Optically switchable second harmonic generation in a liquid crystal thin film within femtoliter volume

Nematic liquid crystal (NLC) is one of most useful soft matters. Because the molecular orientation can be controlled electrically, NLC is widely applied to display devices. It is known that NLC exhibits strong second-harmonic-generation (SHG) due to its orderly arranged molecules. The strength of SHG is strongly dependent on the angle between the incident beam polarization and the NLC molecular orientation, so the SHG in NLC can be switched on/off by rotating the NLC director. However, it is very difficult to control the orientation of NLC director electrically within a micrometer spatial domain. In this report, we demonstrated the orientation control of NLC with sub-micrometer spatial resolution based on optical Freedericksz transition (OFT) combined with a high-numerical-aperture objective. We used azo-dye doped NLC to reduce the intensity threshold of OFT with 473-nm excitation. Interestingly, we found that the threshold of OFT increases with tighter focuses. This effect can be explained by the intermolecular forces from the NLC molecules around the focal spot. By incorporating both the blue laser and a femtosecond near-infrared laser into an optical scanning microscope, we have successfully demonstrated switch of SHG inside a NLC thin film. Note that SHG is confined within femtoliter focal volume due to its intrinsic nonlinearity. That is, we have achieved an ultrasmall switch of nonlinear optical signal in NLC. This work will find applications in optical communication as well as optical-base storage system.