Intense femtosecond light pulses in mid-infrared for spectroscopic and technical applications

First experiments in vibrational spectroscopy on water demonstrate the high potential of the system for applications.

Ultrashort light pulses represent an important tool in basic research and have also found their way into numerous optical technologies. The infrared spectral range with wavelengths longer than 1 µm plays a key role in optical communication, while pulses with wavelengths of up to 300 µm are required in optical measurement and analysis technology and in imaging techniques.

Extremely short pulses with only a few oscillation cycles of the light wave ("few cycle" pulse) are a particular technical challenge. Their generation requires precise control of the optical phase and their propagation conditions. Few-cycle pulses at wavelengths longer than 10 µm are important for fundamental studies of the non-equilibrium properties of condensed matter, i.e., solids and liquids, and exhibit a high application potential, for example in optical materials processing. As a result, the generation of such pulses is a cutting-edge research topic.

In the journal Optica, researchers from the Max Born Institute in Berlin report on a new light source that delivers ultrashort infrared pulses beyond 10 µm wavelength with record parameters. The extremely compact system is based on the concept of optical parametric chirped pulse amplification (OPCPA), in which a weak ultrashort infrared pulse is amplified by interaction with an intense pump pulse of shorter wavelength in a nonlinear crystal. In the novel light source, pump pulses of about 3 ps duration at a wavelength of 2 µm drive a three-stage parametric amplifier with a pump energy of 6 mJ. The amplified pulses at a wavelength around 12 µm have an energy of 65 µJ and a duration of 185 fs, corresponding to a peak power around 0.4 gigawatts (1 GW = 10⁹ W) within about 5 optical cycles of the light wave (Fig. 1). In the 1 kHz train the pulses are highly stable and of excellent optical beam quality (Fig. 1). Output power and repetition rate of the system are scalable.

The potential of this unique source was demonstrated in experiments on liquid water. For the first time, hindered rotations, so-called librations, of water molecules were excited to such an extent that their optical absorption decreased significantly (Fig. 2). From the analysis of this absorption saturation, a lifetime of the librational excitation of 20 to 30 fs is estimated.