1. Introduction

Over the past several years, mid-infrared (MIR) lasers at ~3 μm have gained attention for a variety of applications including remote sensing and dental surgery due to the vibration absorption band of water in this spectral region [1, 2]. In addition, the ~3 μm lasers can act as the pumping source of optical parametric oscillation (OPO) [3].

Such lasers can be realized by doping Er³⁺ or Ho³⁺ as activator ions. However, the crystals of doping Er³⁺ ions can only realize a single wavelength of laser output in a specific host material, such as 2.94 μm in Er: YAG crystal [4] and 2.79 μm in Er: YSGG crystal [5]. The doping concentration of Er³⁺ ions is also generally required to be higher than 30 at%, which causes difficulties in achieving high-quality laser crystals [6]. In contrast, Ho³⁺ is a well-known candidate for ~3 μm lasers owing to the ⁵I₆ → ⁵I₇ transition, which can realize lasers of 2.8-3.1 μm in various host materials [7–10], such as oxide YAlO₃ (YAP), Y₃Al₅O₁₂ (YAG), and fluoride LiYF₄ (LYF). However, the ~3 μm emission cannot be obtained efficiently from Ho³⁺-singly-doped hosts due to the two bottle-necks [11]. On the one hand, Ho³⁺ activator ions lack suitable absorption bands matching well with current laser diode (LD). On the other hand, the fluorescence lifetime of the upper ⁵I₆ level is considerably shorter than that of the lower ⁵I₇ level. Fortunately, there are two main schemes to remove the bottleneck effect. One is choosing Yb³⁺ ions as a sensitizer to absorb and transfer pumping energy to the Ho³⁺ ions [12–14]. The other way is co-doping with Pr³⁺ ions as a deactivator to quench the lower level of Ho³⁺: ⁵I₇, while hardly depopulate the upper level of Ho³⁺: ⁵I₆. Good results have been achieved in Ho, Pr: LLF crystal with the lifetime of the upper ⁵I₆ level decreasing from 1.8ms to 1.47ms and the lifetime of the lower ⁵I₇ level decreasing from 16ms to 1.97ms [15, 16].

Host material is another factor that should be considered to get powerful ~3 μm emission from Ho³⁺. As we know, Y₃Al₅O₁₂ (YAG) single crystal is one of the most popular laser crystals due to its excellent optical, thermal, mechanical as well as physicochemical properties [17, 18]. In this work, Lu₃Al₅O₁₂ (LuAG) single crystal, an isomorphic material of YAG, was chosen as the host matrix for reasons: (i) LuAG has a large manifold splitting, leading to a low thermal occupation factor for the lower laser level [19]. (ii) the molar mass difference between Lu³⁺ and doped rare earth ions for LuAG is slighter, which would make a weaker decease in thermal conductivity of rare ions doped LuAG [20]. (iii) LuAG is harder than YAG and has a higher melting point, which is believed to have a higher damage threshold [21]. Therefore, the Yb, Ho, Pr: LuAG crystal may be a potential laser material for realizing 2.8-3.1 μm lasers.

To our knowledge, there is still no report on the growth of Yb, Ho, Pr: LuAG crystal or the ~3 μm emission in this crystal up to now. In this paper, the high optical quality Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG laser crystals were successfully grown by the Czochralski (Cz) method for the first time. Yb³⁺ and Pr³⁺ were demonstrated to enhance the Ho³⁺: ⁵I₆ → ⁵I₇ ~3 μm emission by efficient energy transfer (ET) from Yb³⁺: ²F_5/2 to Ho³⁺: ⁵I₆ and Ho³⁺: ⁵I₇ to Pr³⁺: ³F₂. The absorption spectra, J-O parameters, emission spectra, fluorescence decay curves and transfer mechanism of the Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals were compared and analyzed.

2. Experiments

The Ho³⁺ (2 at. %) single doped, Yb³⁺ (10 at. %)/ Ho³⁺ (2 at. %) co-doped and Yb³⁺ (10 at. %)/ Ho³⁺ (2 at. %)/ Pr³⁺ (0.5 at. %) co-doped LuAG crystals were grown by the traditional Czochralski method with intermediate frequency induction heating. Lu₂O₃ (99.999%), Al₂O₃ (99.999%), Yb₂O₃ (99.999%), Ho₂O₃ (99.999%) and Pr₆O₁₁ (99.999%) were used as raw material for crystal growth. Mix all the ingredients and put into the mixer for stir 20 hours. Then, the mixture was pressed into disks and heated in air at 1300°C for 15 h for synthesizing the polycrystalline material. After that, the polycrystalline material was loaded into a 60-mm-diameter iridium crucible for crystal growth. The crystal was grown in nitrogen gas (with high purity of 99.99%) environment. The <001> oriented seed of dimension 4 × 4 × 35 mm³ was used for crystal growth. The rotation rate and pulling speed were kept at 8-12 rpm and 0.6-0.8 mm/h, respectively. After growth, the crystals were cooled to room temperature slowly for 53 hours and annealed at 1300°C in air for 24 h in order to prevent crystal crack as well as oxidize some Yb²⁺ ions.

Samples with dimension of Φ30 mm × 1 mm were cut from the crystals and then polished on both faces for spectroscopic measurements. The inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to measure the concentrations of Yb³⁺, Ho³⁺, and Pr³⁺ ions in the as grown crystals. The absorption spectrums of the as-grown crystals in the wavelength of 300-2250 nm were recorded by a UV-vis-NIR spectrophotometer (UV-3150, Shimadzu, Japan). The fluorescence spectra in range of 1800 nm to 2200 nm, 2700 nm to 3050 nm and fluorescence decay profiles of the three crystals samples were acquired by Edinburgh Instruments FLS920 and FSP920 spectrophotometers with a laser diode (LD) as the pump source (Yb, Ho: LuAG and Yb, Ho, Pr: LuAG excited at 970 nm, while Ho: LuAG excited at 637nm), and an optical parametric oscillator pulse laser. All measurements were done at room temperature.

3. Results and discussions

For the distribution coefficient measurement, the sample was cut from the upper part of the as-grown crystals, and then was ground to powder in an agate mortar. The single doped crystal was measured to be (2.01 ± 0.01) at. % ((2.872 ± 0.14) × 10²⁰ ions/cm³) of Ho³⁺. The double doped crystal was measured to be (10.5 ± 0.01) at. % ((1.501 ± 0.002) × 10²¹ ions/cm³) of Yb³⁺, and (2.05 ± 0.01) at. % ((2.929 ± 0.14) × 10²⁰ ions/cm³) of Ho³⁺. The triply doped crystal was measured to be (10.7 ± 0.01) at. % ((1.530 ± 0.002) × 10²¹ ions/cm³) of Yb³⁺, (2.05 ± 0.01) at. % ((2.929 ± 0.14) × 10²⁰ ions/cm³) of Ho³⁺ and (0.36 ± 0.01) at. % ((0.515 ± 0.14) × 10²⁰ ions/cm³) of Pr³⁺, respectively. Based on measurement results, the segregation coefficient of Yb³⁺, Ho³⁺, and Pr³⁺ ions in LuAG crystal is about 1.07, 1.02, and 0.72.

The absorption spectra of Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals in the wavelength range of 300-2250 nm is shown in Fig. 1, and the absorption peaks are basically the same among these crystals except in the range of 850-1050 nm as well as 1340-1570 nm. On one side, the absorption peaks centered at wavelength 970 nm with very large intensity correspond to the transitions of Yb³⁺ ions from the ground state ²F_7/2 to the exited states ²F_5/2, which makes this crystal propitious to be pumped by commercialized InGaAs LD, indicating that importing of Yb³⁺ ions is expected to provide an efficient excitation channel to Ho³⁺ ions. On the other side, the absorption peaks located at wavelength from 1340 to 1570 nm correspond to the transitions of Pr³⁺ ions from the ground state ³H₄ to the excited states ³F₃ and ³F₄. The efficient ET will occur from the ⁵I₇ excited state of Ho³⁺ to the ³F₂, ³H₆ states of Pr³⁺ due to the fact that a strong overlap between the ⁵I₈ → ⁵I₇ absorption transition of Ho³⁺ and the ³H₄ → ³F₂, ³H₆ absorption transition of Pr³⁺ in the wavelength range of 1800-2100 nm [16]. The absorption cross-sections of Yb³⁺ ions at 936 nm are calculated to be 5.93 × 10⁻²¹cm² for the Yb, Ho, Pr: LuAG.

 

Fig. 1 Room temperature absorption coefficient of the Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals. The inset shows samples of as-grown (a) Ho: LuAG (b) Yb, Ho: LuAG (c) Yb, Ho, Pr: LuAG crystals.

Download Full Size | PDF

The Judd-Ofelt (J-O) theory is applied to analyze the room temperature absorption spectrum, and then the branching ratios of the various transitions and the radiative lifetime of the level can be evaluated [22, 23]. The details of J-O analysis are similar to that reported in Refs [24]. The Judd-Ofelt intensity parameters Ω_2,4,6, (shown in Table 1) of Ho³⁺ can be figured out utilizing the reduced matrix elements of the unit tensor operators provided by Brian M. Walsh al [25]. It is well known that Ω₂ is affected by the symmetry of rare-earth ions site. The value of Ω₂ drops with the improved symmetry [26]. The larger Ω₂ of Ho³⁺ in our Yb, Ho, Pr: LuAG crystal indicates that a lower symmetry surrounding Ho³⁺ ions is caused by the introduction of Yb³⁺ and Pr³⁺ ions [27]. Further calculation about the fluorescence branching ratio β of ⁵I₆ → ⁵I₇ transition in the Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals are about 12%, which is much higher than Ho: LuAG, indicating that the co-doping of Yb³⁺ and Pr³⁺ is beneficial to the ~3 μm fluorescence emission efficiency of Ho³⁺.

Table 1. Judd-Ofelt parameters Ω_2,4,6, branching ratio β, life-time of ⁵I₆ and ⁵I₇ level for Ho³⁺ ions (τ_m and τ_r are the measured and calculated radiative lifetime)

View Table

The fluorescence spectra of the Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals in the range of 1800-2200 nm and 2700-3050 nm is shown in Fig. 2. The sampling interval of fluorescence spectra is 1 nm in the range of 1800-2200nm and the range of 2700-3050 nm is 5 nm. The observed many emission bands around 2760, 2785, 2810, 2860, 2890, 2950, and 3010 nm are assigned to the transition of ⁵I₆ to ⁵I₇ stark sublevels of Ho³⁺ ions. By comparing, it is evident that the emission intensities of Yb, Ho, Pr: LuAG is almost nine times that of the Ho: LuAG crystal and twice as much as that of the Yb, Ho: LuAG crystal at 2.95 μm. However, emission in the range of 1800-2200 nm is reverse. This enhanced 2.95 μm emission and reduced ~2 μm emission in the Yb, Ho, Pr: LuAG crystal is mainly attributed to the quenching effect of the Ho³⁺: ⁵I₇ by energy transition (ET) to Pr³⁺: ³F₂. Thus, output of 2.8-3.1 μm lasers may be realized on the Yb, Ho, Pr: LuAG crystal.

 

Fig. 2 (a) ~3 μm emission spectra of Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals; The inset shows the emission cross-section spectra of Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals in the wavelength of 2700-3050 nm. (b) ~2 μm emission spectra of Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals.

Download Full Size | PDF

To further study the effect of Yb³⁺ and Pr³⁺ co-doping on the ~3 μm photoluminescence of Ho³⁺, the corresponding emission cross sections are subsequently calculated by Fuchtbauer-Ladenburg equation [28]:

The fluorescence decay curves of the Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals excited by OPO pulse lasers show a single exponential decay behavior, which are shown in Fig. 3. By single-exponential fitting, the fluorescence lifetime τ_meas of the ⁵I₆ and ⁵I₇ manifold in Yb, Ho, Pr: LuAG crystal is 0.13 ms and 3.73 ms, respectively. Despite the lifetime of Ho³⁺: ⁵I₆ in Yb, Ho, Pr: LuAG crystal decreases a little from 0.15 ms to 0.13 ms due to the energy transfer process Ho³⁺: ⁵I₆ → Pr³⁺: ³F₄ (ET2), the Yb, Ho, Pr: LuAG crystal exists a quicker attenuation of the lower level lifetime compared with the ⁵I₇ lifetimes of the Ho: LuAG (⁵I₇: 11.15 ms), Yb, Ho: LuAG (⁵I₇: 11.18 ms), which is mainly attributed to the energy transfer from the Ho³⁺: ⁵I₇ to the Pr³⁺: ³F₂ + ³H₆ (ET3), as well as the up-conversion (UC) process and strong excited-state absorption (ESA) from Ho³⁺: ⁵I₇ → ⁵F₅, as shown is Fig. 4. Meanwhile, energy transfer from Yb³⁺: ²F_5/2 to Ho³⁺: ⁵I₆ (ET1) under a 970 nm LD excitation is benefit to make the possibility of population inversion for Ho³⁺: ⁵I₆ → ⁵I₇. These processes not only depopulate the Ho³⁺: ⁵I₇, but also recycle energy to Ho³⁺: ⁵I₆ by nonradiative transition (NT), which consequently makes contribution to 2.95 μm emission. The present experimental results show that the lifetime of the upper level (⁵I₆: 0.13 ms) of Ho³⁺ ions is still much shorter than lower level (⁵I₇: 3.73 ms) for Yb, Ho, Pr: LuAG crystal. However, two improvements can be made in future research work: (i) The concentration ratio of doped ions between Yb³⁺, Ho³⁺, Pr³⁺ ions can be further optimized, which may further enhance the fluorescence intensity of 3 μm, and shorten the lifetime of the lower level (Ho³⁺: ⁵I₇); (ii) The crystal quality can be further improved by optimizing the crystal growth process, which may further improve the spectral performances of the as-grown crystals.

 

Fig. 3 Fluorescence decay curves of Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals for the ⁵I₆ and ⁵I₇ manifolds.

Download Full Size | PDF

 

Fig. 4 Energy level diagram of Yb³⁺, Ho³⁺, Pr³⁺ triply doped system. UC: up-conversion, ESA: excited state absorption, NT: nonradiative transition, ET: energy transfer.

Download Full Size | PDF

On the one hand, it is worth noting that the energy transfer from Yb³⁺ to Ho³⁺ ions is a phonon-assisted process due to the fact that the small energy mismatch between the ²F_5/2 level of Yb³⁺ and ⁵I₆ level of Ho³⁺ [29]. The ability of a donor (Yb³⁺) to transfer its energy to an acceptor (Ho³⁺) is an important factor to evaluate the donor (Yb³⁺) as a sensitizer. On the other hand, co-doping Pr³⁺ deactivation ion can be effective in depopulation of ⁵I₇ level of Ho³⁺, because its energy level ⁵I₇ is adjacent to ³F₂ level of Pr³⁺. The energy transfer efficiency of ET2 and ET3 processes can be estimated from the measured lifetime of the 2.95 μm and 2 μm emissions by the following equation: η = 1 - τ_Yb/Ho/Pr/τ_Yb/Ho [30], where τ_Yb/Ho and τ_Yb/Ho/Pr are the Ho³⁺ lifetimes monitored at 2.95 μm and 2 μm, respectively. It can be calculated that the value of η_ET2 = 13.3%, η_ET3 = 66.6%. Therefore, under 970 nm pumping scheme, co-doping of both Yb³⁺ and Pr³⁺ with Ho³⁺ can turn on the possibility of excellent ~3 μm lasers.

4. Conclusion

In conclusion, an efficient emission at ~3 μm was observed in the Yb, Ho, Pr: LuAG crystal under the excitation of a common 970 nm LD. The relevant absorption and emission cross sections as well as the fluorescence lifetimes were calculated and compared with Ho: LuAG and Yb, Ho: LuAG crystals. Studies showed that Yb³⁺ ions acted as a sensitizer owing to the up-conversion process, excited-state absorption from Ho³⁺: ⁵I₇ → ⁵F₅, and energy transfer via Yb³⁺: ²F_5/2 → Ho³⁺: ⁵I₆, which not only increased the absorption cross section but also benefit to make the possibility of population inversion for Ho³⁺: ⁵I₆ → ⁵I₇. In addition, with the existence of Pr³⁺ ions, the ~2 μm emission is weak due to the fact that Pr³⁺ depopulates the Ho³⁺: ⁵I₇ level and has little influence on the Ho³⁺: ⁵I₆ level at the same time, which also benefits the possible population inversion for Ho³⁺: ⁵I₆ → ⁵I₇. The results and analyzed energy transfer indicate co-doping of Yb³⁺, Pr³⁺ with Ho³⁺ in LuAG crystal could be considered as an excellent candidate for mid-infrared lasers.