Optical-absorption spectra of PbSe and PbS spherical quantum dots have been studied both theoretically and experimentally. Exact theoretical analysis of the energy spectra and optical transitions has been carried out in the framework of four-band k•p-method with full account of the anisotropy effects. It is demonstrated that strong anisotropy of the energy bands of bulk PbSe and PbS results in pronounced optical transitions in quantum dots, which are forbidden in isotropic approximation. These anisotropy-induced transitions have been clearly observed in the measured absorption spectra of the PbSe and PbS quantum dots.

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Coherent acoustic phonon oscillation is observed in PbSe quantum dots embedded in phosphate glass by femtosecond pump-and-probe. The size dependence of the oscillation is investigated. Distinct low-frequency peaks are observed in Raman spectrum for the same samples. The size-dependence of the frequencies is well explained by elastic sphere model, but the observed modes are different to each other for coherent phonon and Raman scattering. Coherent phonon measurement and Raman scattering are found to give complementary information on confined acoustic phonons in this system. Semiconductor quantum dots ͑QDs͒ have attracted considerable attention in recent years. 1,2 Three-dimensional confinement of electrons to a nanometer-size space changes their continuous energy spectrum to discrete one. It not only modifies electronic state, but also modifies phonon spectrum to size-dependent discrete lines, and the electron-phonon interaction among them is thought to be changed also. The size-dependence of the exciton-phonon interaction in quantum dot has been the subject of extensive research. Experimental studies on the optical, 3,4 or acoustic 5-8 phonon modes in QDs have been carried out mainly by Raman scattering. Persistent spectral hole-burning ͑PSHB͒ spectroscopy was also successfully applied to QDs. 9,10 During the last decade, coherent oscillation of the confined vibrational modes became to be observable directly in time domain. There are some reports on the observation of coherent optical phonons in semiconductor QDs, CdSe QDs, 11 InP QDs, 12 etc. While the high energy optical phonon forms sidebands in optical spectrum, low energy acoustic phonon rather contributes to excitonic dephasing and homogeneous broadening, 13 which plays a crucial role in nonlinear device applications. Therefore a detailed understanding of the mechanism of generation and damping of confined acoustic phonon in QDs is important. Coherent oscillation of confined acoustic phonon in QDs was first reported by Krauss and Wise in 1997. 14 They observed temporal oscillation of the transmission in femtosecond pump-and-probe measurement on 3 nm PbS QDs dispersed in PVA ͑polyvinyl alcohol͒. They also measured Raman scattering of the same sample to compare with coherent phonon. However, as they mentioned in the paper, they were not able to systematically study the size-dependence, because size control in the polymer matrix was difficult. Therefore, the size-dependence of the relation between coherent acoustic phonon and Raman spectrum has not been not studied yet. In this Rapid Communication, we detect coherent acoustic phonon signal in QDs embedded in glass matrix with high sensitivity. We measured size-dependence of coherent phonon and Raman scattering. The complementary relation of these measurements was clearly shown.For coherent phonon measurements, we performed femtosecond pump-and-probe. A mode-locked Ti:sapphire oscillator was used to produce pump and probe pulses, whose pulse width was about 30 fs. Center wavelength of the near transform-limited...

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It has been demonstrated that thermally poled regions of silicate glasses are capable of blocking penetration of silver ions into the glasses in the course of ion exchange processing. This allows using locally poled regions of the glass surface as thick dielectric masks in the formation of ion exchanged structures, like channel optical waveguides, in glass substrates. This approach provides easy multiplication of the structures and, being based on the thermal electric field imprinting of the masks, allows avoiding both lithography with the use of liquid chemicals and the influence of metal masks on the spatial distribution of the ions penetrating the glass in the ion exchange.