a novel laser crystal in the rare-earth ion doped ... - OSA Publishing

Exploration of Yb3+:ScBO3- a novel laser crystal in the rare-earth ion doped orthoborate system Dazhi Lu,1 Zhongben Pan,2 Haohai Yu,1 Huaijin Zhang,1,* and Jiyang Wang1 1

State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, 27 Shanda Nanlu, Jinan 250100, China 2 Institute of Chemical Materials and Advanced Materials Research Center, Key Laboratory of Science and Technology on high energy laser, China Academy of Engineering Physics, 64 Mianshanlu, Mianyang 621900, China *[email protected]

Abstract: Growth of optical-quality crystals with a borate anionic group and small-diameter cations is challenging. By compensating for the volatilization of boracic acid, an optical-quality Yb3+:ScBO3 laser crystal suitable for laser applications has been grown by the Czochralski method. The thermal and spectral properties of Yb3+:ScBO3 were experimentally and theoretically studied, including the thermal expansion, thermal diffusivity, specific heat, thermal conductivity, and the absorption and lowtemperature emission spectra. The results indicate that this crystal exhibits a thermal conductivity that increases with temperature, and large energy storage properties that indicate promise for utilization in moderate power lasers. A continuous-wave Yb3+:ScBO3 crystal laser has, to the best of our knowledge, been demonstrated for the first time, with a maximum output power of 106 mW. All the results are indicative of a novel promising laser medium for applications in low and even moderate power lasers. ©2015 Optical Society of America OCIS codes: (160.3380) Laser materials; (140.3615) Lasers, ytterbium; (300.6280) Spectroscopy, fluorescence and luminescence.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

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Received 27 May 2015; revised 8 Jul 2015; accepted 10 Jul 2015; published 21 Jul 2015 1 Aug 2015 | Vol. 5, No. 8 | DOI:10.1364/OME.5.001822 | OPTICAL MATERIALS EXPRESS 1822

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1. Introduction Borate crystals including commercialized LBO [1], BBO [2] and GdCOB [3] have proven to be excellent laser host and nonlinear optical materials. The class RBO3 (R = Sc, Y, La-Lu) belongs to the orthoborate family, and has attracted a great deal of attention due to its moderately symmetrical structure and excellent physical and chemical properties. In this system, YBO3 [4], LaBO3 [5], GdBO3 [6], and LuBO3 [7] have been investigated as novel promising scintillators, phosphors [8] and laser host materials [5], which is in turn a motivation for the study of ScBO3 with small-diameter Sc3+ cations located at the six-fold #241864 © 2015 OSA


coordinated site. In fact, crystals with large- or small-diameter cations, such as La3+ and Sc3+ = are atypical and possess interesting optical properties [9, 10], since the cations induce changes in the crystal field and the structure. However, these changes also generate either spiral crystal growth or volatilization of raw materials, which in turn constrains the production of optical-quality crystals. Because of the volatilization of B2O3, the growth of a ScBO3 crystal with high optical quality is still a challenge, and limits any investigation of its properties or possible applications. Based on previous studies on Cr3+ doped ScBO3 crystals, we observed that Sc3+ ions also contribute to a favorable long radiative lifetime of the active ions that are present [11–13]. The Yb3+ active ion is interesting due to its two simple electronic states: the 2F7/2 ground state and the 2F5/2 excited state. Advantages include low quantum defect, broad absorption and emission bandwidths, weak concentration quenching, no excited state absorption, etc. Up to now, Yb3+-doped bulk crystals have shown their capability for ultrashort pulse generation, high-power, and high-efficiency output compared with Nd3+-ion doped crystals [14–17]. Moreover, the effective ionic radius of Yb3+ and Sc3+ is 0.868 and 0.745 Å, respectively [18], with a coordination number of six (CN = 6), which results in homogeneous doping of the Yb3+ ions and decreases lattice deformation in highly-doped Yb3+:ScBO3 crystals. However, perhaps hindered by limitations on the production of any high-quality crystals, only a few spectral properties of Yb3+:ScBO3 have been reported. In the same way that spectral properties determine possible use in a laser, the thermal properties determine the allowable laser power range and the laser configuration design. In this paper, a Yb3+:ScBO3 crystal was grown by the Czochralski method, and was characterized by measuring the spectral and thermal properties of the high-quality crystal. Additionally, a continuous-wave (CW) Yb3+:ScBO3 laser was experimentally realized. 2. Experimental details 2.1 Crystal growth, phase characterization, elemental composition, mechanical and thermal properties The Yb3+:ScBO3 single crystal shown in Fig. 1(b) was grown by the Czochralski method under an argon atmosphere in an iridium crucible. An excess quantity of 3 wt% H3BO3 was added to compensate for the evaporation of B2O3 during the growth process. In Fig. 1(a), a polycrystal which had many cracks was obtained using a platinum wire as the seed. In order to reduce the crack density and in view of the Czochralski growth process, a three-fold c-axis seed was cut from the prepared cracked crystal. After optimizing the growth process, an optical quality crystal was grown which was well-crystallized except for some subsidiary white surface matter due to the intense volatilization, as shown in Fig. 1(b). The sample used for the laser experiments is shown in the inset of Fig. 1(b). The structure of the as-grown crystals was investigated by X-ray powder diffraction (XRPD) at room temperature, and the results are shown in Fig. 2. For comparison, the XRPD of a standard ScBO3 crystal [19] is also presented in this figure. From the data, we found that the grown crystal is in space group R 3 C belonging to the trigonal system. The cell parameters are calculated to be: a = b = 4.776 Å and c = 15.405 Å. Compared with the cell parameters of the standard ScBO3 crystal [19] with a = b = 4.748 Å and c = 15.262 Å, it should be noted that the crystal structure and cell parameters do not obviously change when using Yb3+ ion doping. The theoretical density of the crystal is determined to be 3.812 g/cm3. Based on results obtained by the buoyancy method, the experimental density of this crystal is 3.787 g/cm3, which is in good agreement with the theoretical value. The concentration of elemental Yb3+ in the as-grown crystal was measured and calculated using the polycrystalline material as a standard. The effective segregation coefficient of Yb3+ was determined to be 0.95. All the experimental data on phase characterization and elemental composition confirms our preliminary thoughts about Yb3+ ion doping.

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Fig. 1. (a) Yb3+:ScBO3 crystal grown by platinum wire seed and (b) with a seed cut along cdirection; Inset of (b) is sample for laser experiments.

Fig. 2. X-ray powder diffraction pattern of Yb3+:ScBO3 crystal and standard PDF#79-0097.

For microhardness measurement, a DHV-1000 digital microhardness tester equipped with a diamond pyramidal indentor was employed. The load was stressed along three directions (a, b and c) at a value of 0.5 kg and a holding time of 10 s. A polished sample with dimensions of

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4 × 5 × 6 mm3 (a × b × c) was used for the measurement. After testing, the microhardness was calculated using the following equation:

H v = 1.8544 P / d 2

(1)

where Hv is the Vickers hardness number, P is the applied load and d is the diagonal length of the impression. For thermal expansion measurements, a thermal mechanical analyzer (Diamond TMA, Perkin-Elmer Co.) was employed to obtain the average linear thermal expansion tensor components of the as-grown crystal. The precision of the measurements was 0.02 μm. During thermal expansion measurements, the sample was heated at a steady rate of 5 K·min−1 and the expansion ratio versus temperature curves along the b- and c-axes were recorded. For specific heat capacity measurement, a simultaneous thermal analyzer (Perkin-Elmer Diamond: DSC) was used in the differential scanning calorimetry mode. The sample was held at 293.15 K for 15 minutes, heated to about 573.15 K at a constant rate of 10 K·min−1, and then held at 573.15 K for 15 minutes. The specific heat was calculated using software supplied by Perkin-Elmer Co. Thermal diffusivity analysis was performed with a laser flash apparatus (NETZSCH LFA447 Nanoflash). The samples were coated with graphite on opposite sides for even heating. When a short pulse heats one side of the sample, the temperature on the opposite surface is measured with an IR detector, from which the diffusivity coefficient is calculated. Based on the measured specific heat, density and thermal diffusion coefficient at different temperatures, the thermal conductivity was calculated. 2.2 Spectroscopic properties, laser damage threshold and laser performance The room temperature (RT) absorption spectrum was measured with a spectrophotometer (JASCO, Model V-570) having a spectral resolution of 0.2 nm. The sample was cut along the a-axis, polished on the two opposite sides normal to that orientation and had dimensions of 4 × 4 × 2 mm3. The RT and low temperature (79 K) fluorescence spectra were measured on a 0.5 mm thick sample by an Edinburgh Instruments FLS920 fluorescence spectrometer whose resolution is 0.2 nm. The RT fluorescence lifetime was also measured with the 0.5 mm thick sample using the time-correlated single-photon counting (TCSPC) method. The pump source was a tunable Opolette (HE) 355 II (5 ns, 20 Hz) laser, and the measurements were made with the FLS920 fluorescence spectrometer equipped with an ANDO Shamrock SR-303i highresolution optical spectrum analyzer. A Q-switched laser (ICT Laser Work Station, Piano 2000) with a wavelength of 1064 nm and a pulse width of 10 ns was used for laser damage threshold measurements. The laser was focused into a polished sample with dimensions of 4 × 4 × 2 mm3 through a focusing lens with a focal length of 100 mm. The sample was attached to a precision translation stage (Zolix Inc.) and moved toward the focal point until damage was observed. Then the laser spot size in the crystal and the pulse energy were measured in order to calculate the damage threshold. The CW laser was designed using a plano-concave resonator. Figure 3 shows the experimental laser configuration. The pump source was a fiber-coupled diode laser with an output wavelength of 976 nm and a core diameter of 200 μm. The numerical aperture (NA) of the fiber was 0.22. The front mirror (M1) was coated for a transmittance of over 99.5% at 950-990 nm, and high reflectance at 1000-1100 nm (R > 99.9%). A concave mirror (M2) with a radius of curvature of 50 mm and a transmittance of 5% at 1000-1100 nm was used as the output coupler. The laser crystal was a polished and uncoated Yb3+:ScBO3 sample cut along the a-axis with dimensions of 3 × 3 × 2 mm3 as shown in the inset of Fig. 1 (b). The sample was enclosed by indium foil and attached to a water-cooled copper block. The temperature of the cooling water was maintained at 2°C. The length of the cavity was optimized at about 45 mm. The average laser output power was measured using a Newport laser power meter

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(Model 1916-R), and the spectrum was recorded with an optical analyzer (YOKOGAWA Model AQ6370C).

Fig. 3. Schematic diagram of experimental laser setup.

3. Experimental results and discussion

3.1 Thermal and mechanical properties The Vickers hardness of Yb3+:ScBO3 along the a-, b- and c-axis was measured to be 7.05, 6.85 and 10.67 GPa, respectively. From these results, we conclude that the hardness of Yb3+:ScBO3 is anisotropic with comparable values along the a-and b-axes. The values are smaller than those of YAG (12.7 GPa) [20] but larger than those of YVO4 (4.8 GPa) [21], which indicates that Yb3+:ScBO3 has moderate hardness and can be easily cut and polished. The specific heat (Cp) of Yb3+:ScBO3 versus temperature was measured over the range from 293.15 K to 573.15 K. As shown in Fig. 4 (a), the value increases with temperature with a midrange value of 0.73 J·g−1·K−1 at 330 K. In comparison with Yb3+: YCOB (0.75 J·g−1·K−1) and Yb3+:YVO4 (0.62 J·g−1·K−1) [23, 24], Yb3+:ScBO3 has a relatively large specific heat, which indicates that it will not be greatly affected by the heat generated during the lasing process and that it should have a high damage threshold. The thermal expansion coefficient αij of a crystal is a symmetric second-rank tensor [25]. Since Yb3+:ScBO3 belongs to the trigonal system, based on Neumann’s principle the thermal expansion coefficient tensor can be expressed as: 0   α11 0   (2) 0 α 0 11    0  0 α 33   There are two independent thermal expansion coefficients, α11 and α33. Figure 4(b) shows the thermal expansion curves of Yb3+:ScBO3 plotted versus temperature. These two thermal expansion curves remain almost linear over the range from 303.15K to 774.15K. By calculation, the linear thermal expansion coefficients are found to be: α11 = 1.05 × 10−6 K−1 and α33 = 10.24 × 10−6 K−1. It is worth noting that the value along the c-axis is almost 10 times larger than that along the a- and b- axes, which means that the ScBO3 crystal exhibits considerable anisotropy in the thermal expansion and could easily be cracked during the crystal growth process. The density of Yb3+:ScBO3 at different temperatures can be calculated using the following equation:

ρ=

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ρ0 m m 1 = = a b c a Δ Δ Δ Δ abc a0 b0 c0 (1 + )(1 + )(1 + ) (1 + )(1 + Δb )(1 + Δc ) a0 b0 c0 a0 b0 c0

(3)


where ρ 0 = 3.812 g/cm3 is the theoretical density of the crystal, and the values of and

Δa Δb , a0 b0

Δc can be obtained from the thermal expansion coefficient values measured above. The c0

data were plotted in the form of circles and fitted to a linear equation with the result: ρ = (−5 × 10−5)T + 3.83, where T is the temperature in Kelvin.

Fig. 4. Thermal properties of Yb3+:ScBO3 versus temperature (a) specific heat, (b) thermal expansion coefficient, (c)thermal diffusion coefficient and (d) density variation of Yb3+:ScBO3 with temperature.

Similar to the thermal expansion coefficient, the thermal diffusivity λij is also a secondrank tensor and there exist only two independent values. The results are shown in Fig. 4 (c). Figure 4 (d) is a plot of the data and the linear fit to the density of Yb3+:ScBO3 at different temperatures from 302.15 K to 774.15 K. The thermal conductivity (k) was calculated based on the formula: k = λij ρ C p

(4)

When the temperature increases from room temperature to 571.15 K, the thermal conductivity along the a- and c-axis increases from 3.61 W·m−1·K−1 to 3.86 W·m−1·K−1 and 2.98 W·m−1·K−1 to 3.36 W·m−1·K−1, respectively, as the temperature increases from 302. 15 K to 774.15 K.

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Fig. 5. Thermal conductivity of Yb3+:ScBO3 versus temperature.

It is worth noting that the thermal conductivity increases with increasing temperature, as shown in Fig. 5. This behavior is typical of glasses and disordered crystals. The increase in the thermal conductivity with increasing temperature indicates that this crystal holds promise for use in applications in moderate power lasers. 3.2 Spectroscopic Characteristics The RT absorption and the fluorescence spectra are shown in Fig. 6(a). The strongest absorption peak is located around 964.8 nm, with an absorption cross-section of 0.13 × 10−20 cm2 and a full-width at half-maximum (FWHM) of 19.86 nm. The wide absorption bandwidth is quite suitable for pumping by diode laser sources. As shown in Fig. 6(a), the zero photon line is located at 964.8 nm, as verified by measurements at 79 K. In order to explore the transition between the two lowest levels of the 2F7/2 and 2F5/2 manifolds, the energy levels of Yb3+ ions in ScBO3 crystal are shown in Fig. 6(b) [14]. The RT emission cross-section was calculated based on the Füchtbauer-Ladenburg (F-L) method with the formula [26, 27] expressed as:

σ em ( λ ) =

λ 4 I (λ )

8π cn 2τ rad I ( λ ) d λ

(5)

where I (λ) is the spectral emission intensity of the Yb3+ ions, τrad is the radiative lifetime of the upper energy level, c is the velocity of light in vacuum, and n is the refractive index at the emission wavelength [27]. The strongest emission peak (λpeak) is located around 1021 nm with an emission cross-section of σ em of 0.12 × 10−20 cm2 and a FWHM of 24 nm, almost two times larger than that of Yb:YAG (10 nm) [28] and Yb:YGG (11nm) [29, 30], both of which possess outstanding femtosecond mode-locking properties.

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Fig. 6. (a) The absorption cross-section (RT) spectrum, emission cross-section spectra measured at RT and at low temperatures (79 K) based on F-L Method and (b) energy level scheme.

Fig. 7.

σg (λ )

of Yb3+:ScBO3 crystal versus wavelength.

The effective gain cross-section of Yb3+:ScBO3 is also shown in Fig. 7. It was calculated from the formula as follows: σ g ( λ ) = β σ em ( λ ) − (1 − β ) σ abs ( λ )

(6)

where β is the fraction of doped Yb3+ ions excited to the upper manifold. If σ g ( λ ) = 0, we

obtain the minimum value of β min (1021 nm ) at λ = 1021 nm , calculated as follows:

β min (1021 nm ) =

σ abs (1021 nm )

σ abs (1021 nm ) + σ em (1021 nm )

= 0.1

(7)

The figure shows the positive range of the effective gain cross-section calculated with β = 0.1, 0.15, 0.2, 0.4, 0.6, 0.75. For β = 0.75, the positive range covers from 970 nm to 1100 nm, which is comparable with those of Yb3+:YAG (950-1080 nm) [31], Yb3+:CYB (960-1100 nm) and Yb3+:CaGB (960-1100 nm) [30, 32]. The latter crystals all exhibit excellent #241864 © 2015 OSA


properties in tunable and femtosecond laser applications. On the basis of Fourier’s theorem, and using a hyperbolic secant function for the pulse shape, the relationship between the shortest pulse duration (tFWHM) and the gain bandwidth ( Δν ) can be shown to be [33]: tFWHM ⋅ Δν = 0.315

(8)

It can be concluded that Yb3+:ScBO3 can theoretically support a pulse duration of 65 fs for β = 0.75.

Fig. 8. The RT fluorescence lifetime of Yb3+:ScBO3 fitted using exponential functions. Inset. Fluorescence lifetime with log (intensity) vs time and linear fit.

The fluorescence lifetime was determined to be 5.25 ms, as shown in Fig. 8. The fluorescence lifetime can be calculated by fitting an exponential function [34] as follows: i

F (τ ) = A + B j × e

 t −  τj 

   

(9)

j =1

where A is the background noise of the signal, and Bj and τj are the amplitude and decay time of the jth exponential component, respectively. In the present work shown in Fig. 8, we found a single-exponential function (where i = 1) fit the fluorescence decay curve well. To confirm this view, the logarithm of the intensity was linearly fitted, as shown in the inset of Fig. 8. Both decay curves show a single-exponential profile, which indicates that re-absorption, cross relaxation and multisite effects are relatively weak [35]. The fluorescence time of this crystal is 5 times greater than that of Yb3+:YAG (1.01 ms) [30] and 2 times greater than Yb3+:YCOB (2.28 ms) [36, 37]. In Table 1, we compare the present data with the results reported by DeLoach et al [27]. The main difference appears in the zero-line value, so it was checked at 79 K in order to confirm our results. Table 1. Comparisons between DeLoach’s and our work Reference

λpump (nm)

λext (nm)

[27] This work

966 964.8

1022 1021

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σ ext (10−20 cm2) 0.19 0.11

τem (ms) Measured

Zl/Zu

Zero-line λZL (nm)

4.80 5.25

1.0 1.16

974.6 964.8


In general, Yb3+:ScBO3 has a smaller σ em value and a longer lifetime, both of which indicate that the crystal has good energy storage capacity and so can be classified as a promising Q-switching laser material. In addition, Yb3+:ScBO3 may also have favorable properties for generating ultrashort pulses due to its large FWHM value. Using the method mentioned in Section 2.2, the damage threshold of this crystal was measured to be 509 MW/cm2 at a wavelength of 1064 nm with a pulse width of 10 ns. A comparison with Yb3+:YVO4 and Yb3+: YAG that is shown in Table 2, indicates that the threshold for Yb3+:ScBO3 is nearly half the value of the former two crystals. This result indicates that Yb3+:ScBO3 should only be used in lasers with low and moderate power. In order to make a clear assessment of the integrated performance of Yb3+: ScBO3, comparisons among Yb3+: YAG, Yb3+: YVO4 and the former are made shown in Table 2. Table 2. Comparisons of Yb3+: YAG, Yb3+: YVO4 and Yb3+: ScBO3 −1

Specific Heat (J g K ) Thermal Conductivity (W m−1 K−1) Laser Damage Threshold (MW/cm2) Absorption Wavelength (nm) Emission Wavelength (nm) FWHM at emission peak (nm) Emission Cross-sections (10−20 cm2) Fluorescence lifetime (ms) References

Yb3+: YAG 0.63 6.8 1530 942 1031 10 2.0 1.01 [30, 38]

Yb3+: YVO4 0.62 5.23 1000 985 1013 31 0.9 1.1 [24, 39]

Yb3+: ScBO3 0.73 3.4 509 966 1022 24 0.12 5.25 This work

3.3 Laser performance Applying the design shown in Fig. 3, a CW laser has, to the best of our knowledge, been demonstrated for the first time, and the laser performance is shown in Fig. 9. From this figure, it can be seen that the pump threshold is 1.8 W, and the maximum output power is 106 mW at a wavelength of 1.064 μm under an absorbed pump power of 3.09 W. The optical-to-optical conversion efficiency was calculated to be 3.4% and the slope efficiency was 8.2%. We believe that laser performance can be further improved by optimizing the design of the cavity, including consideration of mode-matching between the fundamental laser radiation and pump radiation in the crystal, optimized transmission of the output couplers, and suitable coating of the Yb3+:ScBO3 crystal surface in order to reduce reflection during laser oscillation in the cavity.

Fig. 9. Laser performance. (a) Average output power versus incident absorbed pump power. (b) Laser spectrum.

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4. Conclusion

In this paper, a high-optical-quality Yb3+:ScBO3 crystal was successfully grown using the Czochralski method. The crystal structure and elemental segregation coefficient were studied by XPRD and XRF, both of which demonstrated the homogenous doping of the Yb3+ ions and a small lattice distortion. The thermal properties, including specific heat, thermal expansion coefficient, thermal diffusion coefficient, and thermal conductivity were also investigated. The thermal survey indicates that the as grown crystal has appropriate thermal properties to hold promise for application in low and even moderate power lasers. The absorption and fluorescence spectra were also measured and the stimulated emission cross-sections were calculated by the F-L method. Finally, CW laser operation at 1.064 μm was, to the best of our knowledge, demonstrated for the first time. All these results indicate that Yb3+:ScBO3 is a very promising optical material for future laser applications because of its thermal properties, small emission cross-sections, large bandwidth and long fluorescence lifetime. Acknowledgments

The authors wish to thank Professor R. I. Boughton, Department of Physics and Astronomy of Bowling Green State University, for discussions and linguistic advice. This work is supported by the National Natural Science Foundation of China (Nos. 51422205 and 51272131), the Natural Science Foundation for Distinguished Young Scholars of Shandong Province (2014JQE27019) and Taishan Scholar Foundation of Shandong Province, China.

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