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Au:BaTi[O.sub.3] composite films comprising hexagon-shaped Au nanoparticle arrays covered with BaTi[O.sub.3] matrix were fabricated by double-layer nanosphere lithography and pulsed laser deposition technique. The optical nonlinearity of the composite film was determined using the Z-scan technique at a wavelength of 532 nm and a pulse duration of 25ps. The third-order nonlinear optical susceptibility, [[chi].sup.(3)], was found to be 2.9 x [10.sup.-8] esu, which is comparable with the best values in metal-dielectric films comprising randomly distributed spherical particles but with much higher metal concentrations. The local electric field enhancement in and near the particles was investigated using the dipole discrete approximation method. OCIS codes: 160.4330, 190.4720, 310.1860, 320.5390.

The effect of external-cavity length on self-mixing signals in a multilongitudinal-mode Fabry-Perot laser diode (FP-LD) was investigated experimentally. It has been shown that the output waveforms of self-mixing signals vary periodically when the length of the external cavity changes. This result agrees well with our theoretical calculations for the self-mixing effect of two adjacent longitudinal modes in a FP-LD. Moreover, the time-averaged output intensities of self-mixing signals has also been measured and compared with theoretical analysis. OCIS codes: 140.5960, 280.3340, 280.3420.

By use of a photopolymerization-diffusion model, the diffraction efficiency of photopolymerizable recording dry films prepared from hyperbranched polyisophthalesters as polymeric binders was investigated. The recording characteristics of these films, i.e., spatial frequency, polymeric binder structure, exposure intensity, and modulation depth, are discussed in detail. For a given total exposure dose the diffraction efficiency first increases and then decreases with increasing exposure intensity, and this effect becomes more remarkable as the unsaturated concentration of polymeric binder increases. An optimum total exposure dose of 36 mJ [cm.sup.-2] and an exposure intensity of 0.4 mW [cm.sup.-2] were determined. A modulation depth of 1 was found to produce the highest diffraction efficiency. Longer-lasting gratings could be obtained by use of polymeric binders with higher cross-linking densities. OCIS codes: 160.5470, 350.5130.

A single-frequency Q-switched erbium-doped fiber ring laser is implemented for producing a single frequency with 25-Mhz laser linewidth, Q-switched pulses with a high peak power at 1557.5 nm. The single longitudinal mode is selected by a distributed Bragg reflector fiber laser, and a fiber-optic Mach-Zehnder interferometer acts as a Q-switch. The peak power and the average power of the Q-switched pulses vary with the repetition rate. OCIS codes: 140.0140, 140.3500, 140.3510, 140.3540, 060.2410.