Publication date: 3rd July 2020
Light-emitting nanocrystals are highly attractive for novel lighting applications and quantum optics. To exploit their full potential, precise knowledge of their excitation and emission spectra as well as their underlying physical mechanism is crucial. Ensemble inhomogeneity washes out many phenomena such as fine-structure splitting, blinking, and spectral diffusion. Consequently, single-particle studies are pivotal to deepen the understanding. Over the past years, these studies have focused on measuring the emission lifetime and emission spectra of individual particles which restricts extracted information to the lowest excited state. While excitation spectra can provide additional insight into higher excited states, they are inherently much more difficult to measure on single particles. A sequential probing of the different wavelengths is necessary to obtain spectral information and especially blinking thus obscures the single-particle excitation spectrum. We present a novel approach to measure excitation spectra based on a broadband light source and a rapidly tunable narrow filter. We perform repeated spectral excitation scans at rates above 100 Hz to average out effects of blinking. During our scans, we record not only the wavelength that caused the emission of specific photons but also the corresponding arrival times. We identify moments in time where the individual quantum dot was in a “bright” or in a “dim” state. The rapid scanning allows us then to extract the separate excitation spectra of transiently occurring states of the quantum emitter. We leverage this to gain further insight into the origin of blinking in CdSe/CdS/ZnS quantum dots that show multiple discrete dim states. We observe a bright, a grey, and a dark state that occur in the same quantum dot. We attribute the bright and the dark state to excitonic emission while the grey state shows a change in the excitation spectrum that we attribute to a trion state.
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