Reflection Filters Research Articles

Improving the color gamut of gold nanostructures using vertical tandem nano-disks

Metallic nanoparticles have been used in structural coloration applications since they provide an alternative for the synthesis of color filters, with extended color gamut, enhanced color saturation, and brightness. These optical properties can be further improved when integrating nanoparticles into coupled dimers that give rise to enhanced hybridized plasmon resonances. In this contribution, gold nano-disks are split into Au/SiO2/Au layered structures to enhance its plasmon resonances and evaluate its improved performance as reflective color filters. To do so, the reflectance spectra of the layered disks were obtained with numerical simulations, identifying high amplitude peaks in the visible band and low-amplitude peaks in the near infrared one, related to the hybridized resonances. By choosing the appropriate size of the intermediate dielectric and metallic layers, as well as the near-field coupling, peaks can be tuned along the visible and near infrared bands, respectively. The chromatic coordinates of the addressed nanostructures obtained from the reflectance spectra show that the nanostructures hold color gamut that extends toward magenta, orange, yellowish, and reddish shades of colors.

Resonant-mode engineering for additive reflective structural colors with high brightness and high color purity

We present quad-layered reflective structural color filters generating vivid additive primary colors by controlling a mode number in a Fabry–Perot (FP) cavity and an anti-reflective (AR) coating layer, thus accomplishing high spectral contrast which is highly demanded in creating sharp colors. The reflection brightness of fabricated structural color filters is over 78% and a color gamut is comparable to the standard color gamut (sRGB). Higher-order resonant modes are exploited yielding a narrow passband with strong suppression of the reflection at shorter and longer wavelength ranges for a green color, while red and blue colors are produced by employing fundamental resonant modes. Besides, the structural color filters maintain both high brightness and high color purity at oblique incidence angles up to 40° due to a small angle of refraction by a cavity medium with high refractive index. Moreover, a large-scale fabrication is enabled owing to the simplicity of a device structure, where thin film deposition is used. The scheme presented in this work may open the door to a number of applications, such as reflective displays, imaging devices, colored photovoltaics, and decorations.

Solution‐Processed Disordered Plasmonic Surfaces as Optics for Infrared Imaging

AbstractIn recent years, thermal imaging and sensing technologies have seen dramatic increases in usage for a range of applications. However, the material cost and manufacturing complexity of infrared optics remain a major barrier toward their democratization. Here, a solution‐processed plasmonic reflective filter (PRF) is presented as a scalable, disordered, and low‐cost thermal infrared (TIR) optic. The PRF selectively absorbs sunlight and specularly reflects TIR wavelengths, with a performance comparable to state‐of‐the‐art infrared optics made of materials like Germanium. Unlike the latter, however, the PRF is fabricated using low‐cost materials and a “dip‐and‐dry” chemical synthesis technique, which enables orders of magnitude lower manufacturing costs. The PRF's optical functionality and integration into infrared imaging systems are experimentally demonstrated. The chemical synthesis technique also affords exceptional spectral tuneability and material compatibility compared to traditional fabrication methods. The PRF's tuneable and broadband TIR yield can be augmented by inexpensive dielectric or polymeric filters to yield novel capabilities such as wide‐area ambient temperature surveys. Practically, the PRF represents a significant advance toward democratizing the benefits of thermal imaging and sensing. Scientifically, it represents a previously unexplored optical functionality of disordered materials, and a new direction for versatile chemical synthesis in designing optical components.

Energy-efficient design for green indoor OWC-IoT systems using passive reflective filters and machine learning-assisted quality prediction

Energy-efficient design for green indoor OWC-IoT systems using passive reflective filters and machine learning-assisted quality prediction

Multispectral Narrowband Frustrated Total Internal Reflection Filter with Inclusions of Plasmonic Nanoparticles

A spatial-frequency thin-film filter with inclusions of nanoparticles operating in the visible range of the spectrum is investigated. The effect of nanoparticles embedded in the central and lateral layers of the frustrated total internal reflection filter on the spectral characteristics, considering the frequency dispersion, is investigated. It is shown that plasmonic effects cause the splitting of the filter bandwidth into a set of narrow-band spectral lines and the angular splitting of the incident beam into a set of output beams. It is demonstrated that due to the difference in the resonance conditions for s- and p-polarization waves, the spectral lines of transparency do not coincide, which indicates the possibility of using the filter as a polarizer.

StainedSweeper: Compact, Variable-Intensity Light-Attenuation Display with Sweeping Tunable Retarders.

Light Attenuation Displays (LADs) are a type of Optical See-Through Head-Mounted Display (OST-HMD) that present images by attenuating incoming light with a pixel-wise polarizing color filter. Although LADs can display images in bright environments, there is a trade-off between the number of Spatial Light Modulators (SLMs) and the color gamut and contrast that can be expressed, making it difficult to achieve both high-fidelity image display and a small form factor. To address this problem, we propose StainedSweeper, a LAD that achieves both the wide color gamut and the variable intensity with a single SLM. Our system synchronously controls a pixel-wise Digital Micromirror Device (DMD) and a nonpixel polarizing color filter to pass light when each pixel is the desired color. By sweeping this control at high speed, the human eye perceives images in a time-multiplexed, integrated manner. To achieve this, we develop the OST-HMD design using a reflective Solc filter as a polarized color filter and a color reproduction algorithm based on the optimization of the time-multiplexing matrix for the selected primary color filters. Our proof-of-concept prototype showed that our single SLM design can produce subtractive images with variable contrast and a wider color gamut than conventional LADs.

A New Interrogation System for FBG Sensors based on Midband Filter with Correction

In this paper, a midband interrogation system for Fiber Bragg Grating (FBG) sensors is presented and its numerical demonstration is reported. The midband interrogation system is based on a finite reflection filter with automatic correction. The filter is used to interrogate the FBG sensors. The strain FBG sensors were used to demonstrate the effectiveness of the proposed systems. The results demonstrate that the proposed system of interrogation has a great resolution (±1 pm) with good stability.

Vertically stacked quantum well diodes for multifunctional applications

Dual-functioning multiple quantum well (MQW) diodes can simultaneously transmit and receive information through visible light. Here, we report vertically stacked red, green, and blue (RGB) MQW diodes for light detection and display applications. Both blue and green MQW diodes are monolithically integrated with distributed Bragg reflector (DBR) filters to realize the separation of light. The versatile RGB MQW transmitter/receiver system not only creates full-color display but also effectively separates RGB light into various colors. These results open feasible routes to generate multifunctional device for the development of full-color display and light receiver.

2 µm band four-wavelength-switchable narrow linewidth fiber laser enabled by fs-laser direct-written polarization-dependent parallel fiber Bragg gratings

We propose and experimentally demonstrate a four-wavelength-switchable single-longitudinal-mode (SLM) narrow linewidth thulium–holmium co-doped fiber laser (THDFL) using two polarization-dependent parallel fiber Bragg gratings (PD-PFBGs). The PD-PFBGs, fabricated using femtosecond (fs) laser direct-writing technology in a standard single-mode fiber (SMF) via a point-by-point method, are used as a four-channel reflection filter. Two FBGs are inscribed in parallel in the fiber core along the axial direction and are uniquely positioned symmetrically on either side of the centerline. This configuration enables polarization-dependent multi-channel filtering capability, which further allows for polarization-control-based four-wavelength-switchable operations of the THDFL. SLM lasing is accomplished by utilizing a simple dual-ring sub-cavity filter. An exceptional output performance of the THDFL is achieved, including an optical signal-to-noise ratio (SNR) of >72 dB, maximum power and wavelength fluctuations of 0.350 dB and 0.024 nm, respectively, and a linewidth of <2 kHz, for all four single-wavelength operations lasing at ∼2000 nm. These performance indicators suggest that the THDFL can be applied in free-space optical communication, atmospheric monitoring, and Lidar.

Reflective Full‐Color Structural Colors with High Brightness and High Purity Based on Ge2Sb2Se4Te1 Chalcogenide Film

AbstractThis study proposes a general strategy for constructing an asymmetric Fabry–Perot structure based on an ultra‐thin composite absorber, Ni/Ge2Sb2Se4Te1, to produce reflective full‐color structural colors with high brightness and purity. The composite absorber effectively enhances the strong interference effect of thin film, which can significantly reduce the reflection bandwidth of the target band and the reflectivity of the non‐target band. Under the premise of optimizing the five‐layer base structure, full‐color structural colors can be tuned by only changing the thickness of the LaTiO3 layer. Using the ion‐assisted electron beam evaporation technique, a simple and efficient film deposition process, six color devices (namely red, orange, yellow, green, blue, and purple) are successfully prepared with reflection peaks exceeding 90%. The chromaticity coordinates of the proposed high‐purity red, green, and blue (RGB) samples are (0.554, 0.339), (0.280, 0.600), and (0.164, 0.075), respectively. These coordinates are fairly close to the standard RGB color coordinates used in liquid crystal displays. This device has a simple structure with a novel material combination and a low production cost, which makes it feasible for mass production using just one coating run process. It has excellent application potential in various fields such as micro‐nano displays, anti‐counterfeiting measures, reflective color filters, and decorations.

Investigation of angle-insensitive grating color filters at periods much smaller than the wavelength of incidence.

We designed and simulated one-dimensional (1D) and two-dimensional (2D) reflective grating color filters inside the aluminized polyethylene (PE) film. The filters have several advantages: high angle insensitivity (up to 45° for the 1D filter, 40° for the 2D filter), high reflectance at non-resonant wavelengths, deep resonance dips, and a large color gamut. Both structures are characterized by with their grating periods being much smaller than the wavelength of incidence. A grating modal analysis was utilized to reveal the physical mechanism behind such structures that exhibit angle-insensitive spectral responses which are favored in the fields of color display and packaging.

Reflective color filters based on SiO2/Cu multilayer stacks

Due to strong inter-band transitions, the optical response of copper exhibits a high light absorbance in the visible region of the spectrum, recovering blue wavelengths and reflecting a reddish color. In this contribution, copper is split into subwavelength thin films, which are separated by silicon dioxide dielectric layers to take advantage of photonic bandgaps, which subtract bands from the reflected light and, thus, give rise to a wide range of reflective colors, including blue. Using numerical simulations, we investigate the photonic bandgaps of SiO2/Cu stacks, enabled by Fabry–Pérot transmission resonances, which demonstrate the color tunability of these structures.

All-polarization-maintaining, efficiently wavelength-tunable, Er-doped mode-locked fiber laser with a compact reflective Lyot filter.

Wavelength tunable mode-locked fiber lasers have highly potential applications in precision spectroscopy, nonlinear microscopy and photonic sensing. Here, we present a compact and thermal-sensitive reflective Lyot filter and utilize it for all-polarization-maintaining efficiently wavelength-tunable Er-doped carbon-nanotube-mode-locked laser for the first time, to the best of our knowledge. The output wavelength of the laser can be tuned from 1544.46 nm to 1572.71 nm, with a wide tuning range of 28.25 nm, and a remarkable tuning efficiency of 0.589 nm/°C, when the angle-spliced fiber is only 8.2 cm and the free spectral range of the filter is 31.32 nm. Dual-wavelength mode-locking is also achieved at boundary temperatures when increasing the pump power. Due to its compact size and reflection configuration, the proposed reflective Lyot filter is promising for realizing highly efficient wavelength tuning and multiwavelength generation in all-polarization-maintaining fiber lasers where reflective filters are needed.

Dual-Function Meta-Grating Based on Tunable Fano Resonance for Reflective Filter and Sensor Applications.

Localized surface plasmon resonance (LSPR)-based sensors exhibit enormous potential in the areas of medical diagnosis, food safety regulation and environmental monitoring. However, the broadband spectral lineshape of LSPR hampers the observation of wavelength shifts in sensing processes, thus preventing its widespread applications in sensors. Here, we describe an improved plasmonic sensor based on Fano resonances between LSPR and the Rayleigh anomaly (RA) in a metal-insulator-metal (MIM) meta-grating, which is composed of silver nanoshell array, an isolation grating mask and a continuous gold film. The MIM configuration offers more freedom to control the optical properties of LSPR, RA and the Fano resonance between them. Strong couplings between LSPR and RA formed a series of narrowband reflection peaks (with a linewidth of ~20 nm in full width at half maximum (FWHM) and a reflectivity nearing 100%) within an LSPR-based broadband extinction window in the experiment, making the meta-grating promising for applications of high-efficiency reflective filters. A Fano resonance that is well optimized between LSPR and RA by carefully adjusting the angles of incident light can switch such a nano-device to an improved biological/chemical sensor with a figure of merit (FOM) larger than 57 and capability of detecting the local refractive index changes caused by the bonding of target molecules on the surface of the nano-device. The figure of merit of the hybrid sensor in the detection of target molecules is 6 and 15 times higher than that of the simple RA- and LSPR-based sensors, respectively.

Bio‐Inspired Homologous Magnesium‐Based Reflective Filters with Angular Heterochromatic Structural Colors

AbstractThe existing color filter implementation schemes are prone to introduce prestress during the preparation process due to the large difference in thermal expansion coefficients of different layers of materials, resulting in poor mechanical stability. Inspired by beetle forewings’ structure, it is proposed to construct asymmetric Fabry–Perot (F‐P) angular heterochromatic structural colors utilizing homologous materials through a continuous deposition method of magnetron sputtering technology. The fabricated large‐area F‐P‐type reflective filters not only exhibit high color saturation but also achieve good bonding ability between the layers with a steady‐state friction coefficient of ≈0.1. The large‐area reflective filter can achieve different colors such as blue, yellow, violet, and green, which is in agreement with the simulation results. This work not only provides a new design method for advanced decorative applications but also provides protection to the substrate.

Synthesis of High-Selectivity Ultra-Wideband Reflective/Absorptive Bandpass Filters With Unequal-Width Three-Coupled Line Sections

In this brief, the unequal-width three-coupled line section is adopted for the design of miniaturized high-selectivity ultra-wideband reflective and absorptive bandpass filters. These bandpass filters can achieve wider bandwidths and higher stopband rejection performances, benefited from the cascaded transmission lines and the short-circuited stubs. With the introduction of the unequal-width three-coupled line sections, extra transmission poles located at the cut-off frequency are generated, resulting in high skirt selectivity. Besides, a grounded resistor is introduced to eliminate out-of-band reflections. The design equations and the exact synthesis procedures of these BPFs are presented and explained. For demonstration, three bandpass filters operating at 3 GHz are design, simulated, and measured, showing an excellent agreement between measurements and the predicted results.

Trans-Reflective Structural Color Filters Assisting Multifunctional-Integrated Semitransparent Photovoltaic Window.

Multifunction-integrated semitransparent organic photovoltaic cells (STOPVs), with high power generation, colorful transmittance/reflectance, excellent UV protection, and thermal insulation, are fully in line with the concept of architectural aesthetics and photoprotection characteristics for building-integrated photovoltaic-window. For the indelible rainbow color photovoltaic window, one crucial issue is to realize the integration of these photons- and photoelectric-related multifunction. Herein, dynamic transmissive and reflective structural color controllable filters, with asymmetrical metal-insulator-metal configurations (20nm-Ag-HATCN-30nm-Ag) through machine learning, were deliberately designed for colorful STOPV devices. This endows the resultant integrated devices with ∼5% enhanced power conversion efficiency than the bare-STOPVs, gifted UV (300-400nm) blocking rates as high as 93.5, 94.1, 90.2, and 94.5%, as well as a superior IR (700-1400nm) rejection approaching 100% for transparent purple-, blue-, green- and red-STOPV cells, respectively. Most importantly, benefiting from the photonic recycling effect beyond microcavity resonance wavelength, a reported quantum utilization efficiency as high as 80%, was first presented for the transparent-green-STOPVs with an ultra-narrow bandgap of 1.2eV. These asymmetrical Febry-Pérot transmissive and reflective structural color filters can also be extended to silicon- and perovskite-based optoelectric devices and make it possible to integrate additional target optical functions for multi-purpose optoelectric devices. This article is protected by copyright. All rights reserved.

Design and Analysis of Broadband Power Delivery Network Noise Absorber for Parallel-Plate Mode Suppression

In this article, a broadband power delivery network (PDN) noise absorber with its 90%-absorption band from 4 to 7.5 GHz is proposed to suppress parallel-plate modes. Due to the ever-increasing switching speed of integrated circuits (ICs) nowadays, a few millimeters separating a chip and the conventional reflective filters may induce significant power noises and cause serious radio frequency interference (RFI) and electromagnetic interference (EMI) problems. In this scenario, PDN noise absorbers can prevent power noises from both reflecting and transmitting, thus avoiding cavity resonances, providing a clean and stable power supply, and achieving almost zero radiation. Four cases were compared to verify all these advantages of the PDN noise absorbers. For conducted noises around 6 GHz, the self-impedance peak is suppressed by 86.9% at the noise source end and the insertion loss is almost the same level at the transmitted end when compared with the case of two rings of the reflective filters. Furthermore, almost 100% of the radiative noises are cleared away by the proposed PDN noise absorbers.

Optical Characterization of a New Facility for Materials Testing under Concentrated Wavelength-Filtered Solar Radiation Fluxes

The materials used to manufacture solar receivers for tower power plants must withstand high fluxes of concentrated solar radiation (from 0.1 to even 1.5 MWm−2) and operate at high operating temperatures (>800 °C). Durability is a key aspect in these systems, which must be ensured under these demanding operating conditions, which also include daily heating–cooling cycles throughout the lifetime of these power plants. So far, to the authors’ knowledge, which wavelengths of concentrated solar radiation have the greatest influence on the mechanisms and speed of aging of materials used in solar receivers has not been analyzed. Yet, such an analysis is pertinent in order to implement strategies that delay or inhibit such phenomena, and, thus, increase the durability of central tower systems’ receivers. To perform such analyses, a new solar furnace was recently designed and installed at the Plataforma de Almería (Spain). This paper describes the components of this new solar furnace. The components are as follows: a heliostat to redirect the direct solar radiation towards a Fresnel lens that concentrates the solar radiation on the material under study, a shutter that allows varying the amount of concentrated solar radiation incident on the Fresnel lens, and reflective filters with selective reflectance that are placed between the Fresnel lens and the material. This paper also describes the procedure and the first results of the energetic and spectral characterization of this new solar furnace. The first experimental results of the characterization of this new test bed using the heliostat and the Fresnel lens showed that concentration ratios of up to 1000 suns (1 sun = 1000 Wm−2) could be achieved. Furthermore, the paper presents the results of the spectral characterization of the test system, using selective reflectance mirrors in the near-visible–IR wavelength range (400–1125 nm) and in the visible–IR red region (700–2500 nm).

Design Compact Absorptive Common-Mode Noise Suppression Filter with Series Unified Circuit

In the PCB process, overcoming common-mode noise radiation is critical. In past years, most studies have focused on a common-mode noise filter (CMNF) that can solve electromagnetic interference in high-speed digital systems by blocking and absorbing common-mode noise radiation. Unfortunately, connecting with any reflective common-mode noise filter (R-CMNF) and reducing the area of an absorptive common-mode noise filter (A-CMNF) are the most troublesome tasks in the PCB process. A novel equivalent circuit is proposed in this research to minimize the complexity of the design and improve accuracy. Detailed analyses of this proposed approach are entirely depicted in this article. The experiment result shows that 9% of fractional bandwidth centered at 2.25 Hz can achieve at least 90% absorption efficiency. With our proposed method, the area of A-CMNF is smaller than in state-of-the-art research.