Broadband Reflection Research Articles

Mechanism study of all-dielectric metamaterial wideband reflector based on quasi-canonical mode

All-dielectric metamaterial broadband reflectors have the advantages of low loss, high reflection efficiency, and compact structure. An in-depth understanding of the formation mechanism of their reflection bands is of great significance in optimizing metamaterial structure and performance, and in further designing novel photonic devices as well. In this work, two all-dielectric metamaterial broadband reflectors are constructed by using discrete silicon nanopillar array and silicon nanopillar array connected with a silicon sublayer. The quasi-normal modes of the metamaterial reflectors are solved. Combining the quasi-normal modes with the scattering matrix theory, we obtain the fitted reflection spectra of the metamaterial, and then reveal that the zero-frequency quasi-normal mode has a substantial influence on the accuracy of the fitting results. A dispersion relation analysis method is proposed to accurately solve the zero-frequency quasi-normal mode. Furthermore, the fitted resonance reflection spectrum and background reflection spectrum are obtained by using high-<i>Q</i>-value quasi-normal mode and low-<i>Q</i>-value quasi-normal mode, respectively. Our results show that the broadband reflection of the considered metamaterial reflectors should be attributed to the background reflection induced by the low-<i>Q</i> quasi-normal modes. The research method proposed in this paper can also be used for discussing quasi bound states in the continuum, Mie resonances, and other resonance phenomena, which provides a new way for interpreting the spectral characteristics of metamaterials.

A bottom-up understanding of the ligand-dominated formation of metallic nanoparticle electrodes with high broadband reflectance for enabling fully solution-processed large-area organic solar cells

Understanding the ligand-dominant impact on silver nanoparticles aids in the realization of compact-packed silver nanoparticle electrodes with high broadband reflectance, resulting in an evaporation-free large-area organic solar cell.

Polymer-stabilized cholesteric liquid crystal films with broadband reflection formed by photomask polymerization

In this paper, polymer-stabilized cholesteric liquid crystal (PSCLC) films with broadband reflection are fabricated by photomask polymerization. Due to the small hole diffraction effect of light, a gradient of UV light intensity is formed in the upper part of the liquid crystal (LC) cell perpendicular to the thickness direction of the glass substrate, which induces the formation of the pitch gradient. The experimental conditions for the optimal reflection bandwidth were obtained by studying the concentration of polymerizable monomers, temperature, time, and UV light intensity of the mask polymerization. The results showed that the reflectance bandwidth was broadened from 293 nm to 508 nm. The method has the advantages of simple preparation process, effective broadening of reflection bandwidth and reusability, which provides new practical guidance for the preparation of PSCLC films with broadband reflectance.

Printing Crack‐Free Microporous Structures by Combining Additive Manufacturing with Colloidal Assembly

To date high printing resolution and scalability, i.e., macroscale component dimensions and fast printing, are incompatible characteristics for additive manufacturing (AM) processes. It is hereby demonstrated that the combination of direct writing as an AM process with colloidal assembly enables the breaching of this processing barrier. By tailoring printing parameters for polystyrene (PS) microparticle-templates,how to avoid coffee ring formation is demonstrated, thus printing uniform single lines and macroscale areas. Moreover, a novel "comb"-strategy is introduced to print macroscale, crack-free colloidal coatings with low viscous colloidal suspensions. The printed templates are transformed into ceramic microporous channels as well as photonic coatings via atomic layer deposition (ALD) and calcination. The obtained structures reveal promising wicking capabilities and broadband reflection in the near-infrared, respectively. This work provides guidelines for printing low viscous colloidal suspensions and highlights the advancements that this printing process offers toward novel applications of colloidal-based printed structures.

Facile Stratification-Enabled Emergent Hyper-Reflectivity in Cholesteric Liquid Crystals.

Cholesteric liquid crystals (CLCs) are chiral photonic materials with selective reflection in terms of wavelength and polarization. Helix engineering is often required in order to produce desired properties for CLC materials to be employed for beam steering, light diffraction, scattering, and adaptive or broadband reflection. Here, we demonstrate a novel photopolymerization-enforced stratification (PES)-based strategy to realize helix engineering in a chiral CLC system with initially one handedness of molecular rotation throughout the layer. PES plays a crucial role in driving the chiral dopant bundle consisting of two chiral dopants of opposite handedness to spontaneously phase separate and create a CLC bilayer structure that reflects left- and right-handed circularly polarized light (CPL). The initially hidden chiral information therefore becomes explicit, and hyper-reflectivity, i.e., reflecting both left- and right-handed CPL, successfully emerges from the designed CLC mixture. The PES mechanism can be applied to structure a wide range of liquid crystal (LC) and polymer materials. Moreover, the engineering strategy enables facile programming of the center wavelength of hyper-reflection, patterning, and incorporating stimuli-responsiveness in the optical device. Hence, the engineered hyper-reflective CLCs offer great promise for future applications, such as digital displays, lasing, optical storage, and smart windows.

Preparation of Flexible Liquid Crystal Films with Broadband Reflection Based on PD&SLC.

A simple and efficient method for the preparation of a film with flexible characteristic and selective reflection of near-infrared light is proposed. Based on the coexistence system (PD&SLC) of polymer dispersed liquid crystals (PDLC) and polymer stabilized liquid crystals (PSLC), it combines the flexibility of PDLC with the selectively reflection of PSLC. Innovative use of step-by-step light curing to achieve microstructural differences in the three-dimensional orientation of the material is proposed. That is, the difference between PDLC and PSLC in the planar orientation, as well as the gradient distribution of cholesteric phase liquid crystal pitch in the cell thickness direction, is observed. While realizing the flexibility of the material, the function of broadening the reflection bandwidth is fulfilled. This method of preparing liquid crystal films is expected to have great potential for applications, such as flexible smart windows, infrared light shielding, and sensors.

Novel graphene oxide derivatives and its effect in broad-band reflectors

ABSTRACT n-Octylyamine (CA) was chemically grafted onto carboxylic groups of graphene oxide (GO) via amidation reaction. The resultant modified GO sheets (GO-CA) were easily redispersed in common organic solvents or liquid crystals (LCs), facilitating the structure characterisation and the device fabrication by solution processing. Broadband reflectors could be obtained by doping different contents of GO-CA nanosheets into chiral nematic liquid crystalline (N*-LCs) media to a photo-polymerisation. Experimental results showed that both the bandwidth of the reflection spectra and the location of the reflection band of the GO-CA-doped composite films were better than the N*-LCs sample without GO-CA. Effects of polymerisation temperatures and doped contents of GO-CA on the broadband reflection of N*-LC composite films were investigated.

Parallel photoreactor development with enhanced photon efficiency and reproducibility based on laws of optics

A lab-scale parallel photoreactor model has been developed for broad applications in organic chemistry. Position, angle, and distance in the reactor are systematically optimized for achieving high reproducibility under the guidance of the Inverse Square Law and Lambert’s Cosine Law, which were first applied in the design of the photoreactor. Improvement of photon efficiency is realized via 3-point irradiation and broad-band light reflector of aluminum. Photon flux is measured with an optical power meter with its display and adjustment suggested being integrated into the photoreactor for the first time.

Transmission properties of one-dimensional symmetric ternary structures of metamaterials and dielectric materials with two different configurations

In this work, we have investigated the tunable transmission properties of a one-dimensional ternary periodic structure designed with meta- and dielectric-materials in two different configurations. To the study the optical properties of considered periodic structure, we have used the transfer matrix method. The two configurations of considered periodic structure show tunable transmission properties with change in the type of configuration, number of layers, and incident angle of the electromagnetic wave. It is noted that the broadband width and the number of peaks and transmittance increase with an increase in the incident angle, where the peaks becomes more confined in a narrow region. An increase in number of unit cells reduces the transmission in the peaks. Comparing the order of the symmetry, we find that the transmissions in the peaks are more in the second sequence as compared to the first one, and hence the second one is more suitable for multichannel filters. On the basis of the results, the proposed structures can be used in the large variety of opto-electronic and photonic applications, including multichannel filter, broadband reflectors, and switches.

Preparation of polymer stabilised cholesteric broadband reflection films based on ZIF-8 assisted bidirectional diffusion method

ABSTRACT The polymer-stabilised cholesteric liquid crystal (PSCLC) broadband reflective films are prepared by zeolite imidazole framework-8 (ZIF-8)-assisted bidirectional diffusion combined with light curing, altering the single pitch of cholesteric liquid crystals (CLCs). By changing temperature, chiral compounds and ultraviolet (UV) absorbing dyes adsorbed on porous ZIF-8 are able to diffuse bidirectionally into the CLCs’ system and produce concentration gradients, thereby inducing a pitch gradient, and the structure of broad reflection bandwidth can be fixed through UV irradiation. In addition, the effects of various factors, such as UV irradiation intensity, UV irradiation time, mass fraction of polymerisable monomers and curing temperature on the pitch gradient of the films are systematically investigated, and the maximum reflection bandwidth of the prepared films is 1274 nm. There is no doubt that new and modified processes and techniques provide significant possibilities for preparing and applying PSCLC films.

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

Optically-Controlled Terahertz Multifunctional Polarization Conversion Metasurface with Reflection and Transmission Modes.

In the terahertz band, how integrating multiple functions into a device with a tiny unit structure is a challenge. In this paper, an optically-controlled multifunctional linear polarization conversion metasurface working in the terahertz band is proposed. The reflection and transmission polarization conversion functions can be realized by irradiating the metasurface with pump light with different wavelengths. The metasurface is designed with a multilayer structure, and a photosensitive semiconductor alone is used to control multiple functions, which makes the manipulation of multifunctional devices easy. When the photosensitive semiconductor germanium (Ge) and silicon (Si) are in different states, the metasurface can realize broadband reflection and transmission polarization conversion functions, the corresponding relative bandwidth are 102.4% and 98.9%, respectively, and the work efficiency can be regulated by pump light with different intensity and wavelength. In addition, the working principle of the metasurface is analyzed by eigenmode theory and surface current distributions. The stability of the metasurface to structural parameters and incident angles are discussed.

Coplanar Meta-Surface-Based Substrate-Integrated Waveguide Antennas with Broadband and Low Reflections for K-Band Beam Scanning.

Four novel substrate-integrated waveguide (SIW) antennas are proposed, in order to obtain K-band beam scanning through the coplanar meta-surfaces of properly devised complementary split-ring resonators. More specifically, coplanar rhombus- and hexagon-shaped meta-surfaces replace the metallized via holes in the traditional SIW structure, achieving low reflection and wide bandwidth, respectively. Another trapezoid-shaped meta-surface is introduced, in order to realize good leaky-wave radiation performance with high-gain beam scanning in both rhombus- and hexagon-shaped SIW components. These designs are further extended to two different mixed types of two-row meta-surfaces, with the rhombus and hexagon structures combined in different orders to enhance the complex SIW transmission lines and antennas, which can simultaneously obtain good reflection and bandwidth with different priority, depending on the arrangement. We explain the performance differences with rhombus and hexagon meta-surfaces through the analysis of relevant equivalent circuit models and extracting the effective medium parameters, and we verify the bandwidths and radiations of four SIW antennas both numerically and experimentally. The maximum gains of the four antennas are 18.1 dBi, 17.0 dBi, 18.8 dBi and 17.1 dBi, where the corresponding relative bandwidths are 10.74%, 19.42%, 14.13% and 18.38%. The maximum simulated radiation efficiency and aperture efficiency of the proposed antennas are 91.20% and 61.12%, respectively. Our approach for generating flexible and selectable tuned electromagnetic fields from SIWs is applicable for the development of mm-Wave antennas or sensors on PCB-integrated platforms for highly directive scanning radiation.

Marchenko Green’s Function Retrieval in Layered Elastic Media from Two-Sided Reflection and Transmission Data

By solving a Marchenko equation, Green’s functions at an arbitrary (inner) depth level inside an unknown elastic layered medium can be retrieved from single-sided reflection data, which are collected at the top of the medium. To date, it has only been possible to obtain an exact solution if the medium obeyed stringent monotonicity conditions and if all forward-scattered (non-converted and converted) transmissions between the acquisition level and the inner depth level were known a priori. We introduce an alternative Marchenko equation by revising the window operators that are applied in its derivation. We also introduce an auxiliary equation for transmission data, which are collected at the bottom of the medium, and a coupled equation, which is based on both reflection and transmission data. We show that the joint system of the Marchenko equation, the auxiliary equation and the coupled equation can be succesfully inverted when broadband reflection and transmission data are available. This results in a novel methodology for elastodynamic Green’s function retrieval from two-sided data. Apart from these data, our approach requires P- and S-wave transmission times between the inner depth level and the top of the medium, as well as two angle-dependent amplitude scaling factors, which can be estimated from the data by enforcing energy conservation.

PEDOT:PSS on flexible black silicon for a hybrid solar cell on textured polyimide substrate

This work investigates properties of PEDOT:PSS on flexible black silicon (bSi) for a hybrid solar cell on textured polyimide (PI) substrate. The flexible bSi is formed by thinning down crystalline silicon (cSi) wafers to 65 μm thickness, followed by fabrication of bSi nanowires (NWs) on the wafer surface using one-step metal-catalyzed electroless etching (MCEE) technique. The resulting bSi NWs exhibit an average diameter of around 90–100 nm and length of 900 nm. Then, PEDOT:PSS with a thickness of 150 nm is coated on the flexible cSi and bSi NWs. For texturing of PI, copper-seeding technique is used. The planar and textured PI substrates are then attached to the back of the flexible cSi and bSi. The PEDOT:PSS/flexible bSi on PI substrate shows lower broadband reflection when compared to PEDOT:PSS/flexible cSi. This is due to the presence of bSi NWs on wafer surface which leads to refractive index grading effect. The PEDOT:PSS/flexible bSi solar cell on the textured PI substrate demonstrates conversion efficiency of 2.58%. This is contributed by the increased short-circuit current density (Jsc) in the device (when compared to the device on planar PI), owing to the enhanced light absorption above wavelength of 800 nm.

Ultra-efficiency broadband terahertz polarization converter based on a cross-shaped metamaterial

In this study, we describe the design, fabrication, and characterization of a cross-shaped structure reflective broadband THz polarization converter. The operating bandwidth of the proposed polarization converter was in the range of 0.8–1.6 THz. The polarization conversion rate (PCR) exceeded 85% in the frequency range of 1.07–1.35 THz and was as high as 91% at 1.35 THz. The cross PCR value exceeded 85% in the frequency range of 1.05–1.35 THz and reached a maximum value of 91%. The performance characteristics (bandwidth and PCR) of the proposed polarization converter were compared with those of similar previously reported devices, and the results indicated that the proposed polarization converter exhibits better performance. The proposed THz polarization converter is suitable for a wide range of applications in communication and polarization manipulation devices.

In-Season Monitoring of Maize Leaf Water Content Using Ground-Based and UAV-Based Hyperspectral Data

China is one the largest maize (Zea mays L.) producer worldwide. Considering water deficit as one of the most important limiting factors for crop yield stability, remote sensing technology has been successfully used to monitor water relations in the soil–plant–atmosphere system through canopy and leaf reflectance, contributing to the better management of water under precision agriculture practices and the quantification of dynamic traits. This research was aimed to evaluate the relation between maize leaf water content (LWC) and ground-based and unoccupied aerial vehicle (UAV)-based hyperspectral data using the following approaches: (I) single wavelengths, (II) broadband reflectance and vegetation indices, (III) optimum hyperspectral vegetation indices (HVIs), and (IV) partial least squares regression (PLSR). A field experiment was undertaken at the Chinese Academy of Agricultural Sciences, Beijing, China, during the 2020 cropping season following a split plot model in a randomized complete block design with three blocks. Three maize varieties were subjected to three differential irrigation schedules. Leaf-based reflectance (400–2500 nm) was measured with a FieldSpec 4 spectroradiometer, and canopy-based reflectance (400–1000 nm) was collected with a Pika-L hyperspectral camera mounted on a UAV at three assessment days. Both sensors demonstrated similar shapes in the spectral response from the leaves and canopy, with differences in reflectance intensity across near-infrared wavelengths. Ground-based hyperspectral data outperformed UAV-based data for LWC monitoring, especially when using the full spectra (Vis–NIR–SWIR). The HVI and the PLSR models were demonstrated to be more suitable for LWC monitoring, with a higher HVI accuracy. The optimal band combinations for HVI were centered between 628 and 824 nm (R2 from 0.28 to 0.49) using the UAV-based sensor and were consistently located around 1431–1464 nm and 2115–2331 nm (R2 from 0.59 to 0.80) using the ground-based sensor on the three assessment days. The obtained results indicate the potential for the complementary use of ground-based and UAV-based hyperspectral data for maize LWC monitoring.

High wall-plug efficiency and narrow linewidth III-V-on-silicon C-band DFB laser diodes.

We present recent results on compact and power efficient C-band distributed feedback lasers through adhesive bonding of a III-V die onto a silicon-on-insulator circuit. A wall-plug efficiency up to 16% is achieved for bias currents below 40 mA. The laser cavity is 180 µm long and a single facet output power up to 11 mW is measured at 20 °C by incorporating a broadband reflector in the silicon waveguide at one side of the cavity. Single mode operation at 1567 nm with a side mode suppression ratio of around 55 dB is demonstrated. By controlling the phase of the external feedback, the laser linewidth is decreased to 28 kHz. Measurement result shows a low relative intensity noise below -150 dB/Hz at 60 mA up to 6 GHz. We also report 20 and 10 Gbps data transmission at a bias current of 50 mA at 20 °C and 40 °C, respectively.

Assembly of Guanine Crystals as a Low-Polarizing Broadband Multilayer Reflector in a Spider, Phoroncidia rubroargentea.

The diminishing of the polarization effect is important in the applications of dielectric multilayer reflectors in many optical systems, such as low-loss broadband waveguides, optical fibers, and LEDs. Low-polarizing broadband reflections were identified from birefringent-guanine-crystal-based multilayer reflectors in the skins of some fish. Previous models for these intriguing natural optical phenomena suggested the combined action of two populations of guanine crystals with an orthogonal low-refractive-index optic axis. Here we report a novel realization of polarization-insensitive broadband reflectivity in a spider, Phoroncidia rubroargentea, based solely on the type of guanine crystals with the low-refractive-index optic axis normal to the crystal plates. We examined the three-dimensional structure of the guanine assembly in the spider and performed finite-difference time-domain (FDTD) optical modeling of the guanine-based multilayer reflector. Comparative modeling studies reveal that the biological selection of the guanine crystal type and specific spatial arrangement work synergistically to optimize the polarization-insensitive broadband reflection. This study demonstrates the importance of both crystallographic characteristics and 3D arrangement of guanine crystals in understanding relevant natural optical effects and also provides new insights into similar broadband, low-polarizing reflections in biological optical systems. Learning from relevant biofunctional assembly of guanine crystals could promote the bioinspired design of nonpolarizing dielectric multilayer reflectors.

Research Progress of Cholesteric Liquid Crystals with Broadband Reflection.

Cholesteric liquid crystal (ChLC) materials with broadband reflection are witnessing a significant surge in interest due to their unique ability to self-organize into a helical supra-molecular architecture and their excellent selective reflection of light based on the Bragg relationship. Nowadays, by the virtue of building self-organized nanostructures with pitch gradient or non-uniform pitch distribution, extensive work has already been performed to obtain ChLC films with a broad reflection band. This critical review systematically summarizes the optical background of the ChLCs with broadband reflection characteristics, methods to obtain broadband reflection of ChLCs, as well as the application in this area. Combined with the research status and the advantages in the field, the challenges and opportunities of applied scientific problems in the research direction are also introduced.