Narrowing spectral width of green LED by GMR structure to expand color mixing field

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1 Narrowing spectral width of green LED by GMR structure to expand color mixing field S. H. Tu 1, Y. C. Lee 2, C. L. Hsu 1, W. P. Lin 1, M. L. Wu 1, T. S. Yang 1, J. Y. Chang 1 1. Department of Optical and Photonics, National Central University, Jhongli, Taiwan 321, ROC 2. Optical Science Center, National Central University, Jhongli, Taiwan, ROC jychang@ios.ncu.edu.tw Abstract: The LED s narrow spectral width and enlarged color gamut promotes its application as the backlight for large LCD panels. Compared to blue and red LEDs, the green LEDs have a wider spectral width, the main reason for cutting out undesired parts of the spectrum to enhance the color mixing field. In this study, we adopt guided-mode resonance filter with a narrow linewidth to reduce the FWHM from 3 nm to 2 nm. The asymmetric grating profile, attained by dividing the one-filling-factor binary structure into one with two distinct filling factors within an identical period, is used to control the resonance profile under the normal incidence with TE and unpolarization modes. Meanwhile, strong modulation is achieved by designing a silicon grating on a quartz substrate with air grooves, which is employed to examine the angular tolerance for the stability of the resonance. Finally, the range of color gamut is shown to expand based on various grating designs. 26 Optical Society of America OCIS codes: (.) General. (undetermined) References and links 1. David L. Brundrett, Elias N. Glytsis, and Thomas K. Gaylord, Normal-incidence guided-mode resonant gratings filters: design and experimental demonstration, Opt. Lett. 23, 7-72 (1998) 2. S. Tibuleac and R. Magnusson, Narrow-linewidth bandpass filters with diffractive thin-film layer, Opt. Lett. 26, (21) 3. S. Tibuleac and R. Magnusson, Reflection and transmission guided-mode resonance filter, J. Opt. Soc. Am. A 14, (1997) 4. I. A. Avrutskiıˇ, V. P. Duraev, E. T. N. A. M. Prokhorov, A. S. Svakhin, V. A. Sychugov, and A. V. Tishchenko, Optimization of the characteristics of a dispersive element based on a corrugated wavguide, Sov. J. Quantum Electron. 18, (1988).. S. S. Wang and R. Magnusson, Theory and applications of guided-mode resonance filters, Appl. Opt. 32, (1993). 6. J. Saarinen, E. Noponen, and J. Turunen, Guided-mode resonance filters of finite aperture, Opt. Eng. 34, (199) 7. S. M. Norton, T. Erdogan, and G. M. Morris, Coupled-mode theory of resonant grating filters, J. Opt. Soc. Am. A 14, (1997). 8. David L. Brundrett, Elias N. Glytsis, Thomas K. Gaylord and Jon M. Bendickson, Effects of modulation strength in guided-mode resonant subwavelength gratings at normal incidence, J. Opt. Soc. Am. A vol 17, (2). 9. Y. Ding and R. Magnusson, Use of nondegenerate resonant leaky modes to fashion diverse optical spectra, Opt. Exp.12, (24) 1. S. S. Wang and R. Magnusson, Theory and applications of guided-mode resonance filters, Appl. Opt. 32, (1993). 11. C. L. Hsu, M. L. Wu, Y. C. Liu, Y. C. Lee and J. Y. Chang, Flattened Broad-Band Notch Filters Using Guided-Mode Resonance Associated with Asymmetric Binary Gratings, submitted to IEEE Photonics Technology Letters.

2 1. Introduction At present, cold cathode fluorescent lamps (CCFLs) are still the main-stream source for the backlighting of large LCD displays. However, the rapid development of solid state technology and advances in packaging skills suggests that LEDs will progressively replace CCFLs in the near future. Compared to CCFLs, LEDs have many advantages such as narrow spectrum, good mechanical stability, long lifetime, high frequency dynamical operation, and they are environmentally friendly and mercury-free. As a light source for backlighting, single and multi-colored LEDs exhibited a broader color range and higher efficiency than white light generated by blue LEDs mixing with phosphor. The challenge still remains of continuing continual requirement for a larger color gamut leading to the demand for a narrower spectrum of RGB-LEDs. Of the currently available LEDs, the spectral width of the green LED is wider, the FWHMs are 2, 3 and 2 nm for blue, green and red respectively. If the spectral width could be reduced adequately, this would also head to the color gamut of large display panels be broadened. The well-known thin film filters have been widely employed as a narrow band filters in laser cavities, a light modulators and optical communication components. However to fabricate a narrow linewidth filter, one basically needs several tens or even hundred of layers with tightly stringent tolerances. Here we propose a relatively simple structure, a guided mode resonance (GMR) filter, which operates on a resonance effect, to take place of the conventional thin film filter. The GMR filter is particularly suitable for application in narrowor broad-band spectral controls.[1, 2] Its elemental structure consists of a waveguide layer to couple an incident plane wave into a leaky mode and a grating layer to couple out the leaky mode into the radiation mode. The GMR structures in identical to the characteristics of thin film filter could also be designed to satisfy high transmission or high reflection requirements. Resonant gratings are a powerful tool for filtering applications, but the resonance peak obtained by exciting one single mode does not possess all of the properties required for a useful device. Numerous studies indicate that the coupling and radiation of a GMR filter with a narrow-band is highly sensitive to the wavelength and polarization of light and angle of incidence [4-6]. Norten et al. [7] found that the radiative loss is proportional to the index modulation of the grating and Brundrett et al. [8] further identified for both of the strong and weak modulated gratings in relation to the sensitivity of incident angle. In results demonstrate that strong modulation contributes to the stability of the incident angle without damaging the desired narrow linewidth, which is related to weak modulation. In this letter, we discuss adding a transmission GMR filter to a green LED with a high peak transmissing and narrow linewidth to reduce the spectral width of output field and meanwhile broad the color gamut. To control the unpolarized and multi-angle characteristics of LEDs, we analyze the filtering effect of symmetric and asymmetric GMR structures, and investigate the color gamut based on TE-polarized and unpolarized modes. Finally, the angular tolerance is also discussed. 2. Spectral properties of GMR filters The basic structure of the GMR filter consists at a top grating layer and a waveguide layer on a lower substrate as shown in Figs. 1. The corresponding structural parameters include indices for the grating ridge n g, waveguide n w, and substrate n s, grating period Λ, grating filling factor f, grating depth d g, and waveguide thickness d w. When the phase-matched conditions satisfy an external incident field, resonance occurs, reading to non-transmission and total reflection at the resonance wavelength.

3 Grating n g Waveguide n w Substrate n s Grating n g Waveguide n w Substrate n s Fig. 1. Illustration of basic GMR filters associated with symmetric and asymmetric grating structures on a planar waveguide constructed on a transparent substrate. Fig. 2 shows the proposed arrangement for reducing the spectral width of a green LED chip. As shown in Fig. 2, the green LED chip is placed at the center of an indented sub-mount. A GMR filter is then connected to the LED sub-mount with the fixed green LED chip. When light emitted from green LED chip pass through the GMR filter, the spectral width of output spectrum will be reduced, enhancing the color gamut. Output field Substrate θ GMR structure Green LED chip LED sub-mount Fig. 2. Illustration of the structural arrangement for reducing the output spectral width of a green LED chip. Fig. 3 shows the measured spectrum of the green LED chip discussed in this paper. The peak wavelength is at 2 nm with a FWHM of 3 nm. Since the color gamut can be enhanced by reducing the spectral width, the GMR filter must be designed to restrain the intensity away from the peak wavelength. The rigorous-coupled-wave-analysis (RCWA) method is used to analyze the properties of GMR filters to find the most suitable structure for order to reducing the spectral width. Since the output field of a LED chip is an unpolarized and multi-directional emission, as shown in Fig. 2, the polarization dependence and angular tolerance of the GMR filter are of great concern when designing a GMR structure with which to reduce the spectral width.

4 LED power (a.u.) 3 Measured green LED spectrum Wavelength (nm) Fig. 3. Measured spectrum of a green LED chip. The initial GMR filter is designed using the structure show in Fig. 1. Quartz is used as the substrate with a refractive index of n Quartz =1.46. The SiN x film with a refractive index at n SiNx =2.18 is used as the grating and waveguide materials in the GMR structure. The incident wave used to investigate the transmission properties of a GMR filter is assumed to be an unpolarized plane wave under normal incidence. To excite a resonance at the peak wavelength of the green LED chip, the structure parameters are set to be: grating period =.27 µm, grating thickness d g =.24 µm, waveguide thickness d w =.2 µm. The grating profile is symmetric as shown in Fig. 1, with a filling factor of f=.. Fig. 4 shows the transmission spectrum of the designed GMR filter, obtained using the RCWA method. The two resonance dips beside the LED peak wavelength (as shown in Fig. 4), are due to the polarization dependence corresponding to the grating geometry. Fig. 4(c) shows the spectrum of the light propagating through the designed GMR filter in a normal direction. It is observed that there is no significant reduction in spectral width. In other words the designed GMR structure is not suitable for this reducing the spectral width. An alternative design of GMR structure is needed to overcome this problem. Since a typical GMR filter results in bandstop transmission, a transmission bandpass at LED peak wavelength and a corresponding bandwidth narrower than the spectral width of LED would be a fruitful candidate to reduce the spectral width.

5 GMR filter Power (a.u.) Transmitance Power (a.u.) (c) Wavelength (nm) Fig. 4. Measured spectrum of the Wgreen l LED th ( chip, ) Spectral of the GMR filter and (c) output light passing through the GMR filter. In order to better control on the resonant modes of GMR filters a method different from the usual symmetric grating profiles what result in a monotonous spectral response have been proposed by researchers discussing on asymmetric grating profiles, see Fig. 1. Ding and Magnusson [9] utilized a single-layered asymmetric grating to control the spectral width of bandpass and bandstop filters. They adjusted the filling factor and modulation amplitude. Resonance peaks exist at each edge of stopbands of asymmetric grating. The separation of two GMRs can also be controlled by the width of the bandgap which is associated with the modulation strength [1]. This means that, the optical spectra can be designed by the asymmetric grating and its modulation amplitude. After experimental demonstration by Hsu et al. [11] showed a wide flattened stopband made from a asymmetric silicon grating that successfully formed on a quartz substrate. In this study, we also adopt this concept, taking advantage of the asymmetric profile and the strong modulation to design narrow bandpass filters for the spectral control of green LEDs. The asymmetric grating structure is used to designed the GMR structure which possess a transmission passband at the peak, LED wavelength and a bandwidth narrower than 3 nm. Si with refractive index of n Si =3.48 is used as the grating and waveguide material of the GMR filter on quartz substrate. The incident light is assumed to be a normally incident TE wave this simplifies the analysis and find out suitable parameters to reduce the spectral width. Fig. shows several designed GMR filters associated asymmetric grating profiles.

6 LED power (a.u.) Asymmetric GMR / TE mode GMR filter LED power (a.u.) Asymmetric GMR / TE mode GMR filter LED power (a.u.) Asymmetric GMR / TE mode GMR filter Fig.. Spectra of GMR filters associate with an asymmetric grating structure and corresponding filtered LED spectrum for a normally incident TE plane wave. The structure parameters of GMR filters are as follows: Λ=.42um, f 1 =., f 2=.3, d g=.22 µm, d w=.46 µm, Λ=.44 µm, f 1=.8, f 2=.3, d g=.2 µm, d w =.4 µm, and (c) Λ=.329 µm, f 1 =.1, f 2 =.373, d g =.18 µm, d w=.7 µm. It is observed that the spectral widths of green LED chips are reduced when the propagate through a GMR filter with an asymmetric grating profile, ie. A normally incident TE plane wave. The corresponding reductions in spectral width are described below: in Fig., the FWHM is reduced from 3 nm at a wavelength of 2 nm to FWHM 1.8 nm at 22nm; in Fig. the FWHM is reduced from 3 nm at a wavelength of 2 nm to FWHM 16.9 nm at 18 nm; in Fig. (c) the FWHM is reduced from 3 nm at wavelength 2 nm to FWHM 19 nm at 18 nm. The enhanced color gamut corresponds to the spectral modifications. The angular tolerance will be described in the next section. Figs. 6 shows the spectral responses corresponding to the structures utilized in Figs. for normally unpolarized plane waves. The corresponding spectral reductions are described below: in Fig. 6, the FWHM is reduced from 3 nm at a wavelength at 2 nm to FWHM 14 nm at 21 nm; in Fig. the FWHM is reduced from 3 nm at wavelength 2 nm to FWHM 19 nm at 18nm; in Fig. (c) the FWHM is reduced from 3 nm at wavelength 2 nm to FWHM 19 nm at 19 nm. It is notable that the effects of spectral reduction remain even though the output efficiencies decline due to the polarization dependence. However,due to significant spectral reduction the color gamut is still enhanced. (c)

7 LED power (a.u.) Asymmetric GMR / unpolarization mode GMR filter LED power (a.u.) Asymmetric GMR / unpolarization mode GMR filter LED power (a.u.) Asymmetric GMR / unpolarization mode GMR filter Fig. 6. Spectra of GMR filters associate with an asymmetric grating structure and corresponding filtered LED spectrum with a normally incident unpolarized plane wave. The corresponding structure parameters of GMR filters are identical to the structure utilized in Fig.. (c) 3. Analysis of gamut for LED spectrum Based on above, analysis RCWA methods, the color gamut corresponding to the green LEDs filtered by the designed GMR filters associate, with an asymmetric grating profile is also investigated Fig. 7 shows the CIE x-y chromaticity coordinate system for analyzing the just-noticeable-color differences in the LED with three various filters, Figs. and 6 show a comparison to the original LED. The area inside the triangle represents the achievable color coordinates for an LED lamp using 63 nm red, 4 nm blue and 2 nm green. The color gamut of original LED is 1.22 times than that of the standard NTSC. After propagating through the GMR filters, the position of the upper apex of the triangle move toward the rim of the chromaticity coordinate diagram meaning the color gamut is expanded. This is due to the purer green LED both of TE polarization and unpolarized waves. Although BA2 has the narrowest FWHM, the peak wavelength shifts to a longer wavelength from 2 nm to 22 nm, leading to a smaller color gamut than BA3 and BA4 which shift to 18 nm and 2 nm, respectively. To approximate the real characteristics of LEDs, we analyze the resonance profile using the same GMR structures to and the color gamut for the normal incidence of unpolarized plane wave as shown in Fig. 6. The color gamut of unpolarized input is little bit smaller than the TE mode s, but the filtering intensity shows an obvious decrease.

8 Fig. 7 CIE x-y chromaticity coordinate system for filtered LEDs under TE polarized and unpolarized. BA2 corresponds to the structure utilized in Fig. and 6. BA3corresponds to the structure utilized in Fig. and 6. BA4 corresponds to the structure utilized in Fig. (c) and 6(c). 4. Angular tolerance in normal incidence Considering the large difference of the refractive index between GaN (n GaN ~ 2.) and air, the critical angle for photons to escape from the GaN LED mesa structure is approximately 23. Thus GMR filters designed for LEDs should have a large angular tolerance, which will modify the spectral of LEDs. Brundrett et al. [8] investigated the evolution of the scattering resonance as the index modulation is increased by decreasing the refractive index of the grooves. They recommended adopting a strong modulation to obtain a larger angular tolerance of GMR filer. For the above design, we also using a silicon grating on a quartz substrate with the groove material set to be air. Fig.8 demonstrates the output power as a function of the wavelength and incidence angle under unpolarized incidence. The 2-D diagram in Fig. 8(b, c, d) corresponds to the structures explained in Fig. (a, b, c). The resonance is stable with variations of the incident angle in the three different structures. The optimal designs shown in Fig. 8(c, d) there is near equivalent similar output power intensity around 1.

9 (c) (d) Fig. 8 Analysis of the angular tolerance of a filtered LEDs. the unpolarized mode corresponds to Fig. 6, the TE mode corresponds to Fig., (c) TE mode corresponds to Fig., and (d) TE mode corresponds to Fig. (c).. Conclusions Various GMR filters have been analyzed by simulating their spectral width together with efficiency under TE mode and for unpolarized incidence. The color gamut after mixing RGB- LED is expanded considerably regardless the incident polarization, but the filter intensity needs to be further optimized. The angular tolerance appears to be favorable for multi-angled incidence but still far out ranges the critical angle of the conventional GaN LEDs. In this paper, we suggest the possibility of using a simple GMR structure as a filter to control the spectral width of a green LED. It is anticipated that modulation at the structure will end in discovery of the excited applications. Acknowledgments The authors acknowledge the financial support by grant 94-EC-17-A-7-S1-43 from the Ministry of Economics, Taiwan.