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.