micromachines Article All-Dielectric Color Filter with Ultra-Narrowed Linewidth Kai Xu 1, Yanlong Meng 1,2,* , Shufen Chen 2, Yi Li 1, Zhijun Wu 3 and Shangzhong Jin 1 1 College of Optical and Electronic Technology, China Jiliang University, Hangzhou 310018, China; [email protected] (K.X.); [email protected] (Y.L.); [email protected] (S.J.) 2 Key Laboratory for Organic Electronics and Information Displays and Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts and Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210023, China; [email protected] 3 Fujian Key Laboratory of Light Propagation and Transformation, College of Information Science and Engineering, Huaqiao University, Xiamen 361021, China; [email protected] * Correspondence: [email protected]; Tel.: +86-571-87676264 Abstract: In this paper, a transmissive color filter with an ultra-narrow full width at half of the maximum is proposed. Exploiting a material with a high index of refraction and an extremely low extinction coefficient in the visible range allows the quality factor of the filter to be improved. Three groups of GaP/SiO2 pairs are used to form a Distributed Brag reflector in a symmetrical Fabry-Pérot cavity. A band-pass filter which is composed of ZnS/SiO2 pairs is also introduced to further promote the purity of the transmissive spectrum. The investigation manifests that a series of tuned spectrum with an ultra-narrow full width at half of the maximum in the full visible range can be obtained by adjusting the thickness of the SiO2 interlayer. The full width at half of the maximum of the transmissive spectrum can reach 2.35 nm. Simultaneously, the transmissive efficiency in the full visible range can keep as high as 0.75. Our research provides a feasible and cost-effective way for realizing filters with ultra-narrowed linewidth. Citation: Xu, K.; Meng, Y.; Chen, S.; Li, Y.; Wu, Z.; Jin, S. All-Dielectric Keywords: color filter; ultra-narrow linewidth; distributed Bragg reflection (DBR) Color Filter with Ultra-Narrowed Linewidth. Micromachines 2021, 12, 241. https://doi.org/10.3390/ mi12030241 1. Introduction Color filter is a kind of optical device which plays a significant role in many industrial Academic Editor: Patrice Salzenstein fields, such as spectroscopy instruments, imaging sensors, and displays [1–4]. The precision and sensibility of many optical systems used in those fields are mainly decided by such Received: 17 January 2021 an optical device. Generally, there are two types of optical filters: reflective filter and Accepted: 24 February 2021 transmissive filter [5]. Usually, the reflective filter is realized by a perfect absorber, of which Published: 27 February 2021 the filtered wavelengths are fully absorbed. As a result, its function is embodied in the reflective characteristic. On the contrary, the filtering function of a transmissive filter is Publisher’s Note: MDPI stays neutral obtained by permitting the required wavelengths to pass through. In comparison with with regard to jurisdictional claims in the reflective filter, the transmissive filter is proper to be used in displays, charge-coupled published maps and institutional affil- devices, and hyperspectral imaging. With respect to the transmissive filter, the quality iations. factor or the full width at half of the maximum (FWHM) of the transmissive spectrum is a principal characteristic if we want a filter with high accuracy and resolution [3]. In addition, the efficiency of the filter is also an important characteristic that should be considered. That is because a higher transmittance means a better energy use of incident light, which will be Copyright: © 2021 by the authors. beneficial to reduce the dissipation of power and obtain a high intensity of the signal in Licensee MDPI, Basel, Switzerland. optical systems using these filters. This article is an open access article Up to now, there have been many reports on filters realized by surface plasmons [6–11], distributed under the terms and Fabry–Pérot (FP) microcavities [12–21], grating guided-mode resonance [22–28], and Mie conditions of the Creative Commons scattering [29–35]. However, high quality-factor filters realized by surface plasmons or Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Mie scattering always involve in many sophisticated nanostructures, such as nanohole 4.0/). array metal films [10,36], metal grating [23,37], and nanoparticle arrays. In comparison, Micromachines 2021, 12, 241. https://doi.org/10.3390/mi12030241 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, x FOR PEER REVIEW 2 of 12 Micromachines 2021, 12, 241 2 of 12 array metal films [10,36], metal grating [23,37], and nanoparticle arrays. In comparison, the FP cavity based on multilayered thin films presents many advantages, such as easy to the FP cavity based on multilayered thin films presents many advantages, such as easy to bebe fabricated, fabricated, facilitating facilitating large-scale large-scale producti production.on. In In recent recent years, years, there there have have been been a a large numbernumber of of reports reports on on the the design design of of filters filters base basedd on on the the FP FP cavity cavity [16,20,21,38–41]. [16,20,21,38–41 ].Using Using a dielectrica dielectric layer layer sandwiched sandwiched by by two two metallic metallic layers layers to to form form an an FP FP cavity cavity is is a a convenient way,way, which is also also named named as as metal-dielectric-metal metal-dielectric-metal (MDM) (MDM) structure. structure. However, However, the the strong strong absorptionabsorption of of metallic metallic layers layers limits limits the the quality quality factor factor [39]. [39]. To To further further improve improve the the quality quality factorfactor of of such such structure, structure, a a distributed distributed Bragg Bragg reflection reflection (DBR) (DBR) is is a a preferable candidate to to substitutesubstitute the the metallic metallic reflective reflective mirror. mirror. Sinc Sincee the the DBR DBR structure structure is fabricated by various dielectricdielectric layers, layers, the the absorption absorption in in metallic metallic layers layers can can be beavoided. avoided. So far, So there far, there are still are few still reportsfew reports on filters on filters with withextremely extremely narrow narrow FWHM FWHM (<10 nm) (<10 in nm) the in visible the visible range range with a with rela- a tivelyrelatively simple simple DBR DBR structure. structure. InIn this this paper, paper, we we propose propose a a transmission filter filter structure structure based based on on an an FP FP cavity. cavity. The The reflectivereflective mirror mirror is is composed composed of of GaP/SiO GaP/SiO2 etalon.2 etalon. As a As direct a direct bandgap bandgap semiconductor semiconductor ma- terialmaterial with with a bandgap a bandgap of 2.26 of 2.26eV, GaP eV, GaP has hasa high a high refractive refractive index index in the in visible the visible range range and canand also can be also prepared be prepared by low-temperature by low-temperature atomic layer atomic deposition layer deposition technology technology or chemical or vaporchemical deposition vapor deposition technology, technology, which is which compatible is compatible with other with thin-film other thin-film deposition deposition pro- cessingprocessing [42]. [42 In]. addition, In addition, the the absorption absorption coefficient coefficient is isclose close to to zero zero when when the the wavelength wavelength isis longer longer than than 470 nm. A multilayer composed ofof ZnS/SiOZnS/SiO22 pairspairs is is used used as as a band-pass filterfilter to to improve improve the the quality quality of of the the spectrum. spectrum. Since Since the the materials materials used used have have relatively relatively small small absorptionabsorption in in the the visible visible range, range, it it is is expect expecteded to to obtain obtain a a smaller smaller FWHM FWHM through through structural structural design.design. This This research research not not only only contributes contributes to to the the preparation preparation of of large-area, large-area, high-resolution high-resolution transmissivetransmissive filters filters but but also also helps helps expand expand the the application range of high-refractive-index semiconductorsemiconductor materials materials in in the the optical optical field. field. 2.2. Simulation Simulation Models Models TheThe simulated simulated structure structure in in this this paper paper is is shown shown in Figure 11.. TheThe lightlight incidentsincidents fromfrom thethe top top of of the the device device and and outputs outputs from from a a glass glass su substrate.bstrate. As As shown shown in in Figure Figure 11,, the device’s structurestructure can can be be divided divided into into two two components components—a—a symmetrical symmetrical FP FP cavity cavity and and a a band-pass band-pass filter. The FP cavity is realized by two DBRs and a SiO interlayer. The DBR consists of filter. The FP cavity is realized by two DBRs and a SiO2 interlayer. The DBR consists of several groups of GaP/SiO pairs. The thicknesses of GaP and SiO in each group and several groups of GaP/SiO2 pairs.2 The thicknesses of GaP and SiO2 in2 each group and the numberthe number of groups of groups have have been been optimized optimized for obtaining for obtaining a transmissive a transmissive spectrum spectrum with with an an ultra-narrow FWHM and high intensity. The ZnS/SiO2 multilayer is set on top of the ultra-narrow FWHM and high intensity. The ZnS/SiO2 multilayer is set on top of the cavity cavity as a band-pass filter for the purpose of improving the purity of the transmissive as a band-pass filter for the purpose of improving the purity of the transmissive spectrum.