Abstract
Second harmonic light generation (SHG) was observed from as-deposited silica glass thin films suitable for waveguiding, without the need for an inversion symmetry-breaking poling treatment. Thin film stacks of up to 16 layers of alternating 2% phosphorus-doped and undoped silica glass on silica substrates were prepared and probed with a pulsed Nd:YAG laser at 1064 nm. We observed that even though these structures were not poled, they possess a net second order non-linearity with a value of the order of 0.03 pm/V. The SHG increased with the number of layers (total thicknesses between 4 and 9.6 µm have been tested) and also depended on the thickness ratio between the doped and undoped layers. Annealing at 800°C for 4 hours removed the nonlinearity completely.
Highlights
The creation of an efficient second-order optical nonlinearity (SON) over thicknesses of the order of a few microns near the surface of bulk silica glass is possible by thermal poling at a few hundred °C and imposed electric fields of the order of 50 kV/cm [1]
Apart from the significant increase in the Second harmonic light generation (SHG) from the samples with layers compared to the bare substrate, the first observation is that the Maker fringes do not oscillate significantly with angle, indicating that the SON is thinner than the coherence length between the pump and SHG waves
The second finding is that the SHG is larger by 70% for Type B structures relative to Type A and that it increases with the number of layers
Summary
The creation of an efficient second-order optical nonlinearity (SON) over thicknesses of the order of a few microns near the surface of bulk silica glass is possible by thermal poling at a few hundred °C and imposed electric fields of the order of 50 kV/cm [1]. Our work is completely different because it is based on the inherent asymmetry of the electronic polarizability which occurs automatically at any interface between different materials (leading to surface SHG), multiplied by stacking many such interfaces in such a way that the SHG waves generated from adjacent pairs of interfaces do not cancel We investigate this different approach with plasma-enhanced chemical vapor deposition (PECVD) of silica glass, a fabrication process compatible with optical waveguide fabrication (and with optical fiber preform fabrication). For type A samples, increasing the thickness from 4 to 8 μm should increase the SHG power by a factor of 3.7, while for the type B samples the expected increase in going from 4.8 to 9.6 μm is 3.6 times, both within the standard deviation of the observed ratios (and with the correct trend, i.e. with a smaller ratio for Type B) This implies that in order to increase the SHG output from these samples in transmission, quasi-phase-matching would be required (i.e. periodic “spacer” layers without nonlinearity). In order to investigate the origin of the significant higher SHG observed in Type B samples at thicknesses comparable to Type A samples, annealing and composition studies were carried out
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