The demonstration of a laser operation in calcium fluoride doped with samarium^[1] and uranium ions^[2] has stimulated great interest in alkaline earth fluorides activated with lanthanide ions, especially trivalent neodymium ions. Unfortunately, early works on this subject revealed that the luminescence of neodymium ion clusters, which are easily formed in these crystals, has been completely quenched by the incoherent dipole–dipole energy transfer process^[3]. As a consequence, rare earth-doped alkaline earth fluorides were discarded as laser materials.

Recently, however, interest in these systems has been renewed. The incorporation of ${\mathrm{Y}}^{3+}$, ${\mathrm{La}}^{3+}$, and ${\mathrm{Sc}}^{3+}$ ions resulted in the dissociation of the ${\mathrm{Nd}}^{3+}–{\mathrm{Nd}}^{3+}$ quenching pairs in clusters, thereby critically reducing the luminescence quenching in neodymium-doped alkaline earth fluorides^[4–8]. $\mathrm{Nd},\mathrm{Y}\text{:}{\mathrm{SrF}}_2$ crystals have achieved some interesting laser performances^[9–11]. Continuous-wave (CW) laser operation in a ${\mathrm{Nd}}^{3+}$ (2 at.%) doped ${\mathrm{Y}}^{3+}\text{:}{\mathrm{CaF}}_2$ crystal pumped by a CW Ti:sapphire laser has been observed, although the output power and efficiency were relatively low^[12]. Laser slope efficiencies of 50% in a 0.5% ${\mathrm{Nd}}^{3+}$, 5% ${\mathrm{Lu}}^{3+}\text{:}{\mathrm{CaF}}_2$ crystal and of 33% in a 1% ${\mathrm{Nd}}^{3+}$, 5% ${\mathrm{Y}}^{3+}\text{:}{\mathrm{CaF}}_2$ crystal have been achieved when pumping with a CW Ti:sapphire laser. The former sample, which had a 400 μm fiber-coupled laser-diode pumped laser, showed a slope efficiency of 20%^[13].

In this work, the effects of ${\mathrm{Y}}^{3+}$ co-doping and the doping concentrations of ${\mathrm{Nd}}^{3+}$ on the spectra properties and laser performance of $\mathrm{Nd},\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals were investigated. This research will offer valuable data to find the optimal proportions of ${\mathrm{Nd}}^{3+}/{\mathrm{Y}}^{3+}$ to achieve good laser performance. With doping concentrations of 0.5% ${\mathrm{Nd}}^{3+}$ and 10% ${\mathrm{Y}}^{3+}$, a highly efficient CW laser operation is obtained in a $\mathrm{Nd},\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystal with a slope efficiency over 30% and an output power of 0.9 W. The slope efficiency of 27.82% was also obtained in a 0.6% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystal.

The 0.5% $\mathrm{Nd}\text{:}{\mathrm{CaF}}_2$ and $x(x=0.4\%,0.5\%,0.6\%,0.8\%)$ Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ single crystals were grown using the temperature gradient technique method as described in Ref. [14]. The raw materials are ${\mathrm{NdF}}_3$ (99.99%), ${\mathrm{CaF}}_2$ (99.99%), ${\mathrm{YF}}_3$ (99.99%), and ${\mathrm{PbF}}_2$ (99%). ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Y}}^{3+}$ co-doped ${\mathrm{CaF}}_2$ samples were obtained by cutting and double-face polishing with a thickness of 2 mm. The real concentrations of ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Y}}^{3+}$ were measured by an inductively coupled plasma atomic emission spectrometry analysis with a measurement error of less than 5%. The segregation coefficients of the ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Y}}^{3+}$ ions are about 1.11 and 0.97, respectively. The absorption spectra were recorded by a Jasco V-570 UV/VIS/NIR spectro-photometer. The fluorescence spectra were obtained with a FLSP920 time-resolved fluorimeter grating blazed at 1200 nm and detected by a Hamamatsu near infrared (NIR) photomultiplier tube. The fluorescence decay curves of the samples were obtained by a Tektronix TDS 3052 oscilloscope, and the fluorescence lifetimes were obtained by a fluorescence decay curves fitting.

The room-temperature absorption spectra of the 0.5% $\mathrm{Nd}\text{:}{\mathrm{CaF}}_2$ and the 0.5% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals are shown in Fig. 1. The absorption bands corresponding to the different energy levels for the ${\mathrm{Nd}}^{3+}$ ion are shown in Fig. 1(a). The absorption band around 791 nm, which is usually used for diode pumping corresponding to the absorption transition ${}^4{\mathrm{I}}_{9/2}\to {{}^4\mathrm{F}}_{5/2}+{{}^2\mathrm{H}}_{9/2}$, is shown in Fig. 1(b). For the 0.5% $\mathrm{Nd}\text{:}{\mathrm{CaF}}_2$ crystal, one main peak at 791 nm and two shoulder peaks at 795 and 799 nm are observed. When the 10% ${\mathrm{YF}}_3$ was co-doped, the strongest absorption band was moved to 797 nm.

The room-temperature emission spectra of the 0.5% $\mathrm{Nd}\text{:}{\mathrm{CaF}}_2$ and 0.5% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals are shown in Fig. 2. The inset shows an emission band around 1.06 μm, which corresponds to the emission transition ${}^4{\mathrm{F}}_{3/2}\to {{}^4\mathrm{I}}_{3/2}$ for the 0.5% $\mathrm{Nd}\text{:}{\mathrm{CaF}}_2$ crystal. There are six emission peaks at 1036, 1046, 1062, 1081, 1092, and 1127 nm in the emission spectrum of the single neodymium-doped ${\mathrm{CaF}}_2$ crystal, indicating the diversity of the ${\mathrm{Nd}}^{3+}$ centers. When the 10% ${\mathrm{Y}}^{3+}$ ion is co-doped, the emission intensity is sharply enhanced. Furthermore, only one relatively broad emission band with a FWHM of 30 nm can be observed in the $\mathrm{Nd},\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystal, peaking at 1054 nm with a shoulder at 1063 nm.

The fluorescence lifetimes were measured to be $18.0\pm 0.2$ and $361.0\pm 3.2\mathrm{\mu s}$ for the 0.5% $\mathrm{Nd}\text{:}{\mathrm{CaF}}_2$ and the 0.5% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals, respectively. With the addition of the ${\mathrm{Y}}^{3+}$ buffer ions, the quenching effect is greatly suppressed. The ${}^4{\mathrm{F}}_{3/2}$ radiative emission lifetimes (${\tau }_{\text{rad}}$) of the ${\mathrm{Nd}}^{3+}$ ion in the crystals were calculated by the Judd–Ofelt formalism^[15,16]. The Judd–Ofelt parameters ${\mathrm{\Omega }}_2$, ${\mathrm{\Omega }}_4$, and ${\mathrm{\Omega }}_6$ are obtained from the measured absorption spectra above, as shown in Table 1. Using the Judd–Ofelt parameters, the radiative emission lifetimes were obtained. Then, the emission cross section (${\sigma }_{\text{em}}$) can be calculated using the well-known Fuchtbauer–Ladenburg expression: $${\sigma }_{\text{em}}(\lambda )=\frac{{\lambda }^4}{8\pi c{n}^2\mathrm{\Delta }\lambda {\tau }_{\text{rad}}},$$(1)where ${\tau }_{\text{rad}}$ and $n$ stand for the radiative emission lifetime and the refractive index of the material, respectively. The emission cross section of the 0.5% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystal was calculated to be $4.27\times {10}^{-20}{\mathrm{cm}}^2$. The parameters are shown in Table 1.

The absorption spectra of the four samples of Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ are presented in Fig. 3. The absorption intensity increases with the ${\mathrm{Nd}}^{3+}$ concentrations. However, the absorption cross section first increases and then decreases with a concentration of ${\mathrm{Nd}}^{3+}$ over 0.6%, as shown in Fig. 3(b). The Judd–Ofelt parameters ${\mathrm{\Omega }}_2$, ${\mathrm{\Omega }}_4$, and ${\mathrm{\Omega }}_6$ of these crystals were also obtained, and are listed in Table 2.

Figure 4 shows the emission spectra of the $\mathrm{Nd},\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals around 1054 nm, with nearly the same spectrum characteristics and FWHM of about 30 nm.

The fluorescence lifetimes (${\tau }_{\text{exp}}$) were measured to be $350.0\pm 3.2$, $361.0\pm 3.2$, $359.0\pm 2.7$, and $321.0\pm 3.1\mathrm{\mu s}$ for the four $\mathrm{Nd},\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals with Nd doping concentrations of 0.4%, 0.5%, 0.6%, and 0.8%, respectively. The radiative emission lifetimes (${\tau }_{\text{rad}}$) and emission cross section (${\sigma }_{\text{em}}$) are also calculated and are shown in Table 2. The emission intensity increases when the concentration of ${\mathrm{Nd}}^{3+}$ increases from 0.4% to 0.6%, and then decreases because of the quenching effect. The experimental results agree well with the trends of fluorescence lifetimes. The Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals with concentrations of Nd of 0.5% and 0.6% have relatively larger absorption and emission cross sections and longer fluorescence lifetimes, which are favorable for higher-efficiency laser operations.

The laser experiment was conducted with the setup shown in Fig. 5. A commercial laser diode (nLight Laser, NL-CN-10.0-793-3-F) was employed as the pump source, and its emission wavelength varied from 791 to 799 nm as the output power increased. The bandwidth of the pump source was 1.5 nm (FWHM). After being collimated and focused by the lens, the pump light was imaged into the crystal with a spot size of $50\mathrm{\mu m}*270\mathrm{\mu m}$. Based on the ABCD propagation matrix method, the waist diameter of the laser mode in the crystal was calculated to be 58 μm. Laser experiments were performed with the 0.5% $\mathrm{Nd}\text{:}{\mathrm{CaF}}_2$ and the other four Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ samples. The output mirror transmission is 2%, and concave mirrors M1 and M2 have the same radium of curvature of 10 cm. The laser output powers of the Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals are depicted in Fig. 6, with a dependence on the absorbed pump power. It should be noted that we could not obtain a laser output in the 0.5% $\mathrm{Nd}\text{:}{\mathrm{CaF}}_2$ sample. At the available maximum pump power of 5.09 W, the 0.5% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystal absorbed 55% of the pump power. The laser slope efficiencies and maximum output power of the four Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals are 24.57% and 433 mW, 30.12% and 901 mW, 27.82% and 563 mW, and 24.85% and 468 mW for Nd concentrations of 0.4%, 0.5%, 0.6%, and 0.8%, respectively. Comparing these results with those obtained in similar conditions with a 2% Nd, 2% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystal^[12], the laser slope efficiency and maximum output power, which are obtained with the 2.65% transmittive output coupler, increased by 10 times and 57 times, respectively. When compared to a 1% Nd, 5% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystal^[13], the laser slope efficiency and maximum output power increased by 1.2 times and 11 times, respectively. The 0.5% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystal has the largest slope efficiency and maximum output power of 901 mW. A slope efficiency of 27.82% was also obtained in the 0.6% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystal.

In conclusion, $\mathrm{Nd}\text{:}{\mathrm{CaF}}_2$ crystals co-doped with 10% ${\mathrm{Y}}^{3+}$ have favorable spectroscopic properties, with an emission cross section up to $4.27\times {10}^{-20}{\mathrm{cm}}^2$, an emission lifetime of 360 μs, and a FWHM of the emission band of 30 nm, respectively. Co-doping ${\mathrm{Y}}^{3+}$ effectively breaks the ${\mathrm{Nd}}^{3+}$ quenching clusters, and optical centers in the samples are changed. A diode-pumped, highly efficient laser operation is obtained with 0.5% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ and 0.6% Nd, 10% $\mathrm{Y}\text{:}{\mathrm{CaF}}_2$ crystals, with slope efficiencies over 30% and 27.82%, respectively, and a maximum output power up to 901 mW. These will provide valuable information to find the optimal proportions of ${\mathrm{Nd}}^{3+}/{\mathrm{Y}}^{3+}$ to achieve good laser performances. Further experiments with optimized doping concentrations of ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Y}}^{3+}$ ions and an optimized laser cavity are under way.