A digital holography interferometry was applied for in situ observation of the light-assisted domain reversal in the lithium niobate doped with MgO. The evolution of the light-assisted domain nucleation and growth was reconstructed, and we found a flat-top reversed domain state during the two stages: I. the reversed domain tip propagates in depth and progressively flattens its edge profile; II. the polarization reversal front abruptly changes shape with the formation of two fast-propagating lateral tips. According to experiments and analysis, we find a Gaussian-shaped space charge field and present a model based on a redistribution of the photo-excited carriers to explain the process of light-assisted domain reversal.
© 2011 OSA
Ferroelectric domain engineering in lithium niobate (LiNbO3, LN) crystals is an active field of research, since various applications can be realized with tailored ferroelectric domain patterns. The most prominent example is frequency conversion using quasi-phase matching in periodically poled crystals . All optical switches  and photonic crystals  are enabled by domain structures. For the high coercive field of lithium niobate crystals, the domain reversal is very difficult and the domain pattern is difficult to control. Recently, structured illumination in combination with the application of a suitable external electrical field allows them to transfer the light pattern to a domain pattern, providing a method for domain engineering with lower electric field [4–6].
From visible to ultraviolet range, some scientists have studied the domain reversal with light illumination, including the reduction of switching field, threshold intensities for domain nucleation, self-organized domain patterns with the ultraviolet illumination, etc [7–12]. However, the origin of the manifold light-assisted/ light-induced domain reversal has been not well understood by now. The digital holography (DH) based technique for in situ interferometric analysis of domain reversal process in lithium niobate provides quantitative spatially resolved phase shift data, with non-contact, non destructive and high resolution advantages [13–15]. The phase shift distribution is generated simultaneously by linear electro-optic and piezoelectric effect along the crystallographic c-axis. Such phase shift distribution is numerically reconstructed by the DH method and it is denoted here as the phase-map of the domain structure. With the evolution of the phase-map, we can monitor the domain reversal process, which help us understand the underlying process of the light-assisted/ light-induced domain reversal further.
In this paper, we observe the evolving process of light assisted domain reversal in the magnesium-doped lithium niobate crystals by the digital holographic interferometry method, and find the flat domain boundary at the middle position in the crystal, and propose a light-assisted domain reversal model to explain the domain reversal mechanism with light illumination.
We use a standard liquid-electrode setup that allows for illumination of the crystal during poling with light of a compact Diode-pumped Solid State Nd:YVO4 Laser at the wavelength 532 nm. The sample holder with a 3 × 3 cm2 size window is filled with saturated LiCl solution. Finally, the evolution of domain reversals is imaged by in situ visualization with the digital holographic interferometry . This enables us to obtain the phase shifts during the domain reversal and measure the coercive field (EC) for different intensities. The experiment is conducted with congruent lithium niobate crystal doped with 5 mol% MgO. The wafer is 1 mm in thickness and c-cut with optical quality polishing. The coercive field of crystal without illumination is 5.5 kV/mm. The focal length of the lens used in experiments is 150 mm, and the diameter of focused spot is about 50 micrometers. And the Rayleigh range for the focused beam (~3 mm) was much larger than the crystal thickness, which ensured a relatively uniform spatial distribution over the entire crystal thickness. The image is obtained by a CMOS camera with a resolution of 2048 × 1536 and 5 fps frequency.
The digital holographic interferometry is performed to measure the phase difference. The CMOS camera continuously records the variation in interference patterns. The digital holographic interferometry can be applied to all the recorded interferograms. The variations in the domain distributions are obtained in the crystals during the electric poling. The phase mapping of individual domain nucleation and growth is obtained by the digital holographic interferometry in the crystals. The distinct variations in phase distribution are observed during the individual domain nucleation and growth in the crystals. According to the electro-optic effect in lithium niobate crystal (the converse piezoelectric effect should be neglected for it is smaller to electro-optic effect), the change in refractive index between the reversed domain and un-reversed domain produces a phase shifts, and the depth of the reversed domain can be calculated with the change in refractive index. So we can acquire the mapping evolutions of the reversed domain in the domain reversal process. The electro-optic effect in the LN crystal produces the refractive index change in response to the applied electric field. The steady-state electric field applied to the LN crystal, with the ramping rate of 30 V/mm-s from the beginning of the illumination, is 4500 V/mm in this work.
3. Results and discussions
3.1 The phase mapping of the light-assisted reversed domain
Figure 1 shows the selected sequence of reconstructed three-dimensional mapping of reversed domain distributions during the individual domain nucleation and growth in congruent lithium niobate doped with 5.0 mol% MgO. From the Figs. 1(a)–1(f), we find that the evolution of the lateral dimension for the light-assisted reversed domain is similar to the nucleus ones without light illumination, which start from the center of the spot and expand outside gradually. But the evolution of the longitudinal dimension for the light-assisted reversed domain is obviously different, that the domain nucleus appear firstly in the center of the laser spot on the –c surface, and propagate towards the + c surface with a crest-like shape, but run through the wafer at the peripheral of spot on the + c surface. In the domain reversal process, the domain boundary between the reversed and native domain shows a flat-top state at a moment, which is the most difference compared with normal domain reversal without light illumination . With the different spot size (30~70 μm), we found the similar results.
The phase images are obstructed seriously by the scattering light on the domain walls, which occurs after the domain walls penetrating through the crystal. So we omitted the images during this period. The scattering light on the penetrating domain walls which is ternary-symmetry, is relative to the polarization of the probe light, and not occurs when the domain walls are circular symmetrical (i.e. the domain walls don’t pass through the crystal).
3.2 The characterization of the longitudinal growth
To study the evolution of the longitudinal dimension of the reversed domain, we choose the longitudinal section of the three-dimensional mapping for the reversed domain through the center of the laser spot (see in the Fig. 2 ). From the figure, we find that the nucleus appears at the central of spot region, and the reversed domain appears a cylinder-like shape with a flat top in the spot when the depth of domain is about 500 μm. The lateral size of the flat-top reversed domain is equal to the spot ones. After the flat-top state, the reversed domain spread out the spot, and the domain at the outer of the spot runs faster than the inner ones, which firstly propagates through the crystal. With the poling time increasing, the reversed domain continues to spread outward and finally runs through the sample in the whole spot region.
Figure 3(a) shows the evolution in the depth of the reversed domain for the selected position during the individual light-assisted domain nucleation and growth. From the figure, we find that the reversed domain at different positions has a same depth at a moment. The longitudinal velocity of domain walls via the different position in the light spot at different time is shown in Fig. 3(b). From the figure, we find that the longitudinal velocity is nonlinear. The velocity in the central region increases firstly with the time, and then decreases after the depth equal to the half thickness of the crystal. The velocity at the peripheral spot lags behind the central ones.
For the visible light-assisted domain reversal, it requires a higher light intensity for domain reversal. However, it doesn’t require a focusing light illumination for domain reversal in the ultraviolet band. With the visible light illumination, the absorption of the crystal is very little, so we can ignore the temperature effect as involved in the [5,17]. Similarly, the photovoltaic effect caused by the absorption isn’t the reason for the reduction of the switching field, which is considered in the UV range .
From the former study, the velocity of the domain boundary is proportional to the extra electric field and described by V = V∞ exp (-δ/E) , with V∞ and δ field independent. The Fig. 4 shows the space charge field in the crystal. For V∞ and δ are not constant over the entire range, we only show a relative variation. From the Fig. 4, we find that the space charge field changes over time and the space charge field is a Gaussian-like shape at initial stage. As it is well known, the photorefraction is a general effect in the lithium niobate, that the photon-excited carriers could migrate and construct the space charge field, to vary the external electric field of the domain reversal. So we consider the electric field in the light-assisted domain reversal come from the photorefraction.
According to former and our results, we propose a light-assisted domain reversal model as schematically shown in Fig. 4. Strong illumination leads to photoexcitation of charges, including electrons and holes [19,20]. A concentration gradient is formed which drives electrons into the dark area and holes with lower mobility remain in the illuminated region producing a forward extra electric field near the -c surface, which decreases the switching field locally, and a reversed electric field at the other end which restrains the domain propagating forward (Fig. 5(a) ). The reduction proportion of nucleation field increases exponentially with increasing laser intensity over a threshold value and eventually reaches some saturation value at higher intensities [5,7]. The intensity used in our experiment was about 2.5 × 103 W/cm2, which is beyond the intensity threshold. When an external constant electric field and a focal laser beam are applied, domain nucleation occurred within the central region of the beam spot on the –c surface; at the same time, domains propagate toward the + c face forming head-to-head boundary beneath the surface (Fig. 5(b)).
In the central region of illumination, holes reach a high concentration level, which are easily accumulated by the head-to-head boundary and then screen the depolarization field of the reversed domains. When the reversed domain boundary arrives at the middle of the crystal, the domain walls are strained for the reversed electric field. The domain walls show a flat-top shape at a moment (Fig. 5(c)).At the outer of the spot, the laser intensity decreases rapidly, causing a lower concentration of light excited holes. In this region, the light-assisted reversed domains need to grow deeper and larger in order to accumulate enough holes for boundary charge compensation, some of them even cross the whole thickness. Because of merging with each other, domains with lower wall energy in this region become more stable in the consequent poling process (Fig. 5(d)). Finally, with the steady electric field appling, the light-assisted domain reversal complete as shown in Fig. 5(e)).
In a summary, we investigated the evolution of the light-assisted domain nucleation and growth by digital holographic interferometry, and found a flat-top reversed domain in the crystal and a Gaussian space charge field. From the study about the domain walls mapping and space charge field in crystal, we propose a model to explain the process of light-assisted domain reversal.
This work is supported by the Research Fund for the Doctoral Program of Higher Education of China (200800551019), the Fundamental Research Funds for the Central Universities (65010941 & 65030091), the Tianjin Natural Science Foundation (10JCYBJC02800), the National Basic Research Program of China (2010CB934101), and the Chinese National Key Basic Research Special Fund (No. 2011CB922003). The authors would like to thank the referees for their valuable comments and suggestions.
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