Introduction

Er3+ ion based laser materials operating at around 2.8 μm have attracted increasing interest in recent years because of their various important applications1,2,3,4. Lasers around this specific region can be utilized for medical surgery, dentistry, remote sensing etc, because of the strong water absorption around this spectral region5,6. Moreover, lasers around 2.8 μm are suitable pumping source for far-infrared optical parametric oscillation (OPO), which have broad applications in scientific research, atmospheric pollution monitoring, and directional infrared countermeasure7.

However, the intrinsic self-terminating effect of Er3+ ion impedes the improvement Er3+ ion based mid-infrared lasers. Because the fluorescence lifetime of the initial laser level (Er3+:4I11/2) is considerably shorter than that of the terminal laser level (Er3+:4I13/2), and thus will lead to high threshold, low slope efficiency and even self-terminating of continuous-wave (CW) laser. One major solution to suppress this detrimental effect is to increase Er3+ doping concentration. One key point of this solution is that with the increase of Er3+ concentration, the lifetime of the Er3+:4I13/2 level quenches faster than that of the Er3+:4I11/2 level, thus narrow the lifetime gap of these two levels3,8. Another key point of high Er3+ doping is to enhance the energy transfer process between Er3+ ions by shortening the Er3+ spacing with high doping concentration. However, the drawback of high Er3+ doping is that the crystal quality and thermal conductivity will degenerate significantly with the increasing of Er3+ concentration. Co-doping deactivating ions, such as Pr3+ 1,6,7,9, Nd3+ 2,10, Ho3+ 11 etc, is another way to overcome this “bottleneck” effect by effectively depopulating the Er3+:4I13/2 level, but only a few have achieved laser output. Furthermore, after co-doping deactivating ions the theoretical slope efficiency limit will be lower, because part of the energy of Er3+ ions is depleted by deactivating ions12,13. And deactivating ions also will have inevitable negative effect on Er3+:4I11/2 level due to complicated energy transfer process1,9. Hence, host materials that can achieve mid-infrared CW laser under low Er3+ doping concentration would be more favorable.

In Er3+ based oxide crystals, such as Y3Al5O12 (YAG)11,14,15, Gd3Ga5O12 (GGG)16 and Gd3−xYxSc2Ga3O12 (GYSGG)1,7 etc, the Er3+ doping concentrations vary from 30 at.% to as high as 50 at.%. Lu2O3 crystal has the lowest Er3+ doping concentration of 7at.% among that group8. Such high doping concentration not only significantly decreases the thermal conductivity of crystal but also leads to extremely short lifetime of the upper laser level (Er:4I11/2)8,14. Compared with oxide crystals mentioned above, SrF2 crystal same as CaF2 crystal have an key advantage that is rare earth ions tend to form clusters even when the doping concentration is low17,18. Such clusters additional shorten the distance between Er3+ ions and thus enhances the energy transfer process between them, which is beneficial for achieving mid-infrared laser under low doping concentration. In addition, SrF2 crystal has an even lower phonon energy of ~280 cm−1 than ~322 cm−1 of CaF2 crystal19,20, which is much lower than that of most oxide crystals like YAG (700 cm−1)21. Low phonon energy plays an important role in reducing the probability of non-radiative transition, prolonging the fluorescence lifetime of the upper laser level and improving the laser performance.

Until now the highest reported slope efficiency (η) from Er:CaF2 crystal is 30%5 pumped by a Ti:sapphire laser, which is comparable with those best results reported in other famous Er-doped crystals. Such as 30at,% Er:Gd3Sc2Ga3O12 (GSGG) (η = 36%)22, 15at.% Er:LiYF4 (η = 35%)23, 50at.% Er:YAG (η = 34%)24, and 7at.% Er:Lu2O3 (η = 36%)8. However, the highest reported slope efficiency from diode-pumped Er:CaF2 crystal is much lower (η = 4%)25. There is less reports about Er:SrF2 crystal and the highest reported slope efficiency is 11%25,26.

This work we aim to investigate and compare the spectral properties and laser performance of Er:SrF2 and Er:CaF2 single crystals. We also report dual-wavelength continuous-wave lasers around 2.8 μm in both 4at.% and 10at.% Er:SrF2 single crystals under 972 nm laser diode (LD) end pumping. 4at.% Er:SrF2 demonstrated the best laser performance with a low threshold of 0.100 W, a high slope efficiency of 22.0% and an maximum output power of 0.483 W.

Materials and Methods

4at.% Er:SrF2, 10at.% Er:SrF2, 4at.% Er:CaF2 and 11at.% Er:CaF2 crystals were grown by traditional vertical Bridgman method, using high purity (>99.995%) ErF3, SrF2 and CaF2 crystalline powders as raw materials. The raw materials were ground, mixed, and sealed with additional deoxidant in platinum crucibles during the whole process of growth. The crystal samples were cut and then polished into a size of 10 × 10 × 1.0 mm3 for spectral measurements.

The actual concentrations of Er3+ ions in the grown crystals were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) method. Absorption spectra were measured by a Cary 5000 UV/VIS/NIR spectrophotometer. 1.5 μm, 2.7 μm emission as well as the fluorescence lifetimes of Er3+:4I13/2 level were measured with an Edinburgh FLSP920 fluorescence spectrophotometer, using an 980 LD as excitation source. And the fluorescence lifetimes of Er3+:4I11/2 level were measured with an Edinburgh FLS920 fluorescence spectrophotometer, using an ns OPO laser, emitting at 980 nm, as excitation source. All the measurements were carried out at room temperature.

Results and Discussion

Ions concentrations

The actual concentrations of Er3+ ions in these samples were measured by ICP-AES. The values were 7.96 × 1020, 21.84 × 1020 cm−3 for 4at.% ErSrF2 and 10at.% Er:SrF2, respectively. And 9.72 × 1020, 25.84 × 1020 cm−3 for 4at.% Er:CaF2 and 11at.% Er:CaF2, respectively.

Absorption spectra

The absorption spectra of four crystals, namely 4at.% Er:SrF2, 10at.% Er:SrF2, 4at.% Er:CaF2 and 11at.% Er:CaF2, were illustrated in Fig. 1a. Within the range from 300 nm to 1700 nm, 13 main absorption bands of Er3+ corresponding to the transitions from ground state Er3+:4I15/2 to higher levels were marked in Fig. 1a. The absorption spectra of Er:SrF2 and Er:CaF2 had same numbers of bands located in similar positions. Only the shape of two absorption bands, corresponding to the transition of Er3+:4I15/2 to Er3+:4I11/2 and Er3+:4I15/2 to Er3+:4I13/2, were slightly different.

Figure 1
figure 1

Absortption spectra (a) and cross-sections (b) of 4at.% Er:SrF2, 10at.% Er:SrF2, 4at.% Er:CaF2 and 11at.% Er:CaF2 crystals at room temperature.

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With the absorption coefficients and the actual concentrations of Er3+, the absorption cross-sections around 980 nm were calculated and shown in Fig. 1b. From Fig. 1b, the differences of this band between Er:SrF2 and Er:CaF2 crystals could be seen clearly. Er:CaF2 crystals had two quite even peak points located in 968 nm and 980 nm, respectively. While Er:SrF2 crystals had a slightly narrower band than Er:CaF2 crystals and the peak values of 971 nm were significantly higher than that of 980 nm. The detailed values of the absorption coefficients (α), the absorption cross-sections (σabs) and the full widths at half-maximum (FWHM) of these bands were listed in Table 1.

Table 1 The detailed values of α, σabs and FWHM of the 980 nm absorption bands.
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In this research, an InGaAs LD emitting at 972 nm was employed as the pumping source when proceeding the mid-infrared continuous-wave (CW) laser experiments. Hence, under this pumping condition Er:SrF2 crystals had an advantage over Er:CaF2 crystals.

Emission spectra

The mid-infrared (MIR, 2500–2860 nm) emission spectra of 4at.% Er:CaF2, 11at.% Er:CaF2, 4at.% Er:SrF2 and 10at.% Er:SrF2 crystals were measured with 980 nm LD excitation under the same condition. Both Er:CaF2 and Er:SrF2 crystals had broad MIR emission bands, which were beneficial for achieving ultra-short pulse laser and tunable laser operation.

Based on the Fuchtbauer-Ladenburg equation27,28 and the MIR emission spectra, the MIR stimulated emission cross-sections were calculated and illustrated in Fig. 2:

Figure 2
figure 2

MIR stimulated emission cross-sections of 4at.% Er:CaF2, 11at.% Er:CaF2 (a), 4at.% Er:SrF2 and 10at.% Er:SrF2 (b) crystals at room temperature.

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where λ is the wavelength, I(λ) is the intensity of emission spectrum, I(λ)/∫λI(λ)dλ is the profile function of emission spectrum, c is the speed of light in vacuum, n is the refractive index, and AJ′→J′ (J = 4I11/2, J′ = 4I13/2) is the corresponding radiative transition probability, which is calculated by Judd-Ofelt theory from the absorption spectrum28. If the emission spectrum, refractive index and radiative transition probability are known, then the emission cross-section can be calculated from equation 1.

Four calculated emission cross-sections (σem) at 2727 nm were 0.78 × 10−20, 0.65 × 10−20, 0.65 × 10−20, 0.53 × 10−20 cm2 for 4at.% Er:SrF2, 4at.% Er:CaF2, 10at.% Er:SrF2 and 11at.% Er:CaF2, respectively. It was obvious that 4at.% Er:SrF2 had the highest value of σem (0.78 × 10−20 cm2 at 2727 nm), which was 20% higher than that of 4at.% Er:CaF2. High emission cross-section was favorable in achieving high performance of MIR laser operation.

Figure 3a showed the up-conversion emission spectra of these four crystals measured under 980 nm LD excitation at room temperature. Both up-conversion emission spectra of Er:CaF2 and Er:SrF2 contained green and red emission bands, corresponding to Er3+:2H11/2 + 4S3/2 → 4I15/2 and Er3+:4F9/2 → 4I15/2. However, obvious differences could be told from Er:CaF2 and Er:SrF2. Firstly, in Er:CaF2 with the increase of Er3+ concentration both green and red up-conversion increased quite evenly. While in Er:SrF2 only green up-conversion increased with the increase of Er3+ concentration. Secondly, the proportion of green and red emission was different in Er:CaF2 and Er:SrF2. The proportion of green and red emission were approximately 1:7.8 and 1:5.7 for 4at.% Er:CaF2 and 11at.% Er:CaF2, respectively. And for 4at.% Er:SrF2 and 10at.% Er:SrF2 were 1:4.1 and 1:2.3, respectively.

Figure 3
figure 3

Up-conversion emission spectra at room temperature (a) and energy level diagram of Er3+ ions (b).

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In order to understand the up-conversion emission spectra of Er:SrF2 and Er:CaF2, the energy level diagram of Er3+ was presented in Fig. 3b. The possible mechanisms of energy transfer processes between Er3+ ions were illustrated, which were similar to that proposed by C. Labbe5. Two excited state absorption (ESA) processes, namely ESA 1 and ESA 2. Two energy transfer up-conversion (ETU) processes, namely ETU 1 and ETU 2. And one cross relaxation process (CR). ESA 1: Er3+:4I11/2 + hv → 4F7/2. ESA 2: Er3+:4I13/2 + hv → 4F9/2. ETU 1: Er3+:4I13/2 + 4I13/2 → Er3+:4I15/2 + 4I11/2. ETU 2: Er3+:4I11/2 + 4I11/2 → Er3+:4I15/2 + 4F7/2. CR: Er3+:4S3/2 + 4I15/2 → Er3+:4I9/2 + 4I13/2.

Among these energy transfer processes, ESA 1 and ETU 2 were detrimental because they reduced the population of the upper laser level (Er3+:4I11/2) and then resulted in the green emission. It was worth noting that the phonon energy of CaF2 and SrF2 were about 322 cm−1 and 280 cm−1. And the energy gap between Er3+:4S3/2 and 4F9/2 was about 3000 cm−1. Thus the non-radiative transition probability between Er3+:4S3/2 and 4F9/2 should be quite low. So ESA 2 should be the major process that was responsible for the red emission. ETU 1 was beneficial because it deactivated the lower laser level (Er3+:4I13/2) and partly repopulated the upper one (Er3+:4I11/2), which was the key to suppress the self-termination problem in Er3+ singly-doped crystals. As could be seen that both ETU 1 and ESA 2 relied on the population of lower laser level (Er3+:4I13/2), so strong red emission would have negative effect on ETU 1 process. In conclusion, both green and red up-conversion emission were detrimental to achieving MIR laser operation.

Hence, samples with weaker green and red up-conversion emission intensity would be more suitable for MIR laser operation. 4at.% Er:SrF2 and 4at.% Er:CaF2 crystals might have better laser performance than the other two highly doped crystals.

Fluorescence lifetime

The fluorescence decay curves of Er3+:4I11/2 and Er3+:4I13/2 levels of these four crystals were recorded upon direct excitation to Er3+:4I11/2 at room temperature and presented in Fig. 4.

Figure 4
figure 4

Decay curves of Er3+:4I11/2 level (a) and Er3+:4I13/2 level (b) in 4at.% Er:SrF2, 10at.% Er:SrF2, 4at.% Er:CaF2 and 11at.% Er:CaF2 crystals at room temperature.

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The decay curves of the Er3+:4I11/2 level still fitted single-exponential decay well, which confirmed that quench effect and energy transfer processes had less influence on this level. On the contrary, the decay curves of the Er3+:4I13/2 level showed multi-exponential behavior, which confirmed that the lower laser level was greatly affected by the quench effect and energy transfer processes. Multi-exponential decay curves could be fitted with equation 229,30:

And then, the fluorescence lifetimes were calculated by equation 329,30:

The fluorescence lifetimes of both laser levels (τ1, fluorescence lifetime of the upper laser level, τ2, fluorescence lifetime of the lower laser level) were listed in Table 2. Benefited from lower phonon energy, the τ1 of Er:SrF2 crystals were much longer than that of Er:CaF2 crystals. And longer lifetime of the upper laser level meant easier to achieve population inversion and better energy storage capacity. Even though τ2 were also extended in Er:SrF2 crystals, the values of Δτ/τ1 in Er:SrF2 crystals were still smaller than that of Er:CaF2 crystals.

Table 2 Fluorescence lifetimes (τ1, τ2), MIR emission cross-section (σem) and spectral quality factors (Q) of 4at.% Er:SrF2, 10at.% Er:SrF2, 4at.% Er:CaF2 and 11at.% Er:CaF2 crystals at room temperature.
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With the MIR stimulated emission cross-sections and fluorescence lifetimes of Er3+:4I11/2 level the spectral quality factors (Q) were calculated and listed in Table 2. And the results were 7.45 × 10−20 ms*cm2, 3.89 × 10−20 ms*cm2, 4.83 × 10−20 ms*cm2, 2.62 × 10−20 ms*cm2, for 4at.% Er:SrF2, 4at.% Er:CaF2, 10at.% Er:SrF2, and 11at.% Er:CaF2 crystals, respectively. It was clear that in both SrF2 and CaF2, crystals with lower Er3+ doping level had larger spectral quality factors, which again indicated that 4at.% Er:SrF2 crystal might have better laser performance than 10at.% Er:SrF2. 4at.% Er:CaF2 had already been proven to have the best laser performance among a series of Er:CaF2 crystals in our previous work3.

Three crystals, namely 4at.% Er:SrF2, 10at.% Er:SrF2 and 4at.% Er:CaF2, were chosen in the following laser experiments. Thus, the laser properties of lightly doped and highly doped Er:SrF2 crystals could be compared. Further more, the laser properties of Er:SrF2 and Er:CaF2 crystals could also be compared.

Laser experiments

The CW laser experiments were carried out with a 17 mm long concave-plane laser resonator. The setup was shown in Fig. 5. Two output coupler with different output transmittance (T), T = 1% and T = 3% at 2.7–2.95 μm, were used to obtain the optimum laser output. The laser samples were mounted in a copper block and placed next to the output coupler. The copper block was kept at 10 °C by water cooling. Three uncoated laser samples were in dimensions of 3 × 3 × 10 mm3 for 4at.% Er:CaF2, 4at.% Er:SrF2 and 3 × 3 × 6 mm3 for 10at.% Er:SrF2. Finally, CW laser operations were demonstrated around 2.8 μm under 972 nm LD pumping, and the results were shown in Fig. 6. The average output power was measured by a power meter (30A-SH-V1, Israel).

Figure 5
figure 5

Schematic of the experimental setup for mid-infrared continuous-wave laser operation.

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Figure 6
figure 6

Laser operation of 4at.% Er:SrF2 4at.% Er:CaF2 and 10at.% Er:SrF2, average output power versus absorbed pump power (a) and dual-wavelength of laser emission (b).

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As shown in Fig. 6a, CW laser operations around 2.8 μm were demonstrated in both lightly doped 4at.% Er:SrF2, 4at.% Er:CaF2 crystals and highly doped 10at.% Er:SrF2 crystal. Both 4at.% Er:SrF2 and 4at.% Er:CaF2 demonstrated better laser performance with the T = 3% output coupler than the T = 1% one.

Figure 6b showed that both 4at.% Er:SrF2 and 10at.% Er:SrF2 had dual-wavelength property, which was not found in Er:CaF2 crystals. The CW laser spectra were measured with an optical spectrum analyzer (MS3504i, SOL instruments, Belarus). Two laser wavelengths of 4at.% Er:SrF2 were 2789.3 nm and 2791.8 nm. Two laser wavelengths of 10at.% Er:SrF2 were 2786.4 nm and 2790.7 nm. And the FWHMs of laser spectra were less than 0.20 nm as marked in Fig. 6b.

4at.% Er:SrF2 crystal was proved to have the best laser performance among these three crystals, which was in agreement with the expectations based on spectral parameters. When T = 1%, the maximum output power of 0.293 W corresponding to a low threshold of 0.100 W was obtained with a slope efficiency of 12.5%, which was better than the slope efficient 11% of a 5at.% Er:SrF2 reported by T.T. Basiev25. When T = 3%, the slope efficiency was significantly improved to 22.0% with a better maximum output power of 0.483 W. And the threshold was maintained at 0.100 W, which was surely benefited from the low phonon energy and long fluorescence lifetime of 4at.% Er:SrF2 crystal.

4at.% Er:CaF2 crystal had worse laser performance than 4at.% Er:SrF2 crystal. When T = 1%, the maximum output power of 0.213 W corresponding to the threshold of 0.175 W was obtained with a slope efficiency of 10.6%. When T = 3%, the slope efficiency was improved to 17.2% with a better maximum output power of 0.304 W. However, the threshold also increased to 0.335 W, which was much higher than 4at.% Er:SrF2 crystal.

10at.% Er:SrF2 crystal only achieved CW laser operation with the T = 1% output coupler. Due to the degeneration of crystal quality and thermal conductivity, the maximum output power of 10at.% Er:SrF2 was only 0.057 W corresponding to a slope efficient of 6.8%.

These results confirmed the expectations based on spectral parameters that lightly doped 4at.% Er:SrF2 crystal was a promising candidate for low threshold, high slope efficiency mid-infrared lasers. Better results could be expected after optimizing the crystal growth and coating the samples.

Conclusion

Compared with Er:CaF2 crystals, Er:SrF2 crystals had larger absorption cross-sections, larger MIR emission cross-sections, much longer fluorescence lifetimes and unique dual-wavelength laser properties. Lightly doped 4at.% Er:SrF2 crystal had better spectral parameters than both highly doped 10at.% Er:SrF2 crystal and Er:CaF2 crystals. As expected by spectral parameters, 4at.% Er:SrF2 crystal demonstrated the best laser performance with a low threshold of 0.100 W, a high slope efficiency of 22.0% and an maximum output power of 0.483 W. Hence, lightly doped 4at.% Er:SrF2 single crystal was a promising candidate for achieving for low threshold, high slope efficiency mid-infrared lasers.

Additional Information

How to cite this article: Ma, W. et al. Highly efficient dual-wavelength mid-infrared CW Laser in diode end-pumped Er:SrF2 single crystals. Sci. Rep. 6, 36635; doi: 10.1038/srep36635 (2016).

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