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Abstract
Temperature dependent thermal and spectroscopic properties of Yb:YAlO3 (Yb:YAP) perovskite crystal at temperatures ranging from 77 to 298 K are presented. Thermal properties including specific heat, thermal expansion coefficient and thermal conductivity were investigated. Thermal shock resistance parameters were evaluated and the thermal shock resistance parameters were significantly increased from 0.68 × 106 W m-1 to 12.32 × 106 W m-1 in Yb:YAP as temperature varies from 298 to 77 K. The spectroscopic parameters, such as absorption, fluorescence, lifetime were also studied. The results of our study indicate that Yb:YAP crystal is a promising laser medium for high-power solid-state laser. The calculated cross-sections together with relevant thermal properties provide important information for the design in a new generation of cryogenically cooled near infrared laser.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ytterbium (Yb)-doped materials have been widely used for the construction of diode-pumped solid state lasers since they offer a long fluorescence lifetime in the millisecond range and a low quantum defect. The Yb3+ ions have a simple two-level energy structure and no up-conversion, no excited-state absorption, and no cross relaxation [1]. Therefore, Yb-doped laser materials have great potentials for compact high-doping concentration, high-power diode-pumped solid-state lasers (DPSSLs). However, a major disadvantage of Yb-doped materials is their quasi-three-level nature caused by the thermally induced population of the lower laser level, due to the low separation of the intra-manifold energy levels. This significantly limits the lasing efficiency of a laser system. An approach to overcome this issue is to cool down the gain medium to cryogenic temperatures, which enables “four-level” operation since the thermally induced population of the lower laser level is frozen out.
Cryogenic systems, with a number of important benefits, including the near vanishing of optical distortion in high average power lasers, as well as enhanced spectroscopic and lasing properties [2], have been used to improve the performance of solid-state lasers from the very earliest laser demonstrations. Cryogenically cooled Yb-doped Y3Al5O12 (YAG), CaF2 and LiYF4 (YLF) have been demonstrated to be efficient high-power solid-state laser materials, eliminating ground state absorption, with potential for even higher powers, because of the improvement of thermo-optic effects at low temperatures [3–6]. Over the years as workers have begun to look for other improved hosts for use in DPPSLs, among the various hosts under investigation is YAlO3 (YAP). YAP with a perovskite-type structure has many other attractive characteristics such as high thermal conductivity, excellent physical and chemical properties similar to those of YAG. Moreover, its nature of birefringence dominates any thermal birefringence even when pumped at high average power, so the deleterious effect caused by thermal birefringence can be ignored. Because of the above-mentioned good properties, Yb-doped YAP crystal is a potentially useful laser gain medium in laser-diode-pumped solid-state configurations [7–9].
High average power lasers rely on cryogenic cooling to reduce re-absorption losses and to remove waste heat. Since the simulation and design of such laser systems is a complex task, its successful realization strongly depends on the availability of basic data of the active medium, allowing for the simulation of the laser process to specify the system layout and details of its parts, especially with respect to the pump system. Unfortunately, for many materials, especially for the less investigated Yb:YAP crystal, reliable thermal and spectroscopic data are not broadly available at cryogenic temperatures. This issue is particularly crucial, when considering that such laser might need to be operated at cryogenic temperatures in order to suppress optical distortion in high average power lasers and increase cross sections. It's worth noting that the fluorescence intensity of c-axis crystal is about two times of a- or b-axis crystal [10]. Compared with a- and b-axis crystals, the absorption peak of c-axis crystal at 960 nm is more far from the fluorescence band so that the overlap of both bands is little which can reduce the self-absorption phenomenon. The c-axis crystal is relatively preferential to be chosen for the high-power and high-beam-quality laser.
In this work, we present a complete thermal properties of Yb:YAP crystal along c-axis in the temperature range between 77 and 298 K, including specific heat, thermal conductivity, and thermal expansion. In addition, the thermal shock resistance parameters were also calculated. We also present detailed measurements of the absorption, emission cross sections and fluorescence lifetime of Yb:YAP for various temperatures between 77 and 298 K. The merit factor (M) [11,12] under different temperatures was also systematically calculated. All results will be useful for designing and optimising cryogenically cooled Yb:YAP systems.
2. Materials and methods
The measurements presented in this paper were carried out using 10 at.% Yb3+-doped Yb:YAP crystal along c-axis (space group: Pbnm) and the crystal was grown with Czochralski method. Thermal expansion behavior and thermal conductivity of crystal (6 mm×6 mm×20 mm) were measured in the temperature range of 77-298 K. Thermal expansion was measured in the temperature range of 77-298 K using a thermal dilatometer (Linseis L75) made by Linseis of the Germany and the uncertainty of the thermal expansion is estimated to be within ±1%. Thermal conductivity was measured using a physical property measurement system (PPMS-9) by the steady-state longitudinal heat-flow method [13]. The uncertainty of the present measurements is estimated to be within ±3% for the thermal conductivity. The specific heat of crystal (3 mm×3 mm×1 mm) were measured by a thermal relaxation method [14] using a physical property measurement system (PPMS-14) made by Quantum Design Corporation of the USA with the temperature range from 77 to 298 K. The accuracy of the present measurements is estimated to be within ±1% for the specific heat. A fluorescence spectrometer (Edinburgh FLSP980) with resolution of 0.1 nm was used to measure the fluorescence spectra of with an exciting source of a Xenon lamp and absorption measurements could be carried out using the same setup. The raw data was corrected for Fresnel losses and the spectral response of the system. The Yb:YAP used in the measurements was cut into 10 mm diameter with 1 mm thickness and polished. All measurements were performed at cryogenic temperatures from 77 to 298 K.
3. Results and discussion
3.1 Thermal properties
The fundamental thermal properties (such as specific heat, thermal expansion coefficient and thermal conductivity) of the crystal at cryogenic temperatures play an important role for practical applications in the design and fabrication of laser devices, especially in the high-power regime.
For optical crystals, the damage threshold and the possible applications could be seriously influenced by the specific heat [15]. The measured specific heat versus temperature is illustrated in Fig. 1, from which it can be seen that the specific heat is almost linear with a value range of 0.11-0.57 Jg-1K-1 for Yb:YAP crystal over the temperature range from 77 to 298 K. As seen in Fig. 1, the specific heat decreases progressively as the temperature decreases in the measuring range. The specific heat (0.57 Jg-1K-1) of Yb: YAP at 298 K is similar to those of pure YAG (0.6 Jg-1K-1) and 5 at.% Yb:YAP (0.56 Jg-1K-1) [16,17]. Furthermore, the specific-heat for Yb:YAP and Yb:YAG are close to each other in the range of 77-298 K [17]. The thermal expansion coefficient and thermal conductivity of the Yb:YAP crystal were measured (Fig. 2). It is clearly seen that the thermal expansion coefficient decreases with the reduction in temperature with a value range of 8.21-1.94×10−6/K. The measured thermal expansion for 10 at.% Yb:YAP from 100 to 298 K are consistent with those reported in Ref. [17]. At low temperature, the 10 at.% Yb:YAP has significantly lower thermal expansion than undoped YLF along c-axis but significantly higher than highly doped YAG and LuAG [17]. The thermal conductivity exhibits a tendency of increasing with declining temperature from 298 to 77 K, and its rising occurs more quickly at lower temperatures. The dependence of thermal conductivity on temperature is mainly affected by phonon mean free paths. The phonon mean free paths increase with declining temperature [18], resulting in the increasing tendency of the thermal conductivity. The thermal conductivity of 10 at.% Yb:YAP was measured to be 16.2 W/mK at 100 K, a little larger than the corresponding values of 11.3 W/mK for 5 at% Yb:YLF (‖a) and 13.7 W/mK for 5 at% Yb:YLF (‖c) [19], and close to the value of 16.4 W/mK of 15 at.% Yb:YAG [17]. It's worth noting that thermal conductivity is lower compared to the previous data for Yb:YAP at the same temperature [17], which is mainly attributed to the higher doped concentration of lasing ions. These thermal characterisations at low temperatures are benefit for higher power and improved beam quality.
Fig. 1. Specific heat for Yb:YAP. The curve is line connecting the data points. The accuracy of measurements is estimated to be ±1%.
Fig. 2. Thermal conductivity and thermal expansion coefficient of the Yb:YAP crystal versus temperature. The accuracy of measurements is estimated to be ±3% for thermal conductivity and ±1% for thermal expansion.
Thermal stress arises as a consequence of heat generation inside the laser material during laser operation. A thermal shock resistance parameter RT is defined as [20,21]
where κ is the thermal conductivity, α is the expansion coefficient. So, high thermal conductivity and low thermal expansion coefficient are both required. At room temperature, the κ/α value of Yb:YAP is calculated to be 0.68 × 106 W m−1, which is slightly smaller than that 9.6 at.% Yb:YAG (0.8 × 106 W m−1) [22]. Table 1 summarizes the thermal shock resistance parameter RT for various temperature steps between 77 and 298 K. We can see that thermal shock resistance parameter is significantly larger at cryogenic temperatures than that at room temperature. At 77 K, this parameter for Yb:YAP is calculated to be 12.32 × 106 W m−1, which is 18.12 times more than that at room temperature. Laser operation at cryogenic temperature is expected to exhibit a high resistance to stress fracture and can operate steadily for high power laser with a good beam quality.3.2 Spectroscopic properties
In order to access the potential of Yb:YAP crystal for efficient, high-power operation at low temperatures, we have measured the temperature dependency of the absorption and emission cross-sections. The wavelength dependent absorption cross-sections σabs were obtained by using Beer-Lambert’s law [23], and the emission cross-sections σem were determined by using both Füchtbauerr-Ladenburg (FL) equation [24] as well as the reciprocity method (RM) [25]. Moreover, it is well known that the threshold of a continuous wave laser is inversely proportional to the M factor, which can be calculated from the expression [11,12]
where τf is the fluorescence lifetime and N = 1.98 ×1021 ions/cm3 is the doping concentration of the sample.
Table 1. Thermal shock resistance parameter RT at different temperatures.
The detailed schematic energy-level structure of Yb3+ ions in YAP crystal [26] is illustrated in Fig. 3(a), together with the relevant transitions. Figure 3(b) shows the absorption cross-section spectra within a spectral range between 850 nm and 1050 nm as a function of temperature for Yb:YAP. There were four main absorption bands in the Yb:YAP crystal. Four absorption bands centered at 933, 960, 979, 999 nm were observed. As expected, almost absorption peaks in the pump band show a reduction in bandwidth and become more intense as temperature is lowered with the exception of the absorption peak at 999 nm, which is anomalous.
Table 2 summarizes the peak cross-section values as a function of temperature for four absorption peaks. The strongest absorption peak occurs in 960 nm in the Yb:YAP crystal. At 960 nm and room temperature, we obtain a absorption cross-section value of 0.98 × 10−20 cm2, smaller than the value of 1.04 × 10−20 cm2 reported in [8] and a FWHM linewidth of 6.67 nm. As the temperature is decreased, the FWHM linewidth reduces, reaching 2.6 nm at 77 K (Fig. 4). This width is available for efficient pumping using conventional diode sources. Furthermore, we also show the wavelength difference between 298 and 77 K. A shift to shorter wavelength by 0.6 nm for the 0→10417 cm−1 transition for 960 nm line is observed for the peak position for the lowest-temperature case, as displayed in Fig. 4, which reveals no discernable trends.
Fig. 4. Center wavelength values and FWHM bandwidth values at 960 nm as a function of temperature for Yb:YAP.
Table 2. Peak absorption cross-sections σabs (10−20cm2) for main absorption lines at different temperatures.
The enormous gains in energy efficiency and waste heat reduction make zero-phonon line (ZPL) pumping very attractive for high power laser development so the detailed understanding of the pump line width, wavelength, and absorption cross-section are essential. At 298 K, the ZPL is centred at 979.1 nm, with a peak absorption cross-section of 0.93×10−20 cm2 and a FWHM linewidth of 3.3 nm. As the temperature is decreased, the FWHM linewidth reduces, reaching 0.61 nm at 77 K and the peak position shifts to shorter wavelength, with 978.9 nm measured at 77 K.
As the absolute values for lifetime from our measurements might be influenced by re-absorption effects, the radiative lifetime was calculated to be 0.67 ms according to the method given in Ref. [27]. Using the calculated radiative lifetime mentioned above, we calculated the emission cross section from the F-L formula and the reciprocity method. Figure 5 shows the emission spectra at the five temperatures of 77, 150, 200, 250, and 298 K. The main emission at 979, 999, 1012, and 1038 nm were observed. Although the fluorescence intensity at 999 nm is the strongest, there is a strong absorption at 999 nm.
Fig. 5. Yb:YAP emission cross-section as a function of wavelength from 970 to 1100 nm, and temperatures of 77 (black), 150 (red), 200 (blue), 250 (pink) and 298 (green) K.
As has been observed with the absorption cross sections, decreasing the temperature from 298 K to cryogenic temperatures results in narrower and more intense emission lines. At room temperature, the peak at about 999 nm is the strongest fluorescence peak in the Yb:YAP crystal and the corresponding emission cross section is 1.44 × 10−20 cm2, which is smaller than that of 15 at.% Yb:YAP (1.92 × 10−20 cm2) [8]. From 298 to 77 K, the peak cross section around the 999 nm is increased by a factor of 4. It is worth noting that Yb:YAP crystal loses the bandwidth advantage as the 999 nm emission peak is split in the low temperature range. It can be seen that the emission peaks at 1012 nm and 1038 nm are strongly increased. From 298 to 77 K the peak cross section at 1012 nm is increased by a factor of 1.8 and the peak cross section at 1038 nm is increased by a factor of 1.7. Table 3 summarizes the peak cross-section values as a function of temperature for three emission peaks of Fig. 5. Emission and absorption cross-sections keep their broadband characteristic even at low temperatures in comparison with Yb:YAG and Yb:LuAG. In addition, there are two emission peaks at 1012 nm and 1038 nm, which do not exist in Yb:YAG and Yb:LuAG crystals [28].
Table 3. Peak emission cross-sections σem (10−20cm2) for main emission lines at different temperatures.
The fluorescence lifetimes at 77, 150, 200, 250 and 298 K were characterized by a single exponential decay function and the results are shown in Fig. 6. The inset shows the fluorescence decay curves. The fluorescence lifetime is larger than the calculated radiative lifetime (0.67 ms). It is because that the fluorescence lifetime of Yb3+ is affected much more strongly by re-absorption and radiation trapping effects, which lengthen measured lifetime generally. It should be noted that the fluorescence lifetime is significantly decreased for lower temperature, from 1.88 ms at 298 K dropping to 0.81 ms at 77 K. This is due to a negligible re-absorption effect at low temperatures.
Fig. 6. Measured fluorescence lifetime of Yb3+ ions in YAP as a function of temperature. The inset shows the fluorescence decay curves.
From the values of σabs (the absorption cross-section at 960 nm), σem and τf mentioned above, the M factor can be calculated using Eq. (2), which is also listed in Table 4. Thus, the results of the spectroscopic parameters indicate that Yb:YAP has a lower threshold at cryogenic temperatures. The fluorescence peak at 999 nm has the largest merit factor in three fluorescence peaks including 999 nm, 1012 nm, and 1038 nm but there is strong re-absorption at 999 nm. In particular, over the measured temperature range, this parameters at 1012 nm and 1038 nm are strongly increased, indicating that Yb:YAP crystal is a potential candidate used for efficient lasers when the laser output wavelength is 1012 or 1038 nm.
Table 4. Merit factor (10−22 cm· s) for main emission lines at different temperatures.
4. Conclusion
In conclusion, we present a detailed analysis of specific heat, thermal conductivity and thermal expansion for Yb:YAP from 77 to 298 K. In addition, the enhanced thermal shock resistance parameters indicate that laser operation at cryogenic temperatures exhibits a higher resistance to stress fracture. Further, the absorption spectra, emission spectra and fluorescence lifetimes were measured in the temperature range 77-298 K. It was shown that the absorption and emission peak cross sections are strongly increased for lower temperatures. We believe that these results will be useful in the simulation and design of diode pumped solid-state lasers operated at cryogenic temperatures.
Funding
Practical Training Program of Beijing; National Natural Science Foundation of China (51890864, 61535013); Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (Y8A9021H11); Chinese Academy of Sciences (GJJSTD20180004); National Key Research and Development Program of China (2016YFB0402103).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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