THZ POLARIZER USING TUNABLE METAMATERIALS W. Zhang1, 2, W. M. Zhu1, J. M. Tsai2, G. Q. Lo2, D. L. Kwong2, E. P. Li3 and A. Q. Liu1,2† 1School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore

639798 2 Institute of Microelectronics, A*STAR, Singapore 117685

3Institute of High Performance Computing, A*STAR, Singapore 138632

ABSTRACT

The polarization of electromagnetic (EM) wave plays an essential role in the application of optoelectronics, life science microscopy and photographic display[1]. The conversion of EM wave in one polarized direction to its orthogonal direction is usually quite weak in natural materials. This paper presents a tunable metamaterial which rotates the polarized direction up to 20º in THz regime. The metamaterial can be transformed to its non-superimposable mirror image and tunes the polarization angle, which can be applied as tunable wave plate and optical switch in THz regime. INTRODUCTION

The metamaterial consists of large amount of artificially designed elements which are usually periodically arranged. The metamaterial elements interact with the EM wave relying on the structure, size of and the space among the elements. Therefore, tuning the metamaterial elements provide a reliable approach to control the EM wave. Different properties of the EM wave such as left handed propagation, extraordinary transmission and optical activity has been realized through metamaterial control, which provides wide potential applications such as cloaking, sensing and absorbing, etc.

Different methods are investigated for tuning the metamaterial properties. For instance, the effective refractive index of the metamaterial can be changed by stimulating the nonlinear material consisted metamaterial[2-6] with different external input such as thermal effect[7, 8], electric voltage bias[9, 10] or magnetic field excitation[11]. Changing the arrangement of metamaterial element arrays tunes the coupling between the metamaterial elements[12-16]. A more straight forward way is to reconfigure the metamaterial element shape directly, which changes the interaction between elements and incident wave[17, 18]. In this paper, a single layered metallic element metamaterial is designed, which can be tuned to its mirror image through micromachined actuating. As a result, the transmission from one polarized direction to the orthogonal direction is tuned with transmitted phase well controlled. TUNABLE METAMATERIAL DESIGN

Figure 1(a) shows the metamaterial element with lattice constant of 80 μm × 50 μm. The element consists of x- and y-oriented metal strips forming a “stair” structure. The x-strips are released from the substrate and become movable in the x-direction. The structure initially goes upstairs along y-direction and is defined as the up-state (Fig. 1(b)). It is tuned to the mid-state (Fig. 1(c)) and down-state (Fig. 1(d)) as the x-strips shift. The structure goes downstairs along y-direction in the down-state, which

is equivalent with the up-state’s non-superimposable mirror image (Fig. 1 (e)). The x-strips are connected with micromachined actuators and driven under electrostatic force.

TUNED METAMATERIAL SIMULATION Linearly Polarized Transmittance

The Transmittance of x-polarized incident wave through the metamaterial in the up-, mid- and down-state are calculated using CST microwave studio as shown in Fig. 2. In the mid-state, the transmittance from x-polarized incident wave to x-polarized signal txx, mid is above 30% in the regime between 2.8 THz and 3.4 THz. However, the transmittance from x-polarized incident wave to y-polarized signal tyx, mid is almost 0. Therefore, there is no polarization rotation in the mid-state. On the other hand,

mirror

(b) (e)

x

y

x

y

(d) (c)

(a)

Figure 1. (a) SEM graph of the tunable metamaterial

polarizer; (b)initial-state, (c) mid-state and (d)

final-state of the metamaterial; (f) mirror image of the

initial-state

MEMS 2013, Taipei, Taiwan, January 20 – 24, 2013978-1-4673-5655-8/13/$31.00 ©2013 IEEE 713

the transmittance in the up-state txx, up equals with that in the down-state txx, down. This is because the metamaterial elements in the up-state and the down-state are mutually mirror image and their transmittance amplitudes are the same. In addition, the transmittance from x-polarized incident wave to y-polarized signal in both up- and down-states txy, up and txy, down increases to above 30%, which is comparable with txx, up (txx, down). Therefore, a polarization rotation can be observed in the up- and down-states.

The transmittances of tyx in the up- and down-states are investigated through current flow and E-field distribution at 3.05 THz as shown in Fig.3. High E-field of opposite phase is induced at the two ends of y-strips due to the coupling with both the left and right side x-strips dipole in the up-state (Fig. 3(a)) and down-state (Fig. 3(c)). Therefore, large surface current is generated and results in a significant transmittance of tyx. In addition, the y-strip dose not couple with the x-strip in mid-state (Fig. 3(b)) and therefore tyx will be 0.

Polarization Rotation The polarization rotation angle θ and ellipticity angle χ can be obtained by detecting the circular polarized transmitted wave under a linear polarized incidence [19]

)(2

1

aa

aatan

where the transmitted transmission coefficients of the RCP (+) and LCP (−) components is

)exp(t

ia

The circular transmission from x-polarized incidence is calculated in Fig. 4(a) for both up-state and down-state, from which θ and χ are derived in Fig. 4(b). The polarization rotation angle θ is opposite for up- and down-states which is due to the mirrored orientation. Therefore, it is possible to switch the polarization orientation by a large angle by switching between the two states of metamaterial. A polarization of ±20º is realized at 3.05 THz, which is tuned dynamically through the MEMS actuator.

(a)

(b)

Figure 4. (a)Transmittance of circular polarized

incidence; (b) Polarization rotation and ellipticity

(a)

(c)

(b)

Figure 3. Surface current of the metamaterial at 3.05

THz in the (a) up-state, (b) mid-state and (c)

down-state

Figure 2. Transmittance from x to x polarization in up-

and down-states (black solid line); from x polarization

to y polarization in up- and down-states (green dashed

line); from x to x polarization in mid-states (blue

dashed line); from x to y polarization in up- and

down-states (red dotted line)

(2)

(1)

(3)

(3)

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EXPERIMENT The structure array of the metamaterial is fabricated in

a silicon-on-insulator wafer using the deep reactive ion etching processes. Fig. 5(a) shows the scanning electron microscopic (SEM) image of the switchable metamaterial. The sample consists of a 200 × 125 element array which has an in-plane translational period of 50 μm× 80 μm in x-y plane. The metamaterial element consists of slabs along x- and y- directions which are patterned on the movable silicon beam and fixed silicon bulk, respectively. The gap between x- and y- slabs is 2 μm while the thickness of the slabs is 50 nm. The relative position of the two directional slabs is tunable when the slabs along x-direction are shifted using MEMS actuator. Figure 5(b) shows the close-up view of the metamaterial element without tuning (up-state), which can be reshaped to its mirror image (down-state) as shown in Fig. 5(c) after the MEMS shifting of 25 μm. The transmittances of x-polarized incidence to y-polarized signal are measured using Terahertz time-domain spectroscopy (THz-TDS) when the element is tuned from up-state to mid-state as shown in Fig. 6(a). The metamaterial shows a high transmittance tyx of 40% in the up-state at 3.1 THz. tyx decreases as the x-slab shifts from 0 μm to 12.5 μm, which reconfigures the element from asymmetric structure to symmetric structure. As a result, the transmission from x-polarized wave to y-polarized signal decreases dramatically.

The transmission from x-polarized incidence to both x- and y-polarized signal are compared in up-, mid- and down-states in Fig. 6(b). The spectra of tyx in up-state and down-state are almost the same because the elements in the two states are mutually mirror image. Similarly, tyx in the two states are also the same, realizing more than 40% transmittance. The transmittance drops to the lowest level when the metamaterial tunes to the mid-state. The transmittance of tyx is comparable with txx in up- and down-state, and much smaller than txx in mid-state, therefore, polarization rotation is realized in up- and down-state, which is suppressed when the metamaterial is tuned to mid-state.

CONCLUSIONS In conclusions, a tunable metamaterial based on the MEMS actuating approach is designed, fabricated and tested for polarization rotation. The element of the metamaterial can be tuned from asymmetric structure to

x

y

20 μm

Figure 5. (a) SEM graph of the tunable metamaterial

and the supporting beam connected with the comb

drive; (b) the close view of the up- and (c) down-state of

the metamaterial

(a)

(b) (c)

supporting beam

Figure 6. (a) Measured transmittance from x to y

polarized wave when the x-strip of the metamaterial element shifts by Δs of 0 μm (black solid), 5μm (green dashed), 10 μm (blue dash dot) and 12.5 μm (red dot). (b) Measured transmittance from x to x polarized wave in up- (black solid), mid- (blue solid), down-states (red solid) and from x to y polarized wave in up- (black dash), mid- (blue dash) and down-states (red dash)

(a)

(b)

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symmetric structure, which tunes the transmittance of a linear polarized incident wave to orthogonal polarized wave. The transmittance is also tuned significantly. As a result, a tunable polarization rotation is realized through the metamaterial tuning. Therefore, the designed metamaterial has wide potential applications in tunable polarizer, optical filter and switches. ACKNOWLEDGMENTS

This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research), Singapore, under SERC grant number: IME/10-439651. REFERENCES [1] R. Bentley, Perspectives in Biology and Medicine

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