A dielectric metasurface with line-square compound lattice structure has been fabricated and demonstrated in the terahertz (THz) regime by the THz time-domain spectroscopy and numerical simulation. A polarization dependent electromagnetically induced transparency (EIT) effect is achieved in this metasurface due to the mode coupling and interference between the resonance modes in line and square subunits of the metasurface. Accompany with the EIT effect, a large artificial birefringence effect between two orthogonal polarization states is also observed in this compound metasurface, of which birefringence is over 0.6. Furthermore, the liquid crystals are filled on the surface of this dielectric metasurface to fabricate an electrically tunable THz LC phase shifter. The experimental results show that its tunable phase shift under the biased electric field reaches 0.33π, 1.8 times higher than the bare silicon, which confirms the enhancement role of THz microstructure on the LC phase shift in the THz regime. The large birefringence phase shift of this compound metasurface and its LC tunable phase shifter will be of great significance for potential applications in THz polarization and phase devices.
© 2017 Optical Society of America
In recent years, terahertz (THz) science and technology has been developed in a rapid speed, because of its advantages in sensing , imaging , spectroscopy , and communication . For THz applications and developments, THz functional devices such as THz switches , filters , polarizers [7, 8], and phase shifters [9, 10] are essential. Conventional polarization optics depending on the birefringence of uniaxial crystalline materials [11, 12], which causes phase retardations between the two orthogonal polarized components. However, because of the low birefringence, narrowband, large loss, huge volume, and high prices, these natural crystal materials in the THz frequency range are limited . Therefore, developing the new THz polarization and phase devices are highly in demand.
Recently, the metasurfaces, which are composed of subwavelength metal or dielectric units periodically or quasi periodically arranged on the thin chip, provide new approaches for THz polarization and phase devices at THz frequencies [14, 15]. These microstructures can easily engineer the amplitude, phase, and polarization of light to realize artificial mode birefringence, chiral polarization rotation or dichroism by manipulating their geometries [16, 17]. Nevertheless, the limits of artificial birefringence, bandwidth, and insertion loss always exist in these metasurfaces. The single layer metasurfaces often have a low polarization conversion rate and narrow bandwidth though it is easy to be fabricated [18, 19]. While the multi-layered metallic structures can improve the birefringence and enlarge the bandwidth, but they introduce the low transmittance due to the metallic Drude loss in the multi-interfaces, also have great difficulty in fabrication and cannot be easily controlled by external field [20–22]. Therefore, a new strategy should be proposed to realize high birefringence, low loss, and large tunable range.
Liquid crystals (LCs) have attracted great attentions as an active phase shift material in the THz regime due to its large optical anisotropy [23, 24], which can be flexibly manipulated by thermal, electrical, optical or magnetic field [25–27]. However, in the THz frequency range, the phase modulation depth (or called as tunable range) under an external electric field is still limited, which results in high operating voltage and slow response. For examples, 2π phase shifter based on chiral nematic LCs has been reported by Wang et al. , but it needs an extremely high biased voltage (above 1000V); Yang et al. demonstrated a tunable randomly aligned LC cell can achieve a large THz phase modulation in the weak magnetic field, but it takes a long time to respond . Therefore, faster response, lower external electric field and greater phase shift are the focus of LC phase shifter in the THz frequency range currently, and combining the LC with the artificial microstructure is one of the important research trends of the new tunable THz devices. Buchnev et. al. experimentally demonstrated an intensity modulation depth of 20% and a phase shift of 0.22π in a large-area LC metamaterial . Then a THz spatial light modulator with LC metamaterial was experimentally demonstrated by Savo et al., which had an intensity modulation depth of 75% . These reports show that the transmittance and resonance peak of these devices can be tuned due to the LC realignment by an external electric field. But so far, the active LC phase shift effects within the metallic or dielectric metasurfaces have not been sufficiently studied yet.
In this paper, we investigated a dielectric metasurface with line-square (L-S) compound lattice structure, which has a polarization dependent electromagnetically induced transparency (EIT) and large artificial birefringence effects. Further theoretical studies indicate that this EIT effect is owing to mode coupling between the compound line and square sub-cells. Moreover, by filling the LC on the surface of this dielectric metasurface, its tunable phase shift under the biased electric field is greatly enhanced by the metasurface structure compared with the LC filled on the bare silicon chip.
2. EIT effect in dielectric metasurface with L-S structure
According to the basic theory of EIT analogue in metasurface, we find that the dielectric metasurface with compound lattice can realize polarization dependent EIT effect with a large artificial birefringence effect. Through many numerical simulations, a line and square compound lattice structure was selected, and the detailed geometry parameters were design to make the EIT effect located in the range of 0.6~0.9THz with the largest mode birefringence. At last, the dielectric metasurface of a compound lattice with line and square structures was fabricated on a 500μm thick 10kΩ·cm high resistance Si wafer by the UV lithography and inductively coupled plasma etching. Its SEM images and 3D structure diagram are shown in Fig. 1. The etching depth h = 120μm, and both the width of the line structure and square structure are w = 120μm, while the period in the x direction and y direction are Px = 200μm and Py = 440μm, respectively, as shown in Fig. 1(b). THz waves were normally incident into the structure along z axis, and the polarization direction of the incident waves was along y axis (θ = 90°) as shown in Fig. 1(c), and then the transmission signal of different polarization directions can be detected by rotating the metasurface.
In our experiment, a four parabolic mirror THz-TDS system was used to measure the transmission spectra of our structure. The excitation source of THz-TDS was a 75 fs 800 nm Ti:sapphire laser. The THz pulse was generated by an LT-GaAs photoconductive antenna and was detected by an (110) ZnTe crystal. Our experiments were carried out at room temperature. The transmission spectra can be calculated byFig. 2(a). The effective refractive index spectra of this device can be calculated by 
Experimental effective refractive index and transmission spectra of this structure with different polarization directions are shown in Figs. 2(b) and 2(c). When the polarization direction of incident wave is parallel along the line structure of metasurface, the polarization rotation angle defines as θ = 0°, while the perpendicular is θ = 90°. As shown in Fig. 2(c), when θ = 0°, a resonance dip is located at 0.75THz with –20dB depth. With the increase of polarization rotation angle, this resonance dip gradually changes as a transmission peak at the same frequency point and two new resonance dips generate at both side of this peak. When θ = 90°, this transmission peak is located at 0.72THz and two resonance dips are observed at 0.63THz and 0.84THz. This is a typical EIT spectral line shape. Moreover, as shown in Fig. 2(b), when θ = 0°, the effective refractive index spectrum of the metasurface gradually increases from 3.2 to 3.4 in the frequency range of 0.5~0.7THz; while with the increase of polarization rotation angle, especially when θ = 90°, there are two sudden drops of the refractive index from 3.2 to 3.0 near 0.65THz and from 3.0 to 2.8 near 0.8THz. This phenomenon is also corresponding to the EIT effect, and each resonance is accompanied with the sharp change of the refractive index in this EIT effect. That is, our dielectric metasurface with L-S structure achieved a polarization dependent EIT effect and a large artificial birefringence between two orthogonal polarization states. When the frequency is larger than 0.8THz, the artificial birefringence of this metasurface is over 0.6 (3.4 for 0° polarization and 2.8 for 90° polarization). Based on its large birefringence, this metasurface can be applied as THz wave plate and tunable phase shifter controlled by mechanically rotating the metasurface.
To further study the mechanism of EIT effect, we simulated the transmission spectra and field distributions of our dielectric metasurface at 0.72THz by the commercial software CST Microwave Studio. The transmission spectra with different polarization directions are shown in Fig. 2(d), which agree with the experimental transmission spectra. The corresponding field distribution of θ = 0° and θ = 90° are shown in Fig. 3. As shown in Figs. 3(c)–3(f), both θ = 0° and 90° can excite a strong resonance mode (called as bright mode) in the square structure, while only θ = 90° can generate a weak resonance mode (called as dark mode) in the line structure, and there is no mode in the line structure for θ = 0°. The EIT effect in our dielectric metasurface depends on the coupling between the modes in the square structure and line structure. Compared with Figs. 3(a) and 3(b), when θ = 0°, only the mode located in square structure appears but no mode in the line structure, which means that there is no mode coupling in this case; but when θ = 90°, the bright mode of square structure and the dark mode of the line structure exist at the same time. These two modes field are very close in space and they have the same phases, which leads that a mode coupling and coherent superposition occur in this compound metasurface. Therefore, the original resonance dip changes into a transmission peak due to the constructive interference, which achieves the EIT effect in the 90° polarization but a resonance dip in the 0° polarization.
3. Phase shift enhancement in LC filled dielectric metasurface
Next, we filled LCs on the surface of the dielectric metasurface with L-S structure to form an electrically controlled LC phase shifter. For the proposed LC metasurface device, we used a silica substrate of 0.5mm thickness and our metasurface with L-S structure (or bare silicon substrate for comparison) as the upper and lower substrate, respectively, as shown in Fig. 4(a). The substrates were separated by two thin metal wires with diameters of 1mm, and these two metal wires were also used as a pair of electrodes, and the distance between these two electrodes was 10mm. The LC was filled into the layer between the up substrate and metasurface as a LC layer, and then the photosensitive glue was used to seal this LC cell . So the thickness of LC layer is 1mm, and the total thickness of the device is 2mm, as shown in Fig. 4(b). The LC used in this work is E7, which is a commercially available mixture with positive dielectric anisotropy (Δε>0). As shown in Fig. 5(a), THz waves were normally incident into the structure along z axis, and the polarization direction of the incident waves was along y axis, which was perpendicular to the direction of the external electric field.
Here, we fabricated and measured three different samples of LC filled on the different lower substrates, that is bare silicon substrate, 90° metasurface, and 0° metasurface. The phase and refractive index of samples were changed with the increase of the biased electric field. The schematic diagrams of three samples and their refractive index spectra measured by the THz-TDS system are shown in Fig. 5. The effective refractive index can be also obtained by Eq. (2). For bare silicon structure, the refractive index decreases from 2.192 to 2.166 at 0.7THz when the bias increases from 0 to 12kV/m, and its maximum tunable range of refractive index is −0.026, as shown in Fig. 5(a). For 90°dielectric metasurface, the refractive index decreases from 2.180 to 2.135 as shown in Fig. 5(b), and its maximum tunable range of –0.045 is much larger than that of the bare silicon under the same electric bias. These results indicate the range of refractive index is increased when LC combined to our dielectric metasurface. In addition, since our dielectric metasurface itself has polarization dependent properties, this LC based THz device also has polarization dependent properties. Under the same condition, the refractive index of 0° metasurface increases from 2.039 to 2.062, which is a positively tunable value of only + 0.023 and no enhancement compared with the bare silicon one as shown in Fig. 5(c).
The line-square compound lattice of the metasurface strongly affects the distribution and director of the LC molecules under the external electric field. When there is no biased electric field, the LC molecules are randomly arranged; when the electric field is applied, the molecules start to rotate along the direction of line structure. There are two forces to determine the molecules arrangement: one is the electric field force which makes the LC molecules rotate to the direction of electric field; the other is the surface anchorage force which makes the LC molecules rotate to the direction of line structures. The surface anchorage force is larger than electric field force when the electric field is small, so for both the bare silicon and 90° metasurface cases, the LC molecules rotate to the x direction and the phase of device decrease, while for the 0° metasurface case, the LC molecules rotate to the y direction. For the 90° metasurface case, the two factors are along the same direction, so the phase shift effect is enhanced under the same electric field compared with the bare silicon case. But for the 0° metasurface case, the two factors are orthogonal to each other, so the phase shift changes to the opposite direction and to be weaken under the same electric field.
This phenomenon is also validated in the phase shift as shown in Fig. 6, the phase shift can be calculated by
The purpose of this tunable phase shifter based on LC metasurface is to realize enough tunable phase shifts to 0.25π as a quarter waveplate under a limited LC thickness and limited bias. If LC thickness is too small (<1mm), the maximum phase shift of LC metasurface cannot reach 0.25π. But if it is too thick (>1.5mm), the surface anchorage force from the microstructure of dielectric metasurface becomes weak compared to the electric field force. In this case, the phase shift of LC metasurface has not any enhancement roles compared to the LC phase shifter on the bare Si substrate under the same electric field. Therefore, the thickness of LC layer is critical parameter that is designed optimally as 1mm in this work.
In conclusion, we designed and fabricated a dielectric metasurface with L-S structure. in the THz regime, and the polarization dependent EIT and large artificial birefringence of over 0.6 in this device has been demonstrated by the THz-TDS system and numerical simulation. Further theoretical studies indicate that this EIT effect is owing to the mode coupling between the dark mode in line subunit and the bright resonance mode in the square subunit. Furthermore, for the LC filled dielectric metasurface, its tunable phase shift under the biased electric field reaches 0.33π, which is 1.8 times higher than the bare silicon, so the results show that the THz phase shift of LC is greatly enhanced by the dielectric metasurface under the same biased electric field. Therefore, the large birefringence phase shift and polarization dependent properties of this compound metasurface and its LC tunable phase shifter will be of great significance for potential THz polarization and phase control applications.
National Natural Science Foundation of China (61505088, 61671491); National Basic Research Program of China (Program 973) (2014CB339800); Natural Science Foundation of Tianjin City(15JCQNJC02100); Open Fund of the Key Laboratory of Optical Information Science & Technology (Nankai University)(2017KFKT003).
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