Optical two-dimensional Fourier transform spectroscopy of semiconductors.

摘要

Optical two-dimensional (2D) Fourier transform spectroscopy (FTS) is implemented in the near-IR regime and employed for the study of exciton dynamics, many-body interactions, and disorders in semiconductors. As the optical analog of multidimensional nuclear magnetic resonance, 2D FTS is based on a highly enhanced transient four-wave mixing (FWM) experiment. A FWM signal is generated by a non-collinear three-pulse sequence and heterodyne-detected with a reference to provide both amplitude and phase. With active interferometric stabilization and scanning, the evolution of FWM electric field is coherently tracked and presented on a 2D map of the absorption and emission frequencies. With capabilities that include disentangling congested spectra, identifying resonant coupling, isolating coherent pathways, determining inhomogeneous broadening, and separating complex spectra into real and imaginary parts, 2D FTS is a powerful tool to resolve problems in traditional FWM spectroscopies.;In a typical 2D spectrum of semiconductor quantum wells, diagonal peaks arise from exciton resonances and cross peaks represent their coupling, with features such as cross peak strength and absorption of continuum dominated by many-body interactions. Based on the modified optical Bloch equations with phenomenological terms including excitation-induced dephasing, excitation-induced shift, and local field effect, numerical calculations are performed to reproduce these features and determine the microscopic origin of many-body effects by comparing to the experimental amplitude and real part spectra. The dependence of 2D spectra on the excitation polarization is employed to further explore the many-body interactions. In comparison with microscopic calculations with contributions of Pauli blocking, Hartree-Fock approximation, and higher-order Coulomb correlations, it is found that exciton correlations play the dominant role in the case of cocircular-polarized excitation. With an alternative 2D projection, Raman coherences between excited excitons are isolated. The experimental and calculated 2D spectra in this form also demonstrate the similar result on exciton correlations.