IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 30, NO. 2, JANUARY 15, 2018 189 Transmission Filters Utilizing Cavity Resonances in Bandgap-Engineered Monomaterials Rajorshi Bandyopadhyay and Rajib Chakraborty Abstract—A monomaterial-based resonator structure is proposed here, which can replace the multilayer-based narrow- band transmission filter. This new concept of introducing the effect of multilayered structures of different materials into a single material is based on etching out repeatable structures of two different dimensions on the same material. As the etched-out repeatable structures are of different dimensions, it is possible to obtain periodic layers of two different-effective refractive indices. This type of monomaterial-based optical filters avoids the challenges generally faced while fabricating multilayer structures of different heterogeneous materials having different refractive indices. The dependence of the filter action on the number of bilayers of two different-effective refractive-index materials formed by etching and on the cavity region thicknesses is studied. Although the study is done mainly on lithium niobate on insulator, but it is seen that similar effects occur for materials of varying refractive indices. Index Terms—Band-pass filters, C band, optical resonator, periodic structure, WDM network. I. I NTRODUCTION O PTICAL filters are a vital component in wavelength- division multiplexing (WDM) transmission systems used in optical communications. A lot of work has already been done to design and develop various optical filters that can oper- ate in the infrared (IR) and terahertz (THz) regions [1]–[5]. Narrowband filters have been fabricated by creating a cavity inside a one-dimensional photonic-bandgap (PBG) structure, which is actually a periodic layer of thin films of two dissimilar reflective materials. The choice of materials used depends upon the desired wavelength and can be dielectrics, metals, semiconductors, organic materials [5]–[8]. PBG structures fol- low the Fabry-Perot resonator principle, where the mirrors sur- rounding the cavity are formed by periodic spatial distributions of high- and low-index materials of a suitable refractive-index contrast. These structures utilize the effect of interferences of light to transmit light over a desirable wavelength range. The defect mode within the PBG causes the localization of photons with a corresponding resonant transmittance peak [9]. The chosen materials for these conventional multilayer thin- film interference filters must exhibit good transparency and Manuscript received June 1, 2017; revised November 29, 2017; accepted November 30, 2017. Date of publication December 7, 2017; date of current version January 3, 2018. The work of R. Bandyopadhyay was supported by TEQIP Phase II, University of Calcutta. (Corresponding author: Rajib Chakraborty.) The authors are with the Department of Applied Optics and Photonics, University of Calcutta, Kolkata 700098, India (e-mail: rajorshi.aop@ gmail.com; rcaop@caluniv.ac.in). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2017.2780825 physical robustness in this frequency region. Moreover, the materials used for developing thin-film interference optical filters usually have different thermal expansion coefficients and are fragile to thermal cycles. As a result, multilayer thin-film filters generally suffer from a large stress [4]. Maintaining a proper thickness with subwavelength dimensions is still a challenge. However, these problems can be avoided if the filter can be designed using a single material. In that case, the periodic variation of refractive index and the creation of defects are possible by complete etching of periodic repeatable structures of two different dimensions from a single material. The effective refractive index of each of the periodic layers will be different and so periodic variations of effective refrac- tive index can be achieved. This concept is used here to design narrowband filters on a single material. In this work, we have proposed a filter based on a single material (Lithium Niobate on Insulator, LNOI) with periodic structures of two different dimensions etched in it, which can work as a multilayer thin-film resonant filter. Monomaterial- based multilayer optical filters based on similar etched struc- tures have previously been reported [4], [10]. While in [4] the fabrication involved costly equipment and yielded transmis- sion of 65 ∼ 70%, the structure in [10] involved a simpler fabrication process but the device consisted of three parts and there was no definite relation between the etched structure dimensions among the different parts. In the monomaterial- based optical filter proposed here, the periodic etched struc- tures are considered to be placed in a regular manner, which resembles the characteristics of a conventional PBG structure formed by two different materials. Moreover, these filters can give almost 100% transmission. Similar to multilayer thin-film filters, the quality (Q) value of the proposed monomaterial filter can be improved by increasing the number of bilayers. It is possible to achieve tunability of the proposed filter by changing the etched structure dimensions or by utilizing the electro-optic property of Lithium Niobate. II. NUMERICAL ANALYSIS The finite-difference time-domain (FDTD) method is used here to study the electromagnetic field propagation through the proposed PBG structure. Normal incidence of light is considered for the simulation. As normal incidence of light yields the same result for both transverse electric (TE) and transverse magnetic (TM) modes in dielectrics, the proposed structure is simulated using TE modes of light only. For a linear, isotropic, nondispersive material placed in a source- free region, wave-evolution equations for field values (in both 1041-1135 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.