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To achieve a high transmittance and extinction ratio for the polarized white light-emitting diodes (WLEDs), the package structures of polarized WLEDs that are pumped by blue LEDs are studied. A WLED is implemented using a nano-wire grid polarizer (NWGP) and forms a pumping cavity. Combining the reflection and transmission characteristics of a NWGP, the optimized package structure has an air gap between the phosphor resin and the NWGP. Specifically, the angular dependence of the chromaticity coordinates and spatial uniformity of the radiation pattern are investigated and are more uniform than those of conventional LEDs.
©2013 Optical Society of America
Introduction
Liquid crystal display (LCD) has low brightness and high power consumption because of low light use efficiency, which is caused by energy loss in the light paths throughout LCD panels, especially in the liquid crystal layer, the backlight unit, and the dichroic sheet polarizers. To enhance LCD brightness and reduce power losses, the polarization conversion of unpolarized light is a solution that minimizes effective energy consumption and reduces the number of sheet polarizers used. Recently, reflective polarizing films (RPFs) have been used to recycle the undesired reflected polarization components, by reflecting them back to the light source, proceeding with polarization randomization, and returning the unpolarized light again. This randomization of the undesired polarization through an uncontrollable reflection mechanism may be ineffective and can result in energy loss [1]. Among conventional polarizers, birefringent crystal polarizers and dichroic sheet polarizers are unsuitable for the polarization conversion through reflection, and recycle a portion of light with the unwanted polarization. Multilayer-polymer polarizer film that can be used in polarization conversion has recently been achieved by stretching a multilayer polymer film in two orthogonal directions in the plane of the film to create an anisotropic refractive index and designed thickness in the alternating polymer layers. These polymer birefringent films or RPFs have a strong reflection band, sharp band edges, low absorption loss and uniformity. However, a blue shift of the band edge of the off-axis transmission spectrum and a low extinction ratio are caused by the characteristics of the incident angle, polarization, and the wavelength dependent reflectivity of RPF [2].
Since the development of nano-lithographic technology, sub-wavelength wire-grid grating, or NWGP with nano-scale features has been proposed as an alternative for reflective polarizers [3]. The advantages of a NWGP for visible wavelengths include a wide wavelength band width, a large field of viewing angle and good polarization efficiency. Because of the emission characteristics of WLED on emitting angle, emission wavelength range and polarization, the NWGPs are particular suitable for polarizing WLEDs. The structure of NWGPs is fabricated by a metallic nano-grating on the top surface of a glass substrate, and features exceptional working stability for thermal and UV degradation effects. However, the nano-scale features make the NWGP suspected to be damaged during operation. By reflecting undesired polarization back into the light source through the controlled optical path and recycling the reflected light through polarization randomization, this mechanism for polarization conversion can be used for polarizing WLEDs. However, the values of ER and the transmittance of reflective polarizers are critical to polarization conversion efficiency [3, 4]. The ER and transmittance of the NWGP are also dependent on manipulating the nano-grating structure and metallic material used by the methods of micro-fabrication [5, 6].
To replace cold cathode fluorescence lamps as a light source for LCD backlight units, LEDs, including phosphor-converted LEDs (pc-LEDs) and tri-primary color LEDs, have been used. However, polarized white light can reduce energy losses in the output power through the bottom sheet polarizer of the backlight unit. Although the output light of LEDs is partially polarized [7], the mechanism of the phosphor-converted light emission of pc-LEDs causes the polarization of the outputted white light to be randomized. This study used WLEDs that were composed of a blue LED with a yellow phosphor resin layer. Consequently, this conventional packaging structure design had difficulty meeting the requirements of polarization, emission color stability, correlated color temperature (CCT), and luminance uniformity. For example, the emission color instability of WLEDs can be reduced using an optical filter to re-excite the phosphor resin by reflecting a portion of the blue excitation intensity [8]. Sepsi et al. theoretically studied polarized LEDs with a NWGP and showed that the placing NWGP on the surface of the encapsulation is advantageous [9]. This research proposes a novel package structure for polarized WLEDs with a NWGP to enhance light transmittance, ER, and improve color spatial uniformity. However, yellow phosphor was re-pumped by the reflected blue light with undesired polarization through the implementation of NWGPs. The transmittance of the polarized white light was enhanced by recycling a portion of the blue light, although there was absorption loss for the metallic NWGP.
Experimental procedures
This study used a 150 mcd blue LED (TC-0465-S12-TB, UniLite) with a peak wavelength of 463 nm to excite the yellow YAG phosphor resin. A transparent resin layer (XE14-C3688, Momentive) measuring 100μm thick surrounded and enhanced the output power of the blue LED by reducing the refraction index difference between the LED chip and the surrounding resin. The phosphor resin was 1.43mm thick and contained 1.8% weight percentage of yellow (YAG:Ce) phosphors mixed with transparent resin. The concentration of the yellow phosphor in the resin was adjusted to obtain an optimized weight ratio for a desired white light emission spectrum. The blended phosphor resin was dispensed uniformly onto the transparent resin layer in a lead frame with Ag-coated reflectors. Furthermore, a NWGP was deposited on top of the phosphor resin layer of the WLED with a 0.34mm air gap.
The polarization of the emitted white light waves of WLEDs comprised transverse magnetic field (TM) and transverse electric field (TE) components. However, the orientation of the wires of the NWGP affected the propagation of the blue light that was reflected from the wire-polarizing layer [3, 10]. The TE-polarized waves, which had a polarization parallel to the orientation of the wires, were multiply reflected in numerous directions by the metallic wire polarizing layer. Conversely, the TM-polarized waves with a polarization perpendicular to the orientation of the wires, were transmitted through the NWGP with a 15% absorption loss. Because the emitted light from WLEDs were reflected between the NWGP and the Ag-coated lead frame, hence the NWGP and the lead frame formed a recycling pumping cavity for the reflected blue light. For the visible wavelength, the NWGP should have periods of 100-150 nm, duty cycles of 04-0.6 and heights of 150-200 nm [11]. Specifically, this study used a commercial Al NWGP with a period of 144 nm, a duty cycle of 0.52 and a depth of 210 nm. The TM transmittance of the NWGP gradually decreased when the wavelengths were less than 450 nm because of the 144 nm period, and it was approximately 85% for visible light and at normal incidence. However, the TE transmittance for the all incidence was in the range of 7x10−4 ~7x10−5 (data not shown). The ER was defined as the power density ratio of the TM-polarized wave divided by the TE-polarized wave.
For the NWGP, the metallic wires of the polarizing layer are delicate and susceptive to the environment because of their feature size. As shown in Fig. 1 , a package structure was implemented where the NWGP faced down to the phosphor resin layer to avoid damaging the wire-grid features. Simultaneously, the packaging structure with an air gap enhanced blue light to interact with the phosphor resin and impinge the NWGP from the air, precluding the phosphor resin from degrading the performance of the NWGP. Therefore, the air gap serves to avoid damage, and enhance the extinction ratio and transmittance. The package structure with the air gap effectively formed a white light pumping cavity. For this pumping cavity, the TE- polarized blue light inside the cavity was multiply reflected, with a continuous production of yellow light emission determined by the package structure design. The blue light of undesired polarization (TE) was reflected by the NWGP and impinged into the phosphor resin layer to re-excite the phosphor grains through the scattering effect. This mechanism randomized the polarization of the blue light and re-emitted yellow light. However, after recycling, the NWGP acted again served as a polarizing filter for the incident unpolarized white light. On the other hand, the glass substrate only allowed the TM-polarized visible light to be transmitted at a large incident angle because of the total internal reflection of the TE-polarized waves at the air/glass interface. Furthermore, because the TE-polarized blue light was recycled and the emitted yellow light was re-generated, the color temperature of the WLEDs was reduced lower. In addition, the spatial intensity distribution of the light emissions also was more uniform than that of conventional WLEDs with a halo effect.
For the wavelength range of 380-780nm, the angular dependent transmittance and ER of the polarized WLEDs were measured as shown in Fig. 2 , and the polarized WLED being tested was placed on a rotation stage to vary the viewing angle of the white light. The transmittances of the TE and TM component waves were measured with respect to the viewing angles by using a spectroradiometer (Spebcos 1201, JETI), and the ER was calculated. A dichroic polarizer (52mm E-PL, Kenko) was placed in front of the spectroradiometer and was used as an analyzer. The angular resolution of the computer-controlled rotational mount of the dichroic polarizer was ± 10”. The experimental setup for the measuring the spatial intensity distribution of the packaged WLED was the same as shown in Fig. 2 but without the analyzer.
Results and discussion
For comparison, the output light properties of the polarized WLED and the conventional WLEDs, without a NWGP, were measured. The experimental data showed that the ER of the polarized WLEDs was raised by increasing the viewing angle, whereas the TM transmittance of the NWGP remained nearly constant at 0.85 and gradually decreases to 0.6 for the viewing angles that were greater than 40°. By combining the reflection and transmission characteristics of the NWGP, the proposed package structure of the polarized WLEDs provided a better TM transmittance than that of the WLEDs with an absorptive polarizer. Figure 3 shows the ER versus the viewing angles. The ER of the polarized WLED remained approximately greater than 1000, and gradually decreased for the viewing angles that were greater than 60°. This resulted from decreased TM transmittance and constant TE transmittance of the NWGP at viewing angles greater than 60°. As shown in Fig. 3, the gain of TM transmittance was around 30% comparing with that of the unpolarized WLEDs at viewing angles less than 60° (i.e., the transmittance of the polarized LED is 65%). This results in the enhancement of the phosphor conversion efficiency because of the recycling of the reflected TE wave and the polarization randomization effect. However, by using 3M’s Dual Brightness Enhancement Film (DBEF), the transmitted light increases on-axis luminance of the LCD up to 50~60% for a direct lit backlight [12]. On the other hand, without using the LCD panel's rear polarizer and DBEF, the amount of polarized light available to be transmitted through the LCD panel is 65% for the polarized white LED light source. Therefore, both NWGP and DBEF provide a mechanism to recycle the backlight before entering the LCD panel and result in an improvement in light use efficiency.
This study adopted the International Commission on Illumination's (CIE) 1976 color system, which characterizes colors by luminance parameter L and color coordinates u’ and v’, to specify the points on the chromaticity diagram and offering precise color measurement. Therefore, a CIE-u’v’ chromaticity diagram for the white spectral emission of the WLEDs was used. Figure 4 shows the comparison of the angular dependence color properties for the polarized and conventional WLEDs. For clarity, a number of chromaticity points were labeled with the viewing angle, CCT and the color rendering index (CRI). when the viewing angle was increased from 0° to 70°, the chromaticity coordinates (u’, v’) of the emitted white light varied from (0.214, 0.455) to (0.213, 0.470) with the NWGP, and from (0.209, 0.424) to (0.216, 0.474), without the NWGP. For the viewing angle ranging from 0° to 70°, the CRI of the polarized WLEDs changed from 85 to 83 and from 82 to 83 for the conventional WLEDs. As shown in Fig. 4, the CCT of the polarized WLEDs was lower than that of the conventional WLEDs and approximately 4000K at normal and remained nearly constant with increased viewing angles. Because the CCT of conventional WLEDs decreased with increased viewing angles, the color temperature difference decreased with the increased viewing angles.
Fig. 4 Chromaticity points of the WLEDs with NWGP and conventional WLEDs in CIE u’v’ color space labeled with viewing angle, CCT and CRI.
The variations in the positions of the chromaticity points of the polarized white light emission were attributed to the use of a NWGP. On the other hand, the varying positions of the chromaticity points of the conventional WLEDs resulted from the non-uniform radiation pattern of the blue LED. Based on the chromaticity coordinates, the polarized WLEDs with a NWGP had a smaller shift regarding the location of the chromaticity points than did the conventional WLEDs without a NWGP. These results indicate that NWGP can stabilize the color properties of emitted white light at various viewing angles. Because the TE-polarized light is reflected and the yellow phosphor is re-excited, the ratio of the blue to yellow light emission intensity is reduced. Therefore, the white light emission with lower CCT is achieved for the polarized WLEDs at all viewing angles. By combing the spectral characteristics of NWGPs and the phosphor emission spectrum, the optimized emission spectrum of polarized WLEDs can be obtained using a NWGP design.
Figure 5 shows a comparison of the radiation patterns of the polarized WLEDs with conventional WLEDs at all viewing angles. The radiation pattern of the polarized WLED had better uniformity than did that of the conventional WLEDs. This result attributes to the multiple reflection of the TE wave and the formation of a pumping cavity between the NWGP and the Ag-coated lead frame. Therefore, the uniformity of the transmitted power distribution can be enhanced by implementing of the NWGP. However, the optimized package structure had an air gap between the phosphor resin and the NWGP that was deposited on top of the phosphor resin. Because of the implementation of the NWGP, the package structure of the polarized WLEDs had the advantages of low CCT, high ER, good angular color uniformity, and an improved radiation pattern. For lighting applications, polarized WLEDs can provide white light emission with desired color properties, enhanced spatial uniformity of radiation, good angular color uniformity and wider view angle ranges.
Fig. 5 Comparison of relative radiation intensity versus viewing angle for the polarized and conventional WLEDs.
Conclusions
By combining a NWGP with the packaging structure of conventional WLEDs that are excited by blue LEDs, the proposed polarized WLED packaging structure forms a blue light partially confining cavity structure to enable the pumping and recycling mechanism of TE-polarized waves. Rather than being absorbed by the sheet polarizer used in LCD, the blue photons process yellow light conversion through the pumping cavity formed by NWGPs and the Ag reflector of lead frame; hence phosphor conversion efficiency and transmittance are enhanced. Based on the output characteristic of NWGPs and the optimized package structure of polarized WLEDs, this study proposed a novel approach for fabricating polarized WLEDs with high extinction ratio and transmittance, good angular color uniformity, enhanced spatial uniformity of radiation and wider viewing angles. Also, polarized WLEDs can be designed by combining a NWGP with micro-lenses located on the opposite side of the glass substrate for lighting applications.
Acknowledgment
This work was supported by the National Science Council of the Republic of China under contract No. NSC 99-2221-E-011 −073 and NSC 100-2221-E-011-083.
References and Links
1. M. F. Weber, C. A. Stover, L. R. Gilbert, T. J. Nevitt, and A. J. Ouderkirk, “Giant birefringent optics in multilayer polymer mirrors,” Science 287(5462), 2451–2456 (2000). [CrossRef] [PubMed]
2. J. M. Jonza and D. Dubner Andrew, “Multilayer Polymeric Color-shifting Polarizer Films,” in Optical Security and Counterfeit Deterrence Techniques, SPIE-IS&T Electronic Imaging (SPIE, 2004), 256–263.
3. S. H. Kim, J.-D. Park, and K.-D. Lee, “Fabrication of a nano-wire grid polarizer for brightness enhancement in liquid crystal display,” Nanotechnology 17(17), 4436–4438 (2006). [CrossRef]
4. X. J. Yu and H. S. Kwok, “Optical wire-grid polarizers at oblique angles of incidence,” J. Appl. Phys. 93(8), 4407–4412 (2003). [CrossRef]
5. I. Yamada, K. Takano, M. Hangyo, M. Saito, and W. Watanabe, “Terahertz wire-grid polarizers with micrometer-pitch Al gratings,” Opt. Lett. 34(3), 274–276 (2009). [CrossRef] [PubMed]
6. S.-W. Ahn, K.-D. Lee, J.-S. Kim, S. H. Kim, J.-D. Park, S.-H. Lee, and P.-W. Yoon, “Fabrication of a 50 nm half-pitch wire grid polarizer using nanoimprint lithography,” Nanotechnology 16(9), 1874–1877 (2005). [CrossRef]
7. M. F. Schubert, S. Chhajed, J. K. Kim, E. F. Schubert, and J. Cho, “Linearly polarized emission from GaInN lightemitting diodes with polarization-enhancing reflector,” Opt. Express 15(18), 11213–11218 (2007). [CrossRef] [PubMed]
8. J.-C. Su, S.-F. Song, and H.-S. Chen, “Chromaticity stability of phosphor-converted white light-emitting diodes with an optical filter,” Appl. Opt. 50(2), 177–182 (2011). [CrossRef] [PubMed]
9. Ö. Sepsi, I. Szanda, and P. Koppa, “Investigation of polarized light emitting diodes with integrated wire grid polarizer,” Opt. Express 18(14), 14547–14552 (2010). [CrossRef] [PubMed]
10. D. Kim, “Polarization characteristics of a wire-grid polarizer in a rotating platform,” Appl. Opt. 44(8), 1366–1371 (2005). [CrossRef] [PubMed]
11. F. Meng, J. Chu, H. Han, and K. Zhao, “The design of the sub-wavelength wire-grid polarizer,” in 2007 7th IEEE International Conference on Nanotechnology - IEEE-NANO 2007), 942–946.
12. T. Liu and M. O'Neill, “Increasing LCD energy efficiency with specialty light-management films,” Inform Display 24, 24–30 (2008).
