Extreme Nonlinear Optics (high harmonic generation and attosecond generation)

This method relies on the possibility of FourierSynthesis of discrete lines over a broad spectrum (similar to mode-locking). A Raman medium is characterized by its Raman frequency ω0, which can be the difference between two rotational or vibrational lines. When we apply a laser excitation ωL − ωs close to ω0, we can obtain gain at the stokes (ωL−nω0) and anti-stokes (ωL+nω0) frequencies, the Raman effect gets resonance-enhanced. The distance between these lines determines the repetition frequency τrep = 2π ω0 (as in ML) and therefore we wish to have smaller distances that help us isolate single pulses. For a 100as pulse, we would need a locked bandwidth of about 5 × 10THz. The generated equidistant sidebands (fig. 1) are already phase-locked. In a medium like Hydrogen, about 40 rotational Ramanlines have been obtained, covering all the spectrum from 200nm upto 800nm (fig. 2) with ν0 = 507cm−1, equivalent to 17.6THz. The repetition rate of τrep,vib = 57fs is much more suitable for single pulses than the repetition rate for rotational lines τrep,rot = 8fs. In this example the medium was pumped with approx. 1ps pulses. For the two different wavelengths Nd:YAG (SHG) and Ti:Sapphire (805nm) lasers were used. As the molecular oscillation modulates the index of refraction (optical Kerr effect), the bandwith gets bigger and a white light continuum is generated. That decreases the Raman gain but increases the spectrum of possible lines. Therefore this parameter has to be optimized. The technique is much more efficient than High Harmonic Generation, because a significant part of the energy is pumped in the Raman lines, whereas HHG only has an efficiency of 10−5. Pulse width measurement with autocorrelation methods is possible with these field strenghts.