Measured Lattice Damage and Optical Index Change in KNbo 3
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MEASURED LATTICE CHANGE IN KNbO 3
DAMAGE
AND
OPTICAL
INDEX
R. IRMSCHER, D. FLUCK', CH. BUCHAL, B. STRITZKER AND P. GUNTER" ISI-Forschungszentrum (KFA) JUlich, D-5170 Julich. Germany "Inst. Quantum Electronics, Swiss Fed. Inst. Technology, CH- 8093 ZUrich, Switzerland
ABSTRACT We have used 2-MeV and 200-KeV He implantation into KNbO 3 and LiNbO 3 single crystals to generate a damaged layer of reduced optical index. MeV implantation enables the formation of optical waveguides of several pum thickness, which have been analyzed by dark and bright line spectroscopy in order to determine the refractive index profiles. In addition, 200-keV implants have been used to model the endof-range-damage, which was measured by RBS/Channeling. KNbO 3 and LiNbO 3 show a distinctly different sensitivity of the radiation damage.
INTRODUCTION Oxide single crystals are important for optical applications. KNbO 3 is particularly attractive because it has strong nonlinear properties, which make it useful for frequency doubling. In KNbO 3 unlike LiNbO 3 optical waveguide formation by indiffusion or implantation of Ti is not possible because low temperature phase transitions prevent the application of diffusion or annealing procedures. However, planar waveguides have been successfully produced by MeV-He-ion implantation, which creates a buried layer of lowered refractive index at a depth of several gm 11,21. This optical barrier is formed by end-of-range (EOR) damage of the incident He. It is generally too deep for MeV-He Rutherford-Backscattering (RBS)/ Channeling experiments. Because for He the EOR damage mechanism is virtually the same for 200keV ions as for 2-MeV ions, the damage produced by 200-keV implantation into KNbO 3 can be analyzed by RBS/Channeling and extrapolated to that of the 2-MeV implant. In this paper such a procedure is used to model the damage at 200-keV He implantation into KNbO 3 and to extrapolate it to 2 MeV, in order to correlate the lattice damage with the change of the index of refraction.
Mat. Res. Soc. Symp. Proc. Vol. 201. 1 1991 Materials Research Society
400
EXPERIMENTAL PROCEDURE with He ions at 200 keV and 2 MeV Single crystal KNbO 33 samples 13 were implanted 1 with doses ranging from 5x10 to 1x10 16 /cm 2 at 300 K. The samples implanted at 2 MeV have been analyzed by dark and bright line spectroscopy at a wavelength of 633 nm. We have used the dark mode reflectivity calculation by Chandler to determine the refractive index profiles [3]. The samples implanted at 200 keV were analyzed by RBS/Channeling to quantify the EOR damage as a function of dose. For the RBS/Channeling measurements, 2.8 MeV He ions incident parallel to the axis (the crystallographic direction refers to the cubic phase) were analyzed at a backscattering angle of 1700. The relative lattice damage in percent was defined as the ratio of aligned peak yield at its maximum to the random yield. In order to equate the measured lattice damage at 200 keV with the observed index change at 2 MeV, Monte-Carlo-TRIM calculations were perfor
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