Arrest of self-focusing collapse in femtosecond air filaments: higher order Kerr or plasma defocusing?

Abstract

Experimentally measured conical emission rings on the blue side of the filament supercontinuum of a 800 nm 50 fs pulse in air are reproduced in simulations with plasma and the third-order Kerr as the nonlinear terms. This agreement indicates plasma as the dominant mechanism arresting the self-focusing collapse. The higher order Kerr terms with the recently measured coefficients stop the collapse at a lower intensity than the plasma does and lead to the spherical angle-wavelength spectrum without blueshifted rings.

Optical Nonlinearity in Ar andN2near the Ionization Threshold

“…Subsequent experimental studies of light filaments [12][13][14][15] have not supported the higher-order Kerr model, with one exception [16]. One measurement [13] found that the electron density was two orders of magnitude higher than predicted by a calculation including higher-order nonlinearities, but agreed with a simulation based on plasma defocusing alone [5].…”

“…It would overturn the picture most have of the mechanism behind long-range filamentary propagation of intense ultrashort pulses -as arising from an interplay between self-focusing due to the positive optical nonlinearity from bound electrons and defocusing due to the plasma generated by ionization. The existence of a higher-order Kerr effect would also have implications for the general nonlinear susceptibility in transparent media [7,8], including harmonic generation [9][10][11].Subsequent experimental studies of light filaments [12][13][14][15] have not supported the higher-order Kerr model, with one exception [16]. One measurement [13] found that the electron density was two orders of magnitude higher than predicted by a calculation including higher-order nonlinearities, but agreed with a simulation based on plasma defocusing alone [5].…”

Transition from Plasma-Driven to Kerr-Driven Laser Filamentation

“…As a consequence, the HOKE can ensure self-defocusing in filaments and balance Kerr self-focusing [10], in place of the plasma, especially for short pulses [9]. This result raised an active controversy [11][12][13][14][15][16] in the lack of direct experimental confirmation.…”

On the relative roles of higher-order nonlinearity and ionization in ultrafast light-matter interactions

“…Observations of the HOKE spurred a revision of the accepted paradigm for femtosecond filamentation [3], suggesting that the role of plasma in filaments is not as important as previously thought. A number of papers followed, both supporting (e.g., [4][5][6]) and rejecting (e.g., [7][8][9][10]) this view.…”