Crystal Offers High Fourth-Harmonic Conversion Efficiency

Breck Hitz

Few nonlinear crystals are available whose transmission and nonlinearities are compatible with the efficient generation of the fourth harmonic of Nd:YAG at 266 nm. KBe₂BO₃F₂ (KBBF) has many desirable parameters: It has a high damage threshold and nonlinearity, it is not hygroscopic, and it is transparent in the UV down to 155 nm. But KBBF is a "layer structured" crystal, with individual layers so weakly bonded to each other that it is difficult to grow crystals of high optical quality. Moreover, the KBBF crystals that can be grown are too thin to cut at the appropriate angle for phase-matching fourth-harmonic generation at 266 nm.

Now, researchers associated with the Chinese Academy of Sciences in Beijing and with the State Key Laboratory of Crystal Materials, Shandong University in Jinan, China, have overcome these obstacles and have grown KBBF crystals capable of generating 266-nm light with an energy conversion efficiency greater than 30 percent.

Figure 1. Researchers in China coupled second-harmonic, 532-nm pulses into a KBBF crystal with a prism-coupling system. The incident angle was 11.7°, and the phase-matching angle was 36.4°.

By using a specially designed crucible and furnace, and some additives in the flux systems, the scientists grew KBBF crystals as large as 20 X 10 X 1.8 mm with high optical quality. These crystals still were too thin to cut at the phase-matching angle, so they adopted a prism-coupling system to propagate the light at the proper angle inside the crystal (Figure 1). They employed type I phase-matching, with the 532-nm, second-harmonic light polarized along the X-axis of the crystal.

Figure 2. The output energy varied with the energy and beam diameter of the input. In the graph, squares correspond to 1.5-mm-diameter beams, and circles correspond to 1-mm-diameter beams.

A Q-switched, mode-locked Nd:YAG laser generated 35-ps pulses of 1.06-µm radiation at 10 Hz, which were frequency-doubled in an 8-mm-thick beta-barium borate crystal. The resulting green pulses were focused into the KBBF, where they generated the ultraviolet radiation. The green input power and the ultraviolet output are shown for two different focusing lenses in Figure 2. For an input of 2.73 mJ, the researchers obtained 840 µJ of ultraviolet, corresponding to a conversion efficiency of 30.6 percent.