Quasi-phase-matched backward second-harmonic and sum-frequency generation in periodically-poled lithium niobate

Since quasi-phase-matching (QPM) was introduced in 1962, it has become an important scheme for achieving efficient second-harmonic generation (SHG) in various nonlinear optical materials. Most of the results obtained so far are based on the forward configuration. Recently, it was shown that backward SHG is also possible, i.e. SH beam can propagate in a direction opposite to the fundamental beam. Most recently, we observed backward SHG in periodically-poled LiNbO₃ (PPLN) using subpicosecond laser pulses. However, due to the severe temporal walk-off between the fundamental and SH pulses, the maximum conversion efficiency was only about 0.02%. We present our new results on the backward SHG and sum-frequency generation (SFG) in PPLN using nanosecond laser pulses. Three PPLN samples with different lengths (L=3 mm, 4 mm, and 5.5 mm) were used in the study of backward SHG and another PPLN sample with L=4.2 mm was used to generate backward sum-frequency (SF)