Achieving Wide-Band Linear-To-Circular Polarization Conversion Using Ultra-Thin Bi-Layered Metasurfaces

Home , Pang (surname)

Achieving wide-band linear-to-circular polarization conversion using ultra-thin bi- layered metasurfaces Yongfeng Li, Jieqiu Zhang, Shaobo Qu, Jiafu Wang, Lin Zheng, Yongqiang Pang, Zhuo Xu, and Anxue Zhang

Citation: Journal of Applied Physics 117, 044501 (2015); doi: 10.1063/1.4906220 View online: http://dx.doi.org/10.1063/1.4906220 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/4?ver=pdfcov Published by the AIP Publishing

Articles you may be interested in Optical spin-to-orbital angular momentum conversion in ultra-thin metasurfaces with arbitrary topological charges Appl. Phys. Lett. 105, 101905 (2014); 10.1063/1.4895620

A linear-to-circular polarization converter with half transmission and half reflection using a single-layered metamaterial Appl. Phys. Lett. 105, 021110 (2014); 10.1063/1.4890623

Polarization conversions of linearly and circularly polarized lights through a plasmon-induced transparent metasurface J. Appl. Phys. 115, 243503 (2014); 10.1063/1.4885769

Ultra-wideband polarization conversion metasurfaces based on multiple plasmon resonances J. Appl. Phys. 115, 154504 (2014); 10.1063/1.4869917

Highly circularly polarized electroluminescence from organic light-emitting diodes with wide-band reflective polymeric cholesteric liquid crystal films Appl. Phys. Lett. 90, 211106 (2007); 10.1063/1.2741603

Achieving wide-band linear-to-circular polarization conversion using ultra-thin bi-layered metasurfaces Yongfeng Li,1,a) Jieqiu Zhang,1,a) Shaobo Qu,1 Jiafu Wang,1,a) Lin Zheng,1 Yongqiang Pang,1 Zhuo Xu,2 and Anxue Zhang3 1College of Science, Air Force Engineering University, Xi’an 710051, People’s Republic of China 2Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education, Xi’an Jiao-tong University, Xi’an 710049, People’s Republic of China 3School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an, 710049 Shaanxi, People’s Republic of China (Received 19 July 2014; accepted 7 January 2015; published online 22 January 2015) In this paper, we propose to achieve wideband linear-to-circular (LTC) polarization conversion by ultra-thin bi-layered metasurfaces. As an example, an LTC polarization conversion metasurface operating in 11.4–14.3 GHz is designed and fabricated, which is composed of two layers of metallic pattern arrays separated by a 1.5 mm-thick dielectric spacer. When linearly polarized waves impinge on the bi-layered metasurface, LTC polarization conversion transmission is greater than 90% over a wide frequency range from 11.0 GHz to 18.3 GHz. Meanwhile, the axis ratio is lower than 3 dB in 9.8–18.3 GHz. This wide-band and highly efﬁcient LTC polarization conversion transmission is analyzed theoretically. The measured LTC polarization conversion transmissions are well consistent with the simulated results. VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4906220]

I. INTRODUCTION plasmons and surface plasmon polaritons (SPPs).2 A com- posite metamaterial has been proposed to simultaneously Polarization is an important characteristic of electro- achieve the polarization selectivity and the 90 polarization magnetic waves because of the inherent polarization sensi- rotation for the transmitted waves.1 As for the linear-to- tivity of materials, especially in the visible spectrum.1–4 circular (LTC) polarization conversion, ultra-thin quarter- Conventional polarization manipulations are always realized wave plates based on plasmonic metasurfaces were designed using the wave-plate, which is made of birefringent materials and analyzed.12 A double-ring-chain metamaterial that ena- such as crystalline solids and liquid crystals. However, the bles efficient polarization conversion of terahertz waves has large thickness and the narrow bandwidth prevent them from been presented.13 In the work, the linear-to-linear and linear- being integrated into the micro-optical systems. Currently, to-elliptic polarization conversions can be simply achieved polarization manipulation can be achieved by the anisotropic by altering the dimensional parameters of the unit cells of or chiral materials,5,6 yet still with thickness limitations and metasurfaces. As for LTC polarization conversion, multi- bulky configurations. layer structures are always used. For example, a wideband Until recent years, metasurfaces provide a new way of polarization manipulation. Metasurfaces are periodic or circular polarizer in Ka-band was presented using a multi- quasi-periodic planar arrays of sub-wavelength elements.5–19 layer frequency selective surface based on split rings bisected by a metal strip and the axial ratios are lower than The sub-wavelength unit cells always have more freedoms in 14 manipulating the amplitude and phase of the reflected/ 3 dB in the frequency range of 25.5–36.5 GHz. A metasur- refracted waves.7,8 Because of the ultra-thin thickness and face with the capability of converting the linearly polarized (LP) signal from a source antenna into a circularly polarized low loss of metasurface-based polarization manipulators, 6 many research interests have been concentrated in this field. signal was proposed and studied. For example, the polarization control of the reflected waves In this paper, we achieve wide-band LTC polarization for a reflective metasurface is studied.9 Ultra-wideband conversion transmission using an ultra-thin (1.5 mm-thick) linear-to-linear polarization conversion is achieved using an bi-layered metasurface. The thickness is about k0/14, with k0 ultra-thin reflective metasurface.10 An ultra-thin, broadband, being the central working wavelength. The simulated and and highly efficient tri-layered metasurface-based terahertz experimental results indicate that the polarization conversion polarization converter that is capable of rotating a linear transmission is greater than 90% over a wide frequency polarization state into its orthogonal one has been demon- range from 11.0 GHz to 18.3 GHz, and the simulated axis strated due to the Fabry-perot-like resonance.11 An enhanced ratios of the transmitted waves are less than 3 dB in the fre- optical rotation of the zero-order transmitted light through a quency range of 9.818.3 GHz. The extracted polarization silver film with an array of perforated S-shaped holes was conversion transmission from the measured co- and cross- realized due to the contributions of both the localized surface polarization transmission for linearly polarized wave incidence displays a good agreement with the simulated results. a)Authors to whom correspondence should be addressed. Electronic addresses: The calculated ellipticity angle indicates that the operating [email protected]; [email protected]; and [email protected] bandwidth of the LTC polarization conversion metasurface

0021-8979/2015/117(4)/044501/7/$30.00 117, 044501-1 VC 2015 AIP Publishing LLC

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 117.32.153.182 On: Tue, 17 Nov 2015 13:54:30 044501-2 Li et al. J. Appl. Phys. 117, 044501 (2015) pffiffiffi is from 11.4 GHz to 14.3 GHz. The proposed metasurface 2 TRCP y ¼ ðÞTxy iTyy : (5) may find potential applications in the design of wide-band 2 circular polarization antennas. From (4) and (5), it can be found that in order to achieve high-efficiency LTC polarization conversion transmission II. LINEAR-TO-CIRCULAR POLARIZATION under linear polarization incidence, the amplitudes of the co- CONVERSION METASURFACE and cross-polarization transmission coefficients should be approximately equal and as high as possible. Meanwhile, the A. Linear-to-circular polarization conversion phase difference between the co- and cross-polarization transmission transmission coefficients must be np/2. Suppose a beam of y-polarized wave normally incidents onto a polarization conversion metasurface from the þz B. Design of the LP-to-CP polarization conversion direction. The electric field component of the incident wave metasurface ikzz can be expressed as Ei ¼ Ey ¼ yE^ 0e . The transmitted In order to realize high-efficiency LTC polarization con- electric field can be written as version, we take advantage of the polarization rotation per- formance of metasurfaces. By delicate design, both the phase Et ¼ yT^ yyEi þ xT^ xyEi: (1) and amplitude of co-/cross-polarization transmission coeffi-

The transmission coefficients can be expressed as Tyy ¼ tyy cients can be tailored to manipulate polarization of waves exp(i /yy), Txy ¼ txyexp(i/xy), where tyy and txy are the ampli- according to our will. Here, a class of bi-layered metasurface composed of two metallic gratings in orthogonal directions tudes of transmission coefficients Tyy and Txy; /yy and /xy and two linear polarization rotators is proposed to realize the are the phases of transmission coefficients Tyy and Txy.We LTC polarization conversion transmission. use TLCP-y and TRCP-y to denote the LTC polarization conver- Figure 1 gives schematic views of the designed sion transmission coefficients. tLCP-y, tRCP-y and /LCP-y, bi-layered metasurface, which is composed of two layers of /RCP-y represent the corresponding amplitudes and phases. Then the electric fields of the transmitted LCP wave and metallic pattern arrays [see in Figs. 1(a) and 1(b)] and a RCP wave can be expressed as dielectric spacer between them. In the design, a 1.5 mm thick F4B (e ¼ 2.65 and tand ¼ 0.001) substrate is used as the pffiffiffi r spacer dielectric. The geometrical parameter a is the repeti- 2 ip E ¼ T E e 2 x^ þ E y^ ; (2) LCP LCPy 2 i i tion period of the unit cell, l is the length of the obliquely ffiffiffi placed short metallic wire, and l is the length of the short p 1 2 ip metallic wire. The width of the short metallic wires is w, ERCP ¼ TRCP y Eix^ þ Eie 2y^ : (3) 2 while the width of the continuous metallic wires for the upper and lower layers is w . The optimized values for these dimen- Thus, the LTC polarization conversion transmission 1 sion parameters are all depicted in the caption of Fig. 1. coefficients can be deduced from the LP-to-LP co- and cross-polarization transmission coefficients under linearly C. Simulation and analysis polarized wave incidence, as described below pffiffiffi As for the designed LTC polarization conversion meta- 2 surface, the full-wave numerical simulation was performed TLCP y ¼ ðÞTyy iTxy ; (4) 2 by using CST microwave Studio and the simulated results

FIG. 1. Schematic views of the polarization conversion bi-layered metasurface. (a) Front view of the upper layer of metallic pattern array, (b) front view for the lower layer of metallic pattern array, and (c) the perspective view of the two layers of metallic pattern arrays. The values for the dimension parameters are a ¼ 4 mm, l ¼ 3.3 mm, l1 ¼ 1.2 mm, w ¼ 0.2 mm, and w1 ¼ 0.1 mm.

are given in Fig. 2. The polarization conversion transmis- To discuss the polarization state of the transmitted sions under different polarized waves normal incidence from waves under y-polarized wave incident from þz direction, þz and –z directions are given in Figs. 2(a) and 2(b), respec- four Stokes parameters are introduced below tively. From Fig. 2(a), one can ﬁnd that as a beam of LP 2 2 wave incidents from þz direction, the y-polarized wave is I ¼jtyyj þjtyxj ; (6a) highly transmitted and converted into LCP wave with LTC Q ¼jt j2 jt j2; (6b) polarization conversion transmission (tLCP-y) greater than yy yx 90% over the frequency range from 11.0 GHz to 18.3 GHz. U ¼ 2jt jjt j cos u ; (6c) For the case under the x-polarized incident wave from yy yx diff –z direction, the LTC polarization conversion transmission V ¼ 2jtyyjjtyxj sin udiff ; (6d) (tRCP-x) larger than 90% can be also obtained in the same fre-

quency region of 11.0–18.3 GHz [see Fig. 2(b)]. In addition, where tyy(xy) and /yy(xy) are, respectively, the amplitudes and under the CP wave incidence, the high-efficiency LCP-to-x phases for the co-(cross-)polarization transmissions, /diff is polarization conversion transmission from þz direction and the phase difference between the x component and y compo- RCP-to-y polarization conversion transmission from –z nent of the transmitted wave, i.e., /diff. ¼ /xy /yy. The direction over 90% are all obtained in the frequency range simulated amplitudes and phases for the co- and cross- from 11.0 GHz to 18.3 GHz. polarization transmissions for y-polarized wave incident The LTC polarization conversion transmissions under y- from þz direction are given in Fig. 4(a), where the left axis polarized wave incidence from þz direction with different denotes the amplitudes and the right axis represents the incidence angles h (0 –80 ) are given in Fig. 3(a), in which phases. The parameter A is defined as A ¼ abs(txy)/abs(tyy). If the inset is the depiction of the incidence angle h. It is found the phase difference /diff. ¼ 0 or p, the transmitted wave is that when the incidence angle is less than 40 , the LTC linearly polarized. When A ¼ 1 and /diff. ¼ p/2 are simulta- polarization conversion transmission is almost unchanged. neously satisfied, the transmitted wave is circularly polar- However, the LTC polarization conversion transmission is ized, otherwise the transmitted wave is elliptically polarized. quickly depressed with increasing incidence angle larger In addition, when the phase difference meets 0 < /diff. < p, than 40 except for the polarization conversion transmission the transmitted wave is left-handed polarized and is right- peak at f ¼ 12.1 GHz. In addition, the polarization-angle de- handed polarized when the phase difference is between p pendence of the LTC polarization conversion transmission is and 2p. The polarization ellipse for the transmitted wave is also given in Fig. 3(b), in which the inset illustrates the defined through the polarization azimuth angle a and the el- polarization angle of the incident wave u. The LTC polariza- lipticity angle b, which can be obtained from tion conversion transmissions for LP wave incident from þz U direction with different polarization angles u from 30 to tan 2a ¼ ; (7) 30 are given in Fig. 3(b), where 0 corresponds to the Q y-polarized incident wave. Obviously, the LTC polarization V conversion transmission is extremely sensitive to the polar- sin 2b ¼ ; (8) I ization angles of the incident LP wave. The peak value and the bandwidth of the polarization conversion transmission where the polarization azimuth angle a describes the direc- rapidly decrease with increasing polarization angle u. tion of the principal axis of ellipse. The ellipticity angle b

FIG. 2. The simulated polarization conversion transmission for LP and CP waves illuminating from (a) þz direction and (b) –z direction.

FIG. 3. (a) The incidence angle and (b) polarization angle dependence of the polarization conversion transmission amplitude versus the frequency.

describes the shape of the ellipse. Figure 4(b) gives the cal- are all less than 3 dB. This is in a good agreement with the culated polarization azimuth angles and ellipticity angles simulated ellipticity angle. Figure 5(b) visualizes the polariza- versus frequency from 8 GHz to 20 GHz according to (7) and tion ellipses of the transmitted waves at different frequencies (8), where the inset illustrates the polarization azimuth angle (f ¼ 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, and 19 GHz) for y- and ellipticity angle. It is found that over the frequency range polarized wave incident from þz direction. We can ﬁnd that from 11.4 GHz to 14.3 GHz, the simulated ellipticity angle at these frequencies (f ¼ 12.0–14.0 GHz), the polarization 2b is greater than 80. Therefore, this LTC polarization con- ellipses are all approximately viewed as circles. The principal version metasurface can be considered to operate in axis directions of these polarization ellipses correspond to the 11.4–14.3 GHz. The value of the ellipticity angle 2b can be azimuth angle a given in Fig. 4(b). made to be p/2 enough over a wider frequency range from Figures 6(a) and 6(b) present the simulated distributions 11.0 GHz to 18.3 GHz by optimizing the parameters of the of the surface current on the bi-layered metasurfaces at the structure. Consequently, the transmitted wave is not exactly frequency f ¼ 13.0 GHz for y-polarized wave normally inci- but very close to the circularly polarized wave. dent from þz direction. It can be found that the long metallic The axis ratio of the transmitted wave can be calculated wire in the x-direction of the upper metallic pattern arrays using p ¼ 10 log10(tan(b)). Figure 5(a) gives the calculated and the long metallic wire in y-direction of the lower metal- results. We can ﬁnd that over the wide frequency range from lic pattern arrays act, respectively, as metallic gratings 9.8 GHz to 18.3 GHz, the axis ratios of the transmitted waves placed in y- and x-direction. They act as two polarisers with

FIG. 4. Simulated transmission coefﬁcients and calculated polarization azimuth angle and ellipticity angle: (a) the simulated amplitudes and phases of the co- and cross-polarization transmissions; (b) the calculated polarization azimuth angle a and the ellipticity angle b versus frequency for y-polarized wave incident from þz direction.

FIG. 5. The simulated axis ratios versus frequency (a) and polarization ellipses at different frequencies (b).

orthogonal polarization directions. The obliquely placed with that on the upper oblique metallic wire. Without the metallic wires on the upper and lower layers are all used as obliquely placed metallic wires, the incident y-polarized linear polarization rotators. For y-polarized wave normal wave can not penetrate the bi-layered metasurface directly incidence, the surface current on the upper obliquely placed orthogonal polarization directions of the two metallic gra- metallic wire is ﬁrstly excited by the incident electric ﬁeld, tings on the upper and lower layers. This is similar to the then the surface current on the lower obliquely placed case that another polarizer is put between two orthogonal metallic wire is induced by the surface current on the upper polarisers. What is difference from the one-more-third oblique metal wire. Therefore, the surface current on the polarizer case is that the two obliquely placed metallic wires lower oblique metal wire has a small phase delay compared on the upper and lower layers introduces a 90 phase

FIG. 6. Surface current distributions on the upper layer (a) and lower layer (b) of the bi-layered metasurface, and (c) electric ﬁeld vector distribution on the sides of the metasurface.

FIG. 7. Photograph of the fabricated sample (a) and the experimental measurement setup (b).

difference into the two orthogonal LP components. Hence, phases of the co- and cross-polarization transmission coeffi- the transmitted waves are circularly polarized. This can be cients (Tyy and Txy) were measured under y-polarized wave more clearly observed from the electric field vector distribu- incidence from þz direction, i.e., tyy, txy, /yy, and /xy. Then, tion on the sides of the metasurface shown in Fig. 6(c). the LTC polarization conversion transmission coefficients under y-polarized wave incidence can be extracted from the D. Experimental verification measured co- and cross-polarization transmission coefficients using (4) and (5). Figure 8 gives the extracted ampli- In order to further verify the design, we fabricated a tudes of the LTC polarization conversion transmission sample with the size of 280 280 mm2 using conventional coefficients (tLCP-y and tRCP-y) over a wide frequency range printed circuit board (PCB) technique. Two layers of 0.017- from 8 GHz to 20 GHz. For comparison, the simulated results mm-thick copper metallic pattern arrays are printed on a 1.5- are also given in Fig. 8. It can be found that the measurement mm-thick F4B (er ¼ 2.65, tand ¼ 0.001) substrate on the front results are in good accordance with the simulations except a and back. The photograph of the sample is shown in Fig. tinny blue-shift. The measured results convincingly demon- 7(a), where the inset is the zoom view of an array of 6 6 strate the LTC polarization conversion metasurface working unit cells. The measurement is carried out in a microwave in 11.4–14.3 GHz. The minor blue-shift is mainly due to the anechoic chamber to reduce the influence of noises. The pho- slight permittivity differences of the dielectric substrate used tograph of the measurement setup is given in Fig. 7(b). Two in the simulation and experiment. The whole frequency standard-gain horn antennas are fixed at the two spiral arms range is divided into three sections in the measurement: of a revolving stage. One is working as a transmitter and the 8–12 GHz, 12–18 GHz, and 18–20 GHz. Three groups of other as a receiver. The y- and x-polarized waves are trans- horn antennas are used. In addition, the LTC polarization mitted or received when the horn antennas are placed onto conversion transmissions are extracted from the measured its longer and shorter sides. The sample is placed vertically amplitudes and phases of the co- and cross-polarization at the center of the revolving stage. The amplitudes and transmission coefficients under LP wave incidence. Therefore, large testing errors will be resulted for the LTC polarization conversion transmissions if the measured linear- to-linear co- and cross-polarization transmission coefficients have minor testing errors. Thus, there is a large discrepancy existed between the simulated and experimental results in Fig. 8, especially above 18 GHz.

III. CONCLUSION In summary, we have proposed an ultra-thin bi-layered metasurface to achieve wideband LTC polarization conversion. Numerical simulations indicate that the LTC polarization conversion transmission is over 90% in 11.0–18.3 GHz and the axis ratio for the transmitted wave is lower than 3 dB in the frequency range of 9.8–18.3 GHz. The calculated ellipticity angle of the transmitted wave reveals an LTC polarization conversion metasurface working in 11.4–14.3 GHz. The sample was also fabricated to provide an experimental FIG. 8. The experimental and simulated LTC polarization conversion trans- validation. It has been found that the extracted LTC polariza- missions for y-polarized wave incidence from þz direction. tion conversion transmission from the measured co- and

cross-polarization transmissions is well consistent with the 5M. Feng, J. Wang, H. Ma, W. Mo, H. Ye, and S. Qu, J. Appl. Phys. 114, simulated results. The proposed wide-band ultra-thin LTC 074508 (2013). 6H. L. Zhu, S. W. Cheung, K. L. Chung, and T. I. Yuk, IEEE Trans. polarization conversion metasurface may ﬁnd potential Antennas Propag. 61, 4615–4623 (2013). applications in the wide-band CP antenna by incorporating it 7N. F. Yu, P. Genevet, M. A. Kats, F. Aieta, J. P. Tetienne, F. Capasso, and into LP antennas. Z. Gaburro, Science 334, 333–337 (2011). 8L. L. Huang, X. Z. Chen, H. Muhlenbernd,€ G. X. Li, B. F. Bai, Q. F. Tan, G. F. Jin, T. Zentgraf, and S. Zhang, Nano Lett. 12, 5750–5755 (2012). ACKNOWLEDGMENTS 9X. Artiga, D. Bresciani, H. Legay, and J. Perruisseau-Carrier, IEEE Antennas Wirel. Propag. Lett. 11, 1489–1492 (2012). The authors are grateful to the supports from the 10H. Chen, J. Wang, H. Ma, S. Qu, Z. Xu, A. Zhang, M. Yan, and Y. Li, National Natural Science Foundation of China under Grant J. Appl. Phys. 115, 154504 (2014). Nos. 61331005, 11204378, 11274389, 11304393, and 11N. K. Grady, J. E. Heyes, D. R. Chowdhury, Y. Zeng, M. T. Reiten, A. K. 61302023; the Aviation Science Foundation of China under Azad, A. J. Taylor, D. A. R. Dalvit, and H. T. Chen, Science 340, 1304 (2013). Grant Nos. 20132796018 and 20123196015; the National 12Y. Zhao and A. Alu, Phys. Rev. B 84, 205428 (2011). Science Foundation for Post-doctoral Scientists of China 13L. Cong, W. Cao, Z. Tian, J. Gu, J. Han, and W. Zhang, New J. Phys. 14, under Grant Nos. 2013M532131 and 2013M532221; the 115013 (2012). 14L. Martinez-Lopez, J. Rodriguez-Cuevas, J. I. Martinez-Lopez, and A. E. Natural Science Foundation of Shaanxi Province under Martynyuk, IEEE Antennas Wirel. Propag. Lett. 13, 153–156 (2013). Grant No. 2013JM6005; and the Special Funds for Authors 15M. A. Kats, P. Genevet, G. Aoust, N. F. Yu, R. Blanchard, F. Aieta, Z. of Annual Excellent Doctoral Degree Dissertations of China Gaburro, and F. Capasso, Proc. Natl. Acad. Sci. U.S.A. 109, 12364–12368 (2012). under Grant No. 201242. 16L. J. Black, Y. D. Wang, C. H. de Groot, A. Arbouet, and O. L. Muskens, ACS Nano 8, 6390–6399 (2014). 1Y.-J. Chiang and T.-J. Yen, Appl. Phys. Lett. 102, 011129 (2013). 17J. Hao, Y. Yuan, L. Ran, T. Jiang, J. A. Kong, C. T. Chan, and L. Zhou, 2S. Wu, Z. Zhang, Y. Zhang, K. Y. Zhang, L. Zhou, X. J. Zhang, and Y. Y. Phys. Rev. Lett. 99, 063908 (2007). Zhu, Phys. Rev. Lett. 110, 207401 (2013). 18J. Lin, J. P. B. Mueller, Q. Wang, G. Yuan, N. Antoniou, X. C. Yuan, and 3J. B. Masson and G. Gallot, Opt. Lett. 31, 265 (2006). F. Capasso, Science 340, 331 (2013). 4N. Vieweg, M. K. Shakfa, and M. Koch, Opt. Commun. 284, 1887 19T. Niemi, A. O. Karilainen, and S. A. Tretyakov, IEEE Trans. Antennas (2011). Propag. 61, 3102 (2013).