Exciton–Phonon Spectroscopy of Quantum Dots Below the Single-Particle Homogeneous Line Width

Abstract

We demonstrate that high-dimensionality coherent spectroscopy yields "super-resolved" spectra whereby peaks may be localized far below their homogeneous line width by resolving them across multiple, coherently coupled dimensions. We implement this technique using a fifth-order photon-echo spectroscopy called Gradient-Assisted Multidimensional Electronic-Raman Spectroscopy (GAMERS) that combines resonant and nonresonant excitation to disperse the optical response across three spectral dimensions: two involving excitonic transitions and one that encodes phonon energies. In analogy to super-resolution localization microscopies, which separate spatially overlapping signals in time, GAMERS isolates signals spectrally using combined electronic and nuclear resolution. Optical phonon lines in a colloidal solution of CdSe quantum dots at room temperature separated by less than 150 μeV are resolved despite the homogeneous line width of these transitions being nearly an order of magnitude broader. The frequency difference between these phonon modes is attributed to softening of the longitudinal phonon mode upon excitation to the lowest exciton state. Further, such phonon mode selectivity yields spectra with electronic line widths that approach the single particle limit. Through this enhanced spectral resolution, the GAMERS method yields insights into the nature of coupling between longitudinal optical and acoustic phonons and specific excitonic transitions that were previously hidden.

Excited State Phononic Processes in Semiconductor Nanocrystals Revealed by Excitonic State-Resolved Pump/Probe Spectroscopy

“…62 More information-rich ultrafast time-domain echoes experiments have recently revealed LO phonon energy shifts due to specific excitonic states. 63 In these state-resolved pump/probe experiments, two peaks are clearly visible. Figure 6b shows the fluence dependence of the peak areas.…”

“…Sidebands have previously been interpreted as arising from surface phonons . More information-rich ultrafast time-domain echoes experiments have recently revealed LO phonon energy shifts due to specific excitonic states . In these state-resolved pump/probe experiments, two peaks are clearly visible.…”

Four-Dimensional Coherent Spectroscopy of Complex Molecular Systems in Solution

“…Detecting the overlapped signals simultaneously limits the dynamic range of the measurement because fluctuations in the much (∼1000 times) stronger 4WM signal overwhelms the weak 6WM signal. A significant improvement in sensitivity was achieved by employing a fully noncollinear five-beam geometry in which the 6WM signal was emitted in a background-free direction . An additional advantage of the fully noncollinear geometry is the ability to perform pseudo-phase cycling.…”

“…Rather than fitting to simple exponential decays, it is important to model the oscillatory and nonoscillatory components of the signal simultaneously to prevent low-frequency oscillations from biasing the decay time constants. One approach that was used in prior studies , was to independently model the signal transient Ŝ ( T ) at each ( T 0 , ω τ , ω t ) point in the data set using a sum of complex exponentials. Although simple and sufficient for many cases, this model does not take advantage of the commonality in dynamics across the ( T 0 , ω τ , ω t ) dimensions and therefore overfits the data.…”

“…GAMERS has been applied to a variety of chemical systems, ranging in complexity from organic dyes , to quantum dots (QDs) . It has been demonstrated that GAMERS spectrally isolates distinct coherence pathways by dispersing the signal across as many as four frequency dimensions, yielding specificity that is not possible in 2D ES.…”