Broadband Light Research Articles

Laboratory demonstration of the wrapped staircase scalar vortex coronagraph

Of the over 5000 exoplanets that have been detected, only about a dozen have ever been directly imaged. Earth-like exoplanets are on the order of 10 billion times fainter than their host star in visible and near-infrared, requiring a coronagraph instrument to block primary starlight and allow for the imaging of nearby orbiting planets. In the pursuit of direct imaging of exoplanets, scalar vortex coronagraphs (SVCs) are an attractive alternative to vector vortex coronagraphs (VVCs). VVCs have demonstrated 2 × 10 − 9 raw contrast in broadband light but have several limitations due to their polarization properties. SVCs imprint the same phase ramp as VVCs on the incoming light and do not require polarization splitting, but they are inherently chromatic. Discretized phase ramp patterns such as a wrapped staircase help reduce SVC chromaticity and simulations show it outperforms a chromatic classical vortex in broadband light. We designed, fabricated, and tested a wrapped staircase SVC, and here we present the broadband characterization on the high contrast spectroscopy testbed. We also performed wavefront correction on the in-air coronagraph testbed at NASA’s Jet Propulsion Laboratory and achieved an average raw contrasts of 3.2 × 10 − 8 in monochromatic light and 2.2 × 10 − 7 across a 10% bandwidth.

Novel high-brightness yellow light-emitting phosphor Ba2GdGaO5: Bi3+ for NUV-pumped WLED application

Developing high-brightness luminescence materials is always a significant work, especially for bismuth-based phosphors, due to their incompletely spin-allowed intra-ionic electron transition of 1S0–3P1. In this work, high-brightness light emitting of Bi3+ ion was achieved in Ba2GdGaO5, and its internal quantum efficiency (IQE) reaches up to 96%. Under the near-ultraviolet (NUV) light excitation, Bi3+-activated Ba2GdGaO5 emits a broad-band yellow light peaked at 566 nm with full width at half maximum (FWHM) of 126 nm. Compared with commercial yellow phosphor Y3Al5O12: Ce3+, the as-prepared sample shows a similar emission band, but preforms a better thermal stability. The significant photoluminescence characteristic of Bi3+ ion in Ba2GdGaO5 is demonstrated to be derived from the extra-ionic electronic transition of Bi3+ ion, which is identified as metal to metal charge transfer band (MMCT). This mechanism was discussed in detailed combined with the crystal and electronic structure of Ba2GdGaO5, which is expected to pave an effective way for developing high-brightness bismuth-based phosphors.

Flexible Photothermal Materials with Controllable Accurate Healing and Reversible Adhesive Abilities

As one of the first studied photothermal agents, eumelanin is often used for photothermal materials based on its broadband light absorption ability, biocompatibility, and cheap availability. However, it is still a challenge to conduct in-depth development of the photothermal effect of eumelanin in flexible polymeric composites due to its dispersion problem. Here, we report light-controlled healable and reversible adhesive elastomers using polar liquid cross-linkers to assist eumelanin dispersion into carboxylic styrene butadiene rubber (XSBR). The uniformly dispersed eumelanin and the coordination between Zn2+ and eumelanin bring the elastomer a satisfactory photothermal conversion efficiency (80%), even if only 1 phr eumelanin is added. Under near-infrared (NIR) laser irradiation of 0.55 W cm–2, the temperature of the elastomer with 1 phr eumelanin sharply increases from room temperature to 120 and ∼190 °C (the maximum temperature) in 20 and 90 s, respectively. Benefiting from the efficient photothermal conversion ability and transesterification, the elastomer shows excellent NIR light-controlled healing ability, and its healing efficiency can reach 94%. Based on noncovalent interactions (mainly hydrogen bonds), the elastomer shows temperature-dependent reversible adhesive behavior. The adhesion strength of the elastomer can be adjusted by changing the temperature or irradiating light, thus facilitating the disassembly and reuse of the elastomers. Thanks to these advantages, the obtained elastomers have potential applications in harvesting sunlight for electricity generation and also provide insights into the development of sustainable reversible adhesive materials.

Nanopillar arrays of mesoporous Bi2Se3 and CuInSe2 in layer‐based perovskite solar cells

AbstractThe improvement of light trapping inside the active layer of perovskite solar cells (PSCs) was numerically investigated. The light absorption probability was improved by incorporating periodic arrays of mesoscopic electron‐transporting layer into the absorber layer (CH3NH3PbI3) of the PSCs. Accordingly, chalcopyrite (CuInSe2) and bismuth selenide (Bi2Se3) were introduced in the form of hexagonal pillars with an optimum radius of 35 nm and a height of 292 nm. It was found that the proposed PSCs can significantly extend the broadband light absorption from the visible spectrum to the near‐infrared (NIR) region compared to planar PSCs that have an identical active layer thickness. After optimization, the PSCs based on MP‐CuInSe2 and MP‐Bi2Se3 showed the best performance with an enhancement of respectively 32% and 54 % in the photocurrent density, (with values of 29.94 and 34.93 mA.cm−2), as compared to the planar PSC (with a photocurrent density of 22.69 mA.cm−2). This enhancement resulted from a more effective carrier transport due to the mesoscopic structures.

Photothermal-Photocatalytic CSG@ZFG Evaporator for Synergistic Salt Rejection and VOC Removal during Solar-Driven Water Distillation.

Sunlight-driven interfacial photothermal evaporation has been considered as a promising strategy for addressing global water crisis. Herein, we fabricated a self-floating porous triple-layer (CSG@ZFG) evaporator using porous fibrous carbon derived from Saccharum spontaneum (CS) as a photothermal material. The middle layer of the evaporator is composed of hydrophilic sodium alginate crosslinked by carboxymethyl cellulose and zinc ferrite (ZFG), whereas the top hydrophobic layer consists of fibrous (CS) integrated benzaldehyde-modified chitosan gel (CSG). Water is transported to the middle layer through the bottom elastic polyethylene foam using natural jute fiber. Such a strategically designed three-layered evaporator exhibits a broad-band light absorbance (96%), excellent hydrophobicity (120.5°), a high evaporation rate of 1.56 kg m-2 h-1, an energy efficiency of 86%, and outstanding salt mitigation ability under the simulated sunlight of intensity 1 sun. Adding ZnFe2O4 nanoparticle as a photocatalyst has been proved to be capable of restricting the evaporation of volatile organic contaminants (VOCs) like phenol, 4-nitrophenol, and nitrobenzene to ensure the purity of evaporated water. Such an innovatively designed evaporator offers a promising approach for the production of drinking water from wastewater and seawater.

BC/GO-Ag composite aerogel with synergistic enhanced photothermal performance for efficient solar water evaporation

Solar water evaporation has great potential to alleviate the global freshwater crisis. Among them, photothermal materials have been widely studied as the core part of the solar water evaporation. However, its development is limited due to the complex preparation method and low evaporation efficiency. Herein, the composite aerogels based on graphene oxide (GO) and silver nanoparticles (Ag NPs) were developed by low-temperature-crosslinking method and in-situ reduction method and applied to solar interfacial evaporation. Due to the synergistic effect between the broadband light absorption of GO and the localized surface plasmon resonance effect of Ag NPs, the BC/GO-Ag composite aerogel has an optical absorption rate of 84.1%, an evaporation rate of 2.36 kg m-2h−1, and an evaporation efficiency of 94.5%, which is better than most of the previously reported photothermal materials. In addition, the material could be applied to various complex water environments. The BC/GO-Ag composite aerogel also showed good reusability. BC/GO-Ag composite aerogel is a promising material due to its simple and mild preparation method, environmental protection of raw materials, and high evaporation efficiency.

Ground-based Optical Transmission Spectroscopy of the Nearby Terrestrial Exoplanet LTT 1445Ab

Nearby M-dwarf systems currently offer the most favorable opportunities for spectroscopic investigations of terrestrial exoplanet atmospheres. The LTT 1445 system is a hierarchical triple of M dwarfs with two known planets orbiting the primary star, LTT 1445A. We observe four transits of the terrestrial world LTT 1445Ab (R = 1.3 R ⊕, M = 2.9 M ⊕) at low resolution with Magellan II/LDSS3C. We use the combined flux of the LTT 1445BC pair as a comparison star, marking the first time that an M dwarf is used to remove telluric variability from time-series observations of another M dwarf. We find Hα in emission from both LTT 1445B and C, as well as a flare in one of the data sets from LTT 1445C. These contaminated data are removed from the analysis. We construct a broadband transit light curve of LTT 1445Ab from 620 to 1020 nm. Binned to 3 minute time bins, we achieve an rms of 49 ppm for the combined broadband light curve. We construct a transmission spectrum with 20 spectrophotometric bins each spanning 20 nm and compare it to models of clear, 1× solar composition atmospheres. We rule out this atmospheric case with a surface pressure of 10 bars to 3.2σ confidence, and with a surface pressure of 1 bar to 3.1σ confidence. Upcoming secondary eclipse observations of LTT 1445Ab with the James Webb Space Telescope will further probe the cases of a high-mean-molecular-weight atmosphere, a hazy or cloudy atmosphere, or no atmosphere at all on this terrestrial world.

Multilayer graphene-enabled structure based on Salisbury shielding effect for high-performance terahertz absorption.

Sandwich-type structure based on Salisbury screen effect is a simple and effective strategy to acquire high-performance terahertz (THz) absorption. The number of sandwich layer is the key factor that affects the absorption bandwidth and intensity of THz wave. Traditional metal/insulant/metal (M/I/M) absorber is difficult to construct multilayer structure because of low light transmittance of the surface metal film. Graphene exhibits huge advantages including broadband light absorption, low sheet resistance and high optical transparency, which are useful for high-quality THz absorber. In this work, we proposed a series of multilayer metal/PI/graphene (M/PI/G) absorber based on graphene Salisbury shielding. Numerical simulation and experimental demonstration were provided to explain the mechanism of graphene as resistive film for strong electric field. And it is important to improve the overall absorption performance of the absorber. In addition, the number of resonance peaks is found to increase by increasing the thickness of the dielectric layer in this experiment. The absorption broadband of our device is around 160%, greater than those previously reported THz absorber. Finally, this experiment successfully prepared the absorber on a polyethylene terephthalate (PET) substrate. The absorber has high practical feasibility and can be easily integrated with the semiconductor technology to make high efficient THz-oriented devices.

Significantly boosted photoelectrochemical water splitting performance by plasmonic enhanced Hematite@MOF composite photoelectrodes

Hematite as a catalyst for photoelectrochemical water splitting offers huge potential, due to its high chemical stability, great abundance, and low cost. However, the low water oxidation kinetics and poor charge transportation have hindered progress towards the manufacture of practical water splitting devices. To tackle these problems, a visible light responsive metal-organic framework (MOF) polyhedral zeolitic imidazolate (ZIF-67), and optimised plasmonic Ag nanorods were incorporated into hematite nanostructures to form a three-component heterojunction photoelectrode. The designed photoanode showed dramatically improved light harvesting in the visible range and enhanced charge transport. A mechanistic investigation allowed the deconvolution of the enhanced performance pathways. First, the Hematite@ZIF-67 core-shell p-n junction enables facile charge carrier transfer between ZIF-67 and hematite. In addition, ZIF-67 also provides active sites for water oxidation and boosts surface oxygen evolution reaction (OER) kinetics. Guided by finite-difference time-domain (FDTD) modelling, Ag nanorods with optimised aspect ratio were incorporated between ZIF-67 and hematite. The Ag nanorods facilitate broadband light absorption and surface charge injection, induced by near-field excitation enhancement and plasmonic resonance energy transfer (PRET) pathways. The design and addition of ZIF-67 and Ag nanorods result in superior performance for a hematite-based photoanode for photoelectrochemical (PEC) water oxidation.

Schottky Junction Made from a Nanoporous Au and TiO2 Film for Plasmonic Photodetectors

The generation, transfer, and collection of plasmon-derived hot electrons represent a distinctive pathway for the utilization of solar energy. Herein, we report the construction of a plasmonic photodetector from a metal/TiO2 Schottky junction, featuring using a solution-processed continuous nanoporous Au film (CNAuF) as the light absorption layer and an n-type TiO2 film as the electron acceptation layer. Because of its plasmon properties, the CNAuF can absorb broadband light in the visible region and in turn creates abundant hot electrons (via the decay of its plasmon). Using a CNAuF/TiO2 Schottky junction, these hot electrons can be collected into the TiO2 film to generate a steady-state photocurrent. As a result, the plasmonic photodetector constructed from the CNAuF/TiO2 Schottky junction delivers an appreciable photoresponse to visible light. Taking the photoresponse to 532 nm light as an example, this plasmonic photodetector delivers a high responsivity of 0.06 A/W, a specific detectivity of 3.9 × 109 Jones, and an external quantum efficiency of 13.8%, with a rise time and a decay time being 110 and 120 ms, respectively. The bottom-up nature of the solution-processed strategy offers great flexibility to tune the plasmonic nanostructure and in turn its optical properties, thereby creating many possibilities for the creation of next-generation plasmonic photoelectric devices and beyond.

Design of hierarchical nanohole for efficient broadband light absorption of MAPbI3 perovskite material based solar cell with thin absorber layer

Perovskite solar cell has the advantages of low dosage of toxic materials, flexibility and consumer-oriented, which has significant development potential. Nanophotonic structure shows superior performance to common geometrical structure in controlling light transfer, which can capture and trap light, and thus improving the light absorption while reducing its thickness. In this work, novel wavelength-sized hierarchical nanohole is devised toward maximum broadband light absorption. The hierarchical nanohole can excite quasi-guided modes to improve light absorption, the electric field distribution is shown and demonstrated. The proposed perovskite solar cells with hierarchical nanohole outperform a planar solar cell by 17% in JSC and outperform a perovskite solar cell with one nanohole reference by 6.7% in JSC. In addition, the effects of hierarchical nanohole geometry on performance of perovskite solar cell are investigated. Overall, this work provides a new design for the ultra-thin perovskite solar cell with high energy conversion efficiency.

Tailored chromatic diffraction efficiency of double-layer single-relief blazed gratings.

"Wavelength selectivity" is an important intrinsic property of diffractive optical elements that offers significant application potential. Here, we focus on tailored wavelength selectivity, the controlled efficiency distribution into different specific diffraction orders for selected wavelengths or wavelength ranges from UV to IR using interlaced double-layer single-relief blazed gratings composed of two materials. Dispersion characteristics of inorganic glasses, layer materials, polymers, nanocomposites, and high-index liquids are taken into account to investigate the impact of especially intersecting or partially overlapping dispersion curves on diffraction efficiency in different orders, providing a guideline for material choice depending on the required optical performance. By selecting appropriate combinations of materials and adjusting the grating depth, a wide variety of small or large wavelength ranges can be assigned to different diffraction orders with high efficiency that can be beneficially applied to wavelength selective functions in optical systems also including imaging or broadband lighting applications.

Highly sensitive salinity and temperature measurement based on tapered-SHF MZI fiber laser structure

A novel Mach–Zehnder interferometer (MZI) based on tapered-side-hole-fiber (SHF) fiber laser structure, which has obvious advantages in salinity and temperature measurement sensitivity, is analyzed both theoretically and experimentally. The tapered-SHF structure is used as a sensing element and an optical filter, which is connected to the erbium-doped fiber to form a fiber ring laser (FRL). Based on the linear correspondence between the output spectra of the FRL and the salinities, the salinity around the sensing area is measured by the MZI. The FRL has the unique advantages of high resolution, narrow line width, high optical signal-to-noise ratio, and high stability. Compared with the transmission spectrum of conical SHF in the broadband light source, these advantages are more suitable for sensing applications. The experimental results show that the finer the taper diameter, the higher the sensitivity of salinity and temperature. When the taper diameter is 8.23 μm, the salinity sensitivity of the sensor can reach 0.3347 nm/‰, and the temperature sensitivity can reach −0.4270 nm °C−1.

All-optical image classification through unknown random diffusers using a single-pixel diffractive network

Classification of an object behind a random and unknown scattering medium sets a challenging task for computational imaging and machine vision fields. Recent deep learning-based approaches demonstrated the classification of objects using diffuser-distorted patterns collected by an image sensor. These methods demand relatively large-scale computing using deep neural networks running on digital computers. Here, we present an all-optical processor to directly classify unknown objects through unknown, random phase diffusers using broadband illumination detected with a single pixel. A set of transmissive diffractive layers, optimized using deep learning, forms a physical network that all-optically maps the spatial information of an input object behind a random diffuser into the power spectrum of the output light detected through a single pixel at the output plane of the diffractive network. We numerically demonstrated the accuracy of this framework using broadband radiation to classify unknown handwritten digits through random new diffusers, never used during the training phase, and achieved a blind testing accuracy of 87.74 ± 1.12%. We also experimentally validated our single-pixel broadband diffractive network by classifying handwritten digits “0” and “1” through a random diffuser using terahertz waves and a 3D-printed diffractive network. This single-pixel all-optical object classification system through random diffusers is based on passive diffractive layers that process broadband input light and can operate at any part of the electromagnetic spectrum by simply scaling the diffractive features proportional to the wavelength range of interest. These results have various potential applications in, e.g., biomedical imaging, security, robotics, and autonomous driving.

Reactor design for solar-driven photothermal catalytic CO2 reduction into fuels

Proper reactor design is of vital importance for highly-efficient photothermal catalytic CO2 reduction into solar fuels. With respect to the whole chain from capturing solar energy through energy and mass transfer to the final reduction of CO2 into fuels, this work proposes a systematic model of elaborately designing solar-driven photothermal catalytic reactors based on comprehensively studying the broadband light harvesting, energy and mass transport, and chemical conversion processes in the entire reduction of CO2 by harnessing solar energy. The established numerical model including the optical and electrical simulations of the catalyst nanoparticle as well as the flow, thermal, and chemical reaction field in the reactor is developed to integrate the overall process of the photocatalytic CO2 reduction, so as to analyze the effectiveness of the reactor and further obtain the optimal design strategy. Based on the analysis of the calculated results, methods are put forward to promote the chemical reaction by changing various parameters to regulate the light capture, radiation transfer, temperature field, and reactant supply in the catalyst layer. Most importantly, the designed reactor shows solar energy harnessing-conversion capability with remarkable light absorptivity, energy confinement, and reactants enrichment characteristics, thus realizing outstanding catalytic performance. This innovative reactor design strategy could greatly facilitate the solar-driven photothermal catalytic CO2 reduction process.

Enhancing solar energy harvest by Cu2S/CuCl heteroarrays with enriched sulfur vacancies for photo-rechargeable pseudocapacitors

The use of broadband light energy is important for enhancing photoenergy conversion. However, bifunctional materials that can efficiently harvest solar energy to assist electrochemical energy storage are difficult to prepare. Herein, copper foam (CF)-supported cuprous sulfide (Cu2S) heteroarrays (CS HAs) with enriched sulfur (S) vacancies (VS) were designed for photo-rechargeable pseudocapacitors. A direct hydrothermal sulfurization of CF first resulted in the growth of CS HAs with leaf-like heterostructures. Subsequent partial oxidation of S2− in FeCl3 aqueous solution enriched the VS within the leaf-like structures. In addition, CuCl and Cu2O were formed from Cu2S during FeCl3 treatment to derive a bifunctional photoelectrode with ternary components, thus further broadening the light absorption region. Thus, the optimal CS HAs have demonstrated areal capacitances of 1440−1048 mF cm−2 with the assistance of the photoelectric/thermal effects of light energy, delivering remarkable photoenhancements of 25%–35%. The asymmetrical pseudocapacitors fabricated from the optimal photoelectrodes delivered a high areal capacitance of 670 mF cm−2 after 6000 cycles, suggesting a promising application in transparent light-driven energy devices.

Porous polymeric microparticles foamed with supercritical CO2 as scattering white pigments

Nowadays, titanium dioxide (TiO2) is the most commercially relevant white pigment. Nonetheless, it is widely criticized due to its energy-intensive extraction and costly disposal of harmful by-products. Furthermore, recent studies discuss its potential harm for the environment and the human health. Environment-friendly strategies for the replacement of TiO2 as a white pigment can be inspired from nature. Here whiteness often originates from broadband light scattering air cavities embedded in materials with refractive indices much lower than that of TiO2. Such natural prototypes can be mimicked by introducing air-filled nano-scale cavities into commonly used polymers. Here, we demonstrate the foaming of initially transparent poly(methyl methacrylate) (PMMA) microspheres with non-toxic, inert, supercritical CO2. The properties of the foamed, white polymeric pigments with light scattering nano-pores are evaluated as possible replacement for TiO2 pigments. For that, the inner foam structure of the particles was imaged by phase-contrast x-ray nano-computed tomography (nano-CT), the optical properties were evaluated via spectroscopic measurements, and the mechanical stability was examined by micro compression experiments. Adding a diffusion barrier surrounding the PMMA particles during foaming allows to extend the foaming process towards smaller particles. Finally, we present a basic white paint prototype as exemplary application.

Method of high-precision free-space distance measurement for noncooperative targets.

Aimed at high-precision distance measurement for noncooperative targets in free space, a spatial distance measurement method is proposed. Based on the concept of optical carrier-based microwave interferometry, this method extracts distance information from the radiofrequency domain. The interference model of broadband light beams is established, and the optical interference can be eliminated by using a broadband light source. A spatial optical system with a Cassegrain telescope as the main body is designed to effectively receive the backscattered signal without cooperative targets. A free-space distance measurement system is built to verify the feasibility of the proposed method, and the results agree well with the set distances. Long-distance measurements with a resolution of 0.033µm can be achieved, and the errors of the ranging experiments are within 0.1µm. The proposed method has the advantages of fast processing speed, high measurement accuracy, and high immunity to disturbances as well as the potential for measurement of other physical quantities.

Enhancement of Charge Separation and NIR Light Harvesting through Construction of 2D-2D Bi4 O5 I2 /BiOBr:Yb3+ , Er3+ Z-Scheme Heterojunctions for Improved Full-Spectrum Photocatalytic Performance.

Developing full-spectrum photocatalysts with simultaneous broadband light absorption, excellent charge separation, and high redox capabilities is becoming increasingly significant. Herein, inspired by the similarities in crystalline structures and compositions, a unique 2D-2D Bi4 O5 I2 /BiOBr:Yb3+ ,Er3+ (BI-BYE) Z-scheme heterojunction with upconversion (UC) functionality is successfully designed and fabricated. The co-doped Yb3+ and Er3+ harvest near-infrared (NIR) light and then convert it into visible light via the UC function, expanding the optical response range of the photocatalytic system. The intimate 2D-2D interface contact provides more charge migration channels and enhances the Förster resonant energy transfer of BI-BYE, leading to significantly improved NIR light utilization efficiency. Density functional theory (DFT) calculations and experimental results confirm that the Z-scheme heterojunction is formed and that this heterojunction endows the BI-BYE heterostructure with high charge separation and strong redox capability. Benefit from these synergies, the optimized 75BI-25BYE heterostructure exhibits the highest photocatalytic performance for Bisphenol A (BPA) degradation under full-spectrum and NIR light irradiation, outperforming BYE by 6.0 and 5.3 times, respectively. This work paves an effective approach for designing highly efficient full-spectrum responsive Z-scheme heterojunction photocatalysts with UC function.

Magnetically Assembled Ultrablack Surface with Omnidirectional and Broadband Light Absorption

Ultrablack surfaces with stable and omnidirectional light absorption over a wide spectral range are fundamentally crucial for applications concerning strict optical requirements from high-end optical to solar-heat conversion devices. Inspired by nature, we report a needle-like array structure (NAS) prepared by spraying and self-assembling the magnetic composite ink under an external magnetic field. With high structure regularity and small feature size, the NAS presents extremely low hemispherical reflectance (≤0.3%) over a wide spectral range of 300-2000 nm and stable omnidirectional absorption (incident angle insensitivity up to 70°), which could be one of the darkest surfaces ever reported. The exciting light absorption performance can be attributed to the synergistic effects of (1) structural absorption caused by multiple scattering between array units and (2) strong forward scattering and high light absorptivity of magnetic particles. The NAS exhibits outstanding photothermal conversion for solar harvesting, self-cleaning performance, good flexibility, and thermal-aging resistance, offering an appealing alternative to construct ultrablack surfaces for practical applications.