Femtosecond sources at 1.5 microns and their application for time-resolved spectroscopic studies of semiconductor devices

Optical communication systems require optoelectronic devices and components that operate at a wavelength of 1.5 plm, where standard optical fibers have a minimum transmission loss. Investigating the performance of these devices and understanding the underlying physics requires good sources at this wavelength. In particular, the investigation of their ultrafast nonlinear response, which will ultimately limit the switching speed and modulation characteristics, requires good femtosecond sources at 1.5 plm. A broadband femtosecond source is ideal for variable wavelength pump-probe experiments, which are the standard tool for time-resolved spectroscopy. To this end, a stretchedpulse additive pulse modelocked erbium-doped fiber ring laser was used as a seeding laser for a solid-state color center amplifier, which was pumped by a 1 kHz Q-switched Nd:YAG laser. The seeding pulse train was matched in repetition rate and synchronized to the pump laser. The seeding pulse was passed twice through the gain medium to be amplified by a factor of ~ 10000. The resulting 10 pJ pulses were recompressed to 250 fs duration and focused onto a nonlinear material, yielding an extremely broadband femtosecond continuum. This femtosecond continuum was then passed through a spectral slicer to demonstrate variable-bandwidth tunable femtosecond pulses. In a separate experiment, an additive pulse modelocked color center laser was used to investigate the nonlinear femtosecond dynamics of a polarization insensitive semiconductor optical amplifier. By mixing quantum wells with compressive and tensile strain, both TEand TM-polarized light are equally amplified. This useful device has been introduced as an integrated optics device to act as an amplifier independent of the polarization state of its input. Although this device has isotropic gain by design, it showed a nonlinear dynamic anisotropy. A heterodyne pump-probe technique was used to resolve the polarization dependence of the dynamic nonlinear response of the gain as well as the index. The cooling time of the carriers following optical excitation showed a dependence on the pump polarization. When the pump and probe are both TE-polarized the cooling time is 900 fs, but for all other TE and TM combinations for pump and probe the cooling time is only ~ 700 fs. Calculations showed that in this experiment the measurement is sensitive mainly to hole dynamics. Further band structure calculations show that the states of interest are heavy hole states with light effective mass in the compressive strain well, and light hole states with a heavy effective mass in the tensile strain well. The different cooling time is attributed to this mass difference. The anisotropy is attributed to the polarization dependence of the heating mechanism, dominated by intervalence band absorption. To further investigate the dynamic anisotropy, a technique was devised to measure the induced anisotropy directly. This measurement contains both the induced birefringence and induced dichroism. The measurement results showed a new 7 ps time constant, not apparent in the previous measurements. This longer time constant is attributed to interwell coupling or transport. A residual long-time anisotropy in the measurement, indicates a polarization dependence of the CW saturation of the device. Thesis Advisor: Erich P. Ippen Title: Elihu Thomson Professor of Electrical Engineering To my parents who started me on this path and to my wife and kids who saw me through it