Angular Selectivity Research Articles

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.

Angular selectivity of SOI photodiode with surface plasmon antenna

Angular selectivity of SOI photodiode with surface plasmon antenna

Modifying angular and polarization selection rules of high-order harmonics by controlling electron trajectories in k-space

Recent advances in generation of strong laser pulses have enabled the acceleration of electrons in solids into regions far away from the band edge. Because nonlinear currents can be generated by laser-driven carriers in the non-parabolic region, tailored laser fields may allow control of optical properties of high-order harmonics (HHs). So far, investigations on laser-induced nonlinear optical phenomena have focused on the simple electron motion induced by linearly or elliptically polarized fields. However, more complex trajectories can be important for development of novel optoelectronic devices. Here, we show that a weak laser field (optical frequency is ω2) applied in a direction orthogonal to a strong main field (ω1) enhances certain HH intensity components of bulk GaSe by a factor of 100. Good agreement between the experiments and calculations shows that manipulation of the electron trajectory allows breaking inversion symmetry of the electronic states felt by the accelerated electrons and leading to a modification of selection rules for frequency-mixing processes of HHs. Owing to our usage of non-integer multiples for ω1 and ω2, it is found that the generation of HHs constitutes a novel way of ultrafast control of light polarization and optical switching.

Dynamically tunable coherent perfect absorption based on bulk Dirac semimetal

Coherent perfect absorption (CPA) is realized in metasurfaces composed of crossed vertical bulk Dirac semimetal (BDS) stripes and S i O 2 . Under the illumination of two counter-propagating coherent beams, the coherent absorption is continuously controlled from almost 0 (approximately 6.2 × 10 − 5 ) to 99.97% by changing the beams’ relative phase, which gives a modulation depth of about 1.6 × 10 4 . Furthermore, the coherent absorption can be controlled substantially by varying the relative intensity, the Fermi energy of BDS, and the structural parameters of the metasurfaces. Further research shows that the metasurface has broadband angular selectivity: CPA frequency splits into TE and TM polarization bands under oblique incidence. Our results provide an effective way to manipulate the interaction of light and BDS, and there may be potential applications in coherent detectors and optical modulators.

Polarization-Encrypted Orbital Angular Momentum Multiplexed Metasurface Holography.

Metasurface holography has the advantage of realizing complex wavefront modulation by thin layers together with the progressive technique of computer-generated holographic imaging. Despite the well-known light parameters, such as amplitude, phase, polarization, and frequency, the orbital angular momentum (OAM) of a beam can be regarded as another degree of freedom. Here, we propose and demonstrate orbital angular momentum multiplexing at different polarization channels using a birefringent metasurface for holographic encryption. The OAM selective holographic information can only be reconstructed with the exact topological charge and a specific polarization state. By using an incident beam with different topological charges as erasers, we mimic a super-resolution case for the reconstructed image, in analogy to the well-known STED technique in microscopy. The combination of multiple polarization channels together with the orbital angular momentum selectivity provides a higher security level for holographic encryption. Such a technique can be applied for beam shaping, optical camouflage, data storage, and dynamic displays.

Study of the Effect of Methyldiethanolamine Initiator on the Recording Properties of Acrylamide Based Photopolymer.

The use of Holographic Optical Elements (HOEs) in applications, such as in light shaping and redirection, requires certain characteristics such as a high Diffraction Efficiency, low angular selectivity and stability against UV damage. In order to maximize the performance of the HOEs, photosensitive materials are needed that have been optimised for the characteristics that are of particular importance in that application. At the core of the performance of these devices is the refractive index modulation created during holographic recording. Typically, a higher refractive index modulation will enable greater light Diffraction Efficiency and also operation with thinner devices, which in turn decreases the angular selectivity and the stability of the refractive index modulation introduced during recording, which is key to the longevity of the device. Solar concentrators based on volume HOEs can particularly benefit from thinner devices, because, for a solar concentrator to have a high angular working range, thinner photopolymer layers with a smaller angular selectivity are required. This paper presents an optimisation of an acrylamide-based photopolymer formulation for an improved refractive index modulation and recording speed. This was achieved by studying the effect of the concentration of acrylamide and the influence of different initiators in the photopolymer composition on the diffraction efficiency of holographic gratings. Two initiators of different molecular weights were compared: triethanolamine (TEA) and methyldiethanolamine (MDEA). A fivefold increase in the rate of grating formation was achieved through the modification of the acrylamide concentration alone, and it was also found that holograms recorded with MDEA as the initiator performed the best and recorded up to 25% faster than a TEA-based photopolymer. Finally, tests were carried out on the stability of the protected and unprotected photopolymer layers when subjected to UV light. The properties exhibited by this photopolymer composition make it a promising material for the production of optical elements and suitable for use in applications requiring prolonged exposure to UV light when protected by a thin melinex cover.

Ultrahigh Angular Selectivity of Disorder-Engineered Metasurfaces

Metastructures hold promises for the realization of novel optical functions. Common topologies utilized in the form of metasurfaces featuring simple periodicities, however, are not exploiting the f...

Photo-double-ionization of water at 20 eV above threshold

The photodouble ionization of the water molecule is studied at 20 eV excess energy in a combined experimental and theoretical investigation. In the experiments, two photoelectrons of equal kinetic energy are detected in coincidence after energy and angular selection. On the theoretical side, a generalized Sturmian function approach is implemented to describe accurately the correlated two-electron continuum, while separable products of Moccia orbitals [J. Chem. Phys. 40, 2164 (1964)] are used for the initial electronic state of the water molecule. The theoretical triple-differential cross sections (TDCSs) are averaged over all possible molecular orientations in order to be compared with the experiments. The measured TDCSs display rich angular distributions that are in large part well reproduced by the adopted first-order treatment of the interaction with a two-active-electron target.

Geometric distortion and mixed pixel elimination via TDYWT image enhancement for precise spatial measurement to avoid land survey error modeling

In remote sensing, land cover classification of vegetation and water area from satellite image play a vital role for rural and urban planning and development. Existing algorithms of land cover classification require more sample image datasets for training. For existing algorithms, land cover classification of vegetation and water area is a challenging task because of mixed pixel and geometric distortion over boundary and curvature region. Mixed pixel affects the precise classification and measurement of land cover. Geometric distortion arises due to frame of isotropic and angular selectivity during image acquisition and affects the contour of land cover. In this paper, the proposed transverse dyadic wavelet transform (TDyWT) enhances and classifies vegetation and water area in land cover from LANDSAT image without training datasets. The proposed TDyWT uses Haar wavelet for decomposition and Burt 5 × 7 wavelet for reconstruction. The TDyWT enhances the contour, curvature, and boundary of vegetation and water area in LANDSAT image due to reversible and lifting properties of wavelet. TDyWT removes geometric distortion and spatial scale error of mixed pixel. In traditional land surveying spatial scale error reduction eliminates through total station and error modeling techniques. From the results, the proposed TDyWT algorithm classifies the area of subclass of vegetation and water with the 95% of accuracy with respect to ground truth survey methods.

Stacked volume holographic gratings for extending the operational wavelength range in LED and solar applications

A novel stacking procedure is presented for volume phase holographic gratings (VPHGs) recorded in photopolymer material using Corning Willow Glass as a flexible substrate in order to achieve broader angular and spectral selectivity in a diffractive device with high efficiency for solar and LED applications. For the first time to our knowledge, we have shown a device designed for use with a white LED that has the same input and output angles and high efficiency when illuminated by different wavelengths. In this paper, two VPHGs were designed, experimentally recorded, and tested when illuminated at normal incidence. The experimental approach is based on stacking two individual gratings in which the spatial frequency and slant have been tailored to the target wavelength and using real-time on-Bragg monitoring of the gratings in order to control the recorded refractive index modulation, thereby optimizing each grating efficiency for its design wavelength. Lamination of the two gratings together was enabled by using a flexible glass substrate (Corning Willow Glass). Recording conditions were studied in order to minimize the change in diffraction efficiency and peak diffraction angle during lamination and bleaching. The final fabricated stacked device was illuminated by a white light source, and its output was spectrally analyzed. Compared to a single grating, the stacked device demonstrated a twofold increase in angular and wavelength range. The angular and wavelength selectivity curves are in good agreement with the theoretical prediction for this design. This approach could be used to fabricate stacked lenses for white light LED or solar applications.

Volatile and permanent optical gratings recorded in Bi2TeO5photorefractive crystal under high cw intensity

We report the recording of optical gratings on photorefractive $ {{\rm Bi}_2}{{\rm TeO}_5} $Bi2TeO5 crystals using $ \lambda = 532\,\,{\rm nm} $λ=532nm wavelength light. We studied the behavior of this material under high light intensity and found the presence of fast and slow gratings, both of photorefractive nature and exhibiting quite significant light intensity dependence for the $ 1 - 13\,\,{\rm kW}/{{\rm m}^2} $1-13kW/m2 range. A permanent grating was found after the complete erasure of fast and slow holograms recorded at room temperature. The experimental results show that the diffraction efficiency of the permanent grating increases with the recorded light intensity. The permanent grating performance as an optical Bragg filter was characterized by measuring the angular selectivity approximately 1.0 mrad. We also show that the diffraction efficiency of the permanent grating is quite dependent on the direction of light polarization.

稀土掺杂光功能玻璃及器件应用（特邀）

The luminescence and laser properties of rare earth are produced by the transition of 4f electrons between different energy levels. Due to its special properties, rare earth ions doped optical-functional glass have played important roles in high power laser system, whether as active or passive components. Nd3+, Er3+ doped phosphate laser glass, which have high rare earth ion doping concentration, preparation characteristics with large size and high uniformity, can be used in high energy laser as an important gain medium material; Volume Bragg grating based on Ce3+-doped Photo-Thermo-Refractive glass, is a multifunctional optical components in high power laser system, owing to its excellent wavelength and angular selectivity, high diffraction efficiency, high thermal stability and high damage threshold. In this paper the latest progress of Nd3+, Er3+ doped phosphate laser glass and Volume Bragg grating base on Ce3+-doped Photo-Thermo-Refractive glass were reported.

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.

Enhancing the defect contrast in ECCI through angular filtering of BSEs

In this study, an annular multi-segment backscattered electron (BSE) detector is used in back scatter geometry to investigate the influence of the angular distribution of BSE on the crystalline defect contrast in electron channeling contrast imaging (ECCI). The study is carried out on GaAs and Ge layers epitaxially grown on top of silicon (Si) substrates, respectively. The influence of the BSE detection angle and landing energy are studied to identify the optimal ECCI conditions. It is demonstrated that the angular selection of BSEs exhibits strong effects on defect contrast formation with variation of beam energies. In our study, maximum defect contrast can be obtained at BSE detection angles 53–65° for the investigated energies 5, 10 and 20 keV. In addition, it is found that higher beam energy is favorable to reveal defects with stronger contrast whereas lower energy ( ≤ 5 keV) is favorable for revealing crystalline defects as well as with topographic features on the surface. Our study provides optimal ECCI conditions, and therefore enables a precise and fast detection of threading dislocations in lowly defective materials and nanoscale 3D semiconductor structures where signal to noise ratio is especially important. A comparison of ECCI with BSE and secondary electron imaging further demonstrates the strength of ECCI in term of simultaneous detection of defects and morphology features such as terraces with atomic step heights.

Power Efficient Scheduling and Hybrid Precoding for Time Modulated Arrays

We consider power efficient scheduling and precoding solutions for multiantenna hybrid digital-analog transmission systems that use Time-Modulated Arrays (TMAs) in the analog domain. TMAs perform beamforming with switches instead of conventional Phase Shifters (PSs). The extremely low insertion losses of switches, together with their reduced power consumption and cost make TMAs attractive in emerging technologies like massive Multiple-Input Multiple-Output (MIMO) and millimeter wave (mmWave) systems. We propose a novel analog processing network based on TMAs and provide an angular scheduling algorithm that overcomes the limitations of conventional approaches. Next, we pose a convex optimization problem to determine the analog precoder. This formulation allows us to account for the Sideband Radiation (SR) effect inherent to TMAs, and achieve remarkable power efficiencies with a very low impact on performance. Computer experiments results show that the proposed design, while presenting a significantly better power efficiency, achieves a throughput similar to that obtained with other strategies based on angular selection for conventional architectures.

Passivating Surface States on Water Splitting Cuprous Oxide Photocatalyst with Bismuth Decoration.

To enhance the visible light photocatalystic activity of Cu2O(100) surface, we performed first-principles calculations on the structural, electronic and optical properties of a bismuth (Bi)-decorated Cu2O(100) surface (Bi@Cu2O(100)). It is shown that the Bi prefer to be loaded to the hollow sites among four surface oxygen atoms and tend to individual dispersion instead of aggregating on the surface due to the lowest formation energy and larger distance between two Bi atoms at the surface than the Bi clusters; the coverage of around 0.25 monolayer Bi atoms can effectively eliminate the surface states and modify the band edges to satisfy the angular momentum selection rules for light excited transition of electrons, and the loaded Bi atoms contribute to the separation of photogenerated electron-holes. The relative positions between the band edges and the redox potentials are suitable for photocatalytic hydrogen production from the redox water, and moreover, the optical absorption spectrum indicates a positive response of the Bi0.25@Cu2O(100) to visible light, implying that the Bi0.25@Cu2O(100) is a promising visible light photocatalyst.

Tailoring angular selectivity in SiO2 slanted columnar thin films using atomic layer deposition of titanium nitride

Core-shell slanted nanocolumnar thin films (SCTFs) were produced by e-beam glancing angle deposition of SiO2 and subsequent atomic layer deposition (ALD) of TiN. Conformity of the ALD film over the slanted columns was confirmed by transmission electron microscopy and energy dispersive X-ray spectroscopy mappings. Angle resolved spectrophotometry characterization revealed angle-dependent transmission of the films, akin to their fully metallic counterparts. We found that such inclined core-shell nanocolumn films enable tailoring of the angular selectivity independently from the SCTF thickness and density.

Plasmonic optical trapping of nanoparticles with precise angular selectivity.

In this paper, a plasmonic trapping scheme including a polystyrene nanoparticle with gold cap and a metal tip tweezers was proposed. We numerically investigated the optical trapping behavior of the metal tip to this asymmetric particle. The results show that the metal tip can capture the particle at the position of the gold cap due to the strong plasmonic interaction, while other positions of the particle cannot be captured by metal tip. Furthermore, the trapping angle of the nanoparticle can be adjusted by changing the incident wavelength. Precisely controlling the trapping angle of the nanoparticles in our study has important potential applications of optical tweezers, such as in single molecule manipulation.

Investigation of high-lying even-parity levels of atomic samarium with multi-color photoionization technique: Energies and radiative lifetimes

We have investigated high-lying even-parity energy levels of atomic samarium in the energy region 32,156.3–32,886.4 cm−1 using two-color three-step photoionization technique. The experiments have been carried out in an atomic beam coupled to a linear time of flight mass spectrometer. The analysis of the photoionization spectra has resulted in the identification of twenty-five high-lying even-parity energy levels of atomic samarium. These include the confirmation of fifteen energy levels reported earlier while the remaining ten energy levels are new. The observed relative photoionization signal intensities of the corresponding transitions are also reported. The possible total angular momenta of the newly discovered energy levels have been assigned by the total angular momentum selection rules. Further, the radiative lifetimes of twenty high-lying even-parity energy levels of atomic samarium were measured for the first time by delayed three-color three-photon resonance ionization spectroscopy via a pump-probe method.

Modeling and optimizing the chromatic holographic waveguide display system.

A ray-tracing model is developed based on coupled wave theory for a volume holographic grating, which is the most important element of the holographic waveguide display but not accessibly integrated in current optical design software. The model fully and faithfully represents the angular selectivity, wavelength selectivity, polarization, and other properties for the in-coupling, out-coupling, and expansion gratings. It is especially important that the model is compatible with the current optical design software. In this paper, combining with other mature optical simulation functions of Zemax, integrated models are built for typical holographic waveguide display configurations, including image source, collimation element, gratings, waveguide plates, and approximate eye. It could provide the retina image at different viewing positions, based on which the main performance characteristics of a holographic waveguide display, such as field of view, color uniformity, eye box, and light efficiency, could be easily derived. Consequently, it provides a valuable guiding approach for the design and optimization of holographic waveguide displays.