The Study Of Ultra-Fast Relaxation Phenomena Within Nd:Yag And Hopg Crystals.
A monolithic two-section quantum dot semiconductor laser is differentially pumped to form non-uniform current injection in the gain region. It is shown that the nature of the pulse spectral content can be manipulated through differential gain. This provided a simple and inexpensive method of optimizing the semiconductor laser performance and also demonstrated that mode locked operation can be achieved with a much larger set of injection current/absorber bias voltage pairs than was previously possible with single-pad current injection. Significantly greater absorber bias voltages were available during differential pumping; thereby reducing absorber recovery time as well as the inferred pulse length. The enhanced spectral bandwidth provided by differential pumping provided an electronic method of pulse length reduction. The capacity for wavelength tuning was also demonstrated by differential pumping with a center wavelength shift of approximately 12nm. Additionally, the design and configuration of a femtosecond IR optical parametric oscillator demonstrating continuous pulsed tunability from 1040 to 1150 nm is presented along with the examination of relaxation rates for optically excited electrons from the pump3 ing level (4 F 5 ) to the metastable upper lasing level (4 F 2 ) in neodymium doped 2 Yttrium aluminum garnet (Nd:YAG) laser crystals. The 4 F 5 lifetime was the2 oretically predicted to be approximately 12 picoseconds using the Energy Gap Law. Finally, the ultrafast relaxations of photoexcited carriers in thin films of exfoliated highly ordered pyrolytic graphite (HOPG) were examined using the equal-pulse correlation technique which provides the means to efficiently extract a relatively fast relaxation process obscured by a significantly slower one. In addition, it allows for measurements on the order of one tenth the laser pulse width, extending the temporal range of existing pump probe systems without the need for customized femtosecond lasers or complicated pulse compression systems. The HOPG samples exhibited an extremely fast relaxation due to intraband carrier-carrier thermalization observed at 15 ± 10 fs followed by intraband carrier-phonon interaction on the order of 175 ± 30 fs. A modulation in the absorption depth of 1.4 percent was observed in the samples from partial saturation due to Pauli blocking.
hopg; graphene; carrier dynamics; quantum dot laser; equal pulse correlation
Pollock, Clifford Raymond
Wayno, Frank J; Lipson, Michal
Ph.D. of Electrical Engineering
Doctor of Philosophy
dissertation or thesis