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Abstract
In this work, a new strategy was proposed for active control of mm-thick liquid crystals (LC) cell to realize the polarization manipulation in terahertz (THz) regime, which through the electric field control and static magnetic field pre-anchoring. The LC cell was fabricated by a nematic 5CB LC and two silica substrates that were coated with the graphite layer as the transparent electrode. Under the pre-anchoring of the static magnetic field, the optical axis of LC can be precisely controlled by the variable electric field. By using a THz-TDS with a wire grid polarizer, the output THz polarization from the LC cell can be deduced from the amplitude and phase shift of ±45° components. Here, we systematically analyzed three different outfield configurations. Only if the ±45° components that output from the polarizer have phase shifts, can the polarization state conversion be realized. The results show that the linear to circular or the cross-polarization conversion were realized under the specific electric field. This work provides a new approach for the thick-LC layer anchoring and orientation control, and also the tunable polarization manipulation of THz LC devices.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays, terahertz (THz) technology has attracted considerable attention due to its promising applications in nondestructive detection, security screening, and next-generation wireless communications [1–4]. Compared with the rapid development of THz sources and detectors, THz functional devices are relatively under-developed, such as THz modulators [5], waveguides [6], phase shifter [7], and polarization devices [8]. In particular, phase and polarization are the basic parameters of electromagnetic waves, which can not only carry useful electromagnetic information but also manipulate the propagation and states of light. However, most of the THz polarization devices are severely limited for it cannot achieve the active manipulation once fabricated.
Liquid crystals (LC), as one of the promising materials for tunable phase and polarization control, which exhibits larger birefringence, lower absorption loss and good tunability in the THz regime [9–15]. However, it still has challenges to overcome for the LC-based THz devices. Firstly, the transparent electrodes in the THz band are in high demand, such as the indium tin oxide (ITO) electrodes that are commonly used in optical bands are opaque in the THz band [16]. To solve this problem, some newfangled nanomaterials with high transmittance are proposed, such as Yang et al. demonstrated a tunable THz phase shifter utilizes ITO nan-whisker electrodes [17]. Wang et al. reported a broadband tunable LC THz waveplate driven with porous graphene electrodes and sub-wavelength metal grating [18]. These nanomaterials or metal micro-structure as LC transparent electrodes are well-presented for electronically controlled LC-based THz devices, but they generally require complex manufacturing processes and also need high production cost [19]. Therefore, it is necessary to develop new materials that are employed as LC transparent electrodes in the THz band.
Secondly, to achieve sufficient phase accumulation to realize the THz polarization conversion, the thickness of the LC layer is very large and usually in the sub-millimeter range. The LC molecules of the middle cell cannot well align in a thick LC cell for either the rubbing-alignment or photo-alignment [20–25]. Magnetic fields used to tune LC have a good tunability, which can make the whole LC layer uniformly controlled in an mm-thick LC cell and no additional orientation process is required [26–27]. For example, Yang et al. studied the magnetically controlled THz birefringence in a 3 mm-thick randomly aligned LC cell, which realized a larger phase modulation depth under a weak magnetic field [28]. Recently, Hsieh et al. demonstrated a magnetically tunable THz achromatic quarter-wave plate by 3 mm-thick LC phase retarders [29]. Therefore, these studies indicate that the magnetic field can be used as an efficient way for orienting the thick-LC layer.
In this paper, we adopt a static magnetic field to pre-anchoring the LC layer, realizing the LC from randomly aligned to the uniform arrangement. Based on this, the active control of the LC layer was investigated under the variable electric field. The graphite layer was employed as the LC transparent electrodes, which has a high transmission. Since there is always an orthogonal field forcing on the LC layer, the optical axis of LC can be precisely controlled in this unique configuration. By changing the relative angle between the LC optical axis and the THz polarization direction, the active regulation of the THz polarization state is realized. This work provides an alternative solution towards the active control of the thick LC layer and laid the foundation for efficient manipulation of THz phase and polarization.
2. Results and discussions
2.1 Liquid crystal THz polarization manipulation under the outfield of (B // x, E // y)
As illustrated in Fig. 1(a), the LC cell is composed of two parallel silica substrates that were coated with the graphite layer and infiltrated with 1mm-thick commercial 5CB LC (Jiangsu Hecheng Display Technology Co., Ltd). The graphite layer is used as the transparent electrode for electronically controlled LC, and it is simply fabricated through the natural drying of the graphene solution (G139799, Aladdin Industrial Co., Ltd.). The transmittance of the graphite layer has a high transmission of more than 80% [30], and then the graphite layer can be used as an efficient transparent electrode for THz wave. Compare with the electrode of ITO nanowhisker and PEDOT, graphite has the advantage of convenient manufacturing and low cost. A variable E-field can be applied to LC when a dc bias is applied to the upper and lower graphite electrodes. Here, the E-field intensity of 1 KV/m corresponds to a biased voltage of 1 V. For a uniform pre-alignment of the LC molecules, a pair of NdFeB-permanent magnets is employed, which has a constant intensity of B=0.17 T. We perform the experiments by using a THz-TDS and a wire grid polarizer [31]. Different from the traditional THz-TDS, an additional wire grid polarizer is placed behind the LC cell, which can be rotated to obtain the 45° and −45° polarization signal that passed through the LC cell. Hence, the THz time signals of two orthogonal linear polarized components can be obtained in this method, and then we can conclude the THz polarization state output from the LC cell from the amplitude and phase shift of the ± 45° components. Figure 1(a) and Fig. 1(b) show the simplified schematic diagram and experimental optical path. In this system, the THz waves are y-polarized excited from the photoconductive antenna (PCA), and only the linear polarization along the y-axis can be detected by the ZnTe crystal. A 3D printed mold loaded with NdFeB-permanent magnets is used for magnetically align LC, and a dc-field is applied by the external wires touched with graphite electrodes.
Fig. 1. (a) Schematic diagram of THz phase and polarization measurement through the LC cell under E-field and B-field (B // x, E // y). α represents the angle between the LC molecule with x-axis. The THz polarizer can be rotated from −45° to +45°. (b) Experimental equipment of THz-TDS system with a polarizer, the sample is located in a 3D printed mold loaded with NdFeB-permanent magnets, and dc-field is applied through external wires. (c) Time-domain THz spectra of LC with different electric fields from 0KV/m to 50KV/m (here the polarizer shown in Fig. 1(a) is fixed at 0°). (d) The corresponding refractive index n of LC.
Firstly, we investigate the THz response of the single LC layer under the transverse B-field and longitudinal E-field, here the polarizer shown in Fig. 1(a) is fixed at 0°. The orientation of LC undergoes three processes under the outfield, which as follows: 1, the LC molecules are oriented strictly along the x-axis under the action of B-field when E < 10KV/m; 2, the LC molecules starts to rotate to the y-axis with the increase of the E-field (10KV/m ≤ E < 20KV/m), which has an angle of α to the x-axis; 3, the LC molecules eventually along the y-axis when the E-field is strong enough (E≥20KV/m). Most importantly, once the E-field is removed, the LC molecules will return to the x-axis under the force of B-field. Hence, the optical axis of the LC can be regulated in the x-y plane controlled by the variable E-field and constant B-field. The time domain signals of LC with different orientations are presented in Fig. 1(c), and it has an obvious backward phase-shift with the increase of E-field from 0 to 20KV/m. If we further increase the electric field intensity to E=50KV/m, the time domain spectral lines coincide with the 20 KV/m, and E=20KV/m is the electric field threshold of this device. In the following discussion, we will only show the results of the maximum electric field of 20 KV/m. By using the Fourier transform of the time signals, the corresponding refractive indexes can be calculated by:
where c is the speed of light in a vacuum, d is the thickness of the LC cell, ω=2πf is the circular frequency, and Δφ(ω)=φsam - φref is the phase shift spectrum between the sample and reference, respectively. Figure 1(d) shows the results of refractive indexes of 5CB linearly rise with a slight dispersion from 0.2 THz to 1.6THz. The refractive indexes increase from 1.53 to 1.65 at 1THz when E=0∼20 KV/m, which corresponding to the LC molecules are aligned from the x-axis (α=0°) to the y-axis (α=90°). Here, the incident THz wave is linearly polarized along the y-axis, thereby the case of α=0° and α=90° are corresponding to the LC optical axis is parallel or perpendicular to the incident polarization direction, respectively. Therefore, we can conclude that no and ne are approximately 1.53 and 1.65. The effective refractive index (neff) of y-axis component can be described as a function of α, which follows [32]:Therefore, the evolution of the refractive index of the LC from no to ne can be accurately measured by the outfield configuration (B // x, E // y).
To determine the polarization state transmitted through the LC cell with different LC orientations, we measured the THz time domain signals of ±45° components for LC cell under different E-field, as shown in Fig. 2. In this way, the polarizer shown in Fig. 1(a) is fixed at 45° or −45°. Here, we chose four kinds of E-field of 0, 12, 14, and 20 KV/m. The insets show the schematic diagram of the LC orientation. In Fig. 2(a), the curves of ±45° components are approximately coinciding with each other when the E-field equals to 0 KV/m, and the LC molecules are arranged along the B-field. In the case of E=12 KV/m, it has an obvious time delay between ±45° components with a small difference in amplitude, as shown in Fig. 2(b). Figure 2(c) shows that the time delay decreases and the amplitude difference increases between ±45° components when E=14 KV/m, and the LC molecules are oriented close to the E-field. When the E-field reaches 20 KV/m, the two curves of ±45° components overlap again as shown in Fig. 2(d).
Fig. 2. Experimental measured THz time signals of ± 45° components at (a) E=0 KV/m, (b) 12 KV/m, (c) 14 KV/m and (d) 20 KV/m. The red line represents 45° component and the blue line represents −45°component. Insets: the orientation of LC molecules with the external E-field and B-field.
Through the Fourier transform of the time-domain signals in Fig. 2, we obtain the corresponding amplitude transmission and phase shift spectrum, as shown in Fig. 3. In Fig. 3(a) and Fig. 3(d), it has almost the same transmission of ±45° components and the phase shift closes to 0 from 0.2 to 1.6THz, which good reflects the coincidence of time signals in Fig. 2(a) and Fig. 2(d). And the small difference in transmittance between Fig. 3(a) and Fig. 3(d) comes from the different THz absorption when the LC optical axis is parallel or perpendicular to the incident y-polarization. As to E=14 KV/m, the transmittances are quite different between ±45° components, and the phase shift below 0.3π in the whole testing band, as shown in Fig. 3(c). It's worth noting that the phase shift of Fig. 3(b) has a large dispersion linearly with the frequency, and the phase shift of 0.5π and 1.0π is achieved at 0.9THz and 1.5THz, respectively. Moreover, their transmittances nearly at the same level at these two frequencies. Both of the two points indicate that the circular polarization conversion and orthogonal linear polarization conversion can be achieved at 0.9THz and 1.5THz when E=12 KV/m.
Fig. 3. The amplitude and phase spectrum obtained by using the Fourier transform of the time-domain signals. (a) E=0 KV/m, (b) 12 KV/m, (c) 14 KV/m and (d) 20 KV/m. The green line represents the phase shift spectra between the ±45° components.
To better illustrate the THz polarization states, the output polarization ellipse from the LC layer was given in Fig. 4. The terminal trajectory equation of electric vector E is obtained by the formula [33]:
Fig. 4. The polarization ellipse of electric vector under the E-field of (a) 0 KV/m, (b) 12 KV/m, (c) 14 KV/m and (d) 20 KV/m with the frequency of 0.9 THz and 1.5 THz.
As shown in Fig. 4, the polarization ellipses with typical frequencies of 0.9 THz and 1.5THz were demonstrated under different E-field. Here, the incident THz wave is y-polarized, which means it only has the Ey component and presents as a vertical line in the polarization ellipse. In Fig. 4(a) and Fig. 4(d), the output polarization state does not change and still in the y-polarization because it has no phase shift between the ±45° components (δφ ∼ 0). In Fig. 4(c), the trajectory of the electric vector indicates that the incident linear polarization is all transformed into elliptical polarization due to the inequality in amplitude and δφ<0.3π. In the case of E=12 KV/m, the phase shift of δφ=0.5π and δφ=1.0π is realized with a small difference in amplitude at 0.9 THz and 1.5THz (Fig. 3(b)). Therefore, the incident y-polarizations can be transformed into the circular polarization and x-polarization, which reflected as a circle and a horizontal line in the polarization ellipse, as shown in Fig. 4(b). The above results demonstrate that the active THz polarization manipulation can be realized by controlling the relative angle between the LC optical axis and the incident polarized THz wave under the outfield of (B // x, E // y).
2.2 Liquid crystal THz Polarization manipulation under the outfield of (B // y, E // z) and (B // x, E // z)
In addition, the other two kinds of outfield configurations of (B // y, E // z) and (B // x, E // z) were presented in Fig. 5 and Fig. 6. Here, the B-field also plays the role of pre-align the LC with a constant intensity of 0.17 T, and the E-field was applied along the transmitted direction (k). In Fig. 5(a), the LC optical axis was rotated from y-axis to z-axis and the refractive index changes from ne to no with the increased E-field from 0 to 20KV/m. The THz time signals and the corresponding refractive with different E-field are respectively shown in Fig. 5(b) and Fig. 5(c), which is contrary to the change of the refractive index in Fig. 1(c) and Fig. 1(d). Due to the phase and amplitude components of the orthogonal 45° direction are consistent with each other, so there is no polarization transformation under (B // y, E // z) and the measured THz time signals of ±45° components are all overlapped as shown in Fig. 5(d).
Fig. 5. (a) Schematic diagram of the LC rotation orientation under the action of electric and magnetic fields (B // y, E // z). (b) Time-domain THz spectra of LC with different E-field. (c) The corresponding refractive index n. (d) THz time-domain signals of ±45° components when E-field changes from 0 to 20 KV/m.
Fig. 6. (a) Schematic diagram of the LC rotation orientation under the action of electric and magnetic fields (B // x, E // z). (b) Time-domain THz spectra of LC with different E-field.
In the case of (B // x, E // z), the LC optical axis was rotated from the x-axis to the z-axis with the increase of E-field. Since the effective refractive index of the LC in the y-direction does not change (neff = no), the THz time spectrum has no phase change with any E-field, as shown in Fig. 6(b). Therefore, the polarization state of the THz wave neither transforms nor the refractive index of the LC changes under this configuration.
3. Conclusion
In conclusion, we have experimentally investigated the THz phase and polarization manipulation of LC under the magnetic field per-anchoring and electric field control for the first time. The introduction of a magnetic field can well per-align the LC with an mm-thick layer, and the graphite layer can be well employed as the THz electrodes. By configuring the external field, we have realized three possible orientation states between the LC directions with the incident polarized wave, which can firmly control LC in two orthogonal fields without any additional orientation layer. Under the transverse magnetic field and longitudinal electric field (B // x, E // y), an active THz polarization conversion was achieved. This regulation strategy of LC can be widely used for the development of THz LC devices.
Funding
National Natural Science Foundation of China (62005143, 61971242, 61831012); Natural Science Foundation of Tianjin City (19JCYBJC16600); Young Elite Scientists Sponsorship Program by Tianjin (TJSQNTJ-2017-12).
Disclosures
The authors declare no conflicts of interest.
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