Researchers at the Max Born Institute for Nonlinear Optics and Short-Term Spectroscopy (MBI) have reached a new milestone in the generation of light pulses with just a few optical cycles.
They broke a ten-year-old record and generated light pulses with a pulse length of only 1.5 optical cycles and a peak output of 1.2 terawatts in a new high-energy hollow fiber compressor. The high-energy laser pulses are used to generate intense attosecond pulses in the XUV spectral range, which in turn are used in non-linear XUV spectroscopy studies.
In order to investigate complex charge transfer mechanisms in the formation of a chemical bond or in the course of biologically relevant processes, tools with an extraordinary temporal resolution in the attosecond range (10 -18 s). The isolated attosecond light pulses required for this can be generated by frequency conversion in the extreme ultraviolet (XUV) spectral range. For this purpose, ultrashort intense laser pulses, which comprise only a few vibrations of the electric field, are focused on noble gas atoms. Here, a frequency conversion process is driven, which is referred to as the generation of high harmonic radiation. However, the conversion efficiency of this process is very low and consequently provides very weak attosecond pulses, which are not sufficient for non-linear spectroscopic applications. In order to generate more intense isolated attosecond pulses, high-energy ultrashort laser pulses are therefore required, the wavelength of which is in the near infrared range.
Now researchers at the MBI have taken a big step forward in the energy scaling of the near-infrared light pulses. The group succeeded in spectrally broadening pulses from a titanium sapphire laser that emits at a wavelength of 790 nm and then to a pulse duration of 3.8 fs (1.5 optical cycles) with an energy of 6.1 mJ to compress, which is unprecedented at a kilohertz repetition rate. The peak power of the pulses thus clearly exceeds the terawatt mark (> 10 12 W). This result breaks a 10-year-old record that was achieved at the RIKEN research institute in Japan .
To achieve these results, a new 8.2 meter long compressor beamline was built around a 3.75 meter long, stretched, hollow hollow core fiber (SF-HCF): this fiber contains a spectral broadening as a result of the non-linear interaction between the intense near-infrared light pulses and the helium atoms embedded in the capillary instead. The spectrally broadened light pulses were then temporally compressed by negatively chirped mirrors and characterized by a dispersion scan measurement. The measuring device was built directly into the vacuum beam path, which is also designed for the subsequent generation of high harmonic radiation and XUV experiments. The new HCF compressor is a scaled up version of a device
This new development paves the way for nonlinear attosecond XUV spectroscopy.
 S. Bohman, A. Suda, T. Kanai, S. Yamaguchi, and K. Midorikawa, Generation of 5.0 fs, 5.0 mJ pulses at 1kHz using hollow-fiber pulse compression , Opt. Lett. 35 , 1887-1889 (2010)