Entry Date:
September 16, 2013

Spectroscopy: Multiexciton Generation and Measurement

Principal Investigator Moungi Bawendi


To help guide applications, many of the relevant aspects of the single exciton state structure and fluorescence have been studied extensively. However, a handful of potential applications rely on the less well-understood multiexcitonic states in which two or more electrons and holes have been excited. For example, the biexciton-exciton transition provides the optical gain in nanocrystal-based lasers.

The group was among the first to study the optical properties of multiexciton complexes. Time-resolved photoluminescence spectroscopy of nanocrystal ensembles reveals new features with distinct dynamical and spectral signatures growing in when the sample is excited with strong laser pulses. First a fast feature due to the biexciton appears superimposed on the slow single exciton decay and then a further blueshifted feature is seen at higher power corresponding to emission from higher multiexcitons. The fast nonradiative decay of these multiexcitons is thought due to an enhanced “Auger”-like process whereby an electron-hole pair recombines by simultaneously promoting another one of the available free electrons or holes in a single energy-conserving step.

Multiexciton states are of interest also for their potential use as nonclassical light sources because electrons and holes recombine pairwise so that multiple, possibly correlated, photons are emitted within a short time from a single quantum object. Such light sources have applications for fundamental quantum optics studies and possibly in quantum computation and cryptography. The group has experimentally demonstrated photon pairs due to this cascaded emission of multiexcitons and excitons from a single nanocrystal. Using two single-photon detectors in a Hanbury-Brown-Twiss geometry we have observed pairs of photons emitted from a NC after a single excitation pulse. This constitutes unambiguous proof of the creation of multiexcitons, and could open the door for future applications of multiexcitons in NCs.