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Abstract
Chiral metasurfaces are widely used in imaging and biosensing due to their powerful light field control capabilities. Most of the work is devoted to achieving the goals of chirality enhancement and tunability, but lacks consideration of design complexity, loss, cost, and multi-band operation. In order to alleviate this situation, we propose a pair of dual-frequency giant chiral structures based on all-silicon, which can achieve excellent and opposite spin-selective transmission around 1.09 THz and 1.65 THz. The giant chirality derives from the in-plane electric and magnetic dipole moments excited in different degrees. Theoretically, the maximum circular dichroism at the two frequencies are both as high as 0.34, and the coverage bandwidths of the two giant chirality are 85.5 GHz and 41.4 GHz, respectively. The experimental results are in good agreement with the simulation results. Based on the dual-band giant chiral patterns, the terahertz near-field imaging of different Chinese character images is demonstrated at two frequencies. The frequency-undifferentiated characteristics, good intensity contrast and three-dimensional imaging information are shown by the results. This work provides new ideas for the design of terahertz devices with simple structure and multi-functions, which are expected to be applied in the field of terahertz imaging or multi-channel communication.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The original chirality refers to a special system that lacks mirror symmetry and reverses symmetry. Chiral substances with such asymmetric properties are widespread in nature, such as amino acids [1], DNA molecules [2], and drugs [3]. The chiral substance and its mirror image are collectively referred to as a couple of chiral isomers, and the form of each other is similar to the left and right hands of human beings. It is worth noting that circularly polarized (CP) light in the optical field perfectly conforms to the concept of chirality, and the polarization direction and rotation direction of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light both exhibit the characteristics of chirality. Utilizing the coupling effect between CP light and chiral substance, a unique chiral optical response arises at the historic moment [4,5]. The uniqueness of this optical response originates from the fact that chiral substances exhibit different real and imaginary of refractive indexes for two CP lights, such as optical rotation (OR) [6,7] and circular dichroism (CD) [8,9]. Among them, the CD plays an important role in distinguishing chiral isomers and judging the strength of chirality, so it is widely used in medical [10], catalysis [11,12], communication [13] and other fields [14,15].
However, since the inherent chirality carried by natural materials is very weak and difficult to detect, it is necessary to enhance the CD by some special means. As a two-dimensional (2D) array structure arranged periodically, the metasurface exhibits incomparable advantages [16,17]. It has enough design freedom to achieve this goal. At present, chiral metasurfaces have increased the response intensity of CD by several orders of magnitude [18–21]. For example, the CD metalenses based on spin-selective reflection metasurfaces and the compact photodetectors based on chiral plasma metasurfaces have been successively proposed [22,23]. In addition, it has also been developed in related practical applications such as holographic projection [24], chiral coding [25], near-field imaging [26–29], wavefront control [30,31] and chiral sensing detection [32–34]. In fact, most of the work of CD enhancement is based on the resonance difference between free electrons on the metal surface and incident photons of different CPs, without considering the design complexity, loss, and cost [35–37]. The emergence of dielectric metasurfaces has greatly alleviated this situation. Different from the inherent loss of metals, the dielectric metasurface utilizes its own low absorption and simple process characteristics to overcome the above challenges while improving the utilization and efficiency of incident electromagnetic waves, which means that the optical field can be manipulated more effectively. Li et al. achieved excellent spin-selective terahertz asymmetric transmission by using the dielectric metasurface, and proposed using infrared continuous-wave optical pumping to dynamically tune the amplitude of the asymmetric transmission [38]. However, most of the work about chiral dielectric metasurfaces is devoted to achieving the goals of chirality enhancement and tenability, while there are few reports on the multi-band chiral optical response in the terahertz region. In particular, it is a thorny issue to ensure that the chiral performance is not attenuated in multiple frequency bands.
In the article, a pair of dual-frequency giant chiral structures based on all-silicon are proposed. Through the ingenious multi-shape combination design and the all-silicon plasma etching preparation method, the excellent and opposite spin selective transmission for LCP and RCP waves are achieved near 1.09 THz and 1.65 THz. The reliability of the structural performance is increased by comparing the experimental results and simulation results of the CP transmission spectrum and the CD spectrum. In order to explore the physical mechanism of dual-frequency giant chirality, the in-plane electric and magnetic field distributions of the structure are calculated. The giant chirality at the two frequencies is caused by different in-plane dipole moments. Based on the designed chiral structure, the terahertz near-field imaging of different Chinese character images is demonstrated at 1.09 THz and 1.65 THz, respectively. The good intensity contrast and three-dimensional (3D) imaging information are showed by the results.
2. Results and discussions
2.1 Design and experiment of metasurface with dual-band giant chirality
In the process of exploring the interaction between terahertz waves and metasurfaces, it can be found that the Jones matrix method is a very good method, which can intuitively and effectively reflect the phase, intensity, polarization and propagation mode of the transmitted beam. Therefore, in order to facilitate the design of a metasurface with dual-band giant chirality, we use the Jones matrix method to simulate the propagation of terahertz waves on the metasurface. Considering the CP wave is incident vertically, the electric field of the transmitted wave can be expressed as:
In order to obtain a structural unit with dual-band giant chirality, it is necessary to break both mirror symmetry and n-order rotational symmetry (n > 2) according to the rules. Starting from simple anisotropic structures, a chiral structure can be formed by combining two different anisotropic structures. We expect to be able to obtain the maximum CD, so we need to destroy the mirror symmetry of the structure to the greatest extent. Specifically, the pattern 1 (P1) is formed by combining a symmetrical arc-shaped strip with a rectangular strip with a horizontal rotation angle of 45°, as shown in Fig. 2(a-b). The high-resistance silicon with simple preparation process and negligible loss is selected as the material of the structure. The dielectric constant is set to 11.9. After parameter optimization using the time domain solver in the commercial CST software, the geometric dimensions of the structure are determined as: P = 175 µm, t1 = 200 µm, t2 = 300 µm, w = 17 µm, R1 = 57.5 µm, w1 = 24 µm, θ = 45°. In order to verify our design ideas, the CD spectrums of each design process of P1 are calculated, as shown in Fig. 2(c-f). It can be found that the CD spectrum corresponding to a single anisotropic structure is always 0, while the chiral structure after the combination has two CD peaks with amplitudes exceeding 0.3 and opposite. Interestingly, when the width (w1) of the arc-shaped strip is 16, 20, or 24, the corresponding CD peak numbers are 0, 1, or 2, respectively. This means that the working frequency of the giant chirality can be freely switched between dual-frequency and single-frequency modes by adjusting the width (w1) of the rectangular strip.
Fig. 1. Functional and unit cell schematics of all-silicon metasurfaces with dual-frequency giant chirality. The length and color of the beam represent the intensity and polarization state, respectively. The LCP and RCP waves are marked in blue and red, respectively.
Fig. 2. (a-b) The design process of dual-frequency giant chiral metasurface. (c-e) The simulation results of the CD spectrum corresponding to each design process. (f) The simulation results of the CD spectrum corresponding to different line widths of rectangular strip.
Based on the geometric parameters and structure designed above, an inductively coupled plasma etching (ICPE) method is used to etch a 500 µm thick commercial silicon wafer to prepare the metasurfaces of P1 or pattern 2 (P2). The specific steps are as follows: the silicon wafer is firstly pre-treated, spin-coated and baked to form a uniform, defect-free photoresist film with good adhesion. Secondly, the alignment, exposure and development are carried out to ensure the precise pattern size on the photoresist. Then, an inductively coupled plasma etching machine is used to etch the silicon wafer, and P1 (or P2) is accurately transferred to the surface of the silicon wafer. Finally, the unnecessary photoresist is removed to finish the production of the sample. Figure 3(a1-a2) show scanning electron microscope (SEM) images of the all-silicon metasurfaces (sample size 1.4 cm × 1.4 cm). It can be seen that the P1 or P2 is arranged on the silicon wafer strictly and periodically. The optical path in Fig. 3(b) is used to actually measure the transmission coefficient of the sample. Considering that all the orthogonal polarization components before and after passing through the sample need to be measured, four linear polarizers (LPS) are used here to adjust the polarization state of the transmitted wave. The polarization directions of the polarizers D and B are fixed as the directions of transmitting and receiving terahertz waves, respectively. The purpose of directly measuring the linear polarization transmission coefficient is achieved by rotating the polarizers A and C.
Fig. 3. Simulation and experiment of the dual-frequency giant chiral metasurface. (a) The 2D SEM images and 3D physical photographs of our designed sample (P1 and P2). (b) Schematic of the transmission setup of a terahertz time-domain spectrometer (THz-TDS) that can control the polarization state. (c-e) The simulation results of the transmission spectrum and CD spectrum of P1 and P2 under the incidence of different CP waves. (f-h) The experimental results of the transmission spectrum and CD spectrum of P1 and P2 under the incidence of different CP waves.
Combining Eq. (2) and Eq. (3), both the CP transmission spectrum and the CD spectrum can be calculated. Figure 3 (c-e) show the simulation results of the CP transmission spectrum and CD spectrum of P1 and P2. It can be seen that the two structures have opposite CD peaks with an intensity of 0.34 near 1.09 THz and 1.65 THz. The bandwidths covered by the two giant CDs (absolute value exceeding 0.3) are 85.5 GHz and 41.4 GHz, respectively. The reason is that the main contribution of such great-circle dichroism comes from the difference in cross-polarization. For example, the normalized values of the co-polarization component at 1.65 THz are 0.3 and 0.1, respectively, and the cross-polarization values are 0.05 and 0.66, respectively. Obviously, the squared difference of the cross-polarized component is much larger than the squared difference of the co-polarized component. Note that the difference in the co-polarization component slightly weakens the final CD. However, the contribution of CD at around 1.09 THz becomes the sum of the squared difference of the cross-polarized component and the squared difference of the co-polarized component, which can be known by observing the co-polarization and cross-polarization components near this frequency. From the experimental results, the measured CP transmission spectrum and CD spectrum are very consistent with the simulation results, as shown in Fig. 3(f-h). It should be noted that the normalized value of the experimental results has a slight attenuation and overall frequency shift (including CP transmission spectrum and CD spectrum), which is attributed to errors in optical path testing and etching. In addition, according to the careful observation of the SEM, there are some errors in the geometric dimensions of the structure, but this does not affect the concept and function demonstration of the overall metasurface.
The study of the CP transmission spectrum alone is not enough to fully explain the spin selective transmission of the metasurface. Therefore, the time-domain signal and electric field distribution of the terahertz wave passing through the metasurface unit are monitored, as shown in Fig. 4(a-b). It can be found that the LCP and RCP transmission waves at 1.09 THz and 1.65 THz propagate regularly with changes in the time domain signal. And there is a significant difference in the intensity of the time-domain transmission signal and electric field distribution between the LCP and RCP transmission waves, which is highly consistent with the CP transmission spectrum in Fig. 3.
Fig. 4. The time-domain signal and electric field distribution of the terahertz wave passing through the unit cell at (a) 1.09 THz and (b) 1.65 THz.
In order to understand the physical mechanism that produces the dual-band giant chiral metasurface, we calculated and monitored the distribution and trend of the electric and magnetic fields at the peak frequency of the circular dichroism, as shown in Fig. 5(a-h). In order to understand the physical mechanism that produces the dual-band giant chiral metasurface, the distribution and arrow trends of the electric and magnetic fields at the CD peak frequency are calculated and monitored, as shown in Fig. 5(a-h). It can be found that the electric and magnetic fields of the P1 are both excited by LCP and RCP waves to varying degrees. The strong electric field excited by the LCP waves at 1.09 THz is mainly distributed outside the pattern, which is mainly due to the formation of a standard in-plane electric dipole moment. In contrast, the electric field excited by the RCP waves is obviously weaker, and its behavior mode is the same as the electric field excited by the LCP waves. In addition, by observing the behavior mode of the magnetic field, it can be found that weak magnetic fields of different degrees within the structure are excited by different CP waves. This implies that there is also a difference in the magnetic field caused by the in-plane magnetic dipole moment in the structure (which can be obtained by judging the arrow direction of the magnetic field), but the difference in the magnetic field is not as strong as the difference in the electric field. Therefore, the contribution of spin selective transmission at 1.09 THz is mainly due to the electric field response. Similarly, the behavior mode of electric and magnetic fields at 1.65 THz is the same as that of electric and magnetic fields at 1.09 THz. The difference is that the contribution of spin selective transmission at this frequency is mainly derived from the magnetic field response. But in the end, the macroscopic effect of selective transmission of CP waves at the two frequencies is the same.
Fig. 5. The electric and magnetic field distributions of the designed structure when excited by different circular polarization components at different frequencies. (a-d) The electromagnetic field distribution excited by LCP and RCP waves at 1.09 THz. (e-h) The electromagnetic field distribution excited by LCP and RCP waves at 1.65 THz. (i-l) The electric field distribution of CP terahertz waves with frequencies at 1.09 THz and 1.65 THz passing through the metasurface array. (m-p) The far-field (electric field) images of CP terahertz waves with frequencies at 1.09 THz and 1.65 THz passing through the metasurface array.
We want to actually see the modulation effect of the metasurface array on the CP transmitted wave, so the transmitted electric fields of the entire metasurfaces under the incidence of LCP and RCP waves are monitored, as shown in Fig. 5(i-l). It can be seen that the transmitted electric field under the incidence of RCP waves is obviously stronger than the transmitted electric field under the incidence of LCP waves when the frequency is 1.09 THz. On the contrary, the transmitted electric field of the RCP waves is significantly weaker than the transmitted electric field under the incidence of LCP waves when the frequency is 1.65 THz. This shows that the composition of the array does not affect the previous CP transmission spectrum, which further verifies that the reason for the CD is the spin-dependent selective transmission. It is worth noting that this is a process of continuous and dynamic propagation of terahertz waves, and we only intercept the transmitted electric field at a certain moment. To confirm that incident waves are not adversely affected by diffraction to multiple orders of magnitude in reflected and transmitted space, far-field (electric field) images of CP THz waves at 1.09 THz and 1.65 THz are calculated, as shown in Fig. 5(m-p). Even in the far-field (electric field), the differences in electric field strength between LCP and RCP at 1.09 THz and 1.65 THz are significant, which does not affect this application.
2.2 Undifferentiated near-field imaging demonstration
It is not difficult to find that the metasurface array composed of P1 and P2 has great potential in the field of imaging display. The terahertz near-field imaging of any image can be realized by using the intensity difference of the metasurface array for different spin-polarized waves. To highlight the frequency-undifferentiated characteristics of the designed structure, we choose the two frequencies with the largest absolute value of the CD to design the metasurface array. The Chinese character image 1 and image 2 shown in Fig. 6(a) and 6(b) are selected for near-field imaging demonstration at 1.09 THz and 1.65 THz, respectively. In the construction of metasurface arrays, the positions occupied outside the Chinese character image are all set to the P2, while the positions occupied inside the Chinese character image are all set to the P1, as shown in Fig. 6(c) and 6(d). We expect to show that the LCP and RCP waves passing through P1 are transmitted inefficiently and efficiently respectively, while the LCP and RCP waves passing through P2 are transmitted efficiently and inefficiently respectively.
Fig. 6. The terahertz dual-frequency undifferentiated near-field imaging design and demonstration. (a-b) The original images 1 and 2 are used for imaging at frequencies of 1.09 THz and 1.65 THz, respectively. (c-d) The arrangement of chiral metasurfaces for terahertz near-field imaging. (e-l) The electric field intensity and phase of different CP transmitted waves at 1.09 THz. (m-t) The electric field intensity and phase of different CP transmitted waves at 1.65 THz.
Considering the actual imaging effect, the normalized electric field distribution and arrow direction of the 50 × 50 array metasurface at the z = 720 µm are calculated, as shown in Fig. 6(e-h) and Fig. 6(m-p). In the contrast of intensity, the imaging contrast with frequency-undifferentiated can be observed. At 1.65 THz, the situation is just the opposite. The intensity of all positions corresponding to P1 is lower, while the intensity of all positions corresponding to P2 is higher. And the corresponding transmission intensities of the LCP and RCP waves are complementary. After observing the intensity information, the imaging information of each point of the array in the three-dimensional space can be ensured according to the arrow directions of the electric field. Finally, in order to be able to verify the imaging effect of the designed metasurface array from multiple perspectives, the corresponding phase information at different frequencies are calculated, as shown in Fig. 6(i-l) and Fig. 6(q-t). It can be observed that the phase information of all CP waves has a significant phase difference contributed by the P1 and P2. It is worth noting that the phase information also shows obvious frequency-undifferentiated characteristics. Furthermore, if the metasurface has more units, dual-frequency terahertz imaging can be observed at greater distances.
3. Conclusion
In summary, a pair of dual-band giant chiral structures based on all-silicon are designed by breaking mirror symmetry and rotational symmetry. The structural pattern is composed of symmetrical arc-shaped strips and rectangular strips with a horizontal rotation angle of ±45°. The metasurface allows selective transmission of LCP and RCP waves. In order to increase the reliability of the dual-band giant chirality, the CP transmission spectrum and the CD spectrum are obtained from experiments and simulations, respectively. By exploring the in-plane electric and magnetic field distribution of the structure pattern, it is determined that the selective transmission of LCP and RCP waves is caused by different in-plane electric dipole moments and different in-plane magnetic dipole moments. Based on the designed Chinese character images, the dual-frequency terahertz near-field imaging with frequency-undifferentiated is demonstrated. The good intensity contrast and 3D imaging information are showed by the results from the perspective of intensity and phase. This method provides new ideas for the design of multifunctional optoelectronic devices, and promotes the application of chiral metasurfaces in the field of terahertz imaging.
Funding
Sichuan Province Science and Technology Support Program (2021JDTD0026, 2021ZYD0033); National Natural Science Foundation of China (62175021); Sichuan Science and Technology Major Projects (Grant No. 2019ZDZX0026); Jiangxi Innovative Talent Program.
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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