Coherent Multi-Dimensional Spectroscopy Reveals New Physics of System-Bath Interactions in Metal-Halide Perovskite Quantum Dots

Coherent Multi-Dimensional Spectroscopy (CMDS) is now well established enough to be experimentally tractable for more groups in materials science. But the contributions of CMDS to materials science is in its early stages as the development work has now matured. With this maturity in the method, we are poised to unravel processes in materials that could not be addressed using one-dimensional ultrafast spectroscopies like transient absorption (TA). Transient absorption (TA) spectroscopy as now mature and has grown ubiquitous due to its scientific power and the availability of commercial solutions for laser systems and now for spectrometers. TA spectroscopy is a one-dimensional spectroscopy in that one probes a single energy axis (the probe energy), as a function of pump/probe time delay. In 2D spectroscopy one obtains 1D TA spectra at various pump energies. 2D is a simplification of Coherent Multidimensional Spectroscopy (CMDS), because the coherence of the measurement process is important and is lost in the hole burning analogy. In moving to CMDS, one has two (or more) energy axes. There is the usual probe axis, and now a pump axis, each at some population time. Being multidimensional, the experiment can have multiple energy axes. Being coherent, the experiment can measure coherent signals and enables 2D measurements of amplitude and phase of coherences. CMDS enables many leaps forward past 1D TA spectroscopy.<br/><br/>We apply CMDS to CsPbBr3 metal-halide perovskite quantum dots spanning weakly confined to strongly confined. CMDS enables a number of totally now and remarkable observations. The CMDS data reveals a previously unobserved excitonic doublet which has remarkable properties. This excitonic doublet is shown to support long-lived excitonic coherence, fulfilling a longstanding search for electronic coherence in the CMDS field. The data reveal splittings in the diagonal and the anti-diagonal spectral that are not possible to observe in 1D spectroscopy. These features in the off diagonals reflect couplings between excitons which we show to be time dependent due to lattice structural dynamics. The CMDS lineshape analysis enables disentangling of all the contributions to spectral broadening from homogenous, to inhomogeneous, from static to dynamic. These CMDS experiments provide a number of remarkable observations on a remarkable material.