We propose a switchable achromatic reflector using a long-pitch cholesteric liquid crystal (CLC) whose Bragg reflection wavelength is chosen to be infrared by controlling the pitch of the CLC so that the planar texture is transparent over the entire visible wavelength. By using the light scattering of the focal conic texture, achromatic reflection can be achieved. Both textures are stable at zero electric field and the operating voltage of the proposed CLC device is much lower than that of conventional CLC devices. The proposed switchable reflector, which can be operated at a low voltage with low power, can be applied to reflective displays and to light shutters. By coupling with a reflective polarizer the efficiency of light scattering at the focal conic texture can be enhanced.
© 2010 OSA
Cholesteric liquid crystals (CLCs) have been studied for the realization of various optical devices such as information displays [1–9], light shutters , color filter arrays , and liquid crystal lasers . A CLC has two bistable textures [13,14], namely planar and focal conic textures. In the planar texture, where the helical axes are perpendicular to the surface, the CLC selectively reflects the wavelength determined by its pitch. The chiral and periodic structure of the CLC causes the Bragg reflection [15–17] where the wavelength of the reflected light is determined by the Bragg wavelength, which is given as
CLCs have been applied to reflective displays because of their reflective nature in the planar state [1–8]. In order to realize a reflective display, the planar state and the focal conic textures are used for the bright state and the dark state, respectively. A CLC capable of reflecting visible wavelengths between 380 nm and 780 nm is usually employed. However, the wavelength of the reflected light in the planar texture is dependent on the viewing angle since the selective wavelength is determined by the effective pitch length and the optical anisotropy which are dependent on the viewing angle. Moreover, only a single color can be displayed with a single CLC layer since the bandwidth of the Bragg reflection does not cover the entire range of visible wavelengths. Stacking of CLC layers is required for achromatic reflection [18–20]. Although low power consumption can be achieved by its bistable property, voltages higher than 50 V are still necessary for switching between the two textures because of the short pitch [4,6,21] and current applications are limited to light shutters and mobile sub-panels playing the role as an indicator of simple information because the panels are transparent only in the field-on state and the reflection is colored. Recently, a method using the dual-frequency CLCs have been proposed for light shutters. But, the frequency and the voltage need to be controlled simultaneously and the operating voltages are very high (around 80 V) .
In this paper we propose a CLC device, in which a selective wavelength of the reflected light is shifted to infrared (IR) wavelengths by controlling the pitch. The planar texture of the new proposed CLC device is transparent over the entire visible wavelengths in the field-off state and omni-directional achromatic reflection through light scattering at the focal conic texture can be achieved without a polarizer. Compared to conventional CLC cells that reflect the visible light in the planar state, the proposed CLC device has a longer pitch, of which the operating voltage for switching between the two textures is much lower so that achromatic reflective displays and light shutters with low power consumption can be realized using the proposed device structure. By coupling these with a reflector the light efficiency of the proposed CLC cell at the focal conic texture can be enhanced, by which higher brightness can be obtained for application to reflective displays.
2. Principle and experiments
Figure 1 shows the operation of the long-pitch CLC cell based on the IR reflection at the planar state. When the proposed CLC cell is in the planar texture, as shown in Fig. 1(a), the IR light is reflected because of the Bragg reflection generated by the periodic structure of the CLC with long pitch, by which transparency is achieved over the entire visible wavelengths. The focal conic texture, which reflects the ambient light as seen in Fig. 1(b), shows achromatic light scattering.
To confirm the electro-optic characteristics of the proposed configuration, a CLC cell is fabricated. In the experiment, a homogeneous polyimide (H-PI) alignment layer (PIA-5310, Nissan Chemical) is spin-coated on the top and the bottom indium-tin-oxide (ITO) glass substrates, followed by the baking process for the polyimidization of the H-PI. These are assembled, maintaining the cell-gap of 4.5 μm by using silica spacers. The positive liquid crystal (E7, Δn: 0.2255, Δε: 14.1, Merck) mixed with a chiral material (S811, Merck), of which the mixture ratio is chosen so as to reflect the IR light of 1000 nm, are injected into the empty cell. The pitch of the CLC is 611.8 nm. For comparison, a conventional CLC cell (pitch: 336.5 nm) reflecting the visible light of 550 nm and a homogeneous aligned (HA) liquid crystal cell are also fabricated.
3. Results and discussion
The reflection characteristics of the fabricated CLC cells are indirectly verified by measuring the transmission spectrum since it can be assumed that wavelengths showing the lower transmittance are coincident with those of the high reflections which are measured using the UV-vis spectrophotometer (UV mini 120, Shimadzu). The transmission spectra are shown in Fig. 2(a) , where all the measurements with the CLC cells in the planar state are made without polarizers. Other than reflection by glass substrates, which lowers the transmittance, light at all the wavelengths penetrates intact into the HA liquid crystal cell which makes the cell become transparenct. In the case of a conventional CLC cell (pitch: 336.5 nm) the green light is reflected , of which the transmittance becomes lower at the wavelength of 550 nm. The IR reflection of the proposed CLC cell is confirmed by the decrease in transmittance at wavelengths near 1,000 nm whereas the transmittance of the proposed CLC cell at visible wavelengths is nearly the same as that of a HA liquid crystal cell.
Figure 2(b) shows the measured voltage-transmittance (V-T) curve of the long-pitch CLC cell, whose transmittance is measured after removing the addressing voltage pulse . For an addressing voltage smaller than 5 V, the CLC cell remains in the planar texture. But, when the addressing voltage is increased to 6 V, the cell begins to reflect the incident light because of the scattering by the focal conic domains. When voltages between 8 V and 11 V are applied, the focal conic domains are dominant in the cell so that the lowering of the transmittance caused by the light scattering can be observed. For the addressing voltage higher than 14 V, the CLC cell is switched to the homeotropic texture by the addressing pulse, followed by a relaxation back to the planar texture as soon as the addressing pulse is removed so that it is transparent at visible wavelengths. Compared to the V-T curve of a CLC cell reflecting the visible light , the operating voltages for alteration of the textures are very low. The longer the pitch, the lower the required addressing voltage for the homeotropic texture as well as the focal conic texture.
Figure 3 shows images of the fabricated CLC cells in homeotropic, planar, and focal conic textures which are placed on a sheet of paper printed with the text 'PNU.' Pictures are taken while maintaining a distance of 1 cm between the cell and the paper. For comparison, the original printed paper without a CLC cell is also shown in Fig. 3(a). The homeotropic texture is obtained by applying a voltage of 18 V and the planar texture is obtained by removing the addressing pulse of 18 V. Although a few focal conic domains do not disappear completely at the boundary regions of the cell, the transparency of the planar texture is the same as that of the homeotropic texture, and so the text ‘PNU’ can be read in both textures, as shown in Figs. 3(b) and 3(c). The focal conic texture is obtained when voltages between 8 V and 12 V are applied as soon as the addressing pulse of 18 V is removed. Here, the text ‘PNU’ is not recognized because the ambient light is scattered omni-directionally, as shown in Fig. 3(d).
It is desirable for higher brightness to be obtained in order for this device to be effectively applied to reflective displays. The proposed CLC device can be coupled with a reflector so that forward scattering as well as backward scattering can be utilized to enhance the efficiency of the reflected light. The reflector used in the experiment is a dual brightness enhancement film (DBEF, 3M Vikuiti), used widely as a reflective polarizer. Figure 4 shows the reflected light intensities of a CLC cell coupled with a DBEF film, measured as the viewing angle is varied for a fixed incident angle of −30°. These are compared with that of a CLC cell without a reflective polarizer and a sheet of white paper. At the azimuth angle of 0°, the light intensity of the CLC cell without a reflective polarizer is lower than that of a sheet of white paper over the entire polar angles except at 30° since the high reflection at 30° is caused by the reflection at the surface of the top glass substrate. The surface reflection can be reduced by using the anti-reflection coating. A CLC cell coupled with a DBEF film shows light intensities much higher than that of a CLC cell without a DBEF film at all polar angles. The light intensity is much higher than that of the white paper at polar angles between 10° and 50° and comparable to a white paper at other polar angles. It is expected that further studies on the surface treatment and the optimization of CLC cell parameters, such as the pitch length and the cellgap, can enhance the efficiency of the light scattering at polar angles higher than 50°.
Figure 5 shows images displayed on the proposed prototype 170 × 290 panel employing the long-pitch CLC which is operated using passive matrix addressing. The bright state is obtained by the focal conic texture, which reflects the ambient light and the dark state is obtained by the planar texture because this texture has no reflection for the visible light. Compared to a polymer-networked liquid crystal display in which the bright state is obtained through the light scattering effect , the proposed device can be operated at lower voltages with low power consumption because of its bistable nature. Moreover, the polarizer-free structure leads to the highly bright state and the transparency of the planar texture causes the superior dark state. The measured contrast ratio of the proposed device is higher than 20: 1. The proposed device can be used as a reflective sub-display, which can display simple information with low power, placed on top of a conventional transmissive liquid crystal display because the transparency can be maintained by using a reflective polarizer.
In conclusion, a long-pitch CLC device has been proposed which reflects IR wavelengths in the planar texture and is transparent over the entire visible wavelengths. Through light scattering of the focal conic texture, achromatic reflection is realized. The operating voltages are much lower because of the longer pitch and the efficiency of the scattered light can be enhanced by coupling with a reflective polarizer. It is believed that the proposed device can be applicable to outdoor billboards, digital picture frames, and reflective sub-panels of mobile devices as well as light shutters and reflective displays because of the transparency aspects and the low power operation.
This work was supported by Mid-career Researcher Program through NRF grant funded by the MEST (No. 2010-0000334).
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