Atmospheric optics is one of the earliest fields among all the natural sciences. As a branch of knowledge dealing with an observation and explanation of the colors of the sky, rainbows, halos, and mirages, over hundreds of years it was exclusively concerned with the visible light and the visible spectral range, where atmospheric air is highly transparent. Systematic studies of radiation propagation in Earth’s atmosphere, motivated by the needs of observational astrophysics and performed with an ever-increasing experimental accuracy over many centuries, helped achieve a detailed quantitative understanding of atmospheric transmission in the visible region. In the era of ultrafast laser technologies, enabling the generation of high-power ultrashort field waveforms within an ultrabroad spectral range from the visible to the mid-infrared, a deeper understanding of the group-velocity dispersion (GVD) of atmospheric air is needed. This call includes a quest for anomalous-GVD ranges as a top priority for long-distance signal transmission and remote sensing of the atmosphere
Our experimental studies performed near the edge of the mid-infrared absorption band of atmospheric carbon dioxide reveal a remarkably broad and continuous dispersion anomaly stretching from approximately 3.5 to 4.2 µm, where atmospheric air is still highly transparent and where high-peak-power sources of ultrashort mid-infrared pulses are available. Within this range, anomalous dispersion acting jointly with optical nonlinearity of atmospheric air is shown to give rise to a unique three-dimensional field dynamics, enabling a highly efficient whole-beam self-compression of high-peak-power mid-infrared pulses to few-cycle pulse widths. Ultrashort high-peak-power 3.9-μm laser pulses are shown to exhibit such self-compression dynamics when exposed to the mid-infrared dispersion anomaly of air induced by the asymmetric-stretch rovibrational band of carbon dioxide. Even though the group-velocity dispersion cannot be even defined as a single constant for the entire bandwidth of mid-infrared laser pulses used in experiments, with all soliton transients shattered by high-order dispersion, 100–200-GW, 100-fs, 3.9-μm laser pulses can be compressed in this regime, as our experiments and simulations show, to sub-40-fs subterawatt field waveforms without any detectable energy loss.