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

The second-harmonic generation (SHG) interferometry technique has to date been 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 polarization combination, because a thick local oscillator, such as an α-quartz single crystal, has several disadvantages for this purpose. The birefringence and optical rotatory power of the quartz crystal often require further consideration on the phase relation of the SH lights with different polarizations. Thus, the full determination of the complex χ-components of an organic ultrathin film by the use of the quartz crystal is impractical. After surveying SH active systems, we found that a 2-docosylamino-5-nitropyridine (DCANP) Langmuir-Blodgett (LB) film is the most applicable for the phase measurements. A few nanometers-thick DCANP LB film, which is much thinner than the wavelength of the SH light, yields a negligible phase delay even if birefringence is presented. 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 oscillator.