A novel CGG-type laser crystal of Nd3+:CNGS is characterized, including crystal growth, structure, refractive index, spectroscopic and laser performance. For Nd3+:CNGS crystal, the unit-cell parameters are a = 0.88897 ( ± 8.89 × 10−5) nm, c = 0.497998 ( ± 4.85 × 10−5) nm and V = 0.28219 nm3. The refractive index no and ne of Nd3+:CNGS were calculated to be 1.772 and 1.854 respectively at 1065 nm. The transmittance spectra of Nd3+:CNGS were measured, four main absorption peaks were located at 586 nm, 748 nm, 804 nm and 804 nm respectively. The absorption cross-section in z-axis is 6.08 × 10−20 cm2 around 804nm. The strongest emission peak of the fluorescence spectra was located at 1064 nm with the emission cross-section of 6.81 × 10−20 cm2 for E//x polarization and the fluorescence lifetime was 255.6 μs. Moreover, the continuous-wave laser at 1065 nm was obtained with the maximum output power of 1.43 W in the z-cut sample for the first time to the best of our knowledge, the optical conversion efficiency was 29.3% and the slope efficiency was 31.0%, which indicated that Nd3+:CNGS crystal should be a promising gain material.
© 2015 Optical Society of America
In recent years, single crystals with the Ca3Ga2Ge4O14 (CGG) structure have been reported to be promising piezoelectric materials for fabrication of filters with large pass bandwidths and oscillators with high-frequency stability [1–8]. In 1998, B. V. Mill et.al firstly reported Ca3NbGa3Si2O14 (CNGS) crystal materials, determined its cell parameters and proposed CNGS crystal as one kind of CGG compounds with ordered structure . Compared with disordered structure LGS, LGT and LGN crystals, CNGS are mechanically strong, with higher acoustic velocity, lower acoustic loss and higher electromechanical coupling. Furthermore, CNGS is much cheaper than LGS because the use of the amount of Ga2O3 raw material is reduced by half [10–12]. According to previous reports, CNGS is a promising substrate material of solid-state laser crystal due to its good mechanical properties, big specific heat and small thermal expansion coefficients, and it has no phase transformation from room temperature to melting point, in addition, there is no obvious absorption above 400 nm wavelength [13, 14]. However, it is much to be regretted that no other rare earth doped CNGS crystals have been reported except for Co2+:CNGS until now . In this paper, Nd3+:CNGS was grown by the Czochralski (CZ) method for the first time to the best of our knowledge. We demonstrated the structure of Nd3+:CNGS crystal by the X-ray powder diffraction, reported its refractive index, transmission and absorption spectra and fluorescence spectra, and compared these properties with those of CNGS. Moreover, 1065 nm wavelength continue-wave laser output was realized by using the Nd3+:CNGS crystal.
2. Experiment details
In order to analyze the characteristics of Nd3+:CNGS crystal better, CNGS and Nd3+:CNGS were grown respectively for comparison. The raw materials for the growth of CNGS and Nd3+:CNGS crystals were prepared by mixing 99.99% pure CaCO3, Nb2O5, Nd2O3, SiO2 powders and Ga2O3 powder of 5N purity according to the stoichiometric ratio for 24 h, in order to ensure its purity and uniformity. The mixture was pressed into tablets, and then the tablets were put into a corundum crucible and then heated at 1150 °C for 28 h to decompose CaCO3 completely.
In order to avoid the influence of the evaporation of Ga2O3 from the melt during growth, excess 0.5 g Ga2O3 in every 100 g mixture was added, the crystal growth was carried out under N2 and 1-3 vol% O2 atmosphere using the conventional RF-heated CZ method in an iridium crucible of Φ70 mm . The polling direction was a-axis with the pulling rate of 0.5-1.0 mm/h and the rotating rate of 15-18 rpm. After growth, the crystal was cooled to room temperature at the rate of 10-50 °C/h.
CNGS and Nd3+:CNGS crystals with good transparence were obtained as shown in Fig. 1.We can see that CNGS crystal is a little yellow and Nd3+:CNGS crystal exhibits slight violet. Both the crystals show four strong crystallographic planes, corresponding to (001) and (010) respectively. The Nd3+ concentration in Nd3+:CNGS crystal was measured to be 0.483 at.% by the X-ray fluorescence method.
X-ray powder diffraction (XRPD) was performed on ground crystal powder, as shown in Fig. 2.The XPRD data of two crystal powder samples were collected on a Bruker-AXS D8 ADVANCE X-ray diffractometer equipped with a diffracted beam monochromator set for Cu Kα radiation (λ = 1.54056 Å). Intensities for the diffraction peaks were recorded in the 10°-120° (2θ) range with a step size of 0.02° and reasonable step time setting at room temperature.
The crystal structure was analyzed using Jade Version 5 Analysis software according to the peak values of 2θ in XRPD pattern. It is clear to see that CNGS and Nd3+:CNGS have the same structure, both the crystals belong to trigonal system, P321 space group. CNGS unit-cell parameters are a = 0.809166 ( ± 3.64 × 10−5) nm, c = 0.498118 ( ± 1.65 × 10−5) nm and V = 0.28245 nm3, which are slightly larger than those in Ref , and Nd3+:CNGS unit-cell parameters are a = 0.88897 ( ± 8.89 × 10−5) nm, c = 0.497998 ( ± 4.85 × 10−5) nm and V = 0.28219 nm3.
For optical crystals, refractive index is an important parameter and basic data to decide whether the crystal could be used for the optical device design and optimization. The refractive index of CNGS and Nd3+:CNGS were measured with the accurate refractive index measurement system HR SpectroMaster UV-VIS-IR (Trioptics, Germany). CNGS and Nd3+:CNGS samples were both cut as right-angle prism with apex angle about 22° and kept at 20.16 °C during the measurement.
The transmittance spectra of CNGS and Nd3+:CNGS crystals were recorded on a HITARCHI U-3500 spectrophotometer at room temperature in the range of 190-3200 nm. The scanning speed is 120 nm/min, the sampling interval is 1.00 nm and the slit width is 3.00 nm. The samples of CNGS and Nd3+:CNGS crystals were all cut to the dimensions of 3 × 3 × 2 mm3 and the 3 × 3 mm2 faces perpendicular to the x, y and z directions were all polished. The incident light is parallel to the thickness directions of the testing samples.
A 3.00 × 3.00 × 6.00 mm3 (x × y × z) sample of Nd3+:CNGS crystal was used to measure the emission spectra. The fluorescence emission spectra of Nd3+:CNGS crystal were measured by an EDINBURGH INSTRUMENTS FLS920 spectrophotometer from 800 nm to 1500 nm at room temperature. The exciting source is a stable xenon lamp with the wavelength of 355 nm light. The fluorescence decay curve at 1064 nm was measured with the same spectrophotometer which was pumped by a 10 ns pulsed OPO laser (Opolette 355 II) with the wavelength of 355 nm.
The continuous-wave (CW) laser performance of the Nd3+:CNGS crystal was realized by using a concave-plano resonator. The laser experimental setup is shown schematically in Fig. 3.The pumping source was a fiber-coupled LD with the emission wavelength centered at 808 nm (400 μm fiber core diameter). The output beam of the LD was focused onto the Nd3+:CNGS crystal sample with a spot radius of about 0.4 mm using an optical focus system (46.6 mm focal length). The physical cavity length was about 17 mm. M1 was a concave mirror with a radius of curvature of 250 mm, antireflection (AR) coated at 808 nm on the pump face, high reflectance (HR) coated at the range of 1020 nm to 1200 nm, and high transmittance (HT) coated at 808 nm on the other face. The plane mirror (M2), with the output transmission of 5.0% at 1.06 μm, was used as output coupler. Considering the problems of the crystal defects and the nonuniformity of the internal concentration distribution, we adopted the crystal samples with short length as the laser media. The samples were cut to the dimensions of 3 × 3 × 2 mm3 and the 3 × 3 mm2 faces perpendicular to the x, y and z directions were polished and uncoated. During the experiments, the samples were wrapped with indium foil and mounted on a copper block cooled by water. The cooling water was maintained at a temperature of 17 °C.
3. Results and discussions
3.1 Refractive index
The refractive index of CNGS an Nd3+:CNGS were measured by the minimum-deviation method at 12 different wavelengths ranging from 365 nm to 2325 nm (no and ne of CNGS was not measured for 1529.58 nm).The Sellmeier equation is an empirical relationship between refractive index and wave length for a particular transparent medium, which is used to determine the dispersion of light in the medium. The equation can often be given in the following form for crystal :Table 1.
Figure 4 shows the experimental data and the fitted dispersion curves of CNGS and Nd3+:CNGS prisms over the full transmission range with the Sellmeier equations. From the results, no and ne of Nd3+:CNGS are calculated to be 1.772 and 1.854 respectively at 1065 nm, both of which are a little higher than those of CNGS.
3.2 Transmission and absorption
The transmission spectra are shown in Fig. 5.. It can be seen that the ultraviolet absorption edge of CNGS and Nd3+:CNGS crystals are both 275 nm. From Fig. 5(a), it can be found that the optical transmittances of CNGS crystal in three directions are all almost above 75%, and the transmittance in z-direction is highest. The high transmittance demonstrates that our CNGS crystal is of good homogeneity. In addition, there is on obvious absorption peaks above 340 nm, which is an excellent advantage for laser host materials.
Compared with Fig. 5(a), the transmission spectrum of Nd3+:CNGS crystal appears some absorption peaks, as shown in Fig. 5(b). It can be seen that the corresponding wavelengths of the main absorption peaks are located between 300 nm and 1000 nm. Thus we captured the transmission spectrum of Nd3+:CNGS crystal from 300 nm to 1000 nm, and converted it into the absorption spectrum, as shown in Fig. 6(a), we can clearly identify four main absorption peaks, and the corresponding wavelengths are 586 nm (4I9/2→4G5/2), 748 nm (4I9/2→2F7/2), 800 nm (4I9/2→2H9/2) and 804 nm (4I9/2→4F5/2). From Fig. 6(b), it can be seen that the absorption in z-axis is the strongest one among the three directions, and the absorption cross-section around 804nm is about 6.08 × 10−20 cm2. Therefore, the z-axis can be treated as the most effective pump direction.
3.3 Fluorescence characteristics
The normal light fluorescence spectrum of Nd3+:CNGS crystal is shown in Fig. 7(a).It can be seen that there are three groups of emission peaks in the range from 800 nm to 1500 nm. At 1064 nm, Nd3+:CNGS crystal has the strongest emission peak. The bandwidth (FWHM) is about 16 nm, which is much larger than that of Nd3+:YAG (Δλe = 0.8 nm) and Nd3+:YVO4 (Δλe = 1.1 nm) crystals [17, 18]. Such a large bandwidth shows that Nd3+:CNGS crystal may possibly be used in laser systems to produce femtosecond pulses . The emission peaks of 893 nm and 1345 nm are relatively weak, with the bandwidths (FWHM) of about 40 nm and 30 nm respectively.
The polarized light fluorescence spectrum is shown in Fig. 7(b). E represents the electric field of the emitted light. We can see that, in three polarization directions, the fluorescence intensities near 1064 nm are all the strongest, and the positions of the corresponding peaks at one certain specific wavelength are all the same. However, what is interesting is that the emission peaks for E//x and E//z polarizations near 893 nm are more obvious than that for E//y polarization while the case is quite different near 1345 nm. The fluorescence decay curve of Nd3+:CNGS crystal is shown in Fig. 8By linear fitting, the fluorescence lifetime is found to be 255.6 μs.
The polarized emission cross-sections of Nd3+:CNGS crystal were also calculated by the Fuchtbauer–Ladenburg (F–L) formula ,Fig. 9.It can be seen that the emission cross-sections around 1064 nm and 1345nm for E//x polarization are about 6.81 × 10−20 cm2 and 3.13 × 10−20 cm2 respectively, which are much larger than those for E//y and E//z polarization. Therefore, the laser output will be more inclined to x polarization at 1064 nm and 1345nm. The emission cross-sections around 893 nm are very small for all three polarized directions, which indicate that the difficulty of the laser output near 893 nm.
3.4 Laser performance
The continuous-wave laser performances of Nd3+:CNGS crystal at 1.06 μm in three physical axis directions were demonstrated in Fig. 10 for the first time. The average output power under various absorbed pump power values was measured. It can be seen that the z-cut sample shows the best laser performance among the three directions. For z-cut sample, the pump threshold is 0.27 W, and a maximum output power of 1.43 W was achieved at an absorbed pump power of 4.89 W. The optical-to-optical conversion efficiency was calculated to be 29.3%, and a slope efficiency of 31.0% was achieved. Exceeding this pump level, the output power was saturated. The samples were very short and not coated, and only one output coupling with 5.0% transmission was used in the experiment. Therefore, we believe that more efficient laser output could be realized if the samples and the transmission output coupling are optimized.
With a spectral analyzer, the spectrum of the CW laser in z direction at the absorbed pump power of 4.89 W was recorded and shown in Fig. 11.It can be found that the center wavelength was located at 1065 nm and the half-maximum value of this spectrum is as broad as 1.83 nm, which is much shorter than that of Nd3+:LGS . This is because the crystal of Nd3+:CNGS has little inhomogeneous broadening due to its ordered structure. We also measured the polarized properties of the CW laser in three directions with a polarized beam splitter. The output laser in x and y directions were polarized and the polarized directions were along y-axis and x-axis respectively, which explained the anisotropy of this crystal. However, no polarized emission was observed in z direction, which meant that the thermal-induced birefringence were not obvious.
The crystals with optical quality of CNGS and Nd3+:CNGS crystals have been successfully grown by the Czochralski method, the unit-cell parameters were calculated respectively. The refractive index of CNGS an Nd3+:CNGS were measured by the minimum-deviation method and fitted by the Sellmeier equation. The absorption in z-axis is the strongest one among the three directions with the absorption cross-section of 6.08 × 10−20 cm2 around 804nm. The emission cross-sections around 1064 nm and 1345nm for E//x polarization are about 6.81 × 10−20 cm2 and 3.13 × 10−20 cm2 respectively. In addition, the fluorescence decay curve at 1064 nm was measured, and the fluorescence lifetime was found to be 255.6 μs. Finally, CW laser performances at 1065 nm were demonstrated. A maximum output power of 1.43 W was obtained with the optical conversion efficiency of 29.3% and the slope efficiency of 31.0% in z-cut sample. All these results demonstate that the Nd3+:CNGS crystal has favourable optical properties. It can be indicated that Nd3+:CNGS is a promising material in application of the single crystal photo-elastic modulator by realizing the composite of its good piezoelectric and optical properties.
The authors wish to thank Prof. Q. M. Lu, for the discussion and linguistic advice. This work was financially supported by the National Natural Science Foundation of China (Grant No. 51472147, 91022034 and 51321091), the Natural Science Foundation of Shandong Province (ZR2009FM012) and Independent Innovation Foundation of Shandong University (IIFSDU).
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