The role of self-trapped excitons in polaronic recombination processes in lithium niobate

Abstract

Transient absorption and photoluminescence are experimentally investigated in the polaronic reference system lithium niobate, LiNbO 3 (LN), with the aim to refine the microscopic model of small polaron dynamics in materials with strong electron-phonon coupling. As a unique feature, our study is performed by using two different spectroscopic methods, in crystals with dopants enhancing photorefraction or damage resistance, and over a broad temperature range from 15-400 K. Although being self-consistent for particular experimental conditions, the hitherto used microscopic polaronic models reveal inconsistencies when applied to this larger data set. We show that comprehensive modeling is unlocked by the inclusion of an additional type of polaronic state with the following characteristics: (i) strongly temperature-and dopantdependent relaxation times, (ii) an absorption feature in the blue-green spectral range, and (iii) a Kohlrausch-Williams-Watts decay shape with a temperature-dependent stretching factor β (T) showing a behavior contrary to that of small, strong-coupling polarons. The hypothesis of self-trapped excitons (STEs, i.e. bound electron-hole pairs strongly coupled to Nb 5+ and O 2− within a niobium-oxygen octahedron) and their pinning on defects as the microscopic origin of these characteristics is supported by a spectroscopic linkage of photoluminescence at low (15 K) and elevated (300 K) temperatures and explains the long-lifetime components in transient absorption as due to pinned STEs.

Pulse-induced transient blue absorption related with long-lived excitonic states in iron-doped lithium niobate

“…The formation of O − -hole polarons with strong coupling is in full accordance with the original expectations formulated by Schirmer et al for nearly all polar oxide crystals [6]. Later on, investigations with continuous wave [7] and pulsed laser light [8,9], however, revealed several discrepancies to the hole polaron model. The results of Waasem et al [7] demanded for the existence of further electronic states within the band-gap, being handled as X-center of unknown microscopic nature.…”

“…Similar luminescence phenomena were observed at room temperature using fs-NIR-pulse trains in Mg-doped LN [13] and have been assigned to the recombination of electron and hole polarons [14]. The goal of the work of Messerschmidt et al [9] was to re-address charge-transport phenomena in LN by considering excitonic states as intermediate steps in addition. The authors succeeded to show, that the temperature dependence of transient absorption in LN crystals with Fe (and Mg) doping can be explained comprehensively by considering the presence of further (pinned) excitonic states, e.g., Fe 2+ Li − O − − V Li .…”

“…In what follows, we will first discuss the individual data sets in more detail according to the state-of-the-art knowledge of small polarons with strong coupling and considering the model of (pinned) excitonic states, i.e., according to the previously reported model approach by Messerschmidt et al [9]. Then, we focus on the absorption features of the Fe 2+ Li −O − −V Li excitonic state and will deduce the respective absorption cross-section, peak position and bandwidth.…”

“…The results of Waasem et al [7] demanded for the existence of further electronic states within the band-gap, being handled as X-center of unknown microscopic nature. Quite recently, Messerschmidt et al discussed excitonic states at the origin of long-lived transient blue absorption [9].…”

“…In this paper, we address the open question about peak position, shape, bandwidth and absorption cross-section of the long-lived transient blue absorption under the assumption of self-trapped excitonic states as microscopic origin, i.e., following the general model approach of Ref. [9]. The study is performed using Fe-doped LN crystals according to previously reported pronounced transient blue absorption observed in these crystals.…”