A Spatiotemporal Symphony of Light

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A Spatiotemporal Symphony of Light

In a Nano Optics Breakthrough, Technion Researchers Observe Sound-Light Pulses in 2D Materials for the first time

Professor Ido Kaminer

Haifa, Israel June 11, 2021 – Using an ultrafast transmission electron microscope, researchers from the Technion – Israel Institute of Technology have, for the first time, recorded the propagation of combined sound and light waves in atomically thin materials. The experiments were performed in the Robert and Ruth Magid Electron Beam Quantum Dynamics Laboratory headed by Professor Ido Kaminer, of the Andrew and Erna Viterbi Faculty of Electrical & Computer Engineering and the Solid State Institute.

Single-layer materials, alternatively known as 2D materials, are in themselves novel materials, solids consisting of a single layer of atoms. Graphene, the first 2D material discovered, was isolated for the first time in 2004, an achievement that garnered the 2010 Nobel Prize. Now, for the first time, Technion scientists show how pulses of light move inside these materials. Their findings, “Spatiotemporal Imaging of 2D Polariton Wavepacket Dynamics Using Free Electrons,” were published  in Science following great interest by many scientists.

Light moves through space at 300,000 km/s. Moving through water or through glass, it slows down by a fraction. But when moving through certain few-layers solids, light slows down almost a thousand-fold. This occurs because the light makes the atoms of these special materials vibrate to create sound waves (also called phonons), and these atomic sound waves create light when they vibrate. Thus, the pulse is actually a tightly bound combination of sound and light, called “phonon-polariton.” Lit up, the material “sings.”

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

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

PhD student Yaniv Kurman

“The hybrid wave moves inside the material, so you cannot observe it using a regular optical microscope,” Kurman explained. “Most measurements of light in 2D materials are based on microscopy techniques that use needle-like objects that scan over the surface point-by-point, but every such needle-contact disturb the movement of the wave we try to image. In contrast, our new technique can image the motion of light without disturbing it. Our results could not have been achieved using existing methods. So, in addition to our scientific findings, we present a previously unseen measurement technique that will be relevant to many more scientific discoveries.”

Main authors, L-R: Yaniv Kurman, Raphael Dahan and Professor Ido Kaminer

This study was born in the height of the COVID-19 epidemic. In the months of lockdown, with the universities closed, Yaniv Kurman, a graduate student in Prof. Kaminer’s lab, sat at home and made the mathematical calculations predicting how light pulses should behave in 2D materials and how they could be measured. Meanwhile, Raphael Dahan, another student in the same lab, realized how to focus infrared pulses into the group’s electron microscope and made the necessary upgrades to accomplish that. Once the lockdown was over, the group was able to prove Kurman’s theory, and even reveal additional phenomena that they had not expected.

L-R: Yaniv Kurman and Professor Ido Kaminer

While this is a fundamental science study, the scientists expect it to have multiple research and industry applications. “We can use the system to study different physical phenomena that are not otherwise accessible,” said Prof. Kaminer. “We are planning experiments that will measure vortices of light, experiments in Chaos Theory, and simulations of phenomena that occur near black holes. Moreover, our findings may permit the production of atomically thin fiber optic “cables”, which could be placed within electrical circuits and transmit data without overheating the system – a task that is currently facing considerable challenges due to circuit minimization.” The team’s work initiates the research of light pulses inside a novel set of materials, broadens the capabilities of electron microscopes, and promotes the possibility of optical communication through atomically thin layers.

Illustration of a Sound-Light wave in 2D materials and its measurement using free electrons

“I was thrilled by these findings,” said Professor Harald Giessen, from the University of Stuttgart, who was not a part of this research. “This presents a real breakthrough in ultrafast nano-optics, and represents state of the art and the leading edge of the scientific frontier. The observation in real space and in real time is beautiful and has, to my knowledge, not been demonstrated before.”

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

Prof. Kaminer is also affiliated with the Helen Diller Quantum Center and the Russell Berrie Nanotechnology Institute. The study was spearheaded by Ph.D. students Yaniv Kurman and Raphael Dahan. 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 Prof. James Edgar (Kansas State University), of Prof. Mathieu Kociak (Université Paris Sud), and of Prof. Frank Koppens (ICFO, The Barcelona Institute of Science and Technology).

Click here for the paper in Science

Click here for video demonstrating the research

Breaking the Diffraction Limit

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Breaking the Diffraction Limit
Technion researchers increased a telescope’s resolution without enlarging its mirror

In a new article, Technion – Israel Institute of Technology scientists document findings that could significantly improve the resolution of telescopes. The research, which was performed by Ph. D. student Gal Gumpel and supervised by Dr. Erez Ribak from the Technion Department of Physics, was published in the Journal of the Optical Society of America B, in a feature issue on Astrophotonics.

Dr. Erez Ribak

The resolution of a telescope – how sharp its images are – is the smallest angle between two observed objects, where they can still be visibly separated. The resolution limit is set by diffraction: light rays diffract and scatter around objects in their path, in this case the telescope mirror, as they travel to the focal plane, where the detector (camera) is.  The original angle of the light is blurred, causing the observed object, in this case a star, to look like a fuzzy spot. Thus, two nearby stars will look like overlapping fuzzy spots, which we cannot distinguish any more.

There are two basic ways to reduce diffraction and improve resolution, as set by the uncertainty principle of quantum mechanics. One is reduction of the wave length, for example by illuminating the object by blue light, instead of red; the other is to increase the telescope aperture. Since in astronomy we observe natural light that is out of our control, we cannot reduce the wave length, but we can increase the telescope aperture. Indeed, the giant telescopes being built in recent decades provide very high resolution. In mid-sized telescopes or space telescopes, limited by launcher volume, resolution is still an issue.

Ph. D. student Gal Gumpel

The experiment performed at the Technion is based on amplification of photons (light particles). When a photon, arriving from a star, crosses the telescope aperture, it reaches a light amplifier, a medium of atoms, which responds by stimulating the emission of many additional photons. These stimulated photons are identical to the original photon, both in direction and in wave length. These daughter photons also obey the initial diffraction limit, but by their mere number they allow for a better measurement of the angle at which the original, astronomical photon has crossed the telescope aperture. This is an improvement on direct detection, which is based only on the original photon (without an amplifier), improves the resolution of the telescope without increasing its size.

Such light amplification has been disfavored because the stimulated emission is also accompanied by constant spontaneous emission by the same atoms. The copious spontaneous photons are emitted in all directions, unlike the stimulated ones, creating a bright background, and reducing the achieved increase in resolution. As a result, Gumpel and Ribak had to separately measure also the spontaneous photons. In the lab experiment they blocked the “star” light part of the time, thus measuring only the background, while the rest of the time served to measure both stimulated and spontaneous photons. The image of the object was obtained by subtraction of the background image from the combined image, leaving only the clean image of the source. This is the first time such an experiment is performed with white light, since most light amplifiers (such as those in lasers) operate only at one wave length.

According to the researchers, “one of the possible drawbacks of the method is the loss of sensitivity in the final images, but this is a worthy price to pay for the increased resolution. Moreover, the loss of sensitivity can be overcome partially by increasing the exposure times, namely the observation period.”

Figure 1: From left to right: astronomical photons (white light particles), emitted by a star, pass the telescope aperture and reach a light amplifier containing atoms. An atom hit by a photon emits a larger number of identical stimulated photons (marked in green) that hit the telescope detector at a higher precision than that of the original stellar photon. At the same time, the amplifier emits spontaneous photons (marked in red) which scatter in all directions and hit the detector in such a quantity so as to create a constant background, hiding the stellar amplified photons. To overcome this limitation the researchers measured the average spontaneous emission by blocking the stellar light and taking a picture of the background alone. By subtraction of that background from a picture with the star light, they were able to reconstruct the image of the star at high resolution.

Click here for the paper in Journal of the Optical Society of America B