1. Introduction

Terahertz (THz) wave, with a frequency range of 0.1 to 3.5 THz (wavelength ranges from 86 µm to 3 mm), coincides with millimeter wave at long wavelength and infrared light at short wavelength, which has excellent electronics and photonics properties. THz wave can be used to detect kinds of biological information because the THz wave contains the characteristic frequencies of most biological [1,2]. Moreover, THz waves are also suitable for imaging due to their excellent penetration and low attenuation to many materials [3]. With the development of THz technology, its great application prospects in many important fields such as radar [4], remote sensing [5], high-security data communication and transmission [6,7], atmospheric and environmental monitoring [8,9], real-time biological information extraction and medical diagnosis [10–12] have gradually emerged.

However, in contrast to the more advanced optical and microwave technologies, THz technology has not been well advanced [13]. The limitation of the development of THz technology is primarily a result of the lack of naturally occurring materials that support strong terahertz (THz) wave-matter interaction [14]. Therefore, metamaterial and metasurface have attracted the attention of related researchers in the field of THz technology with great flexibility to electromagnetic properties. Compared to metamaterial, metasurface are constructed by arranging sub-wavelength artificial structures into a planar array which inherits advantages in achieving intricate manipulations of THz wave and compensates deficiencies of metamaterial such as bulky size, difficult fabrication, and high cost. Due to the incomparable modulation ability of the metasurface, many intriguing applications have been realized in the THz band, such as THz modulator [15–17], near-zero reflection [18], perfect absorption [19,20], giant chirality [21,22] and so on.

Chirality is a phenomenon that the structure of an object cannot coincide with its mirror image by rotating or translating, which is ubiquitous in many fields such as optics, biology, chemistry, and life science [23–25]. Due to the different refractive index between left-handed circularly polarized light and right-handed circularly polarized light in the chiral structure, a circular dichroism response occurs when various circularly polarized waves illuminate the chiral material. Compared with traditional materials, chiral metasurface which enable to be obtained by breaking the in-plane symmetry are capable of achieving stronger optical activity. Many polarization devices which are derived from the chiral metasurface have been applied in CD spectroscopy [26–28], optical sensing [29–31], molecules detection [32], and other fields. However, most of the previously designed chiral metasurface are suffered from the narrow band and single function characteristics. Hence, there are some spin-selective THz chiral transmission metasurface that can support dual-band [33] are proposed in recent years, while it is still a challenge to fabricate chiral metamirror with dual-band spin-selective reflections. When noncircular polarization incident wave impinges onto such metamirror, waves with different spin states will undergo different extinction, then the metamirror could reflect only one kind of circularly polarized wave. The chiral metamirror have many applications in the THz band, such as polarimetric imaging, molecular spectroscopy, ultracompact circularly polarized light detector, and polarization-sensitive detection of electromagnetic wave.

In this work, we have demonstrated a dual-band flexible spin-selective flips THz metamirror which has the ability to realize opposite spin selection reflection in two different frequency bands. The design of this metamirror is based on a typical chiral structure, which consists of two asymmetric C-shaped structures. Therefore, the dual-band chirality is achieved by combining these two similar typical chiral structures [34–38]. By comparing the experimental results with the simulated results, the proposed metamirror has two opposite huge circular dichroism (CD) at two different frequency bands, that the absolute values of them are both great. It means that the chiral metamirror realizes the dual-band spin-selective reflection which demonstrates the chirality flip response of the metamirror. Furthermore, we can adjust the lengths and heights of parts of the structure to modulate the value of CD and change the frequency at that CD response occurred. The work in this paper realizes a dual-band manipulation of spin light, and it may contribute to the applications in spin selection, optical sensing, and chiral molecules detection.

2. Design and principle

The schematics of the proposed flexible chirality flip dual-band THz metamirror are shown in Fig. 1(a), which demonstrates a phenomenon absorbing left-handed circularly polarized (LCP) wave while reflecting right-handed circularly polarized (RCP) wave at frequency band ${{f}_1}$. Meanwhile, the absorption and reflection of LCP and RCP at the frequency band are opposite with ${{f}_1}$ when circularly polarized waves illuminate the metamirror in the same way. It means that the flexible chirality flip dual-band THz metamirror has strong circular dichroism (CD) and both realized a chirality flip at frequency bands ${{f}_1}$ and ${{f}_2}$.

 

Fig. 1. Schematics of the proposed flexible chirality flip dual-band THz metamirror. (a) The chiral metamirror shows huge CDs in two bands. (b) The unit cell of the metamirror. (c) The top view of the unit cell.

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The design details of the unit of the metamirror are proposed in Fig. 1(b) and Fig. 1(c). The unit cell is the typical sandwich structure, the metallic pattern structure and the metallic grounded plane are separated by MPI dielectric substrate. MPI is a flexible material with a relative dielectric constant of 3.1 and a loss tangent of 0.006, which has stable dielectric properties, mechanical properties, good radiation resistance, and low cost. The geometrical parameter h = 91 µm and p = 300 µm are the thickness of the substrate and the length of the unit cells. Three C-shaped metallic structure is arranged on the dielectric substrate to achieve the spin-selective chirality flip response, which is a multi-chiral structure designed by combining two similar typical single chiral structures. For the three C-shaped structures, we name the left structure, the middle structure and the right structure as S-1, S-2, and S-3, individually. As Fig. 1(c) shows, the lengths and heights of the three C-shaped structures are a = 94 µm, b = 50 µm, c = 65 µm, l = 75 µm, r = 200 µm, m = 130 µm, n = 160 µm, respectively. The widths of the metallic line are w = 10 µm. The positions of S-1 and S-3 are symmetric about the center axis of the unit and the distance between S-1 and S-3 is d = 12 µm, while the distance between S-1 and S-2 is z = 5 µm.

In terms of electromagnetic response, the physical properties of chiral materials are characterized by cross coupling the electric and magnetic fields along the same direction. The electromagnetic waves satisfy the Rosenfeld criterion [39–44], which can be described as

To verify the chiral optical response of the proposed metamirror, we perform the reflection spectra of the metamirror sample in Fig. 2(a) and Fig. 2(b) which are simulated by electromagnetic simulation software. The unit cell of the investigated sample is simulated by using periodic boundary conditions in which the modeling of the unit cell structure is illustrated in Fig. 1(b) and Fig. 1(c). The dielectric layer material of the simulation model is MPI as mentioned above, and the material of metal layer is aluminum. Figure 2(a) shows the reflection spectra of the metamirror under circular polarized (CP) wave. The ${{r}_{RR}}$ and ${{r}_{LL}}$ denote the amplitude ratio between the RCP (LCP) wave reflected by the metamirror and the RCP (LCP) incident wave. And the ${{r}_{LR}}$ and ${r_{RL}}$ denote the amplitude ratio between the reflected LCP (RCP) wave and the RCP (LCP) incident wave, respectively. The ${{r}_{LR}}$ and ${r_{RL}}$ exhibit similar cross-polarization reflection coefficients in the operating frequency range, while the difference of ${{r}_{LL}}$ and ${{r}_{RR}}$ exhibit the huge chiral response at two frequency bands. At 0.430 THz, the amplitude of ${{r}_{LL}}$ is much lower than ${{r}_{RR}}$, on the contrary, the amplitude of ${{r}_{RR}}$ is much lower than ${{r}_{LL}}$ at 0.578 THz. It means selective reflections of RCP and LCP waves are realized at 0.430 THz and 0.578 THz, respectively. In order to intuitively compare the experimental data with the simulated data, we have displayed the reflection spectra of the structure under linear incident wave in Fig. 2(b). We define ${{r}_{xx}}$ and ${{r}_{yy}}$ as the reflection of the co-polarization, ${{r}_{xy}}$ and ${{r}_{yx}}$ as the reflection of the cross-polarization, where the first subscript represents the polarization of the reflected wave. It is easy to see that ${{r}_{xx}}$ and ${{r}_{yy}}$ have different reflectivity among the working frequency range, in particular at 0.430 and 0.578 THz, while ${{r}_{xy}}$ and ${{r}_{yx}}$ are same throughout the operating frequency band. It is worth mentioning that the differences in the co-polarization ${{r}_{xx}}$ and ${{r}_{yy}}$ are similar to the ${{r}_{LL}}$ and ${{r}_{RR}}$ at 0.430 THz and 0.578 THz. The simulation results of linear polarized incident wave are in good agreement with those of circularly polarized incident wave which further proves the spin-selective reflection of the structure. Figure 2(c) provides the calculated CD of the metamirror under the CP waves to characterize the chirality, where $CD = |{r_{LL}}|- |{r_{RR}}|$. The value of the huge CD achieved -0.65 and 0.50 at 0.430 THz and 0.578 THz, respectively, which is corresponded to the amplitude differences of ${{r}_{LL}}$ and ${{r}_{RR}}$ in Fig. 2(a). What is noteworthy is that “-” denotes the flip of chirality. The simulation circular dichroism (CD) observed in the proposed metamirror are comparable with those previously reported in studies [45,46].

 

Fig. 2. The simulated reflection spectra, circular dichroism (CD) spectra and circular polarization selective efficiency (CPSE) spectra. (a) The simulated reflection of the sample under the circularly polarized wave. (b) The simulated reflection of the sample under the linear polarized wave. (c) The circular dichroism (CD) spectra of the sample under the circularly polarized wave. (d) The circular polarization selective efficiency (CPSE) of the metamirror.

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In order to clearly show the selectivity of the proposed metamirror to the CP waves, the circular polarization selection efficiency (CPSE) [47] is described as

The CPSE of the metamirror is demonstrated in Fig. 2(d). It is obvious that the CPSE is corresponding to the CD response by comparing the Fig. 2(c) and Fig. 2(d). The CPSE has sharp peaks at 0.430 THz and 0.578 THz, which are at the peaks of the CD. The value of the CPSE can reach 0.95 and 0.80 at 0.430 THz and 0.578THz, respectively. Thus, the high value of the CPSE demonstrates the high selectivity of the proposed metamirror to the CP waves at two frequency bands, while the selectivity to the CP waves is significantly weaker than the designed two frequencies at other operating frequency ranges. Although the proposed metamirror works in the THz band, the CPSE simulation results can be compared with the CPSE of previously proposed metasurface in the GHz band [47].

To further reveal the principle of the dual-band intrinsic chirality of the metamirror, the surface currents at resonant frequencies are performed and compared the current distributions under different circular polarization incidences in Fig. 3. At the first resonant frequency 0.430 THz, a pair of currents in opposite directions are excited between S-1 and S-2 with similar amplitudes due to the LCP incident waves illumination, as shown in Fig. 3(a). It is obvious that the current on the S-3 can be ignored compared to the S-1 and S-2. The pair of currents in opposite directions can be viewed as two excited electric dipoles that have a phase difference of π. The LCP wave can be highly absorbed because the radiated energies of the equivalent electric dipoles are offset with each other. As shown in Fig. 3(b), the surface current is different when RCP wave illuminates the structure at 0.430 THz. Under the RCP wave illumination, a pair of equivalent electric dipoles with the same oscillation are excited at S-2 and S-3, while the current on S-1 has opposite orientation with same amplitude flowing to the center of the S-1. The currents on the S-2 and S-3 are equivalent to a pair of excited equivalent electric dipoles having same oscillation, resulting in the high-efficiency reflection of RCP wave. Therefore, a strong chiral response happens at 0.430 THz. Two surface currents on S-1 with similar amplitude and opposite direction flow to both ends of S-1, while a pair of currents in the same direction between S-2 and S-3 can be viewed as an excited pair of equivalent electric dipoles with the same oscillation leading to the high reflection of LCP wave, illuminated by LCP at the second resonant frequency of 0.578 THz, as shown in Fig. 3(c). However, a pair of anti-parallel currents with similar amplitude is generated in the S-1 and S-2 under the RCP wave illuminates at 0.578 THz, while the current on S-3 can be ignored compared to S-1 and S-2, as shown in Fig. 3(d). The energy radiates by a pair of equivalent electric dipoles with opposite oscillation cancels with each other that means the RCP wave can be highly absorbed which contributes to the huge chiral response occurring at 0.578 THz.

 

Fig. 3. Simulated results of the surface current distributions. (a) LCP incident waves at 0.430 THz. (b) RCP incident waves at 0.430 THz. (c) LCP incident waves at 0.578 THz. (d) RCP incident waves at 0.578 THz.

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3. Results and discussions

The fabricated flexible chirality flip dual-band THz metamirror array consisting of 70 × 70 unit cells, whose size is 21 × 21 mm², are fabricated using standard UV lithography. A part of the whole array consisting of 7 × 5 unit cells is in Fig. 4(a), and an enlarged microscopy image of unit cell is also presented as the inset of (a). The size of the unit cell is same as the model simulated in the electromagnetic simulation software.

 

Fig. 4. Experimental methods and test results. (a) The optical micro-graph of part of the fabricated sample. An enlarged microscopy image of unit cell is also presented as the inset of (a). (b) The schematic diagram of measurement. (c) The measured reflection of the sample under the linear polarized wave. (d) The reflection of the sample under the circularly polarized wave obtained from the linear polarization measurements. (e) The measured circular dichroism (CD) spectra of the metamirror. (f) The measured circular polarization selective efficiency (CPSE) of the metamirror.

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Due to the lack of the quarter wave plates in the desired THz band, we cannot measure the circular reflection coefficients (${r_{RL}}$, ${r_{LR}}$, ${r_{LL}}$ and ${r_{RR}}$) directly. Thus, we measure four linear reflection coefficients (${r_{xx}}$, ${r_{yy}}$, ${r_{xy}}$ and ${r_{yx}}$) to exhibit the circular reflection coefficients by using Terahertz time-domain spectroscopy (THz-TDs), the schematic diagram of measurement is shown in Fig. 4 (b). In the experiments, the initial direction of the sample is shown in Fig. 4(a), and the initial direction of the incident polarization wave and the detected polarization wave are both along y-axis, the data of ${r_{yy}}$ can be obtained by testing in this way. The generation and detection of the polarization wave along x-axis are realized by rotating the sample 90 degrees, thus, the data of ${r_{xx}}$ can be obtained. Then, the coefficient ${r_{yx}}$ was obtained by rotating the detector of the THz-TDs 90 degrees and keeping the sample 90 degrees rotating, and the coefficient ${r_{xy}}$ was obtained by keeping the rotation of the detector 90 degrees while the sample was in initial position. Notably, other settings do not change when rotating the detector, so the received signal is orthogonal to the incident signal. Generally, the Jones matrix can be used to represent the reflection of the THz wave. In our experiment, the reflection coefficients of circularly polarized waves can be calculated by the linear reflection coefficients using following equation:

The “+” and “-” in Eq. (3) are defined as the RCP and LCP incident in + z direction, respectively. It is worth noting that the circularly co-polarization reflection coefficient is different from the transmission coefficient, because the reflected and transmitted wave vectors are in opposite directions. Thus, the parameter ${r_{ -{+} }} = {r_{RR}}$, ${r_{ +{-} }} = {r_{LL}}$, ${r_{ -{-} }} = {r_{RL}}$ and ${r_{ +{+} }} = {r_{LR}}$ [48].

Figure 4(c) illustrates the measured amplitude of the reflection coefficients of the four linear polarization components of the sample. All curves are normalized to co-polarization reflection coefficients measured with a sample without structure layer while the materials and parameters of dielectric layer and metallic grounded plane are the same as the actual metamirror. As Fig. 4(c) shows, the trend of spectra are in good agreement with the simulated linear polarization reflection. The ${r_{yx}}$ and ${r_{xy}}$ have same spectra throughout the operating frequency band, while ${r_{xx}}$ and ${r_{yy}}$ have huge differences at 0.333 THz and 0.550 THz. According to Eq. (3), we can obtain the circular polarization reflection coefficients from these four linear reflection coefficients, as shown in Fig. 4(d). It is quite clear that the ${r_{RL}}$ and ${r_{LR}}$ exhibit similar cross-polarization reflection spectra in the operating frequency range, and the two huge differences of ${r_{LL}}$ and ${r_{RR}}$ exhibits the chiral response at 0.333 THz and 0.550 THz. The differences of ${r_{LL}}$ and ${r_{RR}}$ mean that the selective reflections of RCP and LCP are achieved, respectively. Furthermore, we obtain the CD and CPSE spectra from the four circular polarization coefficients, as shown in Fig. 4(e) and Fig. 4(f). In Fig. 4(e), the values of the CD reach -0.22 and 0.31 at 0.333 THz and 0.550 THz, respectively, and the “-” still denotes the flip of the chirality. The circular dichroism (CD) observed in the proposed metamirror are comparable with those previously THz chiral metasurface reported in study [49]. Meanwhile, CPSE reached 0.55 and 0.62 at two frequencies, respectively. It means that the metamirror we proposed has high selectivity of the CP wave. Corresponding to the chirality responses at 0.430 THz and 0.578 THz in the simulation results, the fabricated sample has high chirality responses at 0.333 THz and 0.550 THz. There is a high reflection of RCP and a high absorption of LCP at 0.333 THz in experimental results, which is the same at 0.430 THz of the simulation spectrum. The comparison result shows that the chirality response at 0.333 THz has a large frequency shift compared to the simulation results. Moreover, there is a weak chirality response at 0.310 THz in the simulation results that the experimental results do not show in the designed frequency band. In the simulation results, the chiral response at 0.310 THz is high reflection of LCP and high absorption of RCP, which is inconsistent with that at 0.333 in the experiment. Considering the frequency shift of the chiral response at 0.333 THz, this chiral response may actually occur before 0.3 THz. Since the chirality response at 0.310 THz in the simulation results is much lower than that at 0.430 THz and 0.578 THz, it has no application value, the absence of it in the experimental results has no significant impact on the metamirror. The deviations between the experimental and the simulation likely arise from the fabrication imperfections and measurement imperfections. Although there are some differences between the experimental results and the simulation results, the function of the metamirror is basically consistent with the simulation results, indicating that our proposed metamirror obtains good selective reflection and chirality flip.

4. Conclusions

In summary, we have demonstrated a flexible chirality flip dual-band THz metamirror both theoretically and experimentally, which achieves the selective reflections of circularly polarized wave and realized the chirality flip at two THz frequency bands. The performances of the metamirror are characterized from the perspectives of CD and CPSE, the value of the CD achieved -0.22 and 0.31 at 0.333 THz and 0.550 THz, and the value of CPSE reached 0.55 and 0.62, respectively. The values of CD and CPSE proved the metamirror obtains effective chiral responds and selective reflective capability at these two frequency bands. From the analysis of surface current at resonant frequencies, we find that the properties can be modulated by adjusting the lengths and heights of part of the structure. Furthermore, due to the selection of MPI as the dielectric layer substrate, the metamirror also has good flexibility, which may contribute to the applications in THz devices such as THz flexible electronics and bio-sensors.