Harmonic Generation and Mixing of CaWO4:Nd3+and Ruby Pulsed Laser Beams in Piezoelectric Crystals

Second harmonic generation (SHG) of the CaW${\mathrm{O}}_4$: ${\mathrm{Nd}}^{3+}$ laser line (1.0582μ) has been observed in a variety of appropriately selected crystals. The pulsed beam (of the order of 30 W) is brought to focus within the crystal under investigation, and the second harmonic (5291 Å) is detected with a photomultiplier, photographic film, or occasionally with the eye. Crystals such as BaTi${\mathrm{O}}_3$ that are transparent in the visible but which could not be studied for nonlinear effects with the ruby laser due to absorption of the ruby laser second harmonic (3467 Å) can be conveniently investigated as harmonic generators using the CaW${\mathrm{O}}_4$: ${\mathrm{Nd}}^{3+}$ laser. Ruby and CaW${\mathrm{O}}_4$: ${\mathrm{Nd}}^{3+}$ laser beams have been mixed successfully in a number of crystals producing the sum frequency (4189 Å). In the geometry used, the two laser beams were made parallel to each other with a half-silvered mirror, and then brought to focus in the sample under investigation. In both the harmonic generation and the mixing experiments, potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) were the most efficient nonlinear crystals investigated. The crystallographic directions in KDP and ADP which give velocity matching of the second-order polarization wave and the corresponding light wave have been determined for SHG of the ruby and CaW${\mathrm{O}}_4$: ${\mathrm{Nd}}^{3+}$ laser lines, as well as for mixing of these two laser lines. The SHG and mixing efficiencies are considerably enhanced when velocity matching occurs.