A spinning top of light

Short, rotating pulses of light reveal much about the internal structure of materials. An international team of physicists headed by Prof. Misha Ivanov from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) has now developed a new method to accurately characterize such extremely short light pulses. The research results have been published in Nature Communications.

Light is not the same light: Depending on the type of preparation, it can be in very different ways. Not only its wavelength or color can be chosen. As an electromagnetic wave, light can also have different types of vibration. For example, it can occur in different polarization - either linearly polarized with rectilinear or circularly polarized with circular oscillation of the electromagnetic fields. Above all, extremely short pulses of circularly polarized light waves are ideal for studying very different materials. With today's methods, such pulses can indeed be produced. However, the methods are still at the limits of the technically feasible and the light pulses produced do not always show the desired properties.

A new method now makes it possible to characterize such light pulses with unprecedented precision. The particular difficulty here: The interesting processes in matter, which one wants to examine by irradiation with light pulses, are extremely short. Accordingly short, in the range of about 100 attoseconds (billionth of a billionth of a second), must also be the light pulses. In this tiny period, a light wave performs only a few rotations. When making such ultrashort pulses using novel laser techniques, it can quickly happen that the light waves do not rotate properly.

The idea behind the new method: irradiating an atom or a body with an extremely short, high-energy and circularly polarized light pulse, which pulse is absorbed and an electron pushes out of the body. On the one hand, this electron carries information about the light wave and on the other hand it can provide information about the properties of the examined body. Since the light pulses are circularly polarized, the ejected electrons also perform rotations.

Fig. 1: A quiet sprinkler distributes the water evenly in a circle and the grass grows in a circular pattern - regardless of whether the sprinkler turns clockwise, counterclockwise or randomly. When the wind blows, the grass gets unevenly wet - this is also reflected in its growth. But when the wind changes direction in sync with the sprinkler's rotation, it causes grass growth to be asymmetric. This makes it possible to reconstruct the spin feature of the sprinkler - whether it is a precise, regularly rotating sprinkler or a cheap specimen that rotates at random.

In the investigation, the sprinkler is the short pulse (blue), which lasts only about 10-16 seconds and whose electric field rotates even faster. The "wind" is a linearly polarized and precisely controlled infrared laser field (red). The grass is the measured photoelectron angular distribution (green). The asymmetry in the latter allows for the first time to reconstruct the properties of the ultrashort pulses. (Source: Felipe Morales and Álvaro Jiménez-Galán)

"You can compare the ejected electrons with a one-arm sprinkler that either rotates as desired or repeatedly stuttered and even reverses its direction of rotation," says Misha Ivanov, head of the theory department at the Max Born Institute. Now when the sprinkler runs for a while, it makes the lawn wet around it circularly - regardless of whether it rotates evenly or not. To find out if the sprinkler turns exactly in the desired direction, it is not enough just to look at the lawn. "But if a gusty wind blows in addition, we can tell whether the sprinkler turns evenly or irregularly," says Ivanov. If, for example, a gust of wind alternately occurs from left or right each time the sprinkler's arm is to the left or right, then the lawn will not be circularly wet but will have a diagonal ellipse. A wholly irregularly rotating sprinkler would cast a wind-directed ellipse on the lawn, while a regular-rotating sprinkler would display a crooked ellipse.

The "wind" is an infrared laser pulse whose vibrations are precisely synchronized with the ultrashort pulses. The infrared radiation accelerates the electron either to the left or to the right - just like the wind drops the water.

"With a measurement on the electrons, we can then prove whether the light pulse has possessed the desired uniform rotation or not," says Álvaro Jiménez-Galán, a scientist at the Max Born Institute and first author of the publication in "Nature Communications". "With our method it is possible to determine the properties of ultrashort light pulses with unprecedented precision," says Jiménez-Galán. And once the light pulses are sharply defined, the information from the electron about its place of origin within exotic materials can be read out all the more precisely.

This is particularly important for investigation on a whole range of novel materials. This could include on the one hand superconductors that can conduct electricity without electrical resistance, and on the other hand topological materials that have exotic properties and for their research was awarded the 2016 Nobel Prize in Physics. These materials could be used in a quantum computer or make particularly fast and energy-efficient processors and memory chips possible in normal computers and smartphones.

Although the new sprinkler process exists for the time being only in theory, it should be ready in the near future. "Our specifications correspond to the current state of the art, so there is no reason for an imminent realization in the laboratory," says Ivanov.

Original publication

Attosecond recorder of the polarization state of light

Jiménez Galán, Á., G. Dixit, S. Patchkovskii, O. Smirnova, F. Morales, M. Ivanov

Nature Communications 9 (2018) 850/1-6