We have designed a reflective composite sheet consisting of a birefringent polymer matrix and isolated isotropic or minimally birefringent fibers. The optical properties of the sheet have been investigated in terms of the width, spacing, and thickness of the individual fibers. Commercial software (FDTD Solution) was used to simulate the reflectance of the proposed sheet, and conventional processes such as cast-film extrusion in combination with solid-state drawing were used to manufacture the multilayer composite sheet. The measured and simulated reflectance spectra confirm the feasibility of employing the sheet as a reflective polarizer.
© 2014 Optical Society of America
The latest trends in the worldwide display industry are thinning, lightening, cost reduction, and low-power consumption. Of these, power consumption is a high-stakes issue because it is directly related to energy efficiency. Increasing the energy efficiency is still a challenging task for researchers and liquid-crystal display (LCD) makers, but one possible approach lies in refining the LCD backlight unit, which consumes the majority of the power in an LCD panel. The high power consumption of the LCD backlight is chiefly due to the fact that the LCD itself does not emit light spontaneously.
Recent backlight units consist of a light source, light guide plate, and a series of optical sheets including a diffuser sheet [1–4], collimation film , and reflective polarizer [6–17]. The diffuser sheet is a Gaussian diffuser that mainly diffuses visible wavelengths; the collimation film, or brightness enhancement film, is effective in increasing the normal luminance; and the reflective polarizer, a key component for the brightness enhancement, utilizes the polarization by recycling the reflected beam and can increase the energy efficiency by ~60% .
There are several types of reflective polarizers including nanowire grids [6–8], cholesteric liquid crystals (CLCs) , scatterers [10,11], and multilayers [12–17]. Multilayer-type reflective polarizers, first introduced by 3M, are the type most widely used in LCD technology; they are better known as dual brightness enhancement film (DBEF) [12–17]. The DBEF is produced via coextrusion of alternating layers of two thermally processable polymers followed by uniaxial stretching of the multilayer stack. To obtain reflectivity over a range of wavelengths, the alternating layers have different thicknesses , and consequently, a typical film consists of hundreds of layers. As stacking hundreds of layers requires a significant amount of manufacturing resources, simplified reflective polarizer designs have been sought.
Here, we propose a reflective composite sheet design based on a birefringent polymer matrix containing isolated isotropic or minimally birefringent fibers. We have studied the optical properties of this sheet in terms of the width, spacing, and thickness of the individual fibers and investigated the optical performance using commercial software (FDTD Solution) simulations. Conventional processes such as cast-film extrusion in combination with solid-state drawing are used to manufacture the reflective composite sheet, and we show that the measured and simulated reflectance spectra indicate that the sheet is a feasible reflective polarizer.
2. Structure design and principle of the reflective polarizer in LCD backlight
The proposed reflective composite sheet design is a two-phase system of a polymer matrix and several isolated fibers (Fig. 1). Individual isotropic or minimally birefringent fibers are embedded in a birefringent polymer matrix with ordinary no and extraordinary ne refractive indices. The isotropic or minimally birefringent fibers have an ordinary refractive index (of no) or two minimally birefringent refractive indices (near no), respectively. Positive birefringent properties are characterized by ne > no, where ne is the refractive index in the stretched direction, which is parallel to the fiber direction, and no is the refractive index in the orthogonal direction. Ideally, the stretched refractive indices of the isotropic or minimally birefringent fibers should match the no value of the birefringent polymer matrix but not ne. Here, ne and no values of 1.850 and 1.550, respectively, were assumed for the matrix. These values were obtained from an estimate of the maximum attainable birefringence of a polymer matrix of poly(ethylene naphthalate) (PEN).
The reflective composite sheet works as a reflective polarizer in the following manner. When unpolarized backlight hits the reflective composite sheet, light linearly polarized in the direction orthogonal to the length of the fibers will be transmitted through the sheet because the refractive indices of the matrix and fibers match. However, light linearly polarized in a direction parallel to the length of the fibers will be reflected back for recycling when the layers of thickness λ/4 in a double-layer film are ordered such that the light encounters a high refractive index medium followed by a low refractive index medium. This structure is called a high-reflectance stack or dielectric mirror . According to a previous report , about 50–60% of the perpendicularly polarized light is converted to light polarized in a direction parallel to the length of the fibers by continuous polarization recycling cycles.
3. Optical simulation
The simulations of the reflectance of the proposed structure design were performed using the commercial software FDTD Solution (Ver. 8.6.4, Lumerical Co.), which utilizes a time-domain technique in which the electric and magnetic fields are calculated as a function of time based on the Maxwell equations . Frequency information is obtained via a Fourier transformation during the simulations. The simulations were conducted in the two-dimensional spatial domain shown in Fig. 2. To calculate the reflectance spectra for various fiber widths and spacings, we assumed the incident light was a plane wave with a specific polarization state over the entire visible wavelength region. The width w, spacing d, and thickness tf of the fibers were changed systematically to investigate the effect on the reflectance and optimize the design parameters. In our simulations, the fiber tf and matrix tm thicknesses satisfy a quarter-wave optical thickness, and each layer obeyed for constructive interference conditions. The central wavelength was constant ( nm) in all cases.
Figure 3 shows the simulated reflectance for incident light polarized parallel to the fiber direction and different numbers of polymer layers. In the case of 1 layer, the maximum reflectance is only ~30%, but as the number of layers increases, both the maximum reflectance and the oscillations in the reflectance increase. A reflectance of ~100% is achieved with 40 layers, and although more layers lead to broader reflectance behavior, there is little difference in the reflectance and increasing the number of layers will only increase the manufacturing complexity.
To obtain broad reflectance behavior, most reports suggest a multi-stacking design with a different central wavelength , but this means that the thicknesses of all the layers in the stack need to be changed. While this is an effective method, it is not the only approach for our designed structure. When the width and spacing of the fibers were changed systematically while keeping the thickness constant, broad bandwidth regions appeared in the reflectance spectra. Unless otherwise noted, the following simulations were performed for a sheet of 10 layers. Figure 4(a) shows that when the width and spacing of the fibers are equal to or less than 100 nm, the reflection spectra are similar to those seen in Fig. 3, but for widths and spacings larger than 100 nm, there is a large amount of noise in the spectra. Furthermore, Fig. 4(b) shows that for widths and spacings larger than 1 μm broad bandwidth reflectance spectra are seen, although the maximum reflectance has decreased. The reflectance values for equal fiber widths and spacings of ~3 μm are similar to those for fiber widths and spacings of 100 nm, and we concluded from these simulated results that it would be possible to fabricate a reflective polarizer with this design. In practice, fabricating a well-aligned composite sheet embedded with 100-nm-wide fibers is somewhat difficult, but the results suggest that fibers with a width of more than 3 μm will yield the same reflectivity, and a sheet with fibers of this size is somewhat easier to fabricate.
Figure 5 shows simulated reflectance spectra for sheets of different thicknesses assuming equal widths and spacings of 100 nm and 3 μm. Compared with the quarter-wave optical thickness (red curve), the spectra for quarter-wave optical thicknesses of multiples of 3 (black curve) and 5 (blue curve) have a narrow reflection bandwidth with more oscillations. The thickness factor is a cardinal matter for the experiments.
The proposed reflective composite sheet was fabricated using the islands-in-the-sea (INS) conjugate melt spinning technique. PEN (Kolon) was used as the birefringent polymer matrix, and Tritan copolymester TX2001 (Eastman) was used for the minimal birefringent fibers. PEN is positively birefringent with a high maximum attainable birefringence (), and the ne and no values of the drawn PEN films for a draw ratio of five were approximately 1.85 and 1.55 . In addition, PEN has good transparency and is an excellent material for melt spinning processing. TX2001, which has a refractive index of 1.55, was thus selected as the fiber material to match the ordinary refractive index of PEN. TX2001 also has a good total transmittance of 92% and a low haze of under 1%. Figure 6 shows the refractive indices of drawn TX2001 films as a function of the draw ratio; the extraordinary (●) and ordinary refractive indices (○) are parallel and perpendicular to the drawing direction, respectively. Figure 6 shows that TX2001 has minimal birefringence properties ().
The two polymers were melted from separate extruder flows through a spinneret with 3808 holes and were then combined in the melt inlet of a coat-hanger die. After passing through the melt-distribution manifold of the coat-hanger die, the cast sheet forms a film, which was then wound around a winding roll machine. The respective speeds of the casting roll, cooling roll, and take-off unit were 10, 13, and 15 m/min. Figure 7 shows scanning electron micrographs of the fabricated reflective composite sheet that has a well-aligned structure of 13–14 layers. The fibers, as shown in Fig. 7(b), have diameters of 6–8 μm.
Figure 8 shows the measured reflectance spectra for different numbers of stacked reflective composite sheets. To obtain linearly polarized light in a direction parallel or perpendicular to the length of the fibers, we used an absorption-type linear sheet polarizer. Thus, the measured reflectance spectra closely resemble the transmission spectra of the conventional absorption-type linear polarizer, and so we focus here on the enhanced reflectance amount. Compared with the simulation results, the measured reflectance is somewhat lower. In addition, there is little difference in the spectra for incident light polarized in directions parallel and perpendicular to the lengths of the fibers. The desired performance with an increasing number of sheets is an increasing reflectance for incident light polarized in the direction parallel to the length of the fibers and a decreasing reflectance for perpendicularly polarized incident light. The performance seen here appears to be due to the ordinary refractive indices of the matrix and the fibers being the same and the degree of drawing during the fabrication process. Although we have not been successful in producing ideal polarization characteristics, we believe that it will be possible to use the proposed reflective composite sheet display application if the fabrication conditions are improved.
We have proposed a reflective composite sheet design based on a birefringent polymer matrix and isolated isotropic or minimally birefringent fibers. We found that in simulations for fiber widths and spacings of less than 100 nm, a large reflectance with a narrow bandwidth was obtained. A lower reflectance with a broader bandwidth was obtained in simulations for widths and spacings of ~3 μm. The simulated results suggested that the reflective polarizer design could be implemented, and the feasibility of the reflective composite sheet, fabricated using conventional processes, was confirmed through the measured and simulated reflectance spectra.
This work was supported by the Fundamental R&D Program for Core Technology of materials (10037191, Structuring Technologies for Multi-functionally Converged Nanofibers) funded by the Ministry of Trade, Industry and Energy, Republic of Korea. This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2013R1A1A2061996). The authors would like to express thanks to Prof. Moo-Sung Lee (Chonnam National University) for his helpful guidance.
References and links
1. G. H. Kim, W. J. Kim, S. M. Kim, and J. G. Son, “Analysis of thermo-physical and optical properties of a diffuser using PET/PC/PBT copolymer in LCD backlight units,” Displays 26(1), 37–43 (2005). [CrossRef]
2. G. H. Kim, “A PMMA composite as an optical diffuser in a liquid crystal display backlighting unit (BLU),” Eur. Polym. J. 41(8), 1729–1737 (2005). [CrossRef]
3. H. P. Kuo, M. Y. Chuang, and C. C. Lin, “Design correlations for the optical performance of the particle-diffusing bottom diffusers in the LCD backlight unit,” Powder Technol. 192(1), 116–121 (2009). [CrossRef]
4. T. Kim, S. Kim, D. Y. Lim, and S.-W. Choi, “A novel diffuser sheet comprising nanosized birefringent fibers embedded within an isotropic polymer matrix,” Opt. Commun. 295, 125–128 (2013). [CrossRef]
5. B.-Y. Joo and D.-H. Shin, “Design guidance of backlight optic for improvement of the brightness in the conventional edge-lit LCD backlight,” Displays 31(2), 87–92 (2010). [CrossRef]
6. G. R. Bird and M. Parrish Jr., “The wire grid as a near-infrared polarizer,” J. Opt. Soc. Am. 50(9), 886–891 (1960). [CrossRef]
7. M. Xu, H. P. Urbach, D. de Boer, and H. Cornelissen, “Wire-grid diffraction gratings used as polarizing beam splitter for visible light and applied in liquid crystal on silicon,” Opt. Express 13(7), 2303–2320 (2005). [CrossRef] [PubMed]
8. J.-S. Seo, T.-E. Yeom, and J.-H. Ko, “Experimental and simulation study of the optical performances of a wide grid polarizer as a luminance enhancement film for LCD backlight applications,” J. Opt. Soc. Kor. 16(2), 151–156 (2012). [CrossRef]
9. B. Fan, S. Vartak, J. N. Eakin, and S. M. Faris, “Broadband polarizing films by photopolymerization-induced phase separation and in situ swelling,” Appl. Phys. Lett. 92(6), 061101 (2008). [CrossRef]
10. H. Jagt, Y. Dirix, R. Hikmet, and C. Bastiaansen, “Linear polarizers based on polymer blends: Oriented blends of poly(ethylene-2,6-naphthalenedicarboxylate) and a poly(styrene/methylmethacrylate) copolymer,” Jpn. J. Appl. Phys. 37(8), 4389–4392 (1998). [CrossRef]
11. K. Totani, H. Hayashi, and T. Watanabe, “Scattering-type polarizers consisting of fiber/matrix and methods to enhance polarization property,” Jpn. J. Appl. Phys. 48(8), 082403 (2009). [CrossRef]
13. J. M. Jonza, M. F. Weber, A. J. Ouderkirk, and C. A. Stover, “Polarizing beam-splitting optical component,” U.S. Patent, 5962114, October 5 (1999).
14. Y. Li, T. X. Wu, and S.-T. Wu, “Design optimization of reflective polarizers for LCD backlight recycling,” J. Disp. Tech. 5(8), 335–340 (2009). [CrossRef]
15. M. E. Denker, A. T. Ruff, K. Derks, J. N. Jackson, and W. W. Merrill, “Advanced polarizer film for improved performance of liquid crystal displays,” SID Int. Symp. Dig. Tech. Pap.37, 1528–1530 (2006).
16. M.-Y. Yu, B.-W. Lee, J. H. Lee, and J.-H. Ko, “Correlation between the optical performance of the reflective polarizer and the structure of LCD backlight,” J. Opt. Soc. Kor. 13(2), 256–260 (2009). [CrossRef]
17. B.-W. Lee, M.-Y. Yu, and J.-H. Ko, “Dependence of the gain factor of the reflective polarizer on the configuration of optical sheets,” J. Inf. Disp. 10(1), 28–32 (2009). [CrossRef]
18. F. L. Pedrotti, L. S. Pedrotti, and L. M. Pedrotti, Introduction to Optics (Prentice Hall, 2006), Chap. 22.