TUNABLE METAMATERIAL LENS ARRAY VIA METADROPLETS Q. H. Song1, 2, W. M. Zhu2, W. Zhang2, P. C. Wu2, Z. X. Shen2, Z. C. Yang4, Y. F. Jin4, Y. L. Hao4,

T. Bourouina3, Y. Leprince-Wang1 †, and A. Q. Liu2 † 1Université Paris-Est, UPEM, F-77454 Marne-la-Vallée, France

2School of Electrical and Electronic Engineering, Nanyang Technological University 50 Nanyang Avenue, Singapore 639798

3Université Paris-Est, ESYCOM, ESIEE, Paris F-93162 Marne-la-Vallée, France 4National Key Laboratory of Science and Technology on Micro/Nano Fabrication,

Institute of Microelectronics, Peking University, Beijing 100871, China

ABSTRACT In this paper, a tunable THz lens array based on

reconfigurable metamaterials is reported. The metamaterial consists with 40 × 40 liquid metal microdroplets, in which the shape is tuned under different air pressures. The droplets are formed by injecting the liquid mercury into a pre-designed microchannel network and arranged to focus incident THz wave. The focus spot size can be tuned with different droplet geometries. As a result, a tunable THz lens is constructed and the smallest spot size of 0.54λ is achieved. The tunable metamaterial lens is realized through simple fabrication processes, which has potential applications in flat lens and imaging system. INTRODUCTION

Metamaterials, or rationally designed artificial materials with sub-wavelength scale elements, offers a fantastic platform to control and manipulate the electromagnetic (EM) waves. The sub-wavelength elements, typically metallic patterns, can respond to both the electric field and the magnetic field. Many extraordinary physical phenomena are then induced, such as negative index [1-2], cloaking [3-4], zero epsilon [5-6], giant chirality [7-8], or exotic and useful hyperbolic dispersion anisotropy [9]. Furthermore, as the metamaterial responded frequency highly depends on the size of the sub-wavelength elements, the EM wave of different frequencies can be effectively modulated. Compared to natural materials, metamaterial also has the advantage of real-time dynamically tunable for EM wave modulation. Tunable metamaterials are widely studied using micro-electro-mechanical system (MEMS) [10-12], phase changing materials [13] and liquid crystals [14].

In the previous tunable metamaterials [15-17], the tuning flexibilities, such as the tuning range and the resonant mode switching, are highly dependent on how the sub-wavelength elements are modulated during the tuning process. Among different tuning mechanisms, changing the geometry of the metallic structure of the sub-wavelength element typically results in a dramatic change in the EM properties since the electric and the magnetic response of the metamaterial are directly dependent on the element shapes. Previous works on MEMS tunable metamaterials [18-19] target on the change of the geometry shape of the metal elements by changing the near-field coupling of the metal parts anchored on the

movable islands driven by micromachined actuators. However, it is difficult to reshape the metal structures once it is forged.

Liquid metals with sub-wavelength feature size are recently utilized to construct tunable metamaterials due to their flexibility on reshaping the geometry [20-21]. This pioneer work used a complex microfluidic system for the tuning function. Although it offers individual sub-wavelength element tuning without any metallic electrode contact that can potentially spoil the EM properties of the metamaterials and introduce extra losses, it still suffers many drawbacks due to the complexity of the system and limited tuning speed. Here, an alternative technique is applied to tune the geometry of metal liquid droplet for the THz wave modulation. Based on this technique, a tunable lens array is designed with simple fabrication processes and control systems. The swift tuning on the focus spot size is realized.

PDMS Substrate

PDMS CoverMicro-droplet

PDMS

Liquid

Air Pressure

k HE

Figure 1: (a) Schematic of tunable lens array based on microdroplets. (b) The droplets are confined in PDMS channel. (c, d) The working principle of tuning method by applying air pressure.

978-1-4799-7955-4/15/$31.00 ©2015 IEEE 960 MEMS 2015, Estoril, PORTUGAL, 18 - 22 January, 2015

METAMATERIAL LENS ARRAY DESIGN Figure 1(a) shows the schematic of the lens array based

on random access metamaterials formed by mercury microdroplets with a period of 300 µm. Each lens is formed by 5 mercury droplets, one of which is at the center with four others surrounded. Proper spatial phase distribution can be induced through such structure, which can effectively focus the incident light. By controlling the shape of the droplets, the focus spot can be switched between the focusing and defocusing states. Figure 1(b) shows the liquid mercury being confined in the PDMS channel. The radii of the droplets can be tuned under different air pressures. The air pressure pushes the PDMS channel up, resulting in the increase of the droplet height and decrease of the droplet radius because of surface tension. Therefore, the interaction between the incident THz wave and the droplet metamaterial is changed, which consequently tunes the resonance in the structure and the focusing state of the incident light.

Liquid

Air

500μm

Figure 2: (a) Fabrication results of the micro-droplets with (b) uniform droplets pattern and (c) one single lens of five droplets cluster. The radii of the droplets (d, e, f) can be tuned by applying different air pressure.

The fabrication of the microfluidic system is based on

the polymer soft lithography technique. A 6-inch silicon is cleaned using a piranha solution (H2SO4 + H2O2) and spin coated with a 50-μm SU-8 photoresist (MicroChem, SU-8 50) at 2000 rpm for 30 s using a spin coater (CEE, 200). The silicon substrate is soft baked using a hot plate at 65oC for 6 min and 95oC for 20 min. The substrate is then exposed to UV light for 30 s under the plastic mask using a mask aligner (OAI, J500-IR/VIS). The post expose bake is performed at 65oC for 1 min and 95oC for 5 min after the exposure. The SU-8 layer, which is used as the master of the PDMS channels, is developed using the SU-8 developer

(MicroChem) for 6 min. The PDMS channels are fabricated using the replica molding, which is the casting of PDMS prepolymer against a master and obtaining the negative replica of the master. Three masters with different patterns are fabricated for the metamolecule array layer, respectively. There are altogether three PDMS layers. Two layers with microchannels and one layer without any pattern as the substrate. The microfluidic system is fabricated by plasma bonding the three PDMS layers together.

Figure 2(a) shows the fabrication results of the tunable metamaterial chip with the uniform micro-droplets pattern shown in Fig. 2(b) and one single lens that consists of five independent micro-droplets shown in Fig. 2(c). The channels for mercury and air injection are constructed to control the shapes of the droplets. The metal liquid is injected into the microchannel by syringe pumps. When the pressure is maintained at a low level, the liquid metal fills in the wide microchannel. Then, the air is injected into the narrow channel, which breaks the liquid metal flow into separated droplets and expels the extra liquid. Figure 2(d), (e) and (f) demonstrate the continuous tuning of the droplet radius from 120 to 80 μm by applying different air pressures.

RESULTS AND DISCUSSION

Figure 3: Simulation results of (a) phase and (b) electric field distribution at z direction and x-y cross section, respectively; (c) full width at half maximum at focus point.

Figure 3(a) shows the simulation results of the phase distribution in the propagation direction (right) and the x-y cross-section at the focus point (left). The corresponding

961

electric field distribution is shown in Fig. 3(b). As the boundary condition of the central droplet is different with that of the four surrounded droplets, a different phase delay is formed. The phase distribution cross-section shows a larger phase delay at the center, which steers the light to the center of the cluster. The wavelength of the incident light is 670 μm (0.448 THz in frequency). When the THz wave is incident on the cluster normally with an optimized droplet radius, which is 90 µm in this design, it will be focused at the other side of the metamaterial lens because of the spatial phase modulation. Repetition of the focus is observed in z direction, which is due to the Talbot effect. Figure 3(c) shows the full width at half maximum (FWHM) at the focus point is as narrow as 362 μm (0.54λ).

Figure 4: The full width at half maximum is changed by (a) tuning all the droplets simultaneously, and (b) tuning only the central droplets.

Figure 4 shows the simulation results of the FWHM of

the focus spot in the x-y plane, which can be effectively tuned by air pressing all the droplets in Fig. 4(a) or only the central droplet of each lens in Fig. 4(b). The FWHM becomes lowest when the radius R is 90 µm for all droplets, and when R is 80 µm for the central droplet. As the droplet radius is detuned, the FWHM increases and the spot becomes defocused. Comparing the two tuning mechanisms, the all-droplet tuning approach realizes a much more abrupt FWHM change. Therefore, it switches the focusing state of the

metamaterial lens to the defocusing state more effectively.

CONCLUSIONS In conclusions, a THz tunable lens array based on

mercury droplets is designed, fabricated and numerically demonstrated. The radius of each droplet are tuned from 120 µm to 80 µm, while the FWHM can be controlled to switch the metamaterial lens between the focusing state and the defocusing state. This metamaterial tuning technique is easy and flexible for the EM wave control, which has potential applications on tunable lens array and can be used in imaging system and detectors. ACKNOWLEDGEMENTS

The work is supported by the Environmental and Water Industry Development Council of Singapore (EWI), RPC programme (Grant No.: 1102-IRIS-05-01, 1102-IRIS-05-02, 1102-IRIS-05-04 and 1102-IRIS-05-05)

REFERENCES [1] D. R. Smith, J. B. Pendry and M. C. K. Wiltshire,

“Metamaterials and negative refractive index”, Science, vol. 305(5685), pp.788-792, 2004.

[2] T. Xu, A. Agrawal, M. Abashin, K. J. Chau, and H. J. Lezec, “All-angle negative refraction and active flat lensing of ultraviolet light”, Nature, vol. 497(7450), pp. 470-474, 2013.

[3] D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr and D. R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies”, Science, vol. 314, pp. 977-980, 2006.

[4] W. Cai, U. K. Chettiar, A. V. Kildishev and V. M. Shalaev, “Optical cloaking with metamaterials”, Nature photonics, vol. 1(4), pp. 224-227, 2007.

[5] M. Silveirinha and N. Engheta, “Tunneling of electromagnetic energy through subwavelength channels and bends using ε-near-zero materials”, Phys. Rev. Lett., vol. 97, pp. 157403, 2006.

[6] R. Mass, J. Parsons, N. Engheta and A. Polman, “Experimental realization of an epsilon-near-zero metamaterial at visible wavelengths”. Nat. Photo., vol. 7, pp. 907-912, 2013.

[7] A. V. Rogacheva, V. A. Fedotov, A. S. Schwanecke and N. I. Zheludev, “Giant gyrotropy due to electromagnetic-field coupling in a bilayered chiral structure”. Phys. Rev. Lett., vol. 97, pp. 177401, 2006.

[8] W. Zhang, W. M. Zhu, E. E. M. Chia, Z. X. Shen, H. Cai, Y. D. Gu, W. Ser, and A. Q. Liu, “A pseudo-planar metasurface for a polarization rotator”. Opt. Exp., vol. 22, issue 9, pp. 10446-10454, 2014.

[9] J. Elser, R. Wangberg, V. A. Podolskiy, and E. E. Narimanov, “Nanowire metamaterials with extreme optical anisotropy”, Appl. Phys. Lett., vol. 89(26), pp. 261102-261102, 2006.

[10] W. Zhang, A. Q. Liu, W. M. Zhu, E. P. Li, H. Tanoto, Q. Y. Wu, J. H. Teng, X. H. Zhang, M. L. J. Tsai, G. Q. Lo and D. L. Kwong, “Micromachined switchable

962

metamaterial with dual resonance”. Appl. Phys. Lett., vol. 101(15), pp. 151902-151902, 2012.

[11] W. Zhang, W. M. Zhu, H. Cai, M. L. J. Tsai, G. Q. Lo, D. P. Tsai, H. Tanoto, J. H. Teng, X. H. Zhang, D. L. Kwong and A. Q. Liu, “Resonance Switchable Metamaterials using MEMS Fabrications”, IEEE Journal of selected topics in quantum electronics, vol. 19, pp. 4700306-4700306, 2013.

[12] W. M. Zhu, A. Q. Liu, W. Zhang, J. F. Tao, T. Bourouina, J. H. Teng, X. H. Zhang, Q. Y. Wu, H. Tanoto, H. C. Guo, G. Q. Lo and D. L, Kwong, “Polarization dependent state to polarization independent state change in THz metamaterials”, Appl. Phys. Lett., vol 99, 221102,2011.

[13] N. Yu, P. Genevet, M. A. Kats, F. Aieta, J. P. Tetienne, F. Capasso and Z. Gaburro, “Light propagation with phase discontinuities: generalized laws of reflection and refraction”, Science, vol. 334(6054), pp. 333-337, 2011.

[14] D. H. Werner, D. H. Kwon, I. C. Khoo, A.V. Kildishev and V. M. Shalaev, “Liquid crystal clad near infrared metamaterials with tunable negative-zero-positive reflactive indices”. Opt. Exp. 15(6), 3342-3347, 2007.

[15] A. Q. Liu, W. M. Zhu, D. P. Tsai and N. I. Zheludev “Micromachined tunable metamaterials: a review”, Journal of Optics, vol. 14(11), pp. 114009, 2012.

[16] H. T. Chen, J. P. Willie, J. M. O. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Active terahertz metamaterial devices”. Nat., vol. 444, pp. 597-600,

2006. [17] H. Tao, A. C. Strikwerda, K. Fan, W. J. Padilla, X.

Zhang, and R. D. Averritt. “Reconfigurable terahertz metamaterials”. Phy. Rev. Lett. vol. 103,147401, 2009.

[18] W. M. Zhu, A. Q. Liu, T. Bourouina, D. P. Tsai, J. H. Teng, X. H. Zhang, G. Q. Lo, D. L .Kwong and N. I. Zheludev, “Microelectromechanical Maltese-cross metamaterial with tunable terahertz anisotropy”, Nat. commun., vol. 3, pp. 1274, 2012.

[19] W. M. Zhu, A. Q. Liu, X. M. Zhang, D. P. Tsai, T. Bourouina, J. H. Teng, X. H. Zhang, H. C. Guo, H. Tanoto, T. Mei, G. Q. Lo and D. L. Kwong. “Switchable magnetic metamaterials using micromachining processes” Adv. Mat., vol. 23(15), pp. 1792-1796, 2011.

[20] T. S. Kasirga, Y. N. Ertas, M. Bayindir, “Microfluidics for reconfigurable electromagnetic metamaterials”, Appl. Phys. Lett., vol. 95(21), pp. 214102-214102-3, 2009.

[21] J. A. Gordon, C. L. Holloway, J. Booth, S. Kim, Y. Wang, B. J. James, R. N. David. “Fluid interactions with metafilms/metasurfaces for tuning, sensing, and microwave-assisted”. Phys. Rev. B, vol. 83, 205130, 2011.

CONTACT †Y. L. Wang, Tel:+33-623131398; yamin.leprince@u-pem.fr †A. Q. Liu, Tel: +65-67904336; eaqliu@ntu.edu.sg

963