Application Cases

Experiment Name: Optical Diffraction of Bent-Core Nematic Liquid Crystal Cells

Test Purpose: This chapter conducts optical diffraction experiments on the bent-core nematic (BCN) liquid crystal cells used in the study. It describes the establishment of the optical path for the diffraction experiments, analyzes the relationship between the polarization states of the incident and emergent light, and subsequently presents the grating diffraction efficiency obtained from the experiments along with relevant analysis. This holds significant implications for the potential future application of BCN liquid crystals in tunable liquid crystal gratings and their further development.

Test Equipment: Voltage amplifier, Signal generator, Hot stage controller, Attenuator, Polarizer, Optical power meter, etc.

Experimental Process:

Figure 1: Experimental optical path for measuring the diffraction efficiency of the BCN liquid crystal grating

The optical path for the diffraction experiment is shown in Figure 1. A red laser with a wavelength of 632.8 nm was used. An attenuator was employed to reduce the laser intensity, followed by an adjustable aperture or pinhole to obtain a well-defined beam. Since the laser emits linearly polarized light with a fixed polarization direction, a λ/4 waveplate was used to convert the linearly polarized light into circularly polarized light. Subsequently, a polarizer was used to convert the circularly polarized light back into linearly polarized light. By adjusting this polarizer, linearly polarized light with any desired polarization direction could be obtained. The linearly polarized light diffracts after passing through the grating structure within the liquid crystal cell, and the light intensity is measured using an optical power meter. An adjustable aperture is placed in front of the detector, or the aperture/pinhole is directly attached to the detector, to control the area of light collected by the detector.

The external driving voltage for the liquid crystal cell was provided by a signal generator. Due to the limited voltage amplitude produced by the signal generator, a voltage amplifier was used between the signal generator and the liquid crystal cell to achieve the voltage amplitude required for the experiment. Since the nematic phase of the BCN liquid crystal used exists at temperatures above 90°C, a hot stage (LTS350) was used for heating, and a hot stage controller (TP94) was used to control the temperature of the liquid crystal cell, with a precision of ±0.1°C.

Figure 2: Variation of grating diffraction efficiency with externally applied voltage at different temperatures for liquid crystal test cells 1 to 6 (a) Cell 1 (b) Cell 2 (c) Cell 3 (d) Cell 4 (e) Cell 5 (f) Cell 6

When measuring the diffraction efficiency, the incident polarized light was chosen to have the same polarization direction as the horizontal rubbing direction on the substrate surface of the liquid crystal cell. As shown in Figures 2(a)-2(f), the variation of diffraction efficiency with the amplitude of the externally applied voltage at different temperatures is presented for the six liquid crystal test cells used in the experiment. It can be observed that as the temperature increases, the diffraction efficiency curves shift significantly to the right. Furthermore, within the DC driving voltage range of U ≤ 80 V, except for the curve corresponding to Cell 6 (Figure 2f), the diffraction efficiency curves for the other cells essentially exhibit two local maxima. Based on Figure 2(f), it can be inferred that local maxima would still exist even when the external driving voltage increases beyond U = 80 V. From Figure 2(e), an extremum appears at an external voltage U = 60 V and a cell temperature T = 110°C, but this value is very small. It can be inferred that multiple extrema also exist at other temperatures; however, the diffraction efficiency at these extrema is too low to be detected effectively during experimental measurement.

From Figures 2(a)-2(f), it can be found that as the temperature increases, the first maximum value of the diffraction efficiency initially increases and then decreases. Furthermore, comparing Figures 2(a) and 2(b), 2(c) and 2(d), and 2(e) and 2(f) respectively, it can be seen that the first extremum in the diffraction efficiency curves for Cells 1, 3, and 5 appears more abruptly, unlike the relatively gentler appearance of the first extremum in the curves for Cells 2, 4, and 6. The reason for this phenomenon might be that this grating structure is originally composed of flexoelectric domains. Compared to Cells 2, 4, and 6, Cells 1, 3, and 5 have smaller cell gaps (experimental measurements show the gap is about half of the latter). Flexoelectric domains have a certain volume. When the cell gap is small, these domains may not have sufficient space to rotate or transform, leading to some abrupt changes.

Experimental Results:

From Figures 2(a) to 2(d), it can be observed that no diffracted light intensity was detected when the external driving voltage was U < 20 V, and no diffraction spots were visibly observed. However, under a polarizing microscope, flexoelectric domains, which resemble a grating structure, do exist when the applied driving voltage satisfies 10 V < U < 20 V. The reasons for this discrepancy are twofold: Firstly, at low voltages, although the refractive index perpendicular to the substrate rubbing direction exhibits a periodic distribution in the formed grating structure, the difference between the maximum and minimum refractive indices is small, resulting in an insignificant diffraction effect. Secondly, the polarization direction of the incident light was parallel to the rubbing direction on the substrate surface, and no analyzer was used for the emergent light. In contrast, the flexoelectric domain grating structure was observed under the polarizing microscope with crossed polarizers (analyzer perpendicular to the polarizer). If the polarizer of the microscope is adjusted to be parallel to the substrate rubbing direction and no analyzer is used, the flexoelectric domains cannot be observed through the microscope eyepiece. Therefore, the polarization state of the emergent light must be related to the polarization state of the incident light.

Voltage Amplifier Recommendation: ATA-2161

Figure: ATA-2161 High-Voltage Amplifier Specifications

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