Increasing the lasing efficiency in cholesteric liquid-crystal photonic structures
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Increasing the Lasing Efficiency in Cholesteric LiquidCrystal Photonic Structures S. P. Palto, N. M. Shtykov, M. I. Barnik, and B. A. Umanskiі Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiі pr. 59, Moscow, 119333 Russia email: [email protected] Received July 27, 2009
Abstract—Two types of lasing in cholesteric liquid crystals (LCs) in the range of luminescence of laser dye molecules have been investigated. The first type belongs to the Bragg modes at the photonic band edge, which propagate along the normal to the LC layer. The second type of lasing is related to the modes leaking into the substrate and propagating at small angles to the LC layer. It is shown that the Bragg lasing efficiency can be significantly increased under wideaperture optical pumping. The method proposed for increasing the lasing efficiency is based on suppressing the excitation of leaky laser modes using partially absorbing thin films as the coatings for LCorienting substrates. Both experimental results and the theoretical model of the effect using the numerical simulation data are discussed. DOI: 10.1134/S1063774510020215
INTRODUCTION Lasing in liquid crystals (LCs) is a new and actively developing field in science and technology [1–11]. LC lasers have a number of specific attractive properties. As in conventional liquid dye lasers, the lasing wave length can be arbitrary and tunable in a very wide spec tral range, including UV, visible, and nearIR regions. However, in contrast to cumbersome liquid dye lasers, LC lasers can be very small and controlled by a low electric voltage [6, 7]. This opens up new possibilities for the applications that are related to the formation of fieldcontrolled matrix laser panels. Along with practical applications, there are purely scientific problems related to the insufficient study of the physical processes of exciting lasing in LC photonic media. One such problem is the lasing efficiency in cho lesteric liquid crystals (CLCs) doped with laser dyes. In this paper we report the results of studying one of the spurious processes which reduce the lasing effi ciency at the photonic band edges in CLCs and the measures taken to suppress this process. The case in point is the excitation of undesirable leaky laser modes, when the optical pumping region (aperture) greatly exceeds the CLC layer thickness [8–10]. The term “leaky mode” stems from the fact that these modes are excited due to the deep positive feedback for light prop agation directions at angles close to the total internal reflection angle at the LC/substrate interface. Actually, laser radiation leaks out of the LC film into the glass substrate to propagate in it at very small angles to the substrate surface. In view of the smallness of the angles
and the effect of total internal reflection, the radiation can emerge only from the substrate end faces. The exci tation mechanism of leaky modes was considered theo retically in detail in [8], and this type of lasing was stud ied experimentally in [9, 10].
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