate efficiencies superior to nanowire and nanohole arrays with the same thickness. Our findings indicate that the randomly arranged SiNR arrays fabricated by the simple self-assembly and etching approach can have a significant impact on performance improvement in thin film silicon solar cells.

Intensity tunable infrared broadband absorbers based on VO2 phase transition using planar layered thin films

Plasmonic and metamaterial based nano/micro-structured materials enable spectrally selective resonant absorption, where the resonant bandwidth and absorption intensity can be engineered by controlling the size and geometry of nanostructures. Here, we demonstrate a simple, lithography-free approach for obtaining a resonant and dynamically tunable broadband absorber based on vanadium dioxide (VO2) phase transition. Using planar layered thin film structures, where top layer is chosen to be an ultrathin (20 nm) VO2 film, we demonstrate broadband IR light absorption tuning (from ~90% to ~30% in measured absorption) over the entire mid-wavelength infrared spectrum. Our numerical and experimental results indicate that the bandwidth of the absorption bands can be controlled by changing the dielectric spacer layer thickness. Broadband tunable absorbers can find applications in absorption filters, thermal emitters, thermophotovoltaics and sensing.

Current Approach in Surface Plasmons for Thin Film and Wire Array Solar Cell Applications

Surface plasmons, which exist along the interface of a metal and a dielectric, have been proposed as an efficient alternative method for light trapping in solar cells during the past ten years. With unique properties such as superior light scattering, optical trapping, guide mode coupling, near field concentration, and hot-electron generation, metallic nanoparticles or nanostructures can be tailored to a certain geometric design to enhance solar cell conversion efficiency and to reduce the material costs. In this article, we review current approaches on different kinds of solar cells, such as crystalline silicon (c-Si) and amorphous silicon (a-Si) thin film solar cells, organic solar cells, nanowire array solar cells, and single nanowire solar cells.

Analysis and simulation of nonlinearity and effects of spontaneous emission in Schottky-junction-based plasmonic amplifiers

An amplifier that operates on surface plasmon polaritons has been analyzed and simulated. Nonlinearity behavior and the spontaneous emission effects of the plasmonic amplifier are investigated in this paper. A rate equations approach has been used in which parameters are derived from simulation results of the plasmonic amplifier (Silvaco/ATLAS). Details on the method of this derivation are included, which were not previously reported. Rate equations are solved numerically by MATLAB codes. These codes verify the Silvaco results. The plasmonic amplifier operates on surface plasmons with a free-space wavelength of 1550 nm. Results show that, even without the effect of spontaneous emission, gain of the plasmonic amplifier saturates in high input levels. Saturation power, which can be used for comparing nonlinearity of different amplifiers, is 2.1 dBm for this amplifier. Amplified spontaneous emission reduces the gain of the amplifiers, which is long. There is an optimum value for the length of the amplifier. For the amplifier of this work, the optimum length for the small signal condition is 265 μm.

Optical Dark-Field and Electron Energy Loss Imaging and Spectroscopy of Symmetry-Forbidden Modes in Loaded Nanogap Antennas

We have produced large numbers of hybrid metal-semiconductor nanogap antennas using a scalable electrochemical approach and systematically characterized the spectral and spatial character of their plasmonic modes with optical dark-field scattering, electron energy loss spectroscopy with principal component analysis, and full wave simulations. The coordination of these techniques reveal that these nanostructures support degenerate transverse modes which split due to substrate interactions, a longitudinal mode which scales with antenna length, and a symmetry-forbidden gap-localized transverse mode. This gap-localized transverse mode arises from mode splitting of transverse resonances supported on both antenna arms and is confined to the gap load enabling (i) delivery of substantial energy to the gap material and (ii) the possibility of tuning the antenna resonance via active modulation of the gap material's optical properties. The resonant position of this symmetry-forbidden mode is sensitive to gap size, dielectric strength of the gap material, and is highly suppressed in air-gapped structures which may explain its absence from the literature to date. Understanding the complex modal structure supported on hybrid nanosystems is necessary to enable the multifunctional components many seek.