Hybrid Helix Metamaterials for Giant and Ultrawide Circular Dichroism

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Hybrid helical metamaterials for giant and ultra-wide circular dichroism Ruonan Ji, Shao-Wei Wang, Xingxing Liu, Huijie Guo, and Wei Lu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00575 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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Hybrid helix metamaterials for giant and ultra-wide circular dichroism Ruonan Ji1,2,3, Shao-Wei Wang1,2*, Xingxing Liu1,2,3, Huijie Guo4 and Wei Lu1,2 1 National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, 200083, China 2 Shanghai engineering research center of energy-saving coatings, Shanghai, 200083, China 3 University of Chinese Academy of Sciences, Beijing, 100049, China 4 University of Fudan, Shanghai, 200433, China KEYWORDS Chiral metamaterials, hybrid helix, circular polarizers, localized resonance

ABSTRACT: Circular polarization-resolved imaging and spectroscopy have versatile applications ranging from astronomy, biological and biomedical studies to remote sensing. To achieve real-time imagery with high contrast and rich information, integratable circular polarizers with high extinction ratios, broad operation bandwidths and low insertion loss are urgently desired, while to our knowledge, no structures proposed previously can balance these three vital parameters. In this paper, a concept of hybrid helix metamaterial has been proposed to achieve ultra-wide and giant circular dichroism with low insertion loss simultaneously. By coupling the localized and delocalized resonant modes, it can be realized with a pencil-like configuration to form a broad and strong blocking band for unselected polarization. A giant extinction ratio of 5.9×105 can be obtained with a six-pitch hybrid single-helixes array, which is almost 7 times larger than the best result reported by Behera et al. in 2015. A very high average extinction ratio of 8.5×103 is achieved in the operation band of 3.59.5μm, which is due to an average transmittance as low as 0.2% for the unselected polarizations. More importantly, the top tapered part well-matches the impedance and contributes to high average transmittance of 72% in the whole range. Based on the general scalablility, it is demonstrated experimentally in the microwave range to prove the validity of such a concept. The experimental results agree well with theoretical ones. The proposed concept can also be broadly extended to other chiral metamaterial systems for the design of novel high-performance circular polarizers.

circular-polarizers.12, 16 Whereas, both the operation bandwidth and extinction ratio are limited as the polarization selective effect is originated from the Bragg resonances of periodical arranged unit cells.17 Chiral optical metamaterials exhibit a mixture of electrical and magnetic response excited by the light field, leading to circular dichroism that exceeds the corresponding effects in natural chiral materials by several orders of magnitude. Thus, various kinds of chiral metamaterials from single-layer7, 14, dual-layer18-21, multilayer22 to three-dimensional15, 17, 23-32 were proposed to achieve high-performance circular polarizers. In 2009, uniform gold helices array operating in mid-infrared frequencies has been fabricated by Gansel et al.[7] Although the extinction ratio is barely exceeded five, such unprecedented bandwidth makes helix metamaterials the most promising candidates for circular micropolarizers. Varies structures have been proposed to enhance the performance of helix metamterials, including the tapered helix28, 31, double-helix24-26, N-helix27, 31 and phase-lagged helix32. However, the tapered one only can dramatically improve the extinction ratio at some specific wavelength. The double helix can broaden the operation bandwidth, while the extinction ratio is reduced.24-25

Circular polarization-resolved imaging and spectroscopy can be applied in versatile fields including remote sensing1-3, biological/biomedical4-8, astronomic9 and material science studies10-12. As both human eyes and common detectors are blind to the circular polarization states, circular polarizers are key components for the polarization states resolving in the imaging and spectroscopy systems. Extinction ratio (ER), operation bandwidth (OB) and transmittance of selected polarization are three key parameters for a circular polarizer. ER is defined as the transmittance ratio of the selected polarization to the unselected one, which directly determined the contrast of the imagery or the signal to noise ratio of the spectrum. The operation bandwidth and transmittance of the selected polarization respectively correspond to the amount of acquired information and efficiency. They are expected to be as large as possible. Traditional circular polarizers are bulky, including cascaded linear polarizers and birefringent crystal quarterwave-plates. With sizable configurations, they cannot satisfy the demands of compact micropolarizers array in real-time polarization imaging systems,13-14 which are crucial for living biologic specimen or moving targets detections.12-13, 15 Recently, Cholesteric liquid crystals (CLCs) were proposed to act as micro-

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ACS Photonics were used in the infrared range simulations.35 In this paper, if not mentioned, by default, the helix structures are with left-handed configuration. The operation band is defined as the wavelength range on which the extinction ratio is above 10:1.27 And the average transmittance and extinction ratio were calculated in the operation band.

The N-helix metamaterials with restored rotational symmetry can achieve a high extinction ratio by eliminating the polarization conversion, but with the cost of high insertion loss.32-34 For example, Behera et al. presented a simulated average extinction as high as 8.1×103 with a six-pitch triple uniform helix in 581-1361 nm. This is the highest AER reported so far, whereas the transmittance of the selected polarization is only 48%.32 The phase-lagged helix provides an alternative approach to restore rotational symmetry. Although the low insertion loss is achieved, the extinction ratio enhancement is quite limited.32 Thus, how to improve the extinction ratio while maintaining broad operation band and high selected polarization transmittance (i.e low insertion loss) is a big challenge for the chiral metamaterial based circular polarizers. According to the definition of extinction ratio as the transmittance ratio of right-handed polarized (RCP) to left-handed polarized (LCP) lights (TRCP/TLCP), it is reasonable to reduce the transmittance of unselected circularly polarized light to obtain high extinction ratio. Essentially, the origination of the blocking effect to the unselected polarization in the previous reports is either from the delocalized resonance or the localized resonance. For example, single and N-helix uniform helix metamaterial (UHM) belong to the delocalized type. The broad stop band is originated from the overlapping of the Bragg resonant and delocalized interhelix coupling. Non-periodical arrangement along the axis will inevitably introduce localized resonance. Thus, both the single helix and N-helix with non-uniform radius belong to the localized type. However, as mentioned above, none of them can optimize all the three key parameters of ER, OB and transmittance of selected polarization simultaneously. In this paper, by coupling the localized and delocalized resonant modes, a new concept of hybrid helix metamaterial (HHM) has been proposed to achieve giant and ultra-wide circular dichroism with high transmittance of selected polarization. The origin of the broadband strong circular dichroism has been analyzed, and its validity has been proved experimentally in the microwave range.

Results and Discussion The performance of six-pitch gold UHM, THM and HHM with same dimensions were simulated, and the main performance parameters are concluded in Table 1. As expected, the ER curve of UHM shows a relative steady feature in a wide range, while the maximum ER value is only 13.9. The ER in the range from 5.79.2µm is even below 10. A higher ER can be achieved with triplehelices configuration, but with the cost of insertion loss above 50% (see Table 1). With well-matched impedance, the average RCP transmittance of THM can reach 81%. Whereas, the fluctuant LCP transmittance curve is obviously increasing in the wavelength range above 5.3µm. The ER value reaches the maximum as 2.1×104, while drops down to even less than 10 when the wavelength is above 7.7µm. Therefore, the OB of THM is only from 3.5 to 7.8µm. As for the HHM, the maximum ER can reach 5.9×105. It is even more than four orders of magnitude to the UHM. The average LCP transmittance is as low as 0.2% in the OB of 3.5-9.5µm, leading to a high AER of 8.7×103. As shown in Table 1, the maximum ER value of HHM is almost seven times higher than the triple-UHM. Furthermore, the proposed HHM not only can achieve a higher AER than the much more complicated tripleUHM32 (the highest AER reported in 2015), but also has a 24% higher average RCP transmittance. Thus, HHMs are promising to act as the novel high-performance micro-circular-polarizers, for many important applications, such as space remote sensing, livetumor diagnosis, living biologic specimen imaging and other high-precision target recognitions. Table 1. Performance comparison of different types of helix metamaterials.

Design and Simulation Methods In this work, the localized and delocalized resonances are combined and coupled to achieve not only broad but also strong blocking effect to the unselected polarization. The concept is realized with a hybrid helix metamaterial (HHM) consisting of tapered top and uniform bottom helix as shown in Figure 1(a) and (b). The helix diameters of the top pitches taper from HD2 to HD1, and the helix diameter of the uniform pitch equals to the one of the last tapered pitch HD1. The top tapered pitches with gradually increased radius help to match the impedance to air, thus contribute to low reflection of RCP incident lights as well as high coupling efficiency of LCP incident lights. The top tapered and bottom uniform pitches share the same wire diameter WD, grid spacing GS and length of the helix-period LH to guaranteed only one wire end will exist. For LCP incident, as originated from the delocalized resonance in the bottom uniform part, broadband reflection will occur at the interface. And the strong narrow band localized resonance in the top tapered part is enhanced by this strong reflection. The helix pitches number of tapered and uniform part is HN2 and HN1, respectively. By tailoring the geometric parameters of the two parts, broadband low-transmittance of LCP is achieved. In this way, the three parameters (OB, ER and insertion loss) are optimized simultaneously. Rigorous electromagnetic simulations of the metamaterials were performed by means of commercial software Lumerical FDTD solutions. The permittivity values of gold given by Palik

Type

Maximum

OB

ER

[μm]

Average TRCP

Average TLCP

3.6-5.7

49%

4.2%

11.3

9.2-9.5

76%

13%

12.0

AER

UHM (HN1=6)

13.9

THM (HN2=6)

2.1×104

3.5-7.8

80%

5.7%

2.0×103

5.9×105

3.5-9.5

72%

0.2%

8.7×103

~ 9×104

0.6-1.4

48%

Not mentioned

8.1×103

HHM (HN1=3 HN2=3) Triple UHM[26] (HN1=6)

A pitch of a helix can be treated as a split-ring resonator when being adiabatically pulled one end out of the plane. According to the previous reports, only the incident light with same handedness as the helix structure can induce strong circulating and oscillating electric currents in the structure, while the one with opposite handedness can not. Namely, for RCP incident lights, a helix metamaterial with left-handed configuration is nearly transparent. The small amount of optical loss is mainly originated from the top reflection and non-resonant absorption. Thus, besides increasing the electrical conductivity of the metal helixes, reducing the impedance mismatch to the air is also an effective way to increase

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the RCP transmittance. Then, it is easy to understand that the THM with gradually increasing radius has the highest RCP transmittance, while the UHM has the lowest one.30 However, the situation becomes more complex when the incident light is left-handed polarized. Induced by the LCP incident light field, oscillating electric current loops will generate in each helix pitch (see Figure 2(b)). 17 And the interhelix coupling generates hybrid modes and presents as resonant dips in the transmittance curve as shown in Figure 2(a).36-37 For the UHM, the periodically arranged unit cells lead to delocalized hybrid modes as well as Bragg resonance. As shown in Figure 2(c), the combination of these two delocalized resonances contributes to strong reflection at the top of the UHM. While for the helix metamaterial with nonperiodically arranged unit cells, such as the THM, the hybrid modes with localized field distribution are generated just as shown in Figure 2(d). Moreover, as the field is localized at a specific pitch, the internal resonance of the single pitch dominates the corresponding resonance. Thus, with the increase of helix radius, the oscillator length of induced current increases, which contributes to the red-shift of the resonant dip. That is to say, for two non-periodical arranged helix metamaterials with the same smallest radius HD2, the one with bigger HD1 (the biggest radius) may have broader circular dichroism, which is consistent with the experimental results of Gansel et al.28 While in the proposed HHM with a pencil-like configuration, as shown in Figure 2(e), the electric field shows a localized distribution at the resonant wavelength similar to the THM, but with a stronger intensity. We attribute this phenomenon to the coupling of the localized and delocalized resonant modes. The localized resonant absorption and reflection blocks most of the incident lights, so that the photons transmitting through the uniform part is very limited. Moreover, the transmitted lights are reflected in the interface and trapped by the tapered part again, thereby enhancing the localized field intensity. The delocalized resonance provides a broad LCP reflection band, while the enhanced localized resonance results in a sharper transmittance dip. In this way, a strong and broad LCP blocking band is achieved, thereby the three key parameters of the circular polarizer are successfully balanced. To evaluate the coupling effect, the transmittance curve of a six-pitch HHM was compared with the simple combination of the three-pitch tapered parts and three-pitch uniform parts. The latter one was approximated by multiplying the transmittance curves of the two independent parts. As shown in Figure 3, a simple combination of the two parts results in a lower RCP transmittance. More importantly, the broadband high blocking effect vanishes when the two parts are working independently. It is worth noting that impedance matching is not only essential to improve the RCP transmittance, but also to achieve effective localized resonance. As the high extinction ratio is mainly provided by the strong localized resonance, the THM with decreasing radius still exhibits limited extinction ratio just like the UHM.28 When the order of the two parts in HHM are inverted, with the combined effect of the impedance mismatch and delocalized resonance, most of the incident light will be reflected by the uniform part and few of them can transmit to the tapered part, thereby significantly weakening the coupling between the localized and delocalized resonance mode. Thus, as shown in Figure 3, in the inverted structure, not only the localized resonant dips disappear, but also the LCP average transmittance obviously increases (see Figure 3). Based on the previous works, it is well-known that single-helix metamaterials face the problem of circular polarization conversion due to their broken full rotational symmetry. Owing to the coupling of localized and delocalized resonances, HHM can effectively reduce the transmittance of both LCP and its converted

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polarization. HHM is more attractive than the N-helix ones because it can not only achieve extremely high ER, but also avoid the problem of greatly reduced RCP transmittance and more complicated fabrication process. In principle, the circular dichroism of single-helix metamaterials can be scaled by structure size to any desired wavelength range, provided that the operation wavelength was sufficiently below the metal plasma frequency.17 As the conductivity of the metal in microwave range is quite close to the PEC, the localized resonance is mainly represented as the sharp reflection peak, rather than absorption as the infrared range. This change, however, does not affect the overall performance of the device. The effectiveness of our design in the microwave range has been verified with the simulation results (see Figure S1 in SI), by proportionally magnifying the dimensions of UHM, THM and HHM in three magnitude orders. Therefore, for simplicity, the copper HHM operates in the microwave range has been fabricated to validate the proposed concept. However, the measurement range is only from 7.5 GHz to 10 GHz, as limited by the source and receiver. In the corresponding theoretical simulation, the electrical conductivity of copper was set as σ= 5.8×107 S/m. As depicted in Figure 4, the measured results agree well with the theoretical ones in general. The resonances at 7.75GHz and 9.25GHz are well matched, except that the measured resonant dips are a bit wider and flatter than the simulated ones. Simulated results show that the change of geometrical parameters, such as wire diameter WD, grid spacing GS, length of the helix-period LH and the tapered ratio, are all leading to the wavelength and magnitude shifts of resonant dips (see Figure S2 and S3 in SI). Thus, due to the dimension uniformity resulting from the manufacturing tolerance, the overlapping of the shifted resonant dips finally leads to broader and weaker measured resonant dips. In addition, the transmittance of RCP in the experiment is a bit lower than the theoretical one primarily because of the loss from substrate. The measured maximum extinction ratio and average extinction ratio in the measured frequency range are 5.7×102 and 75, respectively, which are lower than the simulated results as 2.2×103 and 170. These differences may mainly due to the combined effects of the system noise, imperfect fabrication process and material loss. As limited by the setup, the properties have not been measured in the whole operation band, but they should be in accordance with the theoretical ones, according to the well agreed measurement as shown in Figure 4. Besides the single helix metamaterials, the concept of hybrid helix metamaterial may be broadly extended to other similar system, such as double-helix, three-dimensional twisted metamaterials and so on. Previous studies illustrates that double-helices metamaterials have broader OB than the single-helix ones due to the interaction between the two helices.25 This double-helix configuration also brings obvious decrease of extinction ratio and increase of insertion loss as well. As shown in Figure 5, the maximum ER of gold double-uniform-helix metamaterial (DUHM) is 13.4. In the range from 3.5µm to 11.5µm, the average ER and RCP transmittance are only 8.3 and 53%, respectively. Yang et al. proposed to improve the extinction ratio by introducing a heterostructure.26 However, the enhancement is quite limited, while the complexity of the fabrication process is increased significantly. As shown in Figure 5, by introducing the hybrid concept, the performance of the double-helix metamaterials can be remarkably enhanced with the common double-helix preparation process. The maximum ER can reach 9.7×104 with the gold double-hybridhelix metamaterial (DHHM), which is nearly four orders of magnitude higher than the uniform one. Moreover, the average ER can

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reach 7.1×103 while maintaining an average RCP transmittance higher than 70%.

ACKNOWLEDGMENT The authors would like to thank Dr. Bimu Yao from Shanghai Institute of Technical Physics, Dr. Chao Wu, Dr. Zhijie Gong and Prof. Yong Sun from Tongji University for experimental help and helpful discussions.

Conclusions In this paper, a concept of hybrid helix metamaterial realized with a pencil-like configuration has been proposed to achieve giant and broad circular dichroism. The localized resonance of the tapered part and delocalized resonance from the uniform part are coupled to dramatically enhance the blocking effect on LCP, thereby leading to an extremely high and ultra-wide extinction ratio. With such a concept, a gold helix circular polarizer with giant extinction ratio of 5.9×105 and high average extinction ratio of 8.7×103 in the operation band of 3.5-9.5 µm are obtained. Moreover, an average transmittance higher than 70% in the entire operation band to the selected polarization is achieved owing to the well-matched impedance. The concept has been demonstrated in the microwave range with good agreement between the theoretical and experimental results. It can also be extended to other chiral metamaterial systems and generate novel low-profile highperformance circular polarizers.

REFERENCES 1. Gilbert, G. D.; Pernicka, J. C., Improvement of Underwater Visibility by Reduction of Backscatter with a Circular Polarization Technique. Appl. Opt. 1967, 6, 741-746. 2. Van der Laan, J. D.; Scrymgeour, D. A.; Kemme, S. A.; Dereniak, E. L., Detection range enhancement using circularly polarized light in scattering environments for infrared wavelengths. Appl. Opt. 2015, 54, 2266-2274. 3. Kartazayeva, S. A.; Ni, X.; Alfano, R. R., Backscattering target detection in a turbid medium by use of circularly and linearly polarized light. Opt. Lett. 2005, 30, 1168-1170. 4. McNichols, R. J.; Cote´, G. L., Optical glucose sensing in biological fluids: an overview. J. Biomed. Opt. 2000, 5, 5-16. 5. Hendry, E.; Carpy, T.; Johnston, J.; Popland, M.; Mikhaylovskiy, R. V.; Lapthorn, A. J.; Kelly, S. M.; Barron, L. D.; Gadegaard, N.; Kadodwala, M., Ultrasensitive detection and characterization of biomolecules using superchiral fields. Nat. Nanotechnol. 2010, 5, 783-7. 6. Morgan, S. P.; Stockford, I., Surface-reflection elimination in polarization imaging of superficial tissue. Opt. Lett. 2003, 28, 114-6. 7. Khorasaninejad, M.; Chen, W. T.; Zhu, A. Y.; Oh, J.; Devlin, R. C.; Rousso, D.; Capasso, F., Multispectral Chiral Imaging with a Meta-lens. Nano Lett. 2016, 16, 4595-4600. 8. J Scott, T.; Goldstein, D. L.; Chenault, D. B.; Shaw, J. A., Review of passive imaging polarimetry for remote sensing applications. Appl. Opt. 2006, 45, 5453-5469. 9. Chrysostomou, A.; Lucas, P. W.; Hough, J. H., Circular polarimetry reveals helical magnetic fields in the young stellar object HH 135-136. Nature 2007, 450, 71-73. 10. Kacey, C.; Eileen, P. F.; Miki, K.; Werner, K.; Bart, K., Circular dichroism imaging microscopy: application to enantiomorphous twinning in biaxial crystals of 1,8dihydroxyanthraquinone. J. Am. Chem. Soc. 2003, 125, 1482514831. 11. Ruthanne, H.; Swain, E. J.; Hammer, N. I.; Dhandapani, V.; Barnes, M. D., Probing the chiroptical response of a single molecule. Science 2006, 314, 1437-1439. 12. Hsu, W.-L.; Davis, J.; Balakrishnan, K.; Ibn-Elhaj, M.; Kroto, S.; Brock, N.; Pau, S., Polarization microscope using a near infrared full-Stokes imaging polarimeter. Opt. Express 2015, 23, 4357-4368. 13. Nordin, G. P.; Meier, J. T.; Deguzman, P. C.; Jones, M. W., Micropolarizer array for infrared imaging polarimetry. J. Opt. Soc. Am. A 1999, 16, 1168-1174. 14. Bachman, K. A.; Peltzer, J. J.; Flammer, P. D.; Furtak, T. E.; Collins, R. T.; Hollingsworth, R. E., Spiral plasmonic nanoantennas as circular polarization transmission filters. Opt. Express 2012, 20, 1308-1319. 15. Ji, R.; Wang, S.-W.; Liu, X.; Chen, X.; Lu, W., Broadband circular polarizers constructed using helix-like chiral metamaterials. Nanoscale 2016, 8, 14725-14729. 16. Hsu, W.-L.; Myhre, G.; Balakrishnan, K.; Brock, N.; Elhaj, M. I.; Pau, S., Full-Stokes imaging polarimeter using an array of elliptical polarizer. Opt. Express 2014, 22, 3063-3074. 17. Gansel, J. K.; Thiel, M.; Rill, M. S.; Decker, M.; Bade, K.; Saile, V.; von Freymann, G.; Linden, S.; Wegener, M., Gold

Experimental Section We performed the transmission measurements through a slab of copper helixes array. The left-handed copper hybrid helixes were fabricated by mechanical processing with a general dimension tolerance of 5%. Oriented along the z axis, the helixes were periodically fixed in a polystyrene foam slab (nearly lossless, ε≈1) to form a metallic helixes array in the x-y plane. The sample slab contains 25×25 metallic hybrid helixes, each having three top tapered and three bottom uniform pitches along the helix axis (z axis). A pair of standard linearly polarized horn antennas were connected to two ports of a vector network analyzer of Agilent E8362C. The transmittance spectra of the circular polarizations were calculated according to the following Equation.  T T  1  (Txx +Tyy ) +i (Txy -Tyx ) Tc =  rr rl  =   Tlr Tll  2  (Txx -Tyy ) +i (Txy +Tyx )

(T (T

-Tyy ) - i (Txy +Tyx )    xx +Tyy ) - i ( Txy -Tyx ) 

xx

Where the former and latter subscripts corresponding to the polarization of the transmitted and incident wave, and subscripts x, y, r and l represent the x polarized, y polarized, right-handed polarized (RCP) and left-handed polarized (LCP) waves respectively.

ASSOCIATED CONTENT Supporting Information. Scalability of the proposed circular dichroism (Figure S1); Transmittance curves of LCP evolve with the increase of wire diameter WD, grid spacing SG and length of helix-period LH for HHM (Figure S2); Transmittance curves of HHMs with different tapered ratios (Figure S3 and Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Funding Sources This work was partially supported by the Shanghai Science and Technology Foundations (15DZ2282100, 16DZ2290600) and Youth Innovation Promotion Association CAS (2012189).

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helix photonic metamaterial as broadband circular polarizer. Science 2009, 325, 1513-1515. 18. Zhou, J.; Chowdhury, D. R.; Zhao, R.; Azad, A. K.; Chen, H. T.; Soukoulis, C. M.; Taylor, A. J.; O'Hara, J. F., Terahertz chiral metamaterials with giant and dynamically tunable optical activity. Phys. Rev. B 2012, 86, 6335-6335. 19. Wu, L.; Yang, Z.; Cheng, Y.; Lu, Z.; Zhang, P.; Zhao, M.; Gong, R.; Yuan, X.; Zheng, Y.; Duan, J., Electromagnetic manifestation of chirality in layer-by-layer chiral metamaterials. Opt. Express 2013, 21, 5239-5246. 20. Cui, Y.; Kang, L.; Lan, S.; Rodrigues, S.; Cai, W., Giant Chiral Optical Response from a Twisted-Arc Metamaterial. Nano Lett. 2014, 14, 1021-1025. 21. Yuan, W.; Zhang, H.; Cheng, Y., Asymmetric Chiral Metamaterial Multi-Band Circular Polarizer Based on Combined Twisted Double-Gap Split-Ring Resonators. Prog. Electromagn. Res. 2014, 49, 141-147. 22. Zhao, Y.; Belkin, M.; Alù, A., Twisted optical metamaterials for planarized ultrathin broadband circular polarizers. Nat. Commun. 2012, 3, 870-7. 23. Kan, T.; Isozaki, A.; Kanda, N.; Nemoto, N.; Konishi, K.; Kuwata-Gonokami, M.; Matsumoto, K.; Shimoyama, I., Spiral metamaterial for active tuning of optical activity. Appl. Phys. Lett. 2013, 102, 221906 - 221906-4. 24. Yang, Z. Y.; Zhao, M.; Lu, P. X.; Lu, Y. F., Ultrabroadband optical circular polarizers consisting of double-helical nanowire structures. Opt. Lett. 2010, 35, 2588-2590. 25. Shengxi, L.; Zhenyu, Y.; Jing, W.; Ming, Z., Broadband terahertz circular polarizers with single- and double-helical array metamaterials. J. Opt. Soc. Am. A 2011, 28, 19-23. 26. Yang, Y.; Zhenyu, Y.; Shengxi, L.; Ming, Z., Higher extinction ratio circular polarizers with hetero-structured double-helical metamaterials. Opt. Express 2011, 19, 1088610894. 27. Zhenyu, Y.; Ming, Z.; Peixiang, L., How to improve the signal-to-noise ratio for circular polarizers consisting of helical metamaterials? Opt. Express 2011, 19, 4255-4260. 28. Gansel, J. K.; Latzel, M.; Frolich, A.; Kaschke, J.; Thiel, M.; Wegener, M., Tapered gold-helix metamaterials as improved circular polarizers. Appl. Phys. Lett. 2012, 100, 101109-3. 29. Kay, D.; Dennis, L.; Christian, H.; Andreas, T.; ErnstBernhard, K., Circular Dichroism from Chiral Nanomaterial Fabricated by On-Edge Lithography. Adv. Mater. 2012, 24, OP321-OP325. 30. Zhao, Z.; Gao, D.; Bao, C.; Zhou, X.; Lu, T.; Chen, L., High Extinction Ratio Circular Polarizer With Conical DoubleHelical Metamaterials. J. Lightwave Technol. 2012, 30, 24422446. 31. Johannes, K.; Mark, B.; Sven, B.; Martin, W., Tapered Nhelical metamaterials with three-fold rotational symmetry as improved circular polarizers. Opt. Express 2014, 22, 1993619946. 32. Behera, S.; Joseph, J., N-single-helix photonic-metamaterial based broadband optical range circular polarizer by induced phase lags between helices. Appl. Opt. 2015, 54, 1212-1219. 33. Esposito, M.; Tasco, V.; Todisco, F.; Cuscunà, M.; Benedetti, A.; Sanvitto, D.; Passaseo, A., Triple-helical nanowires by tomographic rotatory growth for chiral photonics. Nat. Commun. 2015, 6, 6484-7. 34. Kaschke, J.; Wegener, M., Gold triple-helix mid-infrared metamaterial by STED-inspired laser lithography. Opt. Lett. 2015, 40, 3986-3989. 35. Palik, E. D., Handbook of optical constants of solids. Academic Press: New York, 1985; p 189.

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36. Liu, N.; Fu, L.; Kaiser, S.; Schweizer, H.; Giessen, H., Plasmonic Building Blocks for Magnetic Molecules in ThreeDimensional Optical Metamaterials &dagger. Adv. Mater. 2008, 20, 3859-3865. 37. Li, Y. R.; Ho, R. M.; Hung, Y. C., Plasmon Hybridization and Dipolar Interaction on the Resonances of Helix Metamaterials. IEEE Photonics J. 2013, 5, 2700510-2700510.

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Schematic diagrams of a left-handed HHM (a) and its unit cell (b). Transmittance curves of UHM c), THM (d) and HHM (e). (f) Extinction ratios comparison of three kinds of helix metamaterials. The geometrical parameters are WD=270nm, GS=1.8µm, LH=1.845µm, helix diameter HD1=1.44µm and HD2=180nm, and the pitch number of HN1 and HN2 in UHM, THM and HHM are (6,0), (0,6) and (3,3), respectively. Figure 1 230x261mm (300 x 300 DPI)

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(a) Transmittance curve of LCP incident light. (b) Snapshots of the electric current along the metal wire for the resonant dips of HHM marked in (a). The absolute value of the current is encoded by the curve thickness, red and blue represent positive and negative directions, respectively. Electric field distribution of UHM(c), THM(d) and HHM(e) at the resonant wavelength. The same geometrical parameters were used as mentioned in Figure 1. Figure 2 59x35mm (300 x 300 DPI)

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RCP(a) and LCP (b) transmittance curves of HHM, inverted HHM (IHHM) and simply combination of tapered and uniform parts (UHM*THM). The same geometrical parameters were used as mentioned in Figure 1 Figure 3 77x29mm (300 x 300 DPI)

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Figure 4. Experimental (a) and theoretical (b) transmission spectra of the fabricated HHM. The geometrical parameters are WD=1.3mm, HN1=HN2=3, GS=18mm, LH=9.2mm, HD1=7.2mm and HD2=0.9mm. The inset in Figure (a) shows the fabricated sample. Figure 4 101x43mm (300 x 300 DPI)

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Comparisons of LCP transmittance (a), RCP transmittance (b) and extinction ratio (c) curves of gold doubleuniform-helix and double-hybrid-helix metamaterial. The geometrical parameters are WD=240nm, GS=1.6µm, LH=1.64µm, HD1=1.28µm and HD2=240nm. The values of NH1 and NH2 of DUHM and DHHM were taken as (6,0) and (3,3), respectively. And the refractive index of the substrate was taken 1.5. Figure 5 71x25mm (300 x 300 DPI)

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