Tunable Polarization Conversion and Rotation based on a Reconfigurable Metasurface

Polarization is an important property of electromagnetic (EM) wave and different polarization manipulations are required for varied optical applications. Here we report a reconfigurable metasurface which achieves both the polarization conversion and the polarization rotation in THz regime. The metasurface is reconfigured through the micro-electro-mechanical-systems (MEMS) actuation. The cross polarization transmittance from a linear polarized incidence is experimentally tuned from 0 to 28% at 2.66 THz. In addition, the polarization rotation angle is effectively changed from −12.8° to 13.1° at 1.78 THz. The tunable bi-functional metasurface for polarization conversion and the polarization rotation can be flexibly applied in various applications such as imaging, polarization microscopy and material analysis, etc.

an x-slab is close to the top of a y-slab, and the right side of the x-slab is close to the bottom of another y-slab. This is like a descending stair structure and is named the "DOWN" state of the metasurface. In this state the metasurface is a planar chiral structure. The metamolecule is reconfigured to a "T" structure when all y-slabs are moved along the y-direction with a displacement Δs of (L y − w y )/2 = 12.5 µm as shown in Fig. 1(c). The metasurface now turns into a symmetrical achiral structure and is name the "T" state of the metasurface. The metasurface is further reconfigured to an ascending stair structure as illustrated in Fig. 1(d) when the y-slab is shifted with a total displacement Δs of (L y − w y ) = 25 µm. This is defined as the "UP" state of the metasurface. In this state, the metamolecule is a mirror image of that in the "DOWN" state. The lattice constant, a x and a y , is 80 µm and 50 µm, respectively. The a y is set twice the length of the total displacement (L y − w y ) so that the metamolecule can be consistently transformed from one state to another during the y-slab shifting.
Calculation results and discussions. The conversion of the linear polarization to its orthogonal direction through the metasurface is firstly discussed. An x-polarized THz wave is normally incident on the metasurface and the cross polarized transmittance T yx is numerically calculated. As shown in Fig. 2(a), in the "DOWN" state when Δs = 0, T yx has a peak value of 37% at 2.64 THz. As the y-slabs shift by 5 µm and 10 µm, the peak value drops to 34% and 18%, respectively. T yx in the whole spectrum vanishes when Δs increases to 12.5 µm and the metasurface changes to the "T" state. As the y-slabs further shift in the y-direction, T yx starts to increase and comes back to 37% when Δs is 25 µm and the metasurface changes to the "UP" state. The T yx is the same for a shifting of ∆s 0 and a shifting of (25 µm − ∆s 0 ) because the two states are the mirror image of each other and the couplings between the x-slab and the y-slab are the same.
The origin of the polarization conversion can be explained by investigating the induced magnetic field and the surface current on the metasurface at 2.64 THz under the normal incidence of the x-polarized THz wave as shown in Fig. 2(b-d) for the "DOWN" state, the "T" state, and the "UP" state, correspondingly. In the "DOWN" state, the surface current on the x-slab is directly induced by the incident electric field and is denoted with black arrows. Opposite electric charges are thus induced on the two ends of the y-slabs and the oscillation current is formed and is denoted with red arrows. In the "T" state, the surface current is mainly concentrating on the x-slab with little current on the y-slab due to weak couplings between x-slabs and y-slabs. Therefore, there is no conversion from the x-polarization to the y-polarization. In the "UP" state, the surface current excitation and coupling are the same as that in the "DOWN" state, except that the coupling induced current on the y-slab flows in the opposite direction.
As a 2D chiral structure, the metamolecule does not possess intrinsic chirality 28 . However, the extrinsic chirality of the metasurface can be realized under oblique incidence because the k-vector of the incident wave is not in the normal direction of the 2D chiral surface 29 . This can be further explained by the coupling between the induced electrical dipole and the magnetic dipole as shown in Fig. 3(a) when the EM wave is incident on the metasurface with an angle of α. An electric dipole p is excited in the metamolecule under the incident electric field and emits an electric field E p . Meanwhile, a magnetic dipole m is also induced on the planar chiral structure due to oblique incidence. The m is either parallel or antiparallel with the electric dipole p and reradiates EM waves with electric field E m . The combination of the E p and E m is the total scattering field E s , which is not parallel to the incident E i due to the non-zero E m . Therefore, the direction of the transmitted field E t , which equals to "E i + E s " is rotated relative to the original E i direction. The polarization rotation angle θ and the circular dichroism tan(χ), which is defined as the relative difference of the circularly polarized transmittance, can be expressed as in the Eqns (1) and (2) and where t ++ (t −− ) and ϕ ++ (ϕ −− ) are the transmittance and phase delay of the transmitted left (right) circularly polarized light from a left (right) polarized incident light, respectively. Figure 3(b) and (c) show the numerically calculated polarization rotation and dichroism of the THz wave passing through the metasurface in the "DOWN" state under different incident angles. The polarization rotation and the dichroism are 0 when α = 0° in the entire investigated frequency band, which confirms that there is no chirality in the planar metasurface under the normal incidence. As the incident angle increases, the absolute value θ and the tan(χ) become non-zero, which indicates a polarization rotation and a circular birefringence. The polarization rotation θ is increased up to 13° at 1.82 THz when α increases to 45°, which is an anti-clockwise polarization rotation. The polarization rotation tunability is investigated through reconfiguring the metasurface from the "DOWN" state to the "UP" state as shown in Fig. 4(a) and (b) with α fixed at 45°. The polarization rotation decreases from 13° to 0° at 1.82 THz when the metasurface is tuned from the "DOWN" state to the "T" state. In fact the polarization rotation is 0 in the whole investigated spectrum, because the metasurface in the "T" state is a planar symmetric structure and no extrinsic chirality can be induced. A negative polarization rotation is observed between 1.75 THz and 1.90 THz when the metasurface is tuned to the "UP" sate with the ∆s of 25 µm. Comparing the polarization rotation angle in the "DOWN" state with that in the "UP" state, they have the same angle value with opposite sign. This is because the coupling direction between the electrical field and the magnetic field is reversed when reconfiguring the planar chiral metamolecule to its mirror image structure. Therefore, the polarization rotation is reversed.
Experimental results and discussions. Figure 5(a) shows the scanning electron microscopic (SEM) image of the reconfigurable metasurface, which is fabricated using a MEMS process. The sample consists of a 200 × 125 element array which has a period of 50 μm × 80 μm in x-y plane. The x-slab and the y-slab in the metasurface are patterned on the fixed silicon bulk and movable silicon beam, respectively. The gap between the x-slab and the y-slab is 2 μm. The relative position of the two types of slabs is tunable when the slabs along y-direction are shifted through the integrated MEMS actuator. The tuning time for shifting the slabs for 25 µm is about 2 ms. The insertion of Fig. 5(a) shows the close-up view of the metasurface element in the "DOWN" state, the "T" state, and the "UP" state, respectively.
The cross transmittances of the x-polarized incident wave are measured when the element is tuned from the "DOWN" state to the "T" state as shown in Fig. 5(b). A high transmittance T yx of 28% is obtained in the "UP" state at 2.66 THz. T yx 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 conversion from the x-polarized incident wave to the y-polarized transmitted wave decreases dramatically.   The measured polarization rotation and dichroism of the metasurface under 45° incidence are plotted in Fig. 5(c) and (d), respectively. A polarization rotation peak of 13.1° is observed at 1.78 THz in the "DOWN" state. The rotation decreases to almost 0 when the metamaterial is shifted to the "T" state, in which the metamolecule is a symmetric structure. The polarization rotation is then reversed to a negative value with a further shift of the y-slab. The rotation finally becomes −12.8° when the metasurface is tuned to the "UP" state. The reversed polarization rotation angle between the "DOWN" state and the "UP" state demonstrates the reversed coupling between the x-slab and the y-slab through the state switch.

Conclusion
In conclusion, we have investigated theoretically and experimentally the polarization conversion and polarization rotations through the reconfigurable metasurface. It is demonstrated that significant tunability can be realized by changing the couplings between the metallic components in the metamolecule. The conversion efficiency is variable between 0 and 28% at 2.66 THz under normal incidence while the polarization rotation angle is effectively tunable from −12.8° to 13.1° with an incident angle of 45° at 1.78 THz. As a result, the tunability of the polarization conversion and the polarization rotation can be applied in many real time controlled applications such as the imaging and signal communications, etc.

Methods
Fabrication process. The fabrication of the reconfigurable metasurface starts with a silicon on insulator (SOI) wafer followed by aluminum metal patterning using physical vapor deposition (PVD) and optical lithography processes. The aluminum layer has two functions: one is to be patterned as the metamolecule structure and the other is to be used as the metal contact for the micromachined actuator. Deep reactive ion etching (DRIE) and SiO 2 layer etching processes are then applied to suspend and drive the y-slab on the substrate. Experimental setup. A THz Time-domain spectroscopy (THz-TDS) Teraview spectra 3000 is used to measure the linearly polarized transmittance. This is converted to circularly polarized transmittance for polarization rotation determination. Electrical voltage is applied on a micromachined comb drive to move the y-slabs. The displacement can be expressed as Δx = KV 2 , where K is the actuation coefficient with the value of 0.04 in the design and depends on the structure size of the comb drive. A displacement ∆x of 12.5 µm and 25 µm are obtained from an actuation drive of 16 V and 26 V, respectively, in the measurement.
Scientific RepoRts | 7: 12068 | DOI:10.1038/s41598-017-11953-z material. If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.