1. Introduction

There is a continuing interest in the development of high efficient solid-state lasers operating around 3 μm for their essential various applications. The first mentioned is the laser surgery because of strong water absorption around this spectral region. Moreover, lasers around 3 μm are the suitable pumping sources for far-infrared optical parametric oscillation or optical parametric generation (OPG) [1], which has important applications in scientific research, atmospheric monitoring and directional countermeasure. Solid-state lasers based on the ⁴I_11/2→⁴I_13/2 emission transition of Er³⁺ doped crystalline materials have been extensively researched [2–6] for the ~2.8 μm emission, which can be directly pumped by the commercial high power InGaAs laser diodes emitting around 980 nm. However, the fluorescence lifetime of the upper laser level (Er³⁺: ⁴I_11/2) is rather shorter than that of the terminal laser level (Er³⁺: ⁴I_13/2), resulting in a high threshold, low slope efficiency and even self-terminating of CW laser. To narrow the lifetime gap between the upper and lower level, co-doping deactivating ions such as Pr³⁺, Nd³⁺, Ho³⁺ has been demonstrated a feasible solution [7–9]. Whereas, the lifetime of Er³⁺: ⁴I_11/2 also decreased apparently with the deactivating ions co-doped, and additionally doping deactivating ions will limit the theoretical slope efficiency because part of energy injected is depleted by the deactivating ions [10]. Another effective solution to suppress the self-terminating effect is to increase Er³⁺ doping concentration [11–13]. With the increase of Er³⁺ ions, the Er³⁺: ⁴I_13/2 level will be depopulated and Er³⁺: ⁴I_11/2 level repopulated by the cross-relaxation process (Er³⁺: ⁴I_13/2 + ⁴I_13/2 → ⁴I_9/2 + ⁴I_15/2) between adjacent Er³⁺ ions as illustrated in Fig. 1, which allows for efficient population inversion and enhancement of the quantum efficiency [3,6]. However, higher doping will significantly degenerate the crystal quality and thermal conductivity.

 

Fig. 1 Schematic of the cross-relaxation cooperative process between adjacent Er³⁺ ions.

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Among numbers of monocrystalline host materials available, CaF₂/SrF₂ crystal with fluorite structure outstands for its unique advantage that rare earth ions tend to form cluster at very low doping level [14,15]. Contrary to the quenching effect of Nd³⁺ ions clustering for ~1μm fluorescence [16], the spontaneous clustering additionally shortens the distance between Er³⁺ ions and thus enhances the cross-relaxation process but not brings to much extra stress that may affect the host materials, improving the thermo-mechanical properties of the doped crystals. Furthermore, the heterovalent substitution induces multisite properties of Er³⁺ which gives rise to broad absorption and emission bands. Besides, the low phonon energy of CaF₂ (~322cm⁻¹)/SrF₂ (~280cm⁻¹) crystal [17,18], which plays an important role in reducing the probability of non-radiative transition, endows the upper level with lifetime in the millisecond range. This lifetime is dozens of times that of oxide crystals and beneficial for improving the CW laser performance. In 2002, Labbe et al. [19] obtained Er³⁺: CaF₂ laser pumped by a Ti: Sapphire laser with a slope efficiency of 34% and output power of 23 mW. In 2016, pumped by a laser diode, the dual-wavelength continuous-wave laser was realized in Er³⁺: SrF₂ crystals by Ma et al. [20] with a slope efficiency of 22% and maximum output power of 483 mW.

CaF₂ and SrF₂ have been demonstrated to form continuous solid solution in the whole range of components with the fluorite structure reserved [21] and the cation sites are randomly occupied by Ca²⁺ as well as Sr²⁺ [22,23], resulting in the disordering of local lattice which diversifies the site structures. It is demonstrated that the mixed crystals possess better mechanical and electrical characteristics, whereas their optical properties are intermediate between those of CaF₂ and SrF₂ [24]. It is well known that the luminescent properties of active ions are strongly dependent on the local sites they occupy. Considering that there are various sites in the mixed host crystals which derive from that in CaF₂ as well as SrF₂, it’s probable that the luminescent properties of active ions doped could be regulated between these of both end-member crystals by tuning the ratio of component concentration. Recently, J. Liu et al. [25] demonstrated CW laser operation in Er doped CaF₂-SrF₂ mixed with a maximum output power of 712 mW and a slope efficiency of 41.4%, which is the highest slope efficiency achieved in fluoride crystals with a laser diode pumping. The doping level of 4at. % is far lower than that in other crystals. However, there are few reports about the spectral properties of this promising MIR laser material.

In this work, Er³⁺: Ca_xSr_1-xF₂ (x = 0, 0.5, 0.7, 1) single crystals were successfully grown by the temperature gradient technique (TGT). Powder x-ray diffraction was used to characterized the phase of all samples. Their composition dependent spectral properties including absorption intensity, emission intensity and fluorescence decay were systematically studied here. Laser performance of the Er doped single mixed crystals were compared with different wavelengths pumping.

2. Materials and experiments

4at. % Er³⁺: Ca_xSr_1-xF₂ (x = 0, 0.5, 0.7, 1) crystals were grown by the temperature gradient technique (TGT). High purity (>99.995%) ErF₃, CaF₂, and SrF₂ powders as raw materials were mixed according to the chemical ratios and put into the sealed graphite crucible with additional PbF₂ (1.2wt. %) powder added into as the oxygen scavenger. The mixtures were melted at 1400 °C and then grown with reducing the temperature at a cooling rate of ∼5 °C/h. The crystal samples for spectral measurements were cut into thick disks in the size of ф20 × 1 mm, with end faces polished. All measurements were conducted at room temperature.

The actual concentrations of Er³⁺ in the grown crystals were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) method and the real concentrations of Ca^{2 +}and Sr^{2 +}were measured by X-ray fluorescence spectrometry (AXIOS, PANalytical, Holland). Powder XRD diffractograms (D/max 2550V, Rigaku, Japan) were measured using a Cu Kα radiation as source (λ = 1.542 Å).

Absorption spectra of four crystals were measured by a Lambda 750 UV/VIS/IR spectrometer with a resolution of 0.5 nm and the emission spectra were recorded with an Edinburg FLSP920 spectrometer by using a 976 nm laser diode and collected by thermoelectric cooled InGaAs detector. The fluorescence lifetimes of Er³⁺: ⁴I_11/2 and Er³⁺: ⁴I_13/2 manifold were measured by the same spectrophotometer with Tektronix TDS 3052 oscilloscope, using a pulse LD laser at 976 nm as excitation source.

The CW laser operation was conducted with the setup showed in Fig. 2. The resonant cavity consisted of an input mirror with a 50-mm radius of curvature and an output coupling (OC) plane mirror with transmissions of 1%, 3% and 5% for 2.7 ~2.95 μm. The input mirror was anti-reflection (AR)-coated for 974 nm and high-reflection coated for 2.7 ~2.95 μm. The whole length of the cavity was about 24 mm. The uncoated crystals used in this experiment were cut into 3 × 3 × 7 mm³, with both end faces polished, which were wrapped by indium foil and fixed in a copper block maintained at 12 °C by cycling cooling water. To compare the laser output characteristics, laser diodes emitting at 976 nm and 980 nm, were used as pumping sources.

 

Fig. 2 Schematic of the diode-end-pumped CW laser with a linear cavity.

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3. Results and discussions

The concentration values of Er dopant are 1.128 × 10²¹, 1.064 × 10²¹, 1.054 × 10²¹ and 0.831 × 10²¹ ions/cm³ for Er: CaF₂, Er: Ca_0.7Sr_0.3F₂, Er: Ca_0.5Sr_0.5F₂ and Er: SrF₂ crystals, respectively. The atomic ratios of Ca to Sr are 0.962 and 2.431 for Er^{3 +} : Ca_0.5Sr_0.5F₂ and Er^{3 +} : Ca_0.7Sr_0.3F₂, which agrees well with the nominal concentrations.

Figure 3 presented the Powder XRD diffractograms in 2θ range from 20° to 80°. All diffraction peaks of each sample there can be assigned to the fluorite structure. With x varied from 0 to 1, the diffraction peaks shifted to high angle, indicating that the raw materials formed homogeneous solid solution and the lattice parameter decreased with the molar ratio of Ca. The lattice parameter was calculated by software Jade 6.0 to be 5.787 Å, 5.658 Å, 5.555 Å and 5.465 Å for x = 0, 0.5, 0.7 and 1 respectively, which is in line with Vegard's rule.

 

Fig. 3 Powder XRD patterns of Er: Ca_xSr_1-xF₂ (x = 0, 0.5, 0.7, 1) crystals. The red dot line and black dot line correspond to the diffraction peak positions of pure SrF₂ (PDF chart: 06-0262) and CaF₂ (PDF chart: 35-0816) as a guide for eyes.

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Absorption spectra of four crystals were illustrated in Fig. 4. As we can see, there were 11 main absorption bands in the range of 300 ~1700 nm, corresponding to the transitions from the ground state Er: ⁴I_15/2 to higher energy levels as marked in Fig. 4(a). The bands attributed to the same transition process had the similar position, shape and intensity for each sample owing to the strong shielding effect of 5s² and 5p⁶ closed orbitals. The Absorption cross sections around 980 nm were calculated and shown in Fig. 4(b). The band of Er: CaF₂ consisted of two even peaks centered at 966 nm and 980 nm. However, for Er: SrF₂ the peak centered at 968 nm was obviously higher than the one centered at 980 nm and the peak centered at 980 nm further declined significantly in Er: Ca_0.7Sr_0.3F₂ as well as Er: Ca_0.5Sr_0.5F₂ crystal. It is worth noting that the width of absorption bands for mixed crystals were narrower than that of the pure crystals, the values of which were 22.2 nm and 17.2 nm for Er: CaF₂ and Er: SrF₂, whereas 16.9nm and 16.5nm for Er: Ca_0.7Sr_0.3F₂ and Er: Ca_0.5Sr_0.5F₂, which was contrary to what we had conjectured.

 

Fig. 4 (a) the absorption spectra of the crystals and (b) the absorption cross sections corresponding to the transition of Er³⁺: ⁴I_15/2→⁴I_11/2.

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Figure 5 showed the emission spectra recorded in the range of 1400 ~1700 nm and 2500 ~2900 nm. There was a similar tendency for the variations of the emission intensities with the composition x in both wavelength ranges. With the composition x decreased from 1 to 0, the emission intensity declined first and reached a minimum value at x = 0.5, whereas, got to the maximum when x = 0. The intensities of Er: Ca_0.5Sr_0.5F₂ is rather weaker than the three other crystals, indicating there was a strong change of the local environment of Er³⁺ ions. As demonstrated in Ref [21,22], there were different chemical environments for both F^- and Ca²⁺ in the mixed crystals with different compositions, which may play an important role in the clustering behavior of Er³⁺ and make a big influence on the site structure of the luminescence centers, leading to different luminescent properties.

 

Fig. 5 Emission spectra (a) around 1.5 μm corresponding to the transition of Er: ⁴I_13/2→⁴I_15/2 and (b) around 2.8 μm corresponding to the transition of Er: ⁴I_11/2→⁴I_13/2.

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The MIR stimulated emission cross sections were calculated by the Fuchtbauer-Ladenburg equation:

The fluorescence lifetimes of Er³⁺: ⁴I_11/2 and Er³⁺: ⁴I_13/2 manifold were measured by monitoring 1 μm and 1.5 μm emission. For Er³⁺: ⁴I_11/2 level, the decay curves exhibited single-exponential decay behavior and the lifetimes were fitted to be 5.44 ms, 4.98 ms, 4.25 ms and 8.58 ms. However, there was an obvious rising stage before it turned to fall in the decay curves of Er³⁺: ⁴I_13/2 level as shown in Fig. 6(b), which mainly resulted from the transition of Er³⁺: ⁴I_11/2→⁴I_13/2 with a 976 nm laser diode pumping. Therefore, these decay curves were fitted by Eq. (2) with the influence of Er³⁺: ⁴I_11/2→⁴I_13/2 process taken into consideration.

 

Fig. 6 Fluorescence decay curves of (a) Er³⁺: ⁴I_11/2 manifold and (b) Er³⁺: ⁴I_13/2 manifold in the crystals.

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Fig. 7 Lifetime variation with the composition (1-x) for Er³⁺ doped Ca_xSr_1-xF₂ crystals.

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Table 1. The detailed value of σ_abs, σ_em, τ and FWHM of the absorption bands around 980nm.

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To further investigate the luminescent properties of the crystals with different composition, the up-conversion emission spectra were measured at room temperature as shown in Fig. 8. The variation trend of up-conversion emission intensity with composition x was different from that of 1.5 μm as well as 2.8 μm emission and the intensity differences were considerable obvious. For Er: Ca_0.5Sr_0.5F₂, the intensity of the band centered at 671nm, corresponding to Er^{3 +} : ⁴F_9/2→⁴I_15/2, was apparently lower compared with three other crystals, however that of Er: Ca_0.3Sr_0.3F₂ is rather higher than Er: CaF₂. Weaker up-conversion emission would be more beneficial to mid-infrared (MIR) laser operation [20]. The insert clearly showed that the shape of the emission bands at 671 nm for two mixed crystals is similar, whereas different from that for the pure ones, indicating that there were differences of local environment of luminescence centers between the mixed crystals and the pure ones.

 

Fig. 8 Up-conversion emission spectra for different composition Er-doped single crystals. The insert shows the corresponding normalized emission intensity.

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Laser performances of Er doped pure single crystals, namely CaF₂ and SrF₂ crystals with the same dopant concentration, have been reported in Ref [20,28]. There the CW laser operation was conducted for Er: Ca_0.5Sr_0.5F₂ and Er: Ca_0.7Sr_0.3F₂ crystals and the results were presented in Fig. 9. For Er: Ca_0.5Sr_0.5F₂ crystal with 976 nm pumping, the maximum output power of 712 mW and slope efficiency of 41.4% were obtained with the OC transmission of 3%, which is the highest slope efficiency achieved in fluoride crystals reported in Ref [25]. When the pumping wavelength turned to 980 nm, with the OC transmission of 3%, the maximum output power and slope efficiency were 696 mW and 36.5% respectively, which were a little lower than pumped by 976 nm. For Er: Ca_0.7Sr_0.3F₂ crystal, the maximum output power reached to 738 mW with a slope efficiency of 33.3%, with the OC transmission of 5% pumped by 976 nm. However, when pumped by 980 nm, the best results were obtained with the OC transmission of 1% that the maximum output power was 540 mW and the slope efficiency was 27.7%, apparently poor than that pumped by 976 nm. It shows that different pumping wavelengths have an obvious effect on the laser performance as mentioned in Ref [29] and 976 nm was more suitable for pumping compared to 980 nm. The narrowed lifetime gap between the upper and lower level leads to higher slope efficiency.

 

Fig. 9 The average output power as a function of the absorbed pump power for Er: Ca_0.5Sr_0.5F₂ (a Ref [25], and b) and Er: Ca_0.7Sr_0.3F₂ (c and d) crystals with 976nm and 980nm LD-pumping.

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4. Conclusion

Several Er³⁺ doped Ca_xSr_1-xF₂ single crystals have been grown and powder XRD patterns demonstrated that homogeneous solid solution was formed in the mixed crystals. Spectroscopic investigation shows that there was a narrower absorption band for the mixed crystals compared to the end-member ones, which was contrary to what we had considered. The emission intensities and lifetimes had a similar variation trend with the composition x. The enhanced emission cross section and narrower lifetime gap of Er: Ca_0.5Sr_0.5F₂ contributed to the high slope efficiency and output power of CW laser [25]. Further works need to be done to study the Er: Ca_xSr_1-xF₂ crystals with different compositions to find the optimum concentration ratio.