High Resolution Nonlinear Laser Spectroscopy of Excitation and Relaxation Near the Band Edge in GaAs Quantum Well Structures

The linear and nonlinear optical properties near the fundamental band edge of GaAs quantum well structures are important for applications of these materials to devices as well as providing new insight into the effects of quantum confinement. Excitons dominate the optical spectrum in this region, however, the coupling of excitons to the applied radiation field is modified by dynamical interactions due to the coupling of the exciton to the surrounding crystal lattice and the vacuum radiation field. At room temperature, the exciton is quickly ionized by phonons resulting in an electron-hole plasma which modifies the optical properties due to the effects of bandfilling and exchange that lead to strong optical nonlinearities.1 At low temperature, the exciton is more stable and other effects such as spontaneous emission, tunneling, diffusion, and scattering from phonons and defects modify the optical properties. These processes can result in decay of the excitation as well as decay of the coherence or induced polarization between the initial and the excited state. In this paper, we describe the use of high resolution nonlinear laser spectroscopy based on four-wave mixing (FWM) to obtain lineshapes associated with the nonlinear susceptibility. The measurements provide new understanding of the physical processes associated with the relaxation of the exciton and the dynamics of the optical response. At low temperatures, FWM can often eliminate inhomogeneous broadening leading to a direct measurement of the exciton homogeneous lineshape.