We analyze all-optical switching property of a nonlinear directional coupler (NLDC) having an MQW coupling layer with both nonlinear and linear losses, and examine the effect of nonlinear losses. We use the Galerkin finite element method accompanied by a prodictor-corrector algorithm. The propagation loss along the strongly-coupled NLDC decreases with increasing nonlinear absorption coefficient due to saturation in absorption. A propagation loss of 8.18 dB or 2.38 dB in the bar state of the cross state is much smaller than the bulk loss of MQW structure which exceeds 50 dB. The nonlinear losses lengthen the coupling length and bring it close to that of a lossfree NLDC, while the linear losses shorten. It is found that the property of the cross state is greatly improved by counting the nonlinear losses: The cross-state output power and the output power ratio of two waveguides increase, and the cross state input power, that is, the switching power decreases.

Properties of a strongly-coupled nonlinear directional coupler (NLDC) with a lossy MQW coupling layer is analyzed using the Galerkin finite element method accompanied by a predictor-corrector algorithm. It is shown that the propagation attenuation along the NLDC is considerably smaller than that in the bulk MQW and tends to reduce with the input power. By the presence of losses, the powers guided in two waveguides do not become a maximum and a minimum at the same propagation length, unlike the lossless coupler. The losses make the nonlinear effect weak due to the decrease in guided power, and hence the coupling length decreases and the switching power increases. The extinction ratio of the switching becomes the largest value not in the cases of nonloss and high losses but in the case of moderately high losses, although the switching power is somewhat larger than that of the lossless case.

In this paper, we present a study on the activity of second harmonic generation (SHG) from mixtures of organic nonlinear materials 2-methyl-4-nitroaniline (MNA) and paranitroaniline (pNA), changing the mixing ratio over a wide range. The mixtures show a maximum SHG powder efficiency of 520 ( urea) at a mixing weight ratio of MNA/pNA=40. We also present a setup for the measurement of the powder efficiency that an integrating sphere and a small hemispherical mirror are in contact with the sample cell on the reflecting and transmitting sides, respectively. This setup is conveniently used to effectively collect the second harmonic waves radiated from organic powder samples, since the transmissivity is usually very small due to scattering. The X-ray diffraction patterns indicate that the enhanced SHG activity of the mixtures may be due to some changes of MNA molecules in the direction of the hydrogen bond, though the crystal structure is unchanged. The SHG activity of the mixtures is found to be thermostable and also not to change so much for over two months.

Topographical changes induced by optical near-field around photo-irradiated nanoparticles were attained using a pulsed laser with a large peak power as a light source. The arrayed structure of nanoparticles was transcribed on urethane-urea azo copolymer film as dent structure. The experiments by the pulsed laser of different wavelength showed that the topographical change was caused by the light absorption. The dent diameter and the dent depth changed depending on the diameter of nanoparticles.

Poled polymer films doped with novel nonlinear organic materials, α-cyano unsaturated carboxylic acid (α-CUCA) derivatives, are prepared. Linear and second-order nonlinear optical properties are investigated. It is found that as the value of hyperpolarizability of the derivatives increases, the second-order nonlinear susceptibility of the film increases. Cerenkov-type second harmonic generation (SHG) of Nd: YAG laser is realized in a poled polymer waveguide doped with the α-CUCA material with a slight absorption at doubled wavelength.

The propagation properties of the TE0 mode in an optically nonlinear tapered waveguide are examined and discussed. It is assumed that the nonlinear core has the Kerr-like refractive index of self-focusing type. The size of the tapered waveguide is so chosen that for a small input power, the guided light is cut off on the way to the taper end, and so the transmitted power becomes nearly zero. If the input power exceeds a threshold, the increment of the core index makes the nonlinear guide break away from the cut-off state, and the output power abruptly goes up at the threshold. Hence, the guide can be used as an all-optical threshold device which has a strong point that the very abrupt increase of the output at the threshold can easily be obtained for its length. Some applications of the tapered guide to pulse compression, optical AND gates, and optical differential amplification are also shown. The above abrupt increase gives the differential amplifier with very high gain.