Alternating Langmuir-Blodgett multilayers with low optical losses for Cerenkov-type frequency doubling

Non-centrosymmetric Langmuir-Blodgett multilayers prepared by alternate deposition of the non-linear optically active nitrophenylhydrazone and an inactive polymer (trimethylsilylcelulose) have a non-linear optical susceptibility of 19 pm V −1 independent of film thickness. Optical waveguides 400 monolayers thick have a quite low damping coefficient of 3.5 dB cm −1 at 633 nm. Therefore these multilayers are interesting candidates for frequency doubling in waveguide configurations. Waveguides consisting of 120–150 alternating layers were prepared with a hydrodynamic trough on BK7 glass to demonstrate frequency doubling using the Cerenkov effect. The fundamental wave, the light of a Nd Y-Al garnet laser (1064 nm), was coupled into the waveguide via a prism. The generated green light could be observed leaving the substrate through the substrate edge. To characterized the generated light, the Cerenkov angle and the intensity of the second-harmonic beam were measured. The observed Cerenkov angles are in good agreement with theory. From the measured intensities of the fundamental and second-harmonic waves the conversion efficiency η = P 2ω P ω was determined to be of the order of 10 −4.