Absorption Filter Research Articles

Tunable narrowband shortwave-infrared absorber made of a nanodisk-based metasurface and a phase-change material Ge2Sb2Te5 layer.

A tunable absorber made of a nanodisk-based metasurface is proposed to realize a narrowband shortwave-infrared (SWIR) perfect absorption. By introducing a phase-change material Ge2Sb2Te5 (GST) layer, we produce a selective and active control of the optical response. It is found that the narrowband absorption of 99.9% can be achieved for amorphous GST (aGST) with a modulation depth of 54.6% at 1931 nm, which is attributed to the strong electric dipole resonance in the germanium nanodisks. Moreover, under the aGST state, the full width at half-maximum of 22 nm can be acquired for a normal TM-polarized wave, and such a nanodisk-based absorber enables a tunable operating wavelength by adjusting the geometrical parameters to realize the spectral selectivity. In addition, the nanodisk-based metasurface nanostructure, combined with a dielectric Bragg reflector with alternately stacked SiO2 and TiO2 layers, can realize the SWIR dual-band absorption for aGST and single-band absorption for crystalline GST through the adjustment of electric and magnetic resonances. The designed absorbers have the potential applications in tunable absorption filter, thermal sensing, and optical signal processing.

Effects of Aspect Ratio Heterogeneity of an Assembly of Gold Nanorod on Localized Surface Plasmon Resonance

We examine the effects of aspect ratio (AR) heterogeneity of an assembly of gold nanorods (GNRs) in a colloid on the total cross-section for its light scattering via localized surface plasmons at visible wavelengths. We observe the extraordinary broadening of the extinction spectrum of light through an assembly of GNRs, a colloidal mixture of those having two different ARs. The interparticle distance estimated as ∼1.2-1.3 μm, being greater than the incident wavelength, allows the radiative dipolar coupling to govern the long-range interaction between GNRs. We find that the coupling enhanced local fields can activate the nonresonant polarization of GNRs to turn into a quasi-resonant one. These higher-order effects for GNR polarization can produce the deviation of total cross-section of GNRs assembly beyond the simple sum of an individual cross-section of GNRs that are assumed to have no such long-range coupling. The extraordinary properties of the extinction spectrum need to be taken into account for modulating the spectral distribution of electromagnetic field in photonic devices where an assembly of GNRs is utilized for field enhancement such as those for surface-enhanced spectroscopy, highly efficient photovoltaics, photothermal nanotherapy, and ultrathin absorption filters.

Ultra-multiband terahertz perfect absorber via a metal groove structure

An ultra-multiband terahertz absorber formed by a periodic metallic groove array filled with high-dielectric-constant material is presented. Simulation results show that since multiple Fabry–Pérot cavity modes are excited in the metal groove, the absorber can obtain 15 narrow-band absorption peaks with an average absorptivity over 99.5% at normal incidence. At oblique incidence, all absorption bands still maintain the high absorption above 90% for angles up to 50°. Due to its structural symmetry, the absorber is polarization insensitive. The frequencies and number of the absorption bands can be controlled by turning the depth of the metal groove. In addition, the proposed absorber can be operated as a highly sensitive refractive index sensor. Such an ultra-multiband, polarization-insensitive, and wide-angle absorber structure can be applied in terahertz-related absorption filters, biochemical sensors, or thermal images.

The Impact of Deterministic Signal Loss on OCT Angiography Measurements.

PurposeIn this study, we aimed at investigating the impact of deterministic signal loss on image quality and, thus, on optical coherence tomography angiography (OCTA) measurements performed by the RTVue-XR Avanti System.MethodsAbsorptive filters with different optical densities (ODs) were used to simulate signal loss in a controlled way in 30 eyes from 15 healthy subjects. Scan Quality (SQ), provided by the AngioVue software, was applied as a measure of image quality.ResultsAssessing the effect of decreased light transmittance on SQ values as well as that of attenuated image quality on OCTA parameters revealed a strong systematic dependence between these quantities. Attenuated image quality was associated with significantly decreased macular and peripapillary vessel density (VD) values, and we calculated a correction factor of 2.27% to 3.97% for a one-unit change in SQ for the different VD parameters.ConclusionsOur findings suggest that the influence of systematic changes in image quality on OCTA parameters needs to be considered during patient follow-up in order to make valid assessment of progression.Translational RelevanceFor accurate evaluation of longitudinal changes in OCTA parameters, equal scan quality or using a correction factor is suggested.

Needle-Type Imager Sensor With Band-Pass Composite Emission Filter and Parallel Fiber-Coupled Laser Excitation

This paper presents an implantable fluorescence system with a composite emission filter and fiber-coupled laser excitation. The composite structure of the short-pass interference filter and absorption filters exhibited a band-pass spectrum between 510 and 570 nm, which is close to green fluorescent protein (GFP) emission. A high-quality excitation light was achieved by utilizing a blue laser through a low-numerical-aperture optical fiber. This coupling method is beneficial in delivering a narrow spectrum and controllable irradiation light in a specific area to minimize auto-fluorescence from the tissue. The fabricated lensless system performance is experimentally validated by imaging emission. The proposed device is capable of perceiving fluorescence emission from microspheres and GFP noticeably.

Multifunctional volumetric meta-optics for color and polarization image sensors

Three-dimensional elements, with refractive index distribution structured at subwavelength scale, provide an expansive optical design space that can be harnessed for demonstrating multifunctional free-space optical devices. Here we present 3D dielectric elements, designed to be placed on top of the pixels of image sensors, that sort and focus light based on its color and polarization with efficiency significantly surpassing 2D absorptive and diffractive filters. The devices are designed via iterative gradient-based optimization to account for multiple target functions while ensuring compatibility with existing nanofabrication processes, and they are experimentally validated using a scaled device that operates at microwave frequencies. This approach combines arbitrary functions into a single compact element, even where there is no known equivalent in bulk optics, enabling novel integrated photonic applications.

Innovative Light Therapy: 1. Biological Effectiveness of Polychromatic Polarized Light Transmitted through Interference, Absorption and Fullerene Filters

Innovative Light Therapy: 1. Biological Effectiveness of Polychromatic Polarized Light Transmitted through Interference, Absorption and Fullerene Filters

Dual-Wavelength High-Spectral-Resolution Lidar for Profiling Optical Properties of Aerosol and Cloud

A dual-wavelength high-spectral-resolution lidar (HSRL) based on an iodine absorption filter and a field-widened Michelson interferometer (FWMI) has been developed to profile backscatter and extinction coefficients of aerosols and clouds accurately. This instrument was tested and calibrated on multiple observations in Hangzhou and Zhoushan, respectively, from August 2018 to April 2019. This paper discusses the design and the internal calibration method of the lidar system in detail, with several typical cases of observations and the analysis of these data products. The optical properties of urban aerosols in Hangzhou and the evolvement of clouds in Zhoushan are presented, respectively.

A Novel Circuit Architecture of Bidirectional Common-Mode Noise Absorption Circuit

Techniques of common-mode (CM) noise absorption have been developed in recent years, but very few of them can achieve CM bidirectional absorption. This article proposed a novel circuit architecture that can provide bidirectional CM noise absorption theoretically, meaning that the circuit can absorb CM noises from both sides of differential channels. The essential concept is to design the even–even and even–odd modes of the proposed circuit individually, and by doing so, the bidirectional absorption can be easily realized in a mathematically sound way. In this article, analytical design equations are derived completely, and synthesis procedure is listed in detail. Then, two bidirectional absorptive CM filters are implemented based on the proposed circuit. One of them is fabricated by the printed circuit board process and the other one is fabricated by the integrated passive device process. These two implementations reveal the flexibility of the proposed circuit. Finally, measured results are given as verification, and a comparison table with previous works is demonstrated.

Absorptive angular-selective filters consisting of dielectric multilayers combined with thin absorbing layers.

We propose a concept of angular-selective filters (ASFs) that transmit the incident light in the transmitting angular ranges and mostly absorb in the blocking ranges. The absorptive ASF consists of a transparent dielectric multilayer combined with a thin absorbing layer, utilizing the change in the fringe pattern of the light intensity in the filter dependent on the incident angle. We clarified the design guide to select the suitable multilayer structure and absorbing material. Concrete structures of three types were designed using Cr for the absorbing material on the basis of ${\text{SiO}_2}/{\text{TiO}_2}$SiO2/TiO2 bandpass, shortpass, and longpass filters, exhibiting the absorptive ASF functions with the transmitting/blocking ranges of 0-10°/10-30°, 0-30°/30-60°, and 10-60°/0-10°, respectively. The present multilayered structures provide a great advantage for practical use: no need for two- or three-dimensional patterning using lithographic techniques.

Microbolometer with a multi-aperture polymer thin-film array for neural-network-basedtarget identification.

Infrared imaging spectrometers are frequently used for detecting chemicals at standoff distances. Cost, size, and sensitivity are common tradeoffs in this regime, particularly when deploying infrared imaging arrays. In this work, we develop and characterize an infrared snapshot computational imaging spectrometer that leverages a multi-aperture filtered design. A theoretical model is developed, describing the multiplexed encoding technique. The experimental system is then described, including filter optimization and fabrication. Finally, the performance of the system is tested, leveraging a neural-network-based calibration approach, for various indoor and outdoor detection scenarios involving liquid contaminants. The results of our testing demonstrate that the system can detect room-temperature liquid contaminants under cold sky downwelling radiance conditions. We achieve a false positive rate (FPR) of 0.12% at a true positive rate (TPR) of 95% for silicon oil on sand at 18°C and a FPR of 2% at a TPR of 95% for silicon oil on various substrates at 23°C. Results support the efficacy of using uncooled polymer absorption filters for infrared imaging liquid contaminant detectors.

Rainbow Trapping with Long Oscillation Lifetimes in Gradient Magnetoinductive Metasurfaces

We report a gradient metasurface design at microwave bands as an elegant approach to realize the goal of “rainbow trapping” for the storage of waves involving wave localization and absorption phenomena. A longitudinally placed coplanar waveguide is loaded with gradient metasurfaces on both sides, where split-ring resonators (SRRs) are the basic cell. The same SRRs are arranged along the transverse direction to establish magnetoinductive channels. Waves of different frequencies are coupled to corresponding SRRs at different positions in metasurfaces. Resonant trapping with a long oscillation life time enhances the absorption caused by inherent losses of the materials, thereby suppressing reflections. Both simulations and measurements verify the existence of “rainbow trapping.” The proposed strategy enhances the interaction between waves and matter, opening an avenue for further component designs, including absorptive filters, multiplexers, and buffers.

Higher order lumped element absorptive low‐pass and bandpass filter structures

Here, the authors present a class of lumped-element low-pass and bandpass absorptive filter circuits. To begin with, the authors discuss a new type of absorptive low-pass filter (LPF) structure that has an excellent impedance matching performance in both passband and stopband. This work provides low-pass prototype values for the proposed absorptive LPFs in terms of those for the conventional reflective filters, namely g -parameters, so that the new type of absorptive LPF can be designed in a straightforward manner. In addition, it is shown that traditional low-pass-to-bandpass frequency transformations can be applied to the absorptive low-pass prototype cases in the design of absorptive bandpass filters (BPFs). Second- and third-order LPF and BPFs are designed and measured to verify the new type of absorptive filter and its design method.

Synthesized Method of Dual-Band Common-Mode Noise Absorption Circuits

This paper proposed a method of synthesizing absorptive common-mode filters (A-CMFs) analytically with high absorption efficiency and two designable absorption bands. The proposed method is carried out by a novel circuit architecture and its analysis procedure. In differential mode, the integrity of the differential signals is maintained, and in common mode (CM), two absorption frequency bands can be obtained theoretically. These bands are designable with the analytical equations derived in this paper, so a dual-band or broadband A-CMF design can be easily synthesized by our proposed method. The authors also show that this method can make any one of R-CMFs “upgrade” to an A-CMF by simply adding an additional circuit, and this might be a revolutionary progress for the currently used CM-suppression techniques. To make the proposed concept more understandable, an example is used to demonstrate how the synthesizing methodology works, where a broadband A-CMF with its absorption bands located at 2.4 and 3.6 GHz is designed and implemented. Furthermore, a test sample of this A-CMF is fabricated on a four-layer printed circuit board. It is measured on a probing station, and the measured results are very similar to the simulated results. Finally, a comparison table of all the related works is given, showing that this proposed method really makes a significant improvement for A-CMF techniques. It is worth noting that this circuit design can provide a broadest 95%-absorption band to our best knowledge.

Performance estimation of space-borne high-spectral-resolution lidar for cloud and aerosol optical properties at 532 nm.

Cloud and aerosol contribute with great uncertainty in Earth's radiative budget. There is an urgent need for global 3-D observation of these atmospheric constituents. High-spectral-resolution Lidar (HSRL) can obtain vertical atmosphere profile with high accuracy, hence several space-borne HSRLs are planned to launch in few years. However, as far as we know, the performance evaluation of space-borne HSRL has not been reported yet. In this paper, we present the characteristics of a new designed space-borne HSRL for aerosol and cloud optical property profiling (ACHSRL), which is part of the Aerosol & Carbon Detection Lidar (ACDL) developed in China. The ACHSRL is essentially similar to the famous Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), which is on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). Moreover, the ACHSRL employs an iodine absorption filter as the spectral discriminator. The atmospheric optical properties data observed by CALIOP is used to estimate the performance of ACHSRL. We chose the level 2 profile data (version 4.10) in South Japan in June 2015 to compare the detection uncertainty of ACHSRL and CALIOP. The simulation calculates the uncertainties of ACHSRL and makes a statistic analysis. The analysis result demonstrates that 73.63% of the backscatter coefficient uncertainties are below 40% for ACHSRL. By contrast, the number is 30.72% for CALIOP. As for absolute extinction coefficient errors, the statistics shows that 76.01% of the extinction coefficient uncertainties are lower than 0.2 km-1 for ACHSRL, while that for CALIOP are 56.97%. The assessment shows that ACHSRL could measure the particulate optical properties with better accuracy and compared with CALIOP. The estimation in this study reveals that the next generation space-borne HSRLs have a promising future.

High-Sensitivity Color Imaging Using Pixel-Scale Color Splitters Based on Dielectric Metasurfaces

Conventional color image sensors employing absorptive color filters exhibit low overall light transmission, resulting in limited signal levels per sensor pixel. This issue is becoming critical beca...

Wide field-of-view lensless fluorescence imaging device with hybrid bandpass emission filter

We demonstrate a highly sensitive lensless fluorescence imaging device with a wide field-of-view by using a hybrid bandpass filter composed of interference filters, an absorption filter, and a fiber optic plate. The hybrid filter shows high excitation light rejection characteristics even in a lensless setup. In this study, we fabricated a hybrid bandpass filter and improved fluorescence observation performance for a target with auto-fluorescence. The filter was combined with a large image sensor with an imaging area of 67 mm2. As a demonstration, a brain slice from a green fluorescent protein transgenic mouse was observed and fluorescent cell bodies were detected with the lensless imaging device.

Stochastic perturbation of sub-pico second envelope solitons for Triki–Biswas equation with multi-photon absorption and bandpass filters

This paper studies sub-pico second solitons modeled by Triki–Biswas equation that is a generalized version of Kaup–Newell equation in presence of stochastic perturbation terms, in addition to deterministic perturbations which are due to multi-photon absorption and bandpass filters. The soliton mean free velocity is recovered after analyzing the corresponding Langevin equation.

Compact Wideband Reflective/Absorptive Bandstop Filter With Multitransmission Zeros

Novel wideband reflective bandstop filters (RBSFs) and absorptive bandstop filters (ABSFs) with high stopband rejection (SR) and good frequency selectivity are proposed. Based on a coupled line (CL) with an open-circuited stub, a broad stopband response with three transmission zeros (TZs) is implemented in a compact circuit configuration. Two modified CL structures are loaded at the ports resulting in two additional TZs. In addition, a grounded resistor is introduced to absorb unwanted signals in the stopband. For demonstration, the five-zero RBSFs and three-zero ABSFs operating at 2 GHz are designed and built. The reflective filter shows a 35-dB SR with 79.5% relative bandwidth (RB). The absorptive filter with 69.5% RB of 24-dB SR achieves an all-passband 12.2-dB return loss. Good agreements between simulated and measured results are observed.

Comparison of dichroric with absorptive filters when using alternate light sources to enhance bruises

Background: Using alternate light sources requires a filter to block the illuminating source to allow fluoresced light (with longer wavelength) to be observed.1 Absorptive filters are dye based. Dichroic filters use thin-film interference. It was proposed their selectivity could be beneficial for use with alternate light sources.