Mg-doped congruent composition LiTaO3 (MgLT) crystal, which can be grown by a conventional Czochralski method, has improved properties such as transparent range, thermal conductivity, and coercive field compared to conventional undoped congruent LiTaO3. In this paper, various properties of MgLT including Mg-doping dependence are characterized, and also compared to that of undoped congruent LiTaO3, LiNbO3, and Mg-doped congruent LiNbO3, as a material of high power quasi-phase matching (QPM) device. Up to 3-mm-thick periodically poled MgLT crystal is shown to demonstrate the possibility of large-aperture QPM-MgLT devices. Subsequently, optical parametric oscillation experiments by using periodically poled MgLT are demonstrated to discuss an efficient QPM condition.
©2008 Optical Society of America
Various types of ferroelectric crystals have been used for materials of high-efficiency nonlinear-optical wavelength conversion by using a quasi-phase matching (QPM) technique [1, 2]. Crystals such as LiNbO3 (LN) [3–8], LiTaO3 (LT) [9–15], and KTiOPO4 [16, 17] are typical materials for the QPM devices with periodically poled structures. Although conventional LN crystal with congruent composition shows large nonlinear optical properties, its low damage threshold against photorefractive damage and its high coercive field to invert the crystal polarization have prevented its practical high-power application with large device-aperture. Doping of Mg ions into the congruent LN has drastically changed these situations. Mg-doped congruent LN (MgLN) shows an improved photorefractive-damage resistance and a relatively low coercive field with keeping its large nonlinear coefficients [18–20]. In last several years, many groups including us have reported efficient wavelength conversion such as optical-parametric oscillation (OPO) and second harmonic generation (SHG) using periodically poled MgLN devices [4, 5, 7, 8].
Properties of LT are similar to that of LN. Moreover, LT has a shorter absorption edge at ultraviolet (UV) region compared to that of LN although the nonlinear susceptibility of LT is smaller than that of LN, which is assumed to be suitable for the highly efficient QPM device pumped by a high-power laser source. For instance, we can expect to reduce the crystal damages, caused by the existing small absorption in green or blue region at high power SHG of near infrared lasers, or caused by unwanted, imperfectly phase-matched higher harmonics (for example: SHG from sum frequency wave of pump and signal waves) at high power OPO.
Recently, LT with stoichiometric composition (SLT) has been developed and used for many efficient QPM applications [9–15]. Crystal preparation techniques of SLT by double-crucible-Czochralski (DCCZ) method  and by vapor transport equilibrium (VTE) method [21, 22] have been reported. Also, SLT crystal with Mg-doping shows superior characteristics in various points such as coercive field, thermal conductivity, transparent range, and photorefractive-damage resistance. Detailed reports on the various properties of SLT crystals have been already reported [9–15, 22, 23].
Nonlinear optical coefficient is an important parameter for material of nonlinear optical device. Excellent comparative measurements about the nonlinear coefficients of various LN crystals of congruent and stoichiometric compositions with/without Mg doping were reported by I. Shoji et. al., [20, 23], which concluded that the nonlinear coefficients of the various LN crystals have the same magnitude within the measurement accuracy. From these results, we can expect that the nonlinear coefficients of the various LT crystals also have the same magnitude.
In this paper, we focused on the recently developed highly Mg-doped congruent LT (MgLT) crystal. Previous works on MgLT with a few mol% Mg were reported to obtain high quality crystals with increased optical damage resistance and transparency [24, 25]. The doping of Mg ions into undoped congruent LT can bring improved material properties, which are the same improvements as the Mg-doping into the undoped congruent LN. Also, MgLT as a congruent-composition crystal have abilities of stable and mass production by using a conventional Czochralski method, which is different from stoichiometric-composition crystals using DCCZ method or VTE method. Here, various properties of MgLT including Mg-doping dependence are characterized, and also compared to that of LT, LN and MgLN crystals characterized by the same method. Also, periodical poling of up to 3-mm-thick MgLT crystal is shown to demonstrate the possibility of large-aperture QPM-MgLT devices. Finally, the first OPO experiments by using periodically poled MgLT are presented to compare with the calculation by using a Sellmeier equation for LT.
2. Crystal properties of MgLT
2.1 Transmission range
MgLT crystals with Mg-doping up to 7 mol% were prepared for characterization. In this work, the doping level of Mg means the Mg amount in the melt state at the Czochralski method. Note that all crystals in this paper are provided by Yamaju Ceramics Co., Ltd.. In this paper, Mg-doped congruent LT with Mg-doping from 1 mol% to 7 mol% are labeled as MgXCLT (X=1~7), and undoped congruent LT, undoped congruent LN, and 5 mol% Mg-doped congruent LN are labeled as CLT, CLN, and Mg5CLN, respectively.
The transmission spectra for both ordinary wave (o-wave) and extraordinary wave (e-wave) in UV and mid-infrared (MIR) regions of 1-mm-thick Mg7CLT are measured and compared with that of CLT and Mg5CLN as shown in Fig. 1. The transmission spectra in UV region (measured by U-3500, Hitachi) does not have a significant difference between o-wave and e-wave as shown in Figs. 1(a) and 1(b), and the absorption-edge wavelength (defined as the wavelength of absorption coefficient =10 cm-1) shows small shift by 5 nm from 277 nm in CLT to 272 nm in Mg7CLT. Also, the broad-range absorption around 300 nm region which exists in CLT crystal could be decreased in Mg7CLT. These absorption-edge shift and absorption decrease in Mg7CLT are effective improvements for preventing damages originated by the existing small absorption and the imperfectly phase-matched higher harmonics as noted before. On the other hand, the transmission spectra in MIR region (measured by FT/IR-6300, JASCO) are shown in Figs. 2(c) and 2(d). Although we don’t have a large difference between Mg7CLT and CLT, the merit of Mg7CLT compared to Mg5CLN on the transmission spectra is obvious. Higher energy OPO using QPM-Mg7CLT compared to QPM-Mg5CLN can be expected because of its wide transparent range in MIR region.
2.2 Thermal conductivity
The thermal conductivity κ of crystal is one of important parameters especially for high power laser applications. We previously reported precise evaluations of κ in various laser crystals and nonlinear materials with a quasi-one-dimensional (q1D) flash method . The κ can be derived by κ=ρ C p D, where ρ, C p, and D are density, heat capacity, and thermal diffusivity, respectively. The q1D flash method enables the precise measurement of D, which largely depends on the crystal composition and quality. Further information about the q1D flash method are presented in our previous report . The ρ of 7.4 g/cm3 and 4.6 g/cm3 for LT and LN were used for calculation. The C p of crystals were measured by a differential scanning calorimeter (DSC 204 F1, NETZSCH). Table 1 presents the measured κ of each crystals at 25°C by the q1D flash method with the measurement accuracy of 5%. In general, κ of LT is higher than that of LN, and the Mg-doping to some extent can improve κ in both LT and LN for both x-axis and z-axis. The measured κ of Mg7CLT in z-axis shows higher value than that of both CLT and Mg5CLN by about 10%, which is suitable for high power application using Mg7CLT.
2.3 Coercive field
The coercive field E c means the electric field to invert the crystal polarization of ferroelectric materials, and limits an aperture of the QPM device. The E c in Mg5CLN is reported to be ~1/6 of that in CLN , and the decreased E c of Mg5CLN enabled us to realize a large aperture QPM device [3–5]. Also, Mg5CLN has a decreased electrical resistance compared to CLN, which prevents an accurate measurement of E c using a conventional hysteresis loop measurement as reported by K. Mizuuchi . Another reported method of E c measurement is to use a ramping electric field with a fixed ramping rate, which has been widely used for evaluating E c of various crystals . We have proposed a different E c characterization method using a ramping electric field with various rates (REFVR) as shown in Fig. 2 .
In the REFVR method as shown in Fig. 2(a), the E c is defined as the applied field when the inversion charge (Q) starts to be observed, and measured with various ramping rates. Characterization of E c dependence on the ramping rate S in the REFVR method enables us to evaluate the response dynamics in the field poling of the material. When a ramping electric field with high ramping rate S 1 (>S) is used for the measurement, the coercive field E cf measured in fast-response crystal (such as CLN) will become almost same as E c, measured by using a ramping electric field with ramping rate S as shown in Fig. 2(b). On the other hand, the coercive field E cs measured in slow-response crystal (such as Mg5CLN) will become higher than E c because of the slow response characteristics of the crystal as Fig. 2(c). The ramping rate S in the REFVR method is comparable parameter with the repetition frequency in the conventional hysteresis loop measurement. The REFVR method enables us to realize a comparable study of E c between various electrical-resistance materials. Further information are presented in our previous report .
In this paper, S is fixed to 1 kV/mm-s for the purpose of an initial evaluation in MgLT with various Mg-doping levels. Circular electrodes of 2 mm diameter were used for the characterization. Figure 3 shows E c dependence on Mg-doping in MgLT, measured at room temperature (RT, ~23°C). The E c decreased with increasing the Mg-doping, and reached to ~3.4 kV/mm at 7 mol% doping, which is less than 1/6 of that in CLT (~21.2 kV/mm) and low enough to fabricate a large aperture QPM device. The E c of Mg5CLN was also measured to be 3.7±0.5 kV/mm by the REFVR method at the same condition . The E c of Mg7CLT was comparably low with that of Mg5CLN. Figure 4 presents the temperature dependence of Ec in Mg7CLT. The E c decreased with increasing the crystal temperature, which is same as that of Mg5CLN , and decreased less than 2 kV/mm at 150°C.
MgLT showed improved properties about transmission range, thermal conductivity, and coercive field compared to CLT, which are suitable for the material of high power QPM device.
3. Periodical poling in Mg7CLT crystal
Mg7CLT has a sufficiently low E c to realize a large aperture QPM device with a few mm thickness. As noted before, the E c characteristics of Mg7CLT is similar to that of Mg5CLN. Therefore, we can expect to use the same method of temperature elevated field poling technique for Mg7CLT with that for Mg5CLN as noted in our previous report [4, 19]. Mg7CLT crystals of z-cut with 1 and 3 mm thickness are prepared for demonstration of periodical poling in MgLT. An aluminum electrode of 0.1 µm thickness with periodical structure was fabricated on the crystal by a conventional vacuum evaporation method. The period of the aluminum electrode was set to ~30 µm, which is suitable for optical parametric oscillation/generation using a 1.064 µm pumping laser. The temperature elevated field poling was done at 150°C in an insulation-oil bath. Periodically poled structure could be realized by high voltage application of ~2.8 kV for 1 mm thick crystal and ~8 kV for 3-mm-thick crystal for the first time to Mg7CLT crystal. The process temperature of 150°C in the Mg7CLT is slightly higher compared to that in Mg5CLN (~120°C) as reported in our recent report . Although we tried at a lower process temperature in initial stage of the Mg7CLT poling, we could not obtain a uniform periodic structure in x-y plane of the Mg7CLT. These results mean that the crystal temperature affects not only to the coercive field but also the uniformity in periodical poling process. Figures 5(a)–5(d) show photographs of etched y-face in 3-mm-thick Mg7CLT after the poling process. Although the obtained periodical structures have a wedged shape with a superior periodicity near +z surface to that near -z surface, penetrating structures with ~30 µm period from +z surface to -z surface in 3-mm-thick Mg7CLT can be clearly confirmed. Further improvements for the periodical poling in Mg7CLT are the next step for efficient large-aperture QPM device using Mg7CLT.
4. OPO using 1-mm-thick periodically poled Mg7CLT
An estimation of QPM period by using a Sellmeier equation of material is important to fabricate an efficient QPM device. Although we do not have any Sellmeier equation for MgLT, the Sellmeier equation with temperature-dependent dispersion relation for CLT is reported, which is effective in the region of 0.4–4 µm within a temperature range of 25–300°C . For the purpose of comparison between experimental results of QPM-OPO using Mg7CLT and calculations using a Sellmeier equation of CLT, we demonstrated the first OPO experiments using 1-mm-thick periodically poled Mg7CLT devices of 40-mm length with QPM period of 30.0, 31.0, 32.0, 32.3 µm. A Q-switched Nd:YAG laser (Spectra Physics, LAB-170-30) with 1.064 µm wavelength (Rep. frequency =30 Hz, Pulse duration =10 ns) was used as a pump source of the OPO experiments. Figure 6 presents the measured dependence of OPO output signal wavelength on the QPM period at RT (~23°C), which is compared with the calculation obtained by the Sellmeier equation of CLT . The experimental results at RT are close to the substitutive calculation at -10°C. These results indicate that we can substitute the Sellmeier equation of CLT with the temperature shift of about -33°C for that of Mg7CLT to estimate the QPM period in OPO.
We have characterized the crystal properties of MgLT, and demonstrated the periodical poling in 3-mm-thick Mg7CLT with ~30 µm period. Subsequently, the fist OPO demonstrations using 1-mm-thick periodically poled Mg7CLT could be realized, and discussed the phase matching condition by using a Sellmeier equation for CLT.
The MgLT could be a good candidate for the practical material of high power QPM device, because it can be grown by a conventional Czochralski method and it has improved transparent range, thermal conductivity, and coercive field. The characteristics of coercive field in MgLT is similar to that of MgLN, which is suitable for fabricating an improved large-aperture QPM device using MgLT by the temperature elevated field poling technique developed for MgLN. We can expect the realization of high power OPO using large-aperture periodically poled MgLT in near future.
The authors would like to acknowledge Yamaju Ceramics Co., Ltd. for providing the MgLT crystals. This research was partially supported by a Grant-in-Aid for Young Scientists (B) 19760038, from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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