X-shaped third harmonic generated by ultrashort infrared pulse filamentation in air
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The authors report the measurement of the angularly resolved spectrum of the third harmonic generated in a femtosecond filament in air and its evolution with increasing pump power. Pumped by a focused infrared ultrashort pulse with a carrier wavelength of 1270 nm, a pulse duration of ~ 20 fs, and pulse energy up to 487 劬 the generated third harmonic is composed of an on-axis emission and a conical ring emission. When the pump power is sufficiently high, angularly resolved spectra with significant X-like feature could be observed, indicating the formation of nonlinear X wave at third harmonic.
Applied Physics Letters
Nonlinear Optics and Spectroscopy