A transparent projection screen based on plasmonic Ag nanocubes

Water-phase synthesis of Au and Au-Ag nanowires and their SERS activity

Metal nanowires (NWs) with a diameter of a few nanometers have attracted considerable attention as a promising one-dimensional nanomaterial due to their inherent flexibility and conductive properties and their weak plasmon absorption in the visible region. In a previous paper, we reported the synthesis of ultrathin 1.8 nm-diameter Au NWs using toluene-solubilized aqueous solutions of a long-chain amidoamine derivative (C18AA). This study investigates the effect of different organic solvents solubilized in C18AA aqueous solutions on the morphology of the Au products and demonstrates that solubilizing methylcyclohexane yields thick 2.7 nm-diameter Au NWs and 3.3 nm-diameter Au-Ag alloy NWs. Further, the surface-enhanced Raman scattering sensitivity of ultrathin Au NWs, thick Au NWs, and thick Au-Ag alloy NWs were assessed using 4-mercaptopyridine and found that their enhancement factors are 10⁴-10⁵ and the order is Au-Ag NWs > thick Au NWs > ultrathin Au NWs.

Degradation problem in silver nanowire transparent electrodes caused by ultraviolet exposure

Silver nanowire (AgNW) transparent electrode inevitably encounters ultraviolet (UV) irradiation from the environment, leading to stability and durability problems when in operation. Since UVA is the most abundant UV band and highly penetrating to AgNW related optoelectrical devices, it is crucial to understand the UVA damage caused to AgNWs. In this study, transparent electrodes composed of pristine AgNWs and glass substrates were manufactured with optimized processing parameters, and then used as model samples for aging tests. UVA exposure was conducted at elevated temperatures including 45 °C, 60 °C and 75 °C at 12 ± 5.5% relative humidity (RH) conditions. Comparative aging tests using conditions of damp heat (85 °C/85% RH) and 105 °C without UV (dark conditions) were also conducted. The relationship between optoelectrical property degradation, morphological changes and photo-corrosion was discussed. Under UVA exposure, the sheet resistance of electrodes increased gradually in an induction period before an abrupt change occurred. A nominal sheet resistance value of 200 Ω/sq was considered as a predestined failure of electrical property. It took 16, 24 and 60 h for UVA exposure at 75 °C, 60 °C and 45 °C, respectively, and 288 h by damp heat aging to degrade to the same status of predestined failure. Aging results of dark conditions indicated no degradation effect on AgNWs for 126 d aging. Moisture caused a different mechanism in damaging the capping agents on AgNWs. Nanocubes of silver chloride and sodium chloride were prone to precipitate at higher aging temperature such as 75 °C with UVA exposure. Sulfidation accounted for deterioration of optical transmittance, and occurred significantly at 45 °C with UVA irradiation and under damp heat conditions. The synergistic aging effect of UVA irradiance at elevated temperature on AgNW degradation has been unambiguously demonstrated. The results of this study provide guidelines for the design of optoelectronic devices when utilizing AgNW transparent electrodes.

Sharp selective scattering of red, green, and blue light achieved via gain material's loss compensation

Frequency-selective scattering of light can be achieved by metallic nanoparticle's localized surface plasmon resonance (LSPR). And this property may find an application in a transparent projection screen: ideally, specially designed metallic nanoparticles dispersed in a transparent matrix only selectively scatter red, green and blue light and transmit the visible light of other colors. However, optical absorption and surface dispersion of a metallic nanoparticle, whose size is comparable or smaller than mean free path of electrons in the constituent material, degenerate the desired performance by broadening the resonance peak width (i.e., decreasing frequency-selectivity) and decreasing light scattering intensity. In this work, it is shown that the problem can be solved by introducing gain material. Numerical simulations are performed on nanostructures based on silver (Ag), gold (Au) or aluminum (Al) with or without gain material, to examine the effect of gain material and to search for suitable structures for sharp selective scattering of red, green and blue light. And it is found that introducing gain material greatly improves performance of the structures based on Ag or Au except the structures based on Al. The most suitable structures for sharp selective scattering of red, green and blue light are, respectively, found to be the core-shell structures of silica/Au (core/shell), silica/Ag and Ag/silica, all with gain material.

Effect of Surface Scattering of Electrons on Ratios of Optical Absorption and Scattering to Extinction of Gold Nanoshell

Gold nanoshell's high light scattering and absorption at its resonance wavelength have found applications in biomedical imaging and photothermal therapy. However, at nanoscale, metallic material's dielectric function is affected by nanoparticle's size, mainly via a mechanism called surface scattering of conduction electrons. In this work, the effect of surface scattering of electrons on the ratios of optical absorption and scattering to extinction (which is the sum of the absorption and scattering) of gold nanoshell is investigated. Simulation results for several shell thicknesses are compared. It is found that the electrons' surface scattering increases the optical absorption ratio, and the thinner the shell thickness, the larger the increase in the difference of the absorption ratio between the situations with and without the surface scattering considered. The increase of absorption ratio is then verified by comparing simulation results to experimental measurements for three nanoshells. The parameters of the simulations to fit the experimental measurements show that the damping of conduction electrons in metallic shell geometry is larger than that predicted by the billiard scattering model.

Multipolar Nanocube Plasmon Mode-Mixing in Finite Substrates

Facile control of the radiative and nonradiative properties of plasmonic nanostructures is of practical importance to a wide range of applications in the biological, chemical, optical, information, and energy sciences. For example, the ability to easily tune not only the plasmon spectrum but also the degree of coupling to light and/or heat, quality factor, and optical mode volume would aid the performance and function of nanophotonic devices and molecular sensors that rely upon plasmonic elements to confine and manipulate light at nanoscopic dimensions. While many routes exist to tune these properties, identifying new approaches-especially when they are simple to apply experimentally-is an important task. Here, we demonstrate the significant and underappreciated effects that substrate thickness and dielectric composition can have upon plasmon hybridization as well as downstream properties that depend upon this hybridization. We find that even substrates as thin as ∼10 nm can nontrivially mix free-space plasmon modes, imparting bright character to those that are dark (and vice versa) and, thereby, modifying the plasmonic density of states as well as the system's near- and far-field optical properties. A combination of electron energy-loss spectroscopy (EELS) experiment, numerical simulation, and analytical modeling is used to elucidate this behavior in the finite substrate-induced mixing of dipole, quadrupole, and octupole corner-localized plasmon resonances of individual silver nanocubes.

Resonant scattering of green light enabled by Ag@TiO2 and its application in a green light projection screen

The ability to selectively scatter green light is essential for an RGB transparent projection display, and this can be achieved by a silver-core, titania-shell nanostructure (Ag@TiO₂), based on the metallic nanoparticle's localized surface plasmon resonance. The ability to selectively scatter green light is shown in a theoretical design, in which structural optimization is included, and is then experimentally verified by characterization of a transparent film produced by dispersing such nanoparticles in a polymer matrix. A visual assessesment indicates that a high-quality green image can be clearly displayed on the transparent film. For completeness, a theoretical design for selective scattering of red light based on Ag@TiO₂ is also shown.

Photoassisted bottom-up construction of plasmonic nanocity

Herein, photoassisted self-construction of nanocity as a novel plasmonic metasurface was achieved. It is an ensemble of nanobuildings, and the height of each nanobuilding is greater than its depth. Plasmonic nanocity exhibits a vertical resonance mode in addition to distal longitudinal and proximal longitudinal resonance modes, such that it can be applied to chromatic angular polarizers, sophisticated image recording, and high density data storage. Further growth of nanobuildings to bulky and tall nanocuboids leads to asymmetric and dichroic scattering, which can be applied in security printing.

Light-efficient augmented reality 3D display using highly transparent retro-reflective screen

We propose and demonstrate a light-efficient 3D display using a highly transparent desktop-sized augmented reality screen. The display consists of a specially designed transparent retro-reflective screen and a pair of low-power pico-projectors positioned close to the viewer's eyes to provide stereo views. The transfer screen is an optically clear sheet partially patterned with retro-reflective microspheres for high optical gain. The retro-reflective material buried in the screen reflects incident light back towards the projectors with a narrow scattering angle and facilitates the viewer to perceive a very bright content. The tabletop prototype mainly consists of an in-house fabricated large augmented reality (AR) screen (60  cm×40  cm) and a pair of laser-scanning 30 lumen pico-projectors. The display is tested for different viewing configurations, and different display parameters, such as retro-reflective coefficient, eye-box size, polarization maintainability, stereo crosstalk, and brightness, are examined. The AR prototype display provides 75% optical transparency, exceptional brightness (up to 1000  cd/m² when viewed through beam splitters and 350  cd/m² with bare eyes), and negligible crosstalk in 3D mode (<5% and <1% when viewed through beam splitters and polarizers, respectively) for the working distance of up to 2 m.

Plasmonic Control and Stabilization of Asymmetric Light Scattering from Ag Nanocubes on TiO2

When plasmonic nanoparticles are placed on a highly refractive semiconductor substrate, we can expect three different effects: (i) resonance mode splitting, (ii) asymmetric light scattering based on the split modes, and (iii) site-selective nanoetching due to plasmon-induced charge separation (PICS) at the nanoparticle-semiconductor interface. Here, we develop novel photofunctional materials by taking advantage of those three effects. More specifically, we control the asymmetric scattering of Ag nanocubes on TiO₂ by PICS, so as to develop materials for photodrawing of one-way visible translucent images and multicolor scattering images. The one-way visible translucent images, which are translucent scattering images visible only from the back side, are drawn by anaerobic bottom-selective etching of the Ag nanocubes. For drawing the multicolor scattering images, a scattering color of Ag nanocubes is changed from yellow to green by the anaerobic bottom-selective etching and from yellow to red by aerobic nonselective etching. We also theoretically and experimentally examined the contribution of a possible thermal effect to the nanoetching, and revealed that the contribution is negligible; Ag nanocubes on TiO₂ are stable even at 473 K for 2 h in the dark, whereas the theoretically expected temperature increase under light is less than 1 K. In addition, we developed methods to stabilize the Ag nanocubes by inactivating PICS. When Ag nanocubes on TiO₂ are coated with a thin polymer layer, PICS is decelerated and the stability is improved. Replacing TiO₂ with diamond, which does not accept electrons from plasmonic nanoparticles, is also an effective method to stabilize the nanocubes.

Multifunctional Silicon Optoelectronics Integrated with Plasmonic Scattering Color

Plasmonic scattering from metallic nanoparticles has been used for centuries to create the colorful appearance of stained glass. Besides their use as passive spectral filtering components, multifunctional optoelectronic applications can be achieved by integrating the nanoscatters with semiconductors that generate electricity using the complementary spectral components of plasmonic colors. To suppress the usual degradation of both efficiency and the gamut of plasmonic scattering coloration in highly asymmetric index configurations like a silicon host, aluminum nanodisks on indium tin oxide (ITO) coated silicon were experimentally studied and demonstrated color sorting in the full visible range along with photocurrent generation. Interestingly, the photocurrents were found to be comparable to the reference devices with only antireflection coatings in spite of the power loss for coloration. Detailed investigation shows that ITO serves as both the impedance matching layer for promoting the backward scattering and schottky contact with silicon, and moreover, plasmonic nanoscatters efficiently harvest the complement spectrum components for charge generation. The present approach combines the capacities of nanoscale color sorting and photoelectric converting at a negligible cost of efficiency, thus providing a broad flexibility of being utilized in various optoelectronic applications including self-powered display, filter-free imaging, and colorful photovoltaics.

Direct imaging of isofrequency contours in photonic structures

The isofrequency contours of a photonic crystal are important for predicting and understanding exotic optical phenomena that are not apparent from high-symmetry band structure visualizations. We demonstrate a method to directly visualize the isofrequency contours of high-quality photonic crystal slabs that show quantitatively good agreement with numerical results throughout the visible spectrum. Our technique relies on resonance-enhanced photon scattering from generic fabrication disorder and surface roughness, so it can be applied to general photonic and plasmonic crystals or even quasi-crystals. We also present an analytical model of the scattering process, which explains the observation of isofrequency contours in our technique. Furthermore, the isofrequency contours provide information about the characteristics of the disorder and therefore serve as a feedback tool to improve fabrication processes.

Optically Thin Metallic Films for High-Radiative-Efficiency Plasmonics

Plasmonics enables deep-subwavelength concentration of light and has become important for fundamental studies as well as real-life applications. Two major existing platforms of plasmonics are metallic nanoparticles and metallic films. Metallic nanoparticles allow efficient coupling to far field radiation, yet their synthesis typically leads to poor material quality. Metallic films offer substantially higher quality materials, but their coupling to radiation is typically jeopardized due to the large momentum mismatch with free space. Here, we propose and theoretically investigate optically thin metallic films as an ideal platform for high-radiative-efficiency plasmonics. For far-field scattering, adding a thin high-quality metallic substrate enables a higher quality factor while maintaining the localization and tunability that the nanoparticle provides. For near-field spontaneous emission, a thin metallic substrate, of high quality or not, greatly improves the field overlap between the emitter environment and propagating surface plasmons, enabling high-Purcell (total enhancement >10(4)), high-quantum-yield (>50%) spontaneous emission, even as the gap size vanishes (3-5 nm). The enhancement has almost spatially independent efficiency and does not suffer from quenching effects that commonly exist in previous structures.

Asymmetric optical properties of photocatalytically deposited plasmonic silver nanoparticles

Plasmonic Ag nanoparticles deposited and grown photocatalytically for a sufficient period of time on a TiO₂ thin film scatter blue or reddish light for the front or back incidence, respectively.