Shigeaki Omi

Optical nonlinearities of a high concentration of small metal particles dispersed in glass: copper and silver particles

Third-order nonlinearities of glasses doped with a large number of copper or silver particles are studied by degenerate four-wave mixing with an emphasis on particle-size dependence. The nonlinear susceptibility χ(3) exhibits a peak at the wavelength of the absorption peak, with a maximum value of the order of 10−7 esu. The value of χ(3)/α is roughly independent of the absorption coefficient α and increases as the particle radius increases. This result is well explained by the size dependence of the imaginary part of the dielectric constant of metal particles and the local-field factor. The time response of the nonlinearity exhibits two-component behavior: the fast decay time is shorter than the pulse width of the laser (~12 ps), whereas the slow one is 120–200 ps, being weakly dependent on the radius.

Ultrafast relaxation dynamics of electrons in silver nanocrystals embedded in glass

Abstract Relaxation dynamics of excited electrons in silver nanocrystals with average radii of 3.0 and 4.9 nm embedded in glass has been investigated by femtosecond pump and probe spectroscopy. The surface-plasmon band shows both broadening and red shift with excitation by 200 fs laser pulses. Time response exhibits a two-component decay, and the decay times of the fast and slow components are 2–3 ps and ~ 200 ps, respectively. Time evolution of the nonlinear absorption due to surface-plasmons reflects a cooling process of hot electrons through the electron-phonon interaction. The additional broadening is ascribed to the increase of surface-plasmon damping due to nonequilibrium heating of electrons. When an absolute value of a dielectric constant of silver nanocrystals is decreased with the femtosecond pulse excitation, the surface-plasmon resonance shows a red shift. We demonstrate time response of nonlinear refractive index on the picosecond time scale.

ESR studies of semiconductor-doped glasses☆

Electron spin resonance of semiconductor-doped phosphate glasses has been measured at 77 K. Three types of signal are induced with precipitation of semiconductor nanocrystals in glass host materials. Two of them are attributed to glass defects around semiconductor nanocrystals and the other to S vacancies in CdS nanocrystals. The increase in electron spin resonance absorption with Ar-ion laser irradiation reveals that carriers photoexcited in nanocrystals are trapped not only on nanocrystal defects but also glass defects.

Electron Energy Relaxation Dependent on Size and Matrix in Gold Nanocrystal-Dielectric Composites

We represent size- and matrix-dependent relaxation behaviors of hot electrons in gold nanocrystals. The observed results have been well explained by a model taking into account an electron-breathing vibration interaction in addition to an electron-bulk phonon interaction and thermal diffusion through the interface as electron relaxation pathways.