Two-dimensional double-quantum spectroscopy: peak shapes as a sensitive probe of carrier interactions in quantum wells

We identify carrier scattering at densities below that which has previously been observed in semiconductor quantum wells. These effects are evident in the peak shapes of 2D double-quantum spectra, which change as a function of excitation density. At high excitation densities (>10(9) excitons/cm(2)), we observe untilted peaks similar to those reported in previous experiments. At low excitation densities (<10(8) excitons/cm(2)), we observe narrower tilted peaks. Using a simple simulation, we show that tilted peak shapes are expected in double-quantum spectra when inhomogeneous broadening is much larger than homogeneous broadening, and that fast pure-decoherence of the double-quantum coherence can obscure this peak tilt. These results show that carrier interactions are important at lower densities than previously expected, and that the natural double-quantum peak shapes are hidden by carrier interactions at the excitation densities typically used. Furthermore, these results demonstrate that analysis of 2D peak shapes in double-quantum spectroscopy provides an incisive tool for identifying interactions at low excitation density. (C) 2016 Optical Society of America