Reflection Filters Research Articles

Compact Fluorescence Detection System Based on a Monolithic DBR-Integrated III-Nitride LED Chip.

Portable applications of fluorescence detection systems have gained much attention in various fields and require system components to be small and compact. In this work, we report on a compact fluorescence detection system and demonstrate its application for fluorescence sensing and imaging. The light source and filter are integrated on a single chip for the proposed system, which not only realizes the separation between excitation and fluorescent lights but also improves the light-emitting diode (LED) light extraction efficiency. Furthermore, the detection system allows for a removable sample unit. The results indicate that the performance of the distributed Bragg reflector (DBR) filter based on an amorphous dielectric film is excellent with selection ratios larger than 4600:1. The peak emission wavelength of the LED is 528 nm. The influence of green light leakage can be neglected, and the fluorescent red light is dominant when the fluorescence detection system is used for sensing and imaging. The low-cost and monolithic DBR-integrated III-nitride LED chip makes the proposed architecture a competitive candidate for portable fluorescence detection applications.

The Application of the Dynamic Atmosphere Energy Transport Climate Model (DAET) to Earth’s Semi-Opaque Troposphere

The objective of this work is to apply the Dynamic-Atmosphere Energy Transport (DAET) climate model to a study of the Earth’s semi-opaque troposphere. In this analysis the concept of previous authors has been followed and the Earth’s climate is treated as a single integrated structured system of solar energy collection, thermal energy retention and energy distribution across the Earth’s surface. Unlike previous authors the hemispheric duality of the Earth’s surface is modelled with two separate energy environments of a day lit hemisphere of net energy collection and a dark night surface of net energy loss as fundamental to the design. Using worked examples, it is shown how the Greenhouse Effect results from the summation of two separate physical atmospheric processes, both of which are mathematically equivalent and which together create an energy reservoir within the Earth’s troposphere. These processes are the thermal radiant opacity blocking of radiative physics, and the process of adiabatic convection and conserved energy delivery to far distance of mass-motion physics. Both these processes involve the mathematical infinite summation of halves-of-halves of energy flux and are completely saturated at a surface atmospheric pressure of 1 Bar. It is concluded that the two fundamental controls on terrestrial planetary climate for a given solar system orbit are the downwelling high frequency energy reflection filter of planetary Bond Albedo, and the upwelling low frequency energy bypass to space filter of the Atmospheric Window.

Effects of silicon dioxide as the polar dielectric on the infrared absorption spectrum of the metal-insulator-metal metasurface

Metal-insulator-metal metasurfaces have been widely used as high-performance absorbers in almost all electromagnetic spectral ranges. Their absorption spectra can be engineered by making variations in the geometry of the unit cells and/or by embedding materials with specific optical constants. Including a polar dielectric in their structure is another approach for manipulating their absorption spectra. In this research, we have numerically and experimentally investigated the effect of using silicon dioxide (SiO 2) as a polar dielectric on the absorption spectrum of a metal-insulator-metal metasurface composed of a tri-layer of Ni-SiO 2 -Ni. Our results have shown the presence of absorption peaks in the mid-infrared which are attributed to the excitation of the optical phonons in the SiO 2 spacer layer. Particularly, the excitation of the Berreman mode in the SiO 2 spacer layer was observed and its effect on the total absorption spectrum is studied. The parametric effects of the top patterned Ni layer, the incident angle, and the polarization are also investigated. This study can provide engineering capabilities for the mid-infrared absorbers and reflection filters.

Reflective high-efficiency color filters for wide color gamut LCD

Reflective high-efficiency color filters for wide color gamut LCD

A Compact Reflectionless Low-Pass Filter Based on TSV Technology

This paper presents a compact reflectionless low-pass filter based on through-silicon via (TSV). Its overall structure is composed of TSV inductors and redistribution layer(RDL) capacitor. The high-frequency structure simulator (HFSS) validates the proposed reflection filter. The results show that the filter operates at a frequency of 0-6.6GHz, with The return loss of input and output is less than -14.6dB in all test frequency bands(0-30GHz). In addition, the size of the reflectionless filter is only 0.19×0.23×0.0139 mm3 (0.014× 0.017×0.01 λg3).

Numerical modeling of an integrated non-volatile reflector switch and mode converter switch based on a low loss phase change material (Sb2Se3) in SiN platforms

Programmable integrated photonics is an emerging research field due to its range of applications, from data processing to computing. Phase change materials (PCMs) on waveguides provide enormous flexibility for programmable integrated photonics. These materials show a large contrast in the optical properties (such as refractive index and optical loss) between the two stable states (i.e., amorphous to crystalline) of the PCM. These states are reversible and reproducible with an external stimuli which could be optical, thermal or electrical. Their non-volatile behavior allows PCMs to serve as an active layer for programmable photonics. In this work, we investigate hybrid device architectures utilizing the non-volatile properties of PCMs for integrated programmable photonics in a Si3N4 platform. FDTD modeling was carried out to design two configurations of non-volatile reconfigurable switches: (1) A non-volatile reconfigurable reflector switch consisting of a photonic crystal (PhC) slab waveguide and having a thin layer of Sb2Se3 on top of the waveguide and (2) 1×2 mode converter switch. In the case of a non-volatile reconfigurable reflector switch, the reflectivity of the waveguides is controlled by shifting the mode-gap of the photonic crystal slab waveguide by changing the phase of the PCM. It has applications as a reconfigurable reflective filter in the optical communication system and on-chip smart Bragg mirror. Non-volatile broadband directional switches provide a new paradigm for designing programmable multifunctional nanophotonics, which works in the same way as electronic field-programmable gate arrays. A 1×2 mode converter switch — based on Sb2Se3 as a clad layer on a coupling waveguide of a directional coupler — is proposed. The optical switches proposed in this paper offer very low optical insertion loss (∼ 0.5 dB), low coupling length (∼ 12 µm), broadband operation (∼ 80 nm), small cross talk (∼ 16 dB), and zero static power consumption.

Fabrication of a reflective polarization wavelength filter via anodization of a Ti grating film.

A reflective-type polarization wavelength filter was fabricated by anodizing a Ti grating film. This polarization filter consisted of a Ti film, an anodized titanium oxide film (thickness ∼25nm), and a photoresist grating with a pitch of 410 nm. The reflectance was less than 10% for both polarizations at 490-675 nm at an incident angle of 9°. In addition, the transverse electric (TE) reflectance of the fabricated element was over 40%, whereas the transverse magnetic (TM) reflectance was 25.8% at 375 nm. Upon increasing the incident angle to 75°, the TE reflectance increased to 77.8% at 375 nm, whereas the TM reflectance decreased to 5.9%.

Full-color reflective filter in a large area exploiting a sandwiched metasurface.

Metasurface-based color filters show great potential in imaging devices and color printing. However, it is still a great challenge to meet the high demand for large-area flexible displays with structural color filters. Here, a reflective color filter is developed with a sandwiched metasurface, where the photoresist grating, complementary silver grating and silicon nitride grating are sequentially stacked on the substrate. Analytical results show that bandpass reflective spectra can be achieved due to the combined influence of guided mode resonance and cavity resonance, and full-spectrum colors including three primary colors can be generated by merely varying the period of the metasurface. With only photolithography and deposition technology involved, large-area samples incorporating pixelated metasurfaces are easily fabricated. Metasurfaces with three periods of 540 nm, 400 nm and 320 nm are experimentally obtained having peak reflective efficiency of ∼ 60%, demonstrating red, green and blue colors as theoretical results. A stripe sample with the structural period varying from 250 nm to 550 nm is fabricated in an area of 10 mm × 30 mm, displaying full-color reflections as simulated. Finally, with metasurfaces of three structural periods, the pixelated Soochow University logo is fabricated in a larger area of ∼ 30 mm × 30 mm. Therefore, the proposed structure shows high compatible to roll-to-roll nano-imprinting for large-area flexible displays, with the photoresist film can be easily substituted by UV film in addition.

Switchable dual-functional guided-mode resonant filters based on VO2 phase transition

Here we propose a hybrid dielectric waveguide grating with an ultrathin vanadium dioxide (VO2) thin film for achieving switchable dual-functional GMR filters. We show that, when the VO2 thin film undergoes the insulator-to-metal phase transition, the hybrid dielectric waveguide gratings enable the switch between the reflection and transmission filters. This work expands the research topic of GMR and provides valuable strategy for the design of novel reconfigurable devices for photonics.

Tailored resonant waveguide gratings for augmented reality.

We explore the use of tailored resonant waveguide gratings (RWG) embedded in a glass-like matrix as angularly tolerant tri-band reflection filters under oblique excitation. Through inverse design we optimize 1D grating structures to support multi-frequency narrowband resonances in an otherwise transparent background, ideally suited for augmented reality applications. In particular, we show theoretically and experimentally that a single RWG can be tailored to provide reflection levels larger than 50% under p-polarized excitation at three distinct wavelengths of choice, over a narrow bandwidth and within a substantial angular range around 58° incidence, while simultaneously eliminating ghost reflections from the glass/air interface. Similar performance can be achieved for s-polarization by cascading two RWG's. Moreover, we demonstrate that these metrics of performance are maintained when the devices are fabricated using roll-to-roll techniques, as required for large-area industrial fabrication. Overall, these devices show exciting potential as large-area transparent heads-up displays, due to their ease of fabrication and material compatibility.

Vivid reflective color generation mechanism in Al/AAO/Al configuration

Lithography-free metal-dielectric-metal (M/D/M) configuration has attracted tremendous interest for vivid reflective color generation, especially the M/anodic aluminum oxide (AAO)/Al configuration due to its simple, cost-effective and flexible preparation processes. However, the physical mechanism of structural color generation in M/AAO/Al configuration has not been sufficiently discussed. Here, the Al/AAO/Al configurations with distinct vivid reflective colors were prepared. To reveal the color generation mechanism, the bumpy nanopore Al island film (BAlIpore) was proposed as the top meal layer. The optical properties of BAlIpore and BAlIpore/AAO/Al configurations are investigated by both experimental and finite-difference time-domain (FDTD) method. The results indicate that BAlIpore/AAO/Al configuration can generate vivid reflective colors due to the large enhancement and widening of the absorptive band in the reflective spectrum, which is the result of the strong coupling between the broadband local surface plasmon resonance (LSPR) and destructive interference (thin-film interference) supported by BAlIpore and BAlIpore/AAO/Al, respectively. Furthermore, the optical properties of the Al/AAO/Al configuration with a planar nanopore Al film (PAlpore) as the top metal layer are investigated by FDTD method and the results indicate that the PAlpore/AAO/Al configuration can’t result in vivid reflective colors because of the limit enhancement and widening of the absorptive band in the reflective spectrum, which resulted in the weak coupling between weak broadband non-resonance scattering and F-P resonance supported by the PAlpore and PAlpore/AAO/Al, respectively. The results contribute to our understanding of the vivid reflective color generation mechanism in the M/AAO/Al configuration and can be used to develop a reflective filter, color display, etc.

Facile fabrication of stretchable photonic Ag nanostructures by soft-contact patterning of ionic Ag solution coatings

Abstract We describe a rapid and simple method to create Ag nanostructures by using direct mechanical patterning of ionic Ag ink coating under gentle pressure, then thermal annealing to reduce the ionic Ag ink to a metallic Ag layer. The ionic liquid-phase Ag coating is easily obtained by spin-coating ionic Ag ink that has appropriate Ag concentration and can be either printed or imprinted on the desired substrate by using a soft elastomer patterning mold, then reduced to the Ag nanostructure by subsequent thermal annealing. More specifically, we present two methods: transfer printing and soft nanoimprinting. In transfer printing, the ionic Ag ink is first inked onto the elastomer mold which then contacts the target substrate to transfer the Ag nanopattern. In soft nanoimprinting, the elastomer mold conducts soft imprinting to engineer the ionic Ag ink coating to the Ag nanostructure. We systematically investigate the optimal patterning conditions by controlling the initial Ag ink concentration and the coating, printing, imprinting, and annealing conditions, to derive Ag architecture that has tunable photonic functionality. As an example, we demonstrate polarization-sensitive reflective color filters that exploit shape-tunable Ag nanostructures fabricated by soft nanoimprinting using a controllably-stretched elastomer mold.

Ta/NiO subwavelength bilayer for wide gamut, strong interference structural color

In this paper we demonstrate that Ta/NiO bilayers may be use as high-efficiency, lithography free, reflective structural color filters for generating broad color gamut. Experimental results show that reflectance spectra present deep dips in the visible range, leading to strong structural colors that can be adjusted via the NiO subwavelength layer thickness. Simulation based on thin film interference theory allow to account for the experimental data. We demonstrate that the optical interference effect is still effective when the films are deposited on a flexible substrates such as paper and kapton, enabling to consider flexible color filtering applications.

Spectrally pure far-UVC emission from AlGaN-based LEDs with dielectric band pass filters

AlGaN-based far ultraviolet-C (UVC) light emitting diodes (LEDs) with a peak emission wavelength below 240 nm typically show a long-wavelength tail at >240 nm that is detrimental to the use of the devices for skin-friendly antisepsis. We present the development of far-UVC LEDs with reduced long-wavelength emission using a HfO2/SiO2-based distributed Bragg reflector (DBR) filter. When the DBR filter is directly mounted on an LED package, the long-wavelength emission around 250 nm is reduced by two orders of magnitude while the transmitted output power is reduced down to 18%–27% of the initial value for DBR filters with cut-off wavelengths at 237–243 nm. As the transmission through the DBR filter depends strongly on the angle of incidence of the radiation, the transmitted output power of the spectrally pure far-UVC radiation can be doubled when an additional collimating lens is used on top of the LED package before passing through the filter.

Development of colored perovskite solar cells using cholesteric helicoidal superstructures

Perovskite solar cells (PeSCs) are promising candidates for building-integrated photovoltaics (BIPV) because of their excellent performance and processability. In this study, the first colored PeSCs that use liquid crystalline cholesteric phase-based reflective filters (ChRFs), which were inspired by the cuticle of a beetle, are developed. The reflective wavelengths, bandwidth, and reflectance of ChRFs are easier to modulate than those of reflective filters based on metals or oxides. Reflective wavelengths affect the hue, and they can be modulated by tuning the contents of a chiral dopant. The reflective bandwidth affects the power conversion efficiency, and it can be modulated by tuning the birefringence of the host liquid crystalline molecules. The reflectance also affects the chromaticity, and it can be controlled by modifying the simple optical design of the ChRF structure. ChRFs are suitable for large and flexible PeSCs because they are constructed using only organic components and wet processes.

Angle-tolerant polarization controlled continuous color palette from all-dielectric nanograting in reflective mode

High-performance angle-tolerant polarization tunable reflective color filters with an adopted one-dimensional silicon grating structure are proposed. The polarization-tunable color filtering properties and incident angle-insensitive properties are thoroughly investigated to achieve the angle-tolerant polarization controlled continuous color palettes. Benefiting from high refractive index of silicon, the proposed color filters evidently provide a high angular tolerance of 30° in conjunction with a large polarization-tunable color range. Moreover, full and vivid colors covering the entire visible range can be produced by simply altering the grating thickness. The combination between the all-dielectric nanostructure with a high angular tolerance and the polarization-tunable filtering properties, described in this paper, can be applied in anti-counterfeiting, imaging and filtering technologies, and so forth.

Thin-Film Frustrated Total Internal Reflection Filter with Plasmonic Nanoparticle Inclusions in the Layers

The influence of plasmonic nanoparticles embedded in the central and side layers of the frustrated total internal reflection filter on the resonant transmission of light is analyzed. It is shown that the frequency dispersion causes the splitting of the filter bandwidth and the angular splitting of the incident beam into several output beams.

Fluorescence Enhanced Optical Resonator Constituted of Quantum Dots and Thin Film Resonant Cavity for High-Efficiency Reflective Color Filter.

Conventional color filters selectively absorb a part of the backlight while reflecting or transmitting other light, resulting in the problem of low efficiency and energy wasting. For this problem, a new concept of fluorescence enhanced optical resonator was proposed and verified in this paper. The new structure consists of structural color filter and light-conversion material. Specially, a thin film resonant cavity was designed, and InP/ZnSe/ZnS quantum dots were inserted inside the resonator. When illuminated by sunlight, the novel fluorescence enhanced optical resonator could not only reflect the specific light, but also convert absorbed energy into desired light, leading to the utilization efficiency improvement of solar energy. An all-dielectric red fluorescence enhanced optical resonator was fabricated, with peak equivalent reflectance up to 105%. Compared with a thin film resonator, the enhancement coefficient of the as-proposed structure is about 124%. The new optical structure can utilize solar source efficiently, showing application potential as the next generation of reflective color filters for display.

Humidity- and Temperature-Tunable Metal-Hydrogel-Metal Reflective Filters.

A tunable reflectance filter based on a metal–hydrogel–metal structure responsive to humidity and temperature is reported. The filter employs a poly(N-isopropylacrylamide)–acrylamidobenzophenone (PNIPAm–BP) hydrogel as an insulator layer in the metal–insulator–metal (MIM) assembly. The optical resonance of the structure is tunable by water immersion across the visible and near-infrared range. Swelling/deswelling and the volume phase transition of the hydrogel allow continuous reversible humidity- and/or temperature-induced tuning of the optical resonance. This work paves the way toward low-cost large-area fabrication of actively tunable reversible photonic devices.

Terrestrial laser scanning to reconstruct branch architecture from harvested branches

Abstract Quantifying whole branch architecture is critical to understanding tree function, for example, branch surface area controls woody gas exchange. Yet, due to measurement difficulty, branch architecture of small diameter branches (e.g. <10 cm ) is modelled, subsampled or ignored. Methods that use terrestrial laser scanning (TLS) are now being widely applied to analyse tree and plot‐level tree architecture; however, resolving small diameter branches in‐situ remains a challenge. Currently, it is suggested that accurate reconstruction of small diameter branches can only be achieved by harvest and measurement in controlled conditions. Here we present a new TLS workflow for rapid and accurate reconstruction of complete branch architecture from harvested branches. The workflow sets out scan configuration, post‐processing (including a novel reflectance filter) and fitting of quantitative structure models (QSM) to reconstruct topologically coherent branch models. This is demonstrated on 595 branches (scanned indoors to negate the impact of wind) and compared with 65 branches that were manually measured (i.e. with measuring tape and callipers). Comparison of a suite of morphological and topological traits reveals a good agreement between TLS‐derived metrics and manual measurements where RMSE (%RMSE) for total branch length = 0.7 m (10%), volume = 0.09 L (43%), surface area = 0.04 m2 (26%) and N tips = 6.4 (35%). Scanning was faster and invariant to branch size compared with manual measurements which required significantly more personnel time. We recommend measuring a subsample of tip widths to constrain the QSM taper function as the TLS workflow tends to overestimate tip width. The workflow presented here allows for a rapid characterisation of branch architecture from harvested branches. Increasing the number of branches analysed (e.g. many branches from a single tree or branches from many species globally) could allow for a comprehensive analysis of the ‘missing link’ between the leaves and larger diameter branches.