Time-resolved photoluminescence ͑PL͒ near 540 nm, taken from barium titanate ͑BaTiO 3 ͒ ultrafine powders of ϳ80, 90, and 110 nm in size, was measured at 11 K. Two-exponential functions were found to fit the decay curves well, and the decay lifetimes were composed of one short, about 10 ns, and one long, about 200 ns, time constants, respectively. Results also indicate that the PL peaks show a small blueshift with decreasing BaTiO 3 particle size. The decay process consists of the recombination of self-trapped excitons ͑long lifetime decay͒ and of the electrons in the surface states with the holes in the valence band ͑short lifetime decay͒. The short lifetime component makes comparable contribution to the decay intensity of PL with the long lifetime one.

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We present what is to our knowledge a new approach to generating tunable blue light by cascaded nonlinear frequency conversion in a single LiTaO3 crystal. Simultaneous quasi-phase matching of an optical parametric generation process and a sum-frequency mixing process is achieved by means of structuring the crystal with a quasi-periodic optical superlattice. The spectral (wavelength tuning and bandwidth) and power characteristics of the blue-light generation are studied with a fixed-wavelength 532-nm picosecond laser and a wavelength-tunable nanosecond optical parametric oscillator (OPO) as the pump sources. By tuning the OPO wavelength, we could tune the blue output over approximately 20 nm. Temperature tuning of the blue output at a fixed pump wavelength of 532 nm was limited to approximately 1.5 nm. A maximum blue power of 15 microW was generated at a pump power of 0.5 mW, corresponding to an efficiency of 3%.

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