Abstract

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 ($>{10}^9\mathrm{excitons}/{\mathrm{cm}}^2$), we observe untilted peaks similar to those reported in previous experiments. At low excitation densities ($<{10}^8\mathrm{excitons}/{\mathrm{cm}}^2$), 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.

© 2016 Optical Society of America

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