Xi Wang is with the State Key Laboratory Base of Novel Function Materials and Preparation Science at the School of Material Sciences and Chemical Engineering of Ningbo University, in Ningbo, Zhejiang, China.

Xi Wang

 single crystal was grown by the vertical Bridgman method using spontaneous crystallization. After annealing at 850 °C for 24 h in a muffle furnace under an air atmosphere, the obtained light black crystal became transparent and optically clear. Powder X-ray diffraction (PXRD) confirmed that the as-grown crystal had the structure of scheelite. The segregation coefficients of Tm

 were 0.589 and 0.588, respectively. Under 980 nm excitation, the temperature dependence of the emission intensity of the crystal in the temperature range of 298–473 K was measured. The results show that, because of the thermal quenching effect, the emission intensity decreases as the temperature rises. The possible energy transfer modes between Yb

 ions were analyzed. The emission cross section was calculated to be 1.97 × 10

 by the Füchtabauer-Ladeburg formula, and the fluorescence lifetime was fitted to be 166.67 μs.

In recent years, with the rapid development of near-infrared (NIR) optical amplifiers and mid-IR fiber lasers, new rare earth-doped solid IR luminescent materials have attracted great attention (1–5). Among rare earth ions, the energy level transitions of 

 can generate fluorescence emission in the bands of 1.47– 1.8 μm (6). The glass fiber with Tm

 as the luminescence center is a type of material suitable for the third window fiber amplifier, which has a strong energy cross-relaxation effect between Tm

 stimulated radiation belongs to quasi-three-level, and the emission wavelength is in the wavelength range of 1.47–1.8 μm (8–10). However, because of the concentration quenching effect, the symmetric energy level of the Tm

 ion limits the effective emission of the laser (11,12). The effective way to overcome this shortcoming is to combine with the Yb

 is the most promising ion that can be used in all solid-state laser sources in the NIR spectrum around 1030 nm. Compared with other ions, Yb

 ion is also a good sensitizer, which only has two very simple 

 energy levels and can absorb light near 980 nm (18,19).

In many substrates, such as glass and single crystal, the combination of the Yb

 ion is considered to be an effective composition for laser crystals. The doped crystal needs the radius and electronegativity of the cation of the host crystal and the activated ion to be as close as possible. Moreover, a variety of solid-state lasers have been constructed for eye-safe applications using Yb

 (20–22). With stable chemical properties, good luminescence properties, high thermal stability, and low phonon energy, sodium yttrium molybdate (NaY[MoO

) is considered to be a good luminescent matrix. Considering the above reasons, we studied the Yb

 co-doped system could be introduced in the NaY(MoO

The vertical Bridgman method using spontaneous crystallization is an easily feasible way to obtain crystal samples for measurement (24–26). Here, we used the solid phase synthesis method to synthesize the precursor and the vertical Bridgman method with spontaneous crystallization to grow the NaY(MoO

:Re (Re=Yb/Tm) crystal. The thermal annealing, X-ray powder diffraction analysis, absorption spectrum, and temperature dependence emission spectra of the NaY(MoO

 crystal is reported in detail in this paper.

The raw materials for crystal growth of NaY(MoO

 precursor for crystal growth was prepared by solid-state reaction according to the following stoichiometric relation:

 are the doping concentration. To compensate for the volatilization of MoO

 during solid phase reaction and crystal growth, an additional 0.2at% MoO

 was added. The raw materials were mixed and grounded in a mortar for 2 h. The precursor was put into an alumina crucible and calcined in a muffle furnace at 950 °C for 24 h. The process was repeated, and the samples were held at 1000 °C for 48 h to make an adequate solid-state reaction. The synthesized NaY(MoO

 polycrystalline powder was filled into a platinum crucible. The platinum crucible was then placed in a Bridgman growth furnace built in-house for crystal growth. The schematic of the Bridgman growth furnace used in this study was presented in Figure 1a. The obvious advantage of the vertical Bridgman method is that multiple crystals with a smaller size can be grown simultaneously, which helps save time, energy, and raw materials. In this experiment, the position of the four platinum crucibles in the furnace was shown in Figure 1b. The melting point of the NaY(MoO

 crystal was 1120 °C (27), and the controlled temperature of the furnace was 1250 °C. The lowering speed of the crucibles was fixed at 0.35 mm/h during the crystal process.

FIGURE 1: (a) Schematic of the Bridgman growth furnace, and (b) layout of platinum crucibles in the furnace.

 crystal was confirmed through powder X-ray diffraction (PXRD), using a diffractometer of Bruker D8 with Cu Kα radiation (λ=1.54056 Å) at room temperature. The measurement was carried out with the scanning step of 0.02°, the scanning scope was from 10° to 80°, the high voltage was 40 kV, and the current was 40 mA. A scanning electron microscope (SEM) with Helios G4 CX model and energy spectrum model called the Edax Apollo X was used to detect the content of the elements in the single crystal. The absorption spectrum in the range of 500–2000 nm was measured by a PerkinElmer UV-visible-NIR (UV-vis-NIR) spectrometer photometer (Lambda-950) with the resolution of 1 nm at room temperature. Under the excitation of 980 nm LD laser, the temperature-dependent emission spectra in the range of 1000–1600 nm were recorded with the Edinburgh FLS920 fluorescence spectrometer, in which the temperature of 273 to 473 K was controlled by Oxford Instruments liquid nitrogen with an isothermal titration calorimetry (ITC) temperature controller and software. The luminescence decay curve was measured by the FL 3–111 fluorescence spectrometer using a gate of 5 μs.

Crystal Growth and Powder X-ray Diffraction Analysis

The vertical Bridgman method is a very effective method to obtain the NaY(MoO

 crystal crystallized at the bottom of the crucible, so the shape of the crucible affected the spontaneous crystallization of the crystal. The crystal shape obtained by the vertical Bridgman method was similar to that of a crucible. To obtain the single crystal, the bottom of the platinum crucible was squeezed with an angle of approximately 7°, because the spontaneous crystallization could not eliminate the grain boundaries at the large shoulder angle and diameter.

The as-grown crystal showed two colors: light black from the middle to the bottom, and yellow at the top, in which the yellow at the top is polycrystalline, which is shown in Figure 2a. The reason for the yellow color at the top could be the role of transition metals in the raw materials (28). The light black color should be caused by the oxygen vacancies, which were formed by the growth of the crystal in a closed environment (5,29). This process can be explained by the following equation 2:

 crystal sample with the size of 15 × 5 × 1 mm

After annealing at 850 °C for 24 h in a muffle furnace under air atmosphere, the obtained light black crystal showed uniform colorlessness and transparency. The reason was that oxygen diffused into the lattice, filling and occupying the oxygen vacancies. The mechanism is shown as the following equation 3:

By cutting and polishing, an optical transparent sheet as shown in Figure 2b was obtained, which was used for testing and analysis.

Figure 3 shows the PXRD pattern of NaY(MoO

 powder at room temperature with the NaY(MoO

 standard card (PDF #82-2369) as a reference. All diffraction peaks match well with the NaY(MoO

 standard cards, and no additional diffraction peaks corresponding to any impurities were observed. Figure 4 shows that the crystal belonged to the tetragonal scheelite (CaWO

) structure, and the phase structure of the NaY(MoO

 crystal had not changed by doping other rare earth ions. The NaY(MoO

 single crystal was a molybdate crystal with scheelite structure, belonging to the tetragonal system, with a I4

 space group and two molecules per unit cell. In this lattice, MoO

 tetrahedral ion groups replaced the positions of the WO

 ions randomly occupied the crystal lattice of Ca

 ions did not affect the crystal structure (30–32).

FIGURE 4: Schematic cell structure of tetragonal NaY(MoO

Figure 5 shows the energy-dispersive spectroscopy (EDS) diagram of the as-grown NaY(MoO

 crystal, which confirmed that the crystal was mainly composed of six elements: oxygen, molybdenum, yttrium, sodium, thulium, and ytterbium. Table I shows the atomic percent of the above six elements.

FIGURE 5: EDS analysis of as-grown NaY(MoO

 are the concentrations of rare earth ions in the crystal and raw material, respectively. Table II shows the results of NaY(MoO

. The separation coefficient represents the extent to which the dopant occupies the host ion lattice, which depends on the difference of the charge, radius, and properties between the doped and host ions. The closer the separation coefficient is to 1, the more uniform the concentration within the grown crystal.

In this paper, the segregation coefficients of Tm

 single crystal are 0.589 and 0.588, respectively, indicating that the concentrations of dopants in different parts during the crystal growth are not uniform. It is reasonable to calculate the spectral parameters using the average concentration of doped ions.

Figure 6 shows the absorption spectrum of the annealed NaY(MoO

 crystal sample at room temperature. The spectrum ranging from 500 to 2000 nm consisted of the absorption bands near 692, 798, 1220, and 1753 nm, which were corresponding to the transitions from the ground state 

, and near 866 nm was corresponding to the transitions from the ground state 

FIGURE 6: The absorption spectrum of NaY(MoO

It can be seen from Figure 6 that the absorption of Yb

 ions forms an absorption band between 856 and 1022 nm, which is because in the NaY(MoO

 crystal, the Stark splitting of the crystal field, 

, form the upper and lower two-state clusters, namely the upper-state cluster 

, in which the energy level difference is approximately 10,000 cm

The absorption cross section of rare earth ions is very important for analyzing the performance of solid-state lasers and amplifiers doped with rare earth ions. We can calculate the absorption cross section by the measured optical absorption OD(λ) (7):

(λ) is the optical density, L is the thickness of sample and 

). The wavelengths of the observed absorption peaks and the corresponding absorption cross-sections are listed in Table III.

Temperature-Dependent Emission Spectra and Cross-Section 

The thermal emission stability of the crystal is an important aspect in the NIR laser field. To investigate the temperature-dependent emission spectra of the NaY(MoO

 crystal at different temperatures, the annealed NaY(MoO

 crystal was excited by a 980 nm LD laser in the range of 298–473 K. The emission intensity of the crystal gradually increased from 473 to 373 K, and sharply increased from 373 K to room temperature, which could be observed from 1000 to 1600 nm in Figure 7.

FIGURE 7: Temperature-dependent emission spectra of the annealed NaY(MoO

 contains three additional emission bands near 1112, 1174, and 1455 nm, which are corresponding to 

), respectively. As shown from Figure 7, the emission peak intensity at 1455 nm is close to that at 1495 nm, and the intense emission peaks are centered at 1455 nm.

To explain the temperature dependent photoluminescence, activation energy was calculated through the Arrhenius equation 6 (33):

 is the emission intensity at room temperature; 

 is the emission intensity at measurement temperature; 

 is the activation energy for thermal quenching. The fitting line of In[

-1] versus 1/kT is illustrated in Figure 8. The Δ

 ions was calculated to be 0.301 eV. Compared to our previous work, where the Δ

 was calculated to be 0.234 eV (10), the thermal emission NaY(MoO

) − 1] vs. 1/kT activation energy graph for the thermal quenching of NaY(MoO

When the temperature rose, the carriers, which were fixed in the traps, were released to the emitting center (Yb

). Therefore, the electron-phonon interaction was the main one, exciting Yb

 luminescence center was activated by the interaction of the phonon, which was obtained by the cross release between the excited state and the ground state. Therefore, as the temperature rose, the emission intensity of the crystal decreased. Therefore, the crystal exhibited a non-radiative transition (27). The emission cross section is an important parameter for evaluating the possibility of laser action. A larger emission cross section means a higher light output. The emission cross section σem was calculated by the Füchtabauer-Ladeburg (F-L) formula (34):

 is the refractive index, which can be calculated by the following formula: 

) is the relative fluorescence intensities as a function of wavelength; and 

 is the velocity of light. The peak value of fluorescence cross-sections calculated by equation 7 is 1.97 × 10

 crystal at 1455 nm. The emission cross-section of Yb

 calculated from equation 7, which is compared with the NaGd(MoO4)2 crystal, is listed in Table IV.

G; through spin-orbit coupling, each spectrum branch item was formed. Under the excitation of 980 nm, the NaY(MoO

 crystal has a strong emission band at 1453 nm, which corresponded to the transition from 

). It can be seen from Figure 8 that the intensity of the emission peak at 1453 nm was close to the intensity of the emission peak at 1496 nm, so 1453 and 1493 nm in the NaY(MoO

 crystal can better overlap to form ultra-wide NIR luminescence.

Energy Transfer and Fluorescence Decay Curves

 ions formed an absorption band between 1187~1216 nm and 1701~1800 nm under the pump of 980 nm LD. First, the Yb

 ion absorbed a photon through ground state absorption (GSA) to generate the 

 energy level. Through mass transfer (ET) to Tm

 level through ground state absorption (GSA). The 

 energy levels were similar (the difference is approximately 2000 cm

 energy level through excited state absorption (ESA), and 

 then relaxes through multi-phonon relaxation (MR) and returns to the 

 energy level through ESA, or it can return to the ground state 

 and emit a photon with a wavelength of approximately 795 nm. Tm

 produces a 692 nm up-conversion red light rare earth ion up-conversion luminescence through a radiation transition to 

. Figure 8b shows the energy level diagram and possible up-conversion transition process of the Yb

 is the frequency. Because the energy difference between the transition between Yb

 is a rope assist energy transfer. It can be seen from the above that the up-conversion near-infrared light (

 transition) is a two-photon absorption process. Up-converting blue light (

 transition) are three-photon absorption processes, and up-converting blue light is not reflected in the figure. In addition, the energy levels of 

, 3 are very short-lived. The ions in these energy levels can quickly and non-radiatively transit to the 

 energy levels, making reverse energy difficult to perform because of the processes 2, 4, and 8. The process has high energy transfer efficiency. Therefore, the Yb

 crystal can produce strong near infrared light emission (798 nm) under the excitation of 980 nm LD pump, which is consistent with the transition mechanism in Figure 9.

FIGURE 9: (a) Stark level splitting of Yb

 energy level diagram, and (b) possible up-conversion luminescence mechanism of Yb

The fluorescence attenuation curve of the NaY(MoO

 crystal after annealing was measured as shown in Figure 10. The curve of the crystal can be calculated by fitting an exponential function:

 is the decay time constant of the emission. Hence, the decay time from the emission line 

 crystal after annealing is fitted to be 166.67 μs. The fluorescence lifetime of co-doped sample was shorter, which was mainly because of the energy transition between ions in a shorter time.

FIGURE 10: The fluorescence decay curves of NaY(MoO

 fitted using exponential functions at room temperature.

 ions have been grown by the vertical Bridgman method using spontaneous crystallization. The grown doped NaY(MoO

 crystal appeared dark brown in color, and the PXRD pattern confirms that the crystal structure was tetragonally symmetric with I4

 co-doped crystal enriches the emission band in the near-infrared region, and improves the pump energy absorption efficiency at 980 nm. In addition, as the temperature dropped from 473 K to room temperature under 980 nm LD laser excitation, the emission intensity increased. Therefore, NaY(MoO

crystal as a NIR laser crystal has a strong potential application prospect in optics and photonic devices.

This work was sponsored by National Natural Science foundation of China (No. 61875096 and 61775108).

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