Photorefraction and photochromism in bismuth–magnesium-codoped stoichiometric lithium niobate

Introduction

Lithium niobate (LN) is a work-horse of photorefractive holography. Recently outstanding results on (Bi,Mg)-codoped congruent LN have been reported. As the photochromic effect often degrades the quality of photorefractive devices, our aim is to investigate the dynamics and mechanism of photochromism and photorefraction in (Bi,Mg)-codoped stoichiometric LN, separating their effects on the diffraction efficiency in two-wave mixing experiments.

Methods

Czochralski-grown samples with a Li/Nb ratio of 1.38:1, Bi concentrations of 2 or 4 mol%, and Mg concentrations of 1 or 2 mol% (all data in the melt) were used to investigate both photochromic and photorefractive effects at 405 and 443nm at temperatures between 10 and 45°C. A two-wave mixing simulator program was developed based on a one-center model involving photochromic and various photorefracive mechanisms.

Results

Apart from persistent light-induced absorption reported earlier [1] and small transients on the ms timescale, an important photochromic component was found to be saturating on a subsecond and decaying on a subminute timescale, the recovery governed by an activation energy of ~0.6eV. Photorefractive response was faster with τ≈30ms. The ratio of the photorefractive and photochromic diffraction efficiencies was determined as a function of the input intensity, showing the domination of the photorefractive one for total input intensities above some tens of mW/cm². Estimates of the recombination coefficient and the effective cross-section for electron excitation by photon absorption could be derived.

Conclusions

Despite its pronounced photochromism, LN:(Bi,Mg) is an efficient and fast photorefractive material, even at low intensities. This is due to various Bi-defect complexes promoted by Mg-codoping, introducing localized ground and higher pinned-exciton (exciton-polaronic) levels in the gap, both serving as potential donors [1]. The results are helpful for optimizing holographic systems and obtaining a better understanding of the microscopic behavior of LN:(Bi,Mg).

[1] L. Kovács et al., Phys. Rev. B 109, 214105 (2024)