We observed cw third-harmonic generation in a periodically poled LiNbO(3) crystal by cascading optimally phase-matched second-harmonic and sum-frequency generation. Other processes, such as fourth-harmonic generation, are allowed by the flexibility of quasi-phase matching. We demonstrate a divide-by-nine (1.19- 10.71-microm) frequency chain that uses only two lasers.

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Thirteen new rotational transitions of H 2 O ϩ in the ͑0,0,0͒ level of the X 2 B 1 state have been measured in the wavenumber region between 80 and 200 cm Ϫ1 ͑50 and 120 m͒ by far-infrared laser magnetic resonance ͑LMR͒ spectroscopy. LMR data measured previously between 25 and 90 cm Ϫ1 ͑110 and 400 m͒, as well as optical and infrared combination differences, have been combined with the new LMR data in a weighted least-squares analysis using an A-reduced expression of the rotational-fine structure Hamiltonian. Thirty-two molecular constants were simultaneously determined, some sextic centrifugal distortion parameters and some quartic and sextic spin-rotation parameters for the first time. From this improved set of molecular parameters, very accurate calculations of rotational term values and zero-field predictions of the 1 11 -0 00 transition, including hyperfine structure, have been performed. Moreover, the electronic g-tensors and the hyperfine coupling constants are consistent with ab initio calculations which had been carried out for these constants.

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