April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
1201
Electrically induced red, green, and blue scattering in chiral-nematic thin films Yu-Cheng Hsiao and Wei Lee* Institute of Imaging and Biomedical Photonics, College of Photonics, National Chiao Tung University, Guiren Dist., Tainan 71150, Taiwan *Corresponding author: [email protected] Received January 13, 2015; accepted February 6, 2015; posted February 18, 2015 (Doc. ID 232396); published March 17, 2015 Cholesteric liquid-crystalline materials are abundant in nature such as condensed phases of DNA, plant cell walls, and chiral biopolymers. These self-organized helical structures produce unique optical properties, giving rise to the selective Bragg reflection of colorful light. In this Letter, we focus on the focal conic state of cholesteric liquid crystals and report on stable, tunable, and reversible color switching among red, green, and blue in polymer-stabilized cholesteric films. The experimental results indicate that, with appropriate voltage pulses, the electrically induced color switching of all six routes can be realized in a single cell reflecting green light. The scattered transmissive color persists at zero voltage due to the polymer stabilization. © 2015 Optical Society of America OCIS codes: (160.3710) Liquid crystals; (230.3720) Liquid-crystal devices; (160.1585) Chiral media; (160.4760) Optical properties. http://dx.doi.org/10.1364/OL.40.001201
The use of colors is important in optical technologies and color information. Some color is provided by nature such as plants, butterflies, birds, and beetles via petals, wings, or bodies due to structural variations. These surface structures result in selective reflections and produce beautiful colors as one sees. With a twist arrangement of mesogenic molecules along a helical axis and, in turn, a spatially periodic variation in refractive index, chiralnematic or cholesteric liquid crystals (CLCs) also exhibit Bragg reflections, producing color when the reflective wavelengths are in the visible spectrum. The selective Bragg reflection from a CLC has been exploited in various optical devices such as reflective displays [1,2], color filter arrays [3], multichannel filters [4–6], and mirrorless lasers [7–9]. The reflective color of the CLC can be tuned by varying the temperature or by applying an external field [10]. The color tuning can be achieved particularly by an applied voltage due to its easy operation. For CLCs, pitch elongation or compression by electric field has been utilized to induce a redshift or blueshift of the reflection band [11,12]. Because of the helical structure of a CLC, the wavelength of incident light being reflected is a function of the pitch P 0 . The central wavelength λ0 of the reflected light can be calculated by the equation λ0 hniP 0 , where hni is the average refractive index of the liquid crystal (LC). In recent years, it has been demonstrated that the reflective color can be electrically switched to blue or green from a cholesteric cell initially reflecting in red [13]. Moreover, the reversible lightdirected red, green, and blue reflections in another type of self-organized chiral LC systems, which are known as the blue phase (BP) materials, have also been demonstrated [14]. Indeed, both CLCs and BPs can possess wide reflective ranges in the visible. In this Letter, we present a scattering-transmissive, red-, green-, and blue-switchable device based on a polymer-stabilized CLC (PSCLC) under crossed polarizers, whose working principle is different from the wellknown reflective scheme. The focal conic (FC) state as a state in CLCs is a multi-domain optical scattering state, which is used as the “dark” state in a typical 0146-9592/15/071201-03$15.00/0
bistable CLC display composed of a black, light-absorbing layer beneath the cell without any polarizers [15]. As a matter of fact, the FC state is actually not colorless; color may remarkably appear through scattering under particular polarizations. The reason is that light is a transverse wave with the electric field vibrating perpendicularly to the propagation direction of the beam. If the incident light is linearly polarized, the electric field vibrates in a specific direction in a plane. Under specific conditions such as a cell with a small cell gap, multiple scattering of only a limited extent takes place among the CLC molecules. The emerging lights with residual polarizations are hence not randomly distributed. Based on this property, blueshift of the scattered color occurs in the FC state of a cholesteric cell agitated by an electric field pulse. Accordingly, electrically induced red–green–blue color scattering and color switching are realized in PSCLC films characterized simultaneously by optical stability, electrical tunability, and color reversibility. The PSCLC material used in this study is E7 (Merck) mixed with 3-wt.% prepolymer NOA65 and the chiral dopant R5011 (Merck) at various concentrations. The mixtures were injected into empty cells by capillary action in isotropic phase. Each empty cell is made of two 1.1-mm-thick indium–tin-oxide glass substrates coated with planar-alignment layers, yielding a cell gap of 6.1 0.5 μm. To polymerize the prepolymer, the cell was irradiated by ultraviolet light at 365 nm. The curing intensity was 10 mW∕cm2 , and the curing time was 100 s. The dopant concentrations of 3.03, 2.56, and 2.15 wt.% were adopted, permitting the photo-cured cells to reflect in the blue, green, and red ranges, respectively. The central wavelengths of the Bragg reflections in blue, green, and red correspond to 476 nm, 550 nm, and 654 nm, respectively, as shown in Fig. 1. Note that there are no differences just after the capillary introduction and after the subsequent photo-curing in the corresponding reflection spectra and that the oscillation in the curves arose from the optical interference between the surface boundaries of the CLC films. The transmission spectra of the © 2015 Optical Society of America
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OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Transmittance (%)
100 75 50 25 0 400
3.03 wt% 2.56 wt% 2.15 wt% 500
600
700
800
Wavelength (nm) Fig. 1. Transmittance spectra of three distinctive PSCLCs. The Bragg reflections occur in blue, green, and red from these cells impregnated with chiral dopant at concentrations of 3.03 wt.%, 2.56 wt.%, and 2.15 wt.%, respectively.
Fig. 2. Photographs of a PSCLC (i.e., G cell in this case) placed between crossed polarizers in three colorful FC states exhibiting red, green, and blue scattered light. Schematics of the corresponding FC structures are shown in the bottom.
PSCLCs were acquired with a fiber-optic spectrophotometer (Ocean Optics HR2000+). An arbitrary function generator (Tektronix AFG-3022B) was employed to supply various AC voltages at 1 kHz in the sinusoidal waveform. All experimental data were measured at an ambient temperature of 26 1°C. In order to describe and discuss the experimental results at convenience, the cells that reflect red, green, and blue colors are here designated the R, G, and B cells, respectively. For oblique Bragg reflections from a polycrystalline layer of CLC (rather than a single-domain planar texture), the expression for the wavelength of the reflection maximum was derived by Fergason [10]:
polymer-dispersed LC systems. In contrast, Rayleigh scattering, as originally formulated, is strictly applicable to small, dielectric (non-absorbing), spherical particles, which is generally preferred if applicable, owing to the complexity of the Mie scattering formulation. Here the FC textures are caused by light scattering from the multi-domains of the self-organized anisotropic LC molecules in the PSCLC cell. The appearance of colorful FC states is governed by two factors—Mie scattering and the birefringence property. The Mie scattering leads to the incompletely randomly polarized light in that the cell gap is not considered thick enough for the transmitted light through the cells. On the other hand, the scattering property unavoidably suppresses the brightness and, in turn, color saturation in the FC states. When the applied voltage increases, the LC molecules tend to re-orient themselves parallel to the field. The effective refractive index, thus, decreases with increasing voltage, causing the blueshift of the emerging light. It is clear from Fig. 2 that the blue scattering state exhibited the best color saturation when the G cell sandwiched between two crossed polarizers was switched by a high voltage of 40 V. This result is in agreement with the Rayleigh theory that the differential scattering intensity has a very strong dependence on the wavelength—it is inversely proportional to the fourth power of wavelength. When the high voltage is applied, the “grain” sizes of the multi-domain FC structure are reduced in the cell because the high voltage enforces the LC molecules to be re-oriented vertically [2]. Before the voltage is even high enough to completely unwind the helix and to induce the homeotropic state, it prompts many smaller sized helical ensembles or fragments in the FC structure. Here the initial (undisturbed) state of the G cell is the planar state at 0 V; it exhibits color change from red, green, to blue in various scattering FC states when the switching voltage of ∼5–55 V is applied. As the stimulating voltage goes beyond ∼55 V, the texture appears to be homeotropic, yielding a completely dark state. Figure 3 depicts nine representative images of colorful FC states in R, G, and B cells under a polarizing optical microscope (OLYMPUS BX51-P). For an arbitrary cell, the induced color in the FC state can be altered by varying the applied voltage. The largest extent of color
P 0 hni 1 −1 sin φ 1 −1 sin φ00 ; cos sin sin λ0 hni m 2 hni 2 where the integer m denotes the reflection order, φ represents the angle of incidence, and φ00 stands for the angle of reflection of the monochromatic light. One can see that the CLC with a smaller pitch can strongly reflect or scatter the high-frequency light. With an applied voltage pulse to modulate the angles, red, green, and blue colors in the optically stable FC states can be observed in three specific pitch ranges. It should be noted that Fergason’s equation cannot accurately describe the observed transmissive phenomenon for a scattering state. A rigorous mathematical expression is, therefore, highly desired. Figure 2 shows three photographs of a G cell scattering blue, green, and red light in the FC states under crossed polarizers on a slide film light box to illuminate the cell. The corresponding schematics of field-induced multidomains in the absence of a sustaining voltage are also illustrated in the bottom. Scattering in the FC state can be analyzed by the Mie theory, which encompasses the general spherical scattering solution (absorbing or nonabsorbing) of Maxwell’s equations without a particular bound on particle or domain size. Mie scattering from larger-sized particles or domains reduces to the limit of geometric optics and is weakly wavelength dependent. It produces the almost white or, more precisely, fluffily translucent appearance as seen in many typical
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
Fig. 3. Optical textures of three PSCLCs with chiral dopant concentrations of 2.15, 2.56, and 3.03 wt.%, yielding R, G, and B cells, respectively, in the FC states induced by various applied voltages as presented numerically. All micrographs were taken without a sustaining voltage across the thickness of each cell.
Transmittance (%)
change, covering the spectral range from red to blue, can be found in the G cell that was made to selectively reflect green light in the PSCLC film in the initial planar state. In comparison with the colorfully scattered light observed in the G cell, the R cell and B cell do not possess the sufficiently wide visible scattering range to generate a good variety of colors. The FC domains are larger in the R cell and smaller in the B cell. As a result, the R cell tends to scatter the red color, and the B cell tends to exhibit the blue FC texture under crossed linear polarizers. Here it is worth emphasizing that the color tuning by an applied electric field in the FC state is demonstrated in all of the samples, whereas the G cell distinctively possesses the widest tuning range of the visible spectrum— including red, green, and blue bands—in the scattering FC state. The transmittance spectra of the G cell switched by various applied voltages are illustrated in Fig. 4. Here
30 20 10 30 20 10 30 20 10 30 20 10
5V
15 V
25 V
45 V
500
600
700
Wavelength (nm) Fig. 4. Transmission spectra of a PSCLC; i.e., G cell, sandwiched between two crossed polarizers in various voltageinduced colorful FC states.
1203
the measured transmittance is said to be 100% when the probe beam propagates through the parallel polarizers without a cell being placed between them. The above-mentioned color shift becomes clear from these spectra in various FC states induced by increasing voltage from 5 to 45 V. As the switching voltage increased, the first red FC state appeared at 5 V. The cell then exhibited a light-yellow FC state at 15 V and a green one at 25 V. Finally, the texture appeared in blue when the voltage ascended to 45 V. It is worth reminding that each colorful FC state as seen in the crossed-polarizer scheme is optically stable. The switching from one to another color is direct and reversible by means of an appropriate voltage pulse. In other words, color tuning (from red to blue) and color stability can be simultaneously achieved in the FC state in a PSCLC cell although the chroma or color saturation cannot be expected to go too high because of the inhomogeneity of CLC domain sizes. In summary, we have demonstrated electrically induced red–green–blue scattering in cholesteric thin films due to the voltage-induced change in domain size distributions in the FC state. Under the condition of a controlled optical pitch hniP 0 ∼ 550 nm in a PSCLC cell placed between crossed polarizers, a wide color range can be realized in the scattering state. The color stability, tunability, and reversibility have been unambiguously ascertained. With the nature of CLCs in the FC structure, the experimental results presented in this work may open up some new possible applications for optical elements and color information. This research is financially supported by the Ministry of Science and Technology, Taiwan, under Grant No. NSC 101-2112-M-009-018-MY3. References 1. D. K. Yang, J. L. West, L. C. Chien, and J. W. Doane, J. Appl. Phys. 76, 1331 (1994). 2. S.-T. Wu and D.-K. Yang, Reflective Liquid Crystal Displays (Wiley, 2001). 3. A. Hochbaum, Y. Jiang, L. Li, S. Vartak, and S. Faris, SID Tech. Digest 30, 1063 (1999). 4. Y.-C. Hsiao, C.-Y. Wu, C.-H. Chen, V. Ya. Zyryanov, and W. Lee, Opt. Lett. 36, 2632 (2011). 5. Y.-C. Hsiao, C.-T. Hou, V. Ya. Zyryanov, and W. Lee, Opt. Express 19, 7349 (2011). 6. Y.-C. Hsiao, Y.-H. Zou, I. V. Timofeev, V. Ya. Zyryanov, and W. Lee, Opt. Mater. Express 3, 821 (2013). 7. Y. Huang, Y. Zhou, and S.-T. Wu, Appl. Phys. Lett. 88, 011107 (2006). 8. I. P. Ilchishin, L. N. Lisetski, and T. V. Mykytiuk, Opt. Mater. Express 1, 1484 (2011). 9. I. P. Ilchishin and T. V. Mykytiuk, Mol. Cryst. Liq. Cryst. 589, 105 (2014). 10. P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, 2nd ed. (Oxford University, 1993). 11. H.-P. Yu, B.-Y. Tang, J.-H. Li, and L. Li, Opt. Express 13, 7243 (2005). 12. H. Xianyu, T.-H. Lin, and S.-T. Wu, Appl. Phys. Lett. 89, 091124 (2006). 13. S.-Y. Lu and L.-C. Chien, Appl. Phys. Lett. 91, 131119 (2007). 14. T.-H. Lin, Y. Li, C.-T. Wang, H.-C. Jau, C.-W. Chen, C.-C. Li, H. K. Bisoyi, T. J. Bunning, and Q. Li, Adv. Mater. 25, 5050 (2013). 15. Y.-C. Hsiao, C.-Y. Tang, and W. Lee, Opt. Express 19, 9744 (2011).
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Electrically induced red, green, and blue scattering in chiral-nematic thin films Yu-Cheng Hsiao and Wei Lee* Institute of Imaging and Biomedical Photonics, College of Photonics, National Chiao Tung University, Guiren Dist., Tainan 71150, Taiwan *Corresponding author: [email protected] Received January 13, 2015; accepted February 6, 2015; posted February 18, 2015 (Doc. ID 232396); published March 17, 2015 Cholesteric liquid-crystalline materials are abundant in nature such as condensed phases of DNA, plant cell walls, and chiral biopolymers. These self-organized helical structures produce unique optical properties, giving rise to the selective Bragg reflection of colorful light. In this Letter, we focus on the focal conic state of cholesteric liquid crystals and report on stable, tunable, and reversible color switching among red, green, and blue in polymer-stabilized cholesteric films. The experimental results indicate that, with appropriate voltage pulses, the electrically induced color switching of all six routes can be realized in a single cell reflecting green light. The scattered transmissive color persists at zero voltage due to the polymer stabilization. © 2015 Optical Society of America OCIS codes: (160.3710) Liquid crystals; (230.3720) Liquid-crystal devices; (160.1585) Chiral media; (160.4760) Optical properties. http://dx.doi.org/10.1364/OL.40.001201
The use of colors is important in optical technologies and color information. Some color is provided by nature such as plants, butterflies, birds, and beetles via petals, wings, or bodies due to structural variations. These surface structures result in selective reflections and produce beautiful colors as one sees. With a twist arrangement of mesogenic molecules along a helical axis and, in turn, a spatially periodic variation in refractive index, chiralnematic or cholesteric liquid crystals (CLCs) also exhibit Bragg reflections, producing color when the reflective wavelengths are in the visible spectrum. The selective Bragg reflection from a CLC has been exploited in various optical devices such as reflective displays [1,2], color filter arrays [3], multichannel filters [4–6], and mirrorless lasers [7–9]. The reflective color of the CLC can be tuned by varying the temperature or by applying an external field [10]. The color tuning can be achieved particularly by an applied voltage due to its easy operation. For CLCs, pitch elongation or compression by electric field has been utilized to induce a redshift or blueshift of the reflection band [11,12]. Because of the helical structure of a CLC, the wavelength of incident light being reflected is a function of the pitch P 0 . The central wavelength λ0 of the reflected light can be calculated by the equation λ0 hniP 0 , where hni is the average refractive index of the liquid crystal (LC). In recent years, it has been demonstrated that the reflective color can be electrically switched to blue or green from a cholesteric cell initially reflecting in red [13]. Moreover, the reversible lightdirected red, green, and blue reflections in another type of self-organized chiral LC systems, which are known as the blue phase (BP) materials, have also been demonstrated [14]. Indeed, both CLCs and BPs can possess wide reflective ranges in the visible. In this Letter, we present a scattering-transmissive, red-, green-, and blue-switchable device based on a polymer-stabilized CLC (PSCLC) under crossed polarizers, whose working principle is different from the wellknown reflective scheme. The focal conic (FC) state as a state in CLCs is a multi-domain optical scattering state, which is used as the “dark” state in a typical 0146-9592/15/071201-03$15.00/0
bistable CLC display composed of a black, light-absorbing layer beneath the cell without any polarizers [15]. As a matter of fact, the FC state is actually not colorless; color may remarkably appear through scattering under particular polarizations. The reason is that light is a transverse wave with the electric field vibrating perpendicularly to the propagation direction of the beam. If the incident light is linearly polarized, the electric field vibrates in a specific direction in a plane. Under specific conditions such as a cell with a small cell gap, multiple scattering of only a limited extent takes place among the CLC molecules. The emerging lights with residual polarizations are hence not randomly distributed. Based on this property, blueshift of the scattered color occurs in the FC state of a cholesteric cell agitated by an electric field pulse. Accordingly, electrically induced red–green–blue color scattering and color switching are realized in PSCLC films characterized simultaneously by optical stability, electrical tunability, and color reversibility. The PSCLC material used in this study is E7 (Merck) mixed with 3-wt.% prepolymer NOA65 and the chiral dopant R5011 (Merck) at various concentrations. The mixtures were injected into empty cells by capillary action in isotropic phase. Each empty cell is made of two 1.1-mm-thick indium–tin-oxide glass substrates coated with planar-alignment layers, yielding a cell gap of 6.1 0.5 μm. To polymerize the prepolymer, the cell was irradiated by ultraviolet light at 365 nm. The curing intensity was 10 mW∕cm2 , and the curing time was 100 s. The dopant concentrations of 3.03, 2.56, and 2.15 wt.% were adopted, permitting the photo-cured cells to reflect in the blue, green, and red ranges, respectively. The central wavelengths of the Bragg reflections in blue, green, and red correspond to 476 nm, 550 nm, and 654 nm, respectively, as shown in Fig. 1. Note that there are no differences just after the capillary introduction and after the subsequent photo-curing in the corresponding reflection spectra and that the oscillation in the curves arose from the optical interference between the surface boundaries of the CLC films. The transmission spectra of the © 2015 Optical Society of America
1202
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Transmittance (%)
100 75 50 25 0 400
3.03 wt% 2.56 wt% 2.15 wt% 500
600
700
800
Wavelength (nm) Fig. 1. Transmittance spectra of three distinctive PSCLCs. The Bragg reflections occur in blue, green, and red from these cells impregnated with chiral dopant at concentrations of 3.03 wt.%, 2.56 wt.%, and 2.15 wt.%, respectively.
Fig. 2. Photographs of a PSCLC (i.e., G cell in this case) placed between crossed polarizers in three colorful FC states exhibiting red, green, and blue scattered light. Schematics of the corresponding FC structures are shown in the bottom.
PSCLCs were acquired with a fiber-optic spectrophotometer (Ocean Optics HR2000+). An arbitrary function generator (Tektronix AFG-3022B) was employed to supply various AC voltages at 1 kHz in the sinusoidal waveform. All experimental data were measured at an ambient temperature of 26 1°C. In order to describe and discuss the experimental results at convenience, the cells that reflect red, green, and blue colors are here designated the R, G, and B cells, respectively. For oblique Bragg reflections from a polycrystalline layer of CLC (rather than a single-domain planar texture), the expression for the wavelength of the reflection maximum was derived by Fergason [10]:
polymer-dispersed LC systems. In contrast, Rayleigh scattering, as originally formulated, is strictly applicable to small, dielectric (non-absorbing), spherical particles, which is generally preferred if applicable, owing to the complexity of the Mie scattering formulation. Here the FC textures are caused by light scattering from the multi-domains of the self-organized anisotropic LC molecules in the PSCLC cell. The appearance of colorful FC states is governed by two factors—Mie scattering and the birefringence property. The Mie scattering leads to the incompletely randomly polarized light in that the cell gap is not considered thick enough for the transmitted light through the cells. On the other hand, the scattering property unavoidably suppresses the brightness and, in turn, color saturation in the FC states. When the applied voltage increases, the LC molecules tend to re-orient themselves parallel to the field. The effective refractive index, thus, decreases with increasing voltage, causing the blueshift of the emerging light. It is clear from Fig. 2 that the blue scattering state exhibited the best color saturation when the G cell sandwiched between two crossed polarizers was switched by a high voltage of 40 V. This result is in agreement with the Rayleigh theory that the differential scattering intensity has a very strong dependence on the wavelength—it is inversely proportional to the fourth power of wavelength. When the high voltage is applied, the “grain” sizes of the multi-domain FC structure are reduced in the cell because the high voltage enforces the LC molecules to be re-oriented vertically [2]. Before the voltage is even high enough to completely unwind the helix and to induce the homeotropic state, it prompts many smaller sized helical ensembles or fragments in the FC structure. Here the initial (undisturbed) state of the G cell is the planar state at 0 V; it exhibits color change from red, green, to blue in various scattering FC states when the switching voltage of ∼5–55 V is applied. As the stimulating voltage goes beyond ∼55 V, the texture appears to be homeotropic, yielding a completely dark state. Figure 3 depicts nine representative images of colorful FC states in R, G, and B cells under a polarizing optical microscope (OLYMPUS BX51-P). For an arbitrary cell, the induced color in the FC state can be altered by varying the applied voltage. The largest extent of color
P 0 hni 1 −1 sin φ 1 −1 sin φ00 ; cos sin sin λ0 hni m 2 hni 2 where the integer m denotes the reflection order, φ represents the angle of incidence, and φ00 stands for the angle of reflection of the monochromatic light. One can see that the CLC with a smaller pitch can strongly reflect or scatter the high-frequency light. With an applied voltage pulse to modulate the angles, red, green, and blue colors in the optically stable FC states can be observed in three specific pitch ranges. It should be noted that Fergason’s equation cannot accurately describe the observed transmissive phenomenon for a scattering state. A rigorous mathematical expression is, therefore, highly desired. Figure 2 shows three photographs of a G cell scattering blue, green, and red light in the FC states under crossed polarizers on a slide film light box to illuminate the cell. The corresponding schematics of field-induced multidomains in the absence of a sustaining voltage are also illustrated in the bottom. Scattering in the FC state can be analyzed by the Mie theory, which encompasses the general spherical scattering solution (absorbing or nonabsorbing) of Maxwell’s equations without a particular bound on particle or domain size. Mie scattering from larger-sized particles or domains reduces to the limit of geometric optics and is weakly wavelength dependent. It produces the almost white or, more precisely, fluffily translucent appearance as seen in many typical
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
Fig. 3. Optical textures of three PSCLCs with chiral dopant concentrations of 2.15, 2.56, and 3.03 wt.%, yielding R, G, and B cells, respectively, in the FC states induced by various applied voltages as presented numerically. All micrographs were taken without a sustaining voltage across the thickness of each cell.
Transmittance (%)
change, covering the spectral range from red to blue, can be found in the G cell that was made to selectively reflect green light in the PSCLC film in the initial planar state. In comparison with the colorfully scattered light observed in the G cell, the R cell and B cell do not possess the sufficiently wide visible scattering range to generate a good variety of colors. The FC domains are larger in the R cell and smaller in the B cell. As a result, the R cell tends to scatter the red color, and the B cell tends to exhibit the blue FC texture under crossed linear polarizers. Here it is worth emphasizing that the color tuning by an applied electric field in the FC state is demonstrated in all of the samples, whereas the G cell distinctively possesses the widest tuning range of the visible spectrum— including red, green, and blue bands—in the scattering FC state. The transmittance spectra of the G cell switched by various applied voltages are illustrated in Fig. 4. Here
30 20 10 30 20 10 30 20 10 30 20 10
5V
15 V
25 V
45 V
500
600
700
Wavelength (nm) Fig. 4. Transmission spectra of a PSCLC; i.e., G cell, sandwiched between two crossed polarizers in various voltageinduced colorful FC states.
1203
the measured transmittance is said to be 100% when the probe beam propagates through the parallel polarizers without a cell being placed between them. The above-mentioned color shift becomes clear from these spectra in various FC states induced by increasing voltage from 5 to 45 V. As the switching voltage increased, the first red FC state appeared at 5 V. The cell then exhibited a light-yellow FC state at 15 V and a green one at 25 V. Finally, the texture appeared in blue when the voltage ascended to 45 V. It is worth reminding that each colorful FC state as seen in the crossed-polarizer scheme is optically stable. The switching from one to another color is direct and reversible by means of an appropriate voltage pulse. In other words, color tuning (from red to blue) and color stability can be simultaneously achieved in the FC state in a PSCLC cell although the chroma or color saturation cannot be expected to go too high because of the inhomogeneity of CLC domain sizes. In summary, we have demonstrated electrically induced red–green–blue scattering in cholesteric thin films due to the voltage-induced change in domain size distributions in the FC state. Under the condition of a controlled optical pitch hniP 0 ∼ 550 nm in a PSCLC cell placed between crossed polarizers, a wide color range can be realized in the scattering state. The color stability, tunability, and reversibility have been unambiguously ascertained. With the nature of CLCs in the FC structure, the experimental results presented in this work may open up some new possible applications for optical elements and color information. This research is financially supported by the Ministry of Science and Technology, Taiwan, under Grant No. NSC 101-2112-M-009-018-MY3. References 1. D. K. Yang, J. L. West, L. C. Chien, and J. W. Doane, J. Appl. Phys. 76, 1331 (1994). 2. S.-T. Wu and D.-K. Yang, Reflective Liquid Crystal Displays (Wiley, 2001). 3. A. Hochbaum, Y. Jiang, L. Li, S. Vartak, and S. Faris, SID Tech. Digest 30, 1063 (1999). 4. Y.-C. Hsiao, C.-Y. Wu, C.-H. Chen, V. Ya. Zyryanov, and W. Lee, Opt. Lett. 36, 2632 (2011). 5. Y.-C. Hsiao, C.-T. Hou, V. Ya. Zyryanov, and W. Lee, Opt. Express 19, 7349 (2011). 6. Y.-C. Hsiao, Y.-H. Zou, I. V. Timofeev, V. Ya. Zyryanov, and W. Lee, Opt. Mater. Express 3, 821 (2013). 7. Y. Huang, Y. Zhou, and S.-T. Wu, Appl. Phys. Lett. 88, 011107 (2006). 8. I. P. Ilchishin, L. N. Lisetski, and T. V. Mykytiuk, Opt. Mater. Express 1, 1484 (2011). 9. I. P. Ilchishin and T. V. Mykytiuk, Mol. Cryst. Liq. Cryst. 589, 105 (2014). 10. P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, 2nd ed. (Oxford University, 1993). 11. H.-P. Yu, B.-Y. Tang, J.-H. Li, and L. Li, Opt. Express 13, 7243 (2005). 12. H. Xianyu, T.-H. Lin, and S.-T. Wu, Appl. Phys. Lett. 89, 091124 (2006). 13. S.-Y. Lu and L.-C. Chien, Appl. Phys. Lett. 91, 131119 (2007). 14. T.-H. Lin, Y. Li, C.-T. Wang, H.-C. Jau, C.-W. Chen, C.-C. Li, H. K. Bisoyi, T. J. Bunning, and Q. Li, Adv. Mater. 25, 5050 (2013). 15. Y.-C. Hsiao, C.-Y. Tang, and W. Lee, Opt. Express 19, 9744 (2011).
