Intraband dynamics at the semiconductor band edge: Shortcomings of the Bloch equation method

Quantum plasmon model for the terahertz photoconductivity in intrinsic semiconductor nanowires

A quantum plasmon model for the terahertz photoconductivity in intrinsic semiconductor nanowires is developed. The classical plasmon model assumes the excited electron in semiconductors feels a restoring force generated by a harmonic-oscillator potential. Although it is successfully applied to explain the terahertz photoconductivity in semiconductor nanowires, the classical treatment of the potential weakens accurate theoretical analysis. Here I treat the potential in a full quantum way and present an exact analytical formula for photoconductivity. The formula not only gives more reasonable photoconductivity, but also has the same conciseness when compared with that of the classical plasmon model. The validity of the quantum plasmon model is proved independently by numerical calculations in real space.

Infinite-order excitonic BLOCH equations for asymmetric nanostructures

We present a new exciton-based formalism for calculating the coherent response of asymmetric semiconductor multiple quantum well structures to ultrashort optical pulses valid to infinite order in the optical field and including the self-generated intraband fields. We use these equations to calculate and explain the oscillations with time delay of peaks in the spectrally resolved degenerate four-wave mixing signals from biased semiconductor superlattices, obtaining good agreement with experiment.

Large four-wave mixing of spatially extended excitonic states in thin GaAs layers

We study the size dependence of the nonlinear response of weakly confined excitons for the size region beyond the long wavelength approximation regime. The observed degenerate-four-wave mixing signal of GaAs thin layers exhibits an anomalous size dependence, where the signal is resonantly enhanced at a particular thickness region. The theoretical analysis elucidates that this enhancement is due to the size-resonant enhancement of the internal field with a spatial structure relevant to the nondipole-type excitonic state. These results establish the formerly proposed new type of size dependence of nonlinear response due to the nonlocality induced double resonance.

Mechanism of THz emission from asymmetric double quantum wells

Impulsive interband optical excitation of the lowest two conduction subbands of a suitably engineered GaAs/AlGaAs double quantum well can lead to coherent THz emission. We demonstrate that, contrary to previous expectations, the dominant emission mechanism can involve the beating of either continuum or exciton states, depending on excitation conditions. The coherence of the continuum beats persists for several picoseconds even for excitation an optical phonon energy above the band edge. We attribute this to the small energy difference between the component eigenstates, which substantially reduces the number of relevant scattering events.