Ultrathin-film local oscillator for determination of complex components ofsecond-order nonlinear susceptibility

The second-harmonic generation (SHG) interferometry technique has to date b een used to determine the phase of second harmonic (SH) light generated in a single polarization combination of a polarizer and an analyzer. However, it is not easy to compare phases of the SH light generated by a plural pola rization combination, because a thick local oscillator, such as an alpha-qu artz single crystal, has several disadvantages for this purpose. The birefr ingence and optical rotatory power of the quartz crystal often require furt her consideration on the phase relation of the SH lights with different pol arizations. Thus, the full determination of the complex chi-components of a n organic ultrathin film by the use of the quartz crystal is impractical. A fter surveying SH active systems, we found that a 2-docosylamino-5-nitropyr idine (DCANP) Langmuir-Blodgett (LB) film is the most applicable for the ph ase measurements. A few nanometers-thick DCANP LB film, which is much thinn er than the wavelength of the SH light, yields a negligible phase delay eve n if birefringence is present. The SHG efficiency of the film can be easily controlled by changing the number of layers. Some experimental results are addressed to demonstrate the advantage of the DCANP LB film as a local osc illator.