Multilayer Dielectric Filters Research Articles

Single‐Shot Wavefront Sensing with Nonlocal Thin Film Optical Filters

AbstractConventional wavefront sensors suffer from the fundamental limitation of the space‐bandwidth product and have a trade‐off between their spatial sampling interval and dynamic range. Here, nonlocal thin film optical filters with optimized angle‐ and polarization‐dependent responses are leveraged to circumvent the fundamental limitation, and a robust single‐shot wavefront sensing system with a small spatial sampling interval of 6.9 µm and a large angular dynamic range of 15° is realized. The system only requires inserting two multilayer dielectric filters, fabricated using a mature thin film deposition technique, into a conventional 4‐f imaging apparatus. The polarization‐sensitive nonlocal filters are used to map the 2D phase gradients of the incident light field to the intensity variation of the x‐ and y‐polarized light, respectively, thus enabling single‐shot 2D wavefront reconstruction from images taken by a polarization camera. Such a system may be used for a variety of applications, including high‐resolution image aberration correction, surface metrology, and quantitative phase imaging.

Tunable directional filter for mid-infrared optical transmission switching.

Controlling the spectral and angular response of infrared (IR) radiation is a challenging task of paramount importance to various emerging photonic applications. Here, we overcome these problems by proposing and analyzing a new design of a tunable narrowband directional optical transmission filter. The presented thermally controlled multilayer filter leverages the temperature dependent phase change properties of vanadium dioxide (VO2) to enable efficient and reversible fast optical switching by using a pump-probe laser excitation setup. More specifically, transmission is blocked for high intensity probe lasers due to the VO2 metallic properties induced at elevated temperatures while at low probe laser intensities high transmission through the filter occurs only for a narrowband IR range confined to near normal incident angles. The proposed multilayer composite dielectric filter is expected to have applications in optical communications, where it can act as dual functional infrared filter and optical switch.

Compact meta-spectral image sensor for mobile applications

Abstract We have demonstrated a compact and efficient metasurface-based spectral imager for use in the near-infrared range. The spectral imager was created by fabricating dielectric multilayer filters directly on top of the CMOS image sensor. The transmission wavelength for each spectral channel was selected by embedding a Si nanopost array of appropriate dimensions within the multilayers on the corresponding pixels, and this greatly simplified the fabrication process by avoiding the variation of the multilayer-film thicknesses. The meta-spectral imager shows high efficiency and excellent spectral resolution up to 2.0 nm in the near-infrared region. Using the spectral imager, we were able to measure the broad spectra of LED emission and obtain hyperspectral images from wavelength-mixed images. This approach provides ease of fabrication, miniaturization, low crosstalk, high spectral resolution, and high transmission. Our findings can potentially be used in integrating a compact spectral imager in smartphones for diverse applications.

Transmissive nonlocal multilayer thin film optical filter for image differentiation

AbstractIt is well-known that a Fourier optical system can be used to perform specific computing tasks, such as image differentiation, with a superior speed and power consumption in comparison with digital computers, despite bulky optical components that are often required. Recently, there has been a surge in the interest to design much more compact nanophotonic structures, such as dielectric and metallic thin films, photonic crystals, and metasurfaces with a tailored angle-dependent (nonlocal) optical response, to directly perform image differentiation without additional lenses for Fourier transformation. Here, we present a straightforward platform, a multilayer dielectric thin film optical filter, fabricated using mature wafer-scale thin film deposition technique, with an optimized nonlocal optical response, for isotropic image differentiation in transmission mode for arbitrary input polarization. The proposed thin film filter may be conveniently coated at various transparent surfaces and inserted in machine vision or microscopy systems for enhanced, real-time image processing.

A symbiotic organisms search algorithm-based design optimization of constrained multi-objective engineering design problems

PurposeIn line with computational technological advances, obtaining optimal solutions for engineering problems has become attractive research topics in various disciplines and real engineering systems having multiple objectives. Therefore, it is aimed to ensure that the multiple objectives are simultaneously optimized by considering them among the trade-offs. Furthermore, the practical means of solving those problems are principally concentrated on handling various complicated constraints. The purpose of this paper is to suggest an algorithm based on symbiotic organisms search (SOS), which mimics the symbiotic reciprocal influence scheme adopted by organisms to live on and breed within the ecosystem, for constrained multi-objective engineering design problems.Design/methodology/approachThough the general performance of SOS algorithm was previously well demonstrated for ordinary single objective optimization problems, its efficacy on multi-objective real engineering problems will be decisive about the performance. The SOS algorithm is, hence, implemented to obtain the optimal solutions of challengingly constrained multi-objective engineering design problems using the Pareto optimality concept.FindingsFour well-known mixed constrained multi-objective engineering design problems and a real-world complex constrained multilayer dielectric filter design problem are tackled to demonstrate the precision and stability of the multi-objective SOS (MOSOS) algorithm. Also, the comparison of the obtained results with some other well-known metaheuristics illustrates the validity and robustness of the proposed algorithm.Originality/valueThe algorithmic performance of the MOSOS on the challengingly constrained multi-objective multidisciplinary engineering design problems with constraint-handling approach is successfully demonstrated with respect to the obtained outperforming final optimal designs.

Designing multilayer dielectric filter based on TiO2/SiO2 for fluorescence microscopy applications

This study presents a new construction design of a distributed Bragg reflector (DBR) filter and a Fabry–Pérot (FP) filter by using needle technique as a synthesis method. The optimized DBR and FP filters having a proper number of layers with controlling thickness TiO2/SiO2 are utilized to transmit only a certain narrow band of wavelengths while blocking the others. As a proof of concept, the filters are designed to selectively transmit only a very narrow band of wavelength at 780 nm which is the near infrared (NIR) fluorescent emission from Alexa Fluor 750 dye. The obtained results show that the optimized filters represent advanced spectral performance which can be used to improve the sensitivity and the imaging contrast in fluorescence microscopy.

Design and fabrication of multilayer dielectric film for combination device spatial filter

Spatial low-pass filters are of great importance to high-power laser systems. Different from the conventional multilayer dielectric film spatial filter device that consists of single component, a combination device of spatial filter that includes multiple film components is more appealing due to its higher laser-induced damage threshold. A novel multilayer dielectric film used for combination device spatial filter is presented in this paper. The sensitivity of the multilayer dielectric film was analyzed, and the main studies are given to the design, fabrication and performance analysis of cutoff filter-combination device. The laser-induced damage threshold of the multilayer dielectric films was tested. It is anticipated that the combination of two cutoff filters can be applied in practical applications of devices of high-power laser systems.

Impact of Wavelength Shift in Relative Spectral Response at High Angles of Incidence in Landsat-8 Operational Land Imager and Future Landsat Design Concepts

The Landsat program plays an important role in providing continuous long-term multispectral moderate resolution observations of the earth’s surface. There is an interest by the community to improve the temporal sampling in future generations of the sensors. One way to achieve higher sampling is to collect imagery with a wider swath. For future instruments using multilayer dielectric filters for band selection, this has the further implication that light may enter the filters on the detectors at a higher angle of incidence, which will shift the center wavelength of the spectral bandpasses. Through simulation of a forest environment this paper explored the impact of this effect on measured spectral radiance and the normalized vegetation difference index. The effect is quantified both for angles of incidence seen in the current Landsat-8 Operational Land Imager as well as for pushbroom designs with up to 13° off-axis imaging. Results indicate the effect will be significant compared to instrument noise and comparable to the limit of current requirements on filter manufacturing tolerances.

Simulating radiation thermometer temperature measurement error from the performance change of an interference filter due to polarization effect

A radiation thermometer is an optical instrument in which the dielectric optical filter is a critical component. Dielectric multilayer optical filters are favored in most radiation thermometer designs for selecting a certain wavelength range of the flux. Asa component those filters are affected by different conditions to which they are exposed, which might lead to error in temperature measurement using radiation thermometer. The radiation thermometer is mostly used at normal to the flux source where the interference filter have the possibility of receiving collimated radiant flux (angle of incidence 0°) after passing through a lens, but there are optical set up situation where the lens involved converge part of the flux beam. In a converged beam the incidence angle can be within a range of approximately 0–20°. In this situation the measured temperature will be affected, as polarization introduces some error specifically from the interference filter. The error might be very small for most industrial applications, but quantifying it is beneficial specifically for high accuracy measurement required in high temperature measurement. It will be demonstrated through simulation how the polarization effect can be quantified and be related to temperature error by considering different design conditions of a multilayer interference filter. The paper also demonstrates how a polarization consideration in a multilayer dielectric filter design can minimize this error. It is practically demonstrated that wavelength shift, broadening of a full width at half maximum (FWHM) and reduced transmittance at central wavelength can be determined by comparing the performance of the designed optical filters.

Realization of mode independent multichannel transmission filter by controlling the photon localization in symmetric cavities

Modern optical communication system requires multiple channel transmission in different spectral band for different applications. Here we are proposing a microresonator structure for closely spaced, mode independent multiple channel transmission in 3rd transmission window by controlling the localization of photons in its cavity. To design the proposed structure, the spectral filtering properties of a single cavity planar microresonator are studied initially. In conventional multilayer dielectric filter, the localization of light in the microcavity is generally controlled by controlling the number of layers. Here we have shown that application of electric field across the structure, designed with suitable materials can also affect the localization of light in the cavity as well as the Q values of the transmission spectra. The mode independent characteristic of the structure is found from the study of its transmittance characteristics at different angle of incidence of light. It is found that the results of single cavity structure hold well for multicavity resonator structure also, in the regime of optical communication wavelength range. A method to increase the number of transmission channels is also proposed here keeping the fabrication perspective in mind.

Biomedical bandpass filter for fluorescence microscopy imaging based on TiO2/SiO2 and TiO2/MgF2 dielectric multilayers

We report a design for creating a multilayer dielectric optical filters based on TiO2 and SiO2/MgF2 alternating layers. We have selected Titanium dioxide (TiO2) for high refractive index (2.5), Silicon dioxide (SiO2) and Magnesium fluoride (MgF2) as a low refractive index layer (1.45 & 1.37) respectively. Miniaturized visible spectrometers are useful for quick and mobile characterization of biological samples. Such devices can be fabricated by using Fabry-Perot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. Distributed Bragg Reflectors (DBRs) consisting of alternating high and low refractive index material pairs are the most commonly used mirrors in FP filters, due to their high reflectivity. However, DBRs have high reflectivity for a selected range of wavelengths known as the stopband of the DBR. This range is usually much smaller than the sensitivity range of the spectrometer range. Therefore a bandpass filters are required to restrict wavelength outside the stopband of the FP DBRs. The proposed filter shows a high quality with average transmission of 97.4% within the passbands and the transmission outside the passband is around 4%. Special attention has been given to keep the thickness of the filters within the economic limits. It can be suggested that these filters are exceptional choice for florescence imaging and Endoscope narrow band imaging.

Multispectral Detection with Metal-Dielectric Filters: An Investigation in Several Wavelength Bands with Temporal Coupled-Mode Theory

Multispectral infrared (IR) detection is of great interest to enhance our ability to gather information from a scene. Filtering is a low-cost alternative to the complex multispectral device architectures to which the IR community has devoted much attention. Multilayer dielectric filters are standard in industry, but they require changing the thickness of at least one layer to tune the wavelength. Here, we pursue an approach based on apertures in a metallic layer of fixed thickness, in which the filtered wavelengths are selected by varying the aperture geometry. In particular, we study filters made of at least one sheet of resonating apertures in metal embedded in dielectrics. We will discuss two interesting problems that arise when one attempts to design such filters. First, metallic absorption must be taken into account. Second, the form and size of the pattern is limited by lithography. We will present some design examples and an attempt at explaining the filtering behavior based on the temporal coupled mode theory. That theory models the filter as a resonator interacting with the environment via loss channels. The transmission is solely determined by the loss rates associated with those channels. This model allows us to give a general picture of the filtering performance and compare their characteristics at different wavelength bands.

Modelling of multilayer dielectric filters based on TiO2/SiO2 and TiO2/MgF2 for flourescence microscopy imaging

We report a design for creating multilayer dielectric optical filters based on TiO2 and SiO2/MgF2 alternating layers. We have selected Titanium dioxide (TiO2) for high refractive index (2.5), Silicon dioxide (SiO2) and Magnesium fluoride (MgF2) as a low refractive index layer (1.45 and 1.37) respectively. Miniaturized visible spectrometers are useful for quick and mobile characterization of biological samples. Such devices can be fabricated by using Fabry-Perot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. Distributed Bragg Re-flectors (DBRs) consisting of alternating high and low refractive index material pairs are the most commonly used mirrors in FP filters, due to their high reflectivity. However, DBRs have high re-flectivity for a selected range of wavelengths known as the stopband of the DBR. This range is usually much smaller than the sensitivity range of the spectrometer. Therefore, bandpass filters are required to restrict the wavelength outside the stopband of the FP DBRs. The proposed filter shows high quality with an average transmission of 97 % within the passbands and the transmission outside the passband is around 3 %. Special attention has been given to keep the thickness of the filters within the economic limits. It can be suggested that these filters are exceptionally promising for florescence imaging and narrow-band imaging endoscopy.

A non-polarization short-wave-pass thin film edge filter

Multilayer dielectric thin film edge filter has serious polarization sensitivity under oblique incidence. The cutoff-bands of the s-polarization and p-polarization light in conventional edge filter will separate obviously under 45° oblique incidence, which limits its application. Based on the two chosen materials TiO2 and SiO2, a novel stack structure is proposed to design the non-polarization short-wave-pass thin film edge filter. By using the (4H 4L 4H) as the matching layers, the polarization separation at 3 dB transmittance for the thin film edge filter cutoff-band is less than 1 nm at the incident angle of 45°. In this way, the non-polarization short-wave-pass edge filter is easily designed and fabricated.

Design of an integrated polarization band-pass filter

In this study, an integrated polarization band-pass filter was designed; it consists of a bilayer nanowire-grid polarizer and a multilayer dielectric band-pass filter. The grating parameters were optimized by rigorous coupled-wave analysis theory and analyzed by effective medium theory. Transverse magnetic transmission higher than 81% and extinction ratio higher than 1000 were obtained at 400–500 nm, and extremely low transmission in the other spectrum was also realized.

A Dielectric Multilayer Filter for Combining Photovoltaics with a Stirling Engine for Improvement of the Efficiency of Solar Electricity Generation

In this Letter we outline a dielectric multilayer spectrally selective filter designed for solar energy applications. The optical performance of this 78-layer interference filter constructed by TiOx and SiO2 is presented. A hybrid system combining photovoltaic cells with a solar-powered Stirling engine using the designed filter is analyzed. The calculated results show the advantages of this spectrally selective method for solar power generation.

Pareto Optimal Microwave Filter Design Using Multiobjective Differential Evolution

Microwave filters play an important role in modern wireless communications. A novel method for the design of multilayer dielectric and open loop ring resonator (OLRR) filters under constraints is presented. The proposed design method is based on generalized differential evolution (GDE3), which is a multiobjective extension of differential evolution (DE). GDE3 algorithm can be applied for global optimization to any engineering problem with an arbitrary number of objective and constraint functions. GDE3 is compared against other evolutionary multiobjective algorithms like nondominated sorting genetic algorithm-II (NSGA-II), multiobjective particle swarm optimization (MOPSO) and multiobjective particle swarm optimization with fitness sharing (MOPSO-fs) for a number of microwave filter design cases. In the multilayer dielectric filter design case a predefined database of low loss dielectric materials is used. The results indicate the advantages of this approach and the applicability of this design method.

Patterning of multilayer dielectric optical filters using metal masks fabricated by electroforming and photolithography

Ni films, replacing photoresists, serve as masks in the selective deposition of optical thin films by electron-beam gun evaporation at a substrate temperature of 300°C. Photolithography is adopted herein to define the growth of Ni films by electroforming. Mosaic patterns with a width of 20 µm are employed as red color filters, which are longwave pass. These red filters are formed from alternate SiO2 and TiO2 layers and have a mean transmittance of >90%. The experimental results show that the combined use of photolithography, electroforming, and electron-beam gun evaporation can deposit miniaturized multilayer dielectric coatings with high light transmittance in a hot process.

Sharp edge wavelength filters utilizing multilayer photonic crystals

A novel configuration of dielectric multilayer wavelength filters for enabling sharp cut-off characteristics is proposed. By applying perturbations to the multilayer structures such as corrugation or lateral film isolation, deep optical stopbands can be created as a result of the coupling between the obliquely and horizontally propagating light waves. Numerical simulation by FDTD revealed that the proposed structure had approximately two to three times larger decay constants than that of unmodulated flat multilayer.

Photonic crystal filters for multi-band optical filtering on a monolithic substrate

Many applications require the ability to image a scene in several different narrow spectral bands simultaneously. Conventional multi-layer dielectric filters require control of film thickness to change the resonant wavelength. This makes it difficult to fabricate a mosaic of multiple narrow spectral band transmission filters monolithically. We adjusted the spectral transmission of a multi-layer dielectric filter by drilling a periodic array of subwavelength holes through the stack. Multi-band photonic crystal filters were modeled and optimized for a specific case of filtering six optical bands on a single substrate. Numerical simulations showed that there exists a particular air hole periodicity which maximizes the minimum hole diameter. Specifically for a stack of SiO2 and Si3N4 with the set of filtered wavelengths (nm): 560, 576, 600, 630, 650, and 660, the optimal hole periodicity was 282 nm. This resulted in a minimum hole diameter of 90 nm and a maximum diameter of 226 nm. Realistic fabrication tolerances were considered such as dielectric layer thickness and refractive index fluctuations, as well as vertical air hole taper. Our results provide a reproducible methodology for similar multi-band monolithic filters in either the optical or infrared regimes.