Mapping laser beam with ultrafast currents

Researchers at the Wigner Research Centre for Physics comes with an application of ultrafast currents. They exploit them to map the change of an important laser beam property. This property is called CEP and determines outcomes of laser-matter interactions.

It does not surprise anyone these days that the humanity harnessed control over the electric voltage. Using electronic circuitry, it is possible to drive electric currents here and there in the way the application requires. As a result we live in a world where electronics is used to control everything from building heating system to space flights. However, the speed of the contemporary electronics has a limit. Therefore, scientists try to employ light to control processes, where the electronics lags behind. Light is much faster. Light is an oscillating electric field that changes direction million times faster than current in a conventional electronic circuit. Thanks to its speed, light can be used to control processes, which happen on a timescale comparable to the light wave cycle as, for example, chemical reactions.

That said, the manipulation of the electric field forming the light is a challenging thing. Fortunately, the laser technology is at hand and there is a category of lasers that provide light pulses with a possibility to control the evolution of the electric field within the individual pulses pulses. The scientinsts call this evolution a carrier-envelope phase (CEP). CEP is a property of every single laser pulse. And as pulses go in a laser beam in a sequence, every pulse can have different CEP. In last decades, scientists learned how to control the change of CEP from pulse to pulse, and that paved the way to observation of many intriguing light-field-dependent physics.

A typical experiment realizing a CEP dependent interaction happens in a laser focus, where the light intensity is high enough so that it changes electronic properties of matter. In the vicinity of microscopic laser focus the CEP changes from place to place due to the free space propagation effects. As the CEP is not constant in space the experiments are limited to a volume that is so small that comprises only a handful of molecules. But for successful application of the light-field controlled interactions it is important to be able to scale up the interaction volume.

The scaling of the interaction volume needs a method for measuring the CEP spatial distribution and a way to control it. This is possible from now on, as the team of scientists from the Ultrafast Nanooptics group led by Péter Dombi came up with a solution to measure spatial distributions of CEP of focused laser beams. “We applied our previous experience with driving CEP-sensitive ultrafast currents to construct an on-chip probe. We scan this probe over the laser focus and this way we measure the spatial distribution of the carrier-envelope phase,” says Václav Hanus, a postdoctoral scientist and a member of the group, who implemented and evaluated the measurements: “It is really exciting that now it is possible to perform the laser beam characterization with such an ease. No need of vacuum, nor interferometry. We call it a CEP scanner.”

With the CEP scanner at hand, the team analyzed and demonstrated the ability to shape the CEP distribution. Using light modulation techniques, they could double the volume experiencing constant CEP. “Imagine you want to ensure constant reaction conditions in the whole volume of some sample. The sample can be for example nanoparticles or molecules contained in a fluid of a microchannel. It’s important to have constant CEP all over the place,” explains Václav Hanus.

The findings were published in renowned open access journal Nature Communications. “We are really happy that we have the know-how to this state-of-the-art technology in our labs now. It is the fruit of persistent effort we do in our labs. Now, we have plenty of ideas how to use our CEP scanner in our subsequent research,” concludes Péter Dombi. The research team is grateful for the help in sample preparation to ELI-ALPS Institute in Szeged and their German colleagues.