Creation of coherent excitons in semiconductor quantum structures

柄律 玄

Hyun Byung-Ryoo

Martín Koch, J. Feldmann, G. von Plessen +8 more

Jeffrey T. Remillard

The purpose of this research is to study the relaxation of optically generated excitons and carriers in semiconductor microstructures using four wave mixing (FWM) spectroscopy. The systems studied include CdSSe microcrystallite doped glasses and GaAs/AlGaAs multiple quantum well structures (MQWS). Exposing a semiconductor to optical radiation near the fundamental band gap results in the creation of populations of elementary excitations including electrons, holes, and excitons, and also results in the creation of a superposition state between the ground and excited state of the solid. The work in this dissertation examines the relaxation of the populations of elementary excitations and the relaxation of the coherence between the ground and excited state using four wave mixing spectroscopy. First, the nonlinear optical response of simple two level systems is examined in order to provide insight into the types of line shapes expected from semiconductors. It is shown that the line shape is strongly dependent on how the system is coupled to the reservoir and the consequences of coupling to a reservoir are examined in a FWM measurement made in atomic sodium. The first semiconductor system studied is CdSSe microcrystallite doped glass. This system is shown to have a very slow component to the nonlinear response which has a optical intensity dependence and temperature dependence which suggests that the FWM response in these materials is trap mediated. Room temperature FWM measurements in GaAs MQWS enables the measurement of the carrier recombination time and the ambipolar diffusion coefficient. Using the technique of correlated optical fields, a slow component to the nonlinear response was measured showing an interference profile which suggests a possible shift of the exciton resonance due to the optically generated carriers. At low temperatures (2-20 K), measurements of the exciton line shape and relaxation time were made and evidence for exciton spectral diffusion was found. The low temperature line shapes can be qualitatively reproduced using Modified Optical Bloch equations which include the effects of spectral diffusion.

Si-Cong Tian, Ren-Gang Wan, Chun-Liang Wang +3 more

We propose a scheme for creation and transfer of coherence among ground state and indirect exciton states of triple quantum dots via the technique of stimulated Raman adiabatic passage. Compared with the traditional stimulated Raman adiabatic passage, the Stokes laser pulse is replaced by the tunneling pulse, which can be controlled by the externally applied voltages. By varying the amplitudes and sequences of the pump and tunneling pulses, a complete coherence transfer or an equal coherence distribution among multiple states can be obtained. The investigations can provide further insight for the experimental development of controllable coherence transfer in semiconductor structure and may have potential applications in quantum information processing.

Ciare C. Byeon, Myoung-Kyu Oh, Hoonsoo Kang +6 more

We suggest that supercontinuum can be used for absorption spectroscopy to observe the exciton levels of a semiconductor nano-structure. Exciton absorption spectrum of a GaAs/AlGaAs quantum well was observed using supercontinuum generated by a microstructrured fiber pumped by a femtosecond (fs) pulsed laser. Significantly narrower peaks were observed in the absorption spectrum from 11 K up to room temperature than photoluminescence (PL) spectrum peaks. Because supercontinuum is coherent light and can readily provide high enough intensity, this method can provide a coherent ultra-broad band light source to identify exciton levels in semiconductors, and be applicable to coherent nonlinear spectroscopy such as electromagnetically induced transparency (EIT), lasing without inversion (LWI) and coherent photon control in semiconductor quantum structures.