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Abstract
A reconfigurable metamaterial for chirality switching and selective intensity modulation is demonstrated experimentally. Through simple folding strategy, nonchiral state, single-band chiral states and dual-bands chiral states can be switched. Circular dichroism up to 0.94 is measured with folding angles of 70°. Meanwhile, selective intensity modulation is realized by the combined effect of folding angle and incident angle. The transmission intensity of circularly polarized waves can be modulated by more than 90% at any selected resonating frequency between 8.97 and 10.73 GHz. This work will benefit the researches of foldable metamaterials and have potential applications in the field of reconfigurable devices.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the high degree of freedom and customization, metamaterials have attracted extensive attention in the past decade. Plenty of exotic properties which are not found in natural materials are proposed, such as negative refractive index [1], perfect absorption [2–4], EIT-like effects [5], generation of multiple vortex beams [6], etc. Having special response to circularly polarized waves, chiral metamaterials possess interesting properties, like giant optical activity, circular dichroism (CD), etc. [7,8]. Those remarkable properties make chiral metamaterials have potential in medicine detecting, bio-sensing, circular polarizers, etc. [9–11]. However, fixed chiral responses cannot provide tunable and versatile platforms for practical applications.
Reconfigurable chiral metamaterials have attracted more attention in recent years. In order to control the circularly polarized waves dynamically, various control methods have been studied. For instance, micro electro mechanical systems [12], phase-change materials [13], incident angles [14–16], etc. Folded metamaterial is also a promising direction to realize tunable chirality [17–19]. The process of fabricating 2D structures is simpler than 3D structures’, and much higher chirality can be obtained by folding 2D structures into 3D structures. However, previous solutions of foldable chiral metamaterials usually concentrate on changing the relative inclination of adjacent resonators in both orthogonal directions, whose deformation logic partly leads to some inconvenience in the manufacturing processes and applications.
One of the most common ways in daily life to fold up a piece of paper is periodically folding it back and forth in one direction, for instance, folding fans and traditional folding screens. Inspired by this folding strategy, a foldable metamaterial for switchable chirality (FMSC) is designed and demonstrated. Compared with previous studies, the manufacturing and control process is greatly simplified by this strategy. Meanwhile, strong CD responses are obtained up to 0.98 in simulation and 0.94 in experiment. Also, considerable modulation effects of transmission intensity (from 0.08 to 0.90), which is based on incident-angle sensitivity, are demonstrated experimentally, and the modulated frequency can be adjusted by folding angles. In addition, shrinkable portability, and lightweight are realized simultaneously in this kind of chiral metamaterials. It is convincing that this work will benefit the researches of foldable metamaterials and have potential in the field of multifunctional chiral devices.
2. Design, simulated and experimental method
The FMSC is designed and optimized by using CST Microwave Studio. The unit of proposed FMSC consists of two asymmetrical L-shaped copper patterns on flexible polyimide film. Plane state of the unit with geometric parameters is illustrated in Fig. 1(a), and will expand in x and y direction periodically. The line width of the copper pattern is w and the interval of two patterns is i. The folding angles that every unit folds along the vertical centerline are θ, as that shown in Fig. 1(b). The thickness of the copper pattern and the polyimide film is 0.035 mm and 0.045 mm. In order to facilitate the experiment, some geometric parameters are optimized as below: l1 = 11 mm; l2 = 6 mm; i = 28 mm; py = 11 mm; w = 1 mm. The copper is set as lossy metal with conductivity of 5.8 × 107 s/m. The permittivity of the polyimide film is 3.5 with loss tangent of 0.0027 [20]. The period in x direction, which is px, will be changed with the folding angles and can be calculated as ${p_x} = {p_{x0}}\cdot \textrm{cos}\mathrm{\theta }$. The px0 is the horizontal period of plane state which is 38 mm.
Fig. 1. (a) Single unit of metamaterial with geometric parameters. (b) Folding strategy for switching. (c) Graphical representation of three states at 8.10 GHz with folding angle of 0°, 50°, and -50°.
The strategy to switch chirality is shown in Fig. 1(b). Folded states with folding angles from 0° to 90° and 0° to -90° are defined as Mode 1 and Mode 2 respectively. As shown in Fig. 1(c), there is no asymmetric transmission when the FMSC is at plane state. After the FMSC is folded to 50°, left-hand circularly polarized (LCP) waves at 8.10 GHz can transmit through the sample, while right-hand circularly polarized (RCP) waves cannot. In contrast, when the FMSC is folded to -50°, the LCP waves cannot transmit through while the RCP waves can.
The samples are fabricated through the printed circuit board (PCB) process. The photo of experiment setup is shown in Fig. 2. Some zigzag strips are printed by 3D printing method and placed on the upper and lower sides of the sample to control the folding angle. Meanwhile, folded cardboard with the same folding period is placed under the sample. The proposed devices are tested with the vector network analyzer (N5227A, Keysight Technologies) S-parameter measurement facilities as shown in Fig. 2. During the experiments, the LCP and RCP electromagnetic waves are achieved by two Dual Circular Polarization Horn Antennas (LB-SJ-60180-P03, A-INFO Inc.).
Fig. 2. Photographs of fabricated samples and experiment setup; transmission spectra; and CD spectra; (a) and (e) Plane State; (b) and (f) Mode 1; (c) and (g) Mode 2; (d) and (h) CD spectra of three folding angles.
3. Simulated and experimental results and discussions
Simulated and experimental results of the FMSC are illustrated in Fig. 2. The photographs of fabricated samples and experiment setup are shown at the top of the figure. Here, tLCP and tRCP represent the transmission coefficient of LCP and RCP waves. When the FMSC is in plane state, as shown in Figs. 2(a) and 2(e), the transmission spectra of RCP waves almost coincide with LCP waves. It can be inferred that there is no intrinsic chirality in the unfolded structure. With folding angles of 50°, as shown in Figs. 2(b) and 2(f), the FMSC is more transmittable for LCP waves at low frequencies. At high frequencies, RCP waves can transmit through a little more than LCP waves. With the folding angle of -50°, as shown in Figs. 2(c) and 2(g), the transmission spectra are opposite to those in Figs. 2(b) and 2(f). At low frequencies, LCP waves are blocked and RCP waves can transmit through. These spectra of FMSC prove that the transmission of dual-band circularly polarized waves can be switched by folding method.
In order to investigate the chirality of the metamaterial intuitively, circular dichroism (CD) is used to show the discrepancy of transmission between RCP waves and LCP waves. Here, the CD spectra are calculated by $\textrm{CD} = |{{t_{RCP}}{|^2} - } |{t_{LCP}}{|^2}$ [13,17,18]. As shown in Figs. 2(d) and 2(h), the CD spectra of the unfolded state approximate to the zero axis. High CD responses at two frequency bands can be found in folded states. However, experimental deviations lead to irregular vibrations and numerical differences in the experimental results. For instance, unexpected bends of flexible polyimide film; folding angles do not reach the designed angles precisely; and circularly polarized waves in the experiment are not perfect standard plane waves. Among them, the main factor should be the imperfect control of folding angles, which is caused by the relaxation of polyimide substrate. Although there are some slight deviations between the experiment and simulation, it does not affect the verification of the designed functions in FMSC.
To further discuss the relationship between CD spectra and folding angles. The CD spectra of FMSC with folding angles from -80° to 80° are simulated. As shown in Fig. 3(a), the upper region has opposite CD responses to the lower region. Strong CD responses are excited in low frequencies firstly, and gradually blueshift with the increase of folding angles. Therefore, when folding angles are less than 30°, the SCFM behaves as a single-band chiral device. When the FMSC is folded between 30° and 60°, relatively weak CD responses appear at high frequencies. When the folding angles are larger than 60°, the CD responses in high frequencies disappear and CD responses in low frequencies become higher and redshift gradually. As shown in Fig. 3(b), with the folding angles of 70°, the CD response at 8.12 GHz can reach 0.98 in simulation and is measured as high as 0.94 at 8.32 GHz.
Fig. 3. (a) CD spectrum with folding angles from -80° to 80° and frequency from 6 to 18 GHz. Some angles mentioned in this paper are marked with dotted lines. (b) Simulation and experimental CD spectra when the folding angle is 70°.
So far, chirality switching and strong CD responses are realized by periodic folding in x direction. In the y-z plane, a wide range of transmission intensity modulation is realized by changing incident angle (φ). When the folding angles are 50°, as shown in Figs. 4(a) and 4(b), the transmission of LCP waves at 9.92 GHz have considerable changes in intensity (from 0.03 to 0.98), and RCP waves have a little response. As shown in Figs. 4(c) and 4(d), when the FMSC is folded to -50°, the modulation effect is switched to RCP waves. The transmission of RCP waves can be modulated from 0.01 to 0.97, and LCP waves have a little response.
Fig. 4. Transmission spectra with oblique incident angle (φ) in the y-z plane with graphical representation marked on the top of the figure. (a)-(b) Transmission spectra of Mode 1. (c)-(d) Transmission spectra of Mode 2. (e)-(h) Measured transmission spectra of (a)-(d), respectively.
As shown in Figs. 4(e)–4(h), the transmission spectra of various oblique incident angles are measured. Strong modulation effects (from 0.18 to 0.90) are observed on LCP waves when the folding angles are 50°. When the folding angles are -50°, the transmissions of RCP waves from 0.08 to 0.90 are measured. These experimental results prove that the FMSC can selectively modulate the transmission intensity of circularly polarized waves.
As shown in Fig. 5, the folding angles can also be used to change the modulated resonating frequency. When the incidences are LCP waves, the modulated frequency can be adjusted from 8.97 GHz to 10.73 GHz by tuning the folding angles. The modulation range (maximum difference in modulation intensity) will always maintain above 0.9. Due to the properties of the FMSC, the same frequency modulation effect can be obtained in RCP waves when the folding angles are set between -30° and -60°. This phenomenon can be explained as follows: the equivalent length of the L-shaped resonators on the incident plane becomes smaller with the increase of folding angles, and leads to blueshift of the resonance frequency. Therefore, the FMSC can also serve as a frequency-adjustable circularly polarized wave modulator.
Fig. 5. Simulated modulated frequencies with different folding angles; the modulation ranges are marked on the top of cuboids.
To reveal the underlying physical mechanism in the FMSC, dipolar analysis is used to discuss. From Rosenfeld criterion [21,22], intrinsic optical chirality in natural media can be interpreted as a cross-coupling effect between electric and magnetic fields, and it can be written as the equation $\sum \vec{P}\cdot \vec{M} \ne 0$ where the $\vec{P}$ is electric dipole and the $\vec{M}$ is magnetic dipole. When the combination of electric dipoles and magnetic dipoles are parallel or antiparallel, strong chiral response can be induced.
The current distributions of plane state and folded state (50°, -50°) at the resonant frequencies are studied. As shown in Fig. 6(a), when the FMSC is illuminated by LCP or RCP wave, due to the characteristics of the plane, the surface current distribution always has in-plane electric dipoles, and the magnetic dipoles are always perpendicular to the plane. So, the equation: $\sum \vec{P}\cdot \vec{M} = 0$ can be deduced. From Rosenfeld criterion, although the FMSC at plane state also has geometric chirality, it is not possessed of chiroptical response.
Fig. 6. Surface current distributions of Plane State (a), Mode 1 (b)-(c), and Mode 2 (d)-(e) with folding angle of 0, 50°, and -50° respectively. Red arrows (electric dipoles) along the direction of the current flow and blue arrows (magnetic dipoles) are drawn with the calculated effective value in the middle.
When the FMSC is switched to folded states, the surface current distributions are illustrated in Figs. 6(b)–6(e). It can be found that the angles between effective electric dipole and effective magnetic dipole (which are illustrated in the middle of the figures) is no longer vertical ($\sum \vec{P}\cdot \vec{M} \ne 0$). Due to the cross-coupling effect, the RCP wave at 8.02 GHz cannot pass through the metamaterial in Mode 1. In comparison, the LCP wave at 8.02 GHz is less affected by the FMSC in Mode 1 and can pass through the metamaterial. So, CD responses are achieved from them. Therefore, an effective way to switching the chiroptical response is switching metamaterials between 2D and 3D. Meanwhile, the difference in CD responses’ intensity between 8.02 GHz and 9.92 GHz can be explained by the angle between effective electric dipole and effective magnetic dipole. The angles, which are formed by effective dipoles in Fig. 6(c) and Fig. 6(e), are not 0° or 180°. The angles in Fig. 6(a) and Fig. 6(c) are nearly 0° and 180°. As a result, higher chiral responses will be induced in the combinations which are closer to parallel or anti-parallel state.
Meanwhile, Mode 1 and Mode 2 are mirror-symmetrical with respect to the initial plane and cannot completely overlap each other. According to [22], Mode 1 and Mode 2 should be geometrically chiral and enantiomers to each other, and they will have opposite chiroptical responses as shown in Fig. 2.
To explain the advantages of FMSC, a comparison table is established as shown in Table 1. The number of resonators per unit and the number of creases in every 3*3 units are studied. The number of resonators per unit relate to resonators that need to be tilted. The number of creases relate to the number of folding angles that need to be controlled in foldable metamaterials. Therefore, fewer resonators and fewer creases will facilitate the design of more flexible reconfigure strategies and have more potential to realize reconfigurable folded metamaterial devices, especially in the optical range. As shown in the table, the FMSC has fewer resonators than [17] and [18]. As a result, the FMSC reduced the number of folding angles by 7 to 12 times than previous studies. In addition, it has fewer number of creases angles than the folded metamaterials devices reported which are in nano scales [19,23]. So, it is convincing that this work will benefit to study reconfigurable chiral metamaterials in optical wavelength bands. Furthermore, the intensity modulation which is realized by the combined effect of the folding angle and incident angle expands the application field of the FMSC.
Table 1. Comparison of chiral metamaterials
4. Conclusion
In this paper, a reconfigurable metamaterial is proposed and experimental proved for chirality switching and selective intensity modulation. Nonchiral states and multimode chiral states can be switched with the foldable structure. Strong circular dichroism has been achieved up to 0.98 in simulation and 0.94 in experiment by switching metamaterial from 2D to 3D. Physical mechanism is studied with the dipole analysis method. Meanwhile, the selective intensity modulation of circularly polarized waves is demonstrated by changing incident angle and folding angle. In summary, it is convincing that this work is beneficial to the researches of reconfigurable metamaterials in the future and has potential applications in the field of bio-sensing and chiral devices.
Funding
National Natural Science Foundation of China (61205095); Funding Programs for Youth Teachers of Shanghai Colleges and Universities (SLG11006).
Disclosures
The authors declare no conflicts of interest.
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