Brief Reflections from a Plasma Mirror

With extremely intense laser pulses, the international team of laser physicists generates fast electrons, which in turn emit attosecond light flashes as plasma levels. (Source: T. Naeser, MPQ)

The interaction between extremely powerful laser pulses and matter has opened up entirely new approaches to the gene­ration of ultra­short light flashes lasting for only a few hundred atto­seconds. These extra­ordinarily brief pulses can in turn be used to probe the dynamics of ultrafast physical phenomena at sub-atomic scales. The standard method used to create atto­second pulses is based on the inter­action of near-infrared laser light with the electrons in atoms of noble gases such as neon or argon. Now researchers at the Labora­tory for Atto­second Physics at the Max Planck Institute of Quantum Optics in Garching and Munich’s Ludwig Maxi­milians Uni­versity (LMU), in colla­boration with colleagues at Umeå Univer­sity, have success­fully imple­mented a new strategy for the gene­ration of isolated atto­second light pulses.

In the first step, extremely powerful femto­second laser pulses are allowed to interact with glass. The laser light vaporizes the glass surface, ionizing its consti­tuent atoms and accelerating the liberated electrons to velo­cities equi­valent to an appre­ciable fraction of the speed of light. The resulting high-density plasma made up of rapidly moving electrons, which propagates in the same direction as the pulsed laser light, acts like a mirror. Once the electrons have attained velo­cities that approach the speed of light they become rela­tivistic, and begin to oscillate in response to the laser field. The ensuing periodic defor­mation of the plasma mirror interacts with the reflected light wave to give rise to isolated atto­second pulses. These pulses have an estimated duration of approxi­mately 200 as and wave­lengths in the extreme ultra­violet region of the spectrum (20-30 nanometers, 40-60 eV).

In contrast to atto­second pulses generated with longer laser pulses, those produced by the plasma-mirror effect and laser pulses that have a duration of few optical cycles can be precisely controlled with the waveform. This also allowed the researchers to observe the time course of the generation process, i.e. the oscil­lation of the plasma mirror. Impor­tantly, these pulses are much more intense, i.e. contain far more photons, than those ob­tainable with the standard procedure.

The increased inten­sity makes it possible to carry out still more precise measure­ments of the behaviour of subatomic particles in real time. Atto­second light pulses are primarily used to map electron motions, and thus provide insights into the dynamics of funda­mental processes within atoms. The higher the inten­sity of the attosecond light flashes, the more infor­mation can be gleaned about the motions of particles within matter. With the prac­tical demon­stration of the plasma-mirror effect to generate bright atto­second light pulses, the authors of the new study have deve­loped a tech­nology, which will enable physicists to probe even deeper into the mysteries of the quantum world. (Source: MPQ)

Reference: D. Kormin et al.: Spectral interferometry with waveform-dependent relativistic high-order harmonics from plasma surfaces, Nat. Commun. 9, 4992 (2018); DOI: 10.1038/s41467-018-07421-5

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

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