Nonlinear Optical Absorption and Related Phenomena in Liquid Crystals
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Nonlinear Optical Absorption and Related Phenomena in Liquid Crystals N.V. Tabiryan, U.A. Hrozhyk, H.L. Margaryan, * M.J. Mora, S.R. Nersisyan, S.V. Serak Beam Engineering for Advanced Measurements Co. 686 Formosa Ave. Winter Park, FL 32789 e-mail: [email protected] *
Presently with Yerevan State University, Yerevan, Armenia
1. INTRODUCTION Nonlinear absorption and related phenomena in dye-doped nematic liquid crystals (NLC) have been studied from the standpoint of laser beam control and characterization applications. We have identified three major classes of NLC materials and systems exhibiting nonlinear optical absorption and transmission: NLC doped with dichroic dyes; NLC doped with isotropic dyes operating in crossed polarizer/analyzer pair; and NLC near the critical point of nematicisotropic phase transition. By that we studied azo-dye doped-NLC and isothermal phase transition induced by a laser beam. 2. ORIENTATIONAL NONLINEAR ABSORPTION 2.1. Experimental As a model of a material with strong nonlinear absorption due to laser-induced reorientation of an NLC containing strongly dichroic dye dopants, we used a NLC 5CB, 50-µm thick, oriented homeotropically and doped with a 0.22% antraquinone dye that enhanced its response to the orientational influence of a light beam for about two orders of magnitude1 . This material made possible using low power He-Ne laser for experimentation. The NLC-cell was scanned in the focal region of a lens. All the radiation at the output of the NLC-cell was accumulated by a lens into a power meter, Figure 1. A variable neutral density filter allowed to continuously control the output power of the laser beam to minimize the effects of heating and saturation. The beam was switched-on abruptly and the time dependence of the output power was registered. A characteristic curve is presented in Figure 2. The power at the initial time moment, when the laser beam is switched on, corresponds actually to the power that is transmitted through the cell without nonlinear effects. Due to reorientation of the NLC by the laser yielding in reorientation of the dichroic dye, the output power is decreased and reaches equilibrium in about τ = 2-3 s. This is a characteristic time for the reorientation processes of the NLC. Indeed, τ ~ γL2 /π2 K ~ 2.5 s for the viscosity of NLC γ = 1 Poise, elastic constant K = 10-6 erg/cm and the thickness of the NLC layer L = 50 µm.
CC4.5.1
z Focusing lens Power meter Laser
Nonlinear material
Collecting lens
Figure 1. The schematic of the experiment: the laser beam is focused on an NLC-film, and the radiation at the output of the film is collected into a power meter. The position of the cell is varied in the focal region.
Transmission
1 0.8 0.6 0.4 0.2 0 0
2
4
6
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Time, s Figure 2. Demonstration of the decrease in the output power in time due to nonlinear absorption when the laser beam is abruptly switched-on.
The experiment was performed for lenses of different focal length and for different positions of the NLC in the focal region of the lenses. The resu
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