Photorefractive and photochromic properties of ruthenium-doped Bi12SiO20

Ruthenium can easily be incorporated into Bi12SiO20 (BSO) and an unusually high (1.3 cm−1) photorefractive gain was measured in the diffusion regime with a krypton laser at 647 nm. One particular experiment demonstrated that electron and hole gratings could eventually be formed with different time constants, thus leading to a reduction of the gain. Several complementary spectroscopic techniques were used to characterise our crystals. Magnetic circular dichroism (MCD) demonstrated that Ru substitutes under several valences for Bi in the pseudo-octahedron formed by bismuth and oxygen atoms. This was confirmed by electron paramagnetic resonance detected either classically (EPR at 9 GHz) or optically (ODMR, 35 GHz). Photochromism was investigated via a series of absorption and MCD experiments on oxidized and reduced samples. Our results in the visible spectral range could be understood via a correlation with the behaviour of the Ru5+ and Ru3+ MCD features in the near-IR, under similar illuminations. The primary process with red light is the ionisation of electrons from the VB to the Ru4+/3+ acceptor level and the subsequent capture of the left holes at the Ru5+/4+ level. Under blue light, paramagnetic BiSi4+ is formed via the ionisation of BiSi3+. The charge transfer transitions of Ru5+ were assigned via additional experiments on Ru-doped lithium niobate and garnets. The build-up and decay of photochromism were investigated, three different behaviours being observed, depending upon the initial conditions. The three-valence-two-level model is not adequate to explain the bi-exponential temporal behaviour of photochromism. It is suggested that a third relatively shallow level, possibly associated to iron, plays an important role.