SHG in a slab‐type waveguide with a 2‐methyl‐4‐nitroaniline crystal as a high index top layer

Recently, it has been recognized that organic materials have a very large nonlinear optical constant compared with non‐organic materials (e.g., KDP and LiNbO₃). It has been known that 2‐methyl‐4‐nitroaniline (MNA), among them, has an extremely large SHG tensor, although this has a disadvantage in that the d₁₁ component cannot be matched in phase in the bulk crystal state. When an MNA single‐crystal made by using the vapor‐phase method and shaped in a thin parallelogram, is set on a tapered slab‐type thin‐film waveguide appropriately, and the fundamental wave is applied as a TE wave, it is possible to match the phase of the fundamental wave and the phase of the second harmonic wave at a certain waveguide thickness. Since the refractive index of the MNA is greater than that of the slab‐type waveguide film, an SHG is observed at the resonating film thickness when the high‐refractive‐index top layer is used. The observation of the second harmonic (0.532 μm) was made by using an Nd:YAG pulse laser (1.064 μm) for the fundamental wave. The phase matching point was examined from the dispersion curves which was calculated from the waveguide structure parameter. The results show that this phenomenon is the conversion from the TE first of the fundamental wave to the TE second of the secondary harmonic. The efficiency of the MNA observed was not great since the overlapping of the two wave types is not large.