Coherent Phonon Quantum Dynamics in Semiconductor Nanostructures

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The main aim of this study is to extend our knowledge of ultrafast acoustic phonon generation using femtosecond light pulses to basic semiconducting materials and nanostructures. We use tightly focused excitation and probe light beams a few microns in diameter to allow the detection of coherent phonons with -1 mum lateral spatial resolution and sub-picosecond time resolution. In order to do this we have developed an ultrafast Sagnac interferometer for measurements at normal incidence.The following studies were carried out :1) Design and construction of a Sagnac interferometer for detection of coherent acoustic phonons on picosecond time scales. The design is particularly convenient for normal incidence measurements and is common path, rendering it immune to acoustic and thermal parasitic fluctuations.2) Generation and detection of longitudinal coherent acoustic phonons in bulk GaAs (100). Above band-gap excitation and probe optical beams were incident from the same side of the sample. … More The effect of coherent phonon pulses propagating into the bulk was detected.3) Generation and detection of longitudinal coherent acoustic phonons in ultrathin (<1 mum) GaAs (100) film (thickness 50 nm) on a Al_<0.3>Ga_<0.7>As (100) substrate film.(- 5 mum in thickness). Above band-gap excitation and probe optical beams were incident from opposite sides of the sample, allowing us to temporally separate phonon pulse signals from spurious thermal and purely electronic contributions to the signal.4) Generation and detection of coherent acoustic phonons in periodic GaAs/AlAs superlattices. Superlattices with atomic order repeat lengths were probed. Selective excitation of the GaAs was achieved by tuning the pump beam to around 830 nm. A new excitation configuration, in which the superlattice was cleaved at right angles to the normal direction, was attempted. Signals due to the propagation of longitudinal acoustic phonons in the in-plane direction were observed.In addition, theoretical work on metals carried out in parallel has allowed a first analytical treatment of the nonequilibrium relaxation of a free electron gas. Further work is needed to extend the technique to a wider variety of semiconductors and to higher frequency acoustic generation. up to - 10 THz. Less