Negative dispersion retarder using two negative birefringence films Anoop Kumar Srivastava, Seungbin Yang, and Ji-Hoon Lee* Advanced Electronics and Information Research Center, Division of Electronics Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, South Korea *
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Abstract: The achromatic response and wide viewing angle for varying wavelength of incident light are of long waiting research to be utilized it for the display devices. Such response can be obtained by employing the retarder that exhibits negative birefringence and negative dispersion. In this paper, negative dispersion half-wave retarder and negative dispersion quarter-wave retarder have been demonstrated by optimizing the retardation and the angle between the extraordinary axes of polystyrene and polymethylmethacrylate films. The optimum angles for half and quarter-wave retarders were found to be 40° and 70°, respectively for different retardation values of polystyrene and poly-methylmethacrylate films. ©2015 Optical Society of America OCIS codes: (160.1190) Anisotropic optical materials; (160.3710) Liquid crystals
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Received 24 Mar 2015; revised 27 Apr 2015; accepted 4 May 2015; published 8 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.013108 | OPTICS EXPRESS 13108
1. Introduction Generally, the phase retardation Г≡2πΔnd/λ of the natural birefringent materials decreases with longer λ, called as positive dispersion (PD) of birefringence. Here, Δn is defined as ne-no, where ne and no are the extraordinary and ordinary refractive index, respectively, and d is the thickness of the film. The PD medium also shows a decrease of Re(λ)≡Δnd with longer λ and this limits the bandwidth of the compensation film. There have been many efforts to make a retarder with negative dispersion (ND) of birefringence whose birefringence or Re(λ) get increased with longer λ [1–4]. The ND retarder has a wider bandwidth than PD retarder, thus can be an achromatic retarder with constant phase retardation over the wide range of λ. The well-known method to obtain the ND retarder is stacking PD retarders with different dispersion property of Re(λ) [1–6]. Multilayers of twist-oriented reactive mesogen have been also reported to show ND of birefringence [7,8]. Recently, single layer approaches using copolymers [9–11], biaxial reactive mesogens [12], and smectic liquid crystal-polymer composites have been developed [13–15]. Further, all previous ND retarders have been made of positive birefringence (PB) materials [16]. The PB medium means a substance showing ne>no, thus giving a positive sign of Δn. With this reason, the Rth value defined as [(nx + ny)/2-nz]d was always positive, where nx and ny are the in-plane refractive indices of the retarder and nz is the refractive index to the surface normal direction. To minimize the viewing angle dependence of the liquid crystal display (LCD) devices, the Rth value should be zero and this can be realized provided nz = (nxny)1/2 [16]. Since most of the liquid crystal displays (LCDs) use the liquid crystal (LC) with PB property, the Rth value of the LC layer is positive. Thus, to make the Rth of the LCD to be zero, the Rth value of the compensation film should negative. This can be satisfied by using a retarder which have negative birefringence (NB), ne
