A high power femtosecond laser pulse weakly focussed in a nonlinear medium, like a rare gas, undergoes self-focussing due to the spatial Kerr effect. The on-axis intensity increase is counteracted by ionisation and saturation effects, forming a self-guiding channel significantly longer than the Rayleigh range, up to several meters. Due to self-phase modulation, strong spectral broadening and white light generation is achieved. The supercontinuum can be compressed to a sub 7 fs ultrashort pulse, suitable for high-order harmonic generation.
Exploring the evolution of the filament along the propagation axis, third-order and high-order harmonic generation can be observed directly from a filament without subsequent recompression of the pulse. Regimes of discrete harmonic orders as well as continuous spectra are found, setting the headstone for attosecond pulse generation in filaments.
- Daniel S. Steingrube, Emilia Schulz, Thomas Binhammer, Tobias Vockerodt, Uwe Morgner and Milutin Kovacev; Generation of high-order harmonics with ultra-short pulses from filamentation, Opt. Exp. 17, 16177 (2009)
- Emilia Schulz, Daniel S. Steingrube, Thomas Binhammer, Mette B. Gaarde, Arnaud Couairon, Uwe Morgner and Milutin Kovacev; Tracking spectral shapes and temporal dynamics along a femtosecond filament, Opt. Exp. 19, 19495 (2011)
- Emilia Schulz, Daniel S. Steingrube, Tobias Vockerodt, Thomas Binhammer, Uwe Morgner and Milutin Kovačev; Gradient enhanced third harmonic generation in a femtosecond filament, Opt. Lett. 36, 4389 (2011)
- Tobias Vockerodt, Daniel S. Steingrube, Emilia Schulz, Martin Kretschmar, Uwe Morgner and Milutin Kovacev; Low- and high-order harmonic generation inside an air filament, Appl. Phys. B, in print (2011)
- Daniel S. Steingrube, Emilia Schulz, Thomas Binhammer, Mette B. Gaarde, Arnaud Couairon, Uwe Morgner and Milutin Kovacev; High-order harmonic generation directly from a filament, New J. of Phys. 13, 043022 (2011)
High-Order Harmonic Generation in liquid water droplets
Our group has set up an experiment, creating a determined series of micrometer-sized water-droplets under vacuum condition as target for high-order harmonic generation. This masslimited droplet resembles an excellent interaction-area between target and fundamental radiation, giving the opportunity of a debris-free conversion process combined with a potentially high amount of emitting particles.
Nanometer sized metallic nanoantennae have similar properties in the optical spectral range as classical antennae for radio frequencies. By choosing an optimised antenna geometry the electric field can be enhanced by up to three orders of magnitude. Hence in a small volume laser pulses from an oscillator can reach peak intensities which are sufficient for high harmonic generation.
New possibilities especially in the field of high precision spectroscopy in the UV frequency range are envisaged.