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Abstract
The opto-electrically and electro-optically controllable diaphragm aperture in a poly(N-vinylcarbazole) (PVK) film-coated tandem-90°-twisted nematic liquid crystal (TNLC) cell is investigated. The tandem-90°-TNLC cell assembled by two common 45°-TNLC cells can be adopted for incident linearly polarized lights at any linear polarization direction. The variations of diaphragm aperture made by a PVK film-coated tandem-90°-TNLC cell are demonstrated opto-electrically and electro-optically. The differences in the degree of linear polarization between the tandem-90°-TNLC and single-90°-TNLC cells for demonstrating diaphragm apertures are investigated. The dynamic transmittance, related to orientation of LCs resulting from the built-in electric fields induced by the diffused positive and negative ions in LC bulk, the induced electric fields generated by the diffused ions in LC bulk and conductive PVK films, and the ion-moving induced force, of the PVK film-coated tandem-90°-TNLC cell is studied, and the possible mechanism based on dynamic distribution of ions (impurities) in LC bulk is also proposed. The proposed mechanism of the dynamic transmittance phenomenon can be a useful reference to the developments of other PVK film-coated LC cells. Moreover, the PVK film-coated tandem-90°-TNLC cell can be considered a controllable diaphragm aperture shutter if the above ion-induced issues are overcome.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Twisted nematic liquid crystal (TNLC) is one of the most common display technologies worldwide [1,2]. The versatile TNLC can be applied in several electro-optical devices, such as light shutters, asymmetrical devices, and gratings [3–11]. The angle between the polarization direction of incident linearly polarized lights (LPLs) and LC director at the entrance plane of a TNCL cell is defined as the β angle. A single-90°-TNLC cell can completely rotate the polarization direction of incident LPLs for 90°, and the β angle must be limited to 0° or 90°. A tandem-90°-TNLC cell comprising two conventional 45°-TNLC cells can be used to eliminate the dependence on β angle and incident LPL wavelength [11].
Poly(N-vinylcarbazole) (PVK) film-coated LC cells have been widely investigated recently. PVK is a photoconductive polymer. In brief, the effective applied direct current (DC) field in a LC cell fabricated by two PVK film-coated indium tin oxide (ITO) glass substrates is reduced due to the built-in electric field induced by the ion-aggregation onto the outer surfaces of PVK films. The ion-aggregation onto the PVK film surfaces result from the applied DC field [12–18]. The induced built-in electric field is reduced by illumination with suitable UV light to increase the effective applied electric field in LC bulk.
Diaphragm apertures are commonly adjusted by mechanically tuning several movable opaque blades [19]. Various alternative methods to control diaphragm apertures are widely investigated. The electrically and opto-thermally adjustable liquid diaphragm apertures have been successfully demonstrated [20,21]. The micro-electro-mechanical system technology is also applied for the stepwise tuning of diaphragm apertures [22]. Diaphragm apertures are also adjusted using polymer-dispersed LCs and TNLCs [14,23–27]. Fuh et al. reported the smart optical control of diaphragm aperture using a single PVK film-coated TNLC cell with a reverse bullseye density filter [14]. Such diaphragm aperture is available only when the β angle is 0° or 90°. On the basis of their results, we will demonstrate the β-independent diaphragm aperture based on a PVK film-coated tandem-90°-TNLC cell.
In this paper, the opto-electrically and electro-optically controllable diaphragm aperture in a PVK film-coated tandem-90°-TNLC cell was investigated. The tandem-90°-TNLC cell consisting of two conventional 45°-TNLC cells can be used for incident LPLs with any linear polarization direction. Thus, such a LC diaphragm aperture was β independent. The variations of the diaphragm aperture fabricated by the PVK film-coated tandem-90°-TNLC cell were demonstrated opto-electrically and electro-optically. The degree of linear polarization (DoLP) of the output light through the diaphragm aperture made by a tandem-90°-TNLC and a single-90°-TNLC cells was studied. The DoLP of the former should theoretically be higher than that of the latter. The dynamic transmittance, which is related to the orientation of LCs resulting from the built-in electric fields induced by the diffused positive and negative ions in LC bulk, the induced electric field generated by the diffused ions in LC bulk and conductive PVK films, and the ion-moving induced force, of the PVK film-coated tandem-90°-TNLC cell is investigated, and the corresponding possible mechanism based on dynamic distribution of ions (impurities) in LC bulk is proposed. We will focus on the investigation of the ions (impurities) distribution in the system just after the applied DC field is turned off. The proposed mechanism of the dynamic transmittance phenomenon can be a useful reference to the further studies of other PVK film-coated LC cells. Furthermore, the PVK film-coated tandem-90°-TNLC cell can also work as a controllable diaphragm aperture shutter if the aforementioned ion-induced issues can be solved.
2. Fabrication of a tandem-90°-TNLC cell
The adopted nematic LCs were HFW59200-200 (Fusol-material, Taiwan), whose ordinary and extraordinary refractive indexes were no = 1.489 and ne = 1.597, respectively, at 20 °C and λ = 589 nm. The photoconductive material, ∼1.64 wt% PVK powder, was mixed homogeneously in ∼98.36 wt% chlorobenzene solvent. The solution was spin-coated onto two ITO-coated glass substrates and then baked at 120 °C for 60 min in an oven. Subsequently, the two PVK-coated substrates were mechanically rubbed. The coated-PVK film can be mechanically rubbed to achieve planar alignment of LCs with an easy axis perpendicular to the rubbing direction [15,16]. The two empty 45°-TNLC cells, each of them was assembled by the two substrates, were then filled with the adopted nematic LCs. The measured cell gap of one cell was ∼17.5 µm, and that of the other was ∼17.6 µm. The cell gaps were measured by multiple beam interference using a white light lamp [11,28]. The cell gap difference between the two cells must be as small as possible for demonstrating the tandem-90°-TNLC cell with good performance [11]. Finally, the tandem-90°-TNLC cell was assembled by arranging the two 45°-TNLC cells based on the configuration shown in Fig. 1, which presents the schematic configuration of the tandem-90°-TNLC cell comprising 45°-TNLCs#1 and 45°-TNLCs#2. As illustrated in Fig. 1, the yellow double-head arrows on the top and bottom substrates of the two 45°-TNLC cells, which are respectively notated as At and Ab, represent the LC alignment directions. The white double head arrow indicates the projection of the Ab of the 45°-TNLCs#2 onto the top substrate of the 45°-TNLCs#1. The key is that the At of the 45°-TNLCs#1 must be perpendicular to the Ab of the 45°-TNLCs#2. The bottom and top substrates, coated with PVK film in the 45°-TNLCs#1 and 45°-TNLCs#2, respectively, were mechanically rubbed along the y-axis, and the angles between the At (Ab) in 45°-TNLCs#1 (45°-TNLCs#2) and + y-axis was 135° (45°), as shown in Fig. 1.
3. Experimental setup
Figure 2 shows the experimental setup to measure the normalized transmittance as a function of applied DC voltage (T–V) curve of a tandem-90°-TNLC cell illuminated with a purple laser (λ = 405 nm) at different intensities. The intensities of the employed He–Ne and purple lasers were controlled by neutral density filters (NDF), and the lasers were expanded by beam expanders and filtered by iris#1 and iris#2, respectively. An apodizing reverse bullseye (ARB) filter (Edmund optics, USA) was placed between the iris#2 and the lens, as shown in Fig. 2. A focusing lens was placed behind the ARB filter. The focusing lens was used to shrink the light spot diameter of the purple laser illuminated onto the tandem-90°-TNLC cell, and simultaneously enhance the purple laser intensity. The light spot of the He-Ne laser after passing through the iris#1 was tuned to be smaller than that of the focused purple laser illuminated onto the tandem-90°-TNLC cell for T-V curve measurement [Fig. 4]. The beam size of the purple laser after passing through the lens illuminated onto the tandem-90°-TNLC cell was approximately 0.35 cm. The purple laser was used to reduce the induced built-in electric field to increase the effective applied DC field in the tandem-90°-TNLC cell via the photo-conductive effect. The conductivity of the coated PVK film also increased with the increase of the illumination of purple laser intensity [14]. The used He–Ne and purple lasers were simultaneously illuminated onto the tandem-90°-TNLC cell during the following experiments. Figure 3 presents the optical density of a light passing through the ARB filter as a function of the position of the ARB filter, diameter of which is 25 mm [Data shown in Fig. 3 are obtained from Edmund Optics 2014]. The optical density range of the ARB filter is 0.04–2.0, and the intensity of purple laser gradually decreases from the center to the edge after passing through the ARB filter. Because the purple laser intensity gradually decreases from center to edge due to the ARB filter, the measured total power (mW), instead of the intensity (mW/cm2), of the purple laser will be given in all following discussion. However, the purple laser beam spot was not a perfect circle and the surface treatments of the LC cells were not uniform, so the shape of the diaphragm aperture demonstrated in the following experiments was not a perfect circle. The inset in Fig. 2 shows that the two 45°-TNLC cells of the tandem-90°-TNLC cell were simultaneously applied with the same DC electric field by a function generator with a voltage amplifier. The transmission axes of the polarizer and analyzer were along x- and y-axes, respectively. The arrangement of the LC alignment directions of the two 45°-TNLC cells shown in Fig. 2 was identical to that depicted in Fig. 1.
Fig. 2. Experimental setup drawn using Microsoft Powerpoint for measuring the normalized transmittance as a function of the applied DC voltage curve by illumination with different purple laser powers.
Fig. 3. Optical density as a function of the position of the used ARB filter. D (25 mm) is the diameter of the filter. The maximum optical density (end density) is 2.0.
Fig. 4. (a) T–V curves of three cases with illuminations of various purple laser powers and (b) T–P curves of two cases with the applications of two different DC electric fields onto the PVK film-coated tandem-90°-TNLC cell. The experimental setup is presented in Fig. 2.
4. Results and discussion
Figure 4(a) shows the measurement of the T–V curve of the tandem-90°-TNLC cell illuminated with various purple laser powers based on the experimental setup plotted in Fig. 2. The blue curve represents the T–V curve of the tandem-90°-TNLC cell without purple laser illumination. The orange and gray curves describe the T–V curve of the tandem-90°-TNLC cell continuously illuminated with a purple laser at the powers of ∼30 and ∼65 mW, respectively. The threshold voltage (Vth) and operation voltage (Vop) decreased with the increase of purple laser power because the induced built-in electric field in the LC bulk was reduced as the purple laser intensity was increased. The three curves approached ∼0% when all LCs applied with suitable voltages were rotated to be perpendicular to the substrates. The T–V curves plotted in Fig. 4(a) are similar with those in Refs. [14] and [17]. Figure 4(b) shows the measurement of the transmittance as a function of power of purple laser (T–P) curve of the tandem-90°-TNLC cell applied with two different DC fields based on the experimental setup depicted in Fig. 2. The red and green curves represent the T–P curve of the tandem-90°-TNLC cell continuously applied with DC fields of 18 and 27 V, respectively. Based on the experimental results marked in the black dashed circle in Fig. 4(b), with the illumination of the same purple laser power, the transmittance of the red curve was maintained at 100%, whereas that of green curve decreased from 100% to ∼76%. This finding can be understood because the applied DC fields of 27 V (18 V) was higher (lower) than the Vth of the LCs, as shown in the green and red curves in Fig. 4(b), respectively. The transmittances depicted as green and red curves remained invariant when the power of the purple laser was higher than ∼25 and ∼45 mW, respectively. Notably, the LC orientation under the conditions shown in the red and green curves were not completely perpendicular to the substrates. The two illumination powers, ∼25 and ∼45 mW, were defined as the operation power (Pop) of the purple laser for the two cases. Thus, the tandem-90°-TNLC cell must be applied with a high voltage DC field to reduce Pop. The transmittance of the red curve is higher than that of the green one because the applied DC field of green curve (27 V) is higher than that of red curve (18 V). Accordingly, the LCs rotated by the applied electric field of 27 V should be more perpendicular to the substrates than those rotated by the applied electric field of 18 V. Finally, the normalization method for the curves plotted in Fig. 4 is described as follows. The maximum/minimum transmittances of the three curves plotted in Fig. 4(a) were normalized to 100%/0% individually, and the maximum transmittances of the two curves shown in Fig. 4(b) were also normalized to 100% individually. This is because the background light intensities of the measurements of the five curves were different. The minimum transmittances of the red and green curves depicted in Fig. 4(b) were not normalized to 0%, because, on the basis of the gray curve in Fig. 4(a), the normalized transmittances are not 0 when the applied DC voltages are 18 and 27 V.
Variations of the ring-type-diaphragm aperture of the tandem-90°-TNLC cell shown in Fig. 5 are discussed as follows. The experimental setup to investigate the variations of ring-type-diaphragm aperture of the tandem-90°-TNLC cell was identical to that shown in Fig. 2, except for the following two conditions. An electrically polymer laser speckle reducer (EP-LSR, Optotune Corp.) for He-Ne laser was placed between the NDF and the beam expander, and the photo-detector was replaced by a screen to observe the variations of the diaphragm aperture. EP–LSR was used to reduce the speckle onto the screen for improved observation. The beam size (diameter) of the He-Ne laser adjusted by iris#1, and that of the focused purple laser, illuminated onto the tandem-90°-TNLC cell, were approximately 0.65 and 0.35 cm, respectively. The beam size of the He-Ne laser must be larger than that of the purple laser illuminated onto the tandem-90°-TNLC cell for demonstrating the switching of the ring-type aperture. Because the purple laser beam spot was not a perfect circle and the surface alignment of the cell was not uniform, the shape of dark region shown in Fig. 5 was not a perfect circle.
Fig. 5. Variation of ring-type-diaphragm aperture of the tandem-90°-TNLC cell by (a) application with various DC fields and continuously illumination with a constant purple laser power (∼65 mW) and (b) illumination with different purple laser powers and continuous application with a constant DC field (27 V).
Figure 5(a) shows the variations of the ring-type-diaphragm aperture of the tandem-90°-TNLC cell under cross polarizers tuned by the continuous illumination with a purple laser with a constant power of ∼65 mW and the applications of various DC fields (0–27 V), and such aperture demonstrated here is called the opto-electrically controllable diaphragm aperture. Given that the intensity distribution of the purple laser gradually decreased from center to edge after it was passed through the ARB filter, the Vth gradually increased from center to edge along the radial direction, as qualitatively shown in Fig. 6. The Vth_cen and Vth_edge represent the Vth at the center and edge of the purple laser spot [Vth_cen < Vth_edge], respectively. Thus, the LCs around the center area started rotating to be perpendicular to the substrates when the applied DC field was 9 V. Because the Vth gradually increased from center to edge due to the gradient purple laser intensity, the LCs outside the center area were not rotated to be perpendicular to the substrates because the applied DC field of 9 V was not high enough. Accordingly, when the applied DC field was further increased, an increasing number of LCs with the distribution along the radial direction were rotated to be perpendicular to the substrates. Figure 5(b) shows the variations of the ring-type-diaphragm aperture of the tandem-90°-TNLC cell under cross polarizers switched by the continuous application of a constant DC field (27 V) and the illuminations of a purple laser with different purple laser powers, and such an aperture demonstrated here is called the electro-optically controllable diaphragm aperture. As shown in Fig. 6, on the basis of Figs. 4(a) and 5(b), illumination with the purple laser (2 mW) reduced the Vth of LCs around the center area, and the LCs there can be rotated to be perpendicular to the substrates by the constant applied DC field (27 V). The Vth gradually increased from center to edge because of the decreasing intensity of the purple laser from center to edge, so the applied DC field (27 V) was not high enough to rotate LCs outside the center area to be perpendicular to the substrates with the illumination of purple laser (2 mW). Accordingly, when the illumination intensity of purple laser was increased to reduce the Vth outside the center area, an increasing number of LCs there along the radial direction can be further rotated to be perpendicular to the substrates. In summary, the diameters of the dark region can be tuned opto-electrically and electro-optically, as shown in Figs. 5(a) and 5(b), respectively.
Fig. 6. Schematic of the distribution of Vth onto the TNLC cell illuminated with a purple laser through an ARB filter.
Figures 7(a) and 7(b) present the method for measuring the DoLP as a function of the β angle (DoLP-β) curve of the tandem-90°-TNLC cell and the single-90°-TNLC cell, respectively. The two substrates of the single-90°-TNLC cell were coated with PVK films. The definitions of the yellow and white colorful double-head arrows and the corresponding notations indicated in Fig. 7 are identical to those shown in Fig. 1. A photo diode was placed behind the analyzer to measure the transmittance, and the probe beam used here was a He–Ne laser. The double-head red and green arrows represent the transmission axis of the polarizer and analyzer, respectively. Both of the β angles in Figs. 7(a) and 7(b) were set to 0°, which is defined as the angle made by the double-head red arrow and x-axis. The transmission axis of the analyzer was initially set to be perpendicular to that of the polarizer. The DoLP values of the output LPLs were measured as follows. For simplicity, we assume that the polarization directions of incident LPLs with various β angles are 90° rotated after passing through the tandem-90°-TNLC cell. Thus, the polarization rotation angle error, which is defined as the angle between the expected polarization direction (90°) and the major-axis direction of the output elliptically polarized light, was neglected here [11]. Accordingly, the transmission axis of the polarizer was rotated from 0° to 90° with the transmission axis of the analyzer always being perpendicular and parallel to that of the polarizer to find the maximum (Tmax) and minimum (Tmin) transmittances, respectively. Finally, the DoLP values were calculated based on the following equation, DoLP = (Tmax − Tmin)/(Tmax + Tmin). Figure 8 shows the DoLP-β curves of the tandem-90°-TNLC cell (blue curve) and a single-90°-TNLC cell (green curve). Clearly, the DoLP values shown by the blue curve were higher than those depicted by the green curve when the β angle was shifted from around 20° to 70°. Theoretically, the polarization rotation resulting from the tandem-90°-TNLC cell is β independent, and all of the DoLP values of the output LPLs plotted by the blue curve should be close to 1 [11]. The experimentally obtained DoLP values of the output LPLs lower than 1 shown in the blue curve (the maximum/minimum DoLP was ∼0.91/∼0.79), and the DoLP values of the output LPLs (β angle from around 0° to 20° and around 70° to 90°) shown in the green curve which were higher than those shown in the blue curve, could be caused by the errors in the fabrication and assembly of the two 45°-TNLC cells. These errors also caused polarization rotation angle error after the linearly polarized He-Ne laser was passed through the tandem-90°-TNLC cell [11]. The fabrication errors could be due to the imperfectly precise LC alignment directions onto the substrates of 45°-TNLCs#1 and 45°-TNLCs#2, the At of the 45°-TNLCs#1 was not fully perpendicular to the Ab of the 45°-TNLCs#2 (Fig. 7), and other errors.
Fig. 7. Experimental setup for measuring the DoLP-β curves of (a) the tandem-90°-TNLC cell and (b) the single-90°-TNLC cell. The definitions of all the yellow and white arrows and the corresponding notations are identical to those shown in Fig. 1.
Fig. 8. DoLP-β curves of the tandem-90°-TNLC cell (blue curve) and the single-90°- TNLC cell (green curve).
The blue curve shown in Fig. 9(a) shows the dynamic transmittance of the normally white tandem-90°-TNLC cell with time when the two 45°-TNLC cells were illuminated with a purple laser with a power of ∼60 mW (diameter of purple laser was ∼0.35 cm) and applied with a DC field of 27 V by adopting the experimental setup described in Fig. 5. The orange curve indicates the statuses of the applied DC field, that is, VDC_on (27 V) and VDC_off (0 V). The transmittance quickly decreased to the minimum when the DC field was turned on, and then transmittance gradually increased during the VDC_on status. The possibly mechanism of the dynamic transmittance phenomenon is proposed as follows. The increased transmittance could result from the built-in electric field induced by the applied DC field so that the effective electric field in LC bulk was reduced. The increase of transmittance stopped when the induced built-in electric field approached the maximum. On the other hand, the transmittance suddenly decreased when the applied DC field was turned off, and then slowly increased to the original value when the residual induced built-in electric field was gradually consumed and disappeared. The dynamic transmittance marked by the black dashed circle shows that the LCs were disturbed, so a vibration occurred during the increasing transmittance. Here, we will deduce the possible mechanism (Fig. 10) to describe the sudden decrease of the transmittance after the applied field was turned off as shown in Fig. 9(a) according to Refs. [12], [18], [29–34].
Fig. 9. (a) Dynamic transmittance (blue curve) of the tandem-90°-TNLC cell with PVK films coated onto the ITO films and the statuses (orange curve) of the applied DC field, VDC_on (27 V) and VDC_off, with time when the tandem-90°-TNLC cell was in normally white mode. (b) Dynamic transmittance (green curve) of the commercial 90°-TNLC cell without any coated PVK film onto the ITO films and the statuses (orange curve) of the applied a suitable DC field, VDC_on and VDC_off, with time when the commercial 90°-TNLC cell was in normally white mode. (c) Details of the dynamic transmittance of (b) within the time interval between 126 and 129 s.
Fig. 10. (a) Most of positive (negative) ions aggregate on the substrates applied with negative (positive) potential when the VDC is turned on. (b) Positive (negative) ions move normally in bulk when the VDC is just turned off. (c) Different electric fields/force applied onto the LCs in bulk and those close to the substrates. The purple glow around PVK films in (a), (b) and (c) represent they are continuously illuminated by purple laser.
The possible mechanism (Fig. 10) based on the dynamics of ions in LC bulk and those close to the substrates is discussed. The ions discussed as follows result from the impurities of the adopted materials in LC bulk. Figure 10(a) schematically shows the aggregations of the positive and negative ions in LC bulk onto the outer surfaces of the PVK films and those in the PVK films onto their inner surfaces, and when the LC cell is applied with a DC electric field and illuminated with a purple laser. The purple glow around the PVK films shown in Figs. 10(a)–10(c) represents that they are continuously illuminated by a purple laser. The diffusion rates of positive and negative ions toward the substrates as shown in Fig. 10(a) could depend on the illumination of purple light intensity. This is because the conductivity of the PVK film illuminated with a strong purple laser is relative high comparing with that illuminated with a weak purple laser [14]. In Fig. 10(a), the force to drift positive and negative ions from the LC bulk to outer surfaces of the coated PVK films results from the suitable applied DC field. The conductivity of PVK film without illumination of a purple laser is low, so nearly all positive and negative ions on the outer surfaces of the PVK films are not discharged [12,18]. On the basis of the blue curve plotted in Fig. 4(a), the required voltage to initiate the rotation of LCs is higher than at least 21.6 V, so the built-in electric fields in the ∼17.5 and ∼17.6 µm-thick PVK film-coated TNLC cells screen the external DC field up to 1.23 V/µm (21.6 V divided by the averaged cell gap). For reference, Kaczmarek et al. reported that the built-in electric field induced by the charges on PVK-LC interfaces can screen the external DC field up to at least 1.9 V/µm [12]. The difference of the two screen voltages could depend on ions/charges concentrations, cell gaps, LC materials, etc. The conductivity of PVK film increases with the increase of the intensity of the purple laser illumination, so parts of the positive and negative ions aggregating on the outer surfaces of PVK films are discharged. Accordingly, the effective DC field increases with the increase of conductivity of the coated PVK films [12,14,17]. On the basis of gray curve shown in Fig. 4(a), the required voltage to initiate the rotation of LCs is higher than at least 9 V, so the built-in electric field can still screen the external DC field, up to 0.51 V/µm (9 V divided by the average cell gap). The diffusion rate of ions in LC bulk could depend on the strength of the effective DC field [29,34]. Accordingly, we deduce that the diffusion rate of ions in LC bulk increases with the increase of the conductivity of the coated PVK films. When the applied VDC is turned off [12,18,29], the induced built-in electric field still existed for a short time. Figure 10(b) schematically shows that the positive and negative ions just start to diffuse from the substrates to the LC bulk. Moreover, because the direction of the built-in electric field (blue arrow) is normal to the substrates, the ion-moving direction (black arrow) is believed to be perpendicular to the substrates [32,33]. Hence, the diffusing ions provide forces (green arrow) normal to the substrates to help align the LCs close to the substrates to be parallel to the ion-moving direction. When the ions diffuse from the outer surfaces of the PVK films, the induced electric fields [red arrows in Fig. 10(b)] generated by the diffused positive and negative ions in LC bulk and the open-circuit potential conductive PVK films, keeping being illuminated with a purple laser, should be considered [34]. Actually, the strength of such induced electric fields, perpendicular to the substrates, should be strong because the distances between the positive (negative) ions in LC bulk and the negative (positive) ions in the PVK films onto their inner surfaces are small. Overall, refer to Fig. 10(c), we deduce that the built-in electric fields (blue arrows), caused by the diffused positive and negative ions in LC bulk do align the LCs in bulk to be perpendicular to the substrates; on the other hand, the ion-moving induced forces (green arrows), and the forces provided by the induced electric fields, generated by the diffused positive/negative ions in LC bulk and the negative/positive ions in the conductive PVK film (red arrows), help align LCs close to the substrates to be perpendicular to the substrates as much as possible. Figure 10(c) represents the profile of 45°-TNLCs#1 for example. It is important to note that the LCs close to substrates are difficult to be completely rotated to be perpendicular to the substrates due to the surface anchoring energy (SAE) provided by the mechanically rubbed PVK films [1,35]. If the SAE is strong enough, the LCs close to the PVK films keep orienting to the original directions. The two fields and the ion-moving induced force cause the sudden decreases of the transmittance as shown in Fig. 9(a). The above discussion on the details shown in Fig. 10(b) is valid when the positive and negative ions just start normally diffusing to LC bulk from the outer surfaces of PVK films. Moreover, when the positive and negative ions further diffuse to the LC bulk, because the diffusion rates of positive and negative ions with different mass/shapes are different, the ions could diffuse toward various directions. Thus, the directions of the two electric fields (blue and red arrows) and ion-moving induced force (green arrows) shown in Fig. 10(b) are no longer perpendicular to the substrates [29,34]. Such electric fields and forces along various directions could disturb the orientation of LCs in bulk. Therefore, regarding the vibrations, marked as the black dashed circles, of the dynamic transmittance, shown in Fig. 9(a), we deduce that they are caused by the dynamic diffusion of ions in LC bulk to disturb the LCs, and/or by other factors [18]. Finally, the positive and negative ions randomly diffuse in LC bulk and PVK films to reach electronic equilibrium, so all induced electric fields (forces) vanish. The transmittance shown in Fig. 9(a) smoothly increases to the maximum eventually.
Furthermore, to study the influences of PVK films on the ions diffusion, Fig. 9(b) shows the dynamic transmittance (green curve) of a normally white mode of commercial 90°-TNLC cell (cell gap ∼5 µm, Instec Inc.) without any PVK film coating and with the application of a suitable DC field by adopting the experimental setup used in Fig. 7(b). The materials of the planar alignment films and conductive electrodes used in the 90°-TNLC cell are polyimide and ITO, respectively (data from Instec Inc.). The LC used in Fig. 9(b) was identical with that used in Fig. 9(a). The trends of dynamic transmittance shown in Figs. 9(a) and 9(b) are similar, two sudden decreases of the transmittance as shown in Fig. 9(b) can be observed at around 58 s and 128 s [18]. For example, Fig. 9(c), depicting the details of the dynamic transmittance of Fig. 9(b) within the time interval between 126 and 129 s, shows that the transmittance quickly decreased to the minimum when the DC field was turned off, and then the transmittance increased during the VDC_off status. In comparison with Fig. 9(a), the possible reasons that it takes relatively long required duration to gradually increase the transmittance during the VDC_on status and no vibration occurs in Fig. 9(c) after the DC field was turned off include the diffferent voltages of the applied DC field, the ion concentrations in bulk, alignment film material, LC material, cell fabrication, and the gap thickness [12,18,29,36–38].
By combing the results shown in Figs. 5(a) and 9(a), the PVK film-coated tandem-90°-TNLC cell can be considered a controllable diaphragm aperture shutter [39,40]. Shutter speed and aperture diameter of a diaphragm aperture shutter are controlled simultaneously using the same component. On the basis of Fig. 5(a), the PVK film-coated tandem-90°-TNLC cell with the continuous illumination of a purple laser at a constant power, the aperture diameter can be controlled by applying the voltage of the applied DC field, and the duration of shutter (shutter speed) in close state can be controlled by tuning the duration of the application of the applied DC field [40]. Table 1 summaries the operation of the shutter made by a PVK film-coated tandem-90°-TNLC cell in normally white (ring-type-diaphragm aperture) mode. However, to approach such controllable diaphragm aperture shutters, the issues depicted in Fig. 10 should be overcome. A possible method to solve the issues is briefly discussed as follows [29–31,41,42]. The PVK film-coated tandem-90°-TNLC cell can be immediately applied with another suitable reverse DC (DCrev) pulse, whose polarity is opposite to the operated DC field, just after the DC field is turned off. The polarities of the operated DC field and the DCrev pulse are opposite. Accordingly, such a DCrev pulse could eliminate the built-in electrical field, and cause the ions aggregating onto the PVK films to diffuse toward various directions into the LC bulk to reduce other fields and forces depicted in Fig. 10. The duration and the amplitude of the applied DCrev pulse should be optimized to avoid generating another undesirable built-in field.
Table 1. Operations of the shutter made by a PVK film-coated tandem-90°-TNLC cell in normally white mode.
5. Conclusions
The opto-electrically and electro-optically controllable diaphragm aperture in a PVK film-coated tandem-90°-TNLC cell was investigated. The electro-optic properties of the diaphragm aperture made by a PVK film-coated tandem-90°-TNLC cell and the differences of DoLP values between the tandem-90°-TNLCs and the single-90°-TNLC cell were also explored. The β-independent diaphragm aperture made by a tandem-90°-TNLC cell shows more potential for application in various optics fields than that made by a single-90°-TNLCs cell. Moreover, PVK film-coated tandem-90°-TNLC cell can work as a controllable diaphragm aperture shutter if the ion-induced issues [Fig. 10(b)] can be solved. Future work can focus on the studies of the deduced mechanism of Fig. 10, such as the electric field caused by ions in LC bulk and conductive PVK films, ion-moving induced force, influences of various purple laser intensities on the PVK conductivity and ion diffusion rate in LC bulk and the dynamic distribution of ions (impurities). The proposed mechanism (Fig. 10) can also be a useful reference to the dynamic transmittance investigation of other PVK film-coated LC cells. On the basis of the results in Fig. 9, the comparisons of dynamic transmittance of TNLC cell coated with PVK films and those coated with different alignment films, such as other types of polyimides, polyvinyl alcohol, and ITO, with applications of various DC fields should be further quantitatively studied [12,18,31,36–38]. Moreover, the proposed doughnut-shaped aperture can be applied as a tunable aperture to produce doughnut-shaped light pulse to modify the morphology of the amorphous silicon film [43], a special aperture for photography to produce doughnut-shaped bokeh [44,45], etc..
Funding
Ministry of Science and Technology, Taiwan (MOST) (MOST 106-2112-M-008-002-MY3).
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