1. Introduction

During the last decade, broadband near-infrared (NIR) 1000–1700 nm emission materials are of crucial importance to develop new lasers and optical amplifiers due to their potential applications in astrophysics, biomedicines, next generation optical communication system, material processing, and laser guide stars [1–6]. Bismuth (Bi), the heaviest stable element in the periodic table, is an ideal luminescence center in an extremely broad spectral range from 1000 nm to 1700 nm. At present, there are numbers of works have demonstrated ultrabroad NIR emissions in Bi doped glasses and fibers, and the univalent Bi cation (Bi⁺) is recognized as the origin of NIR emission [7–9]. In particular, the broadly tunable fiber laser has also been initially obtained from 1150 nm to 1300 nm [8,9].

Nevertheless, the Bi doped glasses and fibers suffer from some unavoidable drawbacks, for example, it is easy to phase-segregated, de-crystallize, and devitrified in the process of material preparation, which would result in reducing the luminous efficiency; it cannot be achieved with high Bi concentration, leading to a low gain factor and efficiency of lasers; furthermore, the ubiquity of multiple valence states (from positive to negative) and species (from single ion to cluster ion) of Bi in glasses or fibers give rise to the competition among different light-emitting wavelengths, bringing on the low NIR luminous efficiency.

Fortunately, the problems above-mentioned can be partly improved in the Bi doped crystals. Compared to the glass with disorder structure, the crystal with order environment may be favorable for providing a stabilized phase and a possibility of high Bi doping concentrations due to the existence of certain crystal lattice site for Bi ion. So for, broadband emission has been observed from Bi doped crystalline BaF₂ [10], PbF₂ [11], SrB₄O₇ [12], RbPb₂Cl₅ [13], and so on [14–16]. However, broadband emission at the wavelength range of blue-green were also observed in above-mentioned crystals [10–16], indicating that multiple luminescent centers (such as Bi³⁺, Bi²⁺, Bi⁺, and so on) are concomitant even in crystals. Therefore, great efforts are urgently needed to stabilize the Bi element to an individual valence state (Bi⁺). In view of the importance of Bi doped material system, especially for the applications as laser and amplifier media, we will deal with it in details.

Here, we are turning our attention to the Bi-Ta co-doped Gd_0.1Y_0.9AlO₃ (Bi/Ta: GYAP) crystal. On the one hand, the co-doping of Ta⁵⁺ ion in Bi: GYAP crystal can act as an appropriate charge compensated ion for Bi ion, which is beneficial to stabilize more Bi element to an individual valence state (Bi⁺). In generally, Bi ions would take up the Gd³⁺ or Y³⁺ sites in GYAP crystal, resulting in forming easily Bi³⁺ ions due to the same valence state between Bi³⁺ and Gd³⁺ (or Y³⁺). Because the radius of Ta⁵⁺ (64 pm) is larger than that of Al³⁺ (53.5 pm), and smaller than that of Gd³⁺ (93.8 pm) and Y³⁺ (88 pm), after the co-doping with Ta⁵⁺ ions, which would also take up the Gd³⁺ or Y³⁺ sites more easily than the Al³⁺ site in GYAP crystal to form Ta_Gd²⁺ or Ta_Y²⁺ ions, giving rise to the generation of positive bivalent charge in GYAP crystal. For purpose of realizing the electrically neutral of the system, the Bi³⁺ would change easily to Bi⁺ for forming Bi_Gd²⁻ or Bi_Y²⁻ ions due to the instability between Bi³⁺ and Bi⁺. On the other hand, the GYAP crystal, which is a solid solution of two crystals: YAlO₃ (YAP) and GdAlO₃ (GdAP), has properties similar to YAP crystal with excellent physicochemical and thermal properties. Furthermore, the GYAP crystal possesses low phonon energy (500–600 cm⁻¹ [17]), which is beneficial to reduce multi-phonon de-excitation processes and increase the fluorescence emission efficiency. The most important, owing to the large radius gap between Bi ion (103 pm) and Y³⁺ ion (88 pm), it might be difficult to achieve high concentration Bi ions doping in YAP crystal, while the radius of Gd³⁺ ion (93.8 pm) is bigger than that of Y³⁺ and more close to Bi ion, therefore, the GYAP crystal was chosen, where a fraction of Y³⁺ ions are substituted by Gd³⁺ ions, which may bring about a better coexistence between Bi ions and GYAP crystal. In addition, the radius of Ta⁵⁺ ion (64 pm) is smaller than that of Gd³⁺, and Y³⁺ ions, which may induce an effect of radius compensation when Ta⁵⁺ and Bi are co-doped into the GYAP crystal. The Bi and Ta co-doped germanium oxide glasses with a broadband emission around 1310 nm has been reported by M. Peng [18]. However, up to now, to our knowledge, it is the first time to provide an effective method for stabilizing the Bi element to Bi⁺ valence state by co-doped with Ta⁵⁺ ions in GYAP crystal on the strength of the charge balance mechanism in this work. Bi: GYAP and Bi/Ta: GYAP crystals were successfully prepared. An enhanced NIR emission from Bi/Ta: GYAP crystal was observed for the first time. The emission and excitation properties of those as grown crystals were investigated so as to explore the contribution of Ta⁵⁺ in enhancing the NIR emission of Bi/Ta: GYAP crystal. The optical investigations of the Bi/Ta: GYAP crystal were also made to demonstrate its feasibility for application in NIR solid state laser.

2. Experimental section

The 0.5 at. % Bi single doped, and 0.5 at. % Bi/ 0.5 at. % Ta co-doped GYAP crystals were grown by the Czochralski method. The powders of Al₂O₃, Y₂O₃, Gd₂O₃, Bi₂O₃, and Ta₂O₅, all with a purity of 99.999%, were used as raw materials. The mixture was completely mixed for 30 h, then pressed into disks and heated for 40 h at 1340 °C to forming polycrystalline powers. Then the bulks was put in a 60 mm diameter iridium crucible for crystal growth. The crystal was grown in a nitrogen gas environment along the b axis. The pull speed and rotation rate were 0.5–0.8 mm/h, and 12–16 rpm, respectively. The concentration of Bi, and Ta ions in the as-grown crystals were measured by the inductively coupled plasma atomic emission spectrometry analysis (ICP-AES). The doping concentration of Bi in the Bi: GYAP crystal was measured to be 0.18 at.% (3.54×10¹⁹ ions/cm³). The concentrations of Bi and Ta in the Bi/Ta: GYAP crystal were 0.19 at.% (3.58 ×10¹⁹ ions/cm³) and 0.61 at.% (1.14×10²⁰ ions/cm³), respectively. The corresponding effective segregation coefficient in the Bi/Ta: GYAP crystal were calculated to be 0.38 for Bi, and 1.21 for Ta, respectively. The NIR fluorescence spectra in the wavelength of 850–1700 nm and visible spectra in the wavelength of 300–800 nm the for the as-grown crystals were record with Edinburgh Instruments FLS920 by an InGaAs detector, and a photomultiplier (PMT) detector, respectively. All the measurements were taken at room temperature.

3. Experimental results and discussion

The NIR emission spectra of Bi: GYAP and Bi/Ta: GYAP crystals under the excitation of 600 nm is shown in Fig. 1. It is evident to see that the fluorescence spectrum cover a very wide spectral region from 900 nm to 1700 nm, which is assigned to the Bi⁺ ions. In particular, unlike Bi doped glass fibers with only one peak, there are eight emission bands of Bi doped GYAP crystals, which center at around 1005, 1095, 1195, 1280, 1375, 1490, 1560, and 1625 nm, respectively. This difference of spectral shape may result from the distinction of structure between glass and crystal. But overall, the NIR emission spectra of Bi doped GYAP crystals are broad enough to be used for generating NIR wide tuning and short pulse lasers. Furthermore, it is obviously to see that the emission intensity of the crystal codoped with Ta⁵⁺ is at least two times that of the crystal without Ta⁵⁺ codoping, indicating that the codoped of Ta⁵⁺ ion can efficiently enhance the NIR emission of Bi: GYAP crystal.

 

Fig. 1 NIR emission spectra of Bi: GYAP and Bi/Ta: GYAP crystals under the excitation of 600 nm.

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On the other hand, in order to investigate more about the influence of the Ta⁵⁺ co-doping on the luminescence of Bi ions, we have measured the visible emission spectra, which are shown in Fig. 2. It can be seen that there are two emission bands centered at 468 nm and 698 nm in the wavelength range of 350–600 nm and 600–800 nm, respectively, for both the Bi single and Bi/Ta double doped GYAP crystals. According to the report by M. Srivatava [19], these two luminescence bands is originated from Bi²⁺ (698 nm) and Bi³⁺ (468 nm) ions. Furthermore, as the Fig. 2 shows, these two fluorescence emission intensities are dramatically reduced with the co-doping of Ta⁵⁺ ions. Combined the above-mentioned enhanced NIR fluorescence emission with the co-doping of Ta⁵⁺ ions, it points out a fact that after the co-doping with Ta⁵⁺ ions in Bi: GYAP crystal, the concentration of Bi⁺ ions increases (enhanced NIR emission), while the concentration of Bi²⁺ and Bi³⁺ ions decreases (subdued 698 nm and 468 nm emissions). Actually, the Bi ions in GYAP crystal are subsistent as Bi⁺, Bi²⁺, and Bi³⁺ ions. Among them, the Bi³⁺ ions are predominant because of the identical valence state between the Bi³⁺ and the Gd³⁺ (or Y³⁺) in GYAP crystal. However, the co-doping of Ta⁵⁺ ion would give rise to redundant positive divalent ion Ta_Gd²⁺ (or Ta_Y²⁺) because the some of the Gd³⁺ (or Y³⁺) sites would be occupied by Ta⁵⁺. As we know, charge balance is a universal law. Therefore, in order to realize the electrically neutral of the system, the Bi³⁺ would change easily to Bi⁵⁺ for forming Bi_Gd²⁻ (or Bi_Y²⁻) ions due to the instability and coexistence between Bi³⁺ and Bi⁺ ions. As a consequence, the visible fluorescence emission (468 nm) derived from the reduced Bi³⁺ ions would be dropped off, while the NIR fluorescence emission resulted from the increase Bi⁺ ions would be enhanced.

 

Fig. 2 Visible emission spectra of Bi: GYAP and Bi/Ta: GYAP crystals.

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As is well-known that the NIR emission of Bi⁺ ions is strongly hinged on the excitation wavelength because of the existence of multiple sites of the active centers in crystals. For better understanding the NIR active centers in Bi/Ta: GYAP crystal, the excitation spectra of Bi/Ta: GYAP crystal with emission at around 1005, 1195, 1280, and 1490 nm were measured and shown in Fig. 3. It is clear to see that in the wavelength range of 600–700 nm (where we denoted as Band D as shown in Fig. 3), these four excitation spectra are very similar. In particularly, as Fig. 3 also shows, there are three well-marked excitation bands, where we denoted as Band A (230 nm), B (340 nm), and C (460 nm), belonging to the emission of 1280, 1005, and 1195 nm, respectively. In detailed, the excitation band A (230 nm) only appears in the excitation spectra of the 1280 nm emission. Similarly, the excitation band B (340 nm) only appears in the excitation spectra of the 1005 nm emission. Further, the excitation band C (460 nm) can be seen both in the excitation spectra of the 1005 and 1195 nm emissions. These different of the excitation spectra show the emission bands at around 1005, 1195, 1280, and 1490 nm may be resulted from three different Bi⁺ active centers in the Bi/Ta: GYAP crystal.

 

Fig. 3 Excitation spectra of Bi/Ta: GYAP crystal with emission at around 1005, 1195, 1280, and 1490 nm.

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In order to further study the optical properties of the multiple Bi⁺ active centers in the Bi/Ta: GYAP crystal, the luminescence emission spectra under the excitation at 230 and 340 nm were measured and shown in Fig. 4. As it shows, the 1005 nm emission band can be seen for both the excitation of 230 and 340 nm. However, the 1005 nm emission under being excited by 340 nm is stronger than that under being excited by 230 nm. Moreover, the emission band of 1280 nm can only be found under the excitation of 230 nm, which is in consistent with the results of Fig. 3, indicating that the emission bands at around 1005, and 1280 nm may be dated from two different Bi⁺ active centers in the Bi/Ta: GYAP crystal.

 

Fig. 4 NIR emission spectra of Bi/Ta: GYAP crystal under the excitation at 230 and 340 nm.

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On the other hand, the luminescence emission spectra under being excited from 425 to 485 nm (Excitation Band C as Fig. 3 shown) were also measured, and the results are shown in Fig. 5. It can be seen that the fluorescence emission intensities change obviously at the wavelength range around 1195 nm, while change little at the other wavelength bands. Moreover, with the increase of the excitation wavelength from 425 nm, the fluorescence emission intensity at around 1195 nm quickly enhances, rising to a maximum under the excitation wavelength of 465 nm. After that, the fluorescence emission intensity decrease rapidly with the increasing of the excitation wavelength. This evolution of fluorescence emission intensity is much consistent with the excitation spectra shown in Fig. 3, indicating again that the emission band at around 1195 nm may be derived from the third Bi⁺ active center in the Bi/Ta: GYAP crystal.

 

Fig. 5 NIR emission spectra of Bi/Ta: GYAP crystal under the excitation from 425 to 485 nm.

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In addition, we also measured the fluorescence emission spectra of Bi/Ta: GYAP crystal under the exciting wavelength from 625 nm to 685 nm, and the results are shown in Fig. 6. It can be seen that the fluorescence emission intensities from 900 nm to 1700 nm integrally enhance with the increase of exciting wavelength, which is consistent with the rising excitation intensities from 625 nm to 685 nm as shown in Fig. 3 (B and D). It also indicating that the NIR fluorescence emission of Bi/Ta: GYAP crystal can be efficaciously enhanced with the red shift of exciting wavelength after 625 nm.

 

Fig. 6 NIR emission spectra of Bi/Ta: GYAP crystal under the excitation from 625 to 685 nm.

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In order to study the polarized spectral properties of the Bi/Ta:GYAP crystal, the NIR emission spectra for three light polarizations parallel to the a, b, and c crystallographic axes under the excitation of 600 nm are shown in Fig. 7. It is evident to see that the relative intensity of fluorescence emission peaks are orientation dependent. As it shows, the fluorescence spectra cover from 900 nm to 1700 nm can be seen for a, b, and c polarizations. In particular, they all possess the same maximal emission band of around 1005 nm. However, compared with c polarization, the fluorescence emission intensities from 1200 nm to 1700 nm drop off more rapidly for the a and b polarizations.

 

Fig. 7 Polarized NIR emission spectra of Bi/Ta:GYAP crystal.

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It is noteworthy that the Bi⁺, Bi²⁺, and Bi³⁺ ions are concomitant even in the 0.5 at.% Bi/0.5 at.%Ta: GYAP crystal. Increasing the doping concentration of Ta⁵⁺ ions may be an effective means to increase the Bi⁺ concentration in GYAP crystal. However, if the concentration of Ta⁵⁺ is too large, it is very hard to obtain the Bi/Ta: GYAP crystal with high optical quality, which is disadvantage to practical application. As a result, it is better to choose a modest Ta⁵⁺ ions concentration. However, it is short of relevant evidence to achieve the ideal concentration ratio between Bi and Ta ions at present, and follow-up studies must be conducted. Therefore, in our future experiment research, we will concentrate on optimizing the concentration ratio between Bi and Ta ions to realize more efficient NIR fluorescence emission in Bi/Ta: GYAP crystal.

4. Conclusion

In conclusion, Bi single and Bi/Ta double doped GYAP crystals were successfully grown by the Czochralski method. An intense NIR emissions with broad FWHM at the wavelength range from 900 nm to 1700 nm were obtained in these present crystals for the first time, to the best of our knowledge. Compared with the Bi:GYAP crystal, the Bi/Ta: GYAP crystal presents an enhanced NIR luminescence emission, while has a descending visible fluorescence emission. It was demonstrated that the introduced Ta⁵⁺ ion can effectively lead to the change of valence states from Bi³⁺ to Bi⁺ based on the charge compensation mechanism, which is the root of enhanced NIR fluorescence emission. Further, the NIR fluorescence emission properties of the Bi/Ta: GYAP crystal under different exciting wavelengths were investigated in details. It is demonstrated that the NIR fluorescence emission peaks at around 1005, 1195, and 1280 nm were ascribed to three different Bi⁺ active centers in the Bi/Ta: GYAP crystal. It has been suggested that the Bi/Ta: GYAP crystal is a promising material for NIR broadband laser applications.