Molecular reorientation based diffraction in a hybrid liquid crystal system applicable for two different laser sources simultaneously
Introduction
Liquid crystals (LC) are highly nonlinear optical materials due to their sensitive property activating even under relatively low optical fields. Several nonlinear mechanisms investigated so far have revealed the promising characters of these materials. The difference in refractive indices, for fields polarized along, and perpendicular to, the director axis brings about a large birefringence property, which is an opportunity for various applications [1]. Director axis reorientation based effects causing the change of refractive index and observation of several interesting dynamic and storage wave-mixing effects have been extensively studied so far [1], [2], [3], [4], [5], [6]. Compared with others, LC based systems require lower characteristic voltages to be applied for the realization of molecular gratings and relatively lower light power for efficient modulation of refractive index. It is experimentally proved that doping a small amount of dye decreases the required threshold of molecular reorientation further in LC materials [7]. This phenomenon has potential applications such as holographic data storage. Because of the large broadband birefringence of nematic liquid crystals, it is obvious that these highly sensitive films could be applied in a variety of image-processing systems operating with low optical power. Since many dyes that will cover the entire visible spectrum exist, such dye-doped nematic films are highly promising candidates for application as broadband optical modulators and limiters, and other adaptive optics and coherent wave-mixing devices. This work concentrates on the co-usage of two different dyes at the same time in the structure of the same system so that the system would be appropriate for the simultaneous usage of two lasers.
In dye-doped samples light–molecule interaction could take place in different mechanisms dependent on the type of the dye. The anthraquinone derivatives and azo dyes are two important dye categories studied widely. Even though the discussion is still going on about the definite mechanism of dye enhanced molecular reorientation, effect of anthraquinone based dyes is well explained by Jánossy effect [8], [9] unlike the azo dye based reorientation, which is being explained by trans–cis photoisomerization [3], [5]. Methyl red (MR) is a favorite agent in azo dye category and it is extensively studied up to now. This is perhaps the most nonlinear optical material known. Such extraordinarily large nonlinearity enables the performance of several all-optical switching, limiting, image modulation, and sensing processes at unprecedented low threshold powers [10], [11], [12], [13], [14], [15]. In fact Disperse Blue 14 (DB14) is another important dye, which is an anthraquinone derivative and also investigated in literature [4]. In the scope of this work two of these dyes were used together to reach our aim that is the construction of a system compatible for different lasers of different wavelengths at the same time.
Section snippets
Experimental
Before the construction of the cells, indium tin oxide (ITO) covered glass substrates were spin coated with polyvinyl alcohol (PVA) at 2000 rpm and they were cured at 50 °C for ∼2 h. The thickness of the coating is ∼100 nm and these coating layers were exposed to surface treatment of unidirectional rubbing with velvet in order to obtain preliminary molecular orientation. The ultimate form of the constructed cell is planar with roughly 2° rubbing tilt. Measurement cells were made up of two glass
Results and conclusion
The character of the systems was investigated in terms of the diffraction signals depending on applied DC voltage. The origin of diffraction is the molecular reorientation happening in bright regions and grating is formed with bright–dark periodicity reinforced by interference pattern. Self-diffraction spots were considered in experiments and diffraction efficiency was measured as the intensity ratio of the first-order diffraction beam to the incident beam in the absence of one side of the two
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