Transmission Filters Research Articles

Structural color filters with compensated angle-dependent shifts.

Structural color filters use nano-sized elements to selectively transmit incident light, offering a scalable, economical, and environmentally friendly alternative to traditional pigment- and dye-based color filters. However, their structural nature makes their optical response prone to spectral shifts whenever the angle of incidence varies. We address this issue by introducing a conformal VO2 layer onto bare aluminum structural color filters. The insulator-metal transition of VO2 compensated the spectral shift of the filter's transmission at a 15° tilt with 80% efficiency. Unlike solutions that require adjustment of the filter's geometry, this method is versatile and suitable also for existing structural filters. Our findings also establish tunable materials in general as a possible solution for angle-dependent spectral shifts.

A Global-Local Error-Driven -Adaptive Scheme for Discontinuous Galerkin Time-Domain Method With Local Time-Stepping Strategy

In this paper, a highly efficient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -adaptive scheme has been developed for discontinuous Galerkin time domain (DGTD) method with local time stepping (LTS) strategy. In the proposed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -adaptive procedure, the dual-fold error driven scheme has been developed. Compared to the global error range corresponding to the preset base order range, the local maximal error in each discretized element is used to determine the allowable local range of the base order. The local error in each element is compared with the local error range to determine which operation, i.e., the base order increasing or decreasing, is implemented. The resulting two error thresholds are insensitive in the dynamic <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -adaptive procedure. When applying the proposed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -adaptive scheme into the LTS method, a simple grouping approach in terms of the time steps related to the base orders has been developed to rapidly perform the marching-on-in time procedure. Several numerical examples including scattering from a missile, radiation of a ridge gap waveguide (RGW) antenna and transmission of a cylindrical cavity filter are given to demonstrate good accuracy and high efficiency of the proposed method.

Study of rib protection devices based on resistant muffler theory for transcostal focused ultrasound therapy

High-intensity focused ultrasound (HIFU) is one of the effective means of treating cancer. One of the current challenges in HIFU treatment is how to minimize thermal damage to the rib during transcostal HIFU treatment and ensure that sufficient energy is delivered to the target area. In this paper, a sound insulation device mounted in front of the rib is proposed to address this challenge. The device is designed based on resistant anechoic theory, combining both reflective and resonant sound energy-blocking mechanisms to enhance the transmission loss and filter bandwidth. The structural parameters of the device are optimized using an orthogonal test method. A finite element method was used to simulate the sound and temperature fields of the sound insulation device with and without different rib spacings. The simulation results demonstrate a significant decrease in sound pressure at the ribs under the protection of the sound insulation device, with no significant difference in sound pressure at the target area. Consequently, the temperature at the ribs decreases, while the temperature at the target area tissue is guaranteed. Furthermore, transcostal HIFU experiments are conducted using real pig ribs and the experimental results validate the accuracy of the simulation results.

Utility of Spectral Filtering to Improve the Reliability of Marine Fauna Detections from Drone-Based Monitoring.

Monitoring marine fauna is essential for mitigating the effects of disturbances in the marine environment, as well as reducing the risk of negative interactions between humans and marine life. Drone-based aerial surveys have become popular for detecting and estimating the abundance of large marine fauna. However, sightability errors, which affect detection reliability, are still apparent. This study tested the utility of spectral filtering for improving the reliability of marine fauna detections from drone-based monitoring. A series of drone-based survey flights were conducted using three identical RGB (red-green-blue channel) cameras with treatments: (i) control (RGB), (ii) spectrally filtered with a narrow 'green' bandpass filter (transmission between 525 and 550 nm), and, (iii) spectrally filtered with a polarising filter. Video data from nine flights comprising dolphin groups were analysed using a machine learning approach, whereby ground-truth detections were manually created and compared to AI-generated detections. The results showed that spectral filtering decreased the reliability of detecting submerged fauna compared to standard unfiltered RGB cameras. Although the majority of visible contrast between a submerged marine animal and surrounding seawater (in our study, sites along coastal beaches in eastern Australia) is known to occur between 515-554 nm, isolating the colour input to an RGB sensor does not improve detection reliability due to a decrease in the signal to noise ratio, which affects the reliability of detections.

Differential magnetic probe based on balanced filter with high electric field suppression

AbstractA differential magnetic field probe based on a balanced filter is proposed to improve electric field suppression. The probe comprises a rectangular metal loop for detection and a balanced filter transmission. Three 1/4λ stepped impedance transmission lines are designed between the detective part and balanced filter for impedance matching. The balanced filter can suppress the common mode signals generated by a normal electric field but has little influence on the differential signals detected by a tangential magnetic field thus, the electric field suppression of the magnetic probe can be enhanced. The measurement results show that the working frequency of the proposed magnetic probe ranges from 10 to 20 GHz, and the electric field suppression ratio is over 35 dB from 10 to 15 GHz.

Low-Loss Si-Based Dielectrics for High Frequency Components of Superconducting Detectors

Silicon-based dielectric is crucial for many superconducting devices, including high-frequency transmission lines, filters, and resonators. Defects and contaminants in the amorphous dielectric and at the interfaces between the dielectric and metal layers can cause microwave losses and degrade device performance. Optimization of the dielectric fabrication, device structure, and surface morphology can help mitigate this problem. We present the fabrication of silicon oxide and nitride thin film dielectrics. We then characterized them using Scanning Electron Microscopy, Atomic Force Microscopy, and spectrophotometry techniques. The samples were synthesized using various deposition methods, including Plasma-Enhanced Chemical Vapor Deposition and magnetron sputtering. The film's morphology and structure were modified by adjusting the deposition pressure and gas flow. The resulting films were used in superconducting resonant systems consisting of planar inductors and capacitors. Measurements of the resonator properties, including their quality factor, were performed.

Frequency-encoded two-photon excited fluorescence microscopy.

Two-photon excited fluorescence (2PEF) microscopy is the most popular non-linear imaging method of biomedical samples. State-of-the art 2PEF microscopes use multiple detectors and spectral filter sets to discriminate different fluorophores based on their distinct emission behavior (emission discrimination). One drawback of 2PEF is that fluorescence photons outside the filter transmission range are inherently lost, thereby reducing the imaging efficiency and speed. Furthermore, emission discrimination of different fluorophores may fail if their emission profiles are too similar. Here, we present an alternative 2PEF method that discriminates fluorophores based on their excitation spectra (excitation discrimination). For excitation we use two lasers of different wavelengths (ω1, ω2) resulting in excitation energies at 2ω1, 2ω2, and the mixing energy ω1+ω2. Both lasers are frequency encoded (FE) by an intensity modulation at distinct frequencies while all 2PEF emission is collected on a single detector. The signal is fed into a lock-in-amplifier and demodulated at various frequencies simultaneously. A customized nonnegative matrix factorization (NNMF) then generates fluorescence images that are free of cross talk. Combining FE-2PEF with multiple detectors has the potential to enable the simultaneous imaging of an unprecedented number of fluorophores.

Integration of tunable solar spectral directional absorption and switchable near-infrared transmission filters based on phase-change materials

In this work, we propose a one-dimensional layered structure to realize the integration of various applications of solar energy spectrum based on phase-change materials (PCMs). Through the effective combination of PCMs and the designed structure, a tunable solar spectral directional absorption and switchable near-infrared band sensor can be simultaneously obtained on one sample. The research provides creative possibility for the applications of angle and polarization information of solar spectrum and narrowband filtering sensing.

Mid-infrared plasmonic filter arrays using nanoholes in gold and silver films.

Plasmonic filters based on subwavelength nanohole arrays are an attractive solution for creating arrays of filters with varying passbands in a single lithography step. In this work, we have developed a fabrication method which allows fabrication of nanohole arrays in silver by use of a thin layer of aluminum oxide, which serves the dual purpose of both capping layer and hardmask for metal patterning. We demonstrate arrays of gold and silver mid-infrared plasmonic filters, fabricated on silicon, intended for use in optical filter blocks or for future integration with infrared imagers. The filter arrays are designed for the wavelength range 2-7 µm, and exhibit peak filter transmission efficiencies around 70%.

Advanced control of an acousto-optic filter performance through arbitrary RF signals.

The proposed study consists of using the capabilities of an arbitrary waveform generator to create a specific radio frequency (RF) signal to be applied to an acousto-optic tunable filter based on quasicollinear interaction. By controlling the duration and shape of the acoustic pulse, the filter transmission function can be changed in terms of selectivity and side-lobe levels. The measurement results show good agreement with the simulation model based on the fast Fourier transform. In particular, the Hamming window has been found to be a good option for suppressing side lobes in the transmission function, even when using high acoustic powers.

Dual-broadband high-efficiency polarization conversion metasurface based on asymmetric transmission

A dual-broadband and high-efficiency polarization conversion metasurface based on asymmetric transmission is proposed, which is composed of two orthogonal metal gratings, metal resonator and FR-4 dielectric spacer. The simulated results show that the designed metasurface can convert x-polarized incident waves into y-polarized waves in the frequency ranges of 4.64–14.37 GHz and 17.02–37.23 GHz, and the corresponding polarization conversion ratio (PCR) exceeds 90%. The relative bandwidths of the two bands are 102.37% and 74.51%, respectively. The influence of structural parameters on the transmissive polarization conversion performance is studied. The bandwidths of the two bands can be easily modulated by varying the structural parameters of l, n, m and w. The excellent performance of dual-broadband and high-efficiency covers C, X, Ku, K and Ka bands, which can also be well maintained in the oblique incidence range of 0°–50°. Moreover, the mechanism of transmissive polarization conversion is analyzed. The prototype is fabricated and the experimental results are basically consistent with the numerical simulation. The proposed metasurface has potential application value in polarization imaging radar or radiometer, radar, transmission array antennas and filter.

THz transmissive all-dielectric guided-mode resonance filter with high Q and tunability using suspended gratings.

Most currently available THz narrowband filters employ metal that introduces loss, or work in reflection mode, which limits their scope of application. Here, a transmissive all-dielectric guided-mode resonance filter in the THz region is presented. It contains a suspended grating layer and a waveguide layer, separated by an air layer. A fabrication process of the filter is proposed. Simulation results show that the designed filter exhibits excellent transmittance of ∼97.5% with a high Q value of ∼1500 at 1.64THz. Furthermore, this transmission peak is surrounded by a wide and flat sideband with width of ∼0.75T H z and transmission below 10%. Moreover, tunability of the filter is realized by geometric scaling and by varying the thickness of the air layer. Using geometric scaling, the filtering frequency can be widely tuned from 0.54 to 1.64THz, covering the 625-725 and 780-910GHz wireless communication windows. Additionally, fine tuning achieved by varying the air layer thickness could be used to compensate for a tiny shift of the designed filtering frequency caused by errors introduced in the fabrication process. The Q value can be further boosted to ∼11,500 by adding another layer of waveguide. Due to its transmissive nature and high-Q resonant mode with a wide sideband and tunability, the presented filter exhibits great potential in THz applications such as spectroscopy, imaging, and communication.

Design of non-Gaussian multispectral shortwave infrared filters assessed by surface spectral reflectances on the ECOSTRESS library.

This paper addresses the multispectral filter design problem for spectral ranges where a viewing subspace is not defined. The methodology of color filter design is extended to this case, which allows the optimization of custom filter transmittance that meets the physical constraints of available fabrication methods. Multispectral shortwave infrared filters are then designed for two scenarios: spectral reconstruction and false-color representation. The Monte Carlo method is used to verify the filter performance degradation due to deviations in fabrication. The results obtained indicate that the proposed method is useful for designing multispectral filters to be fabricated using generic processes without any additional constraints.

광섬유 빗살 필터의 사분파장 지연기 연속 조합을 이용한 협대역 다파장 스펙트럼의 연속 주파수 조정에 관한 연구

We propose an optical fiber comb filter based on a polarization-diversity loop structure, which can continuously adjust the narrowband multiwavelength spectrum. The proposed comb filter consists of a polarization beam splitter(PBS), a successive combination of quarter-wave retarders(QWRs), a set of a half-wave retarder and a QWR, and two polarization-maintaining fiber(PMF) segments of equal length. For the convenient adjustment of the narrowband multiwavelength spectrum, the successive combination of QWRs was located between the PBS and the first PMF segment. The transmittance function of the proposed filter was theoretically derived using the Jones calculus. Based on the filter transmittance, we theoretically predicted the orientation angle sets of four wave retarders, which allow the extra phase difference φ of the transmittance function to vary from 0˚ to 315˚ with increments of 45˚. Then, the proposed filter successfully demonstrated the predicted results by obtaining ∼0.0915 ㎚ average wavelength shift and a high linearity of 0.99878.

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.

All-dielectric metasurface-based color filter in CMOS image sensor

The development of all-dielectric metasurfaces vigorously prompts the applications of optical metasurfaces for a CMOS (Complementary Metal-oxide-semiconductor) image sensor. Dielectric nanostructures, due to their resonant behavior and its tunability, offer the possibility to be assembled into flexible and miniature spectral filters, which could potentially replace conventional pigmented and dye-based color filters. In this paper, a transmission filter based on a GMR (Guided Mode Resonance) mode of all-dielectric metasurface is adopted as an example to study the effect of application scenario of integrated CMOS image sensor chip and manufacturing mistakes, such as the small size of pixel, special dielectric environment around, surface roughness and structural defects. Numerical calculations based on the FDTD (finite difference time domain) method are used to study the relationship between the robustness of the metasurfaces and the optical resonant modes. More important, a method simplifying different manufacturing mistakes as basic geometry structure during the FDTD simulation is also proposed and demonstrated. This work deepens our understanding of the working mechanism of all-dielectric metasurfaces and paves the way for their use in CMOS image sensor.

Far ultraviolet mirrors for aurora imaging: design and fabrication.

The emission lines of 140-180nm are auroral bands of N 2 Lyman-Birge-Hopfield, and they have been imaging targets of many satellites that need reflective mirrors. To obtain good imaging quality, the mirrors also should have excellent out-of-band reflection suppression as well as high reflectance at working wavelengths. We designed and fabricated non-periodic multilayer L a F 3/M g F 2 mirrors with working wave bands of 140-160nm and 160-180nm, respectively. We used a match design method and deep search method to design the multilayer. Our work has been utilized in the new wide-field auroral imager of China, and the application of these notch mirrors with excellent out-of-band suppression reduces the utilization of corresponding transmissive filters in the optical system of space payload. Furthermore, our work provides new routes for the design of other reflective mirrors in the far ultraviolet region.

Improving transmittance of long-wave infrared guided-mode resonant filters

The long-wave infrared (LWIR) spectral region spanning from 8 to 12 μm is useful for many scientific and industrial applications. Many of these applications require use of either a bandpass or a bandstop filter that can be realized by the guided-mode resonance (GMR) effect with subwavelength periodic features in layered dielectric materials transparent in the LWIR. The GMR filters operating in the LWIR region are fabricated by depositing an amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on a polished zinc selenide (ZnSe) substrate. In general, the backside of a ZnSe substrate with refractive index 2.41 is uncoated causing a 17% Fresnel-reflection loss in the light transmitted through the filter due to a large impedance mismatch at the ZnSe/air interface. Because we use such filters in the LWIR laser experiments for notch filtering, to improve the filter transmittance we used ZnSe substrates coated on one-side with broadband antireflection coating (ARC) covering the 7 to 12 μm spectral range to fabricate GMRFs with one-dimensional (1D) Ge ZC WGG. We employed high-spatial resolution e-beam lithography and reactive-ion etching nanofabrication techniques to achieve high-performance large-area (12 × 12 mm2) 1D notch filters with subwavelength periods. We characterized polarization dependent spectral performance of the prototype filters with both coherent and incoherent incident light using a tunable quantum cascade laser system that spans the 7 to 12 μm region, and a Fourier transform infrared spectrometer with collimated incident beam to achieve close to 15% improvement in the peak transmittance as well as significant reduction in coherent noise compared to our earlier results with GMRFs without ARC. Here, we present the filter design simulation and measurement results.

Electronically Tunable UWB Band Stop Filter Using Varactor Diode

An electronically tunable Ultra-Wideband (UWB) Band Stop Filter (BSF) using the varactor diode is investigated. The open stubs and Step Impedance Resonator (SIR) topology are utilized to obtain UWB bandstop response. The concept of variable capacitive loading to get tuning, which is achieved by a varactor diode (SMV2019-079LF) and a biasing network is clearly elucidated. The validation of the |S| parameter of the proposed tunable BSF filter is carried out using the filter's transmission line model (TLM), consequently, even and odd mode impedances are calculated analytically, and a lumped equivalent circuit model of the proposed BSF filter is extracted. Moreover, the filter is fabricated, and corresponding |S| parameters are measured which offers a UWB response (2.44 GHz–9.65 GHz) with five Transmission Zeros (TZ) and has tuning of 1 GHz frequency range (8.65 GHz–9.65 GHz) at the upper edge. The proposed miniaturized BSF has a size of 0.23λg × 0.32λg. The simulation results of the electromagnetic (EM)/distributed model, and the measured results of the fabricated prototype are in good agreement. Additionally, a very high skirt factor (SF) of 380 dB/GHz is achieved.

A multi-color video-ophthalmoscopes allows to measure the spectral distribution of light absorption of blood in the human retina.

Based on our previously developed mono-color video-ophthalmoscope a multi-color video-ophthalmoscope was developed. Using narrow band transmission filters, this instrument allows to measure the pulsatile cardiac cycle induced blood volume changes in the human retina for any wavelength in the sensitivity range of the used CMOS-camera. In this key experiment, video sequences (8 s, 25 fps, 200 frames) of the optic nerve head (ONH) were acquire for seven wavelengths between 475 nm and 677 nm one after the other. After image registration of all frames of each video sequence (to compensate for eye movements) and trend correction (to compensate for slow intensity changes), the amplitude of the cardiac cycle induced light intensity changes (pulsatile absorption amplitude PAA) can be calculated for all seven wavelengths. The results confirmed that the spectral distribution of PAA (λ) follows the distribution of the light absorption of blood. The measured values correspond to the absorption of a thin blood layer of about 0.5 μm thickness.