Unfazed by Phase Shifts

Artificial view on how anovel type of detector enables the oscillation profile of light waves to be precisely determined. (Source: P. Rosenberger, MPQ / LMU)

Light waves propa­gate with a velocity of almost 300,000 km per second, and the wavefront oscillates several hundred trillion times in that same interval. To work with light, one must control it – and that requires precise knowledge of its behaviour. It may even be necessary to know the exact position of the crests or valleys of the light wave at a given instant. Now, a team at the Labora­tory for Attosecond Physics at the Max Planck Institute for Quantum Optics and the Unive­rsity of Munich LMU has developed a new technique, which does precisely that, and enables physicists to characterize pulses of laser light with breath­taking speed.

In the case of visible light, the physical distance between successive peaks of the light wave is less than 1 micrometer, and peaks are separated in time by less than 3 femto­seconds. Researchers based at the Laboratory for Atto­second Physics (LAP) are now in a position to measure the exact location of such peaks within single ultrashort pulses of infrared light with the aid of a newly developed detector. Such pulses, which encompass only a few oscil­lations of the wave, can be used to investigate the behaviour of molecules and their constituent atoms, and the new detector is a very valuable tool in this context. Ultra­short laser pulses allow scientists to study dynamic processes at molecular and even subatomic levels. Using trains of these pulses, it is possible first to excite the target particles and then to film their responses in real time. In intense light fields, however, it is crucial to know the precise waveform of the pulses. Since the peak of the oscil­lating (carrier) light field and that of the pulse envelope can shift with respect to each other between different laser pulses, it is important to know the precise waveform of each pulse.

The team, which was led by Boris Bergues and Matthias Kling, has now made a decisive breakthrough in the characteri­zation of light waves. Their new detector allows them to determine the phase, i.e. the precise positions of the peaks of the few oscillation cycles within each and every pulse, at repetition rates of 10,000 pulses per second. To do so, the group generated circularly polarized laser pulses in which the orientation of the propa­gating optical field rotates like a clock hand, and then focused the rotating pulse in ambient air.

The inter­action between the pulse and molecules in the air results in a short burst of electric current, whose direction depends on the position of the peak of the light wave. By analyzing the exact direction of the current pulse, the researchers were able to retrieve the phase of the “carrier-envelope offset”, and thus reconstruct the form of the light wave. Unlike the method conven­tionally employed for phase deter­mination, which requires the use of a complex vacuum apparatus, the new technique works in ambient air and the measure­ments require very few extra components. “The simpli­city of the setup is likely to ensure that it will become a standard tool in laser tech­nology”, explains Matthias Kling.

“We believe that this technique can also be applied to lasers with much higher repetition rates and in different spectral regions,” says Boris Bergues. “Our metho­dology is of particular interest in the context of the charac­terization of extremely short laser pulses with high repetition rates, such as those generated at Europe’s Extreme Light Infra­structure (ELI),” adds Matthias Kling. When applied to the latest sources of ultrashort laser pulses, this new method of waveform analysis could pave the way to techno­logical break­throughs, as well as permitting new insights into the behaviour of elementary particles in the fast lane. (Source: MPQ)

Reference: M. Kubullek et al.: Single-shot carrier–envelope-phase measurement in ambient air, Optica 7, 35 (2020); DOI: 10.1364/OPTICA.7.000035

Link: Strong-Field Dynamics, Laboratory for Attosecond Physics LAP, LMU Munich and Max Planck Institute for Quantum Optics, Garching, Germany

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