Hyper-Raman spectroscopy is used to investigate low frequency vibrations of various silica glasses. A strong boson peak is observed. The corresponding modes are inactive in infrared and Raman spectra, and are nonacoustic in nature. The shape of this boson peak essentially matches the total density of vibrational states (DOS), with a constant coupling coefficient C. This and other indications suggest that these modes actually dominate the DOS of silica.
The frequencies of the softest polar optic phonons of tetragonal SrTiO3 are accurately determined using high-resolution hyper-Raman spectroscopy on oriented-domain crystals. The splitting of the polar soft mode in the tetragonal phase is quantitatively related to the 105 K structural transition, i.e., to the rotation of the oxygen octahedra and to the associated macroscopic strains. In addition to quantum fluctuations, these tetragonal distortions are found to be essential in preventing the ferroelectric transition.
We demonstrated a novel two-dimensional photonic crystal (PC) based Symmetric Mach Zehnder type all-optical switch (PC-SMZ) with InAs quantum dots (QDs) acting as a nonlinear phase-shift source. The 600- ?m-long PC-SMZ having integrated wavelength-selective PC-based directional couplers and other PC components exhibited a 15-ps-wide switching-window with 2-ps rise/fall time at a wavelength of 1.3 ?m. Nonlinear optical phase shift in the 500-?m-long straight PC waveguide was also achieved at sufficiently low optical-energy (e.g., ??phase shift at ~100-fJ control-pulse energy) due to the small saturation energy density of the QDs, which is enhanced in the PC waveguide, without using conventional measures such as SOAs with current-injected gain. The results pave the way to novel PC- and QD-based photonic integrated circuits including multiple PC-SMZs and other novel functional devices.
Optical interband and intraband transitions in semiconductor quantum dots ͑QD's͒ are analyzed theoretically. It is found that three-dimensional confinement essentially modifies intraband matrix elements of the electron-photon interaction in the QD's as compared to the bulk materials. This effect is quite important for the multiphoton processes. It is shown that two competitive types of two-photon transitions with different selection rules are in the QD's of noncentrosymmetric semiconductor and amplitudes of these transitions differently depend on light polarization, electron and hole effective masses, and QD radius as well. Analytical expressions for the two-photon absorption coefficient are derived for the strong confinement regime, taking into account the size distribution of the nanocrystals. ͓S0163-1829͑96͒00235-4͔
Straight single-line-defect photonic crystal (PC) waveguides on GaAs slabs with lengths of 1, 4, and 10 mm have been fabricated. By controlling the Al content of a sacrificial AlGaAs clad layer and the wet etching duration, a PC core layer with a very smooth surface was obtained. Atomic force microscope images indicate that the roughness on the top surface is less than 1 nm. An extremely low propagation loss of 0.76 dB/mm for the GaAs-based PC waveguide was achieved.
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