Lightwave in Super-Slow Motion

Simulation of a carrier-envelope phase (CEP) in a focused, broadband, pulsed Gaussian beam. (Source: FAU / FSU)

Physicists at Friedrich-Alexander-Uni­versity Erlan­gen-Nürnberg and Friedrich Schiller Uni­versity Jena have managed to capture the behaviour of extremely short laser pulses during focusing by means of very high spatial and temporal reso­lution. The results are of fundamental relevance to under­standing the inter­actions between light and matter and will make it possible to control electron movements and chemical reactions to an extent that was previously not feasible. These insights into funda­mental physics will parti­cularly profit further research into new radia­tion sources and in the field of light wave electronics.

Ultra­short light pulses with such a wide optical spectrum range that the beams appear white are in common use nowadays. Among other things they are used to examine the retina of the eye while in physics they are employed to control processes at the atomic level and analyse them in slow motion. In almost all these appli­cations, the white laser pulses need to be focused. As it is the speci­fic form of the light wave that determines how electrons, for example, will move within it, it is essential to know what the focused laser beam actually looks like in detail.

In order to better under­stand why, think of a ship in stormy seas. The helmsman not only has to know how high and how long the waves are but also has to keep an eye on incoming waves in order to know when they will hit the ship in order to find a safe path up to the crest of the wave on one side and down on the other. In the same way, it is important for re­searchers to know how and where the maximum of a light wave will strike electrons in an experiment or appli­cation in order to have a targeted influence on them. The changes to and propa­gation of light waves in an electrical field take place on a time scale of a few hundred atto­seconds. Until recently, it was not possible to measure the exact distri­bution of the wave troughs and peaks at the focus of a laser beam on this time scale.

The researchers in Erlan­gen and Jena have now achieved this by focusing laser pulses onto a nano­metre-sharp metal tip, causing the tip to emit electrons. These electrons act as a kind of sensor that enables the researchers to interpret the exact form of the light wave. Almost 130 years ago, the French physicist Louis Georges Gouy (1854-1926) observed and described a phase shift that occurred during the focusing of mono­chromatic light when inter­ference was intro­duced. This effect was named the Gouy phase after its dis­coverer and for a long time it was assumed that the effect would be the same in the case of white laser spectra, which consist of many colours of light. The results obtained in the joint project have added to our under­standing of the effect. (Source: FAU)

Reference: D. Hoff et al.: Tracing the phase of focused broadband laser pulses, Nat. Phys., online 10 July 2017; DOI: 10.1038/nphys4185

Link: Inst. for Optics and Quantum Electronics, Friedrich-Schiller-University Jena, Jena, Germany

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