Fig. 1: The degenerate four-wave mixing geometry.
From: Highly efficient THz four-wave mixing in doped silicon
a The energy-level scheme for DFWM with two excitation photons from beam 2, a stimulated emission from beam 1 and an output photon in beam 3. The left hand process involves two virtual excited states, and the right hand permutation is the strongest near to a resonance with the ground state \(\left|g\right\rangle\) and excited state \(\left|e\right\rangle\). b A camera image of the beam at the sample position is shown superimposed on the sample. Far-field images were taken by scanning an iris after collimating, which requires careful conversion from space to angle. Each image has been normalised to the peak power density, and the scale factors for the far-field images are indicated relative to beam 1. Note that the far-field image of the beam 3 has only been scaled by a very small factor in this example (×3), i.e. the DFWM efficiency is very high. The phase matching condition is also shown
