A new light source generates ultra-short infrared pulses around 12 µm wavelength with unprecedented peak intensity and stability.
First applications in vibrational spectroscopy on water demonstrate the high application potential of the system.
Ultrashort light pulses are an important tool in basic research and have also found their way into numerous optical technologies. The infrared spectral range with wavelengths greater than 1 µm plays a key role in optical communication, but light with wavelengths of up to 300 µm is also used in optical measurement and analysis technology and in imaging processes.
A particular technical challenge are extremely short pulses, in which the light waves only have a few oscillation cycles ("few cycle" pulses). Their generation requires precise control of the phase of the light waves and their propagation conditions. Few-cycle pulses at wavelengths longer than 10 µm are essential for fundamental studies of the non-equilibrium properties of condensed matter, i. H. of solids and liquids and have a high application potential, for example in optical material processing. For these reasons, the generation of such impulses is a highly topical research topic.
Researchers at the Max Born Institute in Berlin report in the Optica journal on a new light source that delivers ultra-short infrared pulses beyond the wavelength of 10 µm with record parameters. The extremely compact system is based on the concept of optical parametric amplification. 'Optical Parametric Chirped Pulse Amplification' (OPCPA) in which a weak ultrashort infrared pulse is amplified by interaction with an intense shorter wavelength pump pulse in a nonlinear crystal. In the novel light source, pump pulses with a duration of approx. 3 ps and a wavelength of 2 µm drive a three-stage parametric amplifier with a pump energy of 6 mJ. The amplified pulses at a wavelength of around 12 µm have an energy of 65 µJ and a duration of 185 fs, which corresponds to a peak power of around 0.9 W) within approx. 5 optical cycles of the light wave (Fig. 1). The highly stable infrared pulses have a repetition rate of 1 kHz and excellent optical beam parameters (Fig. 1). The output power and repetition rate of the system are scalable.
The potential of this unique source was demonstrated in first infrared 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 can be estimated.