Photoalignment of sub-micrometer periodic liquid crystal polarization grating by using the optical imprinting method.

Photoalignment of liquid crystal polarization grating based on optical imprinting is a promising technique for polarization grating mass production. However, when the period of the optical imprinting grating is in the sub-micrometer level, the zero-order energy from the master grating will become high, and it will strongly affect the photoalignment quality. This paper proposes a double-twisted polarization grating structure to eliminate the zero-order disturbance of master grating and gives the design method. Based on the designed results, a master grating was prepared, and the optically imprinted photoalignment of polarization grating with a period of 0.5µm was fabricated. This method has the advantages of high efficiency and significantly greater environmental tolerance than the traditional polarization holographic photoalignment methods. It has the potential to be used for large-area polarization holographic gratings production.

Multi-focus image fusion algorithm based on random features embedding and ensemble learning.

Multi-focus image fusion algorithm integrates complementary information from multiple source images to obtain an all-in-focus image. Most published methods will create incorrect points in their decision map which have to be refined and polished with post-processing procedure. Aim to address these problems, we present, for the first time, a novel algorithm based on random features embedding (RFE) and ensemble learning which reduced the calculation workload and improved the accuracy without post-processing. We utilize RFE to approximate a kernel function so that Support Vector Machine (SVM) can be applied to large scale data set. With ensemble learning scheme we then eliminate the abnormal points in the decision map. We reduce the risk of entrap into over-fitting predicament and boost the generalization ability by combining RFE and ensemble learning. The theoretical analysis is in consistence with the experimental results. With low computation cost, the proposed algorithm achieve high visual quality as the state-of-the-art(SOTA).

Dual-task convolutional neural network based on the combination of the U-Net and a diffraction propagation model for phase hologram design with suppressed speckle noise.

In this paper, a dual-task convolutional neural network based on the combination of the U-Net and a diffraction propagation model is proposed for the design of phase holograms to suppress speckle noise of the reconstructed images. By introducing a Fresnel transmission layer, based on angular spectrum diffraction theory, as the diffraction propagation model and incorporating it into U-Net as the output layer, the proposed neural network model can describe the actual physical process of holographic imaging, and the distributions of both the light amplitude and phase can be generated. Afterwards, by respectively using the Pearson correlation coefficient (PCC) as the loss function to modulate the distribution of the amplitude, and a proposed target-weighted standard deviation (TWSD) as the loss function to limit the randomness and arbitrariness of the reconstructed phase distribution, the dual tasks of the amplitude reconstruction and phase smoothing are jointly solved, and thus the phase hologram that can produce high quality image without speckle is obtained. Both simulations and optical experiments are carried out to confirm the feasibility and effectiveness of the proposed method. Furthermore, the depth of field (DOF) of the image using the proposed method is much larger than that of using the traditional Gerchberg-Saxton (GS) algorithm due to the smoothness of the reconstructed phase distribution, which is also verified in the experiments. This study provides a new phase hologram design approach and shows the potential of neural networks in the field of the holographic imaging and more.

Liquid-crystal-polymer binary diffractive optical elements with a sub-micrometer feature size realized by a contact polarization holography.

In this Letter, a contact polarization holographic photoalignment method is proposed. In the holographic recording, a phase mask is contacted with a photoalignment film, making light carrying wavefront information interfere with reference light in the near-field region to realize polarization holographic pattern recording with a sub-micrometer feature size. The relevant theoretical derivation is given, and holographic recording of a 0.4 µm feature-size phase mask is realized. The proposed method can conveniently realize liquid-crystal binary diffractive optical elements with a sub-micrometer feature size. Off-axis diffraction can also be realized by superimposing the grating information by changing the angle between the substrate and the interference light.

Diffuser screen with flat-top angular distribution of scattered light realized by a dual-beam holographic speckle.

Holographic speckle screens with the Gaussian type distribution of scattered light, which are used to increase the viewing angle of the image in projection display systems, result in nonuniform image brightness in different observing positions. In this study, based on Helmholtz-Kirchhoff theory, a dual-beam scattering theory of rough surface is derived. By analyzing the spatial frequency spectrum of the scattered light, it is found that when two laser beams irradiated the ground glass at a certain angle, the resulting speckles recorded on the photoresist can generate a flat-top angular distribution of the scattered light. Speckle screens are fabricated by two light beams at different angles, and the angular intensity distribution of scattered light is measured. The results are in good agreement with the theory. Compared with the Gaussian type diffuser, the energy efficiency of the speckle screen proposed has a 46% increase when the angular luminance uniformity is set to be 80%, which effectively improves the brightness when used in a head up display system.

U-Net convolutional neural network-based modification method for precise fabrication of three-dimensional microstructures using laser direct writing lithography.

In this paper, a modification method based on a U-Net convolutional neural network is proposed for the precise fabrication of three-dimensional microstructures using laser direct writing lithography (LDWL). In order to build the correspondence between the exposure intensity distribution data imported to the laser direct writing system and the surface profile data of the actual fabricated microstructure, these two kinds of data are used as training tensors of the U-Net convolutional neural network, which is proved to be capable of generating their accurate mapping relations. By employing such mapping relations to modify the initial designed exposure intensity data of the parabolic and saddle concave micro-lens with an aperture of 24µm×24µm, it is demonstrated that their fabrication precision, characterized by the mean squared error (MSE) and the peak signal-to-noise ratio (PSNR) between the fabricated and the designed microstructure, can be improved significantly. Specifically, the MSE of the parabolic and saddle concave micro-lens decreased from 100 to 17 and 151 to 50, respectively, and the PSNR increased from 22dB to 29dB and 20dB to 25dB, respectively. Furthermore, the effect of laser beam shaping using these two kinds of micro-lens has also been improved considerably. This study provides a new solution for the fabrication of high-precision three-dimensional microstructures by LDWL.

Single-Phase White-Light-Emitting and Photoluminescent Color-Tuning Coordination Assemblies.

Metal-organic complexes assembled from coordinative interactions are known to be able to display a wide range of photoluminescent behaviors benefiting from an extensive number of metal ions, organic linkers, and inclusion guests, depending on the multifaceted nature of their chemical structures and photophysical properties. In the past two decades, the white-light-emitting (WLE) and photoluminescent color-tuning (PLCT) materials based on the single-phase metal-organic coordination assemblies have merited particular attention and gained substantial advances. In this review, we give an overview of recent progress in this field, placing emphasis on the WLE and PLCT properties realized in the single-phase materials, which covers the origin, generation, and manipulation of different types of photoluminescence (PL) derived from ligand-centered (LC), metal/cluster-centered (MC or CC), excimer/exciplex-based (EX), metal-to-ligand or ligand-to-metal charge-transfer-based (MLCT or LMCT), or guest-included emissions. The coordination assemblies in this topic can be generally classified into three categories [(1) mono/homometallic coordination assemblies based on main group (s,p-block), transition (d-block), or lanthanide (f-block) metal centers, (2) s/p-f-, d-f-, or f-f-type heterometallic coordination assemblies, and (3) guest-included coordination assemblies] for which WLE and PLCT properties can be achieved by virtue of either a wide-band/overlapped emission covering the whole visible spectrum from a single emitting center or a combination of complementary color emissions from multiple emitting centers/origins. Some state-of-the-art assembly methods and successful design models relevant to the above three categories are elaborated to demonstrate how to achieve efficient and controllable white-light emission in a single-phase material through a tunable PL approach. Potential applications in the fields of lighting and displaying, sensing and detecting, and barcoding and patterning are surveyed, and at the end, possible prospects and challenges for future development along this line are proposed.

Multiresponsive UV-One-Photon Absorption, Near-Infrared-Two-Photon Absorption, and X/γ-Photoelectric Absorption Luminescence in One [Cu4I4] Compound.

A new Cu4I4-cluster-based compound is constructed to show multifaceted photoluminescent attributes: (1) ultraviolet (UV)-excited thermo-, mechano-, and rigido-chromic phosphorescence by the OPA (one-photon absorption) pathway, due to the interchanging emissions from cluster-centered (3CC) and halide-to-ligand charge-transfer (3XLCT) excited triplet states, (2) the ability to convert X/γ-ray and near-infrared (NIR) radiation to visible-light emission, in which the heavy Cu4I4 cores serve as the efficient X/γ-PEA (photoelectric absorption) or NIR-TPA (two-photon absorption) trapper and convertor to photons in the visible spectrum from the same emissive triplet states as those produced by UV excitation. This all-in-one compound affords a highly integrated nanolab for understanding and exploiting a wide range of photophysical phenomena simultaneously and is further fabricated into fiber-coupled long-range, in situ cryogenic thermometer and poly(methyl methacrylate) (PMMA)-embedded monolith gel, providing access to advanced applications in multifunctional optical materials and devices.

Ultrasensitive Molecular Detection by Improving the Mode Energy of Graphene Plasmons for Surface Enhanced Infrared Absorption Spectroscopy.

Ultrasensitive detection of molecules by graphene plasmons based surface enhanced infrared absorption spectroscopy (SEIRAS) has attracted considerable research interest in recent years. However, SEIRAS still suffers from low enhancement. Herein, we investigated the crucial factors that determined the enhancement of graphene plasmons based SEIRAS. Through numerical calculations, it found that the enhancement of SEIRAS can be significantly improved by increasing the absorptance of graphene plasmons and the electron relaxation time of graphene. It revealed that such results were related to the mode energy of graphene plasmons. High absorptance and long electron relaxation time would result in high mode energy, which would in turn induce large local electric field to enhance the SEIRAS signal. Moreover, it showed that the resonant center of a molecular vibrational mode can be accurately extracted from the Rabi splitting spectra obtained by sweeping the Fermi energy of graphene. Our study could provide a guidance to improve the enhancement of graphene plasmons based SEIRAS for ultrasensitive molecular detection.

Pressure-Induced Multiphoton Excited Fluorochromic Metal-Organic Frameworks for Improving MPEF Properties.

In multiphoton excited fluorescence (MPEF), high-energy upconversion emission is obtained from low-energy excitation by absorbance of two or more photons simultaneously. In a pressure-induced fluorochromic process, the emission energy is switched by outer pressure stimuli. Now, five metal-organic frameworks containing the same ligand with simultaneous multiphoton absorption and pressure-induced fluorochromic attributes were studied. One-, two-, and three-photon excited fluorescence (1/2/3PEF) can be achieved in the frameworks, which exhibit pressure-induced blue-to-yellow fluorochromism. The performances are closely dependent with the topologies, flexibilities, and packing states of the frameworks and chromophores therein. The multiphoton upconversion performance can be intensified by pressure-related structural contraction. Over ten-fold increment in the 2PA active cross-section up to 2217 GM is achieved in pressed LIFM-114 compared with the 210 GM for pristine sample at 780 nm.

A Metal-Organic Supramolecular Box as a Universal Reservoir of UV, WL, and NIR Light for Long-Persistent Luminescence.

Long persistent luminescence (LPL) materials have a unique photophysical mechanism to store light radiation energy for subsequent release. However, in comparison to the common UV source, white-light (WL) and near-infrared (NIR) excited LPL is scarce. Herein we report a metal-organic supramolecular box based on a D-π-A-type ligand. Owing to the integrated one-photon absorption (OPA) and two-photon absorption (TPA) attributes of the ligand, the heavy-atom effect of the metal center, as well as π-stacking and J-aggregation states in the supramolecular assembly, LPL can be triggered by all wavebands from the UV to the NIR region. This novel designed supramolecular kit to afford LPL by both OPA and TPA pathways provides potential applications in anti-counterfeiting, camouflaging, decorating, and displaying, among others.

White-Light Emission from Dual-Way Photon Energy Conversion in a Dye-Encapsulated Metal-Organic Framework.

The design of white-light phosphors is attractive in solid-state lighting (SSL) and related fields. A new strategy in obtaining white light emission (WLE) from dual-way photon energy conversion in a series of dye@MOF (LIFM-WZ-6) systems is presented. Besides the traditional UV-excited one-photon absorption (OPA) pathway, white-light modulation can also be gained from the combination of NIR-excited green and red emissions of MOF backbone and encapsulated dyes via two-photon absorption (TPA) pathway. As a result, down-conversion OPA white light was obtained for RhB+ @LIFM-WZ-6 (0.1 wt %), BR-2+ @LIFM-WZ-6 (2 wt %), and APFG+ @LIFM-WZ-6 (0.1 wt %) samples under 365 nm excitation. RhB+ @LIFM-WZ-6 (0.05 wt %), BR-2+ @LIFM-WZ-6 (1 wt %) and APFG+ @LIFM-WZ-6 (0.05 wt %) exhibit up-conversion TPA white light under the excitation of 800, 790, and 730 nm, respectively. This new WLE generation strategy combines different photon energy conversion mechanisms together.

ESIPT-Modulated Emission of Lanthanide Complexes: Different Energy-Transfer Pathways and Multiple Responses.

Two series of isostructural lanthanide coordination complexes, namely, LIFM-42(Ln) (Ln=Eu, Tb, Gd, in which LIFM stands for the Lehn Institute of Functional Materials) and LIFM-43(Ln) (Ln=Er, Yb), were synthesized through the self-assembly of an excited-state intramolecular proton transfer (ESIPT) ligand, 5-[2-(5-fluoro-2-hydroxyphenyl)-4,5-bis(4-fluorophenyl)-1H-imidazol-1-yl]isophthalic acid (H2 hpi2cf), with different lanthanide ions. In the coordination structures linked by the ligands and oxo-bridged LnIII 2 clusters (for LIFM-42(Ln) series) or isolated LnIII ions (for LIFM-43(Ln) series), the ESIPT ligand can serve as both the host and antenna for protecting and sensitizing the photoluminescence (PL) of LnIII ions. Meanwhile, the -OH⋅⋅⋅N active sites on the ligands are vacant, which provides availability to systematically explore the PL behavior of Ln complexes with ESIPT interference. Based on the accepting levels of different lanthanide ions, energy transfer can occur from the T1 (K*) or T1 (E*) (K*=excited keto form, E*=excited enol form) excited states of the ligand. Furthermore, the sensitized lanthanide luminescence in both visible and near-infrared regions, as well as the remaining K* emission of the ligand, can be modulated by the ESIPT responsiveness to different solvents, anions, and temperature.

Semiconductive Amine-Functionalized Co(II)-MOF for Visible-Light-Driven Hydrogen Evolution and CO2 Reduction.

A Co-MOF, [Co3(HL)2·4DMF·4H2O] was simply synthesized through a one-pot solvothermal method. With the semiconductor nature, its band gap was determined to be 2.95 eV by the Kubelka-Munk method. It is the first trinuclear Co-MOF employed for photocatalytic hydrogen evolution and CO2 reduction with cobalt-oxygen clusters as catalytic nodes. Hydrogen evolution experiments indicated the activity was related to the photosensitizer, TEOA, solvents, and size of catalyst. After optimization, the best activity of H2 production was 1102 µmol/(g h) when catalyst was ground and then soaked in photosensitizer solution before photoreaction. To display the integrated design of Co-MOF, we used no additional photosensitizer and cocatalyst in the CO2 reduction system. When -NH2 was used for light absorption and a Co-O cluster was used as catalyst, Co-MOF exhibited an activity of 456.0 µmol/(g h). The photocatalytic mechanisms for hydrogen evolution and CO2 reduction were also proposed.

The all-optical modulator in dielectric-loaded waveguide with graphene-silicon heterojunction structure.

All-optical modulators based on graphene show great promise for on-chip optical interconnects. However, the modulation performance of all-optical modulators is usually based on the interaction between graphene and the fiber, limiting their potential in high integration. Based on this point, an all-optical modulator in a dielectric-loaded waveguide (DLW) with a graphene-silicon heterojunction structure (GSH) is proposed. The DLW raises the waveguide mode, which provides a strong light-graphene interaction. Sufficient tuning of the graphene Fermi energy beyond the Pauli blocking effect is obtained with the presented GSH structure. Under the modulation light with a wavelength of 532 nm and a power of 60 mW, a modulation efficiency of 0.0275 dB µm-1 is achieved for light with a communication wavelength of 1.55 µm in the experiment. This modulator has the advantage of having a compact footprint, which may make it a candidate for achieving a highly integrated all-optical modulator.

Epitaxial Growth of Hetero-Ln-MOF Hierarchical Single Crystals for Domain- and Orientation-Controlled Multicolor Luminescence 3D Coding Capability.

Core-shell or striped heteroatomic lanthanide metal-organic framework hierarchical single crystals were obtained by liquid-phase anisotropic epitaxial growth, maintaining identical periodic organization while simultaneously exhibiting spatially segregated structure. Different types of domain and orientation-controlled multicolor photophysical models are presented, which show either visually distinguishable or visible/near infrared (NIR) emissive colors. This provides a new bottom-up strategy toward the design of hierarchical molecular systems, offering high-throughput and multiplexed luminescence color tunability and readability. The unique capability of combining spectroscopic coding with 3D (three-dimensional) microscale spatial coding is established, providing potential applications in anti-counterfeiting, color barcoding, and other types of integrated and miniaturized optoelectronic materials and devices.

Dimension Increase via Hierarchical Hydrogen Bonding from Simple Pincer-like Mononuclear complexes.

A tetradentate symmetric ligand bearing both coordination and hydrogen bonding sites, N(1),N(3)-bis(1-(1H-benzimidazol-2-yl)-ethylidene)propane-1,3-diamine (H2bbepd) was utilized to synthesize a series of transition metal complexes, namely [Co(H2bbepd)(H(2)O)2]·2ClO(4) (1), [Cu(H2bbepd)(OTs(-))]·OTs(-) (2),[Cu(bbepd)(CH(3)OH)] (3), [Cd(H(2)bbepd)(NO3)2]·CH(3)OH (4), [Cd(H(2)bbepd)(CH(3)OH)Cl]·Cl (5), and [Cd(bbepd)(CH(3)OH)2] (6). These complexes show similar discrete pincer-like coordination units, possessing different arrangements of hydrogen bonding donor and acceptor sites. With or without the aid of uncoordinated anions and solvent molecules, such mononuclear units have been effectively involved in the construction of hierarchical hydrogen bonding assemblies (successively via level I and level II), leading to discrete binuclear ring (complex 2), one-dimensional chain or ribbon (complexes 3, 4 and 6) and two-dimensional layer (complexes 1 and 5) aggregates.

Multi-focus plasmonic lens design based on holography.

Multi-focus plasmonic lens with metallic nanoslits of variant widths have great potential applications in optical interconnection, integrated optics and nanophotonics. But the design method with simulated annealing algorithm or Yang-Gu algorithm requires complex calculation and multi focuses are limited to be set on the same output plane. In this paper, we propose a design method based on holography. The desired light field distribution and the incident plane wave can be treated as object wave and reference wave, respectively. So the calculation is relative simple and multi focuses can be located in different output plane. Numerical simulation of multi-focus lens design is performed through finite-difference time-domain (FDTD) method and the result confirms the feasibility of our method.

Surface enhanced Raman scattering substrate with metallic nanogap array fabricated by etching the assembled polystyrene spheres array.

A sensitive surface enhanced Raman scattering (SERS) substrate with metallic nanogap array (MNGA) is fabricated by etching of an assembled polystyrene (PS) spheres array, followed by the coating of a metal film. The substrate is reproducible in fabrication and sensitive due to the nanogap coupling resonance (NGCR) enhancement. The NGCR is analyzed with the finite difference time domain (FDTD) method, and the relationship between the gap parameter and the field enhancement is obtained. Experimental measurements of R6G on demonstrate that the enhancement factor (EF) of the MNGA SERS substrate is increased by more than two fold compared with the control sample.

Experimental study on polarization lens formed by asymmetrical metallic hole array.

A polarization bifocal lens based on the polarization effect caused by asymmetrical hole arrays had been designed, fabricated, and characterized experimentally. By considering the fact that the skin depth of an infrared electromagnetic field inside metal is much shorter than the incident wavelength, a polarization bifocal lens composed of high deep-width ratio metallic holes was realized by using a gold-coated silicon structure to replace the one directly formed on a thick metal film. An infrared optical experiment setup is built based on the secondary imagery method for characterizing the focal length of the designed bifocal lens. The measured focal lengths of the fabricated bifocal lens coincide well with the designed values, which proves the validity for realizing the polarization elements with the proposed structure and the feasibility of the fabrication process.