Coherent nonlinear optics of electron spins in semiconductors

This dissertation presents experimental studies of electron spin coherence in semiconductors. The spin coherence arises from a coherent superposition of electron spin states in the conduction band and can be preserved over remarkably long time and length scales. Electron spin coherence in semiconductors provides an effective model system for investigating fundamental issues of quantum coherences in an interacting manybody system. The robustness of the electron spin coherence also makes it a highly promising platform for optical manipulation of quantum coherences and for the development of coherent quantum devices.; Electron spin coherence induced in the presence of a transverse external magnetic field corresponds to a Larmor precession of the electron spin around the magnetic field. The primary experimental tools for probing and manipulating electron spin coherence are coherent nonlinear optical techniques including transient differential absorption (DA) and time-resolved Faraday rotation (TRFR), which probe, respectively, the oscillations or quantum beats in optical absorption and refractive index induced by the Larmor spin precession.; Nonlinear optical processes in semiconductors are fundamentally modified by inherent manybody interactions between optical excitations. We have developed new experimental techniques based on DA and TRFR to elucidate how these interactions affect and manifest in optical manipulation of electron spins. In an excitonic system, Coulomb interactions between excitons can lead to the formation of bound and unbound two-exciton states. Detailed experimental studies in GaAs and InGaAs quantum wells, along with a phenomenological theoretical analysis, show that the coupling of the spin coherence to the two-exciton states determines the DA and TRFR responses. Nonlinear optical processes via the two-exciton states also lead to a striking difference between closely related TRFR and DA, as revealed by the spectral as well as intensity dependence of the nonlinear optical responses.; We have also demonstrated a spin manipulation scheme that controls the amplitude as well as the phase of the quantum beats from electron spin coherence by exploiting the relative phase between relevant Larmor precessions of electron spins. Surprisingly, the spin manipulation scheme can be more effective in an excitonic system than in a corresponding atomic-like system.