Single lithium niobate crystal provided fourth-harmonic conversion in an experiment by Brett Hooper

Single lithium niobate crystal provided fourth-harmonic conversion in an experiment by Brett Hooper and others at Duke University (Durham, NC). Fourth- and second-harmonic light were generated in a 1-cm-long lithium niobate (LiNbO3) crystal from a free-electron laser emitting at wavelengths from 2 to 2.5 µm. Fourth-harmonic generation typically requires at least two nonlinear crystals. The researchers optimized the experiment for second-harmonic generation, but a small amount of second-harmonic light then produced the fourth harmonic inside the crystal. The conversion efficiency for the fourth harmonic is about 10-6, and the maximum energy output at 500-625 nm was 3.3 nJ. (Appl. Opt. 33(30), 20 Oct 1994, p. 6980)