Last Month's Most Accessed Feature: Taking the pulse: Fast-action laser pulses

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  • Published: Oct 3, 2017
  • Categories: UV/Vis Spectroscopy
thumbnail image: Last Month's Most Accessed Feature: Taking the pulse: Fast-action laser pulses

Repeating

CAPTION

A system that can repeatedly deliver a filtered laser pulse of 0.19 mJ of energy at a repetition rate of 100 kHz has been developed by US researchers. Pulses of 7 femtoseconds duration are available for experiments with this system.

In the couple of decades, extreme ultraviolet (XUV) pulses with durations of hundreds of attoseconds have opened up unprecedented scientific experiments. On this time scale the evolution of electrons in atoms, molecules, and solids has been made possible by using the pump-probe technique.

Pulsing

Pulses in the XUV with attosecond duration can be generated using a strong laser pulse in the visible-near-infrared, VIS-NIR, which then interacts with gaseous atoms through high-order harmonic generation (HHG). In to single out an XUV pulse, the interaction must last about ten femtoseconds, a few oscillations of the electromagnetic field, in other words. Moreover, the precise "temporal shape" of the pulse must be controlled. This is commonly done by amplifying short pulses with a controlled waveform (carrier-envelope phase) in a Ti:Sapphire laser amplifier and then shortening the duration of the pulse with non-linear pulse compression, using a gas-filled hollow-core capillary, for instance.

Unfortunately, such systems are limited to a pulse repetition rate of just a few repeats, about 1-3 kHz. Now, researchers at the Max Born Institute in Germany, working with colleagues at the Norwegian Defence Research Establishment (FFI), have designed and built a laser system capable of operating at much higher pulse repetition rates. The newly developed system could be used to perform pump-probe experiments in attosecond science implementing electron-ion coincidence detection in a reaction microscope.

Amplifying

The new system is based on a non-collinear optical parametric amplifier (NOPA). In such an amplifier, the energy from a strong pump pulse is transferred to a weak signal pulse in an instantaneous non-linear interaction in a crystal. The experimentalist adjusts the gain and bandwidth of the process by controlling the phase-matching conditions, so that all the photons at the signal frequency are emitted in phase and add up coherently as the signal pulse propagates through the crystal. When both the pump and the seed pulses enter the crystal at a small angle, i.e. with non-collinear geometry, the bandwidth peaks and it is then possible to amplify ultrashort pulses that last only a few cycles. Moreover, there is no heating effect so thermal problems are almost negligible, which means a NOPA amplifier could be pushed to much higher repetition rates than is possible with conventional approaches.

Given that attosecond science has already revolutionized the way scientists investigate events where quantum mechanics rules, this new fast-repeating laser system could expand investigations still further. Indeed, this new laser system could facilitate a whole new class of scientific experiments with simple atomic and small molecular systems, as well as high fidelity investigations of more complex molecules.

Related Links

Optics Lett 2017, 42, 2495-2498: "CEP-stable few-cycle pulses with more than 190??µJ of energy at 100??kHz from a noncollinear optical parametric amplifier"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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