Polarization-dependent optical 2D Fourier transform spectroscopy of semiconductors

Abstract

Optical 2D Fourier transform spectroscopy (2DFTS) provides insight into the many-body interactions in direct gap semiconductors by separating the contributions to the coherent nonlinear optical response. We demonstrate these features of optical 2DFTS by studying the heavy-hole and light-hole excitonic resonances in a gallium arsenide quantum well at low temperature. Varying the polarization of the incident beams exploits selection rules to achieve further separation. Calculations using a full many-body theory agree well with experimental results and unambiguously demonstrate the dominance of many-body physics.excitons ͉ many-body effects ͉ ultrafast O ptical excitation of a direct gap semiconductor, such as gallium arsenide (GaAs), produces electron-hole pairs. The Coulomb attraction between the electron and hole can result in a bound state, known as an exciton, with a hydrogenic wavefunction for the relative coordinate. Excitons have a large oscillator strength because of the proximity of the electron and hole and thus can dominate the absorption spectrum close to the fundamental band gap. In GaAs heterostructures, the exciton binding energy is of order 10 meV; thus, excitonic resonances appear only at low temperatures. Excitons and unbound electron-hole pairs exhibit dynamics on a femtosecond-to-picosecond time scale. These timescales, combined with the strong interaction with light, make ultrafast spectroscopy an ideal tool for studying carrier dynamics in semiconductors.Over the last two decades, excitonic resonances in semiconductors have been studied extensively by using ultrafast spectroscopy, primarily transient four-wave-mixing (TFWM) (1, 2). The measurements clearly showed signatures of many-body effects. The first and most prominent was a signal for the ''wrong'' time delay in a two-pulse TFWM experiment. Theoretically, such signals could arise from several effects including local fields (3, 4), biexcitons (5), excitation-induced dephasing (6, 7), or excitation-induced shift (8). Time resolving the signal also provided evidence for many-body contributions (9, 10), although it did not resolve the ambiguity regarding the underlying phenomena.Recent results using optical 2D Fourier transform spectroscopy (2DFTS) to study the exciton resonances have shown that much more information is obtained, promising a more stringent test of the theory (11). 2DFTS traces its roots to NMR (12). Recently, there has been significant progress in translating multidimensional NMR techniques into the infrared and optical domains for the study of vibrations (13) and electronic excitations (14-16) in molecules. Although the usefulness of adding a second dimension was recognized in TWFM studies of semiconductors (17-20), only the intensity of the emitted signal, not the phase-resolved electric field, was measured. The transient absorption experiments clearly show that detecting only the real part of the emitted field is advantageous (21, 22), but the effects of inhomogeneous broadening cannot be removed. 2DFTS combines the best...

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