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Dual-band superposition induced broadband terahertz linear-to-circular polarization converter

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Abstract

A reflective broadband terahertz (THz) linear-to-circular (LTC) polarization converter based on a single-layer ultrathin metasurface is designed and experimentally demonstrated. Two different-size rectangular ultrathin metasurface micro-structures are proposed to realize such a broadband THz LTC polarization converter with bandwidth ranging from 0.832 to 1.036 THz. The phase delay between two orthogonal resonance modes is 90°±5°. These qualities are realized mainly by combining two separated LTC polarization conversion frequencies and the benefit of the coupling between two different-size rectangles. The calculated results indicate that the bandwidth of the LTC polarization converter is controlled via the dimensions and period of the structure. This kind of ultrathin broadband THz polarization converter can be widely applied into wireless communication, imaging, and detection, and can widen the path to designing novel functional THz devices.

© 2018 Optical Society of America

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Figures (11)

Fig. 1.
Fig. 1. (a) Schematic of the broadband THz polarization converter. (b) The corresponding unit cell. The normal incident THz wave is polarized at θ = 45 ° toward the x axis.
Fig. 2.
Fig. 2. (a) SEM of the fabricated structure. (b) Experimental setup.
Fig. 3.
Fig. 3. Simulated and measured results of the broadband THz LTC polarization converter: the reflection coefficients of (a) simulations and (b) measurements; phase delay between two orthogonal components of (c) simulations and (d) measurements.
Fig. 4.
Fig. 4. (a) Simulated and (b) measured ellipticity.
Fig. 5.
Fig. 5. Normalized wavefront trajectory curves of the reflected waves at 0.84, 0.9, 0.96, and 1.02 THz.
Fig. 6.
Fig. 6. Electric field distribution for two orthogonal polarized waves at (a) 0.856 THz, (b) 0.936 THz, and (c) 1.0 THz.
Fig. 7.
Fig. 7. (a) Phase delay and (b) ellipticity for different h 1 . (c)–(g) The normalized wavefront trajectories for different h 1 at f = 1.0 THz with the corresponding phase delay [between these two orthogonal electric fields ( E x and E y )] shown at points A, B, C, D, and E in (a).
Fig. 8.
Fig. 8. (a) Phase delay and (b) ellipticity for different h 2 . (c)–(g) The normalized wavefront trajectories for different h 2 at f = 0.856 THz with the corresponding phase delay [between these two orthogonal electric fields ( E x and E y )] shown at points A, B, C, D, and E in (a).
Fig. 9.
Fig. 9. Phase delay (a) and ellipticity (b) for different thickness of polyimide film.
Fig. 10.
Fig. 10. (a) Phase delay and (b) ellipticity for different periods with size-variational rectangular patterns. L 1 = 0.49 P x , h 1 = 0.42 P x , L 2 = 0.64 P x , h 2 = 0.67 P x , and P y = 2 P x .
Fig. 11.
Fig. 11. (a) Phase delay and (b) ellipticity for different period with size-fixed rectangular patterns. L 1 = 56.8 μm , h 1 = 49 μm , L 2 = 74.9 μm , h 2 = 78 μm , and P y = 2 P x . Inset is the schematic of the THz polarization converter. The centers of the large and small rectangles are located in (0, p y / 4 ) and (0, p y / 4 ), respectively. The black dots in the inset represent the centers of the rectangles.

Equations (2)

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S 0 = | r ˜ x | 2 + | r ˜ y | 2 , S 1 = | r ˜ x | 2 | r ˜ y | 2 , S 2 = 2 | r ˜ x r ˜ y | cos ( ϕ d ) , S 3 = 2 | r ˜ x r ˜ y | sin ( ϕ d ) ,
x 2 a 2 + y 2 b 2 2 x y cos ( ϕ d ) ab = sin 2 ( ϕ d ) ,
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