NONLINEAR OPTICAL PROPERTIES OF K2 RE (NO3)5 . 2 H2O (RE = La, Ce, Pr, Nd)
H. Hellwig, S. Rühle, L. Bohaty
Institute of Crystallography, University of Cologne,
Zülpicher Str. 49 b, D-50674 Köln, Germany,
e-mail: hellwig@kri.uni-koeln.de
Keywords: K2 RE (NO3)5 . 2 H2O, nonlinear optics, maker fringes
In 1993, Ebbers et al. [1] reported outstanding nonlinear optical properties of the orthorhombic K2 La (NO3)5 . 2 H2O and K2 Ce (NO3)5 . 2 H2O. Crystal chemical investigations led to new compounds K2 Nd (NO3)5 . 2 H2O and K2 Pr (NO3)5 . 2 H2O [3]. In our laboratory systematic investigations of crystal physical properties are in progress (e.g. pyroelectric, piezoelectric, thermomechanical and mechanical properties). This contribution presents the results of the nonlinear optical properties.
K2 RE (NO3)5 . 2 H2O (with RE = La, Ce, Pr and Nd) crystallizes in the non-centrosym\-me\-tric point group mm2 (space group: Fdd2) and can be grown in large crystals from aqueous solutions using the standard evaporation technique at 38o C. We determined the refractive indices of all compounds, including mixed crystals of K2 (La1-x, Cex) . (NO3) . 2 H2O with x ~ 0.5. From these data we deduced the conditions for phase matching. Although phase matching is very sensitive to small variations of the refractive indices, all compounds show quite similar variations of the phase matching conditions with wavelength. Of special interest is the existence of non-critical phase matching in the wavelength ranges 1060 - 1150 nm and 600 - 700 nm. While the first range is important for doubling Nd-laser radiation, the second range could be useful in generating blue or ultraviolet radiation, if absorption for the second harmonic is not too strong. Experimental investigations with a fundamental of 1080 nm (Nd-YAP) are in good agreement with the calculated values. Furthermore, we measured the complete tensor describing the second harmonic generation dSHGijk for RE=La, Ce using the Maker fringe technique [4]. While data for RE=La and Ce had been published by Ebbers et al [2], the Pr- and the Nd-compounds had not been measured yet. Because of the difficulties caused by absorption, we were looking for a transparency range for frequency doubling in the near infrared region.
Together with the results of the dSHGijk coefficients, we present precise methods for evaluating Maker fringe data. Because this is a relative method, well-known materials are to be measured as well. We chose quartz as a reference and obtained mean deviation errors of about 2% for 20 measurements. This high reproducibility is achieved employing special methods for refining the refractive indices and a new technique in extracting coefficients from the measured data [5].