Faster Imaging of Nano-Objects

The ability to inves­tigate the dynamics of single particle at the nanoscale and femtosecond level remained an unfathomed dream for years. It was not until the dawn of the 21st century that nano­technology and femtoscience gradually merged together and the first ultrafast microscopy of individual quantum dots (QDs) and molecules was accom­plished. Ultrafast micro­scopy studies entirely rely on detecting nanoparticles or single molecules with lumines­cence techniques, which require efficient emitters to work. However, such techniques cause degra­dation to the sample, as well as, yield little information about the dynamics of the system in the excited state. Only in recent years, the efforts to find an alter­native compatible technique to study fast processes in nano-objects came into the spotlight.

Upon stimulation, two photons emerge from the quantum dot giving detailed information about the dynamics of the excited charges within the quantum dot. (Source: ICFO)

Now, ICFO researchers Lukasz Piatkowski, Nicolò Accanto, Gaëtan Calbris, Sotirios Christodoulou, led by Niek F. van Hulst, in colla­boration with Iwan Moreels from Ghent University, Belgium, report on a technique for studying ultrafast events in individual non-fluorescent nano-objects. In their study, they took individual QDs and rather than waiting for the QD to spon­taneously emit light through photo­luminescence, the team used a sophis­ticated combi­nation of laser pulses to promote individual QDs into excited state and then, force them down, back to the ground state to first: image indi­vidual QDs and second: discern the evolution of the excited charges within the entire photocycle.

Lukasz Piatkowski explains why they used a laser pulse pair to effectively image the dynamics of the QDs: “It is like throwing a ball into a tree; the higher you throw it, the more excited the state. The first laser pulse of the system throws the first ball into the tree. If you are using a photo­luminescence technique it is like you are standing below the tree, and you cannot see what is happening inside the treetop or crown. Thus, you will not know whether the ball starts to bounce down the branches, where, when and how is starts to fall down, if it stomps with something on its way, if it gets caught in an inter­mediate branch, etc. So, in order to see what is happening with the first ball, you need to find another technique that allows you to look into the treetop. The technique we used allowed us to throw a second ball into the tree top – the second laser pulse inter­acting with the QD – to bring the first ball down. Throwing the second ball higher or lower, stronger or weaker, sooner or later after the first ball, we obtain information about the first ball and the structure of the tree.”

In their experiment, the first laser pulse brings indi­vidual QD to the excite state. Then, every few hundreds femto­second, they shot a second laser pulse onto the QD to bring the charges down to ground state, inducing recombination and emission of an extra photon. Hence, for every probe photon they shot into the system, they got two twin photons back. These extra photons allowed the authors not only to image the QDs but also to precisely track the evolution of the excited charges in the QD, unveiling how many charges underwent spontaneous recom­bination, stimulated recom­bination and excited state absorption.

Being able to track excited charges at the nanoscale is of funda­mental importance in nano­technology, photonics and photo­voltaics. The results of the study have proven that ultrafast stimulated emission micro­scopy can be used to study ultrafast processes in individual chromophoric particles that are otherwise unde­tectable through fluores­cence/photo­luminescence techniques. In other words, such study has permitted imaging and studying the dynamics of nano-particles and structures without the need of external fluorescent labels.

Niek van Hulst remarks: “Significant advances are expected in the future within the field of ultra-fast-nano-regime imaging techniques. The first detection of quantum dots using this approach has been out­standing. We now aim to extend this to molecules and bio­molecular complexes, speci­fically photo-synthetic complexes. We are currently working on 3 and 4 pulse schemes to merge the stimulated emission and lumines­cence detection of single systems with 2D-spectro­scopy.” (Source: ICFO)

Reference: L. Piatkowski et al.: Ultrafast stimulated emission microscopy of single nanocrystals, Science 366, 1240 (2019); DOI: 10.1126/science.aay1821

Link: Molecular Nanophotonics, ICFO – Institut de Ciences Fotoniques, Barcelona Institute of Science and Technology, Castelldefels, Spain

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