From transient measurements of exciton dephasing to broad bandwidth optics: Exploring physics on both sides of Heisenberg's time-frequency duality.

摘要

Ultrafast lasers, which emit temporally short pulses with broad energy distributions, provide physicists with a ready example of the duality inherent in Heisenberg's uncertainty principle, ΔνΔ t > 1/4π. Both the broad spectrum and the short time duration of ultrafast laser pulses have been put to use in a variety of experiments. This thesis presents three experiments related to the field of ultrafast optics that together illustrate both sides of the Δν/Δt duality.; The short duration of ultrafast pulses makes them especially useful for studying fast processes in materials. In semiconductor quantum wells, electrons and holes bind together to form excitons, which in GaAs can appear in two energy states, the light-hole (LH) exciton and the heavy-hole (HH) exciton. A coherently excited ensemble of excitons loses its phase coherence over time in a process known as dephasing. I present measurements of the dephasing rate of the LH-HH exciton Raman coherence, as well as measurements of the dephasing rates of the HH and LH excitons, carried out in GaAs/AlGaAs quantum wells using three-beam transient four-wave mixing. These are among the first measurements of the dephasing rate of the LH-HH Raman coherence. I find that, relative to the exciton dephasing rates, the Raman coherence dephases more quickly than expected in the Markoff limit.; The broad bandwidth of ultrafast pulses makes dispersion (the dependence of the index of refraction on wavelength) important in pulsed laser systems and in experiments using short pulses. I present measurements of the second- and third-order dispersion coefficients in liquid water performed using white light interferometry. The measured dispersion coefficients are compared to derivatives of available equations for n(λ) in water.; Finally, I discuss progress toward a proposed nonlinear-optical experiment in the far infrared using single-cycle THz pulses. The broadband nature of THz pulses complicates attempts to complete the proposed experiment and leads to the development of a novel attenuator for use in the far infrared. The so-called Brewster attenuator provides a way to change the electric field strength of a propagating THz pulse while preserving the shape of the broadband THz spectrum.