Wideband modulation instability at combination frequencies in wave beams |
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Authors: | S N Vlasov E V Koposova |
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Institution: | (1) Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia |
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Abstract: | We study theoretically wideband modulation instability at combination frequencies in media having cubic nonlinearity of self-focusing
type along with the higher-order defocusing nonlinearity. It is assumed that in a medium with a purely cubic nonlinearity,
the medium dispersion does not permit modulation instability. In this case, a collapse of the wave field exists if the beam
power is higher than the critical power of self-focusing. The higher-order nonlinearity limits the field at the nonlinear
focus, and the instability at combination frequencies becomes possible. It turns out that the field at the nonlinear focus
increases with increasing excess of the beam power over the critical power of self-focusing. The obtained values of the nonlinear
dielectric permittivity are used for determination of the growth rates of instability at combination frequencies. These growth
rates ensure an increase in the combination fields from noise levels up to values comparable with the field of the high-power
beam. Such an increase takes place if the beam power is severalfold higher than the critical one. The developed theory can
be used for explanation of spectrum superbroadening during self-focusing of sufficiently short laser pulses and high-harmonic
generation.
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Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 6, pp. 522–532, June 2007. |
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