Direct Light Research Articles

Dication Disulfuranes as Photoactivatable Sources of Radical Organocatalysts.

The recent development of photoredox and energy transfer catalysis has led to a significant expansion of visible light-driven chemical transformations. These methods have demonstrated exceptional efficiency in converting a wide range of substrates into radical intermediates and generating open-shell catalytic species. However, the simplification of catalytic systems and the direct generation of highly reactive radical organocatalysts through direct visible light irradiation from stable precatalysts remains largely an unrealized goal. This challenge is mainly due to the limited availability of precatalysts that are responsive to visible light. Herein, we introduce a new class of bench-stable dicationic disulfuranes, which release highly reactive thiyl radicals upon blue light excitation. Spectroscopic and computational studies reveal that this reactivity arises from a combination of structural features and intermolecular interactions. This family of molecules has been employed to catalyze radical cascades previously incompatible with photoredox conditions, enabling the efficient formation of 1,2-dioxolanes and 1,3-hydroxyketones in excellent yields and short reaction times.

Dication Disulfuranes as Photoactivatable Sources of Radical Organocatalysts

The recent development of photoredox and energy transfer catalysis has led to a significant expansion of visible light‐driven chemical transformations. These methods have demonstrated exceptional efficiency in converting a wide range of substrates into radical intermediates and generating open‐shell catalytic species. However, the simplification of catalytic systems and the direct generation of highly reactive radical organocatalysts through direct visible light irradiation from stable precatalysts remains largely an unrealized goal. This challenge is mainly due to the limited availability of precatalysts that are responsive to visible light. Herein, we introduce a new class of bench‐stable dicationic disulfuranes, which release highly reactive thiyl radicals upon blue light excitation. Spectroscopic and computational studies reveal that this reactivity arises from a combination of structural features and intermolecular interactions. This family of molecules has been employed to catalyze radical cascades previously incompatible with photoredox conditions, enabling the efficient formation of 1,2‐dioxolanes and 1,3‐hydroxyketones in excellent yields and short reaction times.

Metasurface folded lens system for ultrathin cameras.

Slim cameras are essential in state-of-the-art consumer electronics such as smartphones or augmented/virtual reality devices. However, reducing the camera thickness faces challenges primarily due to the thick lens systems. Current lens systems, composed of stacked refractive lenses, are fundamentally constrained from becoming thinner due to the presence of empty spaces between lenses and the excessive volume of each lens. Here, we present a lens system using metasurface folded optics to overcome these pervasive issues. In our design, metasurfaces are arranged horizontally on a glass wafer and direct light along multifolded paths inside the substrate. This approach achieves an ultra-slim lens system with a thickness of 0.7 millimeters and 2× thinner relative to the EFL, thereby overcoming the inherent limitations of conventional optical platforms. It delivers quasi-diffraction-limited imaging quality with a 10° field of view and an f number of 4 at an operational wavelength of 852 nanometers. Our findings provide a compelling platform for compact cameras using folded nano-optics.

Numerical Verification of a Polarization-Insensitive Electrically Tunable Far Infrared Band-Stop Meta-Surface Filter

Tunable filters have many potential applications in diverse fields, including high-capacity communications, dynamic beam shaping and spectral imaging. Although providing a high-performance solution for actively tunable devices, metasurface combined with tunable materials faces the great challenges of limited tuning range and modulation depth. Here, we propose a far-infrared tunable band-stop filter based on Fabry-Perot (FP) resonators and graphene surface plasmons. By switching the wavelength of the critical coupling condition of the filter via the gate voltage applied on graphene, achieving the dynamically tunable band-stop filtering at the central wavelengths from 12.4 μm to 14.1 μm with a modulation depth of more than 99%. Due to the symmetry of the proposed meta-atoms, the filter is insensitive to the polarization direction of the incident light. And it can realize more than 85% filtering efficiency within 60° angle of incidence around the vertical direction. By adjusting the geometry of the meta-atoms structure, it is feasible to move the operational range from the near-infrared to terahertz bands.

Micropyramidal Si Photonics–A Versatile Platform for Detector and Emitter Applications

AbstractAnisotropic wet etching of silicon (Si) has been known for decades, but the extraordinary capabilities of this technology to provide highly ordered microphotonic arrays have received limited attention in previous studies. It is shown that these capabilities include: i) a unique self‐terminating etching nature − crucial for achieving uniform arrays over centimeter‐scale wafers, ii) an extraordinary smoothness of the slow‐etching (111) planes, and iii) a precise geometry control including the extent micropyramids or micropyramidal voids are truncated. The combination of these properties transforms this technology into a versatile platform for novel detector and emitter applications in Si photonics. The optical properties of such arrays are studied by finite‐difference time‐domain modeling in two realms represented by different boundary conditions (BCs). Periodic BCs result in Talbot self‐images experimentally observed in this work. Perfectly matched layer BCs describe mesoscale interference effects resulting in the subwavelength focusing properties of micropyramids. Enhancements in the performance of mid‐wave infrared (MWIR) focal plane arrays are demonstrated by monolithic integration of Si micropyramids with silicon‐platinum silicide Schottky barrier photodetectors. Finally, it is shown that Si micropyramidal arrays can be used to enhance light extraction and directionality of quantum sources and infrared scene projectors.

Tunable and polarization-dependent electromagnetically induced transparency analogy in graphene-based terahertz metasurfaces

In this paper, we theoretically and numerically demonstrate a tunable and polarization-dependent electromagnetically induced transparency analogy based on graphene terahertz metasurfaces. The unit cell of the metasurface consists of three-layer graphene strips embedded in a silicon grating. The dynamic adjustment of the transparent window can be achieved by changing the coupling distance between the graphene layers and the polarization direction of the incident lights. The operation mechanism behind the phenomenon can be attributed to the near-field interaction and electromagnetic coupling of modes in graphene strips. Furthermore, the full wave electromagnetic simulations obtained by the finite-difference time-domain method agree well with the theoretical fitting results based on the three-harmonic oscillator model. In addition, by changing the Fermi levels, it can not only realize the outstanding slow-light effects with a maximum group index of 3750 but also obtain the four-frequency asynchronous optical switch function in terahertz regions. Therefore, our proposed metamaterial device may have potential applications in image switching, optical switches, slow-light device, optical communication, and optical storage.

Robust edge photogalvanic effect in thin-film WTe2

The photogalvanic effect (PGE) in Weyl semimetals, such as WTe2 and MoTe2, has been widely observed and is considered a promising phenomenon for advancing Weyl semimetal-based optoelectronic devices. However, as device dimensions continue to shrink, edge effects on photocurrent generation and modulation become increasingly significant and cannot be overlooked. Herein, we have discovered a locally enhanced edge linear photogalvanic effect at the edge of WTe2 thin-film devices using a home-built polarization-modulated scanning photocurrent system, which arises from symmetry breaking. Furthermore, the magnitude and direction of this edge photocurrent are modulated by the polarization direction of the incident light. This research provides valuable insights for the development of polarization-sensitive photodetectors based on layered type-II Weyl semimetals.

Comparison of elastic properties of periodic and aperiodic porous structures produced by additive methods

The aim of this work was to design, print, and examine elastic properties of porous structures with various parameters and beam distributions. These structures, generated using an algorithm, featured three types of lattices: octet, cubic, and Voronoi diagram-based, with different porosities (40 %, 60 %, 80 %) and beam sizes (0.45 mm, 0.66 mm). They were printed using Direct Light Processing and examined using X-ray computed microtomography. Compression tests were conducted to determine the Young’s modulus. The results were compared with computer simulations performed using Abaqus. The tools used, such as 3D printing and X-ray computed microtomography, were described, and the results and conclusions of the tests were presented. As a result, comparable outcomes were obtained for structures with higher beam diameter, while significantly different results were observed for structures with lower beam diameter.

Polarization‐Controlled Diffractions of Submicron Pillar Arrays of Azo Molecular Glass for Image Recording and Reconstruction

Recording and manipulating optical waves with functional structures are crucially important for many applications. Herein, the submicron pillar arrays of an azo molecular glass (IA‐Chol) are explored to show functional synergy of a recording medium and a diffractive optical element. The image recording is achieved through the pillar deformation along the electric‐field oscillation direction of incident light. When illuminated with a polarized beam, the reconstructed images appear in the first‐order diffraction spots of the pillar array with the tailored intensity distributions depending on the states of polarization of the recording beam and the image reconstruction beam. This approach enables several images to be recorded in the adjacent zones of the same pillar array using lights with different polarization directions, and then the images are reconstructed separately or simultaneously upon the polarization directions of the illumination light. Furthermore, the topographic features of the pillar array after the recording are replicated by replica‐molding to the surfaces of polydimethylsiloxane (PDMS) slices as negative replicas and transformed to surfaces of poly(methylmethacrylate) (PMMA) films through hot‐embossing. The PDMS and PMMA replicas are highly transparent in the visible light range and able to produce the reconstructed images with light in a wide‐wavelength extent.

Optimizing Indoor Comfort and Energy Efficiency using Right-Angled Triangular Responsive Facades in Cairo, Egypt

Building energy consumption has been rapidly increasing in recent years due to several factors such as climate change and global population growth. Besides, the majority of buildings are not designed with the consideration of the alteration of the severe conditions of the external surrounding environment, which affects the indoor environment negatively. As a result, excessive HVAC systems are utilized in order to maintain the indoor environment and achieve the indoor human comfort. Thus, large amounts of energy are being consumed and the rates of the energy consumption are increasing rapidly. Responsive architecture is considered as one of the solutions that architects, and façade designers use in order to block the excessive solar radiation and direct natural light and thus enhance the indoor comfort zone. However, the majority of the façade’s pattern designs are not following specific guidelines. This study contributes to the field by identifying an optimal right-angled triangular façade design that effectively enhances indoor thermal comfort, reduces solar radiation, and minimizes energy consumption, thereby providing a practical solution for improving building performance in response to climate change and urban growth challenges. This article will study four different façade pattern cases, which are common in the rotational movement, façade orientation and pattern dimensions; however, they differ in the orientation of the axes of movement. The four-façade pattern proposals will be investigated through simulating the solar radiation, consumed cooling energy and the indoor operative temperature during the maximum solar exposure day. A comparative analysis will be conducted between the results in order to highlight the most efficient right-angled triangular pattern that can be used on the south façade in Cairo, Egypt in order enhance the indoor thermal comfort, enhance the energy consumption rates, reduce the solar radiation and improve the building performance.

Enhancing light trapping for improved efficiency of perovskite solar cells: design and analysis

Abstract A hybrid organic-inorganic perovskite solar cells (PSCs) is explored for its potential to achieve high-efficiency and cost-effective solar energy conversion, especially through improved light management. In this paper, COMSOL Multiphysics is utilized to explore PSCs with light trapping nanostructures which will enhance the optical and electrical properties. A nanometer-scale concave structure is proposed in this paper to direct and capture light. This configuration is compared to a planar structure to see how it affect PSCs optoelectronic performance. Electrical simulations showed that concave nanostructured PSCs increase carrier generation and collect more carriers. The short-circuit current rose from 3.91 mA for the planar structure to 4.95 mA for the concave structure. Power conversion efficiency for concave PSCs increased 11% compared to the planar construction after choosing optimal height of concave structure. Thus, the performance investigation by combined optical and electrical analysis of nanostructures shows that it has the ability to build high-efficiency PSCs with light-trapping approach.

Diurnal changes in cloud cover in eastern Gabon and their impacts on energy balance, light availability and water demand: a case study of the 2022 dry season

Abstract Western Central Africa is atypical of the equatorial domain as the main dry season is cloudier than the rainy seasons. To understand this cloud cover's diurnal evolution, we set-up an infrared camera and acquired measurements of the total cloud cover fraction (TCF) and cloud optical depth at Bambidie, Gabon (0°44’30.5” S,12°58’12.4” O) from May to October 2022. Diurnal variations in TCF can be summarized into four types, mostly discretized through the timing and duration of clouds clearing in the afternoon (Early afternoon Clearing: EaC, Late afternoon Clearing: LaC and Clear Night: CNi) while one type (No Clearing: NoC) shows overcast conditions all day long. Meteorological measurements show that NoC days record 50W/m2 less shortwave incoming surface radiation resulting in daytime temperatures 1°C lower than the seasonal norm, but 20% more diffuse light and 0.5mm/day less ETo. Conversely, EaC days record 50W/m2 more shortwave incoming surface radiation leading to temperatures 1.5°C higher than the seasonal norm, but 40% more direct light. The larger water demand (0.5mm/day more ETo) is partly compensated by more frequent rainfall at night-time. The SAFNWC satellite estimates well capture the TCF variations for most of the 4 types. They confirm that TCF is dominated by very low and low clouds whose dissipation in the afternoon and evolution into fractional and cumuliform convective clouds explains the clearings on EaC and LaC days. Satellite estimates also show that the 4 types of days extracted at Bambidie are representative of a larger-scale cloud cover evolution in Western Central Africa, with a W-E gradient in the timing of afternoon cloud dissipation.

Three-dimensional polarimetric ptychography

A three-dimensional polarimetric ptychography (3D-PP) technique is proposed to reconstruct the residual stress distribution in optical elements in three dimensions by combining polarization imaging and a 3D ptychographic iterative engine (3D-PIE). 3D-PP mainly uses two orthogonal linear polarizers placed along the light direction to capture the photoelastic information within an optical element. Based on phase shift theory, a three-step phase shift method is proposed to record three dark field images, allowing for calculating the isometric line of residual stress within the optical element. This method enables the layer-by-layer measurement of residual stress within thick samples and assesses the distribution of residual stress along the detection axis, enhancing the precision of stress measurements.

Measuring linear birefringence via rotating-sample transmission Stokes spectropolarimetry

Linear birefringence is a fundamental property of optically anisotropic media, defined by the difference in refractive index experienced by light polarized along orthogonal directions. It is usually manifested in microscopically aligned molecular systems, where a preferential direction of light–matter interaction is created. For instance, the anisotropic structure of calcite crystal causes the famous double-refraction phenomenon. Another common example is commercial adhesive tapes, which are polymeric materials possessing birefringent properties due to their manufacturing processes. The intrinsic relation between birefringence and molecular alignment forges a new analytical route to study materials such as polymeric thin films. Therefore, the capacity of measuring linear birefringence and its fast axis is of paramount importance for the science of anisotropic molecular systems. In this contribution, a comprehensive approach to acquire linear birefringence using rotating-sample transmission Stokes spectropolarimetry is presented and applied to transparent adhesive tapes as a case study. The experimental setup comprises a thermal light source and a spectropolarimeter capable of determining wavelength distributions of Stokes parameters. The samples are carefully aligned in a rotating mount and subjected to a fixed broadband vertically polarized light beam. Then, the transmitted light is analyzed using a rotating retarder type of spectropolarimeter. Through systematic variation of the sample’s angular position, the Stokes parameters of transmitted light are measured for each transmitted wavelength as a function of the sample’s angular position. The linear retardance and fast axis direction relative to the tape’s long axis are then determined from the modulation of Stokes parameters over sample rotation. The model derivation, experimental procedure, and signal processing protocol are described in detail, and the approach is verified with a simple correlation between linear retardance and the number of stacked layers of tape.

Polarized and Directional Electroluminescence from Organic Light‐Emitting Devices by Using a Bifunctional Meta‐Electrode

AbstractOrganic light‐emitting devices (OLEDs) with polarized and directional emission are highly desired for their promising applications in optical imaging, optical communication, and immersive visual technologies. Existing implementation methods based on free space bulk optical elements suffer from significant power loss, limiting their practical applications. Here, a bifunctional meta‐electrode is proposed by integrating a metasurface consisting of Ag periodic corrugations onto the indium tin oxide (ITO) anode to simultaneously realize the functions of charge carrier injection and photon management in the OLEDs. Highly polarized and directional light emission is demonstrated while maintaining a high device efficiency using the meta‐electrode. The electroluminescence with a polarization extinction ratio of 5 is achieved by judiciously engineering the polarization‐dependent Purcell factor of the resonant cavity constructed by the meta‐electrode. Meanwhile, the directional light emission with a divergence angle of 10° can be obtained by the efficient out‐coupling of surface plasmon‐polariton mode. The light beam can be collimated in one direction by using 1D nanogratings and in two directions by using 2D nanopillars, respectively.

3D Light-Direction Sensor Based on Segmented Concentric Nanorings Combined with Deep Learning.

High-precision, ultra-thin angular detectable imaging upon a single pixel holds significant promise for light-field detection and reconstruction, thereby catalyzing advancements in machine vision and interaction technology. Traditional light-direction angle sensors relying on optical components like gratings and lenses face inherent constraints from diffraction limits in achieving device miniaturization. Recently, angle sensors via coupled double nanowires have demonstrated prowess in attaining high-precision angle perception of incident light at sub-wavelength device scales, which may herald a novel design paradigm for ultra-compact angle sensors. However, the current approach to measuring the three-dimensional (3D) incident light direction is unstable. In this paper, we propose a sensor concept capable of discerning the 3D light-direction based on a segmented concentric nanoring structure that is sensitive to both elevation angle (θ) and azimuth angle (ϕ) at a micrometer device scale and is validated through simulations. Through deep learning (DL) analysis and prediction, our simulations reveal that for angle scanning with a step size of 1°, the device can still achieve a detection range of 0∼360° for ϕ and 45°∼90° for θ, with an average accuracy of 0.19°, and DL can further solve some data aliasing problems to expand the sensing range. Our design broadens the angle sensing dimension based on mutual resonance coupling among nanoring segments, and through waveguide implementation or sensor array arrangements, the detection range can be flexibly adjusted to accommodate diverse application scenarios.

The significant role of Z-scheme band alignment at the heterojunction for the enhanced photocatalytic H2 production in TiO2/BiNbO4/rGO nanocomposite

An efficient ternary TiO2/BiNbO4/rGO nanocomposite was prepared by a simple wet impregnation process. PXRD confirmed the tetragonal and orthorhombic crystalline natures of titanium dioxide (TiO2) and bismuth niobate (BiNbO4) respectively. The loading of BiNbO4 and reduced graphene oxide (rGO) shifted the light absorption of TiO2 to a longer wavelength from 400 to 430 nm. The suitable conduction band position of BiNbO4 facilitated a favourable band alignment to suppress the electron/hole (e−/h+) recombination in TiO2 via the Z scheme mechanism at the heterojunction. The excited electrons at the CB of TiO2 were easily transported via the fast electron accepting and conducting rGO matrix for the surface redox reactions with the improved charge transfer efficiency, which was confirmed by EIS and PL studies respectively. BiNbO4 and rGO synergistically improved the specific surface area and solar light absorption. EPR analysis confirmed the presence of increased oxygen vacancies at the heterojunction. Hence, the ternary TiO2/BiNbO4/rGO nanocomposite demonstrated an enhanced hydrogen evolution (8910 μ mol h−1 gcat−1) than bare TiO2 (4820 μ mol h−1 gcat−1), bare BiNbO4 (950 μ mol h−1 gcat−1) and TiO2/BiNbO4 (6730 μ mol h−1 gcat−1) under direct solar light. The optimum of 1 wt % BiNbO4 and rGO loaded TiO2 nanocomposite produced twice the H2 production than the bare TiO2. This also further confirms that rGO played a major role during the photocatalytic process by highlighting the potential of metal oxides combined with carbon material as an efficient photocatalyst which prominently enhances the H2 evolution.

Hyperspectral imaging for chloroplast movement detection.

We employed hyperspectral imaging to detect chloroplast positioning and assess its influence on common vegetation indices. In low blue light, chloroplasts move to cell walls perpendicular to the direction of the incident light. In high blue light, chloroplasts exhibit the avoidance response, moving to cell walls parallel to the light direction. Irradiation with high light results in significant changes in leaf reflectance and the shape of the reflectance spectrum. Using mutants with disrupted chloroplast movements, we found that blue-light-induced changes in the reflectance spectrum are mostly due to chloroplast relocations. We trained machine learning methods in the classification of leaves according to the chloroplast positioning, based on the reflectance spectra. The convolutional network showed low levels of misclassification of leaves irradiated with high light even when different species were used for training and testing, suggesting that reflectance spectra may be used to detect chloroplast avoidance in heterogeneous vegetation. We also examined the correlation between chloroplast positioning and values of indices of normalized-difference type for various combinations of wavelengths and identified an index sensitive to chloroplast positioning. We found that values of some of the vegetation indices, including those sensitive to the carotenoid levels, may be altered due to chloroplast rearrangements.

Directional picoantenna behavior of tunnel junctions formed by an atomic-scale surface defect.

Plasmonic nanoantennas have attracted much attention lately, among other reasons because of the directionality of light emitted by fluorophores coupled to their localized surface plasmon resonances. Plasmonic picocavities, i.e., cavities with mode volumes below 1 nm3, could act as enhanced antennas due to their extreme field confinement, but the directionality on their emission is difficult to control. In this work, we show that the plasmonic picocavity formed between the tip of a scanning tunneling microscope and a metal surface with a monoatomic step shows directional emission profiles and, thus, can be considered as a realization of a picoantenna. Electromagnetic calculations demonstrate that the observed directionality arises from the reshaping and tilting of the surface charges induced at the scanning tip due to the atomic step. Our results pave the way to exploiting picoantennas as an efficient way for the far-field probing and control of light-matter interactions below the nanoscale.

Spatial-Dependent Spectral Response of Acousto-Optic Tunable Filters with Inhomogeneous Acoustic Distribution.

The spectral response of an acousto-optic tunable filter (AOTF) is crucial for an AOTF based spectral imaging system. The acousto-optic (AO) interaction within the spatial-distributed area of the acoustic field determines the spectral response of the light incidence. Assuming an ideally uniform acoustic field distribution, phase-matching geometries can be applied to calculate the anisotropic Bragg diffraction in AO interactions, determining the wavelength and direction of the diffracted light. In this ideal scenario, the wavelength of the diffracted light depends solely on the direction of the incident light. However, due to the non-ideal nature of the acoustic field, the wavelength of the diffracted light exhibits slight variations with incident position. In this paper, an analytical model is proposed to calculate the spatial-dependent spectral response of the diffracted light under non-uniform acoustic field distribution. The study computes the variation pattern of the diffracted light amplitude caused by the inhomogeneous acoustic distribution. The theoretical considerations and computational model are confirmed by AOTF frequency scanning experiments. The study demonstrates that the distribution of the acoustic field leads to non-uniform spatial-spectral response in the AOTF, and the spatial AO interaction computational model can provide data support for calibrating AOTF systems in imaging applications.