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Abstract
The use of Tb3+ codoping for the enhancement of the transition of Tm3+:3H4→3F4 ∼1.4 μm emissions was investigated in the Tm3+/Tb3+ codoped (Gd0.5Lu0.5)2SiO5 (Tm/Tb:GLSO) crystal for the first time. The ∼1.4 μm fluorescence emission properties and energy transfer mechanism of the as-grown crystals were investigated in detail. It is found that the codoped of Tb3+ ion in Tm/Tb:GLSO crystal greatly enhances Tm3+:1.4 μm emission under excitation of a common 789 nm laser diode, depopulates the lower laser level of Tm3+:3F4, and has little effect on the upper laser level of Tm3+:3H4 at the same time. The energy transfer efficiency from the Tm3+:3F4 level to the Tb3+:7F0 level is as high as (74.4±1.5)%, indicating that the Tb3+ ion is an effective deactivation ion for enhancing the ∼1.4 μm emission in Tm/Tb:GLSO crystal. These results suggest that Tm/Tb:GLSO crystal may be a promising material for ∼1.4 μm laser under the pump of a conventional 789 nm LD.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the last decades, lasers operating at around 1.4 μm have attracted more and more attention because of its possible applications in laser radar, atmospheric measurements, telecommunication industry, and biomedical systems [1–5]. Tm3+ is a well-known ion with transitions in the near infrared region around 1.4 μm (3H4→3F4) and 1.8 μm (3F4→3H6) [6,7]. However, the 1.4 μm emission cannot be obtained efficiently because the self-terminating effect of Tm3+ ion due to the fact that the longer lifetime of the terminal 3F4 level than that of the initial 3H4 level [8]. To improve the Tm3+:3H4→3F4 emission intensity, several methods have been proposed. Using an upconversion pumping scheme with 1.06 μm lasers to produce a population inversion based on the excited-state absorption (ESA) from 3F4 level to 2F2,3 level [8]. Codoping of Bismuth ions was also found to improve the amplification efficiency at 1460 nm region on account of the effective energy transfer channel from Bismuth to Thulium [9].
Up to now, however, it remains a challenge to achieve intense Tm3+:3H4→3F4 emission for practical laser operation. Fortunately, Tb3+ ions, one of the rare-earth ions, can be used as a feasible alternative to quench the terminal 3F4 level by means of energy transfer. As the energy level scheme illustrated in Fig. 1 shows, after the ions in the Tm3+:3H4 level decay radiatively to Tm3+:3F4 level with ∼1.4 μm emission, the ions in the Tm3+:3F4 level will undergo an effective energy transfer (ET) process to Tb3+:7F0 level because of their neighboring energy level position, which depopulates the Tm3+:3F4 level, making the possibility of population inversion for Tm3+:3H4→3F4. To the best of our knowledge, the use of Tb3+ to deactivate Tm3+ for ∼1.4 μm effective fluorescence emission has not been reported until now.
Fig. 1 Simplified energy level diagram of Tm3+ and Tb3+ co-doped system. ET: energy transfer from Tm3+:3F4 level to Tb3+:7F0 level.
(Gd0.5Lu0.5)2SiO5 (GLSO) crystal, which is considered as a solid solution of two silicates crystals: Gd2SiO5 (GSO) and Lu2SiO5 (LSO), is characterized by a high degree of structural disorder [10,11]. Both compounds crystallize in monoclinic crystal systems, but have different space structures. GSO crystallizes in P21/c space group, while LSO crystallizes in C2/c space group [12]. The structure differences of GSO and LSO would further increase the disorder degree of GLSO crystal, making the crystal possess larger ground-state split, which is benefit to inhomogeneous broadening of the absorption and fluorescence spectra, bringing a large amount of advantages from the standpoint of applications as active media in LD-pumped, continuously tunable, femtosecond, and multi-wavelength generation solid-state lasers [13–15].
In this work, an efficient ∼1.4 μm emission in a Tm3+/Tb3+ codoped GLSO crystal is reported for the first time. Tb3+ ion was demonstrated to be an efficient deactivated ion for Tm3+ ion to greatly facilitate the Tm3+:3H4→3F4 by effective ET from Tm3+:3F4 level to Tb3+:7F0. Besides, the spectroscopic properties of Tm3+/Tb3+:GLSO crystal were also investigated in detail to demonstrate its feasibility for ∼1.4 μm solid state lasers.
2. Experimental section
The 3.0 at.% Tm3+, 0.5 at.% Tb3+ codoped (Gd0.5Lu0.5)2SiO5 (Tm/Tb:GLSO), 3.0 at.% Tm3+ single doped GLSO (Tm:GLSO), and 0.5 at.% Tb3+ single doped GLSO (Tb:GLSO) crystals were grown by the Czochralski method with intermediate frequency induction heating. Lu2O3 (99.999%), Gd2O3 (99.999%), Tb4O7 (99.999%), Tm2O3 (99.999%) and SiO2 (99.999%) were used as raw materials for crystal growth. The mixture was pressed into disks and heated in air at 1300 °C for 20 hours to form polycrystalline powers. Then, the polycrystalline powers were followed loaded into platinum crucible for crystal growth along the b axis. The rotation rate and pulling rate were 10–12 rpm, and 0.8–1.0 mm/h, respectively. The growth atmosphere was pure nitrogen.
The concentrations of Tm3+, and Tb3+ ions in the as grown crystals were measured by the inductively coupled plasma-atomic emission spectrometry (ICP-AES). The doping concentrations of Tm3+ and Tb3+ in the codoped crystal were measured to be (1.50±0.01) at.% ((1.44±0.09)×1020 ions/cm3) and (0.16±0.01) at.%((1.53±0.10)×1020 ions/cm3), respectively. The doping concentration in the single doped crystal was (1.52±0.01) at.% ((1.49±0.09)×1020 ions/cm3) of Tm3+ ions, and (0.17±0.01) at.%((1.57±0.10)×1020 ions/cm3)of Tb3+ ions, respectively. The absorption spectrum of Tm/Tb:GLSO crystal in the range of 600–2500 nm was measured by a UV-VIS-NIR spectrophotometer (ModelV-570, JASCO Co.). The fluorescence spectra and fluorescence decay profiles of the double-doped and single-doped crystals were acquired by Edinburgh Instruments FLS920 and FSP920 spectrophotometers under excitation of 808 nm. All measurement were done at room temperature.
3. Experimental results and discussion
Figure 2 shows the visible-near-IR absorption spectra of Tm:GLSO, Tb:GLSO, and Tm/Tb:GLSO crystals. It is clear to see that in the range of 600–2500 nm, the absorption bands centered at approximately 678, 789, 1209, and 1633 nm, which corresponds to the transitions from 3H6 to 3F3+3F2, 3H4, 3H5, and 3F4 of Tm3+, respectively. The transitions from 7F6 to 7F0+7F1+7F2+7F3 of Tb3+ (centered at around 2194 nm) in the wavelength range of 1800–2500 nm are also clearly shown. In the range of 760–820 nm, there is one strong absorption band located at around 789 nm, which well matches the emitting wavelength of high-power AlGaAs laser diodes (LD).
Based on the absorption spectra, some spectroscopic parameters, which are related to the optical properties of Tm3+, can be obtained by Judd–Ofelt (J–O) theory [16,17]. The experimental line strength for transitions can be obtained by the following expression:
where h is Planck’s constant, c the speed of light, λ̄ the barycenter wavelength of the specific absorption band that corresponds to the J→J′ transition, L the sample thickness, OD(λ) the measured optical density, and n the refractive index of the crystal, respectively. In the J–O theory, the measured absorption line strengths were then used to obtain the J–O intensity parameters Ω2, Ω4 and Ω6 by fitting the set of equations from the corresponding transitions between J and J′ manifolds in the following equation: where J and J′ are the total angular momentum quantum numbers of the initial and final states, respectively, |< S,L,J ‖ U(t) ‖ S′,L′,J′ >|2 are the square of the matrix elements of the tensorial operatou, which were calculated by Carnall et al [18]. Table 1 shows the experimental and calculated line strengths. The intensity parameters Ω2,4,6 of Tm3+ (shown in Table 2) was calculated. 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 [19]. It can be seen that the Ω2 of the Tm3+/Tb3+ co-doped GLSO crystal is higher than that of Tm3+ single doped GLSO crystal, indicating that lower symmetry surrounding Tm3+ ions is caused by the introduction of Tb3+ ions. Moveover, the radiative transition probability (A), and fluorescence branching ratio (β) of different upper levels for the Tm3+/Tb3+ co-doped GLSO crystal can also be calculated by using Ω2,4,6 and the results are shown in Table 3. The fluorescence branching ratio of the 3H4→3F4 transition was also calculated to be 10.3% of the Tm/Tb:GLSO crystal, which is larger than that (7.1%) of the Tm:GLSO crystal, indicating that the codoping of Tb3+ ions has positive influence of the ∼1.4 μm fluorescence emission efficiency of Tm3+ in GLSO crystal.
Table 1. Barycenter wavelengths, and measured and calculated line strengths of Tm/Tb:GLSO crystal.
Table 2. Judd–Ofelt parameters, calculated branching ratio, and lifetime of Tm:GLSO and Tm/Tb:GLSO crystals.
Table 3. Line strengths, branching ratios, and transition probabilities in Tm/Tb:GLSO crystal.
The infrared emission spectra around ∼1.4 μm of Tm:GLSO and Tm/Tb:GLSO crystals under the excitation of 789 nm is shown in Fig. 3(a). The observed five emission bands around 1417, 1450, 1479, 1515, and 1547 nm are assigned to the Tm3+:3H4→3F4 transition. It is clear to see that the emission intensity of the crystal codoped with Tb3+ is at least 1.5 times that of the crystal without Tb3+ codoping. This enhanced fluorescence emission is supposed to be closely associated with the higher fluorescence branching ratio of the 3H4→3F4 transition for the Tm/Tb:GLSO crystal. On the contrary, as is suggested by the infrared emission spectra around ∼1.8 μm (Tm3+:3F4→3H6 transition) shown in Fig. 3(b), the emission intensity from the excited 3F4 state of Tm/Tb:GLSO crystal is almost absent, whereas intense photoluminescence form 3F4 level of Tm:GLSO crystal is observed. This weaker fluorescence emission justifies that Tb3+ ions can be used effectively to depopulate the Tm3+:3F4 level. According to the Fuchtbauer-Ladenburg theory and emission spectra, the ∼1.4 μm emission cross-section can be calculated [20]. It is worth noting that the peak of the emission cross-section in the Tm/Tb:GLSO crystal at 1479 nm achieves (0.36±0.03)×10−20 cm2, which is higher than the result of Tm3+ doped GLSO crystal ((0.22±0.03)×10−20 cm2).
According to the energy level scheme of Tm3+ and Tb3+ ions (shown in Fig. 1), after the crystal is excited to the Tm3+:3H4 level by a 789 nm LD, the ions in the Tm3+:3H4 level will decay radiatively to Tm3+:3F4 level with ∼1.4 μm emission. After that, ions in the Tm3+:3F4 level will decay radiatively to Tm3+:3H6 level with ∼2 μm emission, or undergo an energy transfer process to Tb3+:7F0 because of the small energy mismatch between the 3F4 level of Tm3+ and the 7F0 level of Tb3+, which would depopulate the lower laser level of Tm3+:3F4, and enhance the ∼1.4 μm emission, as shown in Fig. 3. The ability of a deactivation ion (Tb3+) receiving energy from a donor (Tm3+) is an important factor to evaluate the acceptor (Tb3+) as a deactivation ion. To further confirm the energy interaction mechanism, the time-resolved decays of the Tm3+:3H4, and 3F4 levels of the Tm3+ single-doped GLSO and Tm3+/Tb3+ codoped GLSO crystals were measured, and the results are shown in Fig. 4. On the one hand, the measured lifetime of Tm3+:3H4 level in Tm/Tb:GLSO crystal is (64±0.5) μs, which is only (13.5±1.3)% shorter than that of Tm:GLSO crystal ((74±0.5) μs). This comparable lifetime of the Tm3+:3H4 level indicates that the codoping of Tb3+ ions has little influence on the higher laser level of Tm3+. On the other hand, the measured lifetime of the lower laser level of Tm3+:3F4 in the Tm/Tb:GLSO crystal is (1.64±0.02) ms, which is (74.4±1.5)% shorter compared to that of the Tm:GLSO crystal ((0.42±0.02) ms), confirming that Tb3+ ions can be used as effective deactivation ions to depopulate the Tm3+:3F4 level for enhancing the ∼1.4 μm emission. Moreover, the energy transfer efficiency can be estimated by the following equation: ηET =1−τTm/Tb/τTm, where τTm/Tb and τTm are the Tm3+ lifetimes monitored with and without Tb3+ ions, respectively. The energy transfer efficiency from Tm3+:3F4 level to Tb3+:7F0 level was calculated to be as high as (74.4±1.5)%, indicating that the Tb3+ ions can efficiently quench the lower laser level of Tm3+:3F4. Furthermore, based on the measured results above, the upper–laser level 3H4 to lower-laser level 3F4 fluorescence lifetime ratio (τ(3H4)/τ(3F4)) of the Tm/Tb:GLSO crystal was calculated to be (15.2±0.9)%, which is almost three times that of the Tm:GLSO crystal ((4.5±0.1)%). All the results shows that the codoping of Tb3+ ions can efficiently lead to an accelerated depletion of population in the lower-laser level 3F4, enhance the ∼1.4 μm fluorescence emission, and lower the pump intensities to reach inversion.
4. Conclusion
In conclusion, Tm:GLSO crystals codoped with and without Tb3+ ion were successfully grown by the Czochralski method. Compared with the Tm:GLSO crystal, the Tm/Tb:GLSO crystal has a higher fluorescence branching ratio, and stronger fluorescence emission intensity corresponding to the ∼1.4 μm emission from the Tm3+:3H4→3F4 transition. It was also demonstrated that the introduced Tb3+ efficiently depopulates the lower laser level of Tm3+:3F4, and has little effect on the upper laser level of Tm3+:3H4, which is benefited to accelerated depletion of population in the lower-laser level 3F4 for enhancing the ∼1.4 μm emission. Moreover, the energy transfer efficiency from Tm3+:3F4 level to Tb3+:7F0 level was calculated to be (74.4±1.5)%. With these favorable properties, the Tm/Tb:GLSO crystal is suggested to be used as a promising material for ∼1.4 μm laser applications under the pump of a conventional 789 nm LD.
Funding
The National Key Research and Development Program of China (2017YFB1104500); National Natural Science Foundation of China (NSFC)(51702124, 61735005, 11704155); Guangdong Project of Science and Technology Grants (2016B090917002, 2016B090926004); Guangzhou Union Project of Science and Technology Grants (201604040006).
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