Noncritically Phase-matched Secondharmonic Generation In Periodically Domain-inverted AlGaas Waveguides

Here we report, for the first time to our knowledge,' the study of quasi-phase-matched second-harmonic generation (QPM-SHG) in a periodically domain-inverted strontium barium niobate (Sr,~,Ba,~,Nb,O,, SBN) single crystal fabricated by using an external electric field. The starting material is a (unpoled) c-cut SBN single crystal plate with a dimension of 10 mm X 10 mm X 1.0 mm. To facilitate periodic poling, both surfaces were first coated with Cr (50-nm) and Au (150-nm) films. Then, the central area with a dimension of about 6 mm X 6 mm on one of the coated surfaces was patterned with 1500 fine stripes by a standard photolithography and lift-offtechnique, with a period of 4 p m (twice of the coherent length). The poling experiment was carried out in two steps. First, an electric field was applied to the sample for a sufficiently long time to change the sample from a multidomain state to a single-domain state within the patterned region. Then the polarity of the electric field was reversed, the periodic domain inversion was realized by controlling the duration of the poling field. The SHG experiment was done with a tunable OPG/OPA system, which is pumped at 355 nm by a 10-Hz passively and actively mode-locked Continuum PY61 YAG laser pulses with 30-ps duration. The polarized fundamental beam was properly focused and coupled into the crystal through a polished endface with its propagation direction perpendicular to the patterned metallic stripes. The second-harmonic signal was isolated from the fundamental beam with a infrared blocking filter, then collected and focused onto a photodiode detector and monitored with an oscilloscope. Figure 1 shows the dependence of the SHG on the applied voltage (field), which yields a i n . TUESDAY MORNING / CLE0'97 / 59