High harmonic generation

The process of high harmonic generation is an extreme nonlinear process in which ultrafast laser light is upconverted to odd multiples of the fundamental frequency. To produce this spatially and temporally coherent radiation an intense laser pulse is focussed into a gas volume. The generated harmonics extend over the extreme UV region (XUV) with energies up to the keV regime.

Spectrum of HHG radiation generated at JuSPARC_VEGA in a Ne gas target (May 2020). The Al filter absorbs photons with energies larger than 72 eV.

On the microscopic single atom level the process can be described by the so called three step model. The electron is accelerated by the electric field of the laser away from the parent atom. When the electric field reverses its sign the electron gets decelerated until it returns and combines with its parent atom by emitting high energy photons.

Proceeding to a macroscopic multi atom field of view the process of phase matching comes into play. In order to avoid destructive interference between the fundamental and the high harmonics a phase difference close to zero needs to be obtained. In this process intensity, wavelength and pressure of the gas medium play an important role.

This offers a unique opportunity to extend the temporal and spatial resolution to unprecedented fs and nm scales respectively in the same experiment. Access to HHG radiation recently allowed a number of proof-of-principle solid-state and surface physics experiments demonstrating the strength of this approach and bringing new insights to a range of cutting edge scientific questions, including:

  • identification of the laser-assisted surface photoelectric effect
  • first attosecond spectroscopy experiment in a condensed matter system
  • HHG generated XUV pulses for testing thermal transport from nanoscale hotspots
  • ultrafast molecular, plasma and materials dynamics
  • first element-specific lab-based magnetization dynamics studies with femtosecond temporal resolution