Light Redistribution Research Articles

Discovery of Living Optical Networks in Orchid Leaves as Inspiration for Light Harvesting and Redistribution in Soft, Curvilinear Material Formats

AbstractPlants and animals produce highly evolved, hierarchical systems that combine physicochemical material parameters with functional micro‐ and nanostructures to manage energy, motion, and species survival. These systems, besides providing cues on bottom‐up manufacturing of technological structures, can give insight on strategies for efficient energy management. Here is reported the observation of an optically interconnected network formed by the cells of the leaves of the Macodes petola jewel orchid that provides light harvesting and redistribution across its surface. The planar redistribution of light incident on the leaf is enabled by the distinctively rounded shape of the outer epidermal cells and by their arrangement in a short‐range ordered hexagonal lattice. Successful replication of a live leaf in a free‐standing biopolymer film validates the observation and opens avenues for the exploration of non‐traditional energy harvesting interfaces that are soft, conformal, and curvilinear.

Optimization of light response and electron redistribution of active sites by structuring CoS2/MoS2/Ni3S2 heterojunction for highly efficient photo-assisted oxygen evolution

Highly efficient photoelectrocatalysts become one of the current research hotspots for overall water splitting, the design of heterojunctions with a wide response range to sunlight and efficient separation and migration of photo-generated carriers is challenging. Herein, a nanorod CoS2/MoS2/Ni3S2 photoelectrocatalyst with heterojunction is prepared by a one-step hydrothermal method. Under simulated solar irradiation, CoS2/MoS2/Ni3S2 exhibits excellent photoelectrocatalytic activity for OER, with overpotential of 166 mV and the cell voltage of 1.464 V at 10 mA cm−2, and the catalytic performance exhibited a slight decline after 12 hours of stable operation. In addition the STH efficiency reaches 6.6 %. Experiments and DFT calculations demonstrate that CoS2 as a photoactive layer optimizes the light absorption properties of catalyst. Additionally, MoS2 as a co-catalyst modulates the d-band centers and the electronic structure of Co active site, reducing the energy barrier of the rate-determining step. Meanwhile, the II-type heterojunction formed by CoS2 and MoS2 can effectively promote the separation and migration of photo-generated carriers, which further enhances the photoelectrocatalytic OER performance. This work provides a novel insight to design photoelectrocatalysts with transition metal sulfide heterojunction for overall water splitting.

Vacuum Thermoforming of Optically Switchable Liquid Crystalline Elastomer Spherical Actuators.

Liquid crystal elastomer (LCE) actuators are generally limited in shape, size, and quantity by the need for aligning via stretching and fixing via photopolymerizing. A thermoplastic LCE is presented that may be vacuum thermoformed into centimeter-sized hemispheres. The scalable industrial process induces LCE alignment without requiring postfixing. The hemispheres display remarkable properties, actuating with strains around 20% and transitioning from opaque and scattering to highly translucent upon heating: both the physical and optical effects are fully reversible. Simulations reveal the LCE experiences biaxial strains during processing, the magnitude varying as a function of location on the hemisphere: the resulting alignment describing the hemisphere actuation well. The thermoplastic LCE hemispheres may be combined to form complete spheres by simply heating the joint. The hemisphere can also be physically deformed into a ball which can then unfold back into the hemisphere again. By doping the hemispheres with photoswitches, fluorescent or photothermal dyes, devices are formed for light collection and redistribution, addressable water containers that may pour at will, and light-responsive surfing devices. This is the first example of an LCE amenable to high-volume industrial vacuum thermoforming which may lead to intricate 3D-shaped actuators with new functional properties.

Efficient freeform-based pattern projection system for 3D measurements.

For three-dimensional (3D) measurement of object surface and shape by pattern projection systems, we used a hybrid projection system, i.e., a combination of a projection lens and a transmissive freeform to generate an aperiodic sinusoidal fringe pattern. Such a freeform effects a light redistribution, thus leading to an effective and low-loss pattern projection, as it increases the total transmission intensity of the system and has less power dissipation than classical projection systems. In this paper, we present the conception and realization of the measurement setup of a transmissive fringe projection system. We compare the characteristics of the generated intensity distribution with the classical system based on GOBO (GOes Before Optics) projection and show measurement results of different surface shapes, recorded with the new system.

Upconverter Nanoparticles as Plasmon-induced Broadband Light Converters

We show numerically how lanthanide-doped upconverter nanocrystals inserted at optimal positions in adjustable and smooth 2D plasmonic arrays may change and potentially control the whole UV-visible-NIR reflectance spectra displayed by these structures. The analysis and interplay between a custom-built simple 5-levels 2-electrons quantum model and the bare structure have been studied in depth and revealed very fundamental and interesting physics. Essentially, the largely and selectively enhanced white-light excitation field and optimized local density of states at the location of the emitters favor some energy transfer paths and a redistribution of light energy in a broad spectral range. Interestingly, the extent to which the spectra can be modulated owing to the emission properties of such very robust multilevel emitters may find interesting applications and notably allow increased efficiency of emission in Light Emitting Devices or solar light redistribution and collection in Solar cells, where conversions from one color to any other one play a major role.

Chromogenic materials in smart windows for angular-selective filtering of solar radiation

A new approach to the use of chromogenic materials in smart windows to optimize the amount of daylight and solar energy indoors is presented. A smart window with a grating optical filter dinamically adapting to the position of the Sun due to parallel strips of chromogenic materials on two surfaces of window pane(s) provides angular filtering of the solar radiation without using the light redistribution devices. Based on methods for calculating optimum slope angle of the strips on the pane(s), their widths and relative position on two surfaces, equations are obtained for calculating the transmittance of the grating filter with chromogenic strips. Angular and temporal characteristics of the transmittance calculated using these equations demonstrate the angular selectivity of a smart window with chromogenic filter compared to a conventional smart window, fully glazed with chromogenic glass. In addition, such a window attenuates the direct solar radiation in a preset angular range and transmits the diffused radiation providing comfortable daylighting and temperature indoors, unlike conventional smart windows which in an active state attenuate not only direct but also diffused radiation.

Generation of super-high-order petal-like laser beam using Theon-sieve resonator

The Laguerre–Gaussian (LG) mode beam has very important applications in many research fields. Here, the Theon sieve is first introduced into the laser resonator to generate petal-like laser beams by coherently superimposing two high-order LG modes. The effectiveness was verified by GLAD software. The petal-like laser beam is derived from the light field redistribution and coherent superposition caused by the diffraction effect of the Theon sieve. The relationship between the order of the petal-like laser and the cavity structures has also been investigated in detail. Light field operation in the laser cavity greatly simplifies the optical structure and is more beneficial to optical diagnostics and imaging.

Grating optical filters for smart windows: Materials, calculations and prospects

Smart windows with advanced architectural glass coatings providing the comfortable daylight and thermal environment indoors are an important component to improve the energy efficiency of the buildings. Chromogenic and other materials potentially applicable for filtering the solar radiation are reviewed. They have a variety of mechanisms for changing the light transmission depending on change in the ambient conditions or under the influence of electric current. A smart window with additional function of angular filtering of solar radiation without using the blinds or other light redistribution devices is described. Such a window has an optical filter consisting of parallel non-transmissive (absorptive, reflective or scattering, including chromogenics) strips on two surfaces of the pane(s). The filter blocks the direct sunlight partially or completely in a preset angular range and transmits the diffused light providing comfortable day lighting indoors. Methods for calculating the geometrical parameters of the gratings considering the annual and daily change in the solar radiation, the location of the building and the window's azimuth are given. Calculated angular and temporal characteristics of the light transmittance demonstrate the angular selectivity of the transmission of a smart window with grating optical filter compared to a conventional smart window fully glazed with chromogenic glass. A comparative assessment of the potential of various chromogenic and other materials for the use in smart windows, as well as in grating filters for them, is carried out. The future prospects of the field are declared.

Spontaneous Emission in the Anti-Waveguiding VCSEL

Spontaneous emission in a vertical cavity surface emitting laser (VCSEL) have been studied. It was estimated that a significant part of the spontaneous emission (~1/3) in a conventional VCSEL goes into a waveguide mode that propagates in the plane of the Bragg mirrors. The theoretical study of a recently proposed anti-waveguide VCSEL design is presented and the damping of the waveguide mode emission is shown. Moreover we calculate that 3λ/2-cavity AVCSEL has almost the same gain value for vertical mode as conventional λ-VCSEL despite the higher cavity width due to the light redistribution effect while λ/2-cavity AVCSEL has 19% higher gain.

A solar fiber daylighting system without tracking component

A fixed optical fiber solar system for long distance daylighting used in buildings is proposed. Its structure and working principle have been introduced. The proposed system takes the advantages of both light pipe and traditional optical fiber system and overcomes their shortcomings. The receiving surface of the new system is like a spherical sunflower. It has a simple structure, good integrity, flexible light transmission and light redistribution characteristics. Especially, it does not require a tracking device. Both the theoretical analysis and field test have proved that the system has the same reception feature as the traditional optical fiber system with a tracking device. Its graph of the illuminance roughly follows the fluctuation of the curve of the solar irradiance. A small test system with a solar receiving area of 1.69 × 10−2 m2 and receiving angle of 32° was made to verify the feasibility of the fixed optical fiber system. Experimental result had proved that the proposed system is feasible. On average, the output illuminance inside a small test room is up to about 180 lx in a distance of 0.4 m from the bundled end of the fibers with the solar irradiance being about 600 W/m2, approximately 300 lx under approximately 800 W/m2. The simple structure, expectable low cost, almost maintenance-free and a predictable long life of the new system will facilitate the mass production and the viable commercialization of daylighting systems.

Ultrafast dynamic wavefront control for light redistribution via optical Kerr effect

An approach for ultrafast dynamic light redistribution by wavefront control via the optical Kerr effect is proposed. In this approach, an intense pump laser with a temporal profile of a Gaussian pulse train is incident in a bulk Kerr medium or a fused silicon cell filled with liquid Kerr medium to induce a spatiotemporal refractive index of the medium, and meanwhile a signal laser is propagating through the medium. Due to the spatiotemporal refractive index of the medium induced by the pump laser, the wavefront of the signal laser is modulated in both spatial and temporal domains. As a result, the Kerr medium functions as a moving microlens array for the pump laser perpendicular to the signal laser, or as a varifocal lens for the pump laser oblique to the signal laser at a small angle. Consequently, the light field in the focal region can be redistributed rapidly due to the spatiotemporal wavefront modulation to the signal laser by the Kerr medium. Moreover, the response of the optical Kerr effect is on the order of sub-picoseconds, which enables the light redistribution period of the signal laser to be below a few picoseconds. Results indicate that this approach can effectively shape the wavefront of the laser, redistribute the light field in the focal zone, and finally improve the irradiation characteristics of laser beams.

Extraordinary Focusing Effect of Surface Nanolenses in Total Internal Reflection Mode.

Microscopic lenses are paramount in solar energy harvesting, optical devices, and imaging technologies. This work reports an extraordinary focusing effect exhibited by a surface nanolens (i.e., with at least one dimension of subwavelength) that is situated in an evanescent field from the total internal reflection (TIR) of light illuminated to the supporting substrate above the critical angle. Our measurements show that the position, shape, and size of the surface area with enhanced light intensity are determined by the geometry of the nanolens and the incident angle, in good agreement with simulation results. This strong focusing effect of the surface nanolens is shown to significantly promote the plasmonic effect of deposited gold nanoparticles on the lens surface inlight conversion and to vaporize surrounding water to microbubbles by using low laser power. This work further demonstrates that the light redistribution by the surface nanolens in TIR enables a range of novel applications in selectively local visualization of specimens in fluorescence imaging, optical trapping of colloids from an external flow, and selective materials deposition from photoreactions.

Non-destructive analysis on nano-textured surface of the vertical LED for light enhancement

In this work, the nano-textured surface of a GaN-based vertical light emitting diode (VLED) is characterized using a unified framework of non-destructive techniques (NDT) incorporating scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, Photoluminescence (PL), and X-ray diffraction (XRD) to optimize the light output efficiency. The surface roughness of ∼300 nm is revealed by AFM. Compressive stress-state of 0.667 GPa in the GaN surface is indicated by the E2(high) and A1(LO) phonon peak values at 569 cm−1 and 736 cm−1, respectively, in Raman spectrum and the wavelength at 442 nm rather 450 nm in PL spectrum. Without damaging the LED, surface analysis by NDT helps to advance the understanding of the optimized angular light redistribution subject to the high-roughness surface and the negative impacts of the stress induced at the top GaN layer, which leads to the optical efficiency degradation of the VLED. Furthermore, the impact of texturing on underneath n-GaN and MQWs layers is investigated via SEM-based transmission Kikuchi diffraction (TKD) and aberration-corrected scanning transmission electron microscopy (AC-STEM) and revealed a smooth surface morphology and good crystalline quality, indicating that the etch-induced damage by texture engineering does not impair the active region of the VLED. Accordingly, prospective optimizations are suggested in the context of surface engineering for light enhancement in VLEDs.

Structure‐based optics of centric diatom frustules: modulation of the in vivo light field for efficient diatom photosynthesis

The optical properties of diatom silicate frustules inspire photonics and nanotechnology research. Whether light interaction with the nano-structure of the frustule also affects diatom photosynthesis has remained unclear due to lack of information on frustule optical properties under more natural conditions. Here we demonstrate that the optical properties of the frustule valves in water affect light harvesting and photosynthesis in live cells of centric diatoms (Coscinodiscus granii). Microscale cellular mapping of photosynthesis around localized spot illumination demonstrated optical coupling of chloroplasts to the valve wall. Photonic structures of the three-layered C.granii valve facilitated light redistribution and efficient photosynthesis in cell regions distant from the directly illuminated area. The different porous structure of the two sides of the valve exhibited photon trapping and forward scattering of blue light enhancing photosynthetic active radiation inside the cell. Photonic structures of diatom frustules thus alter the cellular light field with implications on diatom photobiology.

Mineral Deposits in Ficus Leaves: Morphologies and Locations in Relation to Function.

Ficus trees are adapted to diverse environments and have some of the highest rates of photosynthesis among trees. Ficus leaves can deposit one or more of the three major mineral types found in leaves: amorphous calcium carbonate cystoliths, calcium oxalates, and silica phytoliths. In order to better understand the functions of these minerals and the control that the leaf exerts over mineral deposition, we investigated leaves from 10 Ficus species from vastly different environments (Rehovot, Israel; Bologna, Italy; Issa Valley, Tanzania; and Ngogo, Uganda). We identified the mineral locations in the soft tissues, the relative distributions of the minerals, and mineral volume contents using microcomputed tomography. Each Ficus species is characterized by a unique 3D mineral distribution that is preserved in different environments. The mineral distribution patterns are generally different on the adaxial and abaxial sides of the leaf. All species examined have abundant calcium oxalate deposits around the veins. We used micromodulated fluorimetry to examine the effect of cystoliths on photosynthetic efficiency in two species having cystoliths abaxially and adaxially (Ficusmicrocarpa) or only abaxially (Ficuscarica). In F. microcarpa, both adaxial and abaxial cystoliths efficiently contributed to light redistribution inside the leaf and, hence, increased photosynthetic efficiency, whereas in F. carica, the abaxial cystoliths did not increase photosynthetic efficiency.

Nanoparticle-crystal towards an absorbing meta-coating

In this paper, a double layer nanoparticle-crystal has been proposed, which shown incident and polarization angle, substrate independences for spectral absorptivity. Such phenomenon originates from the near-field light redistribution and excitation of internal collective oscillating. This kind of nanoparticle-crystal can be made of various types of metal with similar optical responses. A three oscillators mode has been proposed in this paper to understand the shift between global and internal collective oscillating, and verify the physical picture demonstrated. That kind of near-field redistribution result in a prototype of novel meta-coating, and facilitates the large scale application of metamaterial.

Low-dose endothelial monocyte-activating polypeptide-II increases permeability of blood–tumor barrier via a PKC-ζ/PP2A-dependent signaling mechanism

Our previous study demonstrated that low-dose endothelial monocyte-activating polypeptide-II (EMAP-II) induces blood–tumor barrier (BTB) opening via the RhoA/Rho kinase/protein kinase C (PKC)-α/β signaling pathway and that PKC-ζ is involved in this process via other mechanisms. In the present study, using an in vitro BTB model, we detected the exact signaling mechanisms by which PKC-ζ activation affects EMAP-II-induced BTB hyperpermeability. Our results showed that three types of serine/threonine (Ser/Thr) protein phosphatases (PPs), namely PP1, PP2A, and PP2B, were expressed by rat brain microvascular endothelial cells (RBMECs). There was an interaction between PKC-ζ and PP2A in RBMECs. In addition, EMAP-II induced a significant increase in both the expression and the activity of PP2A in RBMECs. Inhibition of PKC-ζ with PKC-ζ pseudosubstrate inhibitor (PKC-ζ-PI) completely blocked EMAP-II-induced PP2A activation. Conversely, inhibition of PP2A with okadaic acid (OA) had no effect on EMAP-II-induced PKC-ζ activation. Like PKC-ζ-PI, OA partially prevented EMAP-II-induced BTB hyperpermeability and occludin redistribution in RBMECs. Neither PKC-ζ-PI nor OA affected EMAP-II-induced phosphorylation of myosin light chain and redistribution of actin cytoskeleton in RBMECs. Taken together, our present study demonstrated that low-dose EMAP-II increases BTB permeability by activating the PKC-ζ/PP2A signaling pathway, which consequently leads to the disruption of TJs and impairment of endothelial barrier function.

Enhanced third harmonic generation using the surface states of light in periodic photonic structures

Third harmonic generation enhancement in periodic photonic structures was experimentally observed and theoretically interpreted. Nonlinear optical effects in opal globular photonic crystals (PC) have been studied under the femtosecond laser pumping. Strong dependence of the third harmonic generation efficiency on the ratio between the central pumping wavelength and the spectral location of the PC band gaps was found. Numerical simulations based on the finite difference time-domain technique for the solution of the Maxwell's equations were applied for investigations of the observed phenomenon origin. The simulation results have shown that the origin of the efficient nonlinear conversion is related with the surface state of electromagnetic field in PC. Interacting with the PC surface the light wavefront distorts coherently, and the effect of structure light focusing appears. Coherent wavefront distortion leads to the strong optical field localization, hence the light intensity within the certain PC regions increases. In case of the band gap pumping dramatic light redistribution appears; very sharp peaks of light intensity emerge in the region of the quartz globules, which leads to the high-efficient PC pumping.

Systematic investigation on light intensification by typical subsurface cracks on optical glass surfaces

By using the finite-difference time-domain method, a systematic investigation has been performed to analyze the light field redistribution due to the modulation from typical subsurface cracks on optical glass surfaces. The involved cracks that can occur during surface processing are mainly of three basic types, including the cone, the radial, and the lateral. It is found out that the modulated light intensification strongly depends on the crack type and especially on if the crack is on the front or rear surface. If the cone crack is on the rear surface or lateral crack is on the front surface, a strong field enhancement as large as 2 orders of magnitude can occur. As for the other four cases, i.e., cone crack on the front surface, lateral crack on the rear surface, and radial crack on both the front and rear surfaces, they do not result in a field enhancement of 2 orders of magnitude, but only several times higher than the incident light field. The field enhancement mechanisms for various crack types are compared and discussed in detail. If only from the perspective of laser damage initiation resulting from a modulated high electric field, the most harmful subsurface cracks may be initially the lateral on the front surface and the cone on the rear surface.

Insights in the laser induced breakdown spectroscopy signal generation underwater using dual pulse excitation — Part II: Plasma emission intensity as a function of interpulse delay

Influence of time delay between two laser pulses on the LIBS (laser induced breakdown spectroscopy) signal inside liquids was investigated and the results are compared with data from literature. Plasma was produced by laser ablation (LA) of aluminum inside water and its emission after the second laser pulse was characterized by spectrally and time resolved detection. Light propagation through the vapor bubble formed by the first laser pulse was studied by measurements of beam scattering and transmission. Optical absorption by the evolving bubble is not significant, but its growth is accompanied by lowering of its refraction index nb with respect to surrounding liquid; this effect increases defocusing both of the incident beam and of the out-coming plasma radiation. Collection efficiency of the secondary plasma emission rapidly degrades with the cavity growth, but close to its full expansion the LIBS signal partially recovers through Snell's reflections at the liquid–vapor interface, which produce a bright spot close to the bubble center. Such a light redistribution allows detecting of the emission from external plasma volume, otherwise deflected out of the collection system. Except for strong line transitions from the main sample constituents, self-absorbed inside the high-pressure cavity, we observed the highest LIBS signal when sending the second pulse well before the bubble is fully expanded. Transitions of the pressure wave through the focal volume, formed by the first laser pulse and reflected from the cell's walls and sample back-plane, enhances the LIBS signal importantly. The measured lifetime of the secondary plasma rapidly decreases with the bubble expansion. Here, we also discuss the optimization of the optical collection system and some analytical aspects of double-pulse (DP) LIBS inside liquids.