Reflection Filters Research Articles

Polarization Reflector/Color Filter at Visible Frequencies via Anisotropic α‐MoO3

Abstract2D van der Waals materials have attracted increasing attention in recent years due to their exciting physical properties and offer new opportunities for creating devices with enhanced or novel functionalities. In particular, α‐MoO3 is an emerging member of the fast‐growing 2D family with strong natural anisotropic optical properties. However, anisotropic optical properties of ‐MoO3 in the visible frequency range remain elusive. Here, α‐MoO3 is investigated as an optical material at the visible frequency (450–750 nm), which exhibits a polarization‐dependent complex refractive index due to the anisotropic crystal structure. As a proof of concept, polarization‐sensitive photonic devices including polarization reflectors and polarization color filters are designed and realized by constructing metal–insulator–metal Fabry–Perot cavities. It is observed that resonance frequencies for designed transmission and reflection filters change up to 25 nm with incident polarization which stems from the polarization‐dependent complex refractive indices of α‐MoO3. The largest contrasts are observed for two orthogonal polarization states parallel to the two orthogonal in‐plane crystal directions. The approach in this study offers new directions for potential applications in the development of polarization‐dependent devices based on 2D van der Waals materials for visible frequencies.

Quasi-total backward reflection with a CRIGF under oblique incidence

We report on a numerical study of cavity resonator integrated grating filters (a grating coupler flanked with two distributed Bragg reflectors) under oblique incidence. At resonance, the diffracted energy is divided equally in four outcoupling directions, versus two directions only for the CRIGF under normal incidence and for the infinite grating coupler. We provide a simple model to demonstrate this fact. Then, we propose a configuration for which all the diffracted energy is reflected backward with respect to the direction of the incident beam, thus realizing a reflection filter free from specular reflectivity.

Structural coloration of stainless steel with planar thin-film surface cavity structure

The esthetic functions of metals are becoming increasingly significant, as their application fields are rapidly extending into mobile electronics, home appliances, surface decoration/art, and building interiors. Herein we present a simple surface structure that produces vivid structural colors on stainless steel (STS). The structure consists of a thin metal layer deposited on a dielectric film, in which the metal layer can adjust the amount of light incident onto the dielectric and strengthens the interference effect. A wide variety of colors, including blue, green, pink, purple, scarlet, and yellow, are produced simply by changing the thicknesses of the dielectric and metal layers. An important finding is that the refractive indices of the thermally evaporated Ag and Au layers are strongly thickness-dependent, which allows color tuning by varying the thickness of the metal layer only. Consequently, color images can also be printed by selectively depositing a metal layer (Au or Ag) on a uniform dielectric film. The results of this study provide a simple, scalable route to produce vivid structural colors on bulk metals and may find diverse applications, including surface decoration, product identification, perfect absorbers, and reflective color filters.

Reflective aperiodic multilayer filters for metrology at XUV sources.

We present a general method for designing XUV aperiodic multilayer mirrors that can mimic a given target spectrum, specifically, the spectral transmission of an XUV optical system. The method is based on minimizing a merit function and using fidelity parameters that quantify the matching of the multilayer reflectivity spectrum with that of the target spectrum. To assess the feasibility of fabricating such a system, we show how to reduce the layer-to-layer thickness variations throughout the aperiodic layer stack. We demonstrate the design method using an example of an EUV optical system composed of 12 identical Mo/Si multilayer mirrors having a reflectivity peak at 13.5 nm. We found that the target spectrum can be mimicked with high fidelity either with a single reflection at an aperiodic multilayer mirror combined with standard absorbing filters or, if required, with two subsequent reflections at a mimic mirror. These examples demonstrate the applicability for metrology at XUV sources, including spectrally proper source imaging. Because our approach is of general applicability, the process can be used to mimic any other narrowband, single-peaked target spectrum in the XUV region.

Design and development of an optical reflective notch filter using the ion assisted deposition technique with stepwise modulated thickness for avionics applications.

This paper presents research work about the design and fabrication of a 44-layer optical reflective notch filter. The performance of the fabricated notch filter was studied at normal (0°) and oblique (45°) incidence angle. In addition, the paper also discusses a three-layer broadband antireflective coating on both sides of the multilayer stack to suppress the ripples in the passband region. The thickness-modulated reflective stack of the filter was designed by using the materials Al2O3 (1.63) and SiO2 (1.46). Optimization of the multilayer stack was carried out by using the damped least-squares algorithm. The theoretical and experimental results from the ion-assisted e-beam deposited samples for single notch reflective filters are presented. Good agreement in the design and experimental results was observed when the deposition process was controlled by time of evaporation. Further, the filter was characterized for the optical properties by using a UV-VIS-NIR spectrophotometer, surface morphology and protective properties using field emission scanning electron microscopy, a coherence correlation interferometer, and water contact angle.

High-purity reflective color filters based on thin film cavities embedded with an ultrathin Ge2Sb2Te5 absorption layer.

A thin film cavity formed by stacking metal–insulator–metal (MIM) continuous layers is of significant interest as a lithography-free and scalable color-filtering structure. Such a cavity can selectively transmit a certain frequency range of incident light, thus producing vivid transmission colors. However, the generation of reflection colors with high purity and reflectivity is a challenge because a cavity in reflection mode resonantly absorbs a narrow range of wavelengths and reflects the remaining spectrum. This study shows that highly pure and reflective colors can be obtained by embedding an ultrathin Ge2Sb2Te5 layer within the cavity. Because the MIM structure exhibits a nonuniform intensity distribution across the insulator layer, the approach is to place the Ge2Sb2Te5 layer in a high-intensity region within the insulator and thereby create another absorption band in addition to the cavity resonance mode. When combined with the refractive-index engineering of the metal layer, this approach leads to red, green, and blue colors having a bandwidth of ∼100 nm and a reflection efficiency of 90%. The results of the study may be effectively utilized in numerous applications, including reflective color filters, colorimetric sensors, and surface decorations.

Highly Precise Birefringence Demodulation to Overcome the Sensing Limitation From the Free Spectral Range on the Fiber Lyot Filter

A high precision demodulation method based on least squares fitting for expanding the temperature measurement range of the fiber Lyot filter has been proposed and experimentally demonstrated. And a compact reflective Lyot filter has also been proposed to verify this method. The filter consists of a polarizer and a section of polarization maintaining fiber (PMF) coated with a gold film at the end. The birefringence of the PMF can be obtained through the least squares method, so that the temperature measurement range is not limited to the free spectral range (FSR) of the periodic interference spectrum. Experimental results show that the proposed method can extend the measurement range by 5.5 times compared to the dip wavelength tracking method. And the demodulation fluctuation of the birefringence is within 10 -7 .

Nanohybrid Materials with Tunable Birefringence via Cation Exchange in Polymer Films

AbstractIn this work, a nanohybrid material based on a freestanding polymeric liquid crystal network capable of postmodification via cation exchange to tune birefringence is proposed. The smectic liquid crystal films can be infiltrated with a variety of cations, thereby changing the refractive indices (ne and no) and the effective birefringence (Δn) of the nanohybrid material, with reversible cation infiltration occurring within minutes. Birefringence could be tuned between values of 0.06 and 0.19, depending on the cation infiltrated into the network. Upon infiltration, a decrease in the smectic layer spacing is found with layer contraction independent of the induced change in birefringence. Potential applications are in the field of specialty optical devices, such as flexible, retunable reflective filters.

Temperature-Independent Gas Pressure Sensor with High Birefringence Photonic Crystal Fiber-Based Reflective Lyot Filter.

A novel temperature-independent gas pressure sensor based on a reflective fiber Lyot filter is presented in this paper. The reflective fiber Lyot filter is simply consist of a fiber polarizer and a segment of hollow-core photonic bandgap fiber (HB-PCF). The HB-PCF plays the role of birefringent cavity in the reflective fiber Lyot filter and works as the sensor head in the gas pressure sensor. Experiment results show that the responses of the sensor to gas pressure and temperature are 3.94 nm/MPa and −0.009 nm/°C, indicating that the proposed gas pressure is sensitive to gas pressure rather than temperature. Coupled with the advantages of simple structure, easy manufacture, high sensitivity and temperature independent, the proposed reflective fiber Lyot filter-based gas pressure sensor holds great potential application in the field of gas pressure monitoring.

Four-wavelength-switchable SLM fiber laser with sub-kHz linewidth using superimposed high-birefringence FBG and dual-coupler ring based compound-cavity filter.

We propose and demonstrate a four-wavelength-switchable erbium-doped fiber laser (4WS-EDFL) with a four-channel superimposed high-birefringence fiber Bragg grating (SI-HBFBG) and a dual-coupler ring based compound-cavity (DCR-CC) filter. Both for the first time, a SI-HBFBG as a four-channel reflective filter is used in a multi-wavelength switchable fiber laser to define wavelength channels and a DCR-CC filter is used to select a single mode from dense longitudinal-modes in a fiber laser. We present in detail how to design, fabricate, and characterize the DCR-CC filter with both theoretical analysis and experimental results, which we believe is the first systematic approach for making a compound-cavity based filter used for selecting single-longitudinal mode (SLM) in a fiber laser. The enhanced polarization hole burning effect in a 2.9 m long erbium-doped fiber, coiled inside a three-loop polarization controller, and the polarization-mismatch-induced losses are introduced into the laser cavity to achieve wavelength-switching operations. We show that the 4WS-EDFL can be switched among fifteen lasing states, including four single-wavelength operations, six dual-wavelength operations, four three-wavelength operations and one four-wavelength operation, all with high stability. For demonstration, in switchable single-wavelength operations, the four SLM lasing outputs measured are all with an optical signal to noise ratio of >80 dB, a linewidth of <700 Hz, a relative intensity noise of ≤-156.7 dB/Hz at frequencies over 3 MHz, an output power fluctuation of ≤0.555 dB and excellent polarization characteristics.

Full‐Color Reflective Filters in a Large Area with a Wide‐Band Tunable Absorber Deposited by One‐Step Magnetron Sputtering

AbstractFull‐color reflective filters for large area applications with potentially unprecedented color saturation and excellent mechanical properties deposited by one‐step magnetron sputtering are proposed. Conventional reflective color filters with multiple layers of dielectric films cannot simultaneously produce a large area and good mechanical properties due to the complex multiple depositions and the difference in the thermal expansion coefficients among the material layers. Herein, full‐spectrum colors are generated by novel Mg‐based reflective color filters in a large area of 2 cm × 2 cm with a high hardness of 9.12 GPa, where the filters include an absorber layer with controllable optical constants and a reflective layer with an amorphous structure. The saturation and hue of the produced colors can be controlled by tuning the optical constants and the thickness of the absorber layer. Additionally, the hardness of the Mg‐based reflective color filters is increased by the reflective metallic glass layers because they are derived from the same material as the absorber layer. This paradigm can pave the way for the efficient fabrication of large area color filtering devices for diverse applications, such as surface decorations, optical components, color display devices, structural color printing, and photovoltaic cells with optimum efficiency.

Integrated flat-top reflection filters operating near bound states in the continuum

We propose and theoretically and numerically investigate narrowband integrated filters consisting of identical resonant dielectric ridges on the surface of a single-mode dielectric slab waveguide. The proposed composite structures operate near a bound state in the continuum (BIC) and enable spectral filtering of transverse-electric-polarized guided modes propagating in the waveguide. We demonstrate that by proper choice of the distances between the ridges, flat-top reflectance profiles with steep slopes and virtually no sidelobes can be obtained using just a few ridges. In particular, the structure consisting of two ridges can optically implement the second-order Butterworth filter, whereas at a larger number of ridges, excellent approximations to higher-order Butterworth filters can be achieved. Owing to the BIC supported by the ridges constituting the composite structure, the flat-top reflection band can be made arbitrarily narrow without increasing structure size. In addition to the filtering properties, the investigated structures support another type of BIC—the Fabry–Perot BIC—arising when the distances between adjacent ridges meet the Fabry–Perot resonance condition. In the vicinity of the Fabry–Perot BIC, an effect similar to electromagnetically induced transparency is observed, namely, sharp transmittance peaks against the background of a wide transmittance dip.

Transmission filter controlled by incident conditions in single-layer waveguide grating structures.

The classical single-layer waveguide grating structure is often designed as a reflection filter under 0° azimuthal angle conditions. A new method for achieving a transmission filter by modulating incident conditions is proposed rather than adjusting structural parameters. A 90° azimuthal angle ensures single resonance peaks, and large incident angles provide a low-transmittance background. When the proposed single-layer waveguide grating structure is irradiated with an incident light azimuthal angle of 90° and incident angle of 86°, numerical results show that a high transmission peak emerges at 1550 nm with linewidth $\Delta \lambda = 7.2\,\,{\rm nm}$Δλ=7.2nm. Moreover, the transmission peak has nearly 100% transmittance. These findings show that modulating the incident conditions is also an effective means apart from optimizing the structural parameters. The classical reflection filter can be reversed into a transmission filter using this method.

Entirely reflective slit spatial filter for high-energy laser systems.

An entirely reflective slit spatial filter is proposed to provide spatial filtering, gain isolation, and ASE mitigation for high-energy laser systems. The traditional circular pinhole is replaced by two orthogonal slits, which lowers the intensity at the spatial filter plane by up to two orders of magnitude, and by using reflective optics we reduce spatial dispersion and eliminate B-integral effects. A ray trace model of the spatial filter shows excellent transmitted wavefront, but also indicates aberrations at the foci from using cylindrical optics at 45°. It is expected that the use of off-axis parabolic mirrors would mitigate this issue but comes at the cost of more complicated, expensive optics and more complex alignment. We created a numerical model based on Fourier optics to explain this effect and guide design requirements to mitigate it. High-quality imaging and filtering capabilities are demonstrated experimentally.

P‐4.3: Reflective Color filters based on subwavelength diffraction gratings

This report discusses a potential reflected color filter for colorful display device. In this design, diffraction gratings could be color filter, reflect specific wavelengths to present blue, green, and red, even cyan and yellow. A 120nm silicon layer lays on a quartz substrate, and the silicon layer was patterned to generate a two‐dimensional circular pillars with different geometric parameters from 180nm pitch to 350nm Meanwhile, it has good tolerance to pillars shape and lattice parameters which are friendly news for volume production. Last but not least, these structures are independence of the polarization and insensitive angular of the incident light of 30°. These RGBCY five primary colors will extend the color gamma to 143% of NTSC. Designed structures can be volume fabricated by nano‐imprint lithography and etching processes.

Picosecond laser writing of [formula omitted] nanocomposite nanogratings for optical filtering

In this article, we propose a novel approach for dielectric nanograting fabrication by direct laser patterning. Possibilities of a well-controlled ultra-short laser recording of Ag-SiO2 nanocomposite gratings and their optical properties are examined. The mechanisms of laser processing involve silver nanoparticle growth in nanoporous silica glass films and laser interference-based formation of a periodic grating-like nanorelief. It is shown that laser energy should stay below the surface ”grooves” formation threshold for laser-inscription of the interference-based grating. Otherwise, another periodic structure oriented parallel to the incident laser polarization appears to erase the interference pattern. The parameter windows required for the controlled fabrication of the obtained structures are determined. The required threshold decay with the number of applied laser pulses is explained by a similarity in the roles of the nanoparticle absorption and surface defect accumulation typically leading to such dependencies. The optical properties of the obtained gratings are shown to depend on the angle between the incidence plane and the grating direction. When these directions coincide, a signal enhancement with a period-dependent blue-shift is revealed in the diffuse scattering spectra. When these directions are perpendicular, the signal is less enhanced, and a red shift is observed. The observed results are promising in short laser fabrication of different optical components, such as reflective optical filters.

Stable and Colorful Perovskite Solar Cells Using a Nonperiodic SiO2/TiO2 Multi-Nanolayer Filter.

While research on building-integrated photovoltaics (BIPVs) has mainly focused on power-generating window applications, the utilization of other underutilized surface areas in buildings, including exteriors, facades, and rooftops, has still not been fully explored. The most important requirements for BIPVs are color, power conversion efficiency (PCE), and long-term stability. In this work, we achieved colorful (red, green, blue, RGB) perovskite solar cells (PSCs) with minimized PCE loss (<10%) and enhanced photostability by exploiting the optical properties of nonperiodic multi-nanolayer, narrow-bandwidth reflective filters (NBRFs). The NBRFs were fabricated by multilayering high-index TiO2/low-index SiO2 in a nonperiodic manner, which allowed devices to demonstrate various colors with effectively suppressed unwanted baseline ripple-shape reflectance. The PCEs of PSCs with nonperiodic RGB-NBRFs were 18.0%, 18.6%, and 18.9%, which represent reductions of only 10%, 7%, and 6% of PCE values, respectively, compared to a black control PSC (20.1%). Moreover, the photostability of the PSCs was substantially improved by using the NBRFs because of ultraviolet blocking in the TiO2 layers. The G-PSC retained 65% of the initial PCE after 60 h of continuous illumination (AM 1.5G one sun) at the maximum power point, whereas the black PSC retained only 30%. Aesthetic color value, low PCE loss, and enhanced photostability of PSCs were simultaneously achieved by employing our NBRFs, making this a promising strategy with potential applicability in power-generating building exteriors.

Standard red green blue (sRGB) color representation with a tailored dual-resonance mode in metal/dielectric stacks

Lithography-free planar structures have been the subject of many studies on structural coloration in recent years because of mass production and large-area applications. Among them, the metal-insulator-metal (MIM) structure follows a subtractive color system. However, the limited gamut of this system hinders a wider application of reflective color filters. We demonstrate a planar metal-insulator-metal-insulator-metal (MIMIM) structure for a tunable dual-resonance mode, by combining each single-resonance mode in MIM structures. By controlling the resonance dips, we obtain standard red/green/blue (sRGB) in the standard color space for modern optical devices. The reflectance spectra, color representation, and fabricated samples verify the dual-resonance tunability and generation of sRGB colors.

Theoretical analysis and simulation of a tunable mid-infrared filter based on Ge2Sb2Te5 (GST) metasurface

Based on an all-dielectric metasurface, we propose a tunable high-efficiency optical filter in the mid-infrared (mid-IR) range, which is composed of Ge2Sb2Te5 (GST) nanodisk arrays. By increasing the crystallization of the GST, the proposed reflective optical filter can work effectively in wavelength range of 3.55–6.14 μm. The theoretical analysis demonstrate that the filter function is originating from the excitation of the magnetic Mie resonance. The simulation results show that the proposed filter possesses another advantage of incident angular independence and polarization independence. These results suggest that the proposed all-dielectric metasurface may find great potential applications in optical filter.

Quality factor based correction for SD-OCT measurements in cataract patients.

To correct peripapillary retinal nerve fibre layer (pRNFL) measurements performed with spectral domain optical coherence tomography (SD-OCT) for low image quality factor (QF) in patients with cataract, using measurements before and after cataract surgery. SD-OCT (Topcon 3DOCT-2000) volume scans of the optic disc of 13 cataract patients were used. A set of three reflective filters with optical density ranging from 0.11 to 0.54 were used. The correlation was calculated between the change in thickness measurements and the change in image quality factor. Changes before and after cataract surgery were analysed. A correction for scans with a lower QF was calculated using an equation which was formulated based on the relationship between the change in thickness measurements and the change in image quality factor. Thirteen right eyes of thirteen cataract patients were included in this study. pRNFL thickness measurements before and after cataract differed significantly (96 versus 99 micron, p<0.01). Preoperative linear regression lines showed a different slope than postoperative regression lines. Corrected pRNFL thickness measurements of before cataract surgery differed significantly with pRNFL thickness measurements after cataract surgery. The presence of cataract influences the QF-pRNFL relationship. The lower the image QF, the higher the pRNFL thickness underestimation. We found a rather curvilinear relationship between QF and pRNFL. Our corrected measurements of the pRNFL thickness in case of lower image QF due to cataract still differed significantly from the pRNFL thickness measurements after cataract surgery.