Lattice Site of Rare-Earth Ions in Stoichiometric Lithium Niobate Probed by OH− Vibrational Spectroscopy

László Kovács,Laura Kocsor,Krisztián Lengyel,Gabriella Dravecz,Ivett Hajdara,Gábor Corradi,Zsuzsanna Szaller

doi:10.3390/cryst7080230

Abstract

Rare-earth (RE = Er3+, Nd3+, or Yb3+) ion-doped stoichiometric LiNbO3 crystals were grown by the Czochralski and the high-temperature top-seeded solution growth methods. For the 0.22–0.87 mol% concentration range of the RE oxides in the melt/solution, in addition to the well-known hydroxyl (OH−) vibrational band in undoped stoichiometric LiNbO3, a new infrared absorption band was observed at about 3500 cm−1, similar to the case of the trivalent optical damage resistant (ODR) dopants In3+ and Sc3+. By comparing the frequencies and polarization dependences of the bands to those detected for ODR ion containing crystals, they are attributed to the stretching vibration of OH− ions in RE3+Nb-OH− complexes. Consequently, above a given concentration threshold, some of the rare-earth ions are assumed to occupy niobium sites in the LiNbO3 lattice. The same model is also suggested for RE-doped congruent LiNbO3 crystals containing over-threshold (>5 mol %) amounts of the Mg-co-dopant.

Highlights

Point defects influencing the physical properties of lithium niobate crystals have been the subject of a vast number of papers presented since their first growth in 1960’s [1]
At low concentrations (0.24 atomic% Er3+ ) the spectrum resembles that of the undoped crystal, with the main absorption band peaking at about 3466 cm−1 with two satellites at ≈3480 and
The sum of the new and the main band corresponds to the full number of OH− molecules in the crystal, which may vary from sample to sample

Summary

Introduction

Point defects influencing the physical properties of lithium niobate crystals have been the subject of a vast number of papers presented since their first growth in 1960’s [1]. Rare-earth ions (RE) are introduced to LiNbO3 to serve as laser activators [8] or, more recently, as active scattering centers in coherent quantum optical experiments [9] Their spectroscopic properties in sLN significantly differ from those observed in cLN; in particular, the linewidths of the optical absorption and luminescence signals are much narrower, due to the more regular crystal structure of sLN [10]. ODR dopants above the photorefractive threshold concentration new OH− absorption bands appear at vibrational frequencies between 3470 and 3540 cm−1 , depending on the valence state of the dopants [4] These bands have been attributed to the vibration of hydroxyl ions in defect complexes containing the ODR dopants at Nb sites (Mn+ Nb -OH− , where Mn+ = Mg2+ , Zn2+ , In3+ , Sc3+ , Hf4+ , Zr4+ or Sn4+ ). The obtained results are discussed based on the comparison with previous experimental findings for both RE and ODR ion doped LiNbO3

Experimental

Results and Discussion

OH spectra ofof crystals containing various amounts of Er

Conclusions