Band-Reject Filters for Bragg Mirror Using Perturbed Chiral Sculptured Thin Films

Transcription

1 Journal of Electrical Engineering 4 (016) 4-9 doi: /38-3/ D DAVID PUBLISHING Band-Reject Filters for Bragg Mirror Using Perturbed Chiral Sculptured Thin Films Zahir Muhammad 1, Mukhtar Ahmad and Li Song 3 1. National Snchrotron Radiation Laborator & Uniersit of Science and Technolog of China, Hefei, Anhui 3009, China. Department of Phsics, Islamia College Uniersit, Peshawar 510, Pakistan 3. National Snchrotron Radiation Laborator & Uniersit of Science and Technolog of China, Hefei, Anhui 3009, China Abstract: We hae theoreticall simulated the optical properties of perturbed chiral STF (sculptured thin film) with and without defect. It was reealed that such STFs could act as band-reject filters with tunable Bragg regime of free space waelengths and different bandwidth through changing the apor incident angle during glancing angle deposition process. These Bragg filters are narrowing due to the change of apor incident angle for circularl polaried light. Interestingl, as the apor incident angle increased, the band-rejected filter ehibited to decrease the bandwidth and shifted in the Bragg regime, indicating that such STF nanostructures hae high potential for behaing as unique polariation insensitie Bragg mirror. Ke words: Bragg phenomenon, sculptured thin films, optical filters, nano-helies, Bragg mirrors. 1. Introduction Sculptured thin films of helicoidall shaped morphologies are called CSTFs (chiral sculptured thin films) [1]. These shapes are fabricated, most commonl, using phsical apor deposition b eaporating and directing the apors from a source towards a rotating substrate tilted at an angle eperimentall. The local tilt or column angle χ growing on the substrate as a function of apor incident angle. Usuall chiral STFs (sculptured thin films) consist of helical nanowires oriented perpendicular to the surface of an substrate. Structurall chiral material, as well as molecules ehibits helical orientation order [, 3]. Structurall chiral materials fabricated as a stacks using thin film technolog [1, 4]. Chiral STFs hae the propert that reflects circularl polaried light of one handedness, but a er little of the other, in the Bragg regime of free space waelength which is called circular Bragg phenomenon and the Bragg regime is called circular Bragg regime [5]. Structurall chiral Corresponding author: Zahir Muhammad, Ph.D., research student, research fields: nanophotonics and nanomaterials, muhammad@student.qau.edu.pk. materials hae the propert of circular Bragg phenomenon [1]. Chiral STFs of multi-section structures produced a spectral hole in the Bragg regime which contained either chiral STFs laer or twist defects or both in the middle [6-8]. The selectie remittances for circular polariation of light hae the CP-dimorphic optical response, propert can be eploited for a ariet of optical filters and sensors [7] using matched chiral STFs. Due to porosit, the chiral STFs are used for optical sensing in humidit and arious chemicals [6, 10, 11]. Circular Bragg phenomenon in chiral STFs has the propert which is used as circular polariation filters [1], which ma be polariation sensitie or insensitie. Circular polariation insensitie band rejection or bandpass filters are fabricating using cascaded chiral STFs [13, 1]. The phase discontinuit in the center of the periodic structure introduces a hole in the Bragg regime of circular Bragg phenomenon which is acting as filters. The spectral hole appears in the Bragg gratings [14] and narrow bandpass filtering in optical fiber communication [8], with increasing the thickness of the chiral STF. The spectral hole in the Bragg regime

2 Band-Reject Filters for Bragg Mirror Using Perturbed Chiral Sculptured Thin Films 5 produced narrow passband which is die out and a new ultranarrow spectrum is occurring in the Bragg regime of free space waelength due to increasing thickness [15]. Multiple spectral holes are fabricated in the Bragg regime b inserting twist defects which are different from each other [15, 16]. Same as the tilt-modulated chiral STF haing the propert of circular Bragg phenomenon and spectral hole designing [15]. These sculptured thin films are porous and its porosit is inersel proportional to the refractie inde of the materials. We use GLAD (glancing angle deposition) technolog which fabricates optical thin films, b controlling the refractie inde and porosit [17]. Here, in this manuscript we see the optical response of perturbed chiral sculptured thin films fabricated b using GLAD process to find out the following issues. We are studing that how the spectrum of the circularl polaried light is controlled oer apor incident angle using perturbed chiral STFs and how the chiral STFs changes the tolerance of the Bragg regime is controlled to fabricate different optical filters in the direction of apor flu using GLAD process? How without and as well as defect STFs are used for the fabrication of Bragg mirror and band-reject filters in different regimes of waelength? The mathematical description for the chiral STFs is presented and described in sec.. The numerical discussion is presented in sec. 3 and concluding remarks are in sec. 4. In ep (iωt) time, dependenc is implicit, with which ω is the angular frequenc and t is the time. Whereas, the ωε µ, /, denoted the waelength and wae-number while ωµ /ε, the intrinsic impedance of free space respectiel. Here ε and µ are the perm-itiit and permeabilit of free space corresp-onddingl. The ectors are denoted b boldface letters and for dadics we hae used twice underlined. The Cartesian unit ectors are represented as u, u, and u.. Theor Let s consider the schematics diagram of the chiral STFs without as well as defect at = 0 structures is shown in Fig. 1. These chiral STFs are periodicall nonhomogeneous. Therefore the permittiit of these STFs is written in the dadic form as: o ref ( ) s ( ) s ( ) ( ) s ( ) s ( ), (1) where the rotation dadic of the STFs is written as: s ( ) u u ( u u u u ) sin( ), and shape dadic of chiral STF is, s ( ) u u u u ( u u ) sin. T T u u ) cos( ) ( u u u u ) cos ( u u () (3) Fig. 1 Schematics of chiral STF haing defect and without defect.

3 6 Band-Reject Filters for Bragg Mirror Using Perturbed Chiral Sculptured Thin Films o ref From Eq. (1), the relatie permittiit dadic of the local orthorhombic smmetr of the () STF is epressed through a dadic function as: ( ) ( ) u u ( ) u u ( ) u u (4) o ref a where ε a,b,c () is scalar relatie permittiit of the STF. In Eq. (1), χ is the tilt angle which aries as a function of depth into the film (along the -ais) which is further as a function of apor incidence angle. In Fig. 1, χ is the tilt relatie to the plane. The relation of column angle χ and deposition angle b c without modulation using Tait s rule, which measures the porosit and inde of refraction using tangent rule for the GLAD process [18] as: 1 1cos sin ( ). (5) The defect structures of chiral STFs from Eq. (1), we can write as: h, (6) where Ω is the structural period and h is structural handedness which can be either +1 or 1 to indicate one of the two tpes of structural handedness. The structural period of the CSTF and the defect at = 0. represents To stud the optical response of these chiral STFs we considered incident plane waes of circular polariation states ecited on the CSTF nanostructures that hae been formulated elsewhere [1]. After using boundar-alue problem to find the reflectances and transmittances whereas, the chiral STF has been modeled using piecewise-uniform approimation [1]. The chiral STF is diided into thin slices parallel to the plane in this technique and the permittiit of the chiral STF is taken to be uniform which is same as of the middle slice. In this technique thickness of slice is taken 0.5 nm after ascertaining that reflectances and transmittances conerged within 0.5% of their alue when the slice thickness was 5 nm and 0.1% when it is 1 nm. 3. Numerical Results and Discussion For numerical results, we considered the principle relatie permittiities ε a,b,c of the perturbed columnar thin film of titanium oide (TiO). This is modulated. The relationship between χ and χ with ε a,b,c has been measured b Hodgkinson et al. [19], for the eperimental data titanium oide as follow, where, a b c / / / / / 1 tan( ) ( ) tan.8818, /, (8) (7) Furthermore, we chose Ω = 00 nm, h = ±1, and χ was kept ariable. The remittance spectrum of circularl polaried light of unperturbed chiral STF without defect as a function of free-space waelength is shown in the Fig.. Fig. shows that light is maimall reflected in the Bragg regime which behaes as a perfect reflector. The transmittance spectrum T RR shows that the unperturbed chiral STF behaes as Bragg mirror in the Bragg regime. Similarl for perturbed chiral STF without defect using GALD process we obsered the same effect but, the bandwidth is decreasing and its center waelength is shifting in the Bragg regime as we increase the apor incident angle. Fig. 3 shows that Bragg regime is shifted but, the phenomenon is still occurring in CSTF without defect on perturbation. The band of perfect reflector and band-reject filters is decreasing due to perturbation. Een further increasing the alue of χ the gien phenomenon is shifted further in the Bragg regime and its bandwidth is more decreasing which are seen from Fig. 4. The transmittance is acting as band-reject filters. Moreoer, b further increasing the apor incident angle through GALD process the gien phenomenon is disappearing

4 Band-Reject Filters for Bragg Mirror Using Perturbed Chiral Sculptured Thin Films 7 Fig. Remittance spectrum as a function of λ of h = ±1, χ = 0 o, Ω = 00 nm, L = 60 Ω and normal incidence. Fig. 3 Same as Fig., ecept χ = 10 o. Fig. 4 Same as Fig., ecept χ = 5 o. Fig. 5 Same as Fig. 4, ecept aring incident angle. in that Braggg regime and shifted in another regimee and will narrower een; it will disappear in the Bragg regime. In Fig. 5, we eamined the effect of incident angle on the remittance spectrum using without defect perturbed chiral STFs. We eamined that, oblique incident angle cannot affect the same phenomenon in the Bragg regime. We can see that, for a same apor incident angle perfect reflector and band-reject filter is occurring for each angle in the same Bragg regime and center waelength.

5 8 Band-Reject Filters for Bragg Mirror Using Perturbed Chiral Sculptured Thin Films Fig. 6 Remittances spectrum of a normall ecited as a function of λ of h = ±1, Ω = 00 nm, L = 30 Ω and χ = {0 o, 5 o, 10 o, 15 o }. Fig. 7 Same as Fig. 6, ecept χ = 10 o and aring incident angle. Furthermore let s use defect nanostructuress of perturbed chiral STF as shown in the Fig. 1b to obsere its optical properties. The computed remittance spectrums for LCP (lift circularl polaried) incident light is represented b R LL and T LL which are computed with a waelength resolution of 0.1 nm as shown in the Fig. 6. The transmittance spectrum T LL from the perturbed defect chiral STFs shows band-reject filters and reflectance R LL shows Bragg mirror. When χ increases from 0, to higher alue the chiral STF still behaes as band-reject filters for same circular polariation states of incident light. Howeer, though increasing alues of χ the bandwidth and the central frequenc are different from each other in the Bragg regime. Moreoer, further increasing the apor incident angle the gien phenomenon will be disappeared and it will be narrower and it will shift to higher regime of free-spacee waelength. The same were obsered for different incident angles. From Fig. 7, it is seen that, the incident angle cannot affect the circular Bragg band-reject filters. Therefore, for different incident angles of same χ, it is iewed that, the filters hae bandwidth and center waelength. 4. Concluding Remarks phenomenon which is fabricating We conclude that, in the Bragg regime we find band-reject filters when a uniform band gap is sandwiched between two identical perturbed chiral STFs or central defect in the perturbed CSTF using circularl polaried light. In the Braggg regime we hae obsered that the perturbed chiral STF permits light of particular polariation state in different regimes using different apor incident angles. In thesee

6 Band-Reject Filters for Bragg Mirror Using Perturbed Chiral Sculptured Thin Films 9 Bragg regimes we reealed that it totall reflects light and no transmission which, fabricated band-reject filters and perfect reflectors. The circular Bragg phenomenon shown here as a wide band-reject filters and polariation insensitie Bragg mirrors for chiral STF with defect. But, it is attained that, different band-reject filters hae different bandwidth and central frequenc using different apor incident angles. Therefore polariation insensitie sensitie band-reject filters and perfect reflectors make polariation insensitie Bragg mirrors for circularl polaried light using perturbed chiral STFs. Acknowledgement The financial support was from the National Basic Research Program of China (014CB ), the National Natural Science Foundatio of China (U13131, U15311, , ) and User with Potential from CAS Hefei Science Center (015HSC-UP00). Z. Muhammad acknowledges the CSC fellowship for financial support. References [1] Lakhtakia A., and Messier, R Sculptured Thin Films Nanoengineered Morpholog and Optics. SPIE Press. [] Li, Q Nanoscience with Liquid Crstals. Springer. [3] Robbie, K., Broer, D. J., and Brett, M. J Chiral Nematic Order in Liquid Crstals Imposed b an Engineered Inorganic Nanostructure. Nature 399 (6738): [4] Miles, M. W., Brian, J. G., and Clarence, C Thin Film Precursor Stack for MEMS Manufacturing. U.S. Patent 1 (7): 495. [5] Muhammad, Z., Farad, M., and Naqi, Q. A Suppression of Circular Bragg Phenomenon in Tilt Modulated Chiral Sculptured Thin Films at Oblique Incidence. Opt. Eng. 53 (11): [6] Wang, F., and Lakhtakia, A Specular and Nonspecular, Thickness-Dependent, Spectral Holes in a Slanted Chiral Sculptured Thin Film with a Central Twist Defect. Optics Commun. 15 (1): [7] Wang, F., and Lakhtakia, A Optical Crossoer Phenomenon due to a Central 90-Twist Defect in a Chiral Sculptured Thin Film or Chiral Liquid Crstal. In Proceedings of the Roal Societ of London A: Mathematical, Phsical and Engineering Sciences 461 (061): [8] Lakhtakia, A., and McCall, M Sculptured Thin Films as Ultranarrow Bandpass Circular Polariation Filters. Optics Commun. 168 (5): [9] Lakhtakia, A Nanometer Structures: Theor, Modeling, and Simulation. SPIE Press. [10] Liu, Y. J., Shi, J., Zhang, F., Liang, H., Xu, J., Lakhtakia, A., Fonash, S. J., and Huang, T. J High-Speed Optical Humidit Sensors Based on Chiral Sculptured Thin Films. Sens. & Actuat. B: Chem. 156 (): [11] Zourob, M., and Lakhtakia, A Optical Guided-Wae Chemical and Biosensors II. Vol. 8. Springer Science & Business Media. [1] Wu, Q., Hodgkinson, I. J., and Lakhtakia, A Circular Polariation Filters Made of Chiral Sculptured Thin Films: Eperimental and Simulation Results. Opt. Eng. 39 (7): [13] Suuki, M., and Taga, Y Integrated Sculptured Thin Films. Jap. J. of App. Ph. 40 (4A): L358. [14] Torres, P., and Valente, L. G. 00. Spectral Response of Locall Pressed Fiber Bragg Grating. Optics Commun. 08 (4): [15] Muhammad, Z., Naqi, Q. A., and Farad, M Spectral Hole Filters Using Tilt-Modulated Chiral Sculptured Thin Films. Opt. Commun. 346: [16] Lakhtakia, A Generation of Spectral Holes b Inserting Central Structurall Chiral Laer Defects in Periodic Structurall Chiral Materials. Optics Commun. 75 (): [17] Jen, Y. J., and Lin, C. F Anisotropic Optical Thin Films Finel Sculptured b Substrate Sweep Technolog. Opt. Ep. 16 (8): [18] Tait, R. N., Sm, T., and Brett, M. J Modeling and Characteriation of Columnar Growth in Eaporated Films. Thin Solid Films 6 (): [19] Hodgkinson, I. J., Wu, Q. H., and Hael, J Empirical Equations for the Principal Refractie Indices and Column Angle of Obliquel Deposited Films of Tantalum Oide, Titanium Oide, and Zirconium Oide. Appl. Opt. 37 (13):