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Applied Optics

Nematic liquid crystal optical amplifier consisting of layered structures

Yanbo Pei, Fengfeng Yao, Jianlong Zhang, Chunfeng Hou, Xiudong Sun

Published May 5, 2009• 1 citation
Publisher

Abstract

Coherent amplification was observed in an optical structure with multilayer C(60) doped nematic films, in which photorefractive gratings working in the Raman-Nath regime were created, leading to an asymmetric energy transfer in two-beam coupling. This multilayer optical structure exhibited exponential dependence of the signal gain on the number of nematic layers when higher-order diffractions were negligibly small. A gain of the 0.1 microW signal as high as approximately 4000 was achieved in the six-layer sample at a low dc voltage of 1.2 V and a low pump beam power of 8 mW. The gain of this ultrahigh sensitive six-layer sample depended strongly on the pump to signal beam power ratio as well as the power of the signal.

DOI
10.1364/ao.48.002707

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Coherent amplification was observed in an optical structure with multilayer C(60) doped nematic films, in which photorefractive gratings working in the Raman-Nath regime were created, leading to an asymmetric energy transfer in two-beam coupling. This multilayer optical structure exhibited exponential dependence of the signal gain on the number of nematic layers when higher-order diffractions were negligibly small. A gain of the 0.1 microW signal as high as approximately 4000 was achieved in the six-layer sample at a low dc voltage of 1.2 V and a low pump beam power of 8 mW. The gain of this ultrahigh sensitive six-layer sample depended strongly on the pump to signal beam power ratio as well as the power of the signal.
Beam coupling theory for the nonlocal Raman-Nath gratings was developed, which showed that the gain initially increased exponentially with the sample thickness, then deviated from the exponential relation and lowered gradually. It was predicted that large thickness and high intensity sensitivity were simultaneously necessary for the sample to obtain larger gain. The quantitative relation between the signal gain and the sample thickness was investigated experimentally by using a multilayer photorefractive liquid crystal structure to increase the overall interaction length, and the experimental result fitted the theory nicely.
Optical bistability has been observed in a new configuration consisting of a nematic liquid crystal (ROTN 403) and a photorefractive crystal (KNbO3). The nematic liquid crystal (NLC), placed between crossed polarizers, serves as an optically controlled gate. The transmitted signal beam is amplified by photorefractive two-wave mixing and sent back onto the NLC in such a way that it changes the transmission of the gate. Bistability of the optical signal in the feedback loop with a large hysteresis is found. The bistable hysteresis curve depends on the nonlinearities of the NLC and of the photorefractive crystal, on the intensity of the storage beam and on the losses in the feedback loop.
Consideration of the coupled wave equations governing three wave mixing in photorefractives and nematic liquid crystals, when both are exhibiting a local response, shows that the main difference between the two is the self phase modulation that occurs in the liquid crystals. It is this difference that explains the dissimilar variation of the exponential gain factor, as a function of inter-beam angle. The other difference between the two media is the dependence of gain on the total incident intensity for the case of nematic liquid crystals and on the detuning frequency for the case of photorefractive media. >
In a highly nonlinear thin film, the presence of the diffracted beams from the pump-probe-beam-induced refractive-index grating modifies considerably the energy coupling between the pump and the probe beams. Using coupled amplitude and phase equations, the roles of various factors such as phases and phase shifts, phase modulations, pump-beam intensity, intensity ratio, interaction length, and losses in the medium in the multiwave-mixing processes are clearly identified. We also present experimental results on the dependence of the probe gain on the pump-probe-beam ratio, the pump intensity, and on the interaction length, using a nematic liquid-crystal film as the nonlinear medium.