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Abstract
The photoluminescence parameters of Nd3+-doped Sr0.95Y0.05F2.05 crystal, which are a function of Nd3+ concentrations, were investigated. Abnormally, the peak emission cross section increased linearly with the lifetime. The quantum efficiency, peak emission cross section and lifetime were significantly improved from 26%, 2.8 × 10−20cm2 and 190μs to 90%, 5.4 × 10−20cm2 and 370μs, respectively. The results suggest that Y3+ ions worked as local lattice structure regulators and barrier layers besides the well known buffer ions.
© 2017 Optical Society of America
1. Introduction
Neodymium-doped fluorites have been discarded in the past as laser materials. Early works on this subject revealed that luminescence of neodymium clusters, easily formed in these crystals, have been deadly quenched by incoherent dipole-dipole energy transfer process [1].
Recently, interest in the system has been renewed. A true continuous-wave (CW) laser has been firstly performed in Nd3+:CaF2 crystal by incorporation of Y3+ ions [2]. A CW efficiency up to be 69% was targeted with a CW Ti-sapphire laser pumping in Nd3+:SrF2 co-doped with Y3+ [3], which was comparable with that of commercial Nd3+:YVO4 crystal [4]. While under the diode (LD) pumped mode of Nd3+:SrF2, the slope efficiencies of 43.5%, 42.7% and 50.6% were achieved when introduced with Y3+, Gd3+ and La3+ ions, respectively [5–7]. In fact, half a century ago Kaminskii et al. has firstly proved that Y3+, Sc3+, La3+, Gd3+ worked as buffer ions could reduce the fluorescence quenching and ameliorate the quantum yields in Nd3+-doped fluorites [8–13].
Particularly, pulses of 103fs were obtained from the LD pumped passively mode-lock technique in Nd3+,Y3+:CaF2 [14], then even shorter of 97fs was achieved in Nd3+,Y3+:SrF2 [15]. The breakthroughs make the Nd3+-doped fluorite crystal be a very promising candidate for ultra-short and ultra-high peak power lasers [16–18].
However, the modulation effects of Y3+ on photoluminescence properties of Nd3+ were rarely studied. In the work, 5 at.% YF3 was chosen as the codopant concentration when taking into account the thermal conductivity, absolute quantum efficiency and spectral properties, then five Nd3+-doped Sr0.95Y0.05F2.05 crystals were investigated. The spectral parameters were realized to be tuned significantly and Y3+ ions worked as more than buffer ions but local lattice structure regulators.
2. Experimental
Temperature Gradient Technique (TGT) method was used to grow high optical quality Nd3+:Sr0.95Y0.05F2.05 crystals, as listed in Table 1, with purity of 99.99% raw materials NdF3, YF3 and SrF2, and growth rate of 1.5K/h, as described in reference [19]. Then samples with same dimensions were prepared for spectral measurement. The real concentrations of dopants were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES), as tabulated in Table 1.
Table 1. Doping and real concentrations of Nd3+ and Y3+ in the crystals.
Room temperature absorption spectra were recorded by a Jasco V-570 UV/VIS/NIR spectro-photometer and fluorescence spectra with a FLS980 time-resolved fluorimeter grating blazed at 1200nm and detected by thermoelectric (TE) cooled InGaAs detector. The excitation was a xenon lamp at 796nm. The lifetime was measured using the fluorimeter and Tektronix TDS 3052 oscilloscope following a flash lamp excitation at 796nm.
The absolute quantum yields were tested with the same instruments and excited at 796nm. A barium sulfate-coated integrating sphere (Edinburgh) was applied as the sample chamber. The entry and output gates of the sphere located in 90° geometry from each other in the plane of the fluorimeter. Both reference and sample emissions were collected in the region of 765nm to 1450nm. There existed one line at 796nm in the reference emission, and four peaks of 796nm, 890nm (4F3/2→4I9/2), 1050nm (4F3/2→4I11/2) and 1330nm (4F3/2→4I13/2) in the sample emission. The yields were thus obtained.
3. Results and discussion
The absorption spectra are presented in Fig. 1. The strongest absorption band of 4I9/2→4F5/2 + 2H9/2 is peaking around 800nm, very suitable for commercial LDs pumping. From sample of A to E with decreasing of Nd3+ concentration, the peak section at 796nm, very sensitive to local symmetries and surroundings was sharply enhanced, which means the higher Y/Nd ratios the lower symmetric local structures of Nd3+ ions. Y/Nd ratios were defined as the ratio of Y3+ doping concentration to that of Nd3+. At the point, the name buffer ions was insufficient, Y3+ ions prefer to contribute as local lattice structure regulators.
Fig. 1 Absorption spectra of the sample. The real Nd3+ concentration was taken into account for reasoning of the absorption cross section.
Just like the absorption, the emission intensity was also highly improved from A to E media when taking the Nd3+ concentration into consideration, as shown in Fig. 2(a). The quenched clusters were completely eliminated by the combination of Nd3+ and Y3+ ions in sample of D and E, and for others there still exists quenching.
Fig. 2 (a) Emission spectra of the crystal, the insert represents the transition of 4F3/2→4I11/2. The sample was measured under the same conditions and the intensity could therefore be compared with each other. (b) The Y/Nd ratios dependent effective linewidth of the 1056nm band. (c) Schematic diagram of the modulation effect of Y3+ on local surroundings of Nd3+.
The emission quenching in A, B and C sample came from, on one hand, the quenched clusters, on the other, the dipole-dipole energy transfer among the quenched or non-quenched centers. As can be seen in Fig. 2(c), the quenched Nd3+-pairs were broken to be Nd3+-Y3+ centers by Y3+ buffer ions. Additionally, the surplus part of Y3+ would act as a layer barrier to blockade the energy migration among different clusters. The thickness of Y3+ buffer layer will aggrandize with higher Y/Nd ratios.
Besides, due to the varied characters of Nd3+ and Y3+, as well as the bonding energy of Nd3+-F- and Y3+-F- [20-21], the local environment of Nd3+ was manipulated to be more distorted. Finally the local surroundings tend to be nearly the same. The effective linewidth (Δλem) of 4F3/2→4I11/2 transition calculated by Eq. (1) was presented to verify the point in Fig. 2(b). The linewidth decreased exponentially with Y/Nd ratios, which means the parameters were adjustable by Nd3+ proportions. For D and E crystal with higher Y/Nd ratios the width was therefore kept to be constant. The unvaried linewidth implied that a multi-site crystal of A has been gradually modulated to be a quasi-single-center system of D and E sample, which agreed well with the aforementioned discussions. Nevertheless, the value of 20nm is much higher than that of Nd3+:YAG (0.8nm), Nd3+:YVO4 (π-1.5nm, σ-3nm) and Nd3+:YLF (2nm) crystals [22–24], and is comparable with that of Nd3+-doped phosphate glasses (21-26nm) [25]. The Nd3+-doped crystals are competent for generation of sub-100fs pulses [15].
where the I(λ) is the emission intensity at the wavelength of λ, Ipeak represents the peak emission intensity. The integrating was performed to 4F3/2→4I11/2 transition.
The decay curves were also tested and shown in Fig. 3. It can be seen the decay line of crystal A is deviated from single exponential expression, which was caused by the serious energy migration. It is impossible to have the real lifetime and only mean lifetime (τA) was obtained by Eq. (2). The other lines were fitted by single exponential.
The I(t) is intensity of the luminescence and t represents time.The lifetime augmented exponentially with Y/Nd ratios, as well as the absolute quantum efficiency, as shown in Figs. 4(a) and 4(b). For D and E sample the time and efficiency inclined to be invariable and about 370μs and 90%, respectively. The lifetime is longer than that of Nd3+-doped phosphate and silicate glasses (270μs-330μs) [25]. The radiative lifetime was calculated by Eq. (3), and presented in Fig. 4(a). The exponentially decreased value means that the transition rate was improved in an exponential manner.
τem and τrad represent the measured and radiative lifetime of 4F3/2 level, respectively.Fig. 4 The Y/Nd ratios dependent (a) fluorescence and radiative lifetime of 4F3/2 level and (b) the absolute quantum yields.
Since Judd-Ofelt theory was not reasonable for a multi-site system of Nd3+-doped fluorites [26-27], the quantum yields were applied to have the radiative lifetime by Eq. (3) and then the peak emission cross section. Based on Eq. (3) the Füchtbauer-Ladenburg equation could be rewritten as,
λpeak represents the peak emission line of 4F3/2→4I11/2, c the velocity of light, n the refractive index of the crystal at the peak emission line and 1.43 was adopted [28], Δλem and β2 respectively the effective linewidth and branching ratio of 4F3/2→4I11/2.
i = 1, 2, 3 were the emissions of Nd3+ from 4F3/2 metastable level to the lower-lying multiplets 4I9/2 at 890nm, 4I11/2 at 1050nm and 4I13/2 at 1330nm, respectively, as shown in Fig. 2(a) and Fig. 5.Fig. 5 The level diagram of Nd3+ ions and three near-infrared emission bands from upper level to the lowers.
The peak emission cross section was thus obtained and presented in Fig. 6(a). The section was self-manipulated to be increased linearly with the growth of lifetime. The abnormal phenomenon should be attributed to the highly enhanced transition oscillator strength and quantum yields. As a result the section and lifetime were improved from 2.8 × 10−20cm2 and 190μs to 5.4 × 10−20cm2 and 370μs, respectively. The products of peak emission cross section and lifetime were enhanced exponentially with Y/Nd ratios in Fig. 6(b), which corresponds to nearly twice the level of phosphate and silicate glasses [25]. The product is inversely proportional to the laser threshold and about 170mW threshold was achieved [5-6], which means the low threshold high efficiency laser would be very promising in the crystal.
Fig. 6 (a) The lifetime dependent peak emission cross section of 4F3/2→4I11/2. (b) The Y/Nd ratios dependence of the products of peak emission cross section and fluorescence lifetime of 4F3/2 level.
4. Conclusion
By adjusting the concentration of Nd3+ the photoluminescence parameters, peak absorption cross section, lifetime, quantum yields and peak emission cross section, can be self-modulated in a large scale. Abnormally, the peak emission cross section increased linearly with the lifetime. It means that Y3+ ions contributed as local lattice structure regulators and barrier layers besides the buffer ions. The idea could also be extended and applied in other rare earth doped fluorites. For example, enhancing the dipole transition probabilities by lowering the local symmetries; stabilizing the valence state of Ce3+, Tb3+, Eu3+ and Yb3+ in a more disordered surroundings; eliminating or utilizing the energy migration, and so on. It is noted that the properties of Nd3+-doped Sr0.95Y0.05F2.05 crystal were tuned to be very desirable for highly efficiency and ultra-short pulses operation.
Funding
The work was supported by the National Key R&D Program of China (2016YFB0402101), the Strategic Priority Program of the Chinese Academy of Sciences (XDB16030000), the National Natural Science Foundation of China (Nos. 61422511, 61635012 and 51432007), and Science and Technology Commission of Shanghai Municipality under Grant agreement 14520710400.
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