All-optical switching and multistability in photonic structures with liquid crystal defects

Abstract

We demonstrate that one-dimensional photonic crystals with pure nematic liquid-crystal defects can operate as all-optical switching devices based on optical orientational nonlinearities of liquid crystals. We show that such a periodic structure is responsible for a modulated threshold of the optical Fréedericksz transition in the spectral domain, and this leads to all-optical switching and light-induced multistability. This effect has no quasi-statics electric field analogue, and it results from nonlinear coupling between light and a defect mode. PACS numbers:The concept of photonic crystals [1] proposed two decades ago [2,3] brought a new paradigm to achieve light propagation control in dielectric media. In such periodic photonic structures tuning may be achieved by using materials which are sensitive to external fields, including temperature, electric field, or light itself. In this context, nonlinear photonic crystals have retained much attention due to possible enhancement of nonlinear effects [4]. Among various nonlinear optical materials that can be implemented in actual photonic crystal devices, liquid crystals (LCs) have been recognized as an attractive alternative material [5] due to their unique sensitivity to external fields. Since then, many tunable photonic crystal devices based on LC tunability have been suggested and implemented either using complete or partial LC infiltration into the periodic dielectric structure. In the first case, the photonic bandgap is tuned due to refractive index changes of the global structure [6], while in the second case a LC-infiltrated layer or hole generates defect modes whose frequencies are controlled by local refractive index changes of LC [7]. The case of complete infiltration is the most studied one, and it was the first to be demonstrated; it concerns thermal [6] and electrical [8] tunability infiltrated one-, two-and three-dimensional photonic structures with LCs. There exist much less studies concerning optical tuning. One can mention the demonstration using photonic LC fibers [9,10], onedimensional [11] or planar [12, 13] photonic crystals using absorbing or dye-doped LCs. In these works the resonant interaction of light induces a change of the order parameter, phase transition or surface-mediated bulk realignment. However the non-resonant case, where well-known orientational optical nonlinearity of LC takes place [14], has only been explored recently in Ref. [15], where the optical Fréedericksz transition (OFT) was studied in a one-dimensional photonic crystal with a nematic liquid crystal (NLC) defect. It was shown that a NLC having a first-order OFT in the single slab case under linearly polarized excitation could be used as an optical switch and operate as an optical diode when the excitation wavelength matches a defect mode frequency. However, such first-order OFT materials are not common and the calculation was made with liquid crystal PAA [15], which has the nematic phase in the typical range of temperature 120 − 135• C and thus prevents from applicat...

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Nonlinear switching of near infrared light in liquid crystal on silicon channel waveguides

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