Abstract
Measurements of second harmonic generation (SHG) by polymer dispersed liquid crystal (PDLC) films have been carried out using nanosecond pulses from a Q-switched Nd:YAG laser. The PDLC films consist of micron-sized droplets of liquid crystal dispersed in a polymer matrix. The liquid crystal-polymer interface breaks inversion symmetry, allowing SHG, and the large surface to volume ratio may enhance the signal intensity. The intensity of the signal depends on the dielectric permittivity difference across the interface, and thus changes in orientational order or configuration of the liquid crystal in the inclusions give rise to changes in the second harmonic intensity. The authors have measured the temperature dependence of the SH signal from PDLC films at temperatures both below and above the nematic-isotropic transition temperature of the liquid crystal in the film. An externally applied electric field can affect the orientation of the liquid crystal, and hence affect the SHG. The modulation of the intensity of the second harmonic signal by a low frequency voltage applied across the film, as well as the effects of droplet size on SHG in PDLC films, have been studied. Possible mechanisms responsible for the measured results are discussed.
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