We report Er3+-doping, Mg2+-codoping and crystal composition effects on emission and absorption cross sections of Er3+-only doped and Er3+/Mg2+-codoped LiNbO3 crystals, in which Er3+ concentration ranges from 0.3 to 2.7 mol% and Mg2+ concentration from 1.2 to 7.4 mol%. The emission cross section spectra were calculated from the measured fluorescent spectra and the absorption cross section spectra were computed using the McCumber relation. The results show that increasing Er3+ concentration tends to decrease the cross section, while codoping with Mg2+ or increasing crystal composition tends to increase the cross section. As the composition is close to the stoichiometry, the cross section value tends to a constant. These effects are related to Er3+ site redistribution induced by doping or crystal composition alteration.
© 2015 Optical Society of America
Er3+-doped LiNbO3 (LN) crystal is a promising substrate material for integrated optics as it combines excellent electro-optic, acousto-optic and nonlinear optical properties of LN with Er3+ laser property. Such an effective combination, together with the possibility of fabricating a waveguide of low loss, enables the broadband amplification and lasing at 1.5 µm. Over the past years, a family of Ti (or vapor ZnO)-diffused Er:LN waveguide lasers (amplifiers) and integrated devices have been demonstrated [1–4 ]. However, the photorefractive effect not only affects the performance of these devices, but also limits both the pumping and operating wavelengths. Although doping with >4.9 mol% MgO can effectively suppress the effect , heavy MgO doping leads to the difficulty in growing high quality single-crystal when bulk codoped with rare earth ions, and the decrease in diffusivity and solid solubility when rare-earth ions are incorporated by diffusion-doping method. It has been reported that a near-stoichiometric (NS) LN crystal needs less amount of MgO (>0.8 mol%) to prevent the photorefractive effect . In addition, the NS crystal also exhibits some other advantages such as lower coercive field, larger electro-optic and nonlinear effects. Thus, an NS Er:LN crystal codoped with moderate MgO concentration would be a promising substrate material.
It is crucial to have the knowledge of Er3+ absorption and emission cross sections. People have reported the knowledge of bulk material [4, 7, 8 ] and waveguide [9, 10 ]. However, previous study focused on the congruent material while a systematic study of composition effect on the cross section could not be found yet. Present work focuses on the study.
2. Experimental description
Singly Er3+-doped and Er3+/Mg2+-codoped congruent LN crystals used in present study were grown by Czochralski method. Tables 1 and 2 summarize the samples and the dopant concentration, determined by neutron activation analysis with an error of ± 5%. The NS crystals were obtained by carrying out Li-rich vapor transport equilibration (1100 °C/100 h) on the as-grown congruent plates. The Li compositions in these NS crystals were determined from either the measured optical absorption edge (OAE) [11, 12 ] or the linewidth of 153 cm−1 phonon . The obtained Li/Nb ratios are summarized in Tables 1 and 2 . The data labeled by the superscript a/b represents the Li/Nb ratio in the congruent/NS crystals. It has been proved that Er3+ ions occupy the Li sites in the LN lattice , and this is also the case for the Mg2+ dopant when its concentration is below the threshold (Cth) . This means that the Er3+ or Mg2+ doping causes different extents of decrease in the Li/Nb ratio. One can see from Tables 1 and 2 that the Li/Nb ratios in the NS crystals are definitely higher than those of the as-grown ones, and are closer to the stoichiometry. For reader’s convenience, the Li/Nb ratios in the ideal stoichiometric Er3+/Mg2+-codoped crystals are also given in Table 2.
All visible, near- and mid-infrared emission spectra were measured by a Horiba Jobin-Yvon Fluorolog-3 double-grating-excitation spectrofluorometer. A 450 W continuous xenon lamp was used as the source to excite all visible, near- and mid-infrared emissions. A rectangular excitation-probe configuration was used. A σ-polarized excitation beam was incident onto the sample along the direction perpendicular to the surface of each Y-cut plate.
The polarized or unpolarized fluorescence was collected along the direction perpendicular to the excitation beam. The spectra of the 0.53, 0.55, 1.5 and 2.7 μm emissions were recorded under the σ-polarized 980 nm (~20 mW) excitation. A couple of 1200 grooves/mm gratings blazed at the wavelength of 750 nm were used to disperse the excitation light. The visible, near- and mid-infrared fluorescence was dispersed by a 1200, 600 and 300 grooves/mm grating, respectively. The 0.86 and 0.98 μm spectra were measured under the σ-polarized 527 nm (~10 mW) excitation and another couple of 1200 grooves/mm gratings blazed at 330 nm were used to disperse the excitation light and the 600 grooves/mm grating to disperse the fluorescence. Both the π- and σ-polarized spectra were recoded for all visible and near-infrared emissions and the recording conditions under the two polarizations were identical for each sample. Due to lack of a polarizer, unpolarized mid-infrared spectrum was recorded. For better detection of relatively weak mid-infrared emission, the excitation light was modulated at a frequency of 140 Hz and a standard lock-in amplifier was used.
3. Results and discussion
The polarization-resolved emission cross section is given by Eq. (1), Arad represents the radiative transition rate. Its value was obtained from the Judd-Ofelt analysis. For the two thermalized states, 2H11/2 and 4S3/2, Arad represents an effective radiative rate .
The absorption and emission cross sections are related by the McCumber relation:9]. For the transitions studied here, the 1/εT has a value of 542.5, 842, 982, 1534.5 and 2722 nm for the bands at 530 + 560(2H11/2 + 4S3/2↔4I15/2), 810 + 860 (2H11/2 + 4S3/2 ↔4I13/2), 980 (4I11/2↔4I15/2), 1530 (4I13/2 ↔4I15/2) and 2700 nm (4I11/2 ↔ 4I13/2), respectively. These 1/εT values are assumed identical for all the crystals studied because the possibility is small that the Er3+ electronic energy structure changes noticeably due to Mg2+ codoping or slight composition alteration (but the Arad value may change from one sample to another).
Comparison reveals that both spectral shape and polarization dependence show little effects of Mg2+-codoping and crystal composition effect. Instead, the cross section value shows definite Er3+ doping, crystal composition and Mg2+-codoping effects. Next, we exemplify the typical 1530 nm electronic transition to demonstrate these effects.
First, we discuss the Er3+ doping effect. Figure 1 shows the σ-polarized Er3+ 1530 nm emission (a) and absorption (b) cross section peaks of five congruent LNs with different Er3+ concentrations of 0.3, 0.9, 1.4, 2.3 and 2.7 mol%. We note that the cross section decreases with increased Er3+ concentration. To more clearly show the dependence, the peaking cross section is plotted against the Er3+ concentration in Fig. 2 . As the Er3+ concentration goes from 0.3 to 2.7 mol%, the decrease is as much as 12%, which is beyond the error (6%).
Second, attention is paid to the Mg2+ codoping effect. Figure 3 shows a comparison of σ-polarized Er3+ 1530 nm cross section peaks of three Er3+/Mg2+-codoped congruent LNs doped with similar Er3+ concentrations (1.16-1.26 mol%) but different Mg2+ concentrations 1.2, 2.4 and 4.8 mol%. For comparison, the peaks of the Er3+ (1.4 mol%)-only doped LN are also shown. It appears that Mg2+ codoping causes the cross section increase by ~10%. To more clearly show the Mg2+-codoping effect, the peaking cross section is plotted against the Er3+ concentration in Fig. 2, together with the results of other samples. The Mg2+ codoping effect appears to be true when we compare the blue asterisk and red ball plots.
Third, we turn to discuss the composition effect. Figure 4 shows σ-polarized Er3+ 1530 nm cross section peaks of five NS LNs doped with different Er3+ concentrations of 0.3, 0.9, 1.4, 2.3 and 2.7 mol%. The peaking cross section is plotted against the Er3+ concentration in Fig. 2, together with the results of other NS samples (the green ball plot is for the Er3+-only doped NS LNs and magenta asterisk curve is for the Er3+/Mg2+-codoped NS LNs). The cross section increases from a congruent to an NS crystal, whether the crystal is Er3+-only doped orEr3+/Mg2+-codoped. As the composition is close to the stoichiometry, the cross section increases by 20% and no longer changes with the concentration of either Er3+ or Mg2+(due to this reason, the 1530 nm peaks of the Er/Mg-codoped NS LNs are not shown for save space).
Similar Er3+-doping, Mg2+-codoping and composition effects are also observed for other transitions 2H11/2↔4I15/2 + 4S3/2↔4I15/2, 4S3/2↔4I13/2 (~0.86 μm), 4I11/2↔4I15/2 and 4I11/2↔4I13/2. We no longer repeat the discussion. For convenience, the cross section values of some typical peaks of these transitions are given in Tables 1 and 2 . Because the transition with the lower polarization shows the features similar to that with the higher polarization, here we give only the cross section values of each transition with the higher polarization.
In words, Er3+ doping causes the cross section decrease by 12% while Mg2+ codoping causes the cross section increase by 10%. As the composition is increased from congruent point to stoichiometry, the cross section increases by 20%. These effects are related to their respective effects on Er3+ site distributions. Site selective spectroscopy study on Er:LN has shown that rare-earth doping, Mg2+ codoping and composition alteration all cause redistribution of non-equivalent energetic Er3+ sites . The site redistribution associated with an increase in Er3+ concentration is similar to that associated with an increase in Li deficiency. Increasing Er3+ concentration tends to increase Li-deficiency and decrease crystal composition. Increasing composition not only reduces cluster site concentration but also increases the absorption upon the light , implying that the cross section of Er3+ transition is larger for the isolated sites while smaller for the clustered sites. Regarding the congruent crystals under study, it is reasonable that the cross section decreases with a rise in Er3+ concentration as high Er3+ concentration increases successively the concentration of clustered Er3+ sites, the probability of cooperative upconversion, which depends on the concentration of clustered site, and the quenching effect on the fluorescent intensity, which is associated with the upconversion effect. For the NS crystals, the composition increase induces the increase of optical absorption and hence the increase of cross section. As composition is close to the stoichiometry, the defects in crystal are almost completely removed, and the defect-related site redistribution no longer changes noticeably from one NS crystal to another, regardless of Mg2+ codoping. Thus, the cross section in the NS composition case tends to a constant and does not change noticeably with the dopant concentration. The Mg2+ codoping effect can be explained similarly. According to the defect model for MgO-doped LN , doping with an Mg2+ concentration below the threshold Cth leads to the composition increase. It is thus comprehensible that, for the Er3+/Mg2+-codoped crystals studied, in which the Mg2+ concentration is lower than Cth, the Mg2+ codoping leads to slight cross section increase. For samples 2, 3 and 4 in Table 2, which are heavily doped with Mg2+, one should observe the cross section decrease because doping with an Mg2+ concentration above Cth causes the defect concentration increase . However, as shown in Fig. 2, the cross section does not reveal such feature because of the lower Er3+ concentration and its small Mg2+ doping effect.
Finally, it is worthy to note that the effects of Er3+ doping, Mg2+ doping and crystal composition match with their effects on the width of spectral line. One can see from Figs. 1, 3 and 4 that the spectral line broadens as more Er3+ ions are doped while narrows as the Mg2+ ions are codoped or the crystal composition is increased. The spectral line broadening/narrowing matches with the cross section decrease/increase.
We have demonstrated crystal composition effect on Er3+ cross section of singly Er3+-doped and Er3+/Mg2+-codoped LNs. Increasing doped Er3+ concentration tends to decrease the cross section, while codoping with Mg2+ or increasing crystal composition tends to increase the cross section. As the composition is close to the stoichiometry, the cross section tends to a constant. These effects are mainly related to the Er3+ site redistribution induced by doping or composition alteration.
This work was supported by the National Natural Science Foundation of China under Project nos. 61377060, 61077039 and 50872089, and by the Research Grants Council of the Hong Kong Special Administrative Region, China, under Project no. 11211014.
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