Researchers observe sound-light pulses in 2D materials for the first time

Research team, from left to right: Yuval Adiv, Yaniv Kurman, Professor Ido Kaminer, Raphael Dahan and Dr Kangpeng Wang. Credit: Technion – Israel Institute of Technology

A spatio-temporal symphony of light

Using an ultra-fast transmission electron microscope, researchers at the Technion – Israel Institute of Technology have, for the first time, recorded the propagation of sound and light waves combined in atomically thin materials.

The experiments were performed in the Robert and Ruth Magid Electron Beam Quantum Dynamics Laboratory headed by Professor Ido Kaminer, the Andrew and Erna Viterbi Faculty of Electrical and Computer Engineering and the Solid State Institute.

Single-layered materials, also known as 2D materials, are themselves new materials, solids made up of a single layer of atoms. Graphene, the first 2D material discovered, was first isolated in 2004, an achievement that won it the 2010 Nobel Prize. Now, for the first time, Technion scientists are showing how pulses of light travel through the air. inside these materials. Their results, “Spatio-temporal imaging of the dynamics of 2D polariton wave packets using free electrons”, have been published in Science following the great interest of many scientists.

Light-sound wave in 2D material

Illustration of a Sound-Light wave in 2D materials and its measurement using free electrons. Credit: Technion – Israel Institute of Technology

Light travels through space at 300,000 km / s. Moving in water or in glass, it slows down by a fraction. But when it moves through some solids in a few layers, the light slows down almost a thousand times. This happens because light vibrates the atoms of these special materials to create sound waves (also called phonons), and these atomic sound waves create light when they vibrate. So the pulse is actually a closely related combination of sound and light, called a “phonon-polariton”. On, the material “sings”.

Scientists projected pulses of light along the edge of a 2D material, producing hybrid sound-light waves in the material. Not only were they able to record these waves, but they also found that the impulses can spontaneously speed up and slow down. Surprisingly, the waves even split into two separate pulses, moving at different speeds.

The experiment was carried out using an ultrafast transmission electron microscope (UTEM). Unlike optical microscopes and scanning electron microscopes, here the particles pass through the sample and then are received by a detector. This process allowed researchers to follow the sound-light wave with unprecedented resolution, both in space and time. The temporal resolution is 50 femtoseconds – 50X10-15 seconds – the number of frames per second is similar to the number of seconds in a million years.

“The hybrid wave travels inside the material, so you can’t observe it using an ordinary light microscope,” Kurman explained. “Most measurements of light in 2D materials are based on microscopy techniques that use needle-shaped objects that scan the surface point by point, but each needle contact disrupts the wave’s motion. we try to image. On the other hand, our new technique makes it possible to image the movement of light without disturbing it. Our results could not have been obtained with the existing methods. Thus, in addition to our scientific discoveries, we present a novel measurement technique that will be relevant for many other scientific discoveries. “

This study was born at the height of the COVID-19 epidemic. During the months of lockdown, when universities were closed, Yaniv Kurman, a graduate student from Professor Kaminer’s lab, sat at his home and performed the mathematical calculations predicting how light pulses should behave in 2D materials and how they might be measured. Meanwhile, Raphael Dahan, another student in the same lab, figured out how to focus infrared pulses in the group’s electron microscope and made the necessary upgrades to achieve it. Once the lockdown was over, the group were able to prove Kurman’s theory and even reveal additional phenomena they weren’t expecting.

Although this is a fundamental scientific study, scientists expect it to have multiple research and industrial applications. “We can use the system to study different physical phenomena that are not otherwise accessible,” said Professor Kaminer. “We are planning experiments that will measure vortices of light, experiments on chaos theory, and simulations of phenomena that occur near black holes. In addition, our findings could enable the production of atomically thin fiber optic “cables”, which could be placed in electrical circuits and transmit data without overheating the system – a task that currently faces considerable challenges due to the minimization of circuits. “

Yaniv Kurman and Ido Kaminer

From left to right: Yaniv Kurman and Professor Ido Kaminer. Credit: Technion – Israel Institute of Technology

The team’s work initiates the search for light pulses within a new set of materials, expands the capabilities of electron microscopes, and promotes the possibility of optical communication through atomically thin layers.

“I was delighted with these results,” said Professor Harald Giessen, of the University of Stuttgart, who was not part of this research. “This represents a real breakthrough in ultra-fast nano-optics, and represents state of the art and the leading edge of the scientific frontier. Observation in real space and in real time is magnificent and has never been demonstrated to my knowledge before.

Another prominent scientist not involved in the study, John Joannopoulos of the Massachusetts Institute of Technology, added that “The key to this achievement lies in the intelligent design and development of an experimental system. This work of Ido Kaminer and his group and colleagues is an essential step forward. It is of great interest both scientifically and technologically, and is of critical importance to the field.

Professor Kaminer is also affiliated with the Helen Diller Quantum Center and the Russell Berrie Nanotechnology Institute. The study was conducted by Ph.D. students Yaniv Kurman and Raphael Dahan. The other members of the research team were Dr Kangpeng Wang, Michael Yannai, Yuval Adiv and Ori Reinhardt. The research was based on an international collaboration with the groups of Professor James Edgar (Kansas State University), Professor Mathieu Kociak (University Paris Sud) and Professor Frank Koppens (ICFO, The Barcelona Institute of Science and Technology).

Reference: “Spatio-temporal imaging of the dynamics of 2D polariton wave packets using free electrons” by Yaniv Kurman, Raphael Dahan, Hanan Herzig Sheinfux, Kangpeng Wang, Michael Yannai, Yuval Adiv, Ori Reinhardt, Luiz HG Tizei, Steffi Y. Woo, Jiahan Li, James H. Edgar, Mathieu Kociak, Frank HL Koppens and Ido Kaminer, June 11, 2021, Science.
DOI: 10.1126 / science.abg9015

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