Single layer retarder with negative dispersion of ... - OSA Publishing

Vol. 24, No. 17 | 22 Aug 2016 | OPTICS EXPRESS 19934

Single layer retarder with negative dispersion of birefringence and wide field-of-view JIYONG HWANG,1 SEUNGBIN YANG,1 YU-JIN CHOI,2 YUMIN LEE,2 KWANGUN JEONG,2,3 AND JI-HOON LEE1,4 1

Division of Electronics Engineering, Chonbuk National University, Jeonbuk 54896, South Korea Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonbuk 54896, South Korea 3 [email protected] 4 [email protected] 2

Abstract: A single layer retarder possessing negative dispersion (ND) of birefringence as well as wide field-of-view (FOV) was long-term objective in optical science. We synthesized new guest reactive monomers with x-shape and mixed them with the host smectic reactive mesogen. The host-guest molecules formed two dimensionally self-organized nanostructure and showed both the ND of birefringence and wide FOV properties. We simulated the antireflection property of a circular polarizer using the optical properties of the retarder. The average reflectance of the retarder was 0.52% which was much smaller than that of the commercial single layer ND retarder 1.83%. © 2016 Optical Society of America OCIS codes: (160.1190) Anisotropic optical materials; (160.3710) Liquid crystals.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

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#268760 Journal © 2016

http://dx.doi.org/10.1364/OE.24.019934 Received 22 Jun 2016; revised 16 Aug 2016; accepted 17 Aug 2016; published 19 Aug 2016


1. Introduction A single layer retarder possessing both negative dispersion (ND) of birefringence and wide field-of-view (FOV) was a long-term objective in optical science [1]. The ND of birefringence means that the birefringence Δn≡nx-ny is in increasing with a longer wavelength of light λ. Here, nx and ny are the principle in-plane refractive indices along the x-and y-axis, respectively. The ND of birefringence also results in the ND of in-plane retardardation which is defined as Re(λ)≡Δnd, where d is the thickness of the retarder. The ND retarder can make the phase retardation which is defined as Г(λ)≡2πΔnd/λ to be constant regardless of λ. There have been many efforts to make the ND retarder [2–10]. The previous ND retarders can be classified into a multi-layers type [2–6] and a single layer type [7–10]. Meanwhile, the retarder with a wide FOV has been also developed and widely used in commercial display compensation films [11–16]. Most of the commercial wide FOV retarders are Lyot-Iand Lyot-II type. The Lyot-Itype retarder is composed of two positive a-plates and one half-wave plate. The slow axes of the a-plates are orthogonally aligned and the half-wave plate is inserted between them [11]. The Lyot-II type retarder is made by one positive a-plate and one negative a-plate [11–14]. The slow axes of two plates are generally orthogonal. Although the retader possessing wide FOV has been studied, all previous researches were limited to the multi-layer approach [11–16]. We need to mention that all retarders showing both ND property and wide FOV were multi-layer type. Thus, the single layer type ND retarder with the wide FOV has not been reported yet. In this paper, we first reported a single layer ND retarder with a wide FOV property. The single layer ND retarder with a wide FOV can be fabricated by a simple process and has a potential of flexible display application owing to its thin thickness. In our previous report [10], we achieved the single layer ND retarder using a 2dimensional self-organization of host smectic liquid crystal and guest monomer. The guest molecules absorbing a longer wavelength of UV were located at the inter-layer space of the host smectic liquid crystals absorbing a shorter wavelength of UV light. Consequently, ne was smoothly decreased with λ, while no was rapidly decreased with λ, resulting in the ND of birefringence. Although the dispersion property of the retarder was close to the ideal achromaticity, the FOV property of the retarder was not wide enough. In this paper, we synthesized new guest molecules with long alkyl chains orthogonally connected to the rigid core [Fig. 1] and investigated their effect on the FOV property. When one of the principal axes of a biaxial retarder is perpendicular to the surface (z-axis) and the other two axes are in the surface plane (xy-plane), the phase retardation Г of the incident light is given by [1], Γ = (k2 z − k1z )d = (γ 2 − γ 1 )k0 d

(1)

γ 1 = k1z / k0

(2)

γ 2 = k 2 z / k0

(3)

k0 = 2π / λ

(4)

where k2z and k1z are the wave vector of the double refracted waves. Neglecting high order terms, Г can be approximated to, Γ = [(nx − n y ) + (nz2 − nx n y )(cos 2 φ / n y − sin 2 φ / nx )(sin 2 θ / 2nz2 )]k0 d

(5)

It should be noted that Г is independent of polar viewing angle θ and azimuthal viewing angle φ provided, nz2 = nx n y

(6)


This condition is also equivalent to the out-of-plane retardation Rth becomes zero. Rth ≡ [(nx n y )1/ 2 − nz ]d ≈ [(nx + n y ) / 2 − nz ]d

(7)

2. Experimental procedure

Fig. 1. Chemical structure of the host smectic RM HCM026 and the guest N1, N2 monomers.

Figure 1 shows the chemical structures of the host smectic reactive mesogen (RM) HCM026 (HCCH) [Fig. 1(a)] and the newly-synthesized guest monomers named as N1 [Fig. 1(b)] and N2 [Fig. 1(c)]. The chemical structure of the molecules were confirmed by NMR and FT-IR methods. The detailed procedure of the synthesis will be announced elsewhere [17]. The HCM026 has a phase sequence of Cryst. (110 °C) SmA (138 °C) N (156 °C) Iso. The guest monomers have a pair of alkyl chains (n = 8) perpendicular to the core and can be intercalated with the host smectic RM. In addition, the imide group (-Ar-N-) in N1 and imine group (-C = N-) in N2 were inserted to absorb a UV light close to the visible range for the rapid decrease of no in the visible wavelength range. We mixed the host smectic RM and the guest N1 and N2 in the cyclopentanone solvent and stirred for 10 min. The 50 and 55 wt% N1-mixed HCM026 showed smectic phase at 109-120 °C and 111-119 °C, respectively. The 40 and 50 wt% N2-mixed HCM026 showed smectic phase at 103-131 °C and 94-121 °C, respectively. To obtain uniform orientation of the RM molecules, a planar alignment polyimide PIAX189-KU1 (JNC) was coated on the substrate and baked at 210 °C for 1 h. Then, the film was unidirectionally rubbed with a cotton cloth. The RM solution was spin-coated on the substrate and the solvent was evaporated at 110 °C for 3 min. Then, a UV light with an intensity of 30 mW/cm2 was exposed to polymerize the RM molecules for 5 min with nitrogen gas purged. The Re(λ) and Rth values were measured with a commercial retardation measurement system Axo-scan (Axometrics). To measure the small angle x-ray scattering (SAXS) spectrum, the RM mixtures were inserted into a glass capillary tube with a thickness of 0.1 mm and then exposed to the UV light at the same temperature where the compensation films were fabricated. 3. Results and discussion Figure 2 shows Re(λ) of the host HCM026, N1-mixed, and N2-mixed HCM026 films. The pure HCM026 showed a positive dispersion (PD) of Re(λ). The 50 wt% N1-mixed HCM026 showed flat dispersion and 55 wt% N1-mixed HCM026 showed ND of Re(λ) [Fig. 2(a)]. On the other hand, the N2-mixed sample showed ND property when the guest concentration was over 40 wt% [Fig. 2(b)]. Thus, the magnitudes of Re(λ) of the N1- and N2-mixture was similar, but their dispersion property was different. Figures 2(c) and 2(d) shows Re(λ) of the corresponding films normalized to Re(550 nm). It was observed that the dRe(λ)/dλ was increased with increasing the cocentration of the N1 and N2 guest molecules. Given the same concentration of the guest molecules 50 wt%, the slope of Re(λ) of the N2-mixed HCM026 was greater than that of the N1-mixed HCM026. The 50 wt% N2-mixed HCM026 film showed Re(450 nm)/Re(550 nm) = 0.75 and Re(650 nm)/Re(550 nm) = 1.15 close to the ideal curve [line in Fig. 2(d)].


Fig. 2. Dispersion of Re(λ) of (a) the HCM026, the N1-mixed HCM026 and (b) the N2-mixed HCM026 vs. λ. (c) and (d) correspond to Re(λ) of the mixtures normalized to Re(550 nm). Table 1. Re(550 nm) and Rth(550 nm) of the HCM026, the 50 wt% N1-mixed-, and the 40 wt% N2-mixed HCM026 films Sample

Re(550 nm)

Rth(550 nm)

Pure HCM026

138 nm

92.5 nm

HCM026 + 50wt% N1

138 nm

47.3 nm

HCM026 + 55wt% N1 HCM026 + 40 wt% N2 HCM026 + 50wt% N2 Commercial ND retarder WR-S (Teijin)

138 nm 138 nm 138 nm 138 nm

41.5 nm 28.8 nm 17.5 nm 70 nm

To examine the viewing angle property of the N1- and N2-HCM026 mixtures, we measured the Rth value of the samples [Table 1]. To compare Rth of the mixtures, Re(550 nm) was set to be 138 nm. The pure HCM026 showed Rth(550 nm) = 92.5 nm which is similar to Rth of the general single layer ND retarders [7–10]. On the other hand, Rth(550 nm) of the 50 and 55 wt% N1-mixed HCM026 was 47.3 and 41.5 nm, respectively, which was about a half of the pure HCM026. Rth(550 nm) of the 40 and 50 wt% N2-mixed HCM026 was 28.8 and 17.5 nm, respectively, which was even smaller than the N1-HCM026 mixture. The Rth(550 nm) of the commercial ND retarder WR-S (Teijin) is about 70 nm. As far as we know, the suggested result is the smallest Rth value which has been reported in the single layer ND retarders. To investigate the performance of the N1- and N2-mixed HCM026, we simulated the reflectance of the circular polarizer for the antireflection (AR) of the organic light emitting diode (OLED) using Table 1 based on the Extened Jones matrix method [Fig. 3]. The circular polarizer is composed of a linear polarizer and a quarter wave plate (QWP). The optic axis of the QWP makes 45° to the transmission axis of the polarizer [17]. The AR film with the QWP made from the pure HCM026 showed a significant light leakage at wide viewing angle and the average reflectance was 3.34% [Fig. 3(a)]. The 55 wt% N1-mixed HCM026 showed better viewing angle dependence and the average reflectance was 0.99% [Fig. 3(b)]. The 50 wt% N2-mixed HCM026 showed the least viewing angle dependence and the average reflectance was 0.52% [Fig. 3(c)]. The reflectance of the 50 wt% N2-mixture was even smaller than that of the commerical single layer ND retarder WR-S 1.83% [Fig. 3(d)]. The


better viewing angle property of the N1- and N2-mixed HCM026 can be explained by the decrease of θ and φ dependence in Eq. (5) due to the reduction of Rth in Eq. (7).

Fig. 3. Simulated reflectance from OLED using a QWP made of (a) the pure HCM026, (b) 55 wt% N1-mixed HCM026, (c) 50 wt% N2-mixed HCM026, and (d) commercial WR-S films. The average reflectance was (a) 3.34, (b) 0.99, (c) 0.52%, and 1.83%, respectively.

To investigate the orientation of the molecules, we measured the SAXS spectra of the pure HCM026 and the N2-HCM026 mixtures [Fig. 4(a)]. Because the N2-HCM026 mixture showed better ND [Fig. 2] and viewing angle properties [Fig. 3], the SAXS data of the N2 mixture was suggested here in after. The pure HCM026 showed the resonance peak at 2δ = 5.4° corresponding to the layer spacing of 1.6 nm. The pure N2 guest showed a resonance at 2δ = 5.1° corresponding to 1.7 nm. The N2-HCM026 mixture showed three resonance peaks at 2δ = 2.9°, 3.8°, and 5.1°, corresponding to 3.1 nm, 2.3 nm, and 1.5 nm, respectively.

Fig. 4. (a) SAXS spectra of the polymerized host HCM026, guest N2, and the 50 wt% N2mixed HCM026. Schematic illustration of the possible molecular orientation where the aromatic core of the N2 is aligned (b) perpendicular or (c) parallel to the smectic layer plane.

The peak of the N2-HCM026 mixture at 2δ = 5.1° can be understood as the domains of HCM026 and N2 molecules. On the other hand, another peaks at 2δ = 2.9° and 3.8° are due to the intercalated structure between the HCM026 and the N2 molecules. We think that two kinds of the molecular orientation are responsible for these two peaks. First, the aromatic cores of the N2 are aligned parallel to the HCM026 molecules and also to the rubbing direction [Fig. 4(b)]. Because the layer spacing of the pure HCM026 and the N2 are about 1.6 nm, this orientation can show the layer spacing similar to 3.1 nm. The other case is the aliphatic chains of the N2 molecules are aligned parallel to the HCM026 molecules with their alkyl chains intercalated [Fig. 4(c)]. The former case is certainly easy to reduce the surface anchoring energy, while the latter case is easy to decrease the elastic energy of the system.


We are planing further experiments to examine the surface effect on the host and guest molecules orientation.

Fig. 5. Schematic illustration of the Rth reduction mechanism by the X-shaped guest molecules.

For the reason of the Rth reduction in the N1- and N2-mixed HCM026 samples, we think some of the guest molecules were vertically oriented as depicted in Fig. 5. Because the alkyl chains of the vertically aligned guest molecules are still intercalated with those of the host molecules, the vertical orientation can exist without a large energy penalty. The vertically oriented molecules result in the increase of nz and consequently, Rth is decreased showing a wide FOV property.

Fig. 6. (a)-(b) POM texture of the pure HCM026 when the rubbing direction was at 0° and 45° to the polarizer, respectively. (c)-(d) POM texture of the 50 wt% N2-mixed HCM026. The scale bars are 100 μm.

We also observed the polarizing optical microscopy (POM) texture of the pure HCM026 [Figs. 6(a) and 6(b)] and 50 wt% N2 mixed-HCM026 [Figs. 6(c) and 6(d)] samples. Both samples showed homogeneous texture without no phase-separated domains. The N1 mixedHCM026 also showed similar homogeneous orientation. Although the resonance peak at 2δ = 5.1° was observed in the N2-HCM026 mixture sample, we could not observe overmicronsized domains showing different texture. This results are distinctly different from our previous result [10] wherein a large fraction of phase-separated polymers were observed. The homogenous texture of the N2-HCM026 mixture seems to be due to the increased miscibility between the constituent molecules by introducing long alkyl chain to the guest molecules. 4. Conclusion To summarize, we demonstrated a single layer retarder with ND of birefringence and wide FOV. The 50 wt% N2-mixed HCM026 film showed Re(450 nm)/Re(550 nm) = 0.75 and Re(650 nm)/Re(550 nm) = 1.15 and Rth(550 nm) = 17.5 nm. The measured Rth value was much smaller than that of the commercial ND retarder about 70 nm. The AR film using the 50 wt% N2-mixed retarder showed 0.52% reflectance and a wide FOV. Funding Ministry of Trade, Industry, and Economy (MOTIE) and Korea Display Research Consortium (10051334); National Research Foundation (2016R1A2B4010361, 2015042417)