Optical frequency tripling using cascading quasi-phasematched nonlinearities in periodically poled lithium niobate

The recent achievement of reliable quasi-phasematching (QPM) in periodically poled crystals, and in particular periodically poled lithium niobate (PPLN), has empowered even more the use of nonlinear optical mixings in frequency connections. As is well-known, QPM gives access to the largest nonlinear coefficients of the material (which are almost never usable for birefringent phasematching - BPM), suppresses walk-off, and gives great flexibility in the choice of the wavelengths involved in the nonlinear mixing. Simultaneous QPM of different interactions in the same crystal is now easy to obtain by poling different regions of the crystal with the different corresponding periods. But even a single-period (50% duty cycle) crystal offers many QPM cascading possibilities, which are impossible most of the time with BPM because of constraints on the optical polarizations. We experimentally realized third-harmonic generation (THG) by cascading second-harmonic generation (SHG: ω→2ω) and sum frequency generation (SFG: ω+2ω→3ω), which of course realizes a 3:1 frequency connection