Liquid crystal devices, phase modulation devices
The liquid crystal device uses a laminated structure with electrodes and magnetic fields to control terahertz waves rapidly, addressing the slow response times of thicker layers and enabling high-speed phase modulation.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- AKITA PREFECTURAL UNIVERSITY
- Filing Date
- 2023-02-01
- Publication Date
- 2026-07-30
AI Technical Summary
Existing liquid crystal devices require thicker layers to control terahertz waves, leading to longer response times and hindering high-speed operation.
A liquid crystal device that controls the orientation of liquid crystal molecules using a laminated structure with electrodes and magnetic fields, allowing for rapid switching of both electric and magnetic fields to control terahertz wave phase.
Enables high-speed control of terahertz waves by reducing the time required for orientation changes of liquid crystal molecules to less than 10 seconds, facilitating faster phase modulation and measurement.
Smart Images
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Abstract
Description
Technical Field
[0001] The present invention relates to a liquid crystal device that performs optical control by controlling the alignment of liquid crystals. The present invention also relates to a phase modulation device that modulates the phase of terahertz waves using this liquid crystal device.
Background Art
[0002] Liquid crystal molecules have anisotropy in dielectric constant (refractive index), and furthermore, the alignment of liquid crystal molecules can be controlled by an applied electric field or the like. Therefore, it is possible to control the phase and polarization direction of light by transmitting it through a liquid crystal material. As a result, a technique for modulating an optical signal is known and is used for modulating visible light in displays and the like.
[0003] On the other hand, terahertz waves having a frequency of about 10 12 Hz, which is significantly different in wavelength (frequency) from visible light, have completely different transmissivity from visible light. For example, the transmissivity of plastic materials and semiconductor materials is very high. Therefore, non-destructive inspection of various industrial products can be performed using terahertz waves. Also in this case, for example, a technique for similarly controlling the phase and the like of terahertz waves is required.
[0004] Therefore, techniques for controlling terahertz waves by controlling the alignment of liquid crystals are described in, for example, Patent Documents 1 and 2. In these techniques, the alignment of the liquid crystal provided between the electrodes is controlled by the voltage (electric field) between the electrodes, and thereby the phase of the terahertz wave transmitted through it is controlled. In this case, an electrode that is transparent to terahertz waves is used, and the terahertz wave is incident from one electrode side and exits from the other electrode side, and the phase of this terahertz wave is controlled by the voltage.
Prior Art Documents
Patent Documents
[0005]
Patent Document 1
[0006] Controlling the optical properties of terahertz waves requires a thicker liquid crystal layer compared to visible light. Generally, the response time during this control is proportional to the square of the thickness, resulting in longer response times, which hinders high-speed operation and prolongs measurement time. Therefore, there has been a demand for liquid crystal devices that can control terahertz waves at high speed using liquid crystals.
[0007] This invention has been made in view of the above-mentioned problems, and aims to provide an invention that solves the above-mentioned problems. [Means for solving the problem]
[0008] In order to solve the above problems, the present invention has the following configuration. The liquid crystal device of the present invention is a liquid crystal device in which the orientation of liquid crystal molecules is controlled, comprising: a liquid crystal cell having a laminated structure in which a liquid crystal layer containing the liquid crystal molecules is sandwiched between two electrodes; a voltage applying means for applying a voltage between the two electrodes; and a magnetic field applying means for applying a magnetic field to the liquid crystal layer along the in-plane direction of the liquid crystal layer, wherein the polarization state of the terahertz wave after it has been transmitted through the liquid crystal cell along the normal direction of the electrodes is controlled by the voltage and the magnetic field. The liquid crystal device of the present invention is characterized in that the on / off switching of the voltage and the on / off switching of the magnetic field are repeatedly controlled alternately. In the liquid crystal device of the present invention, the liquid crystal molecule is a nematic liquid crystal having hydrogen bonds and only alkyl chains at its molecular terminal groups. The phase modulation device of the present invention is characterized by using the liquid crystal device and modulating the phase difference for each polarization direction of the terahertz wave by controlling the voltage and the magnetic field. [Effects of the Invention]
[0009] As the present invention is configured as described above, a liquid crystal device can be obtained that can control terahertz waves at high speed using liquid crystals. [Brief explanation of the drawing]
[0010] [Figure 1] This is a cross-sectional view showing the structure of a conventional liquid crystal device. [Figure 2] This figure shows the molecular structure of the liquid crystal used in the liquid crystal device according to the embodiment. [Figure 3] This diagram schematically illustrates the operating principle of a liquid crystal device. [Figure 4] This is a cross-sectional view showing the structure of a liquid crystal device according to an embodiment of the invention. [Figure 5] This diagram shows the configuration of the measurement system used to measure the operation of a liquid crystal device. [Figure 6] This shows the voltage dependence of the transmittance of a system including a liquid crystal device when no magnetic field is applied. [Figure 7] This is the result of measuring the transmittance of a system including a liquid crystal device over time, starting immediately after the voltage is turned off, when no magnetic field is applied at all. [Figure 8] This shows the results of measuring the transmittance over time of a system including a liquid crystal device when the electric field and magnetic field are alternately switched on and off. [Figure 9] This diagram shows the configuration of a measurement system used to measure the birefringence of a sample using a liquid crystal device. [Figure 10] This shows the results of measuring the transmittance over time of a system containing a liquid crystal device and a sample when the electric field and magnetic field were alternately switched on and off. [Figure 11] This shows the relationship between the orientation angle of liquid crystal molecules and transmittance, calculated for cases with and without a sample (a) and with a sample (b). [Modes for carrying out the invention]
[0011] A liquid crystal device according to an embodiment of the present invention will be described. Here, first, the structure of a conventional liquid crystal device will be described. FIG. 1 is a cross-sectional view showing the structure of this liquid crystal device 9, which is the same as the liquid crystal device described in Patent Document 2. In this liquid crystal device 9, two transparent substrates 10 are provided. On the surface of the substrate 10, a transparent electrode (electrode) 11 thinner than the substrate 10 and an alignment film 12 are sequentially formed. Here, "transparent" means that the transmittance of terahertz waves to be controlled is sufficiently high. As the material of the substrate 10, for example, quartz is used, and as the transparent electrode 11, for example, PEDOT / PSS (poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonic acid)), which is an organic conductive thin film, is used.
[0012] Two substrates 10 having such a laminated structure are provided facing each other with an insulating spacer 13 interposed therebetween at a certain interval, and a liquid crystal layer 15 in which liquid crystal molecules 15A are dispersed in a solvent is provided therebetween. As is well known, by the rubbing treatment, the alignment of the liquid crystal molecules 15A is controlled, and in FIG. 1, the long axis direction of the liquid crystal molecules 15A is controlled in the horizontal direction in the figure (in-plane direction of the transparent electrode 11). The alignment film 12 is formed on the transparent electrode 11, for example, by coating, and the material of the alignment film 12 is appropriately set to enhance this alignment property.
[0013] In addition, voltage application means 22 is connected to the two transparent electrodes 11 to control the electric field between them.
[0014] Figure 2 is a diagram showing the molecular structure of the liquid crystal molecule 15A used here. This liquid crystal molecule 15A is also the same as that described in Patent Document 2, and is a nematic crystal having a hydrogen bond and having only an alkyl chain at the molecular end group. In this liquid crystal molecule 15A, while the anisotropy of the dielectric constant (refractive index) of terahertz waves is large, the anisotropy of the absorption of terahertz waves is small. When controlling the phase, it is preferable that only the phase changes with the control and the change in absorption at this time is small. Therefore, when controlling the phase of terahertz waves particularly by the alignment of the liquid crystal molecule 15A, this material is particularly effective. The thickness of the liquid crystal layer 15 becomes the interval between the transparent electrodes 11 and is, for example, 800 μm. This thickness is large compared to a liquid crystal device that modulates visible light as well.
[0015] Figure 3 is a diagram schematically showing the operation of this liquid crystal device 9 in the structure of FIG. 1. Here, descriptions of the alignment film 12, the spacer 13, etc. are omitted. In FIG. 3, the terahertz wave W whose phase is controlled passes through this liquid crystal device 9 in the vertical direction in the figure. At this time, if the substrate 10 etc. are made of a material with a sufficiently high transmittance of the terahertz wave W and a small optical anisotropy with respect to the terahertz wave as described above, the polarization state or phase after the transmission of the terahertz wave W is adjusted by the alignment of the liquid crystal molecule 15A. The alignment state of the liquid crystal molecule 15A is controlled by the electric field (voltage) between the transparent electrodes 11.
[0016] In FIG. 3(a), the state when there is no electric field is shown, and in FIG. 3(b), the configuration when an electric field E (≠0) is applied is schematically shown. In the case of FIG. 3(a) where no electric field is applied, the alignment state of the liquid crystal molecule 15A is the same as the state immediately after the rubbing treatment (FIG. 1) (the long axis direction is the horizontal direction in the figure), whereas in the case of FIG. 3(b) where the electric field E is applied, the alignment state of the liquid crystal molecule 15A is perpendicular to this (the long axis direction is the vertical direction in the figure). Thereby, the phase of the terahertz wave W after transmission is different between the state of FIG. 3(a) and the state of FIG. 3(b). That is, the alignment of the liquid crystal molecule 15A is controlled by turning on and off the electric field, and the polarization state or phase of the terahertz wave W is controlled.
[0017] The above content is the same as the technology described in Patent Document 2. In contrast, in the liquid crystal device according to the embodiment of the present invention, in addition to controlling the on / off state of the electric field E applied to the liquid crystal layer 15, the on / off state of the application of a magnetic field H is also synchronized and alternately performed. In this case, the direction of the magnetic field H is perpendicular (intersecting) to the electric field E. Figure 4 is a diagram showing the configuration of this liquid crystal device 1 in correspondence with Figure 1. The liquid crystal cell 20 used here is the same as the liquid crystal device 9 in Figure 1, and the substrate 10, transparent electrode 11, alignment film 12, spacer 13, and liquid crystal layer 15 (liquid crystal molecules 15A) are also the same. The magnetic field H in the in-plane direction of the liquid crystal layer 15 (left / right direction in the figure), which is perpendicular to the direction of voltage E application in the liquid crystal cell 20 (up / down direction in the figure), can be applied by magnetic field application means (N pole) 21A and magnetic field application means (S pole) 21B, both of which are composed of coils.
[0018] The operation of this liquid crystal device 1 will now be explained. Figure 5 is a schematic diagram showing the measurement system used in this case. Here, the light source 30 is used, which is a combination of an excitation light source 31, which is a CO2 laser oscillator, and a gas laser oscillator 32 that continuously emits terahertz waves W, which become laser light using the CO2 laser light C emitted by the CO2 laser oscillator as excitation light. The frequency of the terahertz waves W emitted here is, for example, 2.5 THz. These terahertz waves W are transmitted through a polarizer 41, which is a wire grid polarizer, and are then treated as linearly polarized light, with only the polarization component in a specific direction extracted, before being incident on the liquid crystal device 1.
[0019] As described above, a voltage application means 22 is provided for applying (controlling) the voltage (electric field E) between the transparent electrodes 11. In addition, a magnetic field control unit (not shown) is connected to the magnetic field application means (N pole) 21A and the magnetic field application means (S pole) 21B to control the magnetic field H by the current of the coil.
[0020] The terahertz wave W transmitted through the liquid crystal device 1 passes through a polarizer 42 that transmits only the polarization component perpendicular to the polarizer 41, and is then detected by a pyroelectric detector 43 that detects the intensity of the terahertz wave W. In this case, the liquid crystal device 1 imparts a phase difference to the terahertz wave W according to the polarization direction, and this phase difference changes depending on the orientation state, and the detection intensity of the detector 43 changes according to this phase difference. Therefore, this detection intensity changes depending on the orientation state of the liquid crystal molecules 15A in the liquid crystal device 1. In this respect as well, it is the same as the technology described in Patent Document 2.
[0021] First, the operation of the liquid crystal device 1 when the magnetic field H is always turned off will be described. This operation corresponds to the operation of the liquid crystal device described in Patent Document 2. Figure 6 shows the voltage dependence of the transmittance T of the terahertz wave W when the voltage between the transparent electrodes 11 is scanned in the range of 0 to 100 V. Here, the transmittance T is the ratio of the detection intensity when the liquid crystal device 1 is present to the detection intensity when the liquid crystal device 1 is not present in the configuration of Figure 5, and the transmittance T changes according to the change in polarization state due to the liquid crystal device 1. In this case, for example, the value of the transmittance T in the state of Figure 3(a) depends on the settings of the polarizers 41 and 42, and these settings are made as appropriate so that changes in transmittance T due to changes in orientation can be detected.
[0022] Here, the orientation state of the liquid crystal molecules 15A is as shown in Figure 3(a) (major axis horizontal) in the initial state and as shown in Figure 3(b) (major axis vertical) in the final state. Therefore, the upper part of Figure 6 schematically shows the corresponding orientation states corresponding to Figure 3. Furthermore, since the transmittance T is constant in both the initial and final states, as shown in Figure 6, it is considered that the state is as shown in Figure 3(a) from the initial state (0V) to about 30V, and as shown in Figure 6, the state is as shown in Figure 3(b) from 80V onward. In this case, the time required between 30V and 80V was about 2 minutes. Also, as shown in Figure 6, the orientation of the liquid crystal molecules 15A in the intermediate state between the state in Figure 3(a) and Figure 3(b) (range of 30V to 80V) is an intermediate state between Figure 3(a) and Figure 3(b). During this period, the transmittance T fluctuates greatly, especially increasing temporarily before decreasing. This point is also described in Patent Document 2. In Figure 6, "5π / 2", "2π", "3π / 2", "π", "π / 2", and "0" represent the phase differences imparted to the terahertz wave by the liquid crystal layer 15 at the points indicated by the arrows in the graph. In other words, a phase difference of 0 to 5π / 2 can be imparted to the terahertz wave within a voltage range of 0 to 80V.
[0023] On the other hand, Figure 7 shows the time course of the detection intensity from immediately after applying a voltage greater than in Figure 6, 200V, and then instantly turning it off. In this case, the initial state is the state shown in Figure 3(b) (long axis direction is vertical), and the final state is the state shown in Figure 3(a) (long axis direction is horizontal). In this case, the period from the initial state to the final state is approximately 25 minutes. In this measurement, the detection intensity is set to zero when the device is turned on, and in Figure 7, the origin of the horizontal axis (elapsed time 0) does not coincide with the time when the device was turned off. However, the time course of the detection intensity thereafter reflects the time course of the orientation of the liquid crystal molecules 15A, similar to the case in Figure 6. As shown here, when the orientation returns from the vertical direction to the horizontal direction (initial state), the orientation fluctuates rapidly at first, and then fluctuates slowly, eventually returning to the initial state.
[0024] In Figures 6 and 7, the orientation of the liquid crystal molecule 15A shown on the upper side (horizontal and vertical axis) is described as being horizontal in the initial state of Figure 6 and the final state of Figure 7, and vertical in the final state of Figure 6 and the initial state of Figure 7. However, in reality, this orientation is not strictly horizontal or vertical, and the transmittance (detection intensity) corresponding to each orientation state varies according to this variation. However, the changes in the orientation over time (the situation in which transmittance increases or decreases) between these states are similar.
[0025] In other words, in conventional liquid crystal devices, the orientation state of the liquid crystal molecules 15A can be controlled by voltage, but the time required for this change is long, 2 minutes in the case of Figure 6 and 25 minutes in the case of Figure 7.
[0026] In contrast, in the liquid crystal device 1 described above, by using both an electric field E and a magnetic field H, and applying the magnetic field H when the electric field E is off, the time required for the orientation state of the liquid crystal molecules 15A to change can be significantly reduced. Figure 8 shows the measurement results of transmittance T when the on / off cycles of the electric field E (on / off of a 200V voltage) and the on / off cycles of the magnetic field H (500mT) are repeated alternately in a shorter time compared to Figure 7. Here, the on / off timing of the electric field E and magnetic field B, and the orientation state of the liquid crystal molecules 15A at each point in time are shown in the figure. Also, similar to Figure 6, the phase difference imparted to the terahertz wave by the liquid crystal layer 15 is shown. Unlike in Figure 6, this phase difference fluctuates between 0 and 2π. Furthermore, the orientation, especially when off, is estimated to be slightly deviated from the horizontal direction.
[0027] In this case, the time required for the orientation change was 3.1 seconds when the electric field E was switched from off to on, and 9.1 seconds when the magnetic field H was switched from off to on. That is, when the on / off of the magnetic field H perpendicular to the electric field E is used in combination with the on / off of the electric field E, the time required for the orientation change of the liquid crystal molecules 15A is shortened. For this reason, the phase control of the terahertz wave W can be performed at high speed using this liquid crystal device 1. This is thought to be because, by using the application of a high voltage and high intensity magnetic field for a short time, the part in Figure 7 where the orientation changes rapidly immediately after being switched off becomes particularly fast. At this time, the function of this liquid crystal device 1 is the same as that described in Patent Document 2, except for the response speed of the orientation change. For this reason, the same measurements as those described in Patent Document 2 can be performed in a shorter time using this liquid crystal device 1.
[0028] For example, using this liquid crystal device 1, the phase of the terahertz wave W can be controlled and the refractive index of the sample can be measured, similar to the method described in Patent Document 2. As described in Patent Document 2, it is effective to use the material shown in Figure 2, which has low absorption anisotropy of terahertz waves, as the liquid crystal molecule 15A.
[0029] Figure 9 shows the apparatus configuration for measuring the refractive index of sample 100 by modulating the phase of a terahertz wave W. This configuration corresponds to the configuration in Figure 5 where sample 100 is inserted immediately after liquid crystal device 1. A phase difference is applied to the terahertz wave W by both liquid crystal device 1 and sample 100, and the phase difference by the former is variable. In other words, the liquid crystal device 1 functions as a phase modulator in this case. This configuration is the same as the phase inspection apparatus described in Patent Document 2, except for the use of liquid crystal device 1.
[0030] Here, the birefringence (difference in refractive index between the long axis and short axis) Δn of the material in Figure 2 is 0.17, and if the thickness d of the liquid crystal layer 15 is 800 μm, the corresponding retardation (optical path difference length) is 136 μm. When the frequency of the terahertz wave W is 2.5 THz, the phase difference corresponding to this retardation is 415°, which is larger than the phase difference of 360° required in the above measurement.
[0031] Therefore, under these conditions, for example, the phase difference applied to the terahertz wave W can be varied in four ways (π / 2, π, 3π / 2, 2π°) as described in Patent Document 2, and the phase difference generated in sample 100 can be calculated, thereby allowing the value of the birefringence Δn to be determined.
[0032] Here, the angle of the liquid crystal molecule 15A in the liquid crystal device in Figure 3 with respect to the horizontal direction (vertical direction in Figure 9) along the long axis is θ. LC In this case, if no voltage is applied, then θ LC When the initial angle is 0°, and the orientation changes sufficiently when a voltage is applied, then the initial angle becomes θ. LC The angle is 90°. The polarization angle θ1 transmitted through polarizer 41 was set to 45°, and the polarization angle θ2 transmitted through polarizer 42 was set to -45°, which is perpendicular to this. An X-cut quartz substrate was used as sample 100, and its optical axis was set to 90° using the same reference as θ1 and θ2. The birefringence value Δn of this sample measured under these conditions using the same method as in Patent Document 2 was 0.05.
[0033] Figure 10 shows the time course of transmittance T when this measurement was actually performed, corresponding to Figure 8. Figure 8 shows the results when sample 100 is not present in Figure 9, and Figure 10 shows the results when sample 100 is present. In the case of Figure 10 as well, it can be confirmed that the time required for the change in orientation state has been shortened. Specifically, the time required for the change in orientation was 3.4 seconds when the electric field E was turned from off to on, and 11.8 seconds when the magnetic field H was turned from off to on. These values are less than 1 / 10 of those when no magnetic field is used. In other words, the above measurement can be performed at high speed.
[0034] Furthermore, Figure 11 shows the orientation angle θ of the liquid crystal molecule 15A with transmittance T in Figure 9. LC The dependence was calculated using the Jones matrix method. Figure 11(a) shows the result when sample 100 is absent (corresponding to Figure 8), and Figure 11(b) shows the result when sample 100 is present (corresponding to Figure 10). The shape of this change generally matches the shape of the orientation of liquid crystal molecules 15A from horizontal to vertical in Figures 8 and 10. Therefore, the change in the orientation of liquid crystal molecules 15A described above, or the measurement results above, are presumed to be valid. Furthermore, in Figure 10, qualitatively similar results are obtained not only when the electric field E is on to off, but also when the magnetic field H is on to off, indicating that this orientation is also controlled by turning the magnetic field H on and off.
[0035] As described above, by using both the on / off switching of the electric field E and the magnetic field H, the orientation of liquid crystal molecules can be controlled at a much faster rate compared to using only the electric field E. In the above example, the polarization state or phase difference of the terahertz wave was controlled by the liquid crystal, but similar control is possible for any characteristic that can be controlled by the orientation of liquid crystal molecules in terahertz waves, and this control can be performed at high speed with the above configuration.
[0036] Furthermore, although the liquid crystal molecules shown in Figure 2 were used in the above example, it is clear that the above configuration is also effective when using other liquid crystal molecules with similar properties. The materials for the substrate and transparent electrodes (electrodes) can also be appropriately set according to the required properties. [Explanation of Symbols]
[0037] 1.9 Liquid Crystal Devices 10 circuit boards 11 Transparent electrode (electrode) 12. Alignment film 13 Spacers 15 Liquid crystal layer 15A liquid crystal molecule 20 LCD cells 21A, 21B Magnetic field application means 22 Voltage application means 30 light source 31 Excitation light source 32 Gas laser oscillators 41, 42 Polarizer 43 detectors 100 samples C CO2 laser light E electric field H magnetic field W Terahertz waves
Claims
1. A liquid crystal device in which the orientation of liquid crystal molecules is controlled, A liquid crystal cell having a laminated structure in which a liquid crystal layer containing the liquid crystal molecules is sandwiched between two electrodes, A voltage application means for applying a voltage between the two electrodes, A magnetic field application means for applying a magnetic field to the liquid crystal layer along the in-plane direction of the liquid crystal layer, It is equipped with, A liquid crystal device characterized in that the polarization state of the terahertz wave after it has been transmitted through the liquid crystal cell along the normal direction of the electrode is controlled by the voltage and the magnetic field.
2. The liquid crystal device according to claim 1, characterized in that the on / off switching of the voltage and the on / off switching of the magnetic field are repeatedly controlled alternately.
3. The liquid crystal device according to claim 1 or 2, characterized in that the liquid crystal molecule is a nematic liquid crystal having hydrogen bonds and only alkyl chains at its molecular terminal groups.
4. A phase modulation device characterized by using the liquid crystal device according to claim 1 or 2 and modulating the phase difference for each polarization direction of the terahertz wave by controlling the voltage and the magnetic field.