Effect of the orientational disorder on the hyperpolarizability measurement of amphiphilic push-pull chromophores in Langmuir-Blodgett monolayers

First-order hyperpolarizabilties of two new amphiphilic push-pull chromophore derivatives of hemicyanine dyes prepared in Langmuir-Blodgett films and designed for membrane potential imaging of living cells are measured by surface second-harmonic generation (SHG). Assuming uniaxial symmetry, an effective hyperpolarizability coefficient βe is defined, and a procedure to measure this coefficient is described. Relationships between βe and the true hyperpolarizability coefficient β are derived on the basis of a stochastic model taking account of the tilt angle disorder of the chromophores. Maximum discrepancies between the true and the effective hyperpolarizabilities are calculated as a function of the orientation parameter D, and it is shown that the relative difference may not exceed 40% for usual distribution functions of the tilt angle. As a result, the dominant hyperpolarizability coefficient βz,z,z(2) of the chromophores in LB films is found to be in the range 0.3-0.7 × 10−27 esu at 1064 nm fundamental wavelength.