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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 16 — Aug. 1, 2011
  • pp: 15322–15338
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Hologram recording via spatial density modulation of Nb Li 4 + / 5 + antisites in lithium niobate

M. Imlau, H. Brüning, B. Schoke, R.-S. Hardt, D. Conradi, and C. Merschjann  »View Author Affiliations


Optics Express, Vol. 19, Issue 16, pp. 15322-15338 (2011)
http://dx.doi.org/10.1364/OE.19.015322


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Abstract

Hologram recording is studied in thermally reduced, nominally undoped lithium niobate in the time domain from 10 ns to 100 s by means of intense ns pump laser pulses (λ = 532 nm) and continuous-wave probe light (λ = 785 nm). It is shown that mixed absorption and phase gratings can be recorded within 8 ns that feature diffraction efficiencies up to 23 % with non-exponential relaxation and lifetimes in the ms-regime. The results are explained comprehensively in the frame of the optical generation of a spatial density modulation of Nb Li 4 + / 5 + antisites and the related optical features, i.e. absorption as well as index changes mutually related via the Kramers-Kronig-relation. Implications of our findings, such as the electrooptical properties of small bound Nb Li 4 + polarons, the optical features of Nb Li 4 + : Nb Nb 4 + bipolarons, Nb Nb 4 + free polarons and O hole-polarons, the impact of light polarization of pump and probe beams as well as of the polaron density are discussed.

© 2011 OSA

1. Introduction

A steady-state polaron absorption at room temperature in nominally undoped crystals can be created by raising the Fermi level, e.g. by chemical reduction. Then, NbNb4+:NbLi4+ bipolarons are stable. Their optical and/or thermal dissociation leads to bound polarons NbLi4+ and to free NbNb4+ polarons. The latter recombine very fast within several μs with NbLi antisite defects at room temperature. Hence, considerable NbNb4+-densities are present in thermal equilibrium only at rather elevated temperatures [7

7. J. Shi, H. Fritze, G. Borchardt, and K. D. Becker, “Defect chemistry, redox kinetics, and chemical diffusion of lithium deficient lithium niobate,” Phys. Chem. Chem. Phys. 13, 6925 (2011) [CrossRef] [PubMed]

]. Besides, it is possible to generate a pronounced transient small polaron absorption by optical interband excitation of carriers and/or by carrier-release via photo-ionization of defect centers. Number densities of generated small polarons up to tens of 1022 m−3 strongly absorb light in the near-infrared and visible spectral range taking into account the typical polaron absorption cross sections exceeding 4 · 10−22 m2 [8

8. K. L. Sweeney and L. E. Halliburton, “Oxygen vacancies in lithium niobate,” Appl. Phys. Lett 43, 336 (1983). [CrossRef]

10

10. O. Beyer, D. Maxein, Th. Woike, and K. Buse, “Generation of small bound polarons in lithium niobate crystals on the subpicosecond time scale,” Appl. Phys. B 83, 527 (2006). [CrossRef]

].

Obviously, the optical generation of small polarons and their absorption features must be considered for the various fields of applications of lithium niobate in nonlinear photonics [16

16. L. Arizmendi, “Photonic applications of lithium niobate crystals,” Phys. Status Solidi A 201, 253 (2004). [CrossRef]

]. For instance, laser-induced damage mechanisms in lithium niobate may be triggered by small polaron absorption as verified for charged point defects in several prominent oxides widely applied for frequency conversion [17

17. L. E. Halliburton, N. C. Giles, and T. H. Myers, “Final technical report,” Development of nonlinear optical materials for optical parametric oscillator and frequency conversion applications in the near- and mid-infrared, p. A342373 (2003).

19

19. W. Hong, L. E. Halliburton, K. T. Stevens, D. Perlov, G. C. Catella, R. K. Route, and R. S. Feigelson, “Electron paramagnetic resoncance study of electron and hole traps in β-BaB2O4 crystals,” J. Appl. Phys 94, 2510 (2003). [CrossRef]

]. Furthermore, it has been shown that small bound polarons may act as intermediate shallow traps in a multi-step recording scheme for phase holograms featuring non-destructive read-out [20

20. L. Hesselink, S. S. Orlov, A. Lie, A. Akella, D. Lande, and R. R. Neurgaonkar, “Photorefractive materials for nonvolatile volume holographic data storage,” Science 282, 1089 (1998). [CrossRef] [PubMed]

, 21

21. K. Buse, A. Adibi, and D. Psaltis, “Non-volatile holographic storage in doubly doped lithium niobate crystals,” Nature 393, 665 (1998). [CrossRef]

]. Only recently, the optical features of small polarons were shown to play an important role in the prominent bulk photovoltaic effect in Fe-doped lithium niobate [22

22. O. F. Schirmer, M. Imlau, and C. Merschjann, “Bulk photovoltaic effect of LiNbO3:Fe and its small-polaron-based microscopic interpretation,” Phys. Rev. B 83, 165106 (2011). [CrossRef]

].

2. Samples and experimental setup

2.1. Sample preparation and polaronic features

Our studies were performed with single crystals of thermally reduced lithium niobate grown from a congruent nominally undoped melt via Czochralski growth technique (Crystal Technology, Inc.). Dimension, orientation and concentration of residual Fe [23

23. C. Merschjann, B. Schoke, and M. Imlau, “Influence of chemical reduction on the particular number densities of light-induced small electron and hole polarons in nominally pure LiNbO3,” Phys. Rev. B 76, 085114 (2007). [CrossRef]

] for the sample under study as well as parameters for thermal pre-treatment are summarized in Table 1.

Table 1. Dimension, Orientation, Fe Content and Thermal Pre-Treatment Parameters of the Lithium Niobate Sample under Study

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The pre-treated sample features a grayish coloration due to the presence of NbLi4+:NbNb4+-bipolarons (BP) and small bound NbLi4+-polarons (GP) [23

23. C. Merschjann, B. Schoke, and M. Imlau, “Influence of chemical reduction on the particular number densities of light-induced small electron and hole polarons in nominally pure LiNbO3,” Phys. Rev. B 76, 085114 (2007). [CrossRef]

, 24

24. J. Koppitz, O. F. Schirmer, and A. I. Kuznetsov, “Thermal dissociation of bipolarons in reduced undoped LiNbO3,” Europhys. Lett. 4, 1055 (1987). [CrossRef]

] as a result of chemical reduction. The respective polaron number densities N BP and N GP can be simply determined from inspection of the steady-state absorption α and the values for the absorption cross sections σ at λ = 488 nm and λ = 785 nm, i.e. at the wavelengths that characterize the maxima of the bipolaron and bound polaron absorption bands (cf. Ref. [25

25. C. Merschjann, B. Schoke, D. Conradi, M. Imlau, G. Corradi, and K. Polgar, “Absorption cross sections and number densities of electron and hole polarons in congruently melting LiNbO3,” J. Phys.: Condens. Matter 21, 015906 (2009). [CrossRef]

] and references therein). Literature values and estimates for our sample are given in Table 2.

Table 2. Steady-State Absorption αo/e, Polaron Absorption Cross Section σo/e at λ = 488 nm and λ = 785 nm, as well as Polaron Number Densities N BP and N GP

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2.2. Experimental setup

Hologram recording and time-resolved detection of the hologram decay is performed in a two-beam interferometer as schematically depicted in Fig. 1. A pulse of a frequency-doubled YAG:Nd-laser (Innolas Spitlight 600, |k p| = 2π/λ p, λ p = 532 nm, maximum applied intensity Ipmax=760GW/m2, 10 Hz repetition rate, and average pulse duration τ FWHM ≈ 8 ns) was singled out by an electromechanical pulse picker (PP). The pulse was split by a 50% : 50% beam splitter (BS), i.e., equal intensities were adjusted along the two optical paths (I R = I S) yielding a modulation depth of m=2IRIS/(IR+IS)epRepS=epRepS with the light polarization unit vectors epR and epS. A symmetric angle of incidence ΘB was chosen with respect to the sample’s normal thus enabling the recording of unslanted gratings. Beam superposition in the time domain was adjusted by an optical delay stage (DS). The Gaussian spatial beam profiles (2ω 0 ≈ 4 mm) were masked to the sample aperture by a diaphragm (DP). Peak intensity and light polarizations were adjusted by a combination of Glan-Taylor prism (GP) and half-wave plate (λ/2). A fiber-coupled GaAlAs-diode laser (Coherent Cube, |k| = 2π/λ, λ = 785 nm) served for Bragg-matched as well as off-Bragg continuous wave (cw) probing. Its intensity was limited to I 0 = 10 kW/m2, thus not affecting the polaron-density dynamics. The probe-beam angle Θ was precisely controlled by mounting the fiber collimator (FC) onto a motorized rotation stage (Newport, URM 80CC, angular resolution of 0.001°) with an extension arm.

Fig. 1 Scheme of the experimental setup for hologram recording as described in the text. PP: electromechanical pulse picker, BS: 50% : 50% beam splitter, GP: Glan-Taylor prism, λ/2: half-wave plate, M1-M5: dielectric mirrors, DP: diaphragm, D1-D3: Si-PIN diodes, DS: optical delay stage, DSO: digital storage oscilloscope, FC: fiber collimator with wave plate and polarizer.

The intensities of diffracted and transmitted probe beams were determined with Si-PIN diodes (D1, D2) equipped with optics that adjust the beam diameter to the photosensitive area. Small band-width optical filters served for suppression of the pump beam light. Time-dependent data collection in the range from 1μs – 100 s with a time resolution of up to 1 ns by a multi-channel digital storage oscilloscope (LeCroy, Waverunner LT584L) was triggered via a photodiode (D3) detecting pulse light scattered from the sample surface. The time regime 1 ns −1μs is excluded in our study in order to suppress unwanted signal contributions from thermal gratings recorded via the thermo-optic effect [26

26. F. Jermann and K. Buse, “Light-induced thermal gratings in LiNbO3:Fe,” Appl. Phys. B 59, 437 (1994). [CrossRef]

]. The sample temperature was adjusted by a PID-controlled thermoelectric element from room temperature up to 410 K.

Transient absorption was measured in a pump-probe-setup as described elsewhere (cf. [12

12. C. Merschjann, D. Berben, M. Imlau, and M. Wöhlecke, “Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO3,” Phys. Rev. Lett. 96, 186404 (2006).

]). Contrary to the two-beam interferometer, the pump pulse is directed normal to the sample surface while the beam path of the cw probe laser is marginally slanted. Two probing wavelengths at 488 nm and 785 nm were used.

3. Experimental results

3.1. Hologram recording: Bragg-matched probing

Figure 2 highlights the temporal dynamics of the first order diffracted probe beam obtained after hologram recording with a 8 ns-laser pulse.

Fig. 2 Temporal dynamics of the intensity of the first order diffracted beam I (1st) for (a) Kc-axis, e p || c-axis, s-polarization and (b) K || c-axis, e p || c-axis and p-polarization. Recording conditions: λ p = 532 nm, ΘB = 11.5°, spatial frequency Λ ≈ 1.3μm, I p = I R + I S = 380 GW/m2 and 230 GW/m2, respectively. Bragg-matched probing conditions: λ = 785 nm, e || c-axis with (a) s- and (b) p-polarization. The data are normalized to the intensity of the incoming probe beam I 0 and a logarithmic time scale is applied. The solid lines correspond to a fit of a stretched-exponential function Eq. (1) to the data set. The insets sketch the respective recording and probing configurations.

The grating wavevector K was aligned (a) perpendicularly to the polar c-axis with light polarizations of pump and probe beams parallel to c and s-polarization. In (b), K was aligned parallel to the polar c-axis again with light polarizations of pump and probe beams parallel to c but p-polarization. Thereby the recording without (r 331 = 0) and with (r 223 ≠ 0, r 333 ≠ 0) an electro-optic contribution according to the point symmetry 3m of lithium niobate was studied, respectively [27

27. R. S. Weis and T. K. Gaylord, “Lithium niobate: summery of physical properties and crystal structure,” Appl. Phys. A 37, 191–203 (1985). [CrossRef]

].

The intensity I (1st)(t) is normalized to the intensity of the incoming probe beam I 0 and thus corresponds to the efficiency η of the diffraction process. It is plotted on a logarithmic time scale and characteristically shows a non-exponential decay to zero. Obviously, gratings with similar temporal characteristics but severely different efficiencies are recorded in (a) and (b). Fitting a stretched exponential function according to the empirical dielectric decay function by Kohlrausch, Williams and Watts (KWW) [28

28. G. Williams and D. C. Watts, “Non–symmetrical dielectric relaxation behaviour arising from a simple empirical decay function,” Trans. Faraday Soc 66, 80 (1970). [CrossRef]

]
I(1st)(t)I0=I(1st)(t=0)I0exp[(t/τ)β]
(1)
to the experimental data (solid line in Fig. 1) yields the starting amplitude I (1st)(t = 0)/I 0, the decay time τ and the stretching coefficient β given in Table 3.

Table 3. Parameters Obtained by Fit of Eq. (1) to the Data of Fig. 1

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The temporal dynamics were determined for both directions of K as a function of crystal temperature T in the temperature regime 300 – 410 K. The analysis of the individual spectra by fitting Eq. (1) allowed to extract the dependence of the decay time on the sample temperature τ(T). Performing an Arrhenius plot, a linear dependence is found that allowed for the determination of the activation energies E a (see Table 3) yielding an average value of Ē a = (0.57 ± 0.07) eV. As a final note of this subsection we remark, that we were not able to detect any signal in the direction of second or higher order diffraction.

3.2. Hologram recording: off-Bragg probing

In order to verify the presence of volume gratings, the angular sensitivity of the Bragg condition was determined by scanning the diffraction efficiency as a function of the angular detuning ±δΘ. Exemplarily, Fig. 3 shows the spectrum for K || c-axis, commonly called rocking curve, that appeared for probing at 785 nm in an angular detuning range of ±0.25° and at 1 μs. Because of the grating lifetime in the ms-regime, the data points were deduced from a series of individual measurements of the temporal dynamics, each adjusted for a specific read-out angle. The rocking curve reflects a distinct maximum for Bragg-incidence (δΘ = 0°) and a full width at half maximum of δΘFWHM = (0.11 ± 0.02)°.

Fig. 3 Angular dependence of the intensity of the normalized first order diffracted beam I (1st)/I 0 at t = 1μs after the pump pulse as a function of the deviation δΘ of the Bragg-angle ΘB (external values). The solid line represents the result of fitting Eq. (6) to the experimental data.

3.3. Transient absorption

For analysis of the hologram recording mechanism, the temporal dynamics of the light-induced absorption α li was determined for the sample under study and for probing light at 785 nm (Fig. 4). The data were retrieved from the transmitted probe beam intensity as a function of time I t(t) as sketched in the inset.

Fig. 4 The temporal dynamics of the light-induced absorption α li determined for the sample under study and for probing light at λ = 785 nm and λ = 488 nm. The intensity of the pump beam at λ p = 532 nm was I p = 760 GW/m2 with e p || e || c-axis and s-polarization. The solid lines are the results of fitting Eq. (2) to the experimental data sets. The inset sketches the experimental arrangement of pump and probe beams.

The data, which reflect the population density dynamics of NbLi4+, were collected for an intensity of the pump beam of I p = 760 GW/m2 and light polarizations of pump and probing beams parallel to the polar c-axis and s-polarization. Again, a logarithmic scale is chosen for the time range 1 μs – 100 s.

Additionally to the data at 785 nm, the temporal dynamics were also collected for the probe wavelength λ = 488 nm, which contains information about the optical gating process of bipolarons and the optical generation of hole polarons in thermally reduced lithium niobate. Here, the signal decays to zero from a negative starting value, i.e. transient transparency is observed as previously reported and explained [23

23. C. Merschjann, B. Schoke, and M. Imlau, “Influence of chemical reduction on the particular number densities of light-induced small electron and hole polarons in nominally pure LiNbO3,” Phys. Rev. B 76, 085114 (2007). [CrossRef]

]. Both dynamics show a non-exponential decay behavior. Thus, the stretched exponential function
αli=αli0exp[(t/τ)β]
(2)
is fitted to the data sets. Here, αli0 denotes the starting amplitude, τ the characteristic decay time and β the stretching coefficient. From the results of the fitting procedure we get τ and β as given in Table 4. The value α li is the amplitude of the light-induced absorption at 1 μs, which enters our data analysis in the discussion.

Table 4. Parameters Deduced from Fitting Eq. (2) to the Experimental Data in Fig. 4

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From the temperature dependence of the transients at 785 nm we get an activation energy of E a = (0.53 ± 0.07) eV in accordance with literature data [12

12. C. Merschjann, D. Berben, M. Imlau, and M. Wöhlecke, “Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO3,” Phys. Rev. Lett. 96, 186404 (2006).

].

4. Discussion

Our experimental results clearly demonstrate the possibility to record volume holographic gratings in thermally reduced, nominally undoped lithium niobate with unique features: (i) a non-exponential relaxation behavior with stretching coefficient β < 1, (ii) a lifetime in the dark in the ms-regime, (iii) a temperature dependence pointing to a thermally activated relaxation process with activation energy of ≈ 0.57 eV and (iv) highly efficient recording with one single 8 ns-laser pulse at 532 nm. Because of these features, the underlying recording and relaxation mechanisms can not be understood in the frame of well-known photo-induced processes, such as Drude-Lorentz nonlinearities, the thermo-optic/pyroelectric effect [26

26. F. Jermann and K. Buse, “Light-induced thermal gratings in LiNbO3:Fe,” Appl. Phys. B 59, 437 (1994). [CrossRef]

], or the prominent photorefractive effect [29

29. P. Günter and J. P. Huignard, eds., Photorefractive Materials and Their Applications 1: Basic Effects, Springer Series in Optical Sciences (Springer-Verlag, 2006). [CrossRef]

]. The latter can be excluded, as well, due to the absence of a significant number density of Fe3+ as a result of the thermal reduction procedure, due to the rather low Fe impurity content (< 5 ppm) itself and due to the fact that the photovoltaic as well as electro-optic tensor elements are zero from symmetry considerations.

In the following we will follow this small-polaron approach for hologram recording in more detail by first considering the grating recording mechanism within the state-of-the-art picture of small polarons in thermally reduced lithium niobate. It will be the starting point to invest available optical data of literature in order to end-up with a theoretical estimate for the hologram efficiency. The model further will be applied to predict the hologram relaxation dynamics. All estimates can be compared with those listed in the experimental section and allows to further evaluate the validity of the small polaron approach.

4.1. Grating recording mechanism

4.1.1. Homogeneous exposure

Figure 5 summarizes the optical population paths, defect sites and the respective energetic positions that are relevant for the analysis of thermally reduced, nominally undoped lithium niobate exposed to a single ns laser pulse at 532 nm, i.e. the case of spatial homogeneous light exposure, and probing at 785 nm (cf. Refs. [2

2. O. F. Schirmer, M. Imlau, C. Merschjann, and B. Schoke, “Electron small polarons and bipolarons in LiNbO3,” J. Phys.: Condens. Matter 21, 123201 (2009). [CrossRef]

, 3

3. O. F. Schirmer, “O bound small polarons in oxide materials,” J. Phys.: Condens. Matter 18, R667 (2006). [CrossRef]

]).

Fig. 5 Polaron generation in thermally reduced lithium niobate upon exposure to ns laser pulses. Left: optical gating of NbLi4+:NbNb4+ bipolarons into small bound NbLi4+ and free NbNb4+ polarons. Right: two-photon excitation yielding O hole as well as bound and free polarons. The valencies of the individual centers correspond to the sketched location of electrons and holes at the trap centers (left) and in the valence band (right).

In the blue-green spectral range, the groundstate absorption of the unexposed sample is determined by NbLi4+:NbNb4+ bipolarons with absorption features exhibiting a maximum at 2.5 eV. NbLi4+ polarons appear at 1.6 eV and thus affect the transmission of the probing light. The strengths of both absorption features are determined by the respective polaron number densities in our sample (see Table 2). We note that NbLi5+ antisites do not contribute to the optical features.

The number density of NbLi4+ polarons is altered upon light exposure with 2.5 eV via optical gating of bipolarons (left half of Fig. 5) and via two-photon-absorption processes (right half) [23

23. C. Merschjann, B. Schoke, and M. Imlau, “Influence of chemical reduction on the particular number densities of light-induced small electron and hole polarons in nominally pure LiNbO3,” Phys. Rev. B 76, 085114 (2007). [CrossRef]

]. The increase of the bound polaron density is inevitably accompanied with a depopulation of the bipolaron density as well as the appearance of small free NbNb4+ electron and O bound hole polarons (absorption maxima at 1.0 eV and 2.5 eV). Fortunately, we can neglect the contribution of small free NbNb4+ polarons in what follows, because of their comparably small lifetime in the sub-μs regime [33

33. D. Conradi, C. Merschjann, B. Schoke, M. Imlau, G. Corradi, and K. Polgár, “Influence of Mg doping on the behaviour of polaronic light-induced absorption in LiNbO3,” Phys. Stat. Sol. RRL 2, 284 (2008). [CrossRef]

] and the limitation of our temporal dynamics to t ≥ 1 μs (cf. Fig. 2 and Fig. 4).

4.1.2. Inhomogeneous exposure

Taking into account these light-induced processes and the sinusoidal intensity pattern I(x) = I p[1 + cos(|K|x)], that was applied for exposure in our experiments (Kc-axis), we expect the appearance of spatially periodic density modulations of small bound electron and hole polarons as well as of bipolarons as sketched in Fig. 6(a). Obviously, N GP(x) and N HP(x) appear in phase to I(x) while N BP(x) is phase-shifted by π.

Fig. 6 (a) Sinusoidal intensity pattern I(x) applied for exposure in our experiments with average intensity I p = I R + I S and modulation depth unity, spatially periodic density modulations of small bound polarons N GP(x), hole polarons N HP(x) and bipolarons N BP(x). (b) Spatial modulation of the absorption coefficient α(x) with amplitude α 1 and average value of α + α 1. The overall absorption change in the maximum of the fringe pattern α li = 2α 1 is assembled from absorption changes of the individual polaron type: α li,GP, α li,HP and α li,BP. All absorption contributions are related to λ = 785 nm and extraordinary light polarization.

For the particular case of Bragg-incidence and under the assumption of an unslanted, mixed elementary holographic volume grating (mutual phase shift is 0 or π) the diffraction efficiency can be expressed via Kogelnik’s formula [34

34. H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Syst. Tech. J. 48, 2909 (1969).

]:
I(1st)I0=exp(2(α+α1)dhcosΘB)×[sin2(πn1dhλcosΘB)+sinh2(α1dh2cosΘB)]
(3)
with the effective thickness of the recorded hologram d hd and the Bragg angle in the medium ΘB. For the particular case of hologram recording with light polarization of the pump beams parallel c-axis and p-polarization, the factor cos(2ΘB) enters both arguments in the brackets of Eq. (3).

We note that this type of hologram recording does not require any of the further crystal-physical effects present in lithium niobate, such as the electro-optic effect. Nevertheless, diffraction gratings recorded via other mechanisms may appear simultaneously to polaron-density modulations in various recording configurations in lithium niobate. For the particular experimental conditions of our experiments (c(Fe3+) < 5ppm, Kc-axis, s-polarization, t ≥ 1μs) this can be neglected, i. e. the experimental data can be analyzed in the framework of the presented hologram recording mechanism.

4.2. Turnover to data analysis

Let us estimate the diffraction efficiency given by Eq. (3) for the experimental conditions of Fig. 2(a) (probe light at λ = 785 nm, pump beam intensity I p = 380 GW/m2 and Bragg angle ΘB = 11.5°, Kc-axis, extraordinary light polarization and s-polarization) and the groundstate absorption α(785) = (270 ± 10) m−1 (cf. Table 2). For this purpose we need to determine the amplitudes α 1(785) and n 1(785) as well as the hologram thickness d h.

The amplitude α 1(785) can be deduced from the transients in Fig. 4: The initial value of the light-induced absorption α li(785) represents the maximum absorption change in the intensity maximum of the fringe pattern 2 · I p = 760 GW/m2. According to Fig. 6(b), we get α 1(785) = α li(785)/2 = (130 ± 15) m−1 from the values given in Table 4.

Equation (4) can be solved by considering the values of the light-induced absorption at the maximum of the individual polaron absorption bands, i. e. α li(785) and α li(488), that have been experimentally determined for the case of homogeneous exposure (cf. Fig. 4 and Table 4). Further, the values of the individual absorption cross sections are required which are given in Table 2 for bound polarons and bipolarons. For hole polarons σ HP(488) = (4.0 ± 1.0)·10−22 m2 was taken from Ref. [25

25. C. Merschjann, B. Schoke, D. Conradi, M. Imlau, G. Corradi, and K. Polgar, “Absorption cross sections and number densities of electron and hole polarons in congruently melting LiNbO3,” J. Phys.: Condens. Matter 21, 015906 (2009). [CrossRef]

] under the assumption that the absorption features do not differ for ordinary and extraordinary light-polarization. This assumption is justified by inspection of the absorption spectra published in Ref. [35

35. O. F. Schirmer and D. von der Linde, “Two-photon and X–Ray–Induced Nb4+ and O small polarons in LiNbO3,” Appl. Phys. Lett. 33, 35 (1978). [CrossRef]

] and Fig. 2 therein. As a result we get the light-induced alterations of the number densities listed in Table 5.

Table 5. Changes of the Number Densities of Light-Induced Bound Polarons N li,GP, Bipolarons N li,BP, and Hole Polarons N li,HP

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Now, the individual polaron line shapes of the optically induced absorption features can be determined using the band shapes of small polarons. The overall spectral dependence α li(λ) is depicted in Fig. 7 based on data given in Refs. [3

3. O. F. Schirmer, “O bound small polarons in oxide materials,” J. Phys.: Condens. Matter 18, R667 (2006). [CrossRef]

, 24

24. J. Koppitz, O. F. Schirmer, and A. I. Kuznetsov, “Thermal dissociation of bipolarons in reduced undoped LiNbO3,” Europhys. Lett. 4, 1055 (1987). [CrossRef]

, 36

36. B. Faust, H. Müller, and O. F. Schirmer, “Free small polarons in LiNbO3,” Ferroelectrics 153, 297 (1994). [CrossRef]

, 37

37. H. Kurz, E. Krätzig, W. Keune, H. Engelmann, U. Gonser, B. Dischler, and A. Räuber, “Photorefractive centers in LiNbO3, studied by optical–, Mössbauer– and EPR–methods,” Appl. Phys. 12, 355 (1977). [CrossRef]

].

Fig. 7 Spectral dependence of α li(λ) determined from the analysis of the experimentally determined light-induced absorption at 785 nm and 488 nm, the literature data on polaron absorption cross sections and the previously published experimental band shapes of small polarons. The dispersion n li(λ) is calculated from α li(λ) applying the Kramers-Kronig-relation, Eq. (6). For details see text.

With the help of α li(λ), the dispersion of the index change n li(λ) is calculated via the Kramers-Kronig relation [38

38. V. Lucarini, J. J. Saarinen, K.-E. Peiponen, and E. M. Vartiainen, eds., Kramers-Kronig Relations in Optical Materials Research (Springer Verlag, 2005).

]:
nli(ω)=2πP0ωΔκ(ω)ω2ω2dω,
(6)
where P denotes the Cauchy principal value, Δκ = α li(ω) · c/(2 ·ω), ω = 2·π·c/λ, and c is the vacuum speed of light. The resulting spectrum is plotted in Fig. 7. From this dependence, we find an amplitude of the light-induced refractive index modulation at our probe wavelength of n 1(785) = n li(785)/2 = (–3.0±1.0)·10−6. The given error takes into account the experimental error of the absorption data and the limits of the procedure of calculation for λ → ±∞.

Finally, Eq. (3) requires an estimate for the hologram thickness d hd. We determine this value from the analysis of the rocking curve in the configuration K || c-axis (cf. Fig. 3). The pronounced diffraction efficiency ensured a sufficient signal-to-noise ratio for the entire range of angular detuning. This enabled fitting the Kogelnik formula that considers an angular Bragg-mismatch δΘ [34

34. H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Syst. Tech. J. 48, 2909 (1969).

]
I(1st)(δΘ)I0=exp(2(α+α1)dhcosΘB)sin2(ν2+ξ21+ξ2/ν2)
(7)
with
ν=πn1dhcos(2ΘB)/(λcosΘB),ξ=2δΘπndhsinΘB/λ.
to the experimental data. Here, n is the index of refraction of the unexposed sample and n 1 is the index modulation amplitude and the factor cos(2ΘB) accounts for p-polarization. The pronounced efficiency by far exceeds the maximum value of the diffraction efficiency for an absorption grating of 0.037. Hence, contributions of an absorption grating can be neglected in Eq. (7). We note that fitting was performed using a data set with internal angles calculated via Snell’s law.

Knowledge of n, α and α 1 allows to determine d h. Here we find d h = (0.62 ± 0.07) mm. We assume that the hologram thickness for Kc-axis and K || c-axis, both with the same polarization states for recording and read-out, either s- or p-polarization for each of the two experimental configurations, do not differ significantly.

Obviously, the hologram thickness d h is smaller than the crystal thickness. It is therefore necessary to take into account the absorption loss of the diffracted beam within the crystal volume ddh, i.e. the estimate of the diffraction efficiency via Eq. (3) must contain the additional term exp[–2(α + α 1)(ddh)/ cos ΘB]. All values necessary for the calculation and the determined value for I (1 st )/I 0 are summarized in Table 6.

Table 6. Parameters Determined for an Estimate of I (1st)/I 0 via Eq. (3) for Hologram Recording with Kc-Axis, Light Polarization Parallel c-Axis and s-Polarization

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The comparison shows that the estimate η ≈ 6· 10−4 coincides well with the experimentally determined value of η = (4.2 ± 1.0) · 10−4 in the configuration Kc-axis. We note that we neglect fringe pattern deviations from a sinusoidal function due to light scattering and beam profile inhomogeneities. We further did not consider the reported nonlinear response of α li(I p) on the pump beam intensity [23

23. C. Merschjann, B. Schoke, and M. Imlau, “Influence of chemical reduction on the particular number densities of light-induced small electron and hole polarons in nominally pure LiNbO3,” Phys. Rev. B 76, 085114 (2007). [CrossRef]

], which affects the shape of the generated absorption pattern α(x).

4.3. Hologram relaxation

A second characteristic feature of the recorded holograms, that has to be analyzed in the framework of a spatially modulated bound polaron density, is the non-exponential relaxation of the diffraction efficiency. The holograms obey a relaxation time in the ms-regime and an activation energy of Ēa = (0.57 ± 0.07) eV. According to our model approach, the hologram relaxation resembles the temporal decay of the amplitudes n 1(785) and α 1(785), i.e. the decay of the spatial density modulation of small polarons. The relaxation is terminated for n 1 = α 1 = 0, i. e., where the spatial homogeneous polaron density distribution of the groundstate (≡ unexposed sample) is recovered.

The temporal decay of optically generated polaron densities upon spatial homogeneous light exposure in lithium niobate has been studied intensively in recent years by means of transient absorption spectroscopy [12

12. C. Merschjann, D. Berben, M. Imlau, and M. Wöhlecke, “Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO3,” Phys. Rev. Lett. 96, 186404 (2006).

, 23

23. C. Merschjann, B. Schoke, and M. Imlau, “Influence of chemical reduction on the particular number densities of light-induced small electron and hole polarons in nominally pure LiNbO3,” Phys. Rev. B 76, 085114 (2007). [CrossRef]

, 30

30. D. Berben, K. Buse, S. Wevering, P. Herth, M. Imlau, and Th. Woike, “Lifetime of small polarons in iron-doped lithium–niobate crystals,” J. Appl. Phys. 87, 1034–1041 (2000). [CrossRef]

, 31

31. P. Herth, D. Schaniel, Th. Woike, T. Granzow, M. Imlau, and E. Krätzig, “Polarons generated by laser pulses in doped LiNbO3,” Phys. Rev. B 71, 125128 (2005). [CrossRef]

, 40

40. B. Schoke, M. Imlau, H. Brüning, C. Merschjann, G. Corradi, K. Polgár, and I. I. Naumova, “Transient light-induced absorption in periodically poled lithium niobate: small polaron hopping in the presence of a spatially modulated defect concentration,” Phys. Rev. B 81, 132301 (2010). [CrossRef]

]. The decay is determined by thermally activated diffusive hopping transport of the individual small polarons and their mutual recombination. Hopping of polarons can be understood as the transfer of localized carriers between next-neighboring cation sites [4

4. D. Emin, “Phonon-assisted transition rates I. optical-phonon-assisted hopping in solids,” Adv. Phys. 24, 305 (1975). [CrossRef]

, 5

5. D. Emin, “Optical properties of large and small polarons and bipolarons,” Phys. Rev. B 48, 13691 (1993). [CrossRef]

]. The site-specific electronic potentials are mutually separated by an activation barrier W h. According to Austin and Mott [41

41. I. G. Austin and N. F. Mott, “Polarons in crystalline and non-crystalline materials,” Adv. Phys. 18, 41 (1969). [CrossRef]

], it is essentially one half of the polaron binding energy E p in the case of equivalent free small polarons and larger, if the initial site is further stabilized by the influence of an additional defect potential with energy W d: W h = 1/2E p + 1/2W d. Thus, for all small polarons under study in lithium niobate, thermally induced hopping transport prevails.

The hopping process is terminated by charge recombination which commonly takes place within the direct vicinity of an intrinsic defect. The mean hopping path length, given by the sum of the spatial charge displacements of the individual hopping events, thus is determined by the number densities of defects within the sample [12

12. C. Merschjann, D. Berben, M. Imlau, and M. Wöhlecke, “Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO3,” Phys. Rev. Lett. 96, 186404 (2006).

]. The recombination time depends on the type and number of polaronic centers that build the hopping path. We note that the inspection of the polaron number density by means of probing light yields the information on an ensemble of optically generated polarons, their hopping and recombination. The determined relaxation time thus resembles the average value of a very large number of individual recombination paths. The recombination path length of an individual polaron is given by the sum of the spatial charge displacements of all hopping events along this path. It is thus determined by the number density of defects in the specific spatial region of the sample.

The activation energy, that can be deduced from the relaxation kinetics, resembles the energy barrier of the polaron types dominating the recombination paths. Since the spatial length and temporal duration of the path depend on the defect density in a non-trivial way, the non-exponential relaxation behavior is a result of the presence of a spectrum of activation energies, disorder, and spatial recombination path lengths.

In our sample we expect two recombination paths of the optically generated polarons according to the generation mechanisms sketched in Fig. 5: (i) the decay of polarons for the recovery of bipolarons and (ii) the electron-hole-recombination by recombination of bound electron polarons with bound hole polarons.

From these considerations we deduce the following expectation for the relaxation kinetics of the recorded holograms: (i) The relaxation kinetics must resemble the activation energy of small bound polarons, because they dominate both recombination paths. As a consequence, the hologram relaxation time is expected in the ms-time regime at room temperature [12

12. C. Merschjann, D. Berben, M. Imlau, and M. Wöhlecke, “Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO3,” Phys. Rev. Lett. 96, 186404 (2006).

]. (ii) The relaxation kinetics must resemble the statistical superposition of non-exponential relaxation kinetics. At each position of the spatially periodic modulation of the polaron number density, we expect a characteristic non-exponential relaxation behavior. This is due to the dependence of the diffusive hopping transport on the local defect number density. Our probe beam diameter is orders of magnitude larger than the spatial frequency of the polaron modulation. Thus, non-exponential kinetics from different regions of the sample are superimposed on the detector. The situation is similar to the statistical superposition of transient absorption from different regions of the sample in periodically poled lithium niobate [40

40. B. Schoke, M. Imlau, H. Brüning, C. Merschjann, G. Corradi, K. Polgár, and I. I. Naumova, “Transient light-induced absorption in periodically poled lithium niobate: small polaron hopping in the presence of a spatially modulated defect concentration,” Phys. Rev. B 81, 132301 (2010). [CrossRef]

].

Both expectations are indeed verified by our experimental results: while the time-constants determined for homogeneous and inhomogeneous exposure do not differ, we find rather different values of the stretching coefficient β.

4.4. Hologram recording with K || c-axis

Let us now address the data analysis for K || c-axis, which revealed a diffraction efficiency of η = (0.23 ± 0.05) (see Fig. 2 and Table 3). In accordance with the pronounced value of η, fitting of Eq. (7) to the experimental data yields an amplitude of the index modulation up to n 1(785) = (–2.7 ± 0.6) · 10−4.

We first note, that for K || c-axis the linear electro-optic effect is active with non-zero tensor elements r 223 ≠ 0 and r 333 ≠ 0 in this recording geometry. Assuming that the electro-optic effect is the reason for the pronounced diffraction efficiency, an electric field being simultaneously present to the spatial density modulation of small polarons must be present. We may estimate the electric field strength E via
n1(λ)=12neff3(λ)reff(λ)E
(8)
which is valid for index changes in crystals of the point symmetry 3m. Here, n eff(λ) and r eff(λ) denote the effective index of refraction and the effective electro-optic tensor element at wavelength λ.

We now take into account that the light wave vectors propagate at a small angle with respect to the normal of the crystal and that the relation r 333 ≈ 3·r 113 (see e.g. Ref. [42

42. M. Jazbinsek and M. Zgonik, “Material tensor parameters of LiNbO3 relevant for electro- and elasto-optics,” Appl. Phys. B 74, 407 (2002). [CrossRef]

]) holds. Thereby we approximate n eff with the index of refraction for extraordinary light polarization ne and r effr 333. At wavelength of λ = 785 nm, we find ne(785nm) = (2.1776 ± 0.0005) [39

39. D. S. Smith, H. D. Riccius, and R. P. Edwin, “Refractive indices of lithium niobate,” Opt. Commun. 17, 332 (1976). [CrossRef]

]. The electrooptic coefficient has been precisely determined to r 333(632.8nm) = 31.4 pm/V in Ref. [43

43. T. Fujiwara, M. Takahasi, M. Ohama, A. J. Ikushima, Y. Furukawa, and K. Kitamura, “Comparison of electrooptic effect between stoichiometric and congruent LiNbO3,” Electron. Lett. 35, 499 (1999). [CrossRef]

]. Together with the dispersive behavior published in Ref. [44

44. S. Fries and S. Bauschulte, “Wavelength dependence of the electrooptic coefficients in LiNbO3:Fe,” Phys. Status Solidi A 125, 369 (1991). [CrossRef]

] we extrapolate r 333(785nm) = 30.8 pm/V. All values hold for lithium niobate grown from the congruent melt and at room temperature. Then, the estimate for the electric field strength is E=2n1/(ne3r333)17kV/cm.

Such a high electric field strength can not be explained with the diffusion mechanism with a saturation field of E diff = (k B T /e)(2π/Λ) ≈ 1.25 kV/cm. Here, k B is the Boltzmann constant and e the electron charge. We can also exclude drift mechanisms in the absence of an externally applied electric field. Instead, electric fields with a strength up to 100 kV/cm are common for Fe-doped lithium niobate induced via the bulk photovoltaic effect [45

45. A. M. Glass, D. von der Linde, and T. J. Negran, “High-voltage bulk photovoltaic effect and the photorefractive process in LiNbO3,” Appl. Phys. Lett. 25, 233 (1974). [CrossRef]

]. For Fe-doped lithium niobate, this effect has been accounted for by asymmetries in the Nb5+-Fe2+-distances along the ±c-direction. Only recently, a small-polaron-based microscopic interpretation has been established that succeeds in the explanation of the large variety of electrical and optical properties of the bulk photovoltaic effect [22

22. O. F. Schirmer, M. Imlau, and C. Merschjann, “Bulk photovoltaic effect of LiNbO3:Fe and its small-polaron-based microscopic interpretation,” Phys. Rev. B 83, 165106 (2011). [CrossRef]

].

Because of the absence of Fe-doping with significant concentration and a predominant valency 2+ upon thermal reduction, a photovoltaic current may be assigned to the optical generation of small polarons in the samples under study. Such approach on a bulk photovoltaic contribution of small bound polarons was already postulated by Schirmer et al. in 1987 [46

46. O. F. Schirmer, S. Juppe, and J. Koppitz, “ESR–, optical and photovoltaic studies of reduced undoped LiNbO3,” Cryst. Lattice Defects Amorphous Mater. 16, 353 (1987).

]. Our results give evidence for the validity of bound-polaron-based bulk photovoltaic currents by considering the hologram relaxation dynamics, in particular in the view of lifetime and activation energy.

5. Conclusion

Concluding our results and analysis, we succeeded in recording volume holographic gratings in thermally reduced lithium niobate. Taking the dominant intrinsic defect structure and state-of-the-art knowledge on small polarons in lithium niobate into account, a hologram recording mechanism based on the optical generation of a spatially modulated small polaron density is proposed. Following this model, light diffraction is due to the presence of a mixed holographic grating, i.e. the simultaneous presence of an absorption and an refractive index grating. The absorption grating resembles the population density of small polarons and causally affects the index of refraction via Kramers-Kronig relation. Predicted grating features, such as a non-exponential relaxation behavior, a ms-lifetime in the dark, a thermally activated relaxation process and highly efficient ns-pulse-recording, are experimentally verified.

In a second recording configuration it is demonstrated that pronounced holograms with efficiencies up to 23% can be recorded. From symmetry considerations we argue that it is due to a dominant contribution of the linear electro-optic effect. The presence of an electric field is discussed in the frame of our polaron-based model approach and the bulk photovoltaic effect. Considering the polaron formation time of ≈ 100 fs, we deal with a tremendous grating recording sensitivity that is much larger than the well-known photorefractive sensitivity in lithium niobate [20

20. L. Hesselink, S. S. Orlov, A. Lie, A. Akella, D. Lande, and R. R. Neurgaonkar, “Photorefractive materials for nonvolatile volume holographic data storage,” Science 282, 1089 (1998). [CrossRef] [PubMed]

].

Acknowledgments

The authors thank Gerda Cornelsen and Werner Geisler for sample preparation and the Deutsche Forschungsgemeinschaft (projects IM37/5-1, IM 37/7-1, INST 190/137-1 FUGG) and the Deutscher Akademischer Austausch Dienst (project 50445542) for financial support.

References and links

1.

D. Emin, “Polarons,” McGraw Hill Encyclopedia of Science and Technology, 10th edn (McGraw-Hill, 2007), vol. 14, p. 125.

2.

O. F. Schirmer, M. Imlau, C. Merschjann, and B. Schoke, “Electron small polarons and bipolarons in LiNbO3,” J. Phys.: Condens. Matter 21, 123201 (2009). [CrossRef]

3.

O. F. Schirmer, “O bound small polarons in oxide materials,” J. Phys.: Condens. Matter 18, R667 (2006). [CrossRef]

4.

D. Emin, “Phonon-assisted transition rates I. optical-phonon-assisted hopping in solids,” Adv. Phys. 24, 305 (1975). [CrossRef]

5.

D. Emin, “Optical properties of large and small polarons and bipolarons,” Phys. Rev. B 48, 13691 (1993). [CrossRef]

6.

D. Redfield and W. J. Burke, “Optical absorption edge of LiNbO3,” J. Appl. Phys. 45, 4566 (1974) [CrossRef]

7.

J. Shi, H. Fritze, G. Borchardt, and K. D. Becker, “Defect chemistry, redox kinetics, and chemical diffusion of lithium deficient lithium niobate,” Phys. Chem. Chem. Phys. 13, 6925 (2011) [CrossRef] [PubMed]

8.

K. L. Sweeney and L. E. Halliburton, “Oxygen vacancies in lithium niobate,” Appl. Phys. Lett 43, 336 (1983). [CrossRef]

9.

F. Jermann and J. Otten, “Light-induced charge transport in LiNbO3:Fe at high light intensities,” J. Opt. Soc. Am. B 10, 2085 (1993). [CrossRef]

10.

O. Beyer, D. Maxein, Th. Woike, and K. Buse, “Generation of small bound polarons in lithium niobate crystals on the subpicosecond time scale,” Appl. Phys. B 83, 527 (2006). [CrossRef]

11.

P. Herth, T. Granzow, D. Schaniel, Th. Woike, M. Imlau, and E. Krätzig, “Evidence for light-induced hole polarons in LiNbO3,” Phys. Rev. Lett. 95, 067404 (2005). [CrossRef] [PubMed]

12.

C. Merschjann, D. Berben, M. Imlau, and M. Wöhlecke, “Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO3,” Phys. Rev. Lett. 96, 186404 (2006).

13.

S. Sasamoto, J. Hirohashi, and S. Ashihara, “Polaron dynamics in lithium niobate upon femtosecond pulse irradiation: influence of magnesium doping and stoichiometry control,” J. Appl. Phys. 105, 083102 (2009). [CrossRef]

14.

Y. Qiu, K. B. Ucer, and R. T. Williams, “Formation time of a small electron polaron in LiNbO3: measurements and interpretation,” Phys. Status Solidi C 2, 232 (2005). [CrossRef]

15.

J. Carnicero, M. Carrascosa, G. García, and F. Agulló-López, “Site correlation effects in the dynamics of iron impurities Fe2+/Fe3+ and antisite defects NbLi4+/NbLi5+ after a short-pulse excitation in LiNbO3,” Phys. Rev. B 72, 245108 (2005). [CrossRef]

16.

L. Arizmendi, “Photonic applications of lithium niobate crystals,” Phys. Status Solidi A 201, 253 (2004). [CrossRef]

17.

L. E. Halliburton, N. C. Giles, and T. H. Myers, “Final technical report,” Development of nonlinear optical materials for optical parametric oscillator and frequency conversion applications in the near- and mid-infrared, p. A342373 (2003).

18.

M. M. Chirila, N. Y. Garces, L. E. Halliburton, S. G. Demos, T. A. Land, and H. B. Radousky, “Production and thermal decay of radiation-induced point defects in KD2PO4 crystals,” J. Appl. Phys. 94, 6456 (2003). [CrossRef]

19.

W. Hong, L. E. Halliburton, K. T. Stevens, D. Perlov, G. C. Catella, R. K. Route, and R. S. Feigelson, “Electron paramagnetic resoncance study of electron and hole traps in β-BaB2O4 crystals,” J. Appl. Phys 94, 2510 (2003). [CrossRef]

20.

L. Hesselink, S. S. Orlov, A. Lie, A. Akella, D. Lande, and R. R. Neurgaonkar, “Photorefractive materials for nonvolatile volume holographic data storage,” Science 282, 1089 (1998). [CrossRef] [PubMed]

21.

K. Buse, A. Adibi, and D. Psaltis, “Non-volatile holographic storage in doubly doped lithium niobate crystals,” Nature 393, 665 (1998). [CrossRef]

22.

O. F. Schirmer, M. Imlau, and C. Merschjann, “Bulk photovoltaic effect of LiNbO3:Fe and its small-polaron-based microscopic interpretation,” Phys. Rev. B 83, 165106 (2011). [CrossRef]

23.

C. Merschjann, B. Schoke, and M. Imlau, “Influence of chemical reduction on the particular number densities of light-induced small electron and hole polarons in nominally pure LiNbO3,” Phys. Rev. B 76, 085114 (2007). [CrossRef]

24.

J. Koppitz, O. F. Schirmer, and A. I. Kuznetsov, “Thermal dissociation of bipolarons in reduced undoped LiNbO3,” Europhys. Lett. 4, 1055 (1987). [CrossRef]

25.

C. Merschjann, B. Schoke, D. Conradi, M. Imlau, G. Corradi, and K. Polgar, “Absorption cross sections and number densities of electron and hole polarons in congruently melting LiNbO3,” J. Phys.: Condens. Matter 21, 015906 (2009). [CrossRef]

26.

F. Jermann and K. Buse, “Light-induced thermal gratings in LiNbO3:Fe,” Appl. Phys. B 59, 437 (1994). [CrossRef]

27.

R. S. Weis and T. K. Gaylord, “Lithium niobate: summery of physical properties and crystal structure,” Appl. Phys. A 37, 191–203 (1985). [CrossRef]

28.

G. Williams and D. C. Watts, “Non–symmetrical dielectric relaxation behaviour arising from a simple empirical decay function,” Trans. Faraday Soc 66, 80 (1970). [CrossRef]

29.

P. Günter and J. P. Huignard, eds., Photorefractive Materials and Their Applications 1: Basic Effects, Springer Series in Optical Sciences (Springer-Verlag, 2006). [CrossRef]

30.

D. Berben, K. Buse, S. Wevering, P. Herth, M. Imlau, and Th. Woike, “Lifetime of small polarons in iron-doped lithium–niobate crystals,” J. Appl. Phys. 87, 1034–1041 (2000). [CrossRef]

31.

P. Herth, D. Schaniel, Th. Woike, T. Granzow, M. Imlau, and E. Krätzig, “Polarons generated by laser pulses in doped LiNbO3,” Phys. Rev. B 71, 125128 (2005). [CrossRef]

32.

S. Torbruegge, M. Imlau, B. Schoke, C. Merschjann, O. F. Schirmer, S. Vernay, A. Gross, V. Wesemann, and D. Rytz, “Optically generated small electron and hole polarons in nominally undoped and Fe-doped KNbO3 investigated by transient absorption spectroscopy,” Phys. Rev. B 78, 125112 (2008). [CrossRef]

33.

D. Conradi, C. Merschjann, B. Schoke, M. Imlau, G. Corradi, and K. Polgár, “Influence of Mg doping on the behaviour of polaronic light-induced absorption in LiNbO3,” Phys. Stat. Sol. RRL 2, 284 (2008). [CrossRef]

34.

H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Syst. Tech. J. 48, 2909 (1969).

35.

O. F. Schirmer and D. von der Linde, “Two-photon and X–Ray–Induced Nb4+ and O small polarons in LiNbO3,” Appl. Phys. Lett. 33, 35 (1978). [CrossRef]

36.

B. Faust, H. Müller, and O. F. Schirmer, “Free small polarons in LiNbO3,” Ferroelectrics 153, 297 (1994). [CrossRef]

37.

H. Kurz, E. Krätzig, W. Keune, H. Engelmann, U. Gonser, B. Dischler, and A. Räuber, “Photorefractive centers in LiNbO3, studied by optical–, Mössbauer– and EPR–methods,” Appl. Phys. 12, 355 (1977). [CrossRef]

38.

V. Lucarini, J. J. Saarinen, K.-E. Peiponen, and E. M. Vartiainen, eds., Kramers-Kronig Relations in Optical Materials Research (Springer Verlag, 2005).

39.

D. S. Smith, H. D. Riccius, and R. P. Edwin, “Refractive indices of lithium niobate,” Opt. Commun. 17, 332 (1976). [CrossRef]

40.

B. Schoke, M. Imlau, H. Brüning, C. Merschjann, G. Corradi, K. Polgár, and I. I. Naumova, “Transient light-induced absorption in periodically poled lithium niobate: small polaron hopping in the presence of a spatially modulated defect concentration,” Phys. Rev. B 81, 132301 (2010). [CrossRef]

41.

I. G. Austin and N. F. Mott, “Polarons in crystalline and non-crystalline materials,” Adv. Phys. 18, 41 (1969). [CrossRef]

42.

M. Jazbinsek and M. Zgonik, “Material tensor parameters of LiNbO3 relevant for electro- and elasto-optics,” Appl. Phys. B 74, 407 (2002). [CrossRef]

43.

T. Fujiwara, M. Takahasi, M. Ohama, A. J. Ikushima, Y. Furukawa, and K. Kitamura, “Comparison of electrooptic effect between stoichiometric and congruent LiNbO3,” Electron. Lett. 35, 499 (1999). [CrossRef]

44.

S. Fries and S. Bauschulte, “Wavelength dependence of the electrooptic coefficients in LiNbO3:Fe,” Phys. Status Solidi A 125, 369 (1991). [CrossRef]

45.

A. M. Glass, D. von der Linde, and T. J. Negran, “High-voltage bulk photovoltaic effect and the photorefractive process in LiNbO3,” Appl. Phys. Lett. 25, 233 (1974). [CrossRef]

46.

O. F. Schirmer, S. Juppe, and J. Koppitz, “ESR–, optical and photovoltaic studies of reduced undoped LiNbO3,” Cryst. Lattice Defects Amorphous Mater. 16, 353 (1987).

47.

N. V. Kukhtarev, “Kinetics of hologram recording and erasure in electrooptic crystals,” Sov. Tech. Phys. Lett. 2, 438 (1976).

OCIS Codes
(090.7330) Holography : Volume gratings
(160.3730) Materials : Lithium niobate
(160.4670) Materials : Optical materials
(160.4760) Materials : Optical properties
(190.4400) Nonlinear optics : Nonlinear optics, materials
(160.5335) Materials : Photosensitive materials

ToC Category:
Holography

History
Original Manuscript: May 10, 2011
Revised Manuscript: June 16, 2011
Manuscript Accepted: June 16, 2011
Published: July 26, 2011

Citation
M. Imlau, H. Brüning, B. Schoke, R.-S. Hardt, D. Conradi, and C. Merschjann, "Hologram recording via spatial density modulation of NbLi4+/5+ antisites in lithium niobate," Opt. Express 19, 15322-15338 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-16-15322


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References

  1. D. Emin, “Polarons,” McGraw Hill Encyclopedia of Science and Technology , 10th edn (McGraw-Hill, 2007), vol. 14, p. 125.
  2. O. F. Schirmer, M. Imlau, C. Merschjann, and B. Schoke, “Electron small polarons and bipolarons in LiNbO3,” J. Phys.: Condens. Matter 21, 123201 (2009). [CrossRef]
  3. O. F. Schirmer, “O− bound small polarons in oxide materials,” J. Phys.: Condens. Matter 18, R667 (2006). [CrossRef]
  4. D. Emin, “Phonon-assisted transition rates I. optical-phonon-assisted hopping in solids,” Adv. Phys. 24, 305 (1975). [CrossRef]
  5. D. Emin, “Optical properties of large and small polarons and bipolarons,” Phys. Rev. B 48, 13691 (1993). [CrossRef]
  6. D. Redfield and W. J. Burke, “Optical absorption edge of LiNbO3,” J. Appl. Phys. 45, 4566 (1974) [CrossRef]
  7. J. Shi, H. Fritze, G. Borchardt, and K. D. Becker, “Defect chemistry, redox kinetics, and chemical diffusion of lithium deficient lithium niobate,” Phys. Chem. Chem. Phys. 13, 6925 (2011) [CrossRef] [PubMed]
  8. K. L. Sweeney and L. E. Halliburton, “Oxygen vacancies in lithium niobate,” Appl. Phys. Lett 43, 336 (1983). [CrossRef]
  9. F. Jermann and J. Otten, “Light-induced charge transport in LiNbO3:Fe at high light intensities,” J. Opt. Soc. Am. B 10, 2085 (1993). [CrossRef]
  10. O. Beyer, D. Maxein, Th. Woike, and K. Buse, “Generation of small bound polarons in lithium niobate crystals on the subpicosecond time scale,” Appl. Phys. B 83, 527 (2006). [CrossRef]
  11. P. Herth, T. Granzow, D. Schaniel, Th. Woike, M. Imlau, and E. Krätzig, “Evidence for light-induced hole polarons in LiNbO3,” Phys. Rev. Lett. 95, 067404 (2005). [CrossRef] [PubMed]
  12. C. Merschjann, D. Berben, M. Imlau, and M. Wöhlecke, “Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO3,” Phys. Rev. Lett. 96, 186404 (2006).
  13. S. Sasamoto, J. Hirohashi, and S. Ashihara, “Polaron dynamics in lithium niobate upon femtosecond pulse irradiation: influence of magnesium doping and stoichiometry control,” J. Appl. Phys. 105, 083102 (2009). [CrossRef]
  14. Y. Qiu, K. B. Ucer, and R. T. Williams, “Formation time of a small electron polaron in LiNbO3: measurements and interpretation,” Phys. Status Solidi C 2, 232 (2005). [CrossRef]
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