This research grant focuses on the optical properties of semiconductor heterostructures, including quantum wells (QW's), quantum wires (QWR's), quantum dots (QD's) and microcavities (MC's), in the presence of terahertz (THz) electromagnetic fields. This research area, in addition to being of fundamental interest, has received a two-fold impetus from high-speed electronics and from high-bandwidth optical communications. Of particular interest to this research is the regime of low carrier density in which few electrons or holes are excited by the optical beam; the coherence properties of the carriers play an essential role.
Previous reseach considered the dynamics of electron-hole (e-h) pairs excited by ultrafast optical pulses in the presence of THz fields. Because the bandwidth of a sub-ps optical pulse can be in excess of 10 meV, e-h pairs with varying degrees of excess energy (or excitons in different states of their internal motion) are excited by the optical pulse forming a wavepacket. Such studies provide an analogy to several atomic physics phenomena, including the dynamics of Rydberg wavepackets, above-threshold ionization, and high-field harmonic generation. The focus of the proposed work is to explore this analogy further, but more so to push into the domain where intrinsic solid state effects, such as many-body effects and carrier-phonon scattering, begin to make their presence felt by leading to dephasing of the optically excited e-h pairs. Thus, the first optical nonlinearities that kick in as the optical intensity is increased beyond the linear optical regime will be studied. Specifically, how do carrier-carrier and carrier-phonon scattering lead to dephasing of the optically excited electronic excitations, and how does this dephasing modify the spatial motion of e-h wavepackets excited by short optical pulses and driven by THz fields? What is the nature of phonons emitted by such wavepackets; can coherent wavepackets of phonons be launched by THz-driven e-h wavepackets? Can scattering rates themselves be modified by the dynamics of the THz-driven e-h wavepackets?
In addition to being an untapped area of fundamental importance in the spectroscopy of semiconductors, light propagation through THz-modulated quantum wells povides an optical analog for a class of time-domain single-particle quantum tranport phenomena that are otherwise infeasible to study. In particular, tracking the temporal evolution of coherent wavepackets during the tunneling process is of fundamental interest and yet difficult, if not in practice impossible, to access in quantum transport experiments. Fortuitously, there is a close analogy between the scalar classical electromagnetic wave equation and the single-particle Schroedinger equation. This means that in a certain regime one can model by correspondence the quantum mechanical dynamics of a particle by an appropriate light-propagation experiment. This correspondence extends to the phase, i.e., quantum mechanical phase maps to optical phase. Clearly, interferometric experiments are routine in optics but require a tour de force effort in quantum transport. A particularly interesting class of phenomena involves quantum tunneling through a time-modulated potential. The connections between the same type of processes that are associated with the formation of THz sidebands on optical spectra of THz illuminated semiconductors with quantum transport phenomena.
The theoretical research will focus on regimes where neither the QW/atom or optical/transport analogies entirely hold, such as the nonlinear optical regime - where THz-modulated semiconductor heterostructures present new possibilities. Specifically, the propagation of cw and ultrafast optical pulses through heterostructures subjected to pulsed or narrow-band THz fields will be studied. %%% This research grant focuses on the optical properties of semiconductor heterostructures, including quantum wells (QW's), quantum wires (QWR's), quantum dots (QD's) and microcavities (MC's), in the presence of terahertz (THz) electromagnetic fields. This research area, in addition to being of fundamental interest, has received a two-fold impetus from high-speed electronics and from high-bandwidth optical communications. Of particular interest to this research is the regime of low carrier density in which few electrons or holes are excited by the optical beam; the coherence properties of the carriers play an essential role. ***
