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Abstract
The Yb3+:BaGd2(MoO4)4 crystal was grown by the Czochralski method and investigated. The absorption and emission spectra reveal that this crystal can be efficiently excited by a commercial laser diode (LD) around 970 nm and its strongest fluorescent peak locates at 1013 nm. Besides, a significant frequency-doubling effect and a Raman effect of the crystal were observed in the experiment, which reveals that the rare-earth (RE) ions doped BaGd2(MoO4)4 crystal has the potential to generate the self-frequency-doubling laser and self-Raman laser simultaneously.
© 2017 Optical Society of America
1. Introduction
The nonlinear optical effects, such as second harmonic generation (SHG) and stimulated Raman scattering (SRS), are widely used to extend the spectral coverage of existing laser technologies. By introducing the nonlinear crystals into solid-state laser systems, one can obtain deep ultraviolet lasers [1,2], visible lasers [3–6] and the infrared lasers at eye-safe region [7–9]. However, it also makes the systems more complicated and expensive. For example, only one piece of laser crystal is needed in a fundamental laser system at 1064 nm, while three different types of media are needed in a yellow light laser system at 588 nm [10]: a laser crystal, a Raman crystal and a frequency-doubling crystal. Therefore, the materials that possess laser and nonlinear optical functions simultaneously have attracted the researchers’ attention.
Nowadays, the self-frequency-doubling (SFD) lasers and self-Raman lasers have already been realized with a variety of crystalline materials, respectively. The most notable materials for obtaining SFD lasers are Re:LiNbO3 (Re = Nd, Tm) [11,12] and Re:YAl(BO3)4 (Re = Nd3+, Yb3+) [13,14]. They enable the SFD lasers operating in watt-level [15]. Besides, many crystalline materials were found and employed to obtain efficient self-Raman laser, such as Nd3+:YVO4 [16], Nd3+:GdVO4 [17] and Yb3+:KGd(WO4)2 [18]. Nevertheless, few of materials can generate the SFD and self-Raman laser simultaneously, and no relative research has been reported yet according to the best of our knowledge.
The RE ions doped molybdate crystals have larger absorption and emission bandwidth due to their disordered irregular structure and they are considered as promising gain media for solid-state laser [19,20]. Moreover, lots of the molybdate crystals have strong nonlinear optical effects. On one hand, molybdate crystals are analogous to the tungstate crystals which produce efficient stimulated Raman scattering under the laser excitation [21,22]. On the other hand, it has already been proved that the Gd2(MoO4)3 crystal can be used for the second harmonic generation [23,24]. Thus, we have reasons to believe that there should be a special kind of molybdate crystals can generate the SFD and self-Raman laser simultaneously.
In this paper, the Yb3+:BaGd2(MoO4)4 (Yb3+:BGM) crystal grown by the Czochralski method was investigated. With the excitation of a laser diode at 934 nm, a strong fluorescence at infrared region was observed, meanwhile, the second harmonic generation at 467 nm of the excited laser and its first-stokes generation at 550 nm were observed.
2. Crystal growth
First, the starting materials Yb2O3 (99.99%), BaCO3 (99.95%), Gd2O3 (99.99%) and MoO3 (99.95%) were weighted and mixed according to the following chemical equation:
During the mixing process, extra 0.5 wt.% MoO3 was added to compensate the volatilization of MoO3. Then, the mixed raw materials were heated up to 800 °C for 48 h. This mixing step and heating step were repeated for three times to make sure the solid-state reaction can be carried out adequately. Finally, the Yb3+:BaGd2(MoO4)4 single crystal was grown by the Czochralski method. The pulling rate and rotating rate were set to be 1 mm/h and 16 r/min, respectively. After crystallizing, necking, shouldering, isodiametric and winding up, a single crystal with dimensions about Φ23 mm × 40 mm was obtained, shown as inserted images of Fig. 1(a). This sample was cooled down to room temperature slowly in air atmosphere to reduce the color centers inside the crystal.
Fig. 1 Characteristics of the Yb3+:BaGd2(MoO4)4 sample. (a) The XRD result of the sample. The inset depicts the sample that we grew. (b) TG-DSC curve of the sample.
The phase purity of the sample was checked by X-ray diffraction (XRD). As shown in Fig. 1(a), the positions of diffraction peaks of the sample are in good agreement with the standard pattern of BaGd2(MoO4)4 (PDF#36-0192), which confirms the sample has a single monoclinic phase with a scheelite structure. Moreover, the thermogravimetry coupled with differential scanning calorimetry (TG-DSC) of the sample was demonstrated in Fig. 1(b), where one can see that only one endothermic peak (at 1068.9 °C) can be seen in DSC curve, and no polymorphic modification or chemical reaction exists in this system. The concentration of Yb3+ ion was measured to be 9.27 at. % by inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis. However, a few of Tm3+ ions (0.28 at. %) were found in the sample. This impurity would lead to several up-conversion processes.
3. Results and discussion
Since the BaGd2(MoO4)4 crystal has a perfect cleavage in the (010) plane [25], it is easy to obtain a good quality microchip sample from the crystal. In this experiment, a microchip sample with the thickness of 0.91 mm was cut according to the cleavage plane and used for the investigation. The absorbance and fluorescence spectrum of the sample were tested with the spectral solution of 0.54 nm and 0.5 nm, respectively. As shown in Fig. 2(a), the absorption band (2F7/2→2F5/2) of the Yb3+: BaGd2(MoO4)4 crystal is relatively wide and its strongest absorption peak locates at 977.5 nm, which matched well with the commercially- available high power LD at around 970 nm. Figure 2(b) shows the fluorescence spectrum of the Yb3+:BaGd2(MoO4)4 crystal at room temperature with the excitation at 934 nm. As we can see that the strongest fluorescence peak locates at 1013 nm, which belongs to the 2F5/2→2F7/2 transition. These properties indicate the Yb3+: BaGd2(MoO4)4 crystal can be used for generating the infrared laser around 1.0 μm.
Fig. 2 The absorbance cure (a) and the fluorescence cure (b) of the Yb3+: BaGd2(MoO4)4 crystal sample.
The Raman spectrum of the Yb3+:BaGd2(MoO4)4 crystal sample was detected by the Raman confocal microscopic system with a solution of 0.18 cm−1, and shown as Fig. 3. More than fifteen Raman shifting peaks at the region between 50 cm−1 and 1300 cm−1 were observed which are caused by rotating modes, translation modes and vibration modes of (MoO4)4-. The Raman shifting peaks of the sample locate at 323 cm−1, 854 cm−1 and 952 cm−1are relatively strong.
For investigating the ability of SHG and SRS of the sample, a fiber-coupled LD at 934 nm with a core diameter of 100 μm and a numerical aperture of 0.22 was used for the excitation. The excited laser was focused on the microchip sample surface perpendicularly and the excited power was set to be 1.2 W. The sample was mounted in a copper block, and kept in 25 °C. The corresponding emission spectrum of the sample at visible region was obtained by a spectrograph with a photomultiplier, and shown as Fig. 4. The sharp emission peak at 467 nm is the frequency-doubling line of the excitation laser at 934 nm, and the broad emission peak around 474 nm was caused by the energy transfer up-conversion. Since a few of Tm3+ ions were mixed with the sample, the energy transfer process between Yb3+ ions and Tm3+ ions would enable the electrons jumping to the upper energy level of Tm3+ ions and lead to the up-conversion emission (1G4→1H6) [26]. The emission peak at 550 nm should be the Raman scattering of the radiation at 467 nm, which matches with the Raman shift of 323 cm−1 of the Yb3+:BaGd2(MoO4)4 crystal perfectly.
Fig. 4 Emission spectrum of the Yb3+: BaGd2(MoO4)4 crystal sample at visible region (a) and the emission spectrum of the excited laser (b).
To investigate the nature of the frequency-doubling and the Raman shifting of this crystal, we used the same laser source to excite the non-doped BaGd2(MoO4)4 crystal sample. Its emission spectrum at visible region is shown in Fig. 5, where one can see that the frequency-doubling peak at 467 nm and its Raman peak at 550 nm still exist. Besides, without the influence of the rare-earth ion impurity, the Raman peak at ~487 nm that matches the Raman shifting of 85 cm−1 also can be observed clearly.
Fig. 5 Emission spectrum of the non-doped BaGd2(MoO4)4 crystal sample at visible region. The inset depicts the non-doped BaGd2(MoO4)4 crystal that we grew.
The thickness of the Yb3+:BaGd2(MoO4)4 crystal sample is only 0.91 mm, which means the effective length is relatively short for the frequency-doubling and Raman shifting. Besides, the excited laser operated in continue wave status, whose intensity is much lower than that of the Q-switched laser. However, both of these two effects were still observed from the sample, which indicated the Yb3+:BaGd2(MoO4)4 crystal has the potential to generate the self-frequency-doubling laser and self-Raman laser simultaneously. These special characteristics are determined by its structure (shown as Fig. 6). On one hand, as a crystal with the monoclinic structure, the BaGd2(MoO4)4 crystal has a relative low symmetry, and the low symmetry leads to a large effective nonlinear coefficient. On the other hand, as a member of the “molybdate family”, the BaGd2(MoO4)4 crystal exhibit high Raman gain coefficient.
4. Conclusion
In this research, the Yb3+:BaGd2(MoO4)4 crystal was grown by the Czochralski method and its optical characteristics were investigated. Experimental results demonstrated that this kind of crystal has a broad absorption and emission band, and is suitable for generating laser at around 1.0 μm. Besides, the significant frequency-doubling and Raman shifting effects were observed when the crystal sample was excited by the LD at 934 nm, with an effective path length of less than 1 mm. On the basis of the characteristics mentioned above, by considering the advantages of high optical damage threshold and low quantum defect of the crystal, we believe that Yb3+:BaGd2(MoO4)4 crystal can be used as a multi-functional material for generating self-frequency-doubling and self-Raman laser simultaneously.
Funding
National Science Foundation (NSF) (Grant Nos. 61475067, 61605062); the National Natural Science Foundation of China (Grant No. 11575063); Guangdong Project of Science and Technology (Grants Nos.201508010021, 2016B090917002, 2016B090926004); and Guangzhou Union Project of Science and Technology (Grant Nos.201604040006, 201604040007).
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