Exciton Superposition States in CdSe Nanocrystals Measured Using Broadband Two-Dimensional Electronic Spectroscopy

Abstract

Coherent superpositions among eigenstates are of interest in fields as diverse as photosynthesis and quantum computation. In this report, we used two-dimensional electronic spectroscopy (2D ES) to measure the decoherence time of a superposition of the two lowest-energy excitons in colloidal CdSe nanocrystals (cubic phase) in solution at room temperature. In the electron-hole representation, the quantum coherence is, remarkably, a twelve-particle correlation. By comparing the measured 2D ES to simulations, we also explored the effects of inhomogeneous broadening and examined the spectroscopic signatures of biexcitons.

Room-temperature exciton coherence and dephasing in two-dimensional nanostructures

“…2b,d). This result is in marked contrast to previous 2DES studies of more conventional colloidal nanoparticles 24,27 and thus provides further evidence indicating the NPLs are synthesized with a thickness controlled with atomic precision 17 . Gaussian lineshapes generally indicate inhomogeneous broadening, that is, due to a distribution of mean electronic energy gaps throughout the ensemble.…”

“…Except one recent publication 33 , no electronic coherence was reported 24,39 or only with a very fast decoherence time, that is, with oscillations observable mostly within the pulse duration 27 . From the results, it is evident that the pure homogeneous line broadening of the colloidal NPLs enables clear observation of the electronic coherence that is hidden by inhomogeneous dephasing in the case of the CdSe colloidal quantum dots.…”

Photosynthetic light harvesting: excitons and coherence

“…These states are sometimes referred to as possessing a common ground state. This can arise from electronic eigenstates that involve transitions between one or more common molecular orbitals [132]. For instance, excitation of any of the S 1 , S 2 or S 3 states of naphthalene will bleach the other two transitions and that would be detected as cross peaks, because these transitions all derive from linear combinations of single excitations between two HOMOs and two LUMOs.…”

Two‐Dimensional Visible Spectroscopy For Studying Colloidal Semiconductor Nanocrystals

“…Due to the strong interaction with the bath and the ensemble broadening that results in ensemble dephasing ("fake decoherence" [ 78,98 ] ), reports of electronic coherence in nanostructures at room-temperature remain limited. In two recent works, oscillations observed in the amplitude of cross peaks were attributed to electronic coherences between the fi rst two excitonic states of CdSe QDs (1S 3/2 -1S e and 2S 3/2 -1S e ) [70] and between the fi rst 1S and 1P excitonic states. [ 68 ] Some precautions should however be taken in the interpretation of coherent signals in 2DES.…”