We report on the impulsive generation of optical and acoustic phonons in CdTe<sub>0.68</sub>Se<sub>0.32</sub> nanocrystallites embedded in glass, at room temperature. Using ultrafast laser pulses in a pump-probe configuration, we were able to generate coherent vibrations. The energy of our laser was tuned to the absorption edge of the nanocrystals so as to resonantly excite the quantum dots. We identified two longitudinal optical phonons, an optical mode of mixed longitudinal-transverse nature and a longitudinal-like acoustic mode. The frequency, amplitude, decay and phase as a function of excitation energy were determined for the optical modes. These results clearly identify impulsive stimulated Raman scattering as the underlying mechanism of the coherent field generation. The acoustic oscillations are associated with the lowest confined acoustic mode with pseudo angular momentum <i>l</i>=0. We find that the frequency of this mode increases as the laser central energy increases. Since the energy of the exciton at the fundamental gap depends strongly on the particle size, such a behavior is attributed to resonant size-selective excitation of the nanocrystallites. In contrast, spontaneous Raman measurements obtained from the same sample do not show size selectivity and, in addition, the resonant spectra show <i>l</i>=1 and <i>l</i>=2 modes, which are not seen in the pump-probe data. Possible explanations and comparison with other reports are discussed.
A quantum-mechanical many-particle system may exhibit non-local behavior in that measurements performed on one of the particles can affect a second one that is far apart. These so-called entangled states are crucial for the implementation of quantum information protocols and gates for quantum computation. Here, we use ultrafast optical pulses and coherent techniques to create and control spin entangled states in an ensemble of up to three non-interacting electrons bound to donors in a CdTe quantum well. Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of spins.
Although the realisation of femtosecond X-ray free electron laser (FEL) X-ray pulses is still some time away, X-ray diffraction experiments within the sub-picosecond domain are already being performed using both synchrotron and laser- plasma based X-ray sources. Within this paper we summarise the current status of some of these experiments which, to date, have mainly concentrated on observing non-thermal melt and coherent phonons in laser-irradiated semiconductors. Furthermore, with the advent of FEL sources, X-ray pulse lengths may soon be sufficiently short that the finite response time of monochromators may themselves place fundamental limits on achievable temporal resolution. A brief review of time-dependent X-ray diffraction relevant to such effects is presented.
Biological samples, molecular solids and solid state devices have been investigated by Near- Field Scanning Optical Microscopy (NSOM), Near-Field Optical Spectroscopy, and Near- Field Chemical Sensing. We report here on our progress in applying the NSOM technology to various biological and physical systems. Results demonstrating both spatial and spectral resolution as well as image contrast unique to the near-field technique are presented.
A brief review is given of Raman scattering from bound electrons and holes in semiconductor
superlattices. The experiments on Si (donor)-doped and Be (acceptor)-doped
GaAs/AlGaAs quantum-well structures include studies of the dependence of the energy levels
on the position of the impurity in the well and the well-width, and as a function of temperature,
magnetic field and uniaxial stress. The data, showing reasonable agreement with
theoretical predictions, reveal most of the expected features of quantum confinement effects
on the impurity spectra. An extensive list of references to theoretical and related experimental
work is included.