Photo-thermal nonlinearity enhancement in dye-doped liquid crystals by polymer doping

Nonlinear photonic materials play an important role in the development of advanced optical and photonic technologies. Third-order optical nonlinearity is at the basis of effects like self-modulation and threshold phenomena and can be used for fast tuning and optical switching. Dye-doped nematic liquid crystals are characterised by outstanding values of the nonlinear refractive index. However, in resonance conditions, they suffer from energy loss due to absorption. In this work, we demonstrate large photo-thermal nonlinearity in dye-doped linear Polymer-Stabilized Liquid Crystals (PSLCs) and show how to tune their nonlinear refractive index by tailoring the chemical formulation. Dye-doped PSLC are probed with pre-resonant light at very low power and are characterised by the Z-scan technique. This approach, which strongly limits losses, proves to preserve the nonlinear response despite the pre-resonance illumination. Moreover, the linear polymeric structure turns out to enhance the third-order optical nonlinearity and make its response faster. We thus obtain a controllable nonlinear refractive index that reaches values of around 10-5 cm2 & sdot;W-1 with pre-resonant light in a characteristic time of about 1.5 ms, offering low energy loss, remarkable features in terms of nonlinearity and time response that can strongly impact integrated photonics, cryptography and optical computing applications.