Matter At Crossroads

Instantly turning a material from opaque to transparent, or from a conductor to an insulator, is no longer the stuff of science fiction. For several years now, scientists have been using lasers to control the properties of matter at extremely fast rates: during one optical cycle of a light wave. But because these changes occur on the timescale of attoseconds - one-billionth of one-billionth of a second - figuring out how they unfold is extremely difficult. In a new study published in Nature Photonics , Prof. Nirit Dudovich 's team from the Weizmann Institute of Science presents an innovative method of tracking these rapid material changes. This advance in attosecond science, the study of the fastest phenomena in nature, could have a wide variety of future applications, paving the way for ultrafast communications and computing.

If you have ever seen a rainbow, you've seen a practical demonstration of how light slows down and is refracted when it passes through matter, in this case, raindrops. Sunlight is composed of a broad spectrum of colors, each of which experiences a different delay as it passes through the droplets. These differences cause the colors to become separated, producing a radiant rainbow. We tend to think that matter such as glass or water refracts light in a static pattern. However, researchers from leading optical labs around the world have found that a powerful laser can modify the refractive properties of matter, that is, change the extent to which light slows down as it passes through it, on extremely short timescales. Weizmann Institute researchers postulated that if they could measure the subtle laser-induced changes to the delay experienced by light, they would be able to learn how powerful lasers can change the properties of the material so quickly.

""This discovery might lead to the development of the fastest processors possible, which will massively increase the speed at which data is transmitted or processed"

The development of this new measurement method was led by three research students - Omer Kneller, Chen Mor and Noa Yaffe - from Dudovich's lab in Weizmann's Physics of Complex Systems Department. The method uses two laser beams. The first is a powerful one, made up of relatively long pulses, that modifies the optical delay experienced by light in a given material. The other one emits extremely short attosecond pulses, and functions as a slow-motion video camera of sorts. These attosecond pulses come in two copies: one that doesn't interact with the material, serving as a reference, and another that passes through the material, interacting with it and recording the attosecond delays induced by this interaction. When the two copies are ultimately brought together and interfere with one another, this interference enables the researchers to precisely reconstruct the change in the optical delay experienced by light as it passed through the material.

Quantum Waze and superfast computers

In quantum mechanics, a material's properties are determined by its energy levels, which form a kind of energetic ladder. Electrons can move up or down this ladder by gaining or losing exactly the right amount of energy. A powerful laser changes this ladder by modifying the location of its levels; it can cause two levels to unify into one or it can split a single level into two.

Just like navigation apps such as Waze can predict how long a journey from point A to point B will take via any given route, the new method reconstructs the route that an electron has traveled between the different energy levels by measuring the delay experienced by the attosecond pulses. Analyzing the electron's journey in turn allows researchers to learn how the energy levels in a material changed in response to the laser. At first the scientists used the method to learn how the laser changed the properties of single atoms. However, they also present theoretical calculations showing that their new method can be used to reveal the interaction between light and more complex materials.

"Once we can track the 'journeys' of single electrons between energy levels, we can use light to control the properties of a material deliberately and precisely, within hundreds or even dozens of attoseconds," Dudovich says. "This ability might lead to the development of the fastest processors possible, which will massively increase the speed at which data is transmitted or processed. Our new method also has ramifications for basic research: We hope that it will help us create snapshots of electrons in motion, revealing a variety of previously inaccessible quantum phenomena."

Also participating in the study were Nikolai D. Klimkin, Prof. Olga Smirnova, Dr. Serguei Patchkovskii and Prof. Misha Ivanov from the Max-Born-Institut, Berlin; Dr. Michael Krueger from the Technion - Israel Institute of Technology, Haifa; Dr. Doron Azoury from MIT, Cambridge, Massachusetts; Ayelet J. Uzan-Narovlansky from Princeton University, Princeton, New Jersey; Yotam Federman and Dr. Barry D. Bruner from Weizmann's Physics of Complex Systems Department; and Dr. Debobrata Rajak and Prof. Yann Mairesse from the University of Bordeaux, Talence, France.