Absorption cross sections and number densities of electron and hole polarons in congruently melting LiNbO₃

Abstract

The number densities and absorption cross sections of both optically generated and reduction-induced small electron and hole polarons in LiNbO(3) are determined by means of time-resolved pump-multiprobe spectroscopy. The data are obtained for free (Nb(Nb)(4+)) and bound (Nb(Li)(4+)) electron polarons, bound Nb(Li)(4+):Nb(Nb)(4+) electron bipolarons, and bound O(-) hole polarons. The peak absorption cross sections are in the range of σ(pol)≈(4-14) × 10(-22) m(2), comparable to that for Fe(2+). In all cases the ratio of occupied to unoccupied polaronic sites is less than 10(-2).

Small Polaron Hopping in Fe:LiNbO3 as a Function of Temperature and Composition

“…All the data demonstrated the previously-reported behavior [22,23]: after the pulse, a marked increase of the absorption was detected for some of the probe wavelengths, which decay to their dark value after some time, with a dynamics that depends on the probe. The signal at 785 nm was attributed mainly to bound polaron absorption, in view of the spectral dependence of the polaron cross-section, and to a minor extent, to the absorption tail of the free polaron at this wavelength.…”

“…The so-produced polaron population performed a random walk hopping either among defective Nb Li sites (bound polarons) or regular Nb Nb sites (free polarons) until they were trapped by a Fe 3+ ion. Both bound and free polarons contributed to the absorption at 785 nm [23].…”

“…According to the discussion reported elsewhere [17,18,22,23] it can be considered that α TA (785 nm, t) was proportional to the weighted sum of the bound and free polarons populations.…”

“…The polaron decays were studied by light-induced Transient Absorption Spectroscopy (TAS) using the setup described elsewhere [22,23] based on a Q-switched, frequency-doubled Nd:YAG laser at a wavelength of λ = 532 nm with a pulse duration of τ FWHM 8 ns and a maximum pulse energy of E max = (290 ± 10) mJ. The pump intensities in the various experiments were adjusted to maximize the signal to noise ratio of the experimental decay: for Fe:LN 0.02 mol %, I ∼ 60 MW/cm 2 ; for Fe:LN 0.05 mol %, I ∼ 200 MW/cm 2 ; and for Fe:LN 0.1 mol %, I ∼ 40 MW/cm 2 .…”

“…An initial population of 5000 polarons, all forming on a Nb 4+ Nb site just after the pulse, were considered. The TAS signal at 785 nm was computed by considering the weighted sum of the FP and GP populations α LI (785 nm, t) ∝ σ GP (785 nm)N GP (t) + σ FP (785 nm)N FP (t), where σ GP (785 nm) = 7 × 10 −22 m and σ FP (785 nm) = 3.1 × 10 −22 m were the bound and free polaron cross-sections at 785 nm, respectively [23]. The numerically-computed decays were fitted by a KWW function in the experimental range starting from 10 −8 s, and the average lifetime was calculated using Equation (5).…”