Microfluid-based soft metasurface for tunable optical activity in THz wave

Wu Zhang; Wu Zhang; Wu Zhang; Bingzhi Zhang; Xiaohui Fang; Kejun Cheng; Weiqian Chen; Zihuang Wang; Dou Hong; Meng Zhang; Meng Zhang

doi:10.1364/OE.420660

1. Introduction

Since its proposing, the artificially designed planar metasurface has manifested many advantages over the three dimensional (3D) metamaterial, which suffers largely from complicated fabrication process, bulky dimensions and high loss [1–3]. Metasurfaces manipulate and control electromagnetic waves relying on only one active layer, therefore could be compatible with the CMOS technology and advance in cost effectiveness, massive fabrication and device integration, especially for the structure in nanometer feature size. Metasurface has been intensively studied to control the fronts of electromagnetic (EM) waves ranging from microwave to visible light by simply scaling the meta-atom to sub-wavelength dimension [4,5]. The phase delay of each meta-atom is specified by well-established mechanisms such as different mode coupling [6], Berry phase control [7] and waveguide theory [8,9]. Based on the phase delay manipulation, many fascinating applications have been demonstrated, including diffraction-limited focusing [10], vortex beam generation [11], hologram imaging [12] as well as OLED display technology [13], etc.

Besides the phase delay tailoring, the polarization manipulation for EM wave control is also important in lots of areas like chirality sensing [14], optical imaging [15], life science microscopy [16] and multiplexed optical communication [17]. Scientists have demonstrated different polarization manipulations such as polarization conversion [18], versatile polarization generation [19], polarization division multiplexing [20] and faraday rotation enhancement [21]. The polarization rotation, or the optical activity of EM wave, is usually very weak in natural materials. Much effort has been made to enhance the polarization rotation through different approaches like liquid crystal control [22] or 3D chiral metamaterial design [23,24]. On the other hand, as metasurface is usually a flat structure and does not possess 3D chirality, it is hard for it to realize strong optical activity.

In this paper, we proposed a microfluid-based soft chiral metasurface (MSCM) to obtain an actively controllable optical activity in the terahertz (THz) wave. Advanced to the passive metasurface device, the active metasurface manipulates the EM wave in real time. Researchers have demonstrate active metasurfaces through external excitation such as laser pumping [25], electrical bias [26] or temperature control [27]. Another promising approach for the active metasurface is to reshape the meta-atom structure by either shifting meta-atoms through micro/nano-electromechanical systems (MEMS/NEMS) technology [28,29], or patterning the meta-atoms on a stretchable soft substrate [30]. Special fabrication process and treatment is usually required for the soft metasurface realization, and the meta-atom may suffer cracking under stretching. Here, we used the soft polydimethylsiloxane (PDMS) material to construct a periodic microchannel structure, in which liquid metal Galinstan is injected as meta-atoms. In our previous work, the metasurface was reconfigured by driving Galinstan flow in the microchannel structure [31] which realized a phase control of microwave. The control had to be achieved through a complicated valve system, and Galinstan is also susceptible to leave residue in the microchannel when flowing through. Here, the microfluid meta-atom is designed into a 2D chiral structure and bonded on a microfluid pressure system (MPS). When injecting glycerol into the MPS, liquid pressure is applied on the metasurface and the 2D metasurface bends into 3D structure, manifesting a controllable polarization rotation. Thanks to the continuous of the Galinstan, the metal layer does not crack during the bending. No Galinstan residue was left in the microchannel either because the reconfiguration does not rely on the Galinstan flow. The MSCM for polarization rotation can be flexibly applied in various applications such as THz imaging, polarization microscopy and material analysis, etc.

2. Design and principle

The designed MSCM for the tunable optical activity is illustrated in Fig. 1. It consists of three bonded PDMS layers and the side view for the meta-atom is illustrated in Fig. 1(a). In the top PDMS layer, an array of “ oe-29-6-8786-i001 ” shaped microchannels are designed as the active metasurface structure, in which the liquid metal Galinstan is injected. The top view of the “” shape is illustrated in the insertion of Fig. 1(b), where a horizontal channel across the meta-atom is designed to connect neighboring meta-atoms in the same row. Two asymmetrically located arm channels on two sides of the horizontal channel are connected through a vertical channel, making the meta-atom a chiral structure. All rows of the “ oe-29-6-8786-i001 ” channels are connected to the Galinstan Inlet as show in Fig. 1(b), and the injected Galinstan flows along the x-direction as indicated by the black arrows. The Galinstan fills all “” microchannels and then flows out of the Galinstan Outlet on the opposite side of the Galinstan Inlet.

Fig. 1. (a) The side view of the meta-atom of the MSCM and (b) the metasurface deformation.

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In the bottom PDMS layer, one straight microchannel is designed below each “ oe-29-6-8786-i001 ” column along the y-direction. All the bottom microchannels are connected with a Glycerol Inlet and a Glycerol Outlet. When the Glycerol Outlet is open, the injected glycerol flows smoothly through the bottom channels as indicated by the yellow arrows in Fig. 1(b) and the whole structure remains flat as shown in the middle of Fig. 1(a). Then the Glycerol Outlet is closed while the glycerol injection continues, the pressure in the bottom channel is induce which pushes the top PDMS layer upwards. The top PDMS layer is deformed from a flat structure to a bent one with a bending height A as shown in the bottom of Fig. 1(a). The metasurface then becomes a 3D chiral structure. The dimension of one meta-atom is 1 mm${\times} $1 mm in the xy direction and the whole array is 30 mm${\times} $30 mm; the microchannel in the top layer is 20 µm in depth and 100 µm in width; the arm of the “ oe-29-6-8786-i001 ” is designed at length b = 350 µm and has the distance of L = 300 µm with the structure center; the straight microchannels in the bottom PDMS layer is W = 850 µm wide and center aligned with each column of “” meta-atoms in the metasurface layer.

Initially without pressure pumping, the metasurface is a flat 2D chiral structure and the meta-atom does not possess intrinsic chirality. When deformed into a bent 3D structure, optical activity will be induced through the metasurface. This can be explained by analyzing the electrical dipole and the magnetic dipole in the meta-atom as shown in Fig. 2. When a linearly polarized EM wave is incident on the metasurface, an electric dipole p is excited in the meta-atom from the incident electric field E_i and a secondary wave with electric field E_p. Meanwhile, the meta-atom has a non-trivial response to the magnetic field as its two arms are located asymmetrically in the 3D architecture. A magnetic dipole m is therefore induced which has a nonzero component either parallel or antiparallel with the electric dipole p. The magnetic dipole also radiates EM waves with electric field E_m perpendicular to E_p. The combination of the E_p and E_m, or the total scattering field E_s, is not aligned to the incident E_i due to the non-zero E_m. As a result, the polarization of the transmitted field E_t= E_i + E_s, is rotated relative to the original E_i direction.

Fig. 2. Optical activity induced by the interaction between electric and magnetic dipole.

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Fig. 3. Optical activity interpretation with the combination of left and right circularly polarized light.

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Optical activity is characterized by the two parameters of polarization rotation angle θ and the circular dichroism tan(χ), which can be derived from the relative difference of the circularly polarized transmittance, and are expressed as in Eqs. (1) and (2) [32]

(1)$$\theta = ({{\varphi_{\textrm{ ++ }}}\textrm{ - }{\varphi_{\textrm{ -- }}}} )/2$$

and

(2)$$\tan \chi = ({{\textrm{t}_{\textrm{ ++ }}}\textrm{ - }{t_{\textrm{ -- }}}} )/({{\textrm{t}_{\textrm{ ++ }}}\textrm{ + }{t_{\textrm{ -- }}}} )$$

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. This relation can be interpreted with Fig. 3 by investigating an incident EM wave linearly polarized with the form of ${\boldsymbol E} = {E_0}\widehat i\cos \omega t$. The wave can be divided into a left circularly polarized light E_L and a right circularly polarized light E_R. The transmittance of the two polarized light after the light passes through a material can be expressed as

(3)$${{\boldsymbol E}_L} = \frac{{{E_0}{t_L}}}{2}[{\widehat i\cos ({{k_L}z - \omega t} )- \widehat j\sin ({{k_L}z - \omega t} )} ]$$

and

(4)$${{\boldsymbol E}_R} = \frac{{{E_0}{\textrm{t}_R}}}{2}[{\widehat i\cos ({{k_R}z - \omega t} )+ \widehat j\sin ({{k_R}z - \omega t} )} ]$$

where t_L and t_R are the transmittance of the left circularly and the right circularly polarized light, which corresponds to the absorption of the two polarized light. k_L and k_R are the wave numbers, which correspond to the real part of the refractive index of the two polarized light. When t_L = t_R and k_L = k_R, as illustrated in Fig. 3(a), the transmitted left circularly polarized light (red arrow) and right circularly polarized light (green arrow) will have the same amplitude and opposite phase change relative to the original incident phase. Therefore, the polarization of the whole transmitted light (blue arrow) is not changed. For the case t_L = t_R and k_L ≠ k_R, the amplitudes of the transmitted left circularly and right circularly polarized light will still be the same while the phase delays are not the opposite number with each other anymore, therefore the whole transmitted light will remain as linearly polarized, but show a polarization rotation with the rotated angle θ. While for the case t_L ≠ t_R and k_L ≠ k_R, the amplitudes of the transmitted left circularly and right circularly polarized light will be different and the phase delays are not the opposite number, therefore, the whole transmitted light will elliptical wave with the rotated polarization angle θ and the ellipticity tan(χ) being expressed as in Eqs. (1) and (2).

3. Results and discussions

The transmittance of the metasurface is calculated using Microwave Studio of Computer Simulation Technology (CST). In the simulation, Galinstan is characterized as a lossy metal with a conductivity of 3.46 × 10⁶ S/m, and the PDMS has a dielectric constant with the real part ɛ^’ = 2.465 and imaginary part ɛ^” = 0.125, respectively [33,34]. A unit cell boundary condition is used for the calculation and circularly polarized plane wave is incident on the metasurface. The co-polarized transmittances t₊₊ and t_–, and the corresponding phase delays φ₊₊ and φ_– are calculated. The optical activity behavior is then derived by substituting above transmission parameters into Eqs. (1) and (2). Figure 4(a) shows the numerically calculated optical activity of the THz wave passing through the metasurface at different bending height A. The frequency band ranges from 0.16 THz to 0.28 THz, corresponding to a wavelength from 1.88 mm to 1.07 mm. When A = 0, the metasurface is flat and no optical activity is observed. As the metasurface starts to bend, A increases and optical activity becomes nonzero across the spectrum. The rotation angle keeps on increasing as A becomes larger, and a peak value of 30° is obtained at 0.23 THz when A = 80 µm. The ellipticity also increases with A as shown in Fig. 4(b). The polarization rotation and the ellipticity is nonzero, which results from the total effect of the electrical response and the magnetic response as discussed early. The surface currents on the metasurface are compared for the flat structure and the bent structure with A = 80 µm as shown in Fig. 4(e) and (f), respectively. The incident wave is set at frequency of 0.22 THz with a linear polarization along y-direction. In the flat structure, the surface current is concentrated on the vertical microchannel, which is excited by the electrical field of the incident wave. The current on the arm and the horizontal channel only flow along with the current on the vertical microchannel. For its flat dimension, it only responses to the electrical field of the incident wave and no optical activity is induced. While in the bent structure, it can be seen that the electric current on the horizontal channel and the arm channel does not flow along with that on the vertical channel. This can be explained by investigating the surface current direction on the two arms. The surface current on the right side arm flows into the xy plane while that on the left side arm flows out of the xy plane. Therefore magnetic dipole can be generated due to the surface currents in opposite direction. This magnetic dipole has components in both x-direction and y-direction. The y-directional component of the magnetic dipole is aligned with the electrical dipole induced by the y-polarized wave. As discussed in Fig. 2, optical activity can therefore be realized. Similar conclusion applies for the x-polarized incidence. Generally, the absolute values of the polarization rotation angle and ellipticity are nonzero when A ≠0. This corresponds to the case t_L ≠ t_R and k_L ≠ k_R in Fig. 3, which results in a polarization rotated and elliptically transmitted wave. It can be seen that the polarization rotation angle and ellipticity generally increase with a larger A. And it is also interesting to note that the ellipticity is 0 at the incident frequency of 0.19 THz for different A values as shown in Fig. 4(b). This means t_L = t_R and k_L ≠ k_R as indicated in Fig. 3, which results a linearly polarized transmitted wave at a certain polarization rotated angle.

The resonant frequency of both the electric and the magnetic response are highly related to the two asymmetric located arm channels. With different arm lengths, the optical activity is different as shown in Figs. 4(c) and (d) with A being a fixed value of 80 µm. When the length of the arm channel is 0, the unit is a simple symmetric cross structure, and no polarization rotation is induced. As the arm length b = 175 µm, a negative polarization rotation is induced across most of the investigating spectrum range [blue line in Fig. 4(d)]. While when b = 350 µm, the polarization rotation becomes positive from 0.16 THz to 0.23 THz, and negative from 0.23 THz to 0.28 THz. This indicates that the overall optical activity is sensitivity to the length of the arm channel. In addition, when the direction of the arm changes, for example when b = 350 µm changes to b = -350 µm, the chirality of the structure is reversed, therefore the polarization rotation angle and the ellipticity in the whole spectrum is reversed.

Figure 5(a) shows the fabrication process of the microchannel structure. A SU8 photoresist (Microchemicals) layer in thickness of 20 µm was first spin on a Si wafer, followed by the optical lithography process to develop the microchannel pattern. The liquid PDMS with base and curing agent mixing ratio at 10:1 was then spin on the patterned SU8 layer, heated for 4 hours at 65°C and became a solid layer of 50 µm thickness. The solid PDMS layer was peeled off from the Si wafer and bonded with a flat PDMS layer. The two PDMS layers are bonded using an oxygen plasma generator. The two surfaces to be bonded together were treated with oxygen plasma at a power of 25 W for 30 s, then one surface is covered on the other and a tight bonding can be achieved after several seconds. Figure 5(b) shows the photo of MSCM containing 30×30 meta-atoms. Four needles are plugged into the inlets and outlets for Galinstan and glycerol, respectively. The metasurface microchannel under an optical microscope is shown in Fig. 5(c). The microscope is focused on the metasurface layer with the “ oe-29-6-8786-i001 ” microchannel. The MPS layer with straight channel is bonded at the bottom. Figures 5(d) and (e) show the zoom in microchannel structure. There are two narrow microchannels in width of 20 µm connected to the ends of the two arms in each meta-atom. They are designed for the air vent when the Galinstan is injected in to the meta-atom. Galinstan will not penetrate into the two microchannels within a certain driven pressure due to the narrow channel width.

Fig. 4. simulated (a) polarization rotation and (b) the ellipticity at different A; simulated (c) polarization rotation and (d) the ellipticity at different b; (e) simulated surface current on one meta-atom when A=0 and (f) A = 80 µm.

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Fig. 5. (a) The fabrication process of the PDMS layer for the metasurface microchannel; (b) the photo of the metasurface with needles plugged in the PDMS microchannels; (c) the metasurface structure under optical microscope; (d) the meta-atom microchannel for Galinstan injection; (e) the bonding between the metasurface layer and the liquid pressure layer.

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Before the experiment, Galinstan was first injected and filled into all the metasurface meta-atoms. Then glycerol was slowly injected in to the MPS channel with its outlet being open so that the structure was flat initially. After the glycerol filled all the bottom microchannels, the outlet for the glycerol was closed. When glycerol was further injected into the structure, the metasurface layer will be bent up due to the increased pressure in the MPS channel. The bending degree was characterized by the volume ΔV of extra glycerol injected into the MPS channel after the glycerol outlet is closed. A THz Time-domain spectroscopy (THz-TDS) Teraview spectra 3000 was used to measure the optical activity of the metasurface. As the spectroscopy incident a linearly polarized terahertz wave, we considered to convert the linearly polarized transmission to the circularly polarized transmission using the equation [35]

(5)$$\left( {\begin{array}{cc} {{{\mathop t\limits^\sim }_{ +{+} }}}&{{{\mathop t\limits^\sim }_{ +{-} }}}\\ {{{\mathop t\limits^\sim }_{ -{+} }}}&{{{\mathop t\limits^\sim }_{ -{-} }}} \end{array}} \right) = \frac{1}{2}\left( {\begin{array}{cc} {{{\mathop t\limits^\sim }_{xx}} + {{\mathop t\limits^\sim }_{yy}} + i({{\mathop t\limits^\sim }_{xy}} - {{\mathop t\limits^\sim }_{yx}})}&{{{\mathop t\limits^\sim }_{xx}} - {{\mathop t\limits^\sim }_{yy}} - i({{\mathop t\limits^\sim }_{xy}} + {{\mathop t\limits^\sim }_{yx}})}\\ {{{\mathop t\limits^\sim }_{xx}} - {{\mathop t\limits^\sim }_{yy}} + i({{\mathop t\limits^\sim }_{xy}} + {{\mathop t\limits^\sim }_{yx}})}&{{{\mathop t\limits^\sim }_{xx}} + {{\mathop t\limits^\sim }_{yy}} - i({{\mathop t\limits^\sim }_{xy}} - {{\mathop t\limits^\sim }_{yx}})} \end{array}} \right).$$

where the ${\tilde{t}_{ij}}$ is the complex number of transmittance including both amplitude and phase information. In the measurement, two linear polarization plates working in THz regime were used as the polarizer and the analyzer, respectively. The polarizer was aligned with the incident THz wave polarization direction. When aligning the metasurface sample with the polarizer direction or its orthogonal direction, the incident wave for the metasurface can be treated as x-polarized or y-polarized, respectively. By rotating the analyzer properly, the co-polarized transmittances ${\tilde{t}_{xx}}$ and ${\tilde{t}_{yy}}$, and the cross-polarized transmittances ${\tilde{t}_{xy}}$ and ${\tilde{t}_{yx}}$ can be obtained. Then the circularly polarized transmittances are derived according to Eq. (5). The polarization rotation and ellipticity of the metasurface are derived afterwards according to Eqs. (1) and (2) and are plotted in Fig. 6. It can be seen that the polarization rotation is around 0 in the range between 0.1 THz and 0.3 THz when ΔV = 0. However, a couple of small resonances were observed in the spectra for both the polarization rotation angle and the ellipticity. This should be due to the interaction between the resonance in the Galinstan metasurface and the Fabry Perot resonance in the 2 mm thick bottom PDMS layer. Relatively high noise was also noticed in the measurement, which could be due to the weak THz signal, especially when measuring the cross-polarized transmittance. As ΔV increases to 40 µL, a polarization rotation of 5° is observed at 0.19 THz and −7° at 0.24 THz. To estimate the deformation amplitude A at this state, we treated the cross section of the deformed shape as a half ellipse with the long axis of W and short axis of 2A, where W is the bottom microchannel width 850 µm. Therefore the cross section area of the half ellipse S₁ = πWA/4. Considering a total of 30 × 30 meta-atoms and each meta-atom is 1 mm in side length, we can derive the cross section area of the deformed shape S₂ = ΔV/(30×30×1 mm). Letting S₁= S₂, we can estimate A = 67 µm when ΔV = 40 uL. As ΔV increased to 80 µL, A becomes 133 µm, correspondingly. The polarization rotation is significant at 0.19 THz, which reaches 14°. A high ellipticity value of 0.7 is also observed at 0.23 THz. Therefore, the optical activity is demonstrated in the bending chiral metasurface.

Fig. 6. (a) The measured polarization rotation; (b) the ellipticity of the soft metasurface.

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4. Conclusions

In conclusions, we realized an active control of polarization rotation though a microfluid-based soft metasurface. The meta-atom is constructed by injecting liquid metal Galinstan into a planar chiral structured microchannel, and deformed by an external liquid pressure. The metasurface is reconfigured from a 2D planar structure into a 3D chiral structure. Both electrical and magnetic dipole are excited in the meta-atom, the interaction of which induces a giant optical activity. A polarization rotation tuning from 0° to 14° at 0.19 THz was demonstrated when the metasurface was reconfigured from the flat state to the bent state. The tunable metasurface for polarization rotation can be flexibly applied in various applications such as polarization microscopy, bio-detection and material analysis, etc.

Funding

National Natural Science Foundation of China (61905046); China Postdoctoral Science Foundation (2020M683044); Open Fund of Guangdong Provincial Key Laboratory of Information Photonics Technology (GKPT20-08, Guangdong University of Technology).

Acknowledgement

Thanks to the Institute of High Performance Computing, Astar, Singapore.

Wu Zhang initiated the idea, did the numerical calculation, analysed the data and composed the manuscript. Bingzhi Zhang did the experiment and analysed the data. Kejun Cheng performed the numerical calculation. Weiqian Chen, Zihuang Wang joined the experiment; Dou Hong and Xiaohui Fang joined the discussion, Meng Zhang completed the sample fabrication, set up the experiment and composed the manuscript.

Disclosures

The authors declare no conflicts of interest.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Microfluid-based soft metasurface for tunable optical activity in THz wave

Abstract

1. Introduction

2. Design and principle

3. Results and discussions

4. Conclusions

Funding

Acknowledgement

Disclosures

Data availability

References

Data availability

Cited By

Figures (6)

Equations (5)

Optics Express