Time-resolved photoluminescence spectroscopy of ${\mathrm{Nb}}_{\text{Nb}}^{4+}$ and ${\mathrm{O}}^{-}$ polarons in ${\mathrm{LiNbO}}_3$ single crystals

We probe here the optical relaxation properties of Mg-doped wide-band-gap ${\mathrm{LiNbO}}_3$ single crystals with both a high spectral and temporal resolution at cryogenic temperatures. Surprisingly, we observe the photoluminescence to decay in a two-step process: a fast relaxation and a slower one centered around an energy $E_{max}$ = 2.62 $\pm 0.05$ eV. Both decays fit well to the stretched-exponential behavior. Moreover, we are able to associate these energies to the recombination of light-induced ${\mathrm{Nb}}_{\text{Nb}}^{4+}$ and ${\mathrm{O}}^{-}$ small polarons. Also, we checked the stability of our findings by using ${\mathrm{LiNbO}}_3$ single crystals that show on-purpose modified radiative recombination processes, i.e., with a Mg doping both above and below the optical damage resistance threshold, as well as with different poling histories of inverted domains.

Figure 1

Comparison of time-resolved absorption (TRA) and PL spectroscopy (TRPLS) in ${\mathrm{LiNbO}}_3$.

Figure 2

Corrected steady-state PL spectrum in LNO7 for different cryogenic temperatures $T$ for the given excitation laser spectrum denoted as Exc.

Figure 3

Time-resolved PL decay $I(t,\lambda )$ in as-grown LNO7 (a) and normalized decay $I(t,\lambda )/I(t=0,\lambda )$ (b).

Figure 4

Time-resolved PL decay in as-grown LNO7. (a) Raw TRPLS data, (b) corrected TRPLS data, (c) PL spectrum for $\lambda =475$ nm, and (d) PL spectrum for a delay time $t=2\times {10}^{-8}$ s.

Figure 5

Time-resolved PL decay in as-grown LNO3. (a) Raw TRPLS data, (b) corrected TRPLS data, (c) PL decay for $\lambda =475$ nm, and (d) PL spectrum for a delay time $t=2\times {10}^{-8}$ s.

Figure 6

(a) TRPLS graphs for different pump intensities. (b) A clear proportional dependence of the integrated PL for two representative delay times can be observed; thus, nonlinear relaxation processes can therefore be largely excluded.

Figure 7

PL decay in as-grown and singly inverted LNO7 (a) PL decay in LNO3 (d) in LNO3. TRPLS graphs for LNO3 and LNO7: (b) as-grown and (c) singly inverted in the case of LNO7; (d) as-grown and (e) singly inverted in the case of LNO3.

Figure 8

Normalized TRPLS graphs for 7 $\mathrm{mol}$% Mg:${\mathrm{LiNbO}}_3$ for different temperatures $T=200$ K (a) and $T=100$ K (c). The corrected TRPLS graph is given in (b), (d), respectively.

Figure 9

TRPL curves at $\lambda =475\mathrm{nm}$ for a temperature range of $100\mathrm{K}$ to $200\mathrm{K}$. Hereby, dots represent experimental data and lines the fitting curves according to Eq. (3).

Figure 10

Arrhenius plot of the characteristic lifetimes ${\tau }_{1,2}$ of the fast and slow decay versus temperature. Theoretical fit with $E_a^{\left(1\right)}=(140\pm 10)\mathrm{meV}$ and $E_a^{\left(2\right)}=(110\pm 10)\mathrm{meV}$.

Figure 11

Temperature dependence of the stretch parameters ${\beta }_i$ for fast ($i=1$) and slow ($i=2$) transitions.

Figure 12

Temperature dependence of the ratio of fast- (1) and slow- (2) transition PL intensities $I_2/I_1$.

Figure 13

(a) Schematic of radiative recombination of electron and hole polaron in ${\mathrm{LiNbO}}_3$ after hopping transport. (b) Different radiative recombination properties: $①$ single-step hopping recombination, and $②$ several-step recombination after several hopping steps $③$ .