Evidence for Two-Path Recombination of Photoinduced Small Polarons in Reduced LiNbO 3 C. Merschjann, * D. Berben, M. Imlau, and M. Wo ¨hlecke Fachbereich Physik, Universita ¨t Osnabru ¨ck, D-49069 Osnabru ¨ck, Germany (Received 20 December 2005; published 12 May 2006) The recombination of photoinduced free Nb 4 Nb and bound Nb 4 Li small polarons to Nb 4 Li :Nb 4 Nb bipolarons is investigated in nominally pure, reduced LiNbO 3 single crystals by means of excited- state-absorption spectroscopy. We discovered a two-component decay of the light-induced absorption  li t for probe light at   785 nm and moderate pump beam intensities (I p  670 GW=m 2 ). These experimental results give strong evidence for the existence of a two-path recombination of the photo- induced polarons. A corresponding model taking into account hopping charge transport and trapping is presented. DOI: 10.1103/PhysRevLett.96.186404 PACS numbers: 71.38.k, 77.84.Dy Introduction.—Polarons bound to defects are of general interest for the physical behavior of many optical materi- als, including oxide crystals. They are formed by charge carriers at certain shallow traps. Coulomb attraction and lattice distortion tend to localize the carrier at this site [1]. The population of these states under illumination is sup- posed to contribute to nonlinear optical effects, such as optical damage processes in KTiOPO 4 [2], LiB 3 O 5 [3], KNbO 3 [4], LiNbO 3 and LiTaO 3 [5,6]. They further gained importance in the field of photorefraction. A famous ex- ample is the two-step recording of holograms with infrared light in Fe-doped LiNbO 3 , using Fe 2=3 as deep and small Nb 4 Li bound polarons as metastable shallow centers [7]. Even in undoped LiNbO 3 a two-step recording with large diffraction efficiency has been observed after thermal re- duction of the samples. Here the deep center is the stable Nb 4 Li :Nb 4 Nb bipolaron [8], a strongly bound negative-U pair on neighboring cation sites. Because of this rich variety of stable and metastable polarons, LiNbO 3 is an ideal candidate for investigations of the properties of these intrinsic defects and the related charge transport processes. This work focuses on the recombination kinetics of photoinduced metastable small bound and Nb 4 Nb free polar- ons to bipolarons in nominally pure, reduced LiNbO 3 . This congruently melting material shows a Li deficiency which is partly compensated by Nb ions on Li sites [9]. Thermal reduction leads to the formation of stable bipolar- ons with an optical absorption band at 2.5 eV (500 nm) [10,11]. Bipolarons are dissociated by light in the blue- green spectral range (‘‘gating’’ [12]). According to well established small-polaron theory [13] the gating process should excite one electron to an adjacent Nb 5 Nb site, thus forming a metastable small free polaron with a binding energy of 0.5 eV. The corresponding optical absorption is centered at 1.0 eV (1250 nm) [14]. At the same time the second electron remains as small bound polaron with an optical absorption at 1.6 eV (760 nm) [11]. All polaronic absorption bands mentioned here show a large width of 1 eV [11,14]. Up to now, only the appearance of photo- induced bound polarons has been proven experimentally, either by exposure to cw or to pulsed laser light [15]. Information about the existence of photoinduced meta- stable free polarons is missing. Hence, the knowledge about the important charge transport and recombination processes is fragmentary. We gained insight into this question by means of excited-state-absorption spectroscopy. We show measure- ments of the temporal development of the light-induced absorption  li t detected in the blue, red, and infrared spectral range. A two-component decay of  li t is discov- ered at   785 nm for moderate pump beam intensities, which originates from the charge transport and recombi- nation processes of small polarons. The signal amplitude and temporal behavior are investigated for different pump intensities and sample temperatures, also yielding activa- tion energies. Based on our findings, a two-path model for hopping charge transport and recombination of metastable small polarons to bipolarons is developed. Experimental details and results.—Nominally pure samples of congruently melting LiNbO 3 (Crystal Tech- nology, Inc.: c Fe < 5 ppm) were thermally reduced by heating them up to approximately 920 K in vacuum (p< 10 4 mbar) for six hours. The purity of the samples was controlled by instrumental neutron activation analysis. No detectable element was found with an impurity level above 1 ppm. The steady-state absorption at   500 nm i.e., the maximum of the bipolaron absorption band, was  500 nm  410 m 1 . A Q-switched, frequency-doubled Nd:YAG pulse laser (  532 nm,  FWHM  8 ns) served as the pump light source. The low-intensity probe light of an Ar  laser (  488 nm) and of two diode lasers (  785 nm, 1310 nm) propagated through the crystal and were simultaneously detected by three PIN diodes. Ordinary light polarization was chosen for pump and probe beams (e p ? c axis). The signals of the diodes were re- corded for 20 seconds after the laser pulse with a fast digital oscilloscope having a time resolution of 1 ns. After these 20 seconds any light-induced absorption van- ished completely. From the transmitted probe intensity It the transient light-induced absorption was determined via PRL 96, 186404 (2006) PHYSICAL REVIEW LETTERS week ending 12 MAY 2006 0031-9007= 06=96(18)=186404(4) 186404-1  2006 The American Physical Society