Activation and deactivation effects to Ho3+ at ~2.8 μm MIR emission by Yb3+ and Pr3+ ions in YAG crystal

J. Q. Hong; L. H. Zhang; M. Xu; Y. Hang

doi:10.1364/OME.6.001444

1. Introduction

In recent years, ~2.8μm mid-IR (MIR) lasers have attracted much attention for their applications in medical, biological as well as sensing technologies due to the strong vibrational absorption band of water in this spectral region [1–3]. Furthermore, ~2.8μm lasers can also act as efficient pumping sources for longer wavelength MIR lasers and optical parametric oscillators (OPO) [4].

Ho³⁺ is a well-known candidate for ~2.8μm lasers with the transition of ⁵I₆ → ⁵I₇, which has already been discussed and demonstrated in several hosts [5–7]. However, efficient ~2.8μm laser operations of Ho³⁺-singly-doped hosts have been limited by two bottle-necks. One is that the intrinsic absorption of Ho³⁺ can’t match an efficient, currently well-developed laser diodes (LDs) or flash lamp, and the other is that the fluorescence lifetime of the lower ⁵I₇ level is considerably longer than that of the upper ⁵I₆ level. To overcome these deficiencies, on one hand, Yb³⁺ has been considered favorable to improve the absorption efficiencies of Ho³⁺ with commercially available 940 or 970 nm pumping sources, which has been demonstrated in Yb:Ho:YSGG, Yb:Ho:GGG, etc [8–10]. On the other hand, Pr³⁺ has been explored to be feasible to quench the lower level of Ho³⁺: ⁵I₇, while hardly depopulate the upper level of Ho³⁺: ⁵I₆, as good results of ~2.8μm emissions and lasers achieved in studies of Ho,Pr-doped floride fiber [11] and Ho:Pr:LiLuF₄ crystal [12]. Therefore, it is believed that co-doping of both Yb³⁺ and Pr³⁺ with Ho³⁺ could open up the possibility of efficient, high-power ∼2.8μm lasing from Ho³⁺ by suitable high-power diode pumping, which is recently studied in Yb:Ho:Pr:YAP [13].

Host material is another factor that should be considered to get powerful ~2.8μm emissions from Ho³⁺. Among the various crystals, YAG is one of the most popular laser crystals due to its excellent optical, thermal, mechanical as well as physicochemical properties. Optical characterization and laser operations focused on ~2μm emissions have been studied in Ho:YAG and Yb:Ho:YAG [14–16], but to our knowledge, there is still no report on the growth of Yb:Ho:Pr:YAG crystal or the ~2.8μm emission in this crystal up to now. In this letter, the Yb³⁺, Ho³⁺-doped YAG and Yb³⁺, Ho³⁺, Pr³⁺-doped YAG laser crystals were successfully grown. Yb³⁺ and Pr³⁺ were demonstrated to greatly enhance the Ho³⁺: ⁵I₆→⁵I₇ ~2.8μm emission by efficient energy transfer from Yb³⁺: ²F_5/2 to Ho³⁺: ⁵I₆ and Ho³⁺: ⁵I₇ to Pr³⁺: ³F₂, respectively. The absorption spectra, emission spectra, fluorescence decay curve and transfer mechanisms among the ions were investigated for future applications in MIR lasers.

2. Experiments

The 15 at.% Yb³⁺/ 2.5 at.% Ho³⁺-co-doped YAG and 15 at.% Yb³⁺/ 2.5 at.% Ho³⁺/ 1 at.% Pr³⁺-co-doped YAG were grown along the <111> crystal orientation by the Czochralski method with an intermediate frequency induction heating system. Oxide powders of Yb₂O₃ (5N), Ho₂O₃ (5N), Pr₆O₁₁ (5N), Y₂O₃ (5N) and Al₂O₃ (4N) were used as starting materials, which were mixed adequately for 12h and pressed into disks, followed by heating in air for 10h at 1200 °C. Then, the polycrystalline materials were loaded into an Iridium crucible of Φ60 mm for crystal growth in an atmosphere of Ar, which could effectively protect the Ir crucible from oxidization, with a pulling rate of 1.1 mm/h and a rotation speed of 10 rpm. After growth, the crystals were annealed at 1200 °C in air for 20h to oxidize some Yb²⁺ ions, which were formed during growth and made the crystal green, into Yb³⁺. Samples with dimension of 10 × 10 × 3 mm³ were cut from the annealed crystals and then polished on both faces for spectroscopic measurements. The concentrations of Yb, Ho and Pr ions were detected by inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis. The absorption spectra in the range of 300nm-2250nm were recorded by Perkin Elmer Lambda 900. The fluorescence spectra in range of 2700 nm to 3000 nm were measured under 915 nm LD pumping. The fluorescence decay curves were recorded under excitation at 915nm by OPO pulse lasers. All the measurements were taken at room temperature and at the same condition for two crystals.

3. Results and discussions

The doping concentrations in the top of both crystals were measured to be Yb of 15.5at.% and Ho of 2.06at.% for the Yb:Ho:YAG crystal, and Yb of 15.6at.%, Ho of 2.04at.% and Pr of 0.18at. % for the Yb:Ho:Pr:YAG crystal, respectively. Therefor the segregation coefficient of Yb, Ho and Pr in the YAG crystal are approximate to 1.04, 0.82 and 0.18.

The room-temperature absorption spectra of Yb:Ho:YAG and Yb:Ho:Pr:YAG crystals were shown as absorption coefficient in Fig. 1. The absorption peaks of these two crystals are very similar, except the bands around 600nm and in the range of 1340-1570nm, which correspond to the transitions of Pr³⁺: ³H₄→¹D₂ and Pr³⁺: ³H₄→³F₃ + ³F₄, respectively. The broad and strong absorption bands within 850-1050nm centered at 940nm correspond to the transition of Yb³⁺: ²F_7∕2 → ²F_5∕2, which make both crystals suitable to be pumped by commercially available 940 high-power InGaAs LDs. The absorption cross-sections of Yb³⁺ ions can be calculated from

σ_{a b s} (λ) = \frac{O D (λ)}{L \times N_{0} \times \log_{e}},

where

O D (λ)

is the measured absorption optical density as a function of wavelength,

L

is the thickness of sample and

N_{0}

is the number of Yb³⁺ ions per cm³. The results are shown in the right inset of Fig. 1, and the maximum absorption cross-sections are 4.93 × 10⁻²¹cm² and 5.68 × 10⁻²¹cm² at 936 nm for the Yb:Ho:YAG and Yb:Ho:Pr:YAG, respectively. It is no accident that the peak absorption cross-section of Yb:Ho:Pr:YAG is a little greater than that of Yb:Ho:YAG, because the testing sample of Yb:Ho:Pr:YAG is cut from an upper position of the as-grown crystal than Yb:Ho:YAG, as shown in the left inset of Fig. 1. Since the segregation coefficient of Yb is a little greater than 1, the Yb concertration of the test sample of Yb:Ho:Pr:YAG can be a little larger than that of Yb:Ho:YAG.

Fig. 1 Absorption spectra of the Yb:Ho:YAG and Yb:Ho:Pr:YAG crystals.

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The absorption spectra were used to calculate the Judd–Ofelt [17,18] intensity parameters Ω_2;4;6 of Ho³⁺. The obtained Ω_2;4;6 values are shown in Table 1 with comparison of Ho:YAG. As Ω₂ reflects the asymmetry and high-covalence of ligands, it is reasonable that introduction of Yb³⁺ and Pr³⁺ ions leads to a higher Ω₂ value, which indicates a lower symmetry surrounding Ho³⁺ ions [12,19]. With the Ω_2;4;6, several optical parameters, such as the fluorescence branching ratio β and the radiative lifetime $τ_{r}$ are calculated and listed in Table 1 for further calculation. It can be seen that the fluorescence branching ratio β of transition ⁵I₆ → ⁵I₇ for ~2.8μm emission in Yb:Ho:YAG and Yb:Ho:Pr:YAG are approximately 17%, which is higher than that in Ho:YAG. Furthermore, the radiative lifetime $τ_{r}$ of ⁵I₆ manifold in Yb:Ho:YAG and Yb:Ho:Pr:YAG are approximately 4.3ms, which is longer than that in Ho:YAG. The higher fluorescence branching ratio and longer radiative lifetime both indicate that Yb:Ho:YAG and Yb:Ho:Pr:YAG may more easily induce the ~2.8μm emissions. It should be noticed that, the J-O theory does not take into account non-radiative decay, so the $τ_{r}$ [⁵I₇] of Yb:Ho:YAG and Yb:Ho:Pr:YAG are quite similar in calculation.

Table 1. Judd–Ofelt parameters, fluorescence branching ratio and lifetimes of Ho-doped materials.

View Table

The fluorescence spectra of the Yb:Ho:YAG and Yb:Ho:Pr:YAG crystals are shown in Fig. 2. For both crystals, there are three main emission bands around 2780, 2848, and 2944 nm assigned to the Ho³⁺: ⁵I₆ → ⁵I₇. It is evident that the emission intensity around 2780nm of Yb:Ho:Pr:YAG is quite strong, which is nearly 2.8 times that of Yb:Ho:YAG. Although the emission intensity around 2848 of Yb:Ho: Pr:YAG is a little lower than that of Yb:Ho:YAG, the emission intensities around 2944nm of both crystals are comparable. In addition, the left inset of Fig. 2 shows that the ~2μm emission (Ho³⁺: ⁵I₇ → ⁵I₈) of Yb:Ho:Pr:YAG is much weaker than that of Yb:Ho:YAG and the 2780nm emission of Yb:Ho:Pr:YAG, which indicates that co-doping Pr weaken the ~2μm emission and benefit the ~2.8μm emission. Thus, output of ~2.78μm lasers may be realized on Yb:Ho:Pr:YAG.

Fig. 2 Fluorescence spectra of the Yb:Ho:YAG and Yb:Ho:Pr:YAG crystals.

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The emission cross sections can be subsequently calculated by the Fuchtbauer–Ladenburg equation [21]

σ_{e m} (λ) = \frac{β λ^{5} I (λ)}{8 π c n^{2} τ_{r} \int λ I (λ) d λ},

where

I (λ)

is the measured fluorescence intensity at wavelength λ, β is the fluorescence branching ratio as β[⁵I₆→⁵I₇] in Table 1, is the speed of light, n is the crystal refractive index and

τ_{r}

is the radiative lifetime as

τ_{r}

[⁵I₆] in Table 1. The results are shown as the right inset of Fig. 2, and the maximum emission cross section of Yb:Ho:Pr:YAG crystal is estimated to be 1.69 × 10⁻²⁰ cm² at 2780 nm, and that of the Yb:Ho:YAG is 1.23 × 10⁻²⁰ cm² at 2848 nm.

The fluorescence decay curves of Ho³⁺: ⁵I₆ and ⁵I₇ multiplets for the Yb:Ho:YAG and Yb:Ho:Pr:YAG crystals are shown in Fig. 3. The four experiment curves all exhibit a single exponential decaying behavior, and the fitted values are shown in Table 1 as $τ_{m e a s}$ . On one hand, lifetime of ⁵I₆ manifold in Yb:Ho:Pr:YAG is 0.106ms, which is 5% shorter compared with that of 0.112ms in Yb:Ho:YAG, indicating that co-doping of Pr³⁺ has little influence on the higher laser level of Ho³⁺: ⁵I₆. On the other hand, lifetime of ⁵I₇ manifold in Yb:Ho:Pr:YAG is 1.83ms, which is 83% shorter when compared with that of 10.63ms in Yb:Ho:YAG. This remarkable decrease confirms that Pr³⁺ ions are able to depopulate the lower laser level of Ho³⁺: ⁵I₇ for ~2.8μm emission in YAG crystal, which may induce the population inversion and facilitate laser operations. In addition, comparing the $τ_{m e a s}$ of Ho:YAG of [20] and the Yb:Ho:YAG of this study, it can tell that Yb³⁺ may efficiently populate the Ho³⁺: ⁵I₆ while rarely influence the Ho³⁺: ⁵I₇. Lifetime of ⁵I₆ in Yb:Ho:YAG is 149% longer than that in Ho:YAG while lifetime of ⁵I₇ in Yb:Ho:YAG is only 52% longer than that in Ho:YAG. It indicates that due to ET process of Yb³⁺: ²F_5∕2 → Ho³⁺: ⁵I₆ as marked in Fig. 2, Yb³⁺ ions can prolong the higher laser level of Ho³⁺: ⁵I₆ and contribute to the ~2.8μm emission in YAG crystal.

Fig. 3 Fluorescence decay curves of the Yb:Ho:YAG and Yb:Ho:Pr:YAG crystals for the Ho: ⁵I₆ and ⁵I₇ mainfolds.

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To further investigate the Yb³⁺-Ho³⁺ and Ho³⁺-Pr³⁺ energy interaction mechanism, a simplified diagram of energy-level [22] and energy-transfer of Yb³⁺, Ho³⁺ and Pr³⁺ co-doped system is shown in Fig. 4. As the energy gap between Yb³⁺: ²F_5∕2 (~10000 cm⁻¹) and Ho³⁺: ⁵I₆ (~8500 cm⁻¹) is small, efficient ET process from Yb³⁺: ²F_5∕2 to Ho³⁺: ⁵I₆ is expected to occur with the phonon energy of the host bridging this energy difference (∼1500 cm⁻¹), which populates the Ho³⁺: ⁵I₆ level. On the other hand, it is evident that ions in the Ho³⁺: ⁵I₇ level will undergo ET process to the Pr³⁺: ³F₂ level due to rather small energy gap (∼30 cm⁻¹) between these two energy levels, which depopulates the Ho³⁺: ⁵I₇ level. Both ET processes make population inversion for Ho³⁺: ⁵I₆ →⁵I₇ possible and enhance the MIR ~2.8 μm emission. Moreover, efficiency of the energy transfer within Ho³⁺ and Pr³⁺ ions can be estimated by: $η = 1 - \frac{τ_{Y b / H o / \Pr}}{τ_{Y b / H o}}$ , where $τ_{Y b / H o / \Pr}$ and $τ_{Y b / H o}$ are the lifetimes of Ho³⁺: ⁵I₇ for the Yb:Ho:Pr:YAG and Yb:Ho:YAG crystals, respectively. Consequently, the value of η is calculated to be 82.8%, indicating that Pr³⁺ ions can efficiently quench the excited-state population of Ho³⁺: ⁵I₇ by ET process and enhance the ~2.8 μm emission.

Fig. 4 Schematic of energy-level and energy-transfer of Yb³⁺, Ho³⁺ and Pr³⁺ co-doped system.

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4. Conclusion

In conclusion, high-quality Yb:Ho:YAG and Yb:Ho:Pr:YAG crystals were successfully grown using Czochralski method. Absorption spectra show that co-doping with Yb enhances the absorption intensity within 850-1050nm and makes both crystals suitable to be pumped by commercially available high-power InGaAs LDs. Fluorescence spectra exhibit that output of ~2.8μm lasers may be realized on both crystals at different wavelengths with the maximum emission cross section of 1.69 × 10⁻²⁰ cm² at 2780 nm for Yb:Ho:Pr:YAG and 1.23 × 10⁻²⁰ cm² at 2848 nm for Yb:Ho:YAG. Lifetime results and energy transfer analyses indicate that Yb³⁺ ions can prolong the higher laser level of Ho³⁺: ⁵I₆ from 0.045ms of Ho:YAG to 0.112ms of Yb:Ho:YAG and Pr³⁺ ions are able to depopulate the lower laser level of Ho³⁺: ⁵I₇ from 10.63ms of Yb:Ho:YAG to 1.83ms of Pr:Yb:Ho:YAG, both beneficial to population inversion and contributed to facilitate Ho³⁺: ⁵I₆ →⁵I₇ ~2.8μm laser operations. It is suggested that the Yb:Ho:Pr:YAG is a favorable LD pumping active laser media for ~2.8μm solid-state lasers.

Acknowledgments

This work has been supported by the National Natural Science Foundation of China (Grant No.51302283 and Grant No. 51502321), and the Shanghai Natural Science Foundation under Projects (No.13ZR1463400).

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	Ω₂	Ω₄	Ω₆	β[⁵I₆→⁵I₇]	$τ_{r}$ [⁵I₆]	$τ_{r}$ [⁵I₇]	$τ_{m e a s}$ [⁵I₆]	$τ_{m e a s}$ [⁵I₇]	Reference
	(10⁻²⁰ cm²)			(%)	(ms)	(ms)	(ms)	(ms)
Ho:YAG	0.10	2.09	1.72	14.15	3.48	7.82	0.045	7.00	[20]
Yb:Ho:YAG	0.83	2.25	1.12	17.50	4.36	11.15	0.112	10.63	This ork
Yb:Ho:Pr:YAG	1.50	2.13	1.16	17.27	4.24	10.94	0.106	1.83	This ork

Activation and deactivation effects to Ho³⁺ at ~2.8 μm MIR emission by Yb³⁺ and Pr³⁺ ions in YAG crystal

Abstract

1. Introduction

2. Experiments

3. Results and discussions

4. Conclusion

Acknowledgments

References and links

Cited By

Figures (4)

Tables (1)

Equations (2)

Optical Materials Express