Cavity QED with Exciton Polaritons Using Two-Dimensional Coherent Spectroscopy

<p>In semiconductors the band edge absorption properties are dominated by Coulomb bound electron-hole pairs known as excitons. In an isotropic ideal infinite crystal such as a bulk semiconductor the exciton does not decay to a ground state but instead exchanges energy with the internal modes of the electromagnetic field found from the same boundary conditions as the crystal. It is only the leakage of photons out of the crystal due to real crystals having finite size that allows the exciton-light field to decay. This exchange of energy led to the concept of the exciton-polariton which represents the coupled system of exciton and light.</p> <p>In this work we bring novel insight into the behavior of excitons and exciton-polaritons using a new tool for the performing of multidimensional coherent spectroscopy (MDCS) experiments. We first present a new realization of MDCS using a collinear approach to record a nonlinear wave-mixing signal as a photocurrent. This approach has many advantages over conventional approaches to MDCS as it allows for microscopy and the study of single nano objects while recording the phase and amplitude of these signals.</p> <p>To achieve the strong coupling regime in semiconductors a semiconductor microcavity is often grown around a quantum well. When optically excited the quantum well exciton exchanges energy with the electromagnetic field resulting in new normal modes. These normal modes are called exciton-polaritons. The strong coupling regime is typically characterized by an avoided crossing measured as a function of exciton-cavity detuning. In polariton systems, the new normal modes result in quasi particles with very small mass due to the photonic component of the system. As a result the detuning becomes a dispersion relation. We explore the higher lying dispersion curves of exciton-polaritons in a semiconductor microcavity by utilizing a pulse sequence that creates higher order coherences, i.e., coherences between states with an energy dierence of n \&gt; 1. By recording this coherence as a function of detuning we are able to map out the strong coupling for these higher lying states bringing new insight into the nature of strong coupling and the bosonic nature of polaritons.</p>
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University of Colorado Boulder
Boulder, CO
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