Polydiacetylene with the substantial π-electron delocalization along the main chain backbones is one promising nonlinear material for application and fundamental properties of nonlinearity.Utilizing LS technology

uniform thin film with a desired thickness can be obtained easily and inexpensively

which makes convenience for optical device applications.Therefore many attempts have been performed to improve nonlinear optical (NLO) properties of polydiacetylene LS film.In this paper

an eight-monolayer film of poly-10

12 pentacosadiynoic acid (poly-PCDA) about 26 nm thick was prepared on glass substrate using Langmuir-Schaefer method.Then

the third-order NLO properties of poly-PCDA were investigated by Z-scan technique using a femtosecond pulse laser with the pulse duration of 100 fs at 800 nm.A self-focused effect was observed.And the nonlinear refractive index γ

TPA coefficient β

real part of third-order NLO susceptibility Reχ

(3)

and imaginary part of third-order NLO susceptibility Imχ

(3)

of ploy-PCDA were calculated.A very large value of third-order NLO susceptibility χ

(3)

=5.33×10

-9

esu was detected under the average incidence power of 145 mW.This magnitude reaches the same order as that of polydiacetylenes crystal or mictocrystal.We investigated the factors contributed to third-order NLO susceptibility and ascribed primarily them to two factors.First

π-electron aberrance induced by femtosecond laser can be a key mechanism for the third-order NLO poly-PCDA LS film.Second

a large conjugated system requires that the molecule should be as planar as possible without too much torsion or distortion of monomer units in the chain.In our case

longer conjugated chain may form without ligand interference.The high nonlinearity and uniform large area film makes poly-PCDA LS film particularly interesting and attractive for optical device designs.