Hidden Reservoir of Photoactive Electrons in ${\mathrm{LiNbO}}_3$ Crystals

We show that a continuous-wave (cw) pump beam at a wavelength of 532 nm produces substantial light-induced (LI) absorption in the visible range in initially transparent undoped ${\mathrm{LiNbO}}_3$ crystals. The LI absorption coefficient stays linear in the pump intensity $I_{\mathrm{p}}$ up to $I_{\mathrm{p}}^{\max }=48\mathrm{kW}/{\mathrm{cm}}^2$. Together with other features including long-term stretched-exponential relaxation of the LI absorption, it indicates that the present concept of LI electron processes in this important optical material must be revised: the amount of photoactive electrons increases already within the cw intensity range. A quantitative model is proposed that explains the experimental data and employs two-step excitations from filled localized states near the valence band via intermediate deep centers into the conduction band. The introduced localized states serve as a hidden reservoir of electrons.

Figure 1

Temporal evolution of the probe-beam transmittance at ${\lambda }^{*}=785\mathrm{nm}$ (open circles) upon switching the pump beam on and off (solid line).

Figure 2

The steady-state dependence ${\alpha }_{\mathrm{li}}(I_{\mathrm{p}})$ at ${\lambda }^{*}=785$ (a) and 488 nm (b). Four and two measurements at different sample positions are made in (a) and (b), respectively.

Figure 3

The relaxation dependence ${\alpha }_{\mathrm{li}}(t)$ (open circles) at ${\lambda }^{*}=785$ (a) and 488 nm (b) for different pump intensities. The solid lines are stretched-exponential fits (see the text). Note a difference in the horizontal scales in (a) and (b).

Figure 4

Energy diagram for undoped crystals. The arrows indicate the excitation and recombination processes; the subscripts $1c$ and $2c$ indicate transitions $1\to \mathrm{CB}$ and $2\to \mathrm{CB}$. The thicker the line, the larger is the corresponding quantity.

Figure 5

Curves 1 to 4: steady-state dependence $n_2(I_{\mathrm{p}})$ calculated from Eqs. (2) for different pairs ($a$, $b$) and $\overline{n}=N_0$. The straight dotted line is $0.9I_{\mathrm{p}}/I_{\mathrm{p}}^{\mathrm{sat}}$.