Fig. 4
From: Highly efficient broadband terahertz generation from ultrashort laser filamentation in liquids
Simulation of the broadband terahertz generation mechanism and optical spectrum measurements. a Simulated laser-induced supercontinuum spectra obtained from the simultaneous solution of the UPPE with the kinetic equation for the concentration of free electrons in ethanol at laser input energies of 0.0674 µJ (orange line), 0.674 µJ (purple line), and 6.74 µJ (green line). Terahertz radiation is shown to be generated at higher energies (0.674, 6.74 µJ) by the two-color mechanism, where the second harmonic is a part of the supercontinuum. The vertical dashed lines shows the fundamental (800 nm, red), second harmonic (400 nm, blue), and the third harmonic (266 nm, violet) on the frequency scale. b Measurement of UV to infrared spectrum emanated during single-color filamentation of ethanol (magenta) and acetone (blue) demonstrating different ratios of second harmonic generation with respect to the fundamental. c Simulation of the temporal profile of the laser electric field inside the liquid at z = 25 mm. The full electric field (E full) is shown by black lines, the electric fields corresponding to the fundamental (E ω ) and second harmonic (E 2ω ) frequencies are shown by red and blue lines, respectively. d The simulated laser electric fields E full (black line), E ω (red line), and E 2ω (blue line) with expanded time scale between 50–100 fs to show details of the phase variations
