Quantum Light From Superlattices

The formation of superlattices from a monodisperse solution of perovskite nanocrystals during evaporation of the solvent. (Source: J. Schnabl)

Quantum dots (QDs) are bright, luminescent nano­particles that can emit light in very well-defined colors, that can be changed by altering particle size. QDs are used, for example, in the latest generation of LCD televisions for achieving more vivid colors. Maksym Kovalenko and his team work on the next-generation of QDs, made of semi­conducting perov­skites, and arrange these QDs into superlattices, three-dimensional blocks of nearly identical quantum dots. They have now demon­strated a new property of such super­lattices – emission of super­fluorescent light. As a result, these super­lattices can be used as quantum light sources for future quantum appli­cations.

In the group of Maksym Kovalenko, perovskite-type QDs and their optoelectrical properties have been thoroughly inves­tigated during the last years. This type of QD consists of cesium lead halide. The inter­national consortium of researchers could show that indi­vidual perovskite nano­crystals can rapidly emit extremely bright light – a process that occurs faster than in any other type of QD. This property makes perovskite QDs very interesting for use in optical data communi­cation, e.g. in super­computer networks, where every nano­second counts.

Now, researchers from ETH and Empa, in colla­boration with colleagues from IBM Research in Zurich, have discovered yet another intriguing property of these nanocrystals: in contrast to a random ensemble of fluorescent QDs, which emit uncorre­lated photons after some delay, super­lattices of the same QDs can emit bursts of corre­lated photons almost instan­taneously, i.e. they are superfluorescent. Once excited, all dipoles inside the super­lattice self-synchronize before returning to the ground state, thereby emitting all photons simul­taneously in a burst only a few picoseconds after excitation. Such fast emission of light by QDs has never been reported before. These newly discovered super­lattices are therefore parti­cularly interesting as quantum light sources, which could be used e.g. in quantum communi­cation or in quantum sensing appli­cations.

To produce the super­lattices, the researchers use a dispersion of cubic CsPbX3 perovskite nano­crystals, almost all of which have the exact edge length of 9.5 nm. By letting the solvent slowly evaporate, cube-shaped super­lattices are formed spon­taneously, each consisting of up to several million nanocubes. The fluorescent super­lattices are visible under a micro­scope and have an edge length of up to 20 µm. “Such long-range ordered super­lattices could only be obtained from a highly mono­disperse solution of QDs, whose synthesis has been carefully optimized over the last few years” says Maryna Bod­narchuk, a senior scientist at Empa Dübendorf, making these QDs.

“The possi­bility of producing highly correlated photons in such a simple way opens up new and exciting poss­ibilities for funda­mental research, and enables future develop­ments in quantum tech­nology. These newly discovered quantum light sources could become very important for quantum computing, quantum-encrypted communi­cation, or quantum sensors with unpre­cedented accuracy”, says Gabriele Rainò, senior scientist in Kovalenko’s group. (Source: ETHZ)

Reference: G. Rainò et al.: Superfluorescence from lead halide perovskite quantum dot superlattices, Nature, online 7 November 2018; DOI: 10.1038/s41586-018-0683-0

Links: Functional Inorganic Materials, ETH Zurich, Zurich, Switzerland • Thin Films and Photovoltaics, Empa, Dübendorf, Switzerland


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