Peering into Plasma Mirrors

Attosecond flashes of light can be generated on glass surfaces through the process of ionization by a strong laser, which gives rise to a dense mixture of free-moving electrons and nearly quiescent atomic hulls. Every fragment on the glass surface marks the impact of a laser pulse. (Source: T. Naeser)

When light interacts with a mirror which is moving towards it at a speed close to the speed of light, its wavelength is shifted into the extreme ultra­violet region of the spectrum. This effect was first predicted by Albert Einstein. His theory was experi­mentally confirmed almost 100 years later, following the develop­ment of high-intensity laser light sources. Laser physicists at the Laboratory for Atto­second Physics (LAP) at Max Planck Institute for Quantum Optics in Garching (MPQ) and LMU have now charac­terized the phenomenon in detail under controlled conditions, and exploited it to generate high-inten­sity attosecond light flashes. Moreover, they show that these pulses can be shaped with unpre­cedented precision for use in atto­second research.

As a rule, these ultrashort pulses are created by allowing coherent laser light to interact with a sample of a noble gas, such as xenon. However, this method has one serious drawback – the resulting pulses have low energies. An alter­native approach to the generation of atto­second pulses makes use of relativis­tically oscil­lating mirrors. In this case, the light interacts not with a gas, but with a solid surface made of fused silica.

A small portion of the incident light serves to ionize the surface of the glass, thus creating a plasma. This state of affairs can be compared to that found in normal metals, in which a fraction of the electrons can move freely through the material. In fact, this dense surface plasma behaves like a metal-coated mirror. The oscil­lating electric field asso­ciated with the light that impinges on this mirror causes the surface of the plasma to oscil­late at peak velo­cities close to that of light itself. The oscil­lating surface in turn reflects the incident light.

As a consequence of the Doppler effect, the frequency of the incoming light is shifted into the extreme ultraviolet (XUV) region of the spectrum – and the higher the peak velo­cities, the greater the frequency shift. Because the durations of mirror oscil­lations at maximum speed are extremely short, XUV light pulses lasting for a matter of atto­seconds can be spectrally filtered out. Crucially, these flashes have a far greater intensity than those that can be generated by the conven­tional inter­action in the gaseous phase. In fact, simu­lations suggest that they should reach photon energies on the order of kiloelectron volts.

In collaboration with scientists from the ELI (Extreme Light Infrastructure) in Szeged in Hungary, the Foundation for Research & Technology – Hellas (FORTH) in Heraklion (Greece) and Umeå University in Sweden, the team led by Stefan Karsch has been able to gain new and valuable insights into the inter­action of pulsed laser light with relati­vistically oscil­lating solid surfaces. They first analysed the intensity profile and energy distri­bution of the resulting atto­second pulses, and their dependence on the carrier envelope phase of the driving input laser pulse in real time.

“These obser­vations permit us to define the condi­tions required for optimal generation of attosecond light pulses using the oscil­lating plasma mirror,” says Olga Jahn. “We were able to demonstrate that isolated atto­second XUV light flashes can indeed be produced from optical pulses consisting of three oscil­lation cycles.” The LAP team’s findings will enable the procedure required to generate atto­second pulses by means of plasma mirrors to be sim­plified and standardized. The comparatively high intensities achieved open up new oppor­tunities for ultra­violet spectro­scopy, and promise to unveil new aspects of molecular and atomic behavior. (Source: LMU)

Reference: O. Jahn et al.: Towards intense isolated attosecond pulses from relativistic surface high harmonics, Optica 6, 280 (2019); DOI: 10.1364/OPTICA.6.000280

Link: Laboratory for Attosecond Physics LAP, Max Planck Institute for Quantum Optics, Garching, Germany

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