A Spinning Top of Light

Semi-classical sketch of the origin of the photoelectron asymmetry. The IR pulse is polarized and the photoelectron asymmetry is measured along the vertical axis. The XUV pulse rotates counter-clockwise, the momentum distributions are observed. (Source: F. Morales & Á. Jiménez-Galán)

Short, rotating pulses of light reveal a great deal about the inner structure of materials. An inter­national team of physicists led by Misha Ivanov of the Max Born Institute for Non­linear Optics and Short Pulse Spectro­scopy MBI has now developed a new method for precisely charac­terising such extremely short light pulses.

Light can occur in dif­ferent polari­sations, for example either linearly polarised or circu­larly polarised, where the oscil­lations of the electro­magnetic fields follow a line or go round in circles, respec­tively. Above all, extremely short pulses of polarised light waves are excellent for studying many different types of materials. We have methods for producing such pulses, but these methods are already pushing the limits of technical feasi­bility and the light pulses are not always produced with the desired proper­ties.

A new method now allows us to charac­terise these short light pulses with unprece­dented precision. The trouble starts with the fact that the processes of interest taking place inside matter, which we would like to study with our light pulses, are extremely short-lived. Accordingly, the light pulses have to be similarly short, in the range of around 100 atto­seconds. In this short timespan, a light wave can only undergo a few rota­tions. Even using the latest laser methods to produce such ultra­short pulses, it can easily happen that the light wave will not come out rota­ting the right way.

The concept for the new method can be described as follows: one fires an extremely short, high-energy and circu­larly polarised light pulse at an atom or a solid body where, upon being absorbed, the light pulse knocks an electron out of the body. This electron then carries infor­mation about the light wave itself and can further­more reveal clues as to the proper­ties of the sample being examined. Because the light pulses are circu­larly polarised, the ejected electrons also fly off with a rota­ting motion.

“You can compare the electrons being ejected with a one-armed sprinkler, which either continues turning in the direc­tion you want it to, or which keeps stut­tering and even changing its direc­tion,” says Misha Ivanov, Head of the Theory Depart­ment of the Max Born Institute. If the sprinkler is allowed to run for a while, then it will wet the grass in a full circle – irre­spective of whether it rotates consis­tently or not. So, merely looking at the grass will not reveal whether the sprinkler has been turning exactly the way it was desired or not. “But if a gusty wind comes along, then we can distin­guish whether the sprinkler has been turning regu­larly or irre­gularly,” Ivanov says. If the wind blows alter­nately from the left or right each time the arm of the sprinkler faces left or right, then the patch of wet grass will not be circular, but rather elliptical in shape. A sprinkler rotating com­pletely irregularly would magi­cally conjure up an ellipse on the grass stretched in the wind direction, while a regu­larly rotating sprinkler will display a tilted ellipse.

This “wind” is added into the experi­ment in the form of an infrared laser pulse whose oscil­lations are perfectly syn­chronised with the ultra­short pulses. The infrared radiation acce­lerates the electron either to the left or right – just like the wind blows the water droplets. “By measuring the electrons, we can then determine whether the light pulse possessed the desired consis­tent rotation or not,” says Álvaro Jiménez-Galán. “Our method allows one to charac­terise the proper­ties of the ultra­short light pulses with unprece­dented precision,” he adds. And the more precisely these light pulses are charac­terised, the more detailed infor­mation can be derived about the electron’s place of origin within an exotic material.

This is of special signi­ficance when it comes to studying a whole series of novel materials. These could include super­conductors or topo­logical materials that exhibit exotic behaviour. Materials like these could be used to make a quantum computer, for example, or could allow super­fast, energy-effi­cient processors and memory chips to be built into normal computers and smart­phones. The new sprinkler method still only exists in theory for the moment, but ought to be imple­mentable in the near future. “Our require­ments are fully within the latest state of the art, so there is nothing to preclude this from being realised soon,” Ivanov asserts. (Source: MBI)

Reference: Á. Jiménez-Galán et al.: Attosecond recorder of the polarization state of light, Nat. Commun., online 27. Februar 2018; DOI: 10.1038/s41467-018-03167-2

Link: Theory of Attosecond-Physics (M. Ivanov), Max-Born-Institute, Berlin, Germany

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