Iridescence-controlled and flexibly tunable retroreflective structural color film for smart displays.

Structural color materials, which use nano- or microstructures to reflect specific wavelengths of ambient white light, have drawn much attention owing to their wide applications ranging from optoelectronics, coatings, to energy-efficient reflective displays. Although various structural color materials based on specular or diffuse reflection have been demonstrated, neither efficient retroreflective structural colors nor iridescent and non-iridescent colors to different observers simultaneously were reported by existing artificial or natural structural color materials. Here, we show that by partially embedding a monolayer of polymer microspheres on the sticky side of a transparent tape, the spontaneously formed interferometric structure on the surface of air-cushioned microspheres can lead to unique structural colors that remain non-iridescent under coaxial illumination and viewing conditions, but appear iridescent under noncoaxial illumination and viewing conditions. Our findings demonstrate a smart, energy-efficient, and tunable retroreflective structural color material that is especially suitable for nighttime traffic safety and advertisement display applications.

Plasmonic Interferometer Array Biochip as a New Mobile Medical Device for Cancer Detection.

We report a plasmonic interferometer array (PIA) sensor and demonstrate its ability to detect circulating exosomal proteins in real-time with high sensitivity and low cost to enable the early detection of cancer. Specifically, a surface plasmon wave launched by the nano-groove rings interferes with the free-space light at the output of central nano-aperture and results in an intensity interference pattern. Under the single-wavelength illumination, when the target exosomal proteins are captured by antibodies bound on the surface, the biomediated change in the refractive index between the central aperture and groove rings causes the intensity change in transmitted light. By recording the intensity changes in real-time, one can effectively screen biomolecular binding events and analyze the binding kinetics. By integrating signals from multiple sensor pairs to enhance the signal-to-noise ratio, superior sensing resolutions of 1.63×10-6 refractive index unit (RIU) in refractive index change and 3.86×108 exosomes/mL in exosome detection were realized, respectively. Importantly, this PIA sensor can be imaged by a miniaturized microscope system coupled with a smart phone to realize a portable and highly sensitive healthcare device. The sensing resolution of 9.72×109 exosomes/mL in exosome detection was realized using the portable sensing system building upon a commercial smartphone.

Cold Vapor Generation beyond the Input Solar Energy Limit.

100% efficiency is the ultimate goal for all energy harvesting and conversion applications. However, no energy conversion process is reported to reach this ideal limit before. Here, an example with near perfect energy conversion efficiency in the process of solar vapor generation below room temperature is reported. Remarkably, when the operational temperature of the system is below that of the surroundings (i.e., under low density solar illumination), the total vapor generation rate is higher than the upper limit that can be produced by the input solar energy because of extra energy taken from the warmer environment. Experimental results are provided to validate this intriguing strategy under 1 sun illumination. The best measured rate is ≈2.20 kg m-2 h-1 under 1 sun illumination, well beyond its corresponding upper limit of 1.68 kg m-2 h-1 and is even faster than the one reported by other systems under 2 sun illumination.

Ultra-broadband enhancement of nonlinear optical processes from randomly patterned super absorbing metasurfaces.

Broadband light trapping and field localization is highly desired in enhanced light-matter interaction, especially in harmonic generations. However, due to the limited resonant bandwidth, most periodic plasmonic nanostructures cannot cover both fundamental excitation wavelength and harmonic generation wavelength simultaneously. Therefore, most previously reported plasmonic nonlinear optical processes are low in conversion efficiency. Here, we report a strong enhancement of second harmonic generation based on a three-layered super absorbing metasurface structure consisting of a dielectric spacer layer sandwiched by an array of random metallic nanoantennas and a metal ground plate. Intriguingly, the strong light trapping band (e.g. >80%) was realized throughout the entire visible to near-infrared spectral regime (i.e., from 435 nm to 1100 nm), enabling plasmonically enhanced surface harmonic generation and frequency mixing across a broad range of excitation wavelengths, which cannot be achieved with narrow band periodic plasmonic structures. By introducing hybrid random antenna arrays with small metallic nanoparticles and ultra-thin nonlinear optical films (e.g. TiO2) into the nanogaps, the nonlinear optical process can be further enhanced. This broadband light-trapping metastructure shows its potential as a building block for emerging nonlinear optical meta-atoms.

Extremely Cost-Effective and Efficient Solar Vapor Generation under Nonconcentrated Illumination Using Thermally Isolated Black Paper.

Passive solar vapor generation represents a promising and environmentally benign method of water purification/desalination. However, conventional solar steam generation techniques usually rely on costly and cumbersome optical concentration systems and have relatively low efficiency due to bulk heating of the entire liquid volume. Here, an efficient strategy using extremely low-cost materials, i.e., carbon black (powder), hydrophilic porous paper, and expanded polystyrene foam is reported. Due to the excellent thermal insulation between the surface liquid and the bulk volume of the water and the suppressed radiative and convective losses from the absorber surface to the adjacent heated vapor, a record thermal efficiency of ≈88% is obtained under 1 sun without concentration, corresponding to the evaporation rate of 1.28 kg (m2 h)-1. When scaled up to a 100 cm2 array in a portable solar water still system and placed in an outdoor environment, the freshwater generation rate is 2.4 times of that of a leading commercial product. By simultaneously addressing both the need for high-efficiency operation as well as production cost limitations, this system can provide an approach for individuals to purify water for personal needs, which is particularly suitable for undeveloped regions with limited/no access to electricity.

Frozen "Tofu" Effect: Engineered Pores of Hydrophilic Nanoporous Materials.

Frozen tofu is a famous Asian food made by freezing soft bean curds, which are naturally porous to store flavor and nutrients. When the narrow pores of the soft bean curd are saturated with water and then frozen, pore widths expand to generate a completely new porous structure-frozen tofu has visibly wider pores than the initial bean curd. Intriguingly, this principle can be generalized and applied to manipulate micro/nanopores of functional porous materials. In this work, we will manipulate the pore size of nanoporous polymeric photonic crystals based on the phase change between water and ice. Wet-drying and freeze-drying methods were applied to shrink or expand the pore size intentionally. This principle is validated by directly observing the optical reflection peak shift of the material. Owing to the change in pore size, the reflection peak of the polymeric photonic crystal structure can be permanently, and intentionally, tuned. This simple but elegant mechanism is promising for the development of smart materials/devices for applications ranging from oil/water membrane separations, health monitoring, and medical diagnostics to environmental monitoring, anticounterfeiting, and smart windows.

Flat metallic surface gratings with sub-10 nm gaps controlled by atomic-layer deposition.

Atomic layer lithography is a recently reported new technology to fabricate deep-subwavelength features down to 1-2 nm, based on combinations of electron beam lithography (EBL) and atomic layer deposition (ALD). However, the patterning area is relatively small as limited by EBL, and the fabrication yield is not very high due to technical challenges. Here we report an improved procedure to fabricate flat metallic surfaces with sub-10 nm features based on ALD processes. To demonstrate the scalability of the new manufacturing method, we combine the ALD process with large area optical interference patterning, which is particularly promising for the development of practical applications for nanoelectronics and nanophotonics with extremely strong confinement of electromagnetic fields.

Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation.

The study of ideal absorbers, which can efficiently absorb light over a broad range of wavelengths, is of fundamental importance, as well as critical for many applications from solar steam generation and thermophotovoltaics to light/thermal detectors. As a result of recent advances in plasmonics, plasmonic absorbers have attracted a lot of attention. However, the performance and scalability of these absorbers, predominantly fabricated by the top-down approach, need to be further improved to enable widespread applications. We report a plasmonic absorber which can enable an average measured absorbance of ~99% across the wavelengths from 400 nm to 10 µm, the most efficient and broadband plasmonic absorber reported to date. The absorber is fabricated through self-assembly of metallic nanoparticles onto a nanoporous template by a one-step deposition process. Because of its efficient light absorption, strong field enhancement, and porous structures, which together enable not only efficient solar absorption but also significant local heating and continuous stream flow, plasmonic absorber-based solar steam generation has over 90% efficiency under solar irradiation of only 4-sun intensity (4 kW m(-2)). The pronounced light absorption effect coupled with the high-throughput self-assembly process could lead toward large-scale manufacturing of other nanophotonic structures and devices.

Thermally Diffused Al:ZnO Thin Films for Broadband Transparent Conductor.

Here, we report an approach to realize highly transparent low resistance Al-doped ZnO (AZO) films for broadband transparent conductors. Thin Al films are deposited on ZnO surfaces, followed by thermal diffusion processes, introducing the Al doping into ZnO thin films. By utilizing the interdiffusion of Al, Zn, and O, the chemical state of Al on the surfaces can be converted to a fully oxidized state, resulting in a low sheet resistance of 6.2 Ω/sq and an excellent transparency (i.e., 96.5% at 550 nm and higher than 85% up to 2500 nm), which is superior compared with some previously reported values for indium tin oxide, solution processed AZO, and many transparent conducting materials using novel nanostructures. Such AZO films are also applied as transparent conducting layers for AZO/Si heterojunction solar cells, demonstrating their applications in optoelectronic devices.

Phase change dispersion of plasmonic nano-objects.

Phase is an inherent and important feature for coherent processes, which, unfortunately, has not been completely understood for surface plasmon polariton (SPP) and matter interactions. Here we propose a practical approach to extract the phase change dispersion during the interaction between free-space light, SPPs and nanogroove/slit based on far-field information only. Numerical simulation and experimental validation were both presented using nanoslit-groove plasmonic interferometers, agreeing well with theoretical near-field analysis. This approach is generally feasible to extract the intrinsic phase dispersion of other plasmonic nanostructures and can reveal more fundamental features of SPP-matter interactions.

Nanocavity absorption enhancement for two-dimensional material monolayer systems.

Here we propose a strategy to enhance the light-matter interaction of two-dimensional (2D) material monolayers based on strong interference effect in planar nanocavities, and overcome the limitation between optical absorption and the atomically-thin thickness of 2D materials. By exploring the role of spacer layers with different thicknesses and refractive indices, we demonstrate that a nanocavity with an air spacer layer placed between a graphene monolayer and an aluminum reflector layer will enhance the exclusive absorption in the graphene monolayer effectively, which is particularly useful for the development of atomically-thin energy harvesting/conversion devices.

A large-scale lithography-free metasurface with spectrally tunable super absorption.

Recently, periodically patterned metasurfaces have been employed to realize on-chip super/near-perfect optical absorption. However, most reported meta-absorbers rely on top-down micro/nano-lithography, which imposed a serious cost barrier on the development of practical applications, especially in the visible-infrared (IR) domain and at very large scales. Here we report a simple method to manufacture super absorptive metasurfaces using direct sputtering deposition. By controlling the deposition and post thermal treatment conditions, random metallic nanoparticles (NPs) can be formed easily on rigid and flexible substrates to function as the nanoantennas of spectrally tunable meta-absorbers. This low-cost and highly scalable approach would release the manufacturing barrier for previously reported meta-absorbers and therefore enable the development of affordable and large-scale thin-film metamaterial structures and devices.

Nanocavity enhancement for ultra-thin film optical absorber.

A fundamental strategy is developed to enhance the light-matter interaction of ultra-thin films based on a strong interference effect in planar nanocavities, and overcome the limitation between the optical absorption and film thickness of energy harvesting/conversion materials. This principle is quite general and is applied to explore the spectrally tunable absorption enhancement of various ultra-thin absorptive materials including 2D atomic monolayers.

Broadband absorption engineering of hyperbolic metafilm patterns.

Perfect absorbers are important optical/thermal components required by a variety of applications, including photon/thermal-harvesting, thermal energy recycling, and vacuum heat liberation. While there is great interest in achieving highly absorptive materials exhibiting large broadband absorption using optically thick, micro-structured materials, it is still challenging to realize ultra-compact subwavelength absorber for on-chip optical/thermal energy applications. Here we report the experimental realization of an on-chip broadband super absorber structure based on hyperbolic metamaterial waveguide taper array with strong and tunable absorption profile from near-infrared to mid-infrared spectral region. The ability to efficiently produce broadband, highly confined and localized optical fields on a chip is expected to create new regimes of optical/thermal physics, which holds promise for impacting a broad range of energy technologies ranging from photovoltaics, to thin-film thermal absorbers/emitters, to optical-chemical energy harvesting.

Holographic photopolymer linear variable filter with enhanced blue reflection.

A single beam one-step holographic interferometry method was developed to fabricate porous polymer structures with controllable pore size and location to produce compact graded photonic bandgap structures for linear variable optical filters. This technology is based on holographic polymer dispersed liquid crystal materials. By introducing a forced internal reflection, the optical reflection throughout the visible spectral region, from blue to red, is high and uniform. In addition, the control of the bandwidth of the reflection resonance, related to the light intensity and spatial porosity distributions, was investigated to optimize the optical performance. The development of portable and inexpensive personal health-care and environmental multispectral sensing/imaging devices will be possible using these filters.

Rainbow trapping in hyperbolic metamaterial waveguide.

The recent reported trapped "rainbow" storage of light using metamaterials and plasmonic graded surface gratings has generated considerable interest for on-chip slow light. The potential for controlling the velocity of broadband light in guided photonic structures opens up tremendous opportunities to manipulate light for optical modulation, switching, communication and light-matter interactions. However, previously reported designs for rainbow trapping are generally constrained by inherent difficulties resulting in the limited experimental realization of this intriguing effect. Here we propose a hyperbolic metamaterial structure to realize a highly efficient rainbow trapping effect, which, importantly, is not limited by those severe theoretical constraints required in previously reported insulator-negative-index-insulator, insulator-metal-insulator and metal-insulator-metal waveguide tapers, and therefore representing a significant promise to realize the rainbow trapping structure practically.