Temperature-dependent thermal, spectroscopic properties, and laser performance of Nd:YVO4 crystal

Temperature-dependent thermal and spectroscopic properties of Nd:YVO4 crystal at temperatures ranging from 77 to 300 K are presented. Thermal properties including specific heat, thermal expansion coefficient, and thermal conductivity are investigated. The spectroscopic parameters, such as absorption, fluorescence, lifetime are also studied. The calculated absorption and emission cross-sections together with relevant thermal properties provide important information for designing cryogenically cooled near infrared laser. Furthermore, we experimentally explore continuous wave (CW) laser performances under various cryogenic temperatures. A maximum output power of 2.13 W is obtained at an incident pump power of 3.7 W at 150 K, corresponding to an optical-to-optical conversion efficiency of 57.6%.


Introduction
The Nd:YVO 4 (neodymium doped yttrium orthvanadate) crystal is a laser medium widely used in commercial lasers due to its large product of stimulated emission cross-section and fluorescence lifetime at 1064 nm. The product of the two parameters for 1 at. % Nd:YVO 4 is more than twice that of 1 at. % Nd:YAG at room temperature [1], resulting in a larger small-signal coefficient. Therefore, the Nd:YVO 4 crystal has an advantage in high-gain solid-state laser systems.
As we all know, the heat deposition in rod and slab diode pump solid-state laser (DPSSL) causes high temperature gradients between the pumped region and cooled surface, with potentially strong thermal lensing arising, leading to limited output power scaling, and thermal aberrations that increase losses and degrade the beam quality of the output beam. One successful strategy for power-scaling bulk DPSSL is cryogenic cooling, that is to aggressively cool the gain medium to lower temperatures (T < 150 K). Recent developments in cryogenically cooled lasers demonstrated record high optical-to-optical efficiencies and a great potential for further power scaling [2,3], mainly due to the improvement of spectroscopic and thermo-optical performance [4,5].
In the case of cryogenic solid-state laser development, the knowledge of these parameters in the 80-300 K temperature range is critically important for laser design. To the best of our knowledge, the thermal expansion coefficient and thermo-optic coefficient for the YVO 4 single crystal in the 80-320 K temperature range are available [6]. However, only the specific heat and thermal conductivity above room temperature has been studied [7,8]. Also, the absorption and emission cross-sections below room temperature has not been reported, though it is readily available in the temperature range from 290 to 430 K [9,10].
In this work, we report a complete thermo-optic properties of Nd:YVO 4 crystal at cryogenic temperatures, including specific heat, thermal conductivity, and thermal expansion. The spectroscopic characterization related to the laser performance, such as absorption, fluorescence, and lifetime are also studied in detail at 77, 150, and 300 K. Especially, continuous wave (CW) laser performances under various cryogenic temperatures were demonstrated. A maximum output power of 2.13 W was obtained at an incident pump power of 3.7 W at 150 K, corresponding to an optical-tooptical conversion efficiency of 57.6%.

Materials and methods
The measurements presented below have been carried out using 0.5 at.% a-cut Nd:YVO 4 crystals with 20 mm and 1 mm thickness for thermo-optic properties and spectroscopic characterization, respectively. Thermal expansion behavior was implemented using a thermal dilatometer (Linseis L75) and the uncertainty of the thermal expansion is estimated to be within 1%. The specific heat was measured by a thermal relaxation method [11] using a physical property measurement system (PPMS-14). Thermal conductivity was accomplished using a PPMS-9 by the steady-state longitudinal heat-flow method [12]. The uncertainty of the present measurements is estimated to be within 3% for the thermal conductivity and 1% for the specific heat.
A fluorescence spectrometer (Edinburgh FLSP980) was used to characterize the fluorescence spectra of the crystal with an exciting source of a Xenon lamp and absorption measurements can be carried out using the same setup. The spectral features narrow at reduced temperature and excellent resolution is required to fully resolve the lines. In our measurements, spectral resolution was 0.1 nm over the entire region to reduce errors caused by insufficient resolution. All measurements were performed across the temperatures range from 77 to 300 K.

Thermal properties
The measured specific heat versus temperature is illustrated in Fig. 1. The data are fitted to a second-order polynomial of the form in the range of approximately 77-300 K by: It is found that the specific heat decreases progressively as the temperature is droped from 300 to 77 K. At 300 K, the specific heat is about 0.55 J/gK, this value is more three times higher than that at 77 K (0.17 J/gK). It must be also noticed that the value at 300 K is consistent with the value (0.56 J/gK) measured at 300 K for Nd:YVO 4 [9].
The measured thermal conductivity versus temperature is shown in Fig. 2. The data are also fitted to a second-order polynomial of the form in the range of approximately 77-300 K by: At room temperature, the thermal conductivity was measured to be 8.1 W/mK, a little smaller than the corresponding values of 8.9 W/mK for pure YVO 4 [9] and 8.8 W/mK for 0.5 at% Nd:YVO 4 [10]. The thermal conductivity of doped laser materials is strongly affected by the presence of impurities, and in particular, the doping density of lasing ions [13].
Furthermore, the thermal conductivity value exhibits a tendency of increasing with declining temperature due to the decrease in disorder. The thermal conductivity value at 300 K is about 8.1 W/mK, while the thermal conductivity at 77 K is about 23.1 W/mK, almost an increase by a factor of 3 above the room-temperature value.
The thermal expansion coefficient was measured over a temperature range 77 to 300 K, as shown in Fig. 3. It is clearly seen that the thermal expansion coefficient decreases with the reduction in temperature with a value range of 2.11-0.27 × 10 -6 /K, which is consistent with those reported in Ref. [8]. According to Ref. [14], the thermal expansion coefficient for Nd:YVO 4 in the temperature range can be fitted by: At 77 K, the Nd:YVO 4 has significantly lower thermal expansion, benefiting from this characteristic in the highpower laser field because of a reduction in the thermal lens effect caused by thermal expansion of the crystal [15].

Spectroscopic properties
The absorption spectra of Nd:YVO 4 at the three temperatures of 77, 150, and 300 K were measured using a spectrometer mentioned above. From these measurements corrected to Fresnel losses, the absorption cross-section in the wavelength range of 700-950 nm was calculated, shown in Fig. 4. It is found that the absorption bands occur around 739-769, 793-823, and 875-900 nm, and represent absorption from the 4 I 9/2 ground-state manifold to various excited-state manifolds. It is observed that the largest absorption crosssection is located at 808 nm with σab = 1.92 × 10 −19 cm 2 at = −0.46 × 10 −6 +0.94 × 10 −8 T room temperature, which is slightly larger than that of a-axis Nd:YVO 4 and smaller than that of c-axis Nd:YVO 4 [7]. Considering the absorption peak around 808 nm, the peak absorption cross-section at 150 K is about 2.3 times higher as compared to room temperature. The cross-section of the absorption peak at 880 nm at 77 K is increased by a factor of 1.2 than the value at 300 K. It should be noted that most peaks are maximized at 77 K, the absorption peak around 808 nm is maximized at 150 K, suggesting the relatively complex energy level structure of Nd:YVO 4 , and the changing of Boltzmann populations and energy levels with temperature. Therefore, we calculated Boltzman occupation factors at different temperatures, as shown in the inset of Fig. 4. This is attributed to the fact that this peak consists of two lines, 0 (Z1) → 12,366 cm −1 and 173 (Z3) → 12,542 cm −1 , and the fractional population in Z3 starts to diminish quickly below 150 K, while Z1 is gaining over the same temperature range, a twofold increase at 77 K with respect to room temperature. As shown in Fig. 4, all peaks in the pump band show a reduction in bandwidth for lower temperatures. The transition at 808 nm shows narrowing as temperature is lowered and has a FWHM bandwidth of ~ 0.69 nm at 77 K. In addition, the peak absorption wavelength is 809.2 nm at 300 K and shifts only slightly to 809.1 nm at 77 K. The absorption wavelength shifting rate with respect to the temperature is found to be only 4.48 × 10 −4 nm·T −1 .
The emission cross-section was calculated with the measured f luorescence spectrum based on the Fuchtbauer-Ladenburg (F-L) formula [16], as shown in Fig. 5. At room temperature, the peak at about 1064 nm exhibits the strongest fluorescence emission in the Nd:YVO 4 crystal and the corresponding emission cross-section is 8.74 × 10 -19 cm 2 , smaller than the value of 13.7 × 10 -20 cm 2 reported in Ref. [17]. Also visible in Fig. 5 is the peak cross-section at 150 K around the 1064 nm (15.39 × 10 -19 cm 2 ) almost two times higher than that at 300 K. It is worth noting that the peak crosssection slightly reduced as the temperature drops down to 77 K (14.46 × 10 -19 cm 2 ). The FWHM bandwidth is reduced from 1.78 nm at 300 K to about 0.53 nm at 77 K. The peak emission wavelength is centered at 1064.7 nm at 300 K and shifts only slightly to 1064.4 nm at 77 K. The emission wavelength shifting rate is found to be approximately 13.45 × 10 −4 nm-T −1 .
The fluorescence lifetime at all temperatures was characterized by a single exponential decay function. Figure 6 shows the typical results of the measured fluorescence lifetime from 4 F 3/2 → 4 I 11/2 transition with respect to the temperature. The fluorescence lifetime is 90 μs at 300 K, which is close to that reported earlier in Ref. [18]. It can be seen from Fig. 6 that lifetime at upper laser level ( 4 F 3/2 ) decreases when the temperature is lowered. Particularly, the fluorescence lifetime is 90 μs at 300 K and 61 μs at 77 K, respectively. The fluorescence lifetime decreases rather than increases when the sample temperature is lowered, which is consistent with Ref. 16. Excitation migration with impurity quenching may be the cause of the anomalous temperature dependence [19]. Figure 7 schematically depicts the experimental setup for the cryogenic laser system. The gain medium was a 0.5 at.% a-cut Nd:YVO 4 crystal rod with 3 mm in diameter and 10 mm long. The front facet was coated for high reflection (> 99.9%) at 1064 nm and high transmission (> 95%) at 808 nm. The rear facet was coated for partial reflection (90%) at 1064 nm. The rod was wrapped with indium foil and mounted in a copper holder to improve the heat dissipation efficiency. The latter was attached to a cold finger of a temperature-controlled cryostat and placed in a vacuum chamber with two plane-parallel optical windows (M1 and M2) coated for 99.8% transmittance over a wavelength range of 800-1100 nm. The cooling power available was limited to about 5 W. A dichroic mirror M3 with 45° HR coated at 1064 nm and AR at 808 nm was employed to separate the generated laser beam from the residual pump beam. The pump source was a continuous wave 20 W fiber coupled laser diode with a core diameter of 200 µm and a numerical aperture of 0.22, emitting at 808 nm with a spectral bandwidth of ~ 2.5 nm. The pump beam was imaged onto the crystal in 1:2 ratio using a pair of focusing lenses.

Laser performance
The input-output characteristics of the Nd:YVO 4 laser were measured over a temperature range from 290 to 80 K. The threshold pump power as a function of the temperature  Fig. 8(a). At 290 K, it was found to be approximately 0.26 W. As we lowered the temperature from 290 to 80 K, the threshold power attained a minimum value of approximately 0.10 W at 200 K. At 80 K, the threshold increased to approximately 0.25 W. The dependence of the output power P out on the temperature, observed with an incident pump power of 3.7 W, is shown in Fig. 8(b), where we can see that it increases from 1.83 W to a maximum value of 2.13 W when the temperature changes from 290 to 150 K. When the temperature was further decreased to 80 K, the output power dropped to 2.06 W. Also, at the maximum output power of 2.13 W, a maximum optical-to-optical conversion efficiency of 57.6% was achieved. Transverse distributions of the 1064-nm laser at 150 K with the incident pump power of 3.7 W are shown in the inset of Fig. 8(b). The nearly Gaussian distributions at 150 K reveal a great improvement for the laser beam quality at lower temperature.
The measured absorption efficiency for the Nd:YVO 4 crystal around 808 nm as a function of temperature is shown in Fig. 9. Despite the increase in peak absorption cross-section at 808 nm, the actual pump-absorption efficiency reduced due to the narrowing linewidth, going from 83% at room temperature to 67% at 80 K. We believed that the optional temperature for a-cut Nd:YVO 4 laser is the combining effects of the absorption efficiency, the emission cross-section, the degradation of the thermal lensing, and birefringence in the laser crystal.

Conclusion
In this paper, the thermal properties of Nd:YVO 4 crystal, such as thermal expansion, specific heat, and thermal conductivity are systematically investigated with the temperature variation from 77 to 300 K. Further, the absorption spectra, emission spectra, and fluorescence lifetimes are measured in the temperature range 77-300 K. CW laser operation of Nd:YVO 4 at 1064 nm with LD pumping under cryogenic temperature is demonstrated. A maximum output power of 2.13 W is obtained with optical-to-optical conversion efficiency of 57.6% with respect to the incident pump power. Further power scaling can be expected by employing more powerful LD pump source and improving the cryostat. Data availability The datasets generated and/or analyzed during the current study are available from the corresponding author on request.

Competing interests
The authors declare no competing interests.