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Crystals 2018, 8(8), 322; doi:10.3390/cryst8080322

Article
Photorefractive Properties of Molybdenum and Hafnium Co-Doped LiNbO3 Crystals
1
School of Physics, Nankai University, Tianjin 300071, China
2
The MOE Key Laboratory of Weak-Light Nonlinear Photonics and TEDA Institute of Applied Physics, Nankai University, Tianjin 300457, China
*
Authors to whom correspondence should be addressed.
Received: 10 July 2018 / Accepted: 10 August 2018 / Published: 13 August 2018

Abstract

:
A series of LiNbO3: Mo, Hf crystals with 0.5 mol % fixed MoO3 and various HfO2 concentrations (0.0, 2.0, and 3.5 mol %) were grown by the Czochralski technique. The photorefractive properties of the LiNbO3: Mo, Hf crystals were investigated by two-wave coupling measurements and the beam distortion method was employed to obtain the optical damage resistance ability. The UV-visible and OH absorption spectra were also studied. The experimental results imply that the photorefractive properties of LiNbO3: Mo crystals at laser wavelengths of 532, 488, and 442 nm can be greatly enhanced by doping HfO2 over the threshold concentration. At 442 nm especially, the response time of LN: Mo, Hf3.5 can be shortened to 0.9 s with a diffraction efficiency of 46.07% and a photorefractive sensitivity reaching 6.28 cm/J. Besides this, the optical damage resistance at 532 nm is 3 orders of magnitude higher than that of the mono-doped LiNbO3: Mo crystal, which is beneficial for applying it in the field of high-intensity lasers.
Keywords:
LiNbO3 crystal; molybdenum and hafnium co-doping; photorefractive properties

1. Introduction

Lithium niobate (LiNbO3, or LN) crystal is one of the most prominent materials for applications in many practical fields, such as optical modulators [1], holographic storage [2], waveguides [3,4], resonators [5], and integrated optics devices, resulting from its superior and diverse physical performance [6,7]. In recent decades, crystal growth, defect structures, photorefractive properties, phase-matching, and theoretical simulations have been studied in depth and substantial research progress has been reported for LN crystals [7,8,9,10]. Meanwhile, the control of photorefraction, i.e., saturated diffraction efficiency, response time, etc., is one of the most important research fields. The incorporation of transition metal ions, such as Fe, Cu, and Mn, can enhance the photorefractive properties of LN crystals [11,12,13], while Mg, In, Zr, and Hf ions would increase the optical damage resistance of LN crystals [14,15,16]. Recently, Tian et al. reported that hexavalent Mo6+ doping can tremendously enhance the photorefractive properties of LN crystals [17]. Nevertheless, similarly to mono-doped Fe ions, there are still many obstacles, including low optical damage resistance and insufficient response time, which restrict their use in commercial applications. We know that the double or triple co-doping of iron-doped lithium niobate crystals can improve these properties to some extent [18,19,20]. The subsequent work of Tian et al. draws the conclusion that when the concentration of bivalent Mg2+ exceeds the threshold, the response time of LN: Mo, Mg crystal can be greatly shortened (0.22 s at 351 nm), but tetravalent Zr4+ drastically reduces the photorefractive properties of LN: Mo crystal [21]. Depending on whether the effect of the valence state of dopants on the properties of lithium niobate has a common character, it is necessary to add other tetravalent dopants into LN: Mo crystal to explore the inherent law. On the other hand, the doping threshold of tetravalent dopants, such as Hf4+, Sn4+, and Zr4+, is low and the distribution coefficient is close to 1.0, which is favorable for the growth of high-quality crystals. However, based on our previous work experience, it is easier to grow Hf4+-doped LN than Sn4+-doped LN.
In this work, Hf4+ was selected as the co-doped dopant for LN: Mo crystal. Hf4+ and Mo6+ co-doped LN crystals with various Hf4+ doping levels were grown and their photorefractive properties and UV-visible and OHabsorption spectra were investigated. Our goal was to ascertain the impact of Hf4+ on the photorefractive performance of LN: Mo crystal.

2. Materials and Methods

The conventional Czochralski method was applied to grow LN: Mo, Hf crystals [22]. On the basis of previous work, the optimal results were obtained when the doping concentration of Mo6+ was 0.5 mol %, and for Hf4+ the doping threshold is between 2.0 and 2.5 mol % [17,23]. Therefore, we fixed the concentration of Mo6+ at 0.5 mol % and selected the Hf4+ concentrations of 0.0, 2.0, and 3.5 mol %, which are labeled as LN: Mo, LN: Mo, Hf2.0, and LN: Mo, Hf3.5, respectively. The [Li]/[Nb] ratio for all the samples was 48.38/51.62. A congruent LN crystal was also grown for comparison and named CLN. The as-grown crystals were polarized in air at 1150 °C; the polarization current was 30 mA and the duration time was 15 minutes. Finally, we cut 3.0-mm-thick and 1.0-mm-thick y-oriented samples from the crystals and polished them to optical grade.
We used the 3-mm-thick y-oriented plates to study the photorefractive properties of LN: Mo, Hf crystals by the two-wave coupling method. The laser wavelengths we selected for the measurements were 442, 488, and 532 nm from a (continuous wave) CW frequency-doubled solid-state laser, an Ar+ laser, and a He-Cd laser, respectively. An optical splitter was employed to split the extraordinary polarized laser into two equal-intensity beams (400 mW/cm2 each), which were then reflected by mirrors and incident on the crystals at an appropriate crossing angle. In order to utilize the maximum electro-optic coefficient γ33, the grating vector should be kept along the c axis of the crystal. The intensity of the diffracted light Id and transmitted light It was measured directly by the detectors after passing through the crystal. We defined the diffraction efficiency as η = Id/(Id + It) and then fitted the curve of “η” versus time “t” by the function ηt = ηs (1 − e−t/τr)2 to get the saturated diffraction efficiency ηs and the response time τr. Further, the photorefractive sensitivity is defined as S = ((∂√η)/∂t)t = 0/Id. Here, “I” is the recording light intensity and “d” is the thickness of the samples.
The optical damage resistance ability of all samples was investigated by the beam distortion method and a sample size of 3-mm-thick y-oriented plates. When the laser is focused on the crystal, the transmitted beam will become distorted once the laser intensity exceeds a certain value: we consider this intensity as the optical damage threshold of the crystals. We obtained the optical damage threshold for the crystals by directly observing the shape of the transmitted beam spot. The 532 nm wavelength laser (the CNI MGL-III-532 laser, Changchun New Industries Optoelectronics Tech. Co., Ltd., Changchun, China) was selected for this experiment; also, we used a 30-mm lens for focusing the beam into the crystal to obtain a higher light intensity.
The UV-visible absorption spectra were measured by a UV-4100 spectrophotometer (Hitachi Science and Technology, Tokyo, Japan) with a range of 300–800 nm and resolution of 1 nm. A MAGNA-560 FT-IR spectrometer (Thermo Nicolet Corporation, Madison, America) was used for infrared (IR) spectroscopy with a test range of 400–4000 cm−1 and a resolution of 2 cm−1. Both of the samples were 1-mm y-oriented plates for the spectrum measurements and the incoming light was unpolarized.

3. Results

3.1. Two-Wave Coupling Measurements

Figure 1 shows the photorefractive properties of LN: Mo, Hf crystals at the 442, 488, and 532 nm laser wavelengths. The saturated diffraction deficiency (ηs) is described in Figure 1a, the response time in Figure 1b, and the sensitivity in Figure 1c. We can see from Figure 1a that, compared with CLN, the saturated diffraction efficiency of all LN: Mo, Hf crystals is improved. The ηs of each sample measured at 442 nm is higher than that at 488 nm and 532 nm. The ηs of LN: Mo, Hf3.5 crystal can reach 46.07% at the 442 nm laser wavelength, which is about 10 percent higher than that of LN: Mo crystal. However, the ηs slightly decreased for LN: Mo, Hf2.0 with respect to LN: Mo at all three wavelengths and for LN: Mo, Hf3.5 at 488 and 532 nm. As shown in Figure 1b, the response time of all LN: Mo, Hf crystals has been greatly shortened compared with that of CLN crystal. For LN: Mo, Hf2.0, there is no amendment in the response speed compared to LN: Mo crystal. However, for LN: Mo, Hf3.5 crystal, when the concentration of Hf exceeds the threshold, we can obtain the minimum response times of 1.93 s, 2.06 s, and 0.9 s at 532 nm, 488 nm, and 442 nm, respectively, which are several times lower than that of LN: Mo crystal. Moreover, Figure 1c indicates that the sensitivity for all samples is greatly enhanced at 442 nm and slightly different in value at 488 nm and 532 nm, respectively. For LN: Mo, Hf3.5, we obtain the highest sensitivity of 6.28 cm/J at 442 nm.
The above experimental results indicate that the incorporation of Hf4+ above the threshold can greatly enhance the photorefractive properties of LN: Mo crystal.

3.2. Optical Damage Resistance

Figure 2 shows the transmitted light beam distortion of LN: Mo, Hf samples, which were irradiated by a 532-nm wavelength laser for 5 min. CLN crystal is also listed for comparison. As shown in Figure 2d, the optical damage threshold for LN: Mo, Hf3.5 crystal was found to be about 1.54 × 104 W/cm2; which is 3 orders of magnitude higher than optical damage threshold for mono-doped Mo6+ crystals (Figure 2b, the transmitted light beam was blurred at the very low intensity of 5.93 × 10 W/cm2). LN: Mo, Hf2.0 crystal is unable to withstand a laser power density of 1.54 × 104 W/cm2 (Figure 2c).
The LN: Mo crystal has good photorefractive properties, but it exhibits optical damage at a very low light intensity. Obviously, it is not conducive to commercial applications of high laser intensities. The above results suggest that Hf incorporation is an effective way to enhance the optical damage resistance of LN: Mo crystal, and can also advance its application in the field of high-intensity lasers.

3.3. UV-Visible Absorption Spectra

We employed a UV-4100 spectrophotometer to measure the UV-visible absorption spectra. Figure 3 shows the results of the UV-Vis absorption spectra for LN: Mo, Hf crystals with different Hf4+ doping levels along with those of CLN crystal for comparison. The inset figure shows the absorption spectra near the absorption band edge. Compared with CLN crystal, there are noticeable red-shifts for LN: Mo crystals. With an increase in Hf4+ doping concentration, violet-shifts occur for LN: Mo, Hf crystals compared to LN, Mo crystal.
Based on the accumulated results, Mo6+ in the LN crystal structure preferentially occupies the Nb sites and pushes regular Nb5+ to Li sites, forming new defects of MoNb+ [17]. A large number of anti-site NbLi4+ cause a significant red-shift in the absorption edge of LN: Mo crystals. With the doping of Hf4+ in LN: Mo crystal, NbLi4+ will be mostly substituted by Hf4+; thus, a violet-shift of the absorption edge occurs in LN: Mo, Hf3.5 crystal.

3.4. OH Absorption Spectra

The infrared (IR) spectroscopy of samples was measured within the test range of 400–4000 cm−1 and the resolution of 2 cm−1. It is well-known that there is a threshold concentration for optical-damage-resistant dopants in LN crystals, which would result in analogous characteristic movements of the OHabsorption peak [24]. The OH absorption peak of the CLN crystal is about 3484 cm−1, so for comparison we only took values between 3420 cm−1 and 3560 cm−1 and then normalized them. Based on previous reports, the shift of Hf-doped LiNbO3 crystals is very slight [23], and it is necessary to study the internal structure of the OHabsorption peak in more detail. The OH absorption band of CLN crystal is broad, and we can decompose it into three peaks, in which the peak of 3466 cm−1 is thought to be caused by the direct substitution of protons for Li+ ions, while the 3481 and 3489 cm−1 peaks are due to protons occupying the Li vacancy (VLi) in two different ion environments near NbLi4+ [25]. Moreover, according to the shape of the Hf-doped LiNbO3 difference spectra, it is conceivable that the absorption peak can also be decomposed into three peaks [26,27].
The OH absorption spectra of LN: Mo, Hf and CLN crystals are shown in Figure 4. Just as expected, there is only a slight shift of the OH vibration absorption band for both LN: Mo, Hf2.0 and LN: Mo, Hf3.5 crystals. Therefore, the three-decomposed-peaks model was employed. We separately decomposed the absorption bands of LN: Mo, Hf crystals into three peaks by Lorentz fitting. We tried to fit the peak of 3466 or 3475 cm−1 [28] for both LN: Mo, Hf2.0 and LN: Mo, Hf3.5 crystals, but the fit was not good; only when the peaks were located at 3481, 3490, and 3500 cm−1 could we obtain good fittings. We can see in Figure 5a that for LN: Mo, Hf2.0, the peak at 3490 cm−1 is the highest. As shown in Figure 5b, the three fitted peaks of LN: Mo, Hf3.5 crystal are also located at 3481, 3490, and 3500 cm−1, but the highest peak is at 3481 cm−1 and the lowest one is at 3500 cm−1. It is necessary to further discuss the influence of ion locations on the OHstretching vibration spectra.

4. Discussion

According to previous reports, in mono-Hf-doped CLN crystals, due to the strong electronegativity of Hf Li 3 + , the components near 3484 cm−1 are formed by H+ occupying VLi around not only Nb Li 4 + ions but also Hf Li 3 + ions, and the H+’s direct compensation for the negatively charged Hf Nb should be the reason for the component of 3500 cm−1 [25]. Besides this, the OHvibrational frequency is found at 3475 cm−1 in Hf-doped stoichiometric LN crystals, which represents the HfNb–OH defect [28]. What is more, there are Mo4+, Mo5+, and Mo6+ ions in our samples [17]. All these complex factors make it difficult for us to deduce the specific substitution sites in our samples. Compared with previous results [25,28], the difference may be caused by the complicated valence of Mo ions in LN: Mo, Hf crystals; consequently, this needs further investigation.
As reported, Zr4+ cannot improve the response speed of LN: Mo crystals even when its doping concentration exceeds the threshold. When the concentration of Mg2+ exceeds the threshold, very short photorefractive response times of 0.22 s, 0.33 s, 0.37 s, and 1.2 s for 351, 488, 532, and 671 nm, respectively, were obtained [21]. From the present results, when the Hf4+ concentration exceeds the threshold, it can greatly enhance the photorefractive properties of LN: Mo crystal, though the effects are still slightly weaker as compared with those of Mg2+. Firstly, we believe that the defects related to Mo ions (Mo4+, Mo5+, and Mo6+) play the main role of photorefractive centers in our samples. Compared with Mg2+, when the doping concentration of Zr4+ and Hf4+ reaches the threshold, ZrLi3+ and HfLi3+ could not completely replace the MoLi3+ resulting from Zr4+, as Hf4+ and Mo4+ have the same valence. This would lead to a lower concentration of MoNb+ in LN: Mo, Zr and LN: Mo, Hf crystals. In addition, ZrNb–MoNb+ and HfNb–MoNb+ will form more stable defect clusters than MgNb3−–3MoNb+, which actually weaken the role of MoNb+ as a photorefractive center in LN: Mo, Zr and LN: Mo, Hf crystals. Therefore, the photorefractive properties of both samples cannot be significantly enhanced compared with LN: Mo, Mg crystals. Secondly, as for Zr4+ and Hf4+, Hf4+ has the same valance as Zr4+. Checking the periodic table of elements, we can find that Zr and Nb are adjacent elements in the same cycle while Hf is in the next cycle. This shows that Nb and Zr have more similar properties; hence, Zr4+ would be more likely to occupy Nb sites than Hf4+, and it is easier for Hf4+ to replace Mo ions at the Li sites than it is for Zr4+. Consequently, the amount of MoNb+ in LN: Mo, Hf crystals will be greater than that in LN: Mo, Zr crystals; then, LN: Mo, Hf crystals will have better photorefractive properties than LN: Mo, Zr crystals. Finally, these assumptions need further investigation.

5. Conclusions

Lithium niobate crystals co-doped with molybdenum and hafnium were grown and their photorefractive properties were studied with 532, 488, and 442 nm wavelength lasers. The results clearly describe that, different from the incorporation of Zr4+, which drastically reduces the photorefractive properties of LN: Mo crystal, Hf4+ co-doping can greatly enhance the photorefractive properties of LN: Mo crystal when the Hf4+ doping concentration is over the threshold. The doping of hafnium over the threshold is an effective method to improve the photorefractive properties of LN: Mo crystals. In detail, the optical damage resistance of LN: Mo, Hf3.5 crystal was 3 orders of magnitude higher than that of mono-doped Mo: LiNbO3 crystal at the 532 nm laser wavelength. The response time of LN: Mo, Hf3.5 crystal can be shortened to 0.9 s with a diffraction efficiency of 46.07%, and the photorefractive sensitivity can reach 6.28 cm/J at the 442 nm laser wavelength. These results can play a good role in further tuning the performance of LN crystals and promoting their practical application.

Author Contributions

Conceptualization, L.Z., D.Z., and Y.K.; Funding acquisition, D.Z. and Y.K.; Investigation, L.Z.; Methodology, L.Z., D.Z., S.L., S.C., and L.Z.; Project administration, Y.K. and J.X.; Resources, S.L., S.C., L.Z., and J.X.; Writing (original draft), L.Z.; Writing (review and editing), D.Z., S.S., S.W., and H.L.

Funding

This research was funded by the National Natural Science Foundation of China with grants [11674179] and [61705116] and the Program for Changjiang Scholars and Innovative Research Team in University with grant [IRT_13R29].

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Photorefractive properties of Lithium niobate (LN): Mo crystals co-doped with different concentrations of Hf with a light intensity of 400 mW/cm2 per beam at laser wavelengths of 532, 488, and 442 nm. Control LN (CLN) crystal is also presented for comparison. (a) Diffraction efficiency; (b) Response time; and (c) Sensitivity.
Figure 1. Photorefractive properties of Lithium niobate (LN): Mo crystals co-doped with different concentrations of Hf with a light intensity of 400 mW/cm2 per beam at laser wavelengths of 532, 488, and 442 nm. Control LN (CLN) crystal is also presented for comparison. (a) Diffraction efficiency; (b) Response time; and (c) Sensitivity.
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Figure 2. Transmitted laser beam spots of an incident 532-nm wavelength laser after 5 min of irradiation: (a) CLN; (b) LN: Mo; (c) LN: Mo, Hf2.0; (d) LN: Mo, Hf3.5. The light intensity was 59.3 W/cm2 for (a,b) and 1.54 × 104 W/cm2 for (c,d).
Figure 2. Transmitted laser beam spots of an incident 532-nm wavelength laser after 5 min of irradiation: (a) CLN; (b) LN: Mo; (c) LN: Mo, Hf2.0; (d) LN: Mo, Hf3.5. The light intensity was 59.3 W/cm2 for (a,b) and 1.54 × 104 W/cm2 for (c,d).
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Figure 3. Absorption spectra of LN: Mo, Hf and congruent LN crystals, while the inset is the absorption spectra near the absorption edge.
Figure 3. Absorption spectra of LN: Mo, Hf and congruent LN crystals, while the inset is the absorption spectra near the absorption edge.
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Figure 4. The OH spectra of LN: Mo, Hf crystals with different concentrations of Hf4+. The spectrum of CLN is presented for comparison.
Figure 4. The OH spectra of LN: Mo, Hf crystals with different concentrations of Hf4+. The spectrum of CLN is presented for comparison.
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Figure 5. The components of OH absorption spectra of LN: Mo, Hf crystals. (a) LN: Mo, Hf2.0; (b) LN: Mo, Hf3.5.
Figure 5. The components of OH absorption spectra of LN: Mo, Hf crystals. (a) LN: Mo, Hf2.0; (b) LN: Mo, Hf3.5.
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