Investigation of Small Polarons in Reduced Iron-Doped Lithium-Niobate Crystals by Non-Steady-State Photocurrent Techniques

Abstract

Photorefractive iron-doped lithium-niobate crystals (LiNbO 3 : Fe) are of special interest for applications in the fields of optical communication techniques and holographic data storage. Recently, considerable attention has been concentrated on a shallow photorefractive center in LiNbO 3 : Fe, which enables non-volatile storage of holograms by two-color processes [1 to 4]. This center is attributed to the intrinsic Nb 4a5 Li antisite defect (Nb 4a5 on Li site) [5,6]. The center with a trapped electron (Nb 4 Li ) forms a`small polaron'. Because of the short lifetime of these small polarons at room temperature, high light intensities (above 10 kWm À2 ) are required to generate them [1,3].Thermally reduced LiNbO 3 :Fe is of special interest for practical applications, because reduction enlarges the photoconductivity, which makes photorefractive processes faster. However, lifetime and influence of the small polarons grow upon reduction. In this contribution we employ the technique of non-steady-state photocurrents (photo-EMF) to investigate a reduced iron-doped LiNbO 3 crystal. A recent publication shows that this approach is promising for getting more information about shallow centers [7]. Here we check with temperature dependent measurements whether the photo-EMF and light-induced absorption changes li are governed by the same type of defect.An iron-doped LiNbO 3 crystal (iron content c Fe 7  10 24 m À3 , size b  c 4X61  5X31 mm 2 , thickness d 2X21 mm) is reduced by thermal annealing (1000 C, 2 h, vacuum). The absorption coefficient for ordinarily polarized light of the wavelength l 532 nm is 2350 m À1 . Non-steadystate photocurrents are generated in the sample by illumination with two interfering light beams (fringe spacing of the interference pattern L 0X6 mm) of a frequency-doubled Nd : YAG laser (wavelength l 532 nm, incident light intensity I 2 kWm À2 ) [8,9]. The phase of one of the beams is periodically modulated, and an oscillating light intensity pattern is created. This yields an ac current signal which is directly detected by a lock-in amplifier. The crystal is mounted on a temperature-controlled stage which enables adjustment of the temperature in the range from 300 to 375 K. Measurements of the non-steady-state photocurrents I W are carried out for various temperatures T and modulation frequencies W 2%f. The dependence of I W on the frequency W can be described by [10,11] where ( EMF is the lifetime of the charge carriers in shallow traps and ( M is the Maxwell time, which is given by ( M 0 a' ph with the photoconductivity ' ph , the dielectric constant , and the permittivity of free space 0 . Measurements of the light-induced absorption li are carried out using green pump light of an argon-ion laser (wavelength l 514 nm, intensity up to 30 kWm À2 ). Absorption changes are monitored by means of different weak laser beams (wavelengths 633, 670, 785, and 1064 nm, ordinarily polarized). The pump light illuminates the sample shorter than one second to prevent light-induced heating. After switching off...

Birefringence changes induced by thermal cycling in lithium niobate

“…Furthermore, the activation energy for migration of small polarons is around 0.3 eV, Peithmann et al (2000) clearly much lower than what was observed here.…”

Influence of iron doping on spatial soliton formation and fixing in lithium niobate crystals

“…Most recent results concerns the realization of photorefractive optical waveguides in Fe:LN by ion implantation [14]. Using these configurations, the first demonstrators of optical interconnections integrated optical circuits and sensors with a holographic stage have been already reported [15][16][17][18][19][20]. The main limitation of these optical processing schemes is that their full potential is nowadays difficult to miniaturize and integrate in a http://dx.doi.org/10.1016/j.optmat.2014.05.020 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.…”