1. Introduction

Manipulating the spin state or circularly polarized light (CPL) is of great significance in modern photonic applications ranging from quantum computing [1], spin orbit communication [2] and sensing to circular dichroism (CD) spectroscopy [3–5]. In nature, chiral media composed of elements lacking the mirror symmetry such as quartz crystals, acids and proteins exhibit distinct response from light with opposite spin states. These intriguing phenomena typically refer to chiroptical effects, such as optical activity—the polarization rotation of linearly polarized light as it propagates through the media, and CD—the absorption difference between left- and right-handed circularly polarized (LCP and RCP) light. However, the chiroptical effects of natural materials are usually too weak to be detected in the range of optical frequencies [3], hence limiting their potentials for photonic applications. Metamaterial, an artificially-engineered composite material, provides an unprecedented control on the energy flow, phase and polarization state of light at deep subwavelength scale. By designing meta-atoms that break the mirror symmetry, various chiral metamaterials have been intensely investigated in past decades. They have demonstrated impressive properties such as strong optical activity [6], CD enhancement [7–11], chirality-induced negative refractive index [12, 13], asymmetric transmission [14, 15] and superchiral electromagnetic fields [16, 17]. These exciting features have been widely exploited in beam splitter, circular polarizers, bio-sensors, and so on.

The ability to selectively-absorb one specific CPL state is particularly attractive because it has various potential applications in CD spectroscopy, holographic imaging and bio-detecting. To this end, intensive effort has been made to realize spin-selective metamaterial absorbers [18–23]. For example, Li et al. [18] proposed a Z-shaped sliver antenna to strongly absorb one state CPL but reflect the opposite state CPL. By incorporating with hot electron injection, a photodetector for CPL detection is achieved. Kang and associates [21] also proposed a meta-mirror with asymmetric-hole array to spin-selectively absorb the incoming CPL, which has been applied in linear and nonlinear imaging. Up to now, all the proposed metamaterials mentioned above follow the very typical designs of achiral metamaterial absorbers [24], the so-called sandwich structure: a patterned metallic nanostructure in the upper layer, followed by a metallic backplane separated by a dielectric layer. The functions of the metallic backplane are two folds: on one hand, it blocks the transmitted light; on the other hand, it forms a Fabry-Perot-like cavity with the upper metallic layer, in which the light with different spin sates could experience constructive or destructive interference, thus enhancing the spin-selective absorption. However, this type of structure suffers some intrinsic disadvantages. For example, it only allows unidirectional light absorption and the absorption spectrum is usually narrow band due to the resonant nature. Meanwhile, once the chiral metasurfaces are fabricated, their chiroptical responses are fixed. The proposed metasurfaces can only absorb light with one specified CPL state. It still remains a great challenge to realize the conversion of spin-selective absorption for opposite CPL. So far, no practical scheme has been proposed to address this grand challenge.

To achieve a reconfigurable chiral metasurface, one possible approach is to integrate active materials with a passive chiral metasurface. Among various active materials, liquid crystals (LCs) are the most widely used materials for tunable optoelectronic devices spanning from displays to modulators [25]. LCs are a wonderful material exhibiting intermediate phase between crystalline phase and liquid amorphous phase, so they can flow like fluid yet possess the physical properties of crystals. As a gift of nature, LCs show unique properties such as large birefringence, extremely broad spectral range from near UV to microwave regime, excellent biocompatibility due to their organic nature and versatile driving methods. These distinctive features make LCs an excellent candidate to achieve tunable chiral metasurfaces. In our previous studies, we demonstrated LC-loaded nanostructures for tunable photonic devices such as switchers and color filters [26–30]. However, the potential of LCs is far from being fully exploited.

In this work, we demonstrate a planar backplane-free, LC-loaded chiral metasurface absorber that has reconfigurable multiband spin-selective light absorption in the visible to near infrared range. The single layered metasurface can support different plasmonic resonance modes for different CPL states, leading to strong chiroptical effects for both transmitted and reflected light. As a result, multiband spin-selective absorption can be achieved. We show that the far field chiroptical response originates from the near field enhanced optical chirality. Furthermore, the spin-selective absorption could be actively modulated by the LC layer. The LCs simultaneously affect the spin state of CPL and the plasmonic resonance modes. We numerically demonstrate the spin-selective absorption bands could be flipped from one spin state to the other opposite state by electrically changing the alignment of LC molecules. Our proposed design may pave a new way for manipulating CPL and being exploited in exotic devices for imaging or sensing applications.

2. Structure and theoretical model

For both natural and artificial chiral media, the chiroptical effects originate from the cross coupling between electric and magnetic fields. To obtain large chirality, our proposed metasurface is composed of L-shaped silver nanoantenna with two orthogonal arms, as illustrated in Fig. 1. Such a structure breaks the in-plane mirror symmetry and supports different resonant modes [24]. We choose silver instead of gold for the fabricating material of the metasurface since silver has a much higher plasmon frequency [31]. The metasurface structures are situated on a conductive quartz substrate that is coated a 100 nm thick indium tin oxide (ITO) layer. A LC cell is then assembled by integrating the metasurface-situated conductive substrate with another quartz substrate that is subsequently coated an ITO layer and a LC alignment layer (i.e., a mechanically rubbed polyvinyl alcohol (PVA) layer with assumed thickness of 200 nm). A nematic LC is then infiltrated into the cell to have a preferred alignment. Upon applying a bias voltage, the LC molecules can then be re-orientated along the electric field direction. The angle θ between the LC director and z-axis is therefore can be tuned, as shown in Fig. 1(b). Figure 1(c) shows a unit cell of the designed metasurface, which has a period of p = 330 nm. The two arms have the same length of l = 250 nm and thickness of t = 150 nm but different widths of w_x = 150 nm and w_y = 40 nm, respectively.

 

Fig. 1 (a) Schematic of the LC-loaded metasurface. Incident light propagates along –z-axis through the superstrate and LC layer. The inset shows the respective alignment of LC molecules at “OFF” and “ON” states. (b) The tilted angle θ between z-axis and the director of the LC molecule. The dash line represents the director of LC molecules. (c) The unit cell of the metasurface with the detailed structural parameters.

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To investigate the optical response and physical mechanism of the metasurface, full wave electromagnetic simulation was carried out using the finite-difference-time-domain method (Lumerical FDTD Solutions). The single unit metasurface structure was simulated with periodic (x- and y-directions) and perfectly matched layer (PML) (z-direction) boundary conditions. A maximum mesh size of 5 nm was set for simulation to ensure the calculation accuracy. The dispersion curves of Ag and ITO were chosen from Palik [32] and Konig [33] respectively. The refractive index of the quartz substrate is 1.46. The nematic LC E7 was considered in our work, whose wavelength-dependent ordinary and extraordinary refractive indices can be well described by the extended Cauchy model. The detailed description can be found in a previous report [34]. Considering the alignment of LC molecules depicted in Fig. 1(b), the effective permittivity tensor for LC reads [35]:

The transmission and reflection complex coefficients of CPL are obtained from the superposition of transmitted or reflected linearly polarized light. Their relationship can be well described by the Jones matrix under circular coordinate system [36]:

3. Results and discussion

From the above equation, it’s easy to obtain that the total transmittance of RCP and LCP light is$T_R\equiv |t_{++}{|}^2+|t_{-+}{|}^2$ and$T_{\text{L}}\equiv |t_{\text{--}}{|}^2+|t_{\text{+-}}{|}^2$. And the total reflectance of RCP and LCP light is$R_R\equiv |r_{++}{|}^2+|r_{-+}{|}^2$ and$R_L\equiv |r_{--}{|}^2+|r_{+-}{|}^2$. Consequently, the absorption for RCP and LCP light can be obtained through energy conservation and written as$A_R=1-T_R-R_R$and$A_L=1-T_L-R_L$, respectively. Meanwhile, CD is defined by$CD={\tan }^{-1}[(A_L-A_R)/(A_L+A_R)]$. We begin with exploring the optical characteristics of the metasurface when the alignment of LC molecules is perpendicular to the metasurface under a certain bias voltage. For this case, the effective permittivity for LC is:

Figure 2(a) depicts the transmittance spectra for LCP and RCP light within the wavelength range from 600 to 950 nm. We can clearly see a giant difference in transmittance for the two opposite CPL states, indicating significant chirality. The overall transmittance of RCP light is much larger than that of LCP case. Moreover, for LCP light, two distinct dips with near zero transmittance are observed at 739 nm and 845 nm, showing that the incident LCP light is almost completely blocked. As for reflectance spectra shown in Fig. 2(b), the picture is quite different. There are also three different dips for LCP light from 600 to 900 nm, but their locations are not corresponding to those in transmittance spectra, which are at 674 nm, 755 nm and 835 nm. This large and diverse chiroptical effects for both transmitted and reflected light are attributed to the excitation of different plasmonic resonance modes and absorption efficiency in the metasurface under opposite CPL states, which will be discussed in more details in the following context. The corresponding absorption spectra are shown in Fig. 2(c). Interestingly, three pronounced spin-selective absorption bands occur at 675 nm, 747 nm and 838 nm with absorption intensity around 0.65 for incident LCP light. In contrast, the intensity of the corresponding absorption peaks for incident RCP light is only about 0.2. Figure 2(d) shows the CD spectrum with the three corresponding resonant peaks reaching about 25°.

 

Fig. 2 Simulated far-field spectra of the LC-loaded metasurface with a bias voltage that makes the LC molecules homeotropically aligned. (a) Transmittance, (b) reflectance, (c) absorption and (d) CD spectrum for RCP and LCP at normal incidence. In (a)-(c), the red and blue lines represent for the LCP and RCP light, respectively.

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The significant far-field chiroptical effects for both transmitted and reflected light are a consequence of near-field chiral enhancement of the metasurface. To quantitatively evaluate the enhanced optical chirality, the optical chirality can be defined as [17, 37]:

 

Fig. 3 Enhanced optical chirality at the vicinity of the metasurface for the three absorption peaks. The left and right panels show the superchiral fields for LCP and RCP light at the three absorption peaks, respectively.

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The spin-dependent light absorption and the superchiral fields arise from the excitations of different plasmonic resonance modes in the metasurface, which can be attributed to the surface plasmon resonance induced by charge oscillation and accumulation on the metal-dielectric interface. To clearly reveal the physical origins, we calculate the induced charge distribution at different resonances on the metallic interface. Figure 4 shows the corresponding charge distributions of Fig. 3 at one slice of z = 80 nm. It’s not surprising that we can see huge difference about the charge distributions excited by the LCP and RCP light. For example, in Fig. 4(a) and 4(b), at the same wavelength of 631 nm, the LCP-excited charges mainly accumulate at the upper right corner while the RCP-excited ones accumulate at the lower right corner. This observation indicates that the resonant modes are highly dependent on the CPL state. This also holds true for the other two modes. Meanwhile, it’s verified that the induced charges tend to highly concentrate at the corners of the metallic structure, explaining why the superchiral field always occurs at the corners. Another interesting phenomenon can be also observed in Fig. 4. For LCP incident light, there are three pairs of the positive-negative charge distribution areas in all the three resonance modes, while for RCP incident light, there are only two pairs. The induced current in the metallic structure is much stronger with more positive-negative charge distribution areas. And it’s known that the light absorption intensity is primarily attributed to the ohmic dissipation of the induced current. This is the reason for different absorption efficiency of the incident light with opposite spin states and distinct CD as well.

 

Fig. 4 The induced charge distributions at the three absorption peaks with the left and right panels for the LCP and RCP light, respectively. For better comparation, the intensity is normalized by the maximum value of the LCP light induced charge at each absorption peak.

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A distinct advantage for our proposed device is that the LC molecules can be reoriented by an external bias voltage, making the device reconfigurable. In the above discussion, the LC molecules are homeotropically aligned, i.e., perpendicular to the metasurface. This can be usually achieved by applying an external electric field. Once the electric field is removed, the LC molecules will return back to the prealigned state, which is usually parallel to metasurface, hence leading to the change of the effective refractive index of the LC layer. The LC layer here has two major effects. On one hand, as known, the plasmonic resonant wavelength is highly sensitive to the surrounding dielectric medium. The change of the refractive index of the LCs will therefore affect the plasmonic resonances and the absorption performance of metasurfaces subsequently. On the other hand, as the orientation of the LC molecules is changed, the refractive index in the x-y plane becomes inhomogeneous. In such a case, the light with polarization along x- and y-axis will experience different refractive indices of the LCs and hence propagate at different velocities. As the CPL can be decomposed of two orthogonal linearly polarized light, the polarization of CPL is changed with the light propagation through the LC layer accordingly. The phase retardation $\Delta \varphi$ is proportional to the thickness of the LC cell h, and written as$\Delta \varphi \text{=}2\pi h(n_e-n{}_o)/\lambda$, where λ is the light wavelength. When$\Delta \varphi \text{=}\pi$, the spin state of the incoming CPL becomes the totally opposite, leading to the complete conversion of the spin-selective absorption. As a result, the combined two effects of the LC layer provide an efficient way to actively manipulate the spin-selective absorption.

To verify the above reconfigurable mechanism, we further investigate the absorption of the incident LCP and RCP light as a function of the tilting angle of the LC director. The thickness of the cell is set as$h=\lambda /[2(n_e-n_o)]$. Take the resonant wavelength λ = 838 nm as an example, and the corresponding$h\approx 1900$nm. Figure 5(a) shows the absorption performance of the LCP light with the change of the tilted angle of the LC director. The 90° and 0° tilted angles correspond to the “OFF” state with no applied bias voltage and the “ON” state with the switching bias voltage, respectively. As θ increases, the three absorption peaks for the LCP light gradually disappear. In contrast, three absorption peaks for the RCP light appear when θ is larger than 50°, as shown in Fig. 5(b). Interestingly, the spin-selective absorption for the three resonances is completely flipped when the LC layer is switched from the “ON” state to the “OFF” state or vice versa. The resulted CD spectrum with the change of the tilted angle is also depicted in Fig. 5(c). Obviously, the sign of CD is also flipped following the similar behaviour. Meanwhile, there exists a white-color gap of CD = 0 in the CD contour map, manifesting that the chirality of our proposed device could fully vanish by controlling the tilted angle. Figure 5(d) further depicts the absorption spectra for the incident light with two opposite spin states at “ON” state (dash line) and “OFF” state (solid line), respectively. The spin-selective absorption performance is switchable by controlling the bias voltage.

 

Fig. 5 The light absorption bands changing with the tilted angle of the LC director under the incident (a) LCP and (b) RCP light. (c) The corresponding CD spectrum with the changing tilted angle. (d) The absorption spectra at “ON” and “OFF” states. The red and blue lines represent for the LCP and RCP light, respectively. The dash and solid lines represent for the “ON” and “OFF” states, respectively. (e)-(f) The polarization ellipses of the resonant wavelengths λ = 675 nm (e), λ = 747 nm (f) and λ = 838 nm (g) for RCP incident light at “ON” (red line) and “OFF” state (blue line). For λ = 675 nm the major angle is flipped from −0.94° (“ON” state) to −17.55° (“OFF” state). For λ = 747 nm the major angle is flipped from 40.52° (“ON” state) to −68.91° (“OFF” state) and for λ = 838 nm the major angle is flipped from −67.79° (“ON” state) to 60.45°. The polarization states for all the resonant wavelengths change from elliptically clockwise to elliptically counterclockwise when the bias voltage turns from the “ON” state to the “OFF” state.

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Figures 5(e)-5(g) show the polarization ellipse of the incident RCP light at the wavelengths of the three absorption peaks for both “ON” and “OFF” states inside the quartz substrate beneath the metasurface. The red line represents the incident RCP light at “ON” state and the blue one represents that at “OFF” state. It’s clearly seen that the spin state still remains RCP at the “ON” state. Once the bias voltage is removed, the spin state reverses to LCP. In the above analysis, strong plasmonic resonances can be excited at the designed metasurface under LCP light illumination, which further result in the pronounced LCP absorption. This explains why the spin-selective absorption behavior can be switchable/reconfigurable by externally controlling the LC layer. In addition, it’s seen from Fig. 5(a) that the resonance wavelengths show very small redshift as θ increases, indicating that compared to the spin-changing effect of the LC layer, the effect of the change of the LC’s effective refractive index on the plasmonic resonance is nearly ignorable. This could be mainly attributed to the highly-localized confinement of plasmonic resonances at the interface of metasurface, leading to weak interaction with a very thin LC layer. It’s worth mentioning that, in order to accumulate sufficient phase retardation, the LC cell should be thick enough, which may result in lower switching frequency and higher driven voltage. Nevertheless, the thickness of the LC cell could be further decreased by choosing those LCs with a larger birefringence. For example, the commercial LC, Hi-Bi LC (LCM1107, LC Matter Corp.) with n_e = 1.97 and n_o = 1.55, shows the birefringence of as large as 0.42 in the visible light wavelength range.

4. Conclusion

In conclusion, we have demonstrated a LC-loaded metasurface with the capability to reconfigure spin-selective multiband light absorption from the visible to NIR range. The difference in absorption efficiency reaches 40% between the two opposite spin states. This high-contrast, spin-selective absorption originates form the different plasmonic resonance modes excited under different CPL states. Our proposed LC-loaded metasurface absorbers could open a door for some exciting photonic applications. The backplane-free structure allows bidirectional light absorption, showing great potential for spatial light modulation. The reconfigurable spin-selective light absorption shows tremendous potential in CD spectroscopy, pharmacy and enantiomer sorting. The designed metasurface can also work as pixels for detecting or differentiating CPL, which would benefit the chirality-induced holographic imaging, bio-sensing or chirality detecting.