Research presents the detection of energy transfer from excited electrons to the crystal lattice on the femtosecond timescale. Knowledge could contribute to the development of materials that prolong the coherence time
Energy is information. Lengthening the time during which a system is capable of retaining energy before losing it to the local environment is a key goal for the development of quantum information. This interval is called the "coherence time". Several studies have been performed with the aim of retarding decoherence.
A study conducted by researchers at the University of Campinas's Gleb Wataghin Institute of Physics (IFGW-UNICAMP) in São Paulo State, Brazil, in partnership with colleagues at the University of Michigan's Physics Department in Ann Arbor, USA, and Sungkyunkwan University's Advanced Institute of Nanotechnology (SAINT SKKU) in Seoul, South Korea, set out to understand the decoherence process on the femtosecond (10-15 s) timescale. An article describing the results was published in Physical Review Letters.
In the study, interactions between excitons (excited electrons) and phonons (quantum units of vibrational energy in a crystal lattice) were observed on the femtosecond timescale. A femtosecond is one quadrillionth of a second.
The use of a revolutionary ultrafast spectroscopy technique with high temporal and spectral resolution was fundamental to the success of the study, which was supported by FAPESP via a Young Investigator Grant awarded to Lázaro Aurélio Padilha Junior and a project conducted in partnership with the University of Michigan under the aegis of the São Paulo Research Foundation - FAPESP program São Paulo Researchers in International Collaboration (SPRINT).
Padilha was one of the principal investigators for the project, and Diogo Burigo Almeida, then a postdoctoral fellow at Michigan, was one of the main authors. The experiment was performed with semiconducting nanocrystals dispersed in a colloidal solution at cryogenic temperatures.
"We found that when the material is excited [by light], the light it emits changes color in under 200 femtoseconds. This is due to interaction between excitons and phonons. The excitons transfer part of the energy they receive to the crystal lattice. This causes a change of frequency and hence a change of emission color," Padilha told.
Their study was the first to observe this phenomenon. "It had never been observed before because the amount of energy transferred from each exciton to the lattice is tiny, corresponding to 26 millielectron volts (26x10-3 eV), and the process takes a very short time, lasting under 200 femtoseconds (200x10-15 s). Similar phenomena have been observed but on far greater timescales and due to other processes. We accessed hitherto unknown physical relations," he said.
He and his research group long studied semiconductor nanomaterials with sizes between 1 nanometer and 10 nm. A major challenge arises when promoting the growth of these materials, as each individual unit grows differently; hence, the spectrum of light emitted by the material after excitation is broadened, with the various components emitting at slightly different frequencies, and the color of the emission is less precise. When a single particle is isolated, the spectrum becomes narrower, but signal detection is retarded. In other words, spectral resolution is enhanced but at the loss of temporal resolution.
"About five years ago we began working with a technique that can pick out subsets comprising a few thousand identical particles from a set of 1020 nm particles," Padilha said. "This has enabled us to achieve very fine and precise spectral resolution, as well as fine temporal resolution. In this study, we obtained single-particle spectral resolution for a group of particles in an exceptionally short time."
As noted, this experimental solution enabled the researchers to access hitherto unknown physical processes, such as the ultrafast exciton-phonon interaction. It is worth recalling that in condensed matter physics, the phonon is a quasi-particle associated with the quantum of vibration that propagates in a crystal lattice.
There are no immediate technological applications for the results obtained, but in the not-too-distant future, knowledge of physical interactions on the femtosecond timescale can help scientists control the structure of materials such that excitons retain energy from electrical or light impulses for longer periods, retarding decoherence in quantum systems.
"Prolonging coherence is key to the success of devices such as optical switches and single-photon emitters," Almeida said. "Actually, what you aim to do is reduce energy waste to a minimum. When the material changes color, it means it's losing energy. We discovered that this loss is extremely fast. That's what we want to delay."
About São Paulo Research Foundation (FAPESP)
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. You can learn more about FAPESP at http://www.
TU Graz researchers develop new 3D printing for the direct production of nanostructures
14.11.2019 | Technische Universität Graz
Massive photons in an artificial magnetic field
14.11.2019 | Faculty of Physics University of Warsaw
The Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM in Dresden has succeeded in using Selective Electron Beam Melting (SEBM) to...
Carbon nanotubes (CNTs) are valuable for a wide variety of applications. Made of graphene sheets rolled into tubes 10,000 times smaller than a human hair, CNTs have an exceptional strength-to-mass ratio and excellent thermal and electrical properties. These features make them ideal for a range of applications, including supercapacitors, interconnects, adhesives, particle trapping and structural color.
New research reveals even more potential for CNTs: as a coating, they can both repel and hold water in place, a useful property for applications like printing,...
If you've ever tried to put several really strong, small cube magnets right next to each other on a magnetic board, you'll know that you just can't do it. What happens is that the magnets always arrange themselves in a column sticking out vertically from the magnetic board. Moreover, it's almost impossible to join several rows of these magnets together to form a flat surface. That's because magnets are dipolar. Equal poles repel each other, with the north pole of one magnet always attaching itself to the south pole of another and vice versa. This explains why they form a column with all the magnets aligned the same way.
Now, scientists at ETH Zurich have managed to create magnetic building blocks in the shape of cubes that - for the first time ever - can be joined together to...
Quantum-based communication and computation technologies promise unprecedented applications, such as unconditionally secure communications, ultra-precise...
In two experiments performed at the free-electron laser FLASH in Hamburg a cooperation led by physicists from the Heidelberg Max Planck Institute for Nuclear physics (MPIK) demonstrated strongly-driven nonlinear interaction of ultrashort extreme-ultraviolet (XUV) laser pulses with atoms and ions. The powerful excitation of an electron pair in helium was found to compete with the ultrafast decay, which temporarily may even lead to population inversion. Resonant transitions in doubly charged neon ions were shifted in energy, and observed by XUV-XUV pump-probe transient absorption spectroscopy.
An international team led by physicists from the MPIK reports on new results for efficient two-electron excitations in helium driven by strong and ultrashort...
05.11.2019 | Event News
30.10.2019 | Event News
02.10.2019 | Event News
14.11.2019 | Materials Sciences
14.11.2019 | Health and Medicine
14.11.2019 | Materials Sciences