HERALDED SPECTROSCOPY OF SINGLE NANOCRYSTALS REVEALS EXCITON-EXCITON INTERACTION
2Department of Physics and Center for Applied Photonics, University of Konstanz, Konstanz, Germany
3School of Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Neuchatel, Switzerland
Multiply excited states in semiconductor nanocrystals feature intriguing physics and play a crucial role in classical and quantum nanocrystal-based technologies. Photoluminescence (PL) of these states presents a natural probe for their investigation. Nevertheless, direct observation and spectroscopy of emission from multiexcitonic states have proved elusive. The typically dim contribution is overwhelmed by overlapping emission from other excited states, introducing challenges and ambiguities in classical spectroscopic measurements.
While identifying the PL contribution of multiexcitonic states in the spectral or temporal domains is challenging, this is conceptually simple in the temporal-correlation domain. A single nanocrystal under pulsed illumination will emit two photons in rapid succession when relaxing from the doubly-excited state (BX) to the singly-excited state (1X) to the ground state (GS). Thus, post-selecting pairs of photons following the same excitation pulse can unambiguously isolate BX-1X-GS emission cascades. However, this cannot be realized with existing spectroscopic detectors. Current techniques cannot discern the energy and the arrival time of single-photons, in parallel, at the relevant timescales.
We present a new type of spectroscopy - heralded spectroscopy, specifically tailored to tackle this challenge. By introducing single-photon avalanche diode array detectors into a spectroscopic setup, we surpass the fundamental temporal limit of existing spectrometers. Namely, the technique extends spectroscopy’s temporal resolution by six orders of magnitude without compromising spectral resolution or single-photon sensitivity. This groundbreaking function allows the unambiguous isolation of emission cascades originating in multiply excited states of single nanocrystals.
Using heralded spectroscopy, we demonstrate that the attractive interaction between excitons in the prototypical CdSe/CdS nanocrystals correlates with charge-carrier confinement. Moreover, we demonstrate that this attractive energy fluctuates with the spurious electric fields around the particle, and the fluctuations are significant enough to lead to momentary repulsive interaction. Heralded spectroscopy also allowed resolving the controversy regarding exciton-exciton interaction in lead halide perovskite nanocrystals. Contradicting values were published in the literature for the interaction in these materials, ranging from strongly attractive to strongly repulsive. Using our novel approach, we have demonstrated that the value is in the lower range of previously reported attractive values and features a similar correlation with charge-carrier confinement.
The significance of these findings highlights the potential of this new branch of single-particle spectroscopy. Breaking a fundamental limit of spectroscopy, it harbours the potential to continue to expand the capabilities of spectroscopy, and greatly impact the understanding of quantum emitter physics and nanocrystal-based technologies.
The results that will be presented in the talk have been recently published in Nano Letters: https://doi.org/10.1021/acs.nanolett.1c01291, and ACS Nano: https://doi.org/10.1021/acsnano.1c06624.
Author contibution (for Margulis Prize only): G.L., R.T. and D.O. conceived the original idea and designed the experiment. G.L. performed the experiments and developed the data analysis. G.L. and R.T. built the experimental and analysis setup with contributions from S.K.. V.J.Y. advised the data analysis. A.U. designed the linear SPAD array chip with contributions from I.M.A., C.B. and E.C.. C.B. coordinated the microlens development. S.B. designed the FPGA firmware. G.L. wrote the manuscripts with critical feedback from all authors.