Fourth-harmonic generation in a single lithium niobate-crystal with cascaded second-harmonic generation

Tunable second- and fourth-harmonic radiation was generated in a single 1-cm-long lithium niobate (LiNbO(3)) crystal with the Mark III infrared free electron laser at Duke University. The fundamental laser radiation was tuned from 2 to 2.5 µm, yielding 1-1.25-µm radiation (second harmonic) and 0.5-0.625-µm radiation (fourth harmonic). A fundamental-second-harmonic energy conversion efficiency of 66% and a fundamental-fourth-harmonic energy conversion efficiency of 3.3 × 10(-6) were measured. The maximum energy in the fourth harmonic was 3.3 nJ.