Hi there, I'm having a little trouble understanding the "distinguishability" of frequencies in the nonlinear electric susceptibility tensor. As far as I understand, if we have a SHG process with two collinear beams of the same polarization and frequency ω, there is only one susceptibility component 2ω;ω,ω. But if these beams propagate to different directions, still with the same frequency ω, must we label the frequencies as ω1 and ω2 and end up with components 2ω;ω1,ω1 , 2ω;ω2,ω2 , 2ω;ω1,ω2 and 2ω;ω2,ω1? So can the propagation alone make the beams distinct? Thanks in advance!