MODULATION OF THE OPTICAL 2ND-HARMONIC GENERATION IN FERROELECTRIC LIQUID-CRYSTALS

We investigated the temporal dependence of the second harmonic generat ion from a 50 mu m thick homeotropically aligned cell of ferroelectric liquid crystal SCE9 (BDH Ltd.) exposed to a square waveform field var ying between non zero and zero value. The field switches the molecular configuration between the efficient phase matching geometry and the i nefficient helically modulated structure. The power of the second harm onic beam responds to the field on a millisecond time scale. The respo nse time of the signal after switching on the field decreases inversel y proportionally to the amplitude of the field, while the response tim e of the signal after switching off the field is almost field independ ent and has a value characteristic for the spontaneous relaxation of t he helical SmC phase (Goldstone mode). The angular dependence of the modulated second harmonic intensity is almost the same as in the stati c external field. This shows that a completely unwound homogeneous mol ecular structure is formed in a few ms after application of the field.