Determining the Properties of Extremely Short Pulses of Light
Extremely short pulses of circularly polarised light waves can be used to study various types of materials. Such pulses can be produced with current methods, but these methods are pushing the limits of technical feasibility, meaning that light pulses are not always produced with the desired properties. Since the processes to be investigated are exceptionally short in duration, the light pulses must therefore also be short, in the range of around 100 attoseconds (billionths of a billionth of a second). In this minute timespan, a light wave can only undergo a few rotations. When ultrashort pulses are produced, the light waves may easily not rotate the right way.
Electrons comparable with water from one-armed sprinkler
Physicists at MBI have developed a method of firing an extremely short, high-energy and circularly polarised light pulse at an atom or a solid body where the light pulse knocks an electron out of the body upon being absorbed. This electron then carries information about the light wave itself and can reveal clues as to the properties of the sample being examined. Since 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 MBI’s Theory Department. When the sprinkler runs for a while, it wets the grass in a full circle – irrespective of whether it rotates consistently or not. "But if a gusty wind comes along, then we can distinguish whether the sprinkler has been turning regularly or irregularly," Ivanov says. A sprinkler rotating completely irregularly would generate an ellipse on the grass stretched in the direction of the wind, while a regularly rotating sprinkler would display a tilted ellipse.
This "wind" in this method is generated as 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 MBI scientist Álvaro Jiménez-Galán. "Our method allows one to characterise the properties of the ultrashort light pulses with unprecedented precision." This theoretical method is of special significance when it comes to studying novel materials such as superconductors or topological materials and is expected to be implemented in practice.
The research results were published in "Nature Communications".