In optical second-harmonic generation coherent optical radiation of frequency 2ω is generated, through the use of a medium with the appropriate nonlinear susceptibility, from incident coherent optical radiation of frequency ω. For the process to take place efficiently, the phase matching condition (colinear geometry)
k(2ω)=2k(ω)
k(2ω)=2k(ω)
must be fulfilled, where k(ω)and k(2ω)are the wave numbers, in the nonlinear medium, for the coherent radiations at frequencies ω and 2ω, respectively.
bbo crystal second harmonic generation