Fig. 1: THz generation by R-FWM in diamond.

a Energy level diagram, Feynman diagram, and wave-vector diagram describing four-wave mixing with a Raman resonance. b Time variation of the 1332 cm−1 vibrational wave in diamond coherently excited by \(\vec E_1\) and \(\vec E_2\) pulses of different pulse widths. c Calculated THz output spectra at selected θ. d Calculated spatial distributions of different frequency components in the collimated THz output beam for the case of θ = 0.42° in c; each circle describes a Gaussian distribution cut off at e−2 of the maximum