1. Introduction

Recently, rare earth (RE) ions doped crystals that exhibit mid-infrared (MIR) lasing have attracted considerable research interest due to their potential applications in medicine, military, eye-safe lasers, and radar etc [1–3]. Among RE ions, Dy³⁺ is one of the most efficient ion for obtaining MIR emissions due to its energy level structure which shows a series of closely spaced multiplets separated by 1500~3300 cm⁻¹. Also Dy³⁺ exhibits intense absorption bands in the near-infrared region that are suitable for pumping to these multiplets [4]. Mainly, Dy³⁺ two lasing transitions ${}^{6}H{}_{11/2}\to {}^{6}H{}_{13/2}$(4.3~4.4 μm) [5] and ${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ (3.0~3.4 μm) [6] are of particular interest for MIR lasers. The possibility of obtaining laser emission from Dy³⁺ at ~3.0 µm depends on the choice of host crystals, i.e., phonon energies should be relatively small and the orbital coupling of the ion to the lattice should be relatively weak [7]. To date, Dy³⁺ lasing for ${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ transition has been demonstrated in fluoride compounds such as BaYb₂F₈ [6, 8], BaY₂F₈ [7] crystals and ZBLAN fiber [9], and PbGa₂S₄ [4] sulfide crystals mainly due to their low phonon energies of the host crystals (BaY₂F₈:415 cm⁻¹ [10], and PbGa₂S₄:350 cm⁻¹ [11]), which can decrease the non-radiative losses efficiently and thus increasing the quantum efficiency of ${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ transition. However, fluoride and sulfide crystals must be grown in an inert atmosphere to avoid contamination from outside environment, and also their poor mechanical properties seriously limit the enhancement of MIR laser output power and efficiency. On the other hand, oxide crystals are much easier to grow with higher optical quality, and they possess much better physical properties.

YAlO₃ (YAP) crystallizes in a distorted perovskite structure, belonging to orthorhombic system with the space group Pnma-D¹⁶_2h (No.62), Z = 4. The cell parameters are a = 5.330Å, b = 7.375Å, and c = 5.180Å. Its density is 5.35 g/cm³ [12]. As it melts congruently, large size single crystals can be obtained by the Czochralski (CZ) method. This crystal has low phonon energy (570 cm⁻¹ [13]) and exhibits good physicochemical properties. To date, much research work has been devoted to Nd³⁺ [14], Yb³⁺ [15], Er³⁺ [16], Tm³⁺ [17], Ho³⁺ [18] doped YAP crystals as laser gain media. However, despite the afore-mentioned technological interests, Dy³⁺ doped YAP crystal for MIR lasing emission has not been reported yet. The ionic radii of Dy³⁺ (1.03Ǻ) and Y³⁺ (1.02 Ǻ) are almost same, and it has been proved that Dy³⁺ ions are easy to substitute the Y³⁺ ions in the octahedral site coordination of the YAP matrices since the orthorhombic structure of Y³⁺ (in YAP) occupies octahedral site coordination in C_1h point symmetry [19]. In this work, for the first time we report on the strong Dy³⁺ emission in YAP crystal operating on the ${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ transition at 3020 nm and tunable throughout the 2600~3400 nm region, at room-temperature.

2. Experimental

Dy:YAP crystal was grown by the Czochralski method. The 99.999% pure Y₂O₃, Al₂O₃ and Dy₂O₃ powders were appropriately dried and weighed according to the formula Dy_0.02Y_0.98AlO₃. The mixtures were ground and mixed, then pressed into bulks and put into an alumina crucible. The bulks were sintered at 1450 °C for 72 h in air, and then the grinding and sintering processes were repeated twice to confirm the finally synthesized polycrystalline compounds by using X-ray diffraction (XRD) method. And then, the bulks were loaded into an iridium crucible of 70 mm in diameter and 50 mm in height for crystal growth. The crystal growth was carried out in DJL-400 furnace (NCIREO, China) with a a-cut YAP seed. A pulling rate of 1.5 mm/h and rotation rate of 16~20 r.p.m were adopted during the growth. High-purity nitrogen gas was used as a protective atmosphere. The initial growth boundary in the solid-melt was convex toward the melt, such that dislocations and impurities were reduced or eliminated from the crystal. Then the growth boundary became flat. To prevent the crystal from cracking, it was cooled to room temperature very slowly with a rate of 8.0~30.0 °C/h after its growth. Finally, a high optical quality Dy:YAP crystal with size Φ25 × 40 mm² was obtained, which is free from cracks and inclusions, and no scattering centers inside. Owing to the electron trap O^- centers produced in crystal growth, it shows shallow orange color, therefore the crystal was reheated to 1400~1600 °C in O₂ atmosphere, finally its orange color fades and the crystal becomes colorless for spectral measurements. The study on the color change of YAP crystal has been carried out in Ref [20].

The concentrations of dysprosium and yttrium ions in the grown crystal were measured by the inductively coupled plasma-atomic emission spectrometry (ICP–AES) method, and the values obtained are 1.52 wt%, and 52.34 wt%, respectively. Therefore, the effective segregation coefficient K for the Dy³⁺ is calculated to be 0.78 by following the Eq. (1) in Ref [21].

3. Spectral analyses

Samples for spectroscopic measurements were cut from the as-grown Dy:YAP bulk crystal with each face perpendicular to one of the three main crystallographic directions a~c of the crystal. They were optically polished to 5.0 × 5.0 × 1.0 mm³. The polarized absorption spectra were measured by using the Perkin–Elmer UV-VIS-NIR Spectrometer (Lambda 900) at room temperature, with the incident light along the certain direction corresponding to one of crystallographic axis of a~c. The spectral resolution for absorption spectra is 1 nm. The emission spectrum and fluorescence decay curve were recorded at room temperature by using Edinburgh Instrument FSP920 Spectrophotometer, under excitation with a 5 ns pulse of an optical parametric oscillator; the excitation wavelength is 1300 nm since Dy³⁺ shows strong absorption in YAP crystal. The sample used for emission spectrum and decay curve measurement is along a-axis, owing to unable to afford polarized plate in this FSP 920 Spectrophotometer. The spectral resolution for emission spectrum is 10 nm.

The polarization absorption spectra in the range from 300 to 3200 nm are shown in Fig. 1, in which the terminal levels of the corresponding transitions from the ⁶H_15/2 ground state of Dy³⁺ ions were assigned and marked. It can be seen that the absorption spectra are polarization dependent,while the absorption peaks in three directions are similar with a slight difference in the absorption peaks location. Among the absorption bands, at around 1300 nm spectra show strong and broad absorption corresponding to the ⁶H_15/2→⁶H_9/2 + ⁶F_11/2 transition, suitable for pumping by 1.3 µm Nd: YAG laser and laser diodes. The a, b and c polarized absorption cross sections with peaks at 1289 nm, 1312 nm and 1281 nm are 2.43 × 10⁻²⁰ cm², 1.90 × 10⁻²⁰ cm² and 2.21 × 10⁻²⁰ cm², respectively. The evaluated full widths at half maximum (FWHM) values are 46 nm, 52 nm and 48 nm, respectively, and these make Dy: YAP crystal highly appropriate for 1300 nm Nd: YAG laser pumping.

 

Fig. 1 Polarized absorption spectra of Dy:YAP crystal at room temperature.

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The conventional Judd-Ofelt (J-O) theory [22, 23] is used to calculate the spectroscopic parameters of Dy: YAP crystal, such as the integrated absorption coefficients, three intensity parameters, the calculated and experimental line strengths and oscillator strengths, radiative lifetimes etc. Ten absorption bands were used in the calculation of the J–O intensity parameters following the procedure given in Refs [21,24]. The parameters such as the reduced matrix elements of tensor operators are adopted from Ref [25]. and the refractive index of YAP crystal is estimated from the Sellmeier dispersion equation [26]: $n^2(\lambda )=1+\frac{A{\lambda }^2}{{\lambda }^2-B}$, where for the a, b, c crystallographic axes, A = 2.709, 2.678, 2.635 μm² and B = 0.0126, 0.0123, 0.0116 μm², respectively. The magnetic dipole transitions are not much important for the manifolds of Dy³⁺ ions relating to absorption transitions considered, therefore, it is sufficient to calculate the electric dipole transitions only [27]. The calculated average wavelength, refractive indices, integrated absorption coefficients are presented in Table 1.

Table 1. The average wavelength ($\overline{\lambda }$), refractive indices (n), integrated absorption coefficients (Г) for the absorption transitions of Dy³⁺ in YAP crystal.

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And then, the experimental and calculated line strengths (S) and oscillator strengths (f) are acquired and listed in Table 2.The values in Table 2 provide a root-mean-square (RMS) deviation of 0.266 × 10⁻²⁰ cm² for a polarization, 0.525 × 10⁻²⁰ cm² for b polarization, 0.350 × 10⁻²⁰ cm² for c polarization. The values of error rmsΔS and rmsΔf are relatively low and thus the obtained results can be taken as reasonable.

Table 2. Polarized experimental and calculated line strengths (S) and oscillator strengths (f) of Dy:YAP crystal

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A least-square fitting of S_exp to S_cal provides the three J–O intensity parameters Ω_t (t = 2, 4, 6) for each polarization. For the biaxial Dy:YAP crystal, the effective intensity parameters Ω_t,eff should be Ω_t,eff = (Ω_t,a + Ω_t,b + Ω_t,c)/3 [28]. So, three effective J–O intensity parameters of Dy:YAP are obtained and listed in Table 3, together with other Dy³⁺-doped crystals. As seen from Table 3, the three intensity parameters are polarization dependent, Ω₂ value of Dy:YAP crystal is larger than that of some low-phonon-energy chloride, fluoride and sulfide crystals, and this indicates that Dy:YAP crystal is less ionic in character. With the above necessary parameters and the emission squared reduced matrix elements [33], the spontaneous emission probabilities for the transition ${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$are calculated to be 62.491 s⁻¹, 65.594 s⁻¹ and 67.626 s⁻¹ for E//a, E//b, and E//c, respectively. According to [34], the average spontaneous emission probability is calculated to be 65.237 s⁻¹, and thus the radiative lifetime τ_r of the ⁶H_13/2 level is calculated as 15.33 ms.

Table 3. The J–O intensity parameters of Dy³⁺-doped crystals

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The MIR emission spectrum of Dy:YAP crystal under 1300 nm OPO pumping is shown in Fig. 2(a).A broad emission band from 2600 to 3400 nm was observed corresponding to Dy³⁺:${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ transition. The maximum emission cross section is 4.07 × 10⁻²¹ cm² at 3020 nm; the FWHM of this emission band is 520 nm. Figure 2(b) presents the simplified energy level diagram of Dy³⁺ doped YAP crystal. Energy from the 1300 pump source excites the Dy³⁺ ions to its ⁶H_9/2 + ⁶F_11/2 level and then decay radiatively to ⁶H_13/2 level, finally emitting at 3020 nm. This is the first time to observe MIR emission in Dy³⁺ doped oxide YAP crystal. According to Weber [35], in order for materials to emit at long wavelengths, the highest phonon frequencies of the host medium must be less than about 0.2~0.25 times the light frequency. The chloride, sulfide, selenide and telluride have suitably low phonon frequencies to allow for efficient emission beyond 3 microns, such as CaGa₂S₄ and KPb₂Cl₅, whose phonon frequency are as low as 350 cm⁻¹ and 203 cm⁻¹ [11], respectively, while the standard oxide crystals are usually not suitable for this purpose. As an example, the frequency (in wave number units) of 3 micron light is 3333 cm⁻¹, and 20% of that corresponds to 667 cm⁻¹, the approximate maximum phonon vibrational frequency of oxide host should possess in order to facilitate efficient 3 micron luminescence. In this work, the maximum phonon energy of YAP is only ~570 cm⁻¹ [13], such low phonon vibrational frequency leads to a reduced non-radiative decay rates between excited states of Dy³⁺ with small energy separation, and finally ensures an efficient MIR emission.

 

Fig. 2 (a) MIR emission spectrum of Dy:YAP crystal excited by 1300 nm at room temperature. (b) The simplified energy level diagram of Dy³⁺ doped YAP crystal.

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To further explore the energy interaction mechanism, the fluorescence decay curve of the ⁶H_13/2 level in Dy:YAP crystal is measured by monitoring 3020 nm emission, with excitation at 1300 nm. As shown in Fig. 3, the measured decay curve shows singly exponential decaying behavior. The fluorescence lifetime of the ${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ transition of Dy³⁺ was determined to be 8.88 ms. Therefore, the fluorescence quantum efficiency was calculated to be: η = τ_f/τ_r = 8.88/15.33 = 57.9%. Table 4 presents the comparison of several important spectroscopic parameters of several Dy³⁺-doped laser crystals. One can see from Table 4 that the absorption cross-section, emission cross-section and quantum efficiency of Dy:YAP crystal are comparable and even higher than those of Dy:KPb₂Br₅, Dy:CaGa₂S₄and Dy:BaYb₂F₈ crystals, which imply that Dy:YAP crystal is an excellent candidate for MIR laser material by 1300 nm LD pumping.

 

Fig. 3 The fluorescence decay curve of the ⁶H_13/2 level in Dy:YAP crystal at room temperature.

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Table 4. Comparison of the optical spectroscopic parameters of some Dy³⁺-doped laser crystals

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Once the absorption and emission cross-section spectra are derived, the gain cross-section spectrum $G(\lambda )$ can be computed by following equation [36]:

 

Fig. 4 Gain cross-section spectra of Dy³⁺:${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ in the Dy:YAP crystal.

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4. Conclusion

To conclude, a high quality Dy³⁺-doped YAlO₃ single crystal has been successfully grown with the Czochralski method. The polarized absorption spectra, unpolarized fluorescence spectrum as well as decay curves are measured at room temperature. The effective J–O intensity parameters Ω_2,eff, Ω_4,eff and Ω_6,eff are calculated to be 3.482 × 10⁻²⁰, 3.786 × 10⁻²⁰ and 3.712 × 10⁻²⁰ cm², respectively. The polarized line strengths, oscillator strengths, spontaneous emission probabilities as well as the radiative lifetimes of Dy³⁺ ions in YAP crystal are presented. One intense MIR emission extending from 2600 to 3400 nm on the phonon terminated transition ${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ of Dy³⁺ was demonstrated for the first time at room temperature. The maximum emission cross-section is 4.07 × 10⁻²¹ cm² at 3020 nm, and the FWHM of this emission band is 520 nm. The fluorescence lifetime of the ${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ transition of Dy³⁺ was determined to be 8.88 ms and the quantum efficiency evaluated was 57.9%. In addition, the gain emission property of the Dy³⁺:${}^{6}H{}_{13/2}\to {}^{6}H{}_{15/2}$ transition is discussed. The above results indicate that Dy:YAP crystal merits further investigation as a broadly tunable mid-infrared laser material.