The conditions required to achieve bistability in two-mode semiconductor lasers via the nonlinearity associated with gain saturation are discussed. The laser can be switched between the bistable states through coherent or incoherent optical control. Wavelength bistability in such a laser is demonstrated experimentally.

Various methods have been used to measure the particle size and number density of ultrafine bubbles generated by the ultrafine bubble generator, ultrafineGALF. The presence of particles with diameters of about 100 to 200 nm was indicated by every method that we used before. However, absolute identification of these as ultrafine bubbles rather than some other type of particle was not possible because conventional measurement methods using light generated by a laser and scattered by particles do not distinguish dust particles from bubbles. The present series of experiments, using the resonant mass measurement method, was the first to succeed in clearly distinguishing ultrafine bubbles from other particles. This was due to the use of the resonant mass measurement method, which is capable of distinguishing positively buoyant particles (bubbles) from negatively buoyant particles. In addition, the results from the resonant mass measurement method were compared, in terms of particle size distribution, with this from the particle tracking analysis method, which uses a different measurement principle. The particle size distributions yielded by both methods showed a moderate correlation between the number density results obtained by each.

In 2010, we succeeded in measuring the sizes of bubbles generated by our GALF (GAs Liquid Foam) bubble generating system, using particle tracking analysis for the first time, and quantitatively confirmed the generation and presence of ultrafine bubbles measuring around 100 to 200 nm in diameter. After that, we also developed a new technology to generate a high density of ultrafine bubbles and launched our ultrafine bubble generating system (ultrafineGALF) in 2011. This report details several independent measurements of bubbles generated in water by ultrafineGALF, using dynamic light scattering, laser diffraction scattering, particle tracking analysis, and the electrical sensing zone method. It was found that the presence of ultrafine bubbles with a diameter of about 100 to 200 nm could be determined quantitatively using any of these methods.

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We have developed an ultrafine bubble generating system, ultrafineGALF, upgrading the microGALF system to a flow rate of 0.24 m 3 /h. The ultrafineGALF system can generate a dense population of more than 10 9 ultrafine bubbles per ml. The density and size distribution of these bubbles have been measured using a NanoSight measuring instrument, but precision measurement of the number density has become difficult because it now extends beyond the measuring range (1×10 9 /ml) of this instrument. Thus far, the number density of the ultrafine bubbles after dilution has been measured, but few reports are available on the effect of dilution on gas particles, which behave differently from solid particles. In this study, the effect of dilution, which is required to measure the density of ultrafine bubbles at ultra-high densities, was investigated. No large differences due to the use of dilution among three types of samples with different concentrations of ultrafine bubbles were found, although the samples did show slightly different rates of change in the concentration of ultrafine bubbles over time.