Femtosecond pulses with an 8.5 μm wavelength achieved by difference-frequency generation between frequency components of a pulse at 2.1 μm central wavelength


The scope of laser pulse applications in the mid- and long-wavelength IR range has increased significantly in the past few years. Femtosecond laser pulses with a few electromagnetic field cycles are important for conducting ultrafast spectroscopy and experiments investigating the behaviour of nanostructures and solids in the presence of very intense radiation. A group consisting of scientists from the HiLASE Centre and the Massachusetts Institute of Technology (USA) has successfully demonstrated pulse generation centered at wavelength of 8.5 μm having an extensive 6.2 μm bandwidth (at the level of an 1 % maximum). Pulses generated using a simple experimental setup had a few electromagnetic field cycles and a passively stabilized carrier-envelope phase. The last-mentioned property is very important as the carrier-envelope phase of very short pulses significantly influences the interaction between the radiation and the matter. The outcoming pulses with an energy of 2 μJ were achieved as a difference frequency generation between individual frequency components of the input pulse with a central wavelength of 2.1 μm and spectrum width of 0.45 μm (at the level of a 10 % maximum). The difference-frequency generation was performed in an AgGaSe2 crystal suitable for this spectral range. An input pulse of wavelength around 2 μm is much more suitable then the traditionally used pulses with shorter wavelengths (0.8 μm or 1 μm) due to its higher efficiency of conversion into long-wavelength IR range and a smaller absorption of the input radiation in this type of crystals. It has been the first time that the input pulses for such an efficient difference-frequency generation had a wavelength of around 2 μm.

Input and output pulse spectrum (a) Optical spectrum of the broadband driving pulse of 2.1 µm central wavelength, (b) Spectrum of the output pulse obtained by intra-pulse difference frequency generation. Inset shows the AgGaSe2 crystal used for the experiment.