Nonlinear optics with high power femtosecond mid-infrared laser pulses

In this contribution we present the review of recent experimental and numerical investigations in a very new research field of nonlinear optics - interaction of high energy femtosecond mid-infrared laser pulses with matter. Following current developments in the field of optical parametric chirped pulse amplification (OPCPA) we have designed a sub-terawatt OPCPA laser system delivering 10 mJ, 80 fs pulses at the central wavelength of 3.9 μm [1]. Using this unique laser system, a large number of experiments on the interaction of high intensity mid-IR laser pulses with gases and solids were carried out. We show that even in the case of collimated beam propagation of high power mid-IR radiation in air is accompanied by highly nonlinear interaction with the gas which includes not only the lowest for centrosymmetric media third order nonlinearity but also higher fifth and seventh order contributions. Soft focusing of the mid-IR beam in gases leads to the generation of multiple optical harmonics (Fig. 1a) which opens new perspectives for nonlinear molecular spectroscopy. By increasing the gas pressure, filamentation of the mid-IR femtosecond laser pulses was demonstrated and investigated for the first time. In atomic gases like argon, an impressive super-continuum, spanning over three octaves, is generated (Fig. 1b). We show that this continuum builds-up via the efficient generation of low-order harmonics and cascaded four-wave mixing processes [2]. In molecular gases, efficient continuum generation further in the mid-IR spectral range is demonstrated (Fig. 1b), revealing important role of stimulated Raman scattering in filament dynamics.