Surface periodic poling of lithium niobate for efficient non-linear optical waveguide applications

Domain engineering of LiNbO3 has been researched for a range of applications in areas as diverse as second harmonic generation and parametric oscillation, electro-optic Bragg gratings and piezoelectric micro-actuated devices. Non-linear frequency conversion achieved by quasi-phase-matched interaction remains an attractive route for realising efficient coherent blue-green light sources. Such quasi-phase-matching requires precise control of periodic domain inversion, with periods that can be as small as ~2µm for first order conversion via second harmonic generation (SHG) from the near I.R. into the blue spectral region Application of an electric field greater than the crystal's coercive field is the most widely employed route for fabrication of periodically inverted domain structures for non-linear quasi-phase matched interactions. It is experimentally very difficult to achieve such high aspect ratios in bulk poled material of typical thickness ~500µm. The high coercive fields required for domain inversion, together with the inherent non-uniformities and defects present in commercially available materials, restrict the routine applicability of electric field poling to periods of the order of >4-5 µm in samples of this thickness. To circumvent this problem wafers can be thinned to ~100-150 µm to achieve such small domain periods, but this pre-treatment is both undesirable, expensive and for waveguide applications, unnecessary. Several other techniques such as controlled spontaneous backswitching , and the use of multiple short current pulses, have also been successfully used to generate periods of the order of 2.2-3.0 µm in bulk and waveguide geometries respectively. However fabrication of such poled crystals with very small periods particularly of sub-micron scales remains an elusive goal. Achieving large uniform periodically poled areas is difficult: we however have achieved periods of 1µm by our technique which relies on over-poling the sample, thereby achieving the apparently undesirable effect of domain spreading and merging beneath the lithographically patterned photoresist layer. This technique results in superficial or surface domain inversion, which can be used in conjunction with a waveguide geometry for higher conversion efficiencies in non-linear interactions. Second harmonic generation experiments yielding blue light have been performed with surface poled annealed proton exchanged and Ti-indiffused waveguides confirming the utility of our technique