Ultrafast stimulated emission microscopy of single nanocrystals in Science

December 09, 2019

The ability to investigate the dynamics of single particle at the nano-scale 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.

Now, ICFO researchers Lukasz Piatkowski, Nicolò Accanto, Gaëtan Calbris, Sotirios Christodoulou, led by ICREA Prof. at ICFO Niek F. van Hulst, in collaboration with Iwan Moreels (Ghent University, Belgium), have published a study in SCIENCE entitled "Ultrafast stimulated emission microscopy of single nanocrystals", where they 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.

Dr. 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 (photon) throws the first ball (charge in the QD) 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 (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 (how long it took the balls to fall out, where, how, etc.) ".

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.

As ICREA Prof at ICFO 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.
Reference: https://science.sciencemag.org/content/366/6470/1240

ICFO-The Institute of Photonic Sciences

Related Quantum Dots Articles from Brightsurf:

Direct visualization of quantum dots reveals shape of quantum wave function
Trapping and controlling electrons in bilayer graphene quantum dots yields a promising platform for quantum information technologies.

Scientists age quantum dots in a test tube
Researchers from MIPT and the RAS Institute of Problems of Chemical Physics have proposed a simple and convenient way to obtain arbitrarily sized quantum dots required for physical experiments via chemical aging.

'Growing' active sites on quantum dots for robust H2 photogeneration
Chinese researchers had achieved site- and spatial- selective integration of earth-abundant metal ions in semiconductor quantum dots (QDs) for efficient and robust photocatalytic H2 evolution from water.

New insights into the energy levels in quantum dots
Researchers from Basel, Bochum and Copenhagen have gained new insights into the energy states of quantum dots.

What a pair! Coupled quantum dots may offer a new way to store quantum information
Researchers at the National Institute of Standards and Technology (NIST) and their colleagues have for the first time created and imaged a novel pair of quantum dots -- tiny islands of confined electric charge that act like interacting artificial atoms.

Spinning quantum dots
A new paper in EPJ B presents a theoretical analysis of electron spins in moving semiconductor quantum dots, showing how these can be controlled by electric fields in a way that suggests they may be usable as information storage and processing components of quantum computers.

Controlling the charge state of organic molecule quantum dots in a 2D nanoarray
Australian researchers have fabricated a self-assembled, carbon-based nanofilm where the charge state (ie, electronically neutral or positive) can be controlled at the level of individual molecules.

Modified quantum dots capture more energy from light and lose less to heat
Los Alamos National Laboratory scientists have synthesized magnetically-doped quantum dots that capture the kinetic energy of electrons created by ultraviolet light before it's wasted as heat.

Using quantum dots and a smartphone to find killer bacteria
A combination of off-the-shelf quantum dot nanotechnology and a smartphone camera soon could allow doctors to identify antibiotic-resistant bacteria in just 40 minutes, potentially saving patient lives.

Synthesizing single-crystalline hexagonal graphene quantum dots
A KAIST team has designed a novel strategy for synthesizing single-crystalline graphene quantum dots, which emit stable blue light.

Read More: Quantum Dots News and Quantum Dots Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.