Near-stoichiometric zirconium-doped lithium niobate crystals were fabricated and their optical damage resistance was investigated. It was found that these crystals can withstand a light intensity of 20 MW/cm2 at 514.5 nm cw laser, 80 GW/cm2 at 532 nm pulse laser, and 120 kW/cm2 at 351 nm cw laser. The minimum switching field is only 1.00 kV/mm for 0.5 mol% zirconium-doped lithium niobate crystal. These properties suggest that the near-stoichiometric zirconium-doped lithium niobate crystals will be an excellent candidate for quasi-phase matching technique.
© 2011 OSA
Quasi-phase matching (QPM) technique [1,2] is widely used in the field of high efficient laser frequency conversion. The photonic devices based on QPM have an amazing future, which have several features, such as high conversion efficiency, small size, easy to use, etc. Various types of ferroelectric crystals have been used for high-efficiency nonlinear-optical wavelength conversion by QPM technique. Crystals such as LiNbO3 (LN) [3,4], LiTaO3 (LT) , and KTiOPO4 (KTP)  are typical materials for QPM devices with periodically poled structures.
Although conventional normally pure congruent LN (CLN) crystals show large nonlinear optical properties, its low light intensity threshold against laser-induced optical damage (also named photorefraction) and its high coercive field for polarization reversal have prevented its practical high-power applications with large device aperture. Doping of Mg ions into CLN has drastically changed these situations. When the doping concentration up to 4.6 mol% (so-called threshold value), Mg-doped CLN (Mg:CLN) [7,8] shows a greatly improved optical damage resistance and a much lower coercive field while keeping its large nonlinear coefficients. So the periodically poled magnesium-doped lithium niobate (PPMgLN) is the most commonly medium in frequency conversion with QPM technique. However, the distribution coefficient of magnesium ion in Mg:CLN is large (~1.2) and the threshold (usually 5 mol % MgO for CLN) is high, which makes it difficult to grow Mg:CLN crystals with high optical quality.
In the recent years, several doping crystals [9–12] were discovered to have the similar resistance as Mg:CLN against optical damage. Zirconium-doping crystal [13,14] is one of them, which not only suppress optical damage in visible region but also in ultraviolet (UV) region, moreover, the distribution coefficient is close to one. However, the switching field of congruent zirconium-doped lithium niobate (Zr:CLN) is higher than that of Mg:CLN, which is not superior for QPM applications.
In this letter, we investigate the near-stoichiometric zirconium-doped LiNbO3 (Zr:NSLN) crystals. The experimental results show Zr:NSLN crystals not only have a resemble optical damage resistance as near-stoichiometric MgO-doped lithium niobate (Mg:NSLN) but also have a lower switching field than that of Mg:NSLN. These properties make Zr:NSLN an excellent candidate for photonic devices with QPM technique.
ZrO2-doped congruent LiNbO3 crystals (supplied by R&D Center for Photon-Electro Materials of Nankai University) were grown by Czochralski method along the c-axis. The doping concentrations are 0.5, 1.0, and 1.5 mol% ZrO2. The near-stoichiometric crystals were fabricated by vapor transport equilibrium (VTE) technique  with Li-rich powder in a Li/Nb ratio larger than 2.0. These Zr:CLN crystals were cut into 1.2 mm x and c plates, some of them were treated by VTE at 1373 K for 120 hours. Thereafter these Zr:CLN samples are labeled as Zr05C, Zr10C and Zr15C, and the near-stoichiometric ones as Zr05S, Zr10S and Zr15S, respectively. And all samples were polished to 1.0 mm plates with optical grade. For comparison, near-stoichiometric lithium niobate doped with 1 mol% MgO (Mg10S) was also fabricated.
To characterize the composition of lithium in these near-stoichiometric crystals, the UV-visible absorption spectra were measured. It is well known that the ultraviolet absorption-edge exhibit shifts from 320 nm of CLN toward shorter wavelength when the crystal composition closes to stoichiometry . Here, the absorption edge was defined as the wavelength where the absorption coefficient is equal to 20 cm−1. Therefore we measured the UV-visible absorption spectra at room temperature by a Beckman DU-8B spectrophotometer with light transmitting through these 1 mm thick plates. Figure 1 shows the UV-visible spectra of series of lithium niobate crystals. For comparison, 2.0 mol% ZrO2 (above the doping threshold) doped LN (labeled as Zr20C) and CLN were also shown. According to the relationship between Li/Nb ratio and absorption-edge , the composition of near-stoichiometric crystals are 0.998, 0.997 and 0.996 for Zr05S, Zr10S and Zr15S respectively.
3. Experiments and results
3.1 Highly optical damage resistances in visible-UV range
Optical damage is indicated by a transmitted light beam smeared and elongated along the c axis and furthermore by the light intensity decreased in the central part (as shown in Fig. 2(a) and 2(e)). To evaluate the ability of Zr:NSLN crystals against optical damage, we focused the green laser beam (Ar+ laser, wavelength 514.5 nm, e-polarized) onto the x-cut plates and observe the distortion of the transmitted laser beam spot. Figures 2(b) and 2(f) show the images of the transmitted beam spots after 5 minutes irradiation. We can see the beam spots for near-stoichiometric samples don’t distort under the intensity of 20 MW/cm2, which is 6 orders of magnitude higher than that of CLN and equal to Mg10S. This intensity is the maximum value we could afford at present, so we believe the real value will be higher.
A QPM material is often used with high light intensities. In the above experiments, the Zr05S and Zr10S samples are all able to endure the cw irradiation. Further to verify the intensity of optical damage resistance (I ODR) of these samples, the fundamental and second harmonics of a pulsed Nd:YAG laser was employed, the pulse durations of the output (full-width of half-maximum, Gaussian shapes) were approximately 10 ns and it runs at a repetition rate of 1 Hz. The maximum output energy that our laser source can apply is 120 mJ for the fundamental wave, and 60 mJ for the second harmonic wave. The sample was placed at the focus point (800 mm, focal length), at which the light intensity is about 80 GW/cm2, and the transmitted spot were captured by a CCD. From Figs. 2(c) and 2(g) we don’t find light spot distortion for Zr05S and Zr10S. Compared with conventional lithium niobate and Mg:LN, this value is higher, which is close to that of KDP [18,19].
The optical damage resistance of Zr:LN in UV range is the most advantage compared to Mg:LN. We focused the UV laser beam (Ar+ laser, wavelength 351nm, e-polarized, light intensity 1.2 × 105 W/cm2), put the x-cut plates at the rear focal plane of the lens to observe the distortion of transmitted beams. As shown in Figs. 2(d) and 2(h), the light spot is non-distorted. Thus, the intensity threshold of Zr:NSLN is above 105 W/cm2, which is also the highest intensity now in our laboratory.
For conventional lithium niobate crystals, the laser-induced surface-damage (also named as “gray tracking” or “dark traces” ) limits its usage at high light intensities. We investigated the laser-induced surface-damage with the fundamental wave. To avoid the air breakdown, the samples were placed at the distance of 200 mm from the focus point under the pulse irradiation of 1064 nm. Under the intensity of 8 J/cm2, no laser-induced surface-damage was observed, which is similar to Mg:CLN .
The above results indicate that Zr05S and Zr10S have the same ability against optical damage. Further to compare them quantitatively, the light-induced change of refractive index n e of these crystals was measured with two beam interference holographic recording method in the visible and UV range, respectively. We employed the 514.5 nm laser with an irradiation of 450 mW/cm2 and the 351 nm laser with irradiation of 400 mW/cm2 respectively. The refractive index changes resulting from two beam interference holographic recording are listed in Table 1 . For comparison, we cited some results of doped lithium niobate crystals (such as Mg2+, Zn2+, In3+, and Hf4+ with doping concentration above threshold) in early work. From this table, we can see that the change of refractive index of Zr:NSLN is in the orders of 10−7 in visible range, which is similar to that of Zr:CLN above the doping threshold  and smaller than that of other crystals. In UV range, the change of refractive index of Zr:NSLN crystals is in the orders of 10−6, which is about one order smaller than that of the others.
The former results have indicated that the change of refractive index is inversely related the optical damage resistance, so we can deduce the light intensity that Zr05S can withstand without optical damage is highest in lithium niobate families, both in UV and in visible range.
3.2 Lower switching field
Normally, the lower switching field is, the larger the aperture for QPM is. The domain reversal behavior was investigated by the conventional external electric field method in which a poling field was applied with a uniform liquid electrode consisting of LiCl electrolyte. The electric switching field was defined as the one where the corresponding poling current exceeds 0.1 μA. Figure 3 shows the domain switching field in Zr:LN as a function of zirconium doping. For the near-stoichiometric crystals, the switching field increases with increased doping concentration, while for congruent ones it decreases rapidly. The switching field of Zr05S crystal reaches at 1.00 kV/mm, which is roughly one-twentieth of that of CLN and much lower than that of Zr20C (7.2 kV/mm) and Mg:NSLN (2.35 kV/mm) [13,21].
It is generally accepted that the optical damage of lithium niobate is related with the intrinsic defects, such as anti-site niobium (Nb Li) ions and Li vacancies (V Li), some of them act as photorefractive centers. The optical damage resistant dopants will firstly substitute Nb Li ions and occupy Li-sites when the doping concentration below threshold, which reduce the content of Nb Li and then increase the crystal’s resistance to optical damage. But as the doping concentration above threshold, it is still not well understood that the resistance to optical damage increases with increased doping concentration. For near-stoichiometric crystals, the micro-mechanism for the change of optical damage resistance with composition and doping concentration is also not clear. In this study, the doping concentrations of ZrO2 are below the threshold value in our Zr:CLN samples but above threshold in Zr:NSLN crystals. As we known, the generally defined UV absorption edge of LN at the absorption coefficient of α = 20 cm−1 has direct relationship with the concentration of intrinsic defects. From Fig. 1 we can see the absorption-edge has a violet-shift for Zr:CLN crystals and a red-shift for Zr:NSLN when the doping concentration increased. The reason for the violet-shift of Zr:CLN is Zr ions will replace Nb Li when the doping concentration below the threshold, which reduce the content of intrinsic defects. As for Zr:NSLN, the red-shift indicates the content of intrinsic defects increases with increased doping concentration, which decreases the ability of these crystals against optical damage. Therefore, Zr05S has the strongest ability against optical damage among all our samples. This result coincides with the above consequence of the change of refractive index. Zr05S crystal has not only higher optical damage resistance and lower switching field but also less doping concentration, which is benefit to acquire highly optical quality crystals, so it is an excellent substrate for QPM devices.
Utilizing VTE technique, we fabricated near-stoichiometric zirconium-doping LiNbO3 crystals. The experimental results show that the optical damage resistance of these crystals beyond 20 MW/cm2 at 514.5nm cw laser, 80 GW/cm2 at 532nm (10ns) pulse laser, and 120 kW/cm2 at 351 nm cw laser, which is the highest value in the lithium niobate families. The switching filed of these crystals is lower than that of Zr:CLN, and the minimum value is only 1.00 kV/mm. In summary, the Zr:NSLN crystals, especially 0.5 mol% Zr-doped near-stoichiometric LN (Zr05S), have high optical damage resistant capability in visible-UV range and will be an excellent material for QPM devices.
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, the Tianjin Natural Science Foundation (10JCYBJC02800), National Basic Research Program of China (2007CB307002 and 2010CB934101), and the National Advanced Materials Committee of China (2007AA03Z459). The authors greatly thank the referees for their valuable comments and suggestions.
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