Application And Research On Nonlinear Effect Of Liquid Crystals By Femtosecond Laser

Liquid crystals(LCs)are widely used in display,biological detection,optical communication and other fields owing to its excellent optical property and controllability.As one of the key research fields of LCs,the nonlinear optical effects of LCs have attracted more and more attention.Femtosecond laser pulses have very short pulse width and ultra-high peak intensity.Research on nonlinear effect of LCs by femtosecond laser is very significant.In this paper,the terahertz(THz)radiation and fluorescence of liquid crystal induced by femtosecond laser are analyzed,and the generation mechanism is researched based on LC technology and femtosecond laser technology.Firstly,several optical nonlinearity effects of LCs and measurement methods of liquid crystal nonlinearity coefficient are introduced.The nonlinear effect and fluorescence properties of LC NJULDn-4 were analyzed from the perspective of molecular structure.The LC materials contain a large number of highly conjugated rod-like molecules with long π electron conjugated bonds which are preferred to achieve respectively large nonlinearity.Biphenyl groups can also emit fluorescence well.Then,the elliptically polarized THz radiation of LC pumped by 800 nm femtosecond laser pulses is studied.The characteristics of THz radiation generated by optical rectification are analyzed theoretically.And the variation of THz radiation with pump power and polarization angle is studied.The peak intensity of THz electric field increases linearly with the increase of pump power.The director of LC is parallel to pump polarization thereby resulting in a larger THz signal.When the LC director is perpendicular to pump polarization,almost no THz radiation is detected.Additionally,the elliptical polarization of THz radiation relates to the reorientation effect of LC induced by the femtosecond laser.Finally,the fluorescence emission of LC materials pumped by femtosecond laser is studied.The fluorescence and absorption spectra of LC are measured.It is believed that the fluorescence of LCs pumped by femtosecond laser pulses is mostly derived from two-photon absorption comparing to fluorescence emission of LCs pumped by 405 nm continuous laser pulses.Moreover,the relationship between fluorescence intensity and pump power is not always a quadratic increase due to little second harmonic generation in the fluorescence.Its relationship is related to the thickness of LC.Further research shows that,the fluorescence intensity is related to the pump polarization.The fluorescence intensity is strong,when the director of LCs is parallel to pump polarization.The microstructure of LC also directly affects the structure of fluorescence.LCs can emit THz waves based on optical rectification effect;therefore,they can be used as a potential alternative to develop tunable THz sources.Study on fluorescence of LC also helps to expand the research field and application of LC photonics.