II Dephasing-Induced Coherent Phenomena

H. Kalt, Claus F. Klingshirn

Rick P. Trebino, Larry A. Rahn

Dephasieg-induced phenomena are a novel class of effects that are ordinarily forbidden because the two or more relevant quantum-mechanical amplitudes add coherently and cancel out.1 When dephasing is present, however, these amplitudes no longer cancel, and previously unseen effects can be observed. Indeed, a number of novel dephasing-induced phenomena have been seen.2-5 Of particular interest, both for fundamental reasons and for diagnostic purposes, are (third-order) dephasing-induced resonances between equally populated levels of a ground electronic state,6,7 specifically, Zeeman and hyperfine coherences. Working with a sodium-seeded flame, we8 have observed these latter effects and, at higher intensities, subharmonics of them.

H. Kalt, Claus F. Klingshirn

Kyle B. Ferrio

Optically induced coherences reveal key features of nonlinearities in the optical response of semiconductors and can be employed to obtain information about dynamical processes via such techniques as transient four-wave mixing and differential transmission. This thesis presents experimental observations of three previously unrecognized aspects of coherent nonlinear optical phenomena occurring in gallium arsenide and multiple quantum wells: excitation-induced dephasing, two-photon biexcitonic coherence, and electronic Raman coherence. Results are analyzed using optical Bloch equations, which emphasize the potential generalization to other semiconductor systems. Excitation-induced dephasing is similar to classical line broadening. The work in this thesis shows for the first time that this incoherent effect is directly responsible for the dominant coherent optical nonlinearity in bulk gallium arsenide. Exact analytic models show that excitation-induced dephasing accounts for the previously unexplained polarization and density dependence of four-wave mixing signals obtained at the 1s exciton resonances in gallium arsenide. This thesis also presents the first observation of the two-photon biexcitonic coherence in a gallium arsenide multiple quantum well using a modified Michelson phase-conjugate interferometer to directly observe the 770-Terahertz oscillation of the nonradiative superposition of the electronic ground state and the biexciton state. The contribution of the two-photon coherence to the nonlinear optical response is thereby confirmed, in addition to the well-known stepwise biexcitonic response. Finally, the short-lived electronic Raman coherence between the light- and heavy-hole 1s excitons in gallium arsenide is conclusively demonstrated for the first time. The Raman coherence provides direct access to dephasing processes not detectable in conventional linear spectroscopy. Similar non-radiative coherences are related to population trapping, electromagnetically induced transparency, and lasing without inversion in atomic systems, and the new observations in gallium arsenide suggest that analogous phenomena may occur in semiconductors.

Bernhard Rauer

Yukiyoshi Ohtsuki, Yuichi Fujimura

Bath-induced coherence transfer effects on a time- and frequency-resolved resonant light scattering spectrum is theoretically investigated using the Markoff master equation. According to Eberly and Wódkiewicz, a general expression for an experimentally observable spectrum in terms of a molecular response function is derived within the density matrix formalism. To generalize our previous results of the bath-induced coherence transfer which were derived based on a displaced harmonic oscillator model [Y. Ohtsuki and Y. Fujimura, J. Chem. Phys. 91, 3903 (1989)], an eigenstate basis is used to represent a relevant system for investigating characteristics of the transfer. By the present model, we clarify the dependence of the bath-induced coherence transfer on the energy-level structure of the intermediate states associated with the transfer, i.e., energy mismatch effects. It is shown that if the energy mismatch of these states is smaller than dephasing rates, the bath-induced coherence transfer occurs resonantly. In the other cases, the energy mismatch brings about a modulation in the time evolution of the superposition state created by the bath-induced coherence transfer, which strongly diminishes the efficiency of the transfer. The resonance condition is derived analytically and is confirmed by numerical calculations of quantum beats induced by the bath-induced coherence transfer. The possibility of very rapid dephasing of a quantum beat signal which cannot be explained in terms of dephasing rates is also shown, when the transition moments have such values that give π-phase-shifted quantum beats in bath-induced fluorescence.