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Abstract
Ho3+-doped calcium niobium gallium garnet (Ho:CNGG) single crystals have been successfully grown by the micro-pulling-down (µ-PD) method. The crystal structure, spectral properties, and laser performance of Ho:CNGG crystals were investigated. The doped Ho3+ ions showed a broad absorption and fluorescence bands due to the random distribution of Nb5+, Ga3+ and cationic vacancies in the host lattices. Moreover, 1.0 at.% Ho:CNGG single crystal was end-pumped by using a diode-pumped Tm:YAP laser with central wavelength of 1937nm. The maximum output power of 1.4 W with a slope efficiency of 18.7% at 2080nm was obtained preliminarily.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser systems operating in the eye-safe spectral region around 2 µm are widely applied to medicine, environment monitoring, coherent Doppler velocimetry, and military. Furthermore, high-power 2 µm lasers can also be used as efficient pump and seed sources of optical parametric amplifiers in the mid-infrared region [1,2]. Typically, Tm3+- and Ho3+-doped materials are used for laser generation at about 2 µm. Tm3+ ion has been paid more attention for the development of 2.0 µm lasers, since it can be efficiently pumped by the high-performance and well-developed AlGaAs lasers. Nevertheless, the wavelengths of Tm-based lasers are commonly not above 2.0 µm. Fortunately, the wavelengths above 2.0 µm can be generated by the 5I7→5I8 transition of Ho3+ ion, which is favorable particularly for the excitation of zinc germanium phosphide [3]. Additionally, Ho-based lasers have advantages of larger emission cross sections and longer lifetimes in comparison to Tm-based lasers. In the past few years, Tm3+ and Yb3+ ions are often used as a sensitizer of Ho3+ ion to improve the pump efficiency, because Ho3+ ion has no absorption to match the commercially available laser diodes (LD) [4,5]. However, their efficiency is strongly limited mainly due to the energy transfer efficiency as well as up-conversion processes [6]. With the development of high power 1.9 µm laser, resonant (in-band) excitation of Ho3+ ions to the 5I7 state has attracted more and more concern [7–9]. Laser operations have been realized in Ho:Y3Al5O12 (YAG) [10], Ho:LiYF4 [11], Ho:sesquioxides [12–13].
Calcium niobium gallium garnet (CNGG) single crystal is a promising host material in the field of tunable and ultra-short lasers due to the random distribution of Nb5+, Ga3+ and cationic vacancies in the host lattices, which will cause significant inhomogeneous broadening in the absorption and luminescence spectra of the doped rare-earth ions [14]. Furthermore, its melting point is about 1460°C, which makes it possible to synthesize these garnets using platinum crucibles, and it will simplify the growth process and decrease the cost in comparison with other garnet crystals. So far, rare ions doped CNGG crystals have been widely investigated, especially for crystal growth and laser properties of Nd3+, Yb3+-doped CNGG crystals [15–23]. However, only little attention has been paid to Ho3+-doped CNGG crystal. Ryabochkina et al. grew Ho:CNGG single crystal by the Czochralski method and spectroscopic characteristics were investigated [24]. And then, they demonstrated a continuous-wave laser operation of Ho:CNGG crystal with an output power of 2.1 W and slope efficiency of 37% pumped by a laser based on Tm:LiYF4 crystal [25]. However, the Ho:CNGG single crystal has not been grown by the µ-PD method. The µ-PD method has many advantages over the conventional crystal growth methods, such as much faster growth rate, much lower costs, and controllable shape of crystal, which is a promising method for growing gain media for miniature lasers.
In this work, Ho:CNGG crystals were grown by the µ-PD method. Room-temperature spectroscopic properties of Ho:CNGG crystals were measured and discussed. Furthermore, the laser operation on the 5I7→5I8 transition of 1.0 at.% Ho:CNGG crystal grown by the µ-PD method has also been investigated for the first time to the best of our knowledge.
2. Experimental details
2.1. Crystal growth
The CNGG single crystals doped with 0.5 at% and 1.0 at% Ho3+ were grown by the µ-PD method. The CaCO3, Nb2O5, Ga2O3 and Ho2O3 powders with purity of 99.999% were used as starting materials and weighted according to the formula Ca3Nb1.6875Ga3.1875O12 in order to keep charge balance close to its congruent melt composition, besides, Ho2O3 powders were doped together with additional Ga2O3 in a ratio of 3:5 (mol%) to form Ho3Ga5O12. After the compounds ground and mixed, they were pressed into bulks and then sintered at 1300°C for 12 h in a muffle furnace. The polycrystalline Ho:CNGG was melted within a platinum crucible, and then passed through the micro-nozzle at the bottom of the crucible to form a rod shaped micro single crystal. The outer diameter of crucible die was about 2 mm. The CNGG crystal with <111 > orientation was used as seed and the pulling rate was 0.5 mm/min. As shown in Fig. 1 (a) and (b), 0.5 at.% Ho:CNGG crystal with size of Φ2×83 mm3 and 1.0 at.% Ho:CNGG crystal with size of Φ2×90 mm3 were obtained. The crystals were transparent with a light yellow color.
2.2. Characterizations
In order to investigate the structure of Ho:CNGG crystals, small section of the samples were cut from the as-grown crystals and grounded into powder for XRD measurement. And the structure was confirmed by X-ray diffraction (XRD) using an automated Ultima IV diffractometer (Cu target, Kα, Rigaku, Japan) at a scan width of 0.0585° within 2θ=10-90°.
For spectroscopic measurements, the samples were cut from the as-grown crystals and two surfaces perpendicular to the <111>-growth axis were optically polished with the thickness of about 1.3 mm. The absorption spectra from 350 to 2200 nm were measured by a spectrophotometer (Cary 5000, UV-VIS-NIR). The fluorescence spectra, as well as the decay curves were measured by Edinburgh Instruments FLS920C spectrophotometers under the excitation 1136 nm. The excitation source is an optical parametric oscillator laser with excitation pulse length of 5 ns. All the measurements were carried out at room temperature.
3. Results and discussions
3.1. XRD pattern
The XRD patterns of Ho:CNGG crystals are shown in Fig. 2. No other phase was found in the sample and the XRD patterns of Ho:CNGG crystals matched very well with that of the undoped sample, which means the dopant did not change the crystal structure. The data confirmed that the crystalline phase of Ho:CNGG crystals belong to the cubic phase (Ia3d) garnet-type structure, isostructural to Ca3(NbGa)2Ga3O12 garnet (ICSD Card No.261821) [26].
3.2. Spectral properties
The room temperature absorption spectra of Ho:CNGG crystals in the wavelength region of 350-2200 nm are shown in Fig. 3. The seven main absorption bands related to transitions from the ground state 5I8 to the higher excited states of Ho3+ are marked. The absorption coefficients of 0.5 at.% and 1.0 at.% Ho:CNGG crystals were calculated to be 0.31 cm-1 and 0.62 cm-1 at 1922nm, which increase with the increase of Ho3+ doping concentration. The full widths at half-maximum (FWHM) of the absorption peaks at 1922nm for two samples are similar, which are about 58 nm. The broad absorption band at 1922nm is much larger than that of Ho:Lu2O3 crystal (3.0 nm) [12] and Ho:α-NaYF4 (16 nm) [27], which is important for commercially available LD pumping in the 1.9 µm range.
The fluorescence spectra in the range of 1700-2300 nm, excited at 1136 nm, are presented in Fig. 4. The emission bands centered at 2056nm, correspond to the transitions of Ho3+ 5I7→5I8. The FWHM of the emission band is about 167 nm, which is much larger than that of Ho:YAG and Ho:LuAG ceramics [28]. Such a broad and smooth emission band, arising from the disordered crystal structure, can be efficiently utilized to generate ultrafast pulse around 2 µm wavelength.
The fluorescence decay curves from the level 5I7 of Ho3+ ions under the excitation of 1136 nm were measured, as shown in Fig. 5. The decay curves could be fitted with a single exponential decay function well. The fluorescence lifetimes of 0.5 at.% and 1.0 at.% Ho:CNGG crystals were obtained to be 7.07 ms and 7.75 ms. The lifetime values obtained in our study are much larger than that of Tm3+-doped materials, which proves that Ho:CNGG crystal is suitable for laser operation above 2.0 µm.
4. Laser performance
Figure 6 shows a schematic diagram of the experimental setup. The Ho:CNGG laser sample has a dimension 30 mm in length and 2 mm in diameter. The doping concentration is 1.0 at.%. Both surfaces of the sample were mirror-polished, parallel, and uncoated. The sample was wrapped in indium foil and held in a copper heat-sink bonded on a thermal electric cooler (TEC) for maintaining at 20°C. A diode-pumped Tm:YAP laser with emission wavelength of 1937nm was utilized for pumping, which emits 15 W of output power with beam quality $M_x^2$ of ∼1.4 and $M_y^2$ of ∼2.1, respectively. The pump beam diameter was approximately 320 µm. The Ho:CNGG laser resonator is plano-concave with a 200 mm radius of curvature concave output coupler, whose transmissions is 2.5%. The 45° dichroic mirror provides high reflection (HR) at 2.1 µm (R > 99.5%) and high transmission (HT) at 1.94 µm (T > 94%). The physical length of the resonator was about 70 mm.
The laser spectrum of Ho:CNGG crystal is shown in Fig. 7(a). The output laser wavelength was centered at 2080nm. Figure 7(b) illustrates the laser output power versus the pump power for the Ho:CNGG crystal. The maximum continuous wave (CW) output power of 1.4 W was obtained with the incident pump power of 10.3 W. The slope efficiency was 18.7% with a threshold of approximately 3.3 W. The slope efficiency of Ho:CNGG crystal yielded in this study is still lower than that of Ho:CNGG bulk crystal [20]. However, with the improvement of crystal quality and optimizing the cavity design, much better conversion efficiency would be expected.
Fig. 7. The laser spectrum (a) and the laser output power versus the pump power (b) for the 1 at.% Ho:CNGG crystal.
5. Conclusion
In this work, high-quality Ho3+-doped CNGG crystals with different concentrations have been grown successfully by the µ-PD method. The absorption spectra, fluorescence spectra and fluorescence decay curves have been studied at room temperature. The absorption coefficients of 0.5 at.% and 1.0 at.% Ho:CNGG crystals are about 0.31 cm-1 and 0.62 cm-1 at 1922nm, and the FWHM are both 58 nm. The FWHM of the emission band are 167 nm. Broad absorption and emission bands will be beneficial for LD pumping and ultra-short pulse generation. The values of the fluorescence lifetimes are 7.07 ms and 7.75 ms, respectively. CW laser operation above 2 µm was demonstrated in the 1.0 at.% Ho:CNGG crystal pumped by Tm:YAP laser with emission wavelength of 1937nm. The maximum output power of 1.4 W was obtained with a slope efficiency of 18.7%. The results prove the great potential of Ho:CNGG crystal grown by the µ-PD method for highly efficient and ultra-short pulse lasers with emission wavelengths above 2.0 µm.
Funding
National Natural Science Foundation of China (NSFC) (51672190, 61621001); MOE Key Laboratory of Advanced Micro-Structured Materials.
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