Phase-matched second harmonic generation and nonlinear phase shift in a Langmuir-Blodgett film waveguide

Abstract

Wave-guides have been prepared as y-type Langmuir-Blodgett multilayers from 2-docosylamino-5-nitropyridine (DCANP) on quartz glass substrates. The tensor elements of the LB-films as determined by polarisation dependent second harmonic generation (SHG) are χ (2) yyy = 18.6 pm/V, χ (2) yxx = 1.5 pm/V, χ (2) yzz = 6.4 pm/V (± 20%). The wave-guides were fabricated in a way that the second-order susceptibility changes sign at the nodal plane of the first-order wave-guide mode for spolarisation. In such wave-guides efficient second harmonic generation (SHG) was reached via mode conversion at a fundamental wavelength near 1064 nm. The conversion efficiency reached the extraordinary high value of 8%/W which corresponds to a normalised conversion efficiency of 3600 %/(W cm 2 ). In addition, interferometric measurements have been carried out to study the non-linear phase-shift which the fundamental beam experiences due to non-linear interaction in the wave-guide. From these experiments an apparent intensity-dependent refractive index n 2 SHG of 2,6 10 -13 cm 2 /W was calculated. This as about 400 times the intensity-dependent refractive as expected from third-order susceptibility of the isotropic material. From that it can be concluded that the main contribution of the intensity-dependent refractive index is connected to cascading of second-order processes.