当光功率超过自聚焦临界阈值，会伴随自聚焦产生一条等离子体通道，光脉冲在其中能保持几乎不变的直径传输很长一段距离，而不受瑞利长度限制。
光丝，guāng sī
introduction
ultrashort pulses of several millijoule energy can reach GigaWatt powers that exceed the critical power for self-focusing in air. the critical power is defined as the power at which self-focusing via the optical Kerr effect balances beam diffraction. Above the critical power, and according to the paraxial wave theory, self-focusing should lead to a collapse singularity of infinite energy density in a finite distance, sinice both diffraction and self-focusing increase at the same rate with decreasing beam diameter.
light filamentation in air
a general explanation for the formation of light filaments for input peak powers several times the critical power is that, during the initial stages of the propagation, the field breaks up transversely into hot spots of high intensity under the combined actions of the nonlinear Kerr effect and diffraction, namely, modulational instability. The high intensities produced in the hot spots by self-focusing will produce nonlinear absorption via multi-photon ionization and electron plasma generation, along with
plasma defocusing, which in turn both limits the growth of the hot spots and eventually causes them
to terminate. This is the basic mechanism of filament formation, and in our case the typical propagation
length of filaments is of the order of a meter. In addition, due to spatial inhomogeneities in the input
field, the modulational instability initiates hot spots at a range of propagation distances, and filaments are created and disappear along the full propagation distance. It is assumed that static filaments were formed when the self-focusing due to the Kerr effect was balanced by plasma-induced defocusing.
characterization of filaments
There has been a controversy as to the filament size, their conductivityw4,5xand even their existence . The main experimental difficulty in the investigation of filaments is that the pulse intensity
within the filament far exceeds the damage threshold of any reflectingrtransmissive material. The theoretical studies are equally challenging because of the large range of dimensions involved transverse dimensions over a few microns to several centimeters, longitudinal dimensions over the pulse length, tens of microns, to the total propagation length of tens of meters.. Another difficulty resides in the plethora of nonlinear effects involved at these high intensities, including beam diffraction, group-velocity dispersion, self-phase modulation, multi-photon ionization, and plasma absorption and defocusing, along with the scarcity of known material parameters.
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