1. Introduction

The observation of stimulated emission from Ni²⁺-doped MgF² crystal by Johnson, Dietz, and Guggenheim in 1963 [1], provided the first example of a laser with potential for broadband wavelength tuning. Since that, all-solid-state broadband near infrared sources have attracted considerable attention, because of its fundamental applications in biomedicines, telecommunications, and laser guide stars, e.g., solid-state tunable lasers, optical coherence tomography, and compact and versatile amplifiers. In the past 40 years, many efforts have been concentrated on the development of crystals doped with transition metal (TM) ions, such as Ti³⁺, Ni²⁺, Cr³⁺, Cr⁴⁺ and Co²⁺. Unlike rare-earth (RE) ions, the energy levels of the TM ions are strongly influenced by the host materials. Thus, the emission based on 3d-3d transition of TM ions shows larger full width at half maximum (FWHM) than the 4f–4f transition of RE ions. However, such broadband emission is usually baffled by excited-state absorption and nonradiative decay [2, 3]. These drawbacks will severely limit their actual applications.

Recently, a novel active ion (bismuth) has attracted a great interest due to its favorable potential for broadband amplifier and tunable laser [4–11]. A number of glass hosts have been investigated and broadband near-infrared (NIR) luminescence has been reported in silicate [6], germinate [7], phosphate [8], and barium borate glasses [9]. It is exciting that optical amplification and lasing operation were also realized in bismuth doped silica fiber [10]. However, the mechanism of the broadband luminescence is still in controversy. The origin of Bi-related broadband luminescence has been ascribed to Bi⁺, Bi²⁺ or Bi⁵⁺.

In comparison with glass, the definite majority of sites for active ions in crystal are determined by a rigid regular crystal lattice. Thus investigation on Bi-doped single crystal was expected to enable the understanding of the luminescence nature of the bismuth active centers. Moreover, the irreplaceable advantage of crystals over glass is the superior thermal conductivity. Thus Bi-doped single crystal will be more competitive candidates for efficient broadband optical amplifier and tunable lasers. However, there are is few reports on it till now.

In this letter, we report on the spectral properties of Bi-doped BaF₂ crystal. Broadband luminescence peaking at ~1070 and 1500 nm both with full width at half maximum (FWHM) greater than 200 nm was observed.

2. Experimental

BaF₂ crystal doped by 1.0-at. % elemental bismuth (BFB) was grown by the temperature gradient technique. The equipment growing Bi-doped BaF₂ single crystal is the same as described in Ref.12. High purity BaF₂ (99.99%), bismuth powders (99.999%) and NH₄F (99.99%) were selected as raw materials. The content of NH₄F added in the raw materials is 2.0-at. %. It has been proved in fluoride crystals that no additional defect structures will be generated with such an additive content. The purpose of NH₄F additive is to avoid unexpected defect structures resulted from metal bismuth doping. 0.5-wt. % PbF₂ was used as an oxygen scavenger. They were mixed homogeneously in an agate mortar and then transported into a graphite crucible. The growth procedure started to run after the whole furnace was vacuumed to 10^-3 Pa. The temperature was kept at 673 K for 10–20 h to remove water in the raw materials and the furnace chamber. Then 1.1 atm of highly pure Ar gas was injected into the furnace chamber before the procedure continued. After the raw materials were melted, the crystal growth was controlled by lowering the temperature at the rate of 3 K/h. For comparison, a blank BaF₂ crystal (BF) was also grown through the same process. Then the samples were cut into 2 mm in thickness and polished for optical measurements.

Optical absorption spectra were measured with a JASCO V-570 spectrophotometer. The infrared luminescence spectra were obtained with a ZOLIX SBP300 spectrofluorometer with an InGaAs detector excited with 808 nm LD. The excitation and emission spectra, and the fluorescence decay curves in both visible and infrared regions were recorded by using a FLS920 compressor attachment.

3. Results and discussion

Room temperature absorption spectra of BFB and BF crystals are shown in Fig. 1. The inset shows the photographs of the crystals. The BFB crystal is light gray-green in color, while the BF crystal is colorless. Both of them are uniform and transparent. In comparison with BF crystal, apparent absorption bands at 243, 422, 504, 647, and 860 nm can be observed in BFB crystal. These are quite different from those absorption bands observed in Bi-doped glass (usually at ~500, 700, 800 and 1000nm) [7] and Bi³⁺-doped crystals [13]. This may be ascribed to the differences of structure between glass and crystal.

Figure 2 exhibits the NIR photoluminescence spectra of BFB crystal excited at various wavelengths. The gap in the emission band at wavelengths between 1350 to 1400 nm should be ascribed to absorption of OH- adhered to the inside of spectroscopic equipment. As shown in Fig. 2, intense NIR luminescence was observed in BFB crystal by excitation at 500, 700 and 808 nm. While no NIR luminescence was observed in BF crystal. The luminescence is consisted with two broadband emissions around 1050 nm and 1500 nm when the sample was pumped at 500 or 700 nm. But only emission at ~1070 nm was observed when the sample was pumped at 808 nm, while the FWHM of the broadband emission reaches 200 nm. The broadband emission at ~1070 nm is similar to the reported emission around 1100 nm in Bi-doped silica glass. While the broadband emission around 1500 nm shows an analogy to those broadband emissions center at 1450 nm in Bi-doped fibers [14, 15]. In Ref. 15, an absorption bands around 1400 nm corresponding to the emission were also observed. In our work, no absorption bands around 1400 nm can be clearly identified in the absorption spectrum. It is probable that some less pronounced absorption bands was masked by the diffuse absorption or scattering tail in the crystal.

 

Fig. 1. Absorption spectra of BFB, BF crystals and Bi-doped glass[7] in the wavelength region from 200 to 2000 nm. The inset shows the photographs of crystal samples.

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Fig. 2. NIR photoluminescence spectra of BFB crystal excited at 500, 700, and 808 nm, and measured excitation spectra of BFB crystal monitored at 1070, 1280 and 1500 nm

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In addition, although PbF₂ was used as an oxygen scavenger in fabrication of the crystal, no detectable Pb ions remained in the crystal due to the volatilization of lead or lead oxides during crystal growth process. The measurement of Pb concentration was carried with the inductively coupled plasma atomic emission spectrometry (ICP-AES) for four times. No Pb was found in the crystal, while Bi was clearly detected. Therefore, the near-infrared luminescence should be connected with electronic transition in Bi-related centers and not with structural defects of BFB crystal lattice resulted by impurities.

Earlier researches have reported that broadband luminescence should be contributed to the randomly disorder around the active centers as well as the multiple sites of the active centers. The broadband luminescence has been proposed to be contributed to two (emissions center at 1110 and 1240 nm) or three types (emissions center at 1130, 1286 and 1420 nm) active centers [16, 17]. In order to further investigate the detailed attributions of the observed emissions, the corresponding excitation spectra were measured and also shown in Fig. 2. The excitation spectra for the emissions at 1070 and 1500 nm are obvious different. There are two intense bands in the excitation spectra for both the emissions at 1070 and 1500 nm, respectively. This indicates that the observed emission may be originated from two different kinds of Bi-related centers and represented as Bi(I) and Bi(II).

The excitation spectrum monitored at 1280 nm was also measured to evaluate the possibility of coexcitation process for Bi(I) and Bi(II) active centers. As shown in Fig. 2, the emission around 1280 nm can be excited at wavelength region between 500 to 600 nm. Fig. 3 shows the luminescence spectra of BFB crystal excited at above mentioned wavelength region. No single broadband emission but two isolated emissions at 1080 and 1500 nm was found in BFB crystal. The evolution of observed emission should also be due to the distinction of Bi(I) and Bi(II) centers. The luminescent decay monitored at 1070 and 1500 nm, respectively, were also measured with Xe lamp excitation at 580 nm. The curves show a fair consistency with the first-order exponential decay and the lifetime fitted at about 2.5 and 2.1 μs, respectively. In our work, the decay of the observed emission is much shorter than the reported ones for Bi-doped glasses. This may be due to the structural difference between fluoride crystal and oxide glasses, since the fluorescent decay of active centers changes with their local environments.

 

Fig. 3. NIR emission spectra of BFB crystal excited at wavelengths between 500 and 590 nm.

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Fig. 4. Visible luminescence and corresponding excitation spectra of BFB crystal.

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For better understanding of Bi-related active centers in BFB crystal, the visible emission and corresponding excitation spectra were also measured. As shown in Fig. 4, when excited at 254 or 354 nm, BFB crystal shows a broadband blue-green luminescence. The peak maxima of the emission bands for 254 and 354 nm excitations are 470 and 410 nm, respectively. The blue-green luminescence observed in the emission spectra can be assigned to the emission bands of Bi³⁺ ions according to G. Blasse and A. Bril [18]. When excited at 260, 395 and 580 nm, a red luminescence can be observed at ~630 nm in BFB crystal, which can be ascribed to electron transition of Bi²⁺ ions [19]. The decay time of the luminescence from electron transition of Bi²⁺ ions is usual several microseconds, which is close to those of observed NIR emissions.

According to earlier results, Bi-related broadband NIR luminescence should not be assigned to Bi³⁺ ions. In contract, it was suggested that the luminescence should be contributed to bismuth ions with low valence, such as Bi²⁺ (6s²6p) or Bi⁺ (6s²6p2) [20]. Pb⁺ and Pb⁰ ions have the same electron configuration with Bi²⁺ and Bi⁺ ions, respectively. Efficient NIR luminescence have been observed from Pb-related centers in alkaline earth fluoride crystals, which were contributed to Pb⁺(1) center (at ~1060 nm), Pb⁺(1)-Pb²⁺ dimer center (at ~1350 nm) and Pb⁰(2) center (at ~1600 nm), respectively [21]. These Pb-related centers consisted of a Pb⁺ ion or Pb⁰ ion next to one or two anion vacancies, respectively. While the NIR emission with long lifetime (several ms at 77K) from Pb⁺(1)-Pb²⁺ dimer centers only occurred in highly doped samples (2–3 at% of PbF₂). In our work, analogous NIR emissions peaking at 1070 and 1500 nm were observed in BFB crystal, while no emission around 1300 nm was found. This could be ascribed to the low concentration of Bi ion in the BFB crystal. Only ~1 ppm of bismuth can be confirmed in the BFB crystal by ICP-AES analysis, which should due to the volatilization of bismuth during crystal grown process. Furthermore, similar absorption bands to those Pb-doped alkali earth fluorides can be observed in BFB crystal. This indicates that the observed NIR luminescence in BFB crystal may also be ascribed to the electron transition of Bi²⁺ or Bi⁺-related centers, consisting of a substitution Bi²⁺ or Bi⁺ ions adjacent to neighbor fluorine vacancies.

Such assumption can be supported by the performance of BFB crystal after subsequent heat treatment in conventional atmosphere (in air) at 473 K for 30 min. Fig. 5 shows the NIR emission spectra of samples before and after the heat treatment excited at 808 nm, respectively. The inset shows the photographs of the crystal samples. After the heat treatment, the emission intensity of BFB crystal was highly decreased. While the color of BFB crystal turned from gray-green to gray, which is resulted by the disappearance of corresponding absorption bands at visible wavelength region. Such phenomena are similar to those in earlier reports on Bi-doped silica glass and fiber, which have been assigned to the metastability of Bi-related centers [22]. The changes of emission/absorption bands should be ascribed to the structural differences of Bi-related centers before and after heat treatment, such as the modification of the luminescent center into a nonluminescent one. During the process of heat treatment, the number of point defects connected to those Bi-related centers will be highly reduced. This may be resulted by the rearrangement of cation and anion vacancies in BFB crystal lattice. Of course, a weak oxidative process of Bi-related centers may also be contributed to the weakening of the luminescence.

 

Fig. 5. NIR emission spectra of BFB samples before and after heat treatment, respectively. The inset shows the photographs of the samples.

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Recently, broadband NIR luminescence from Pb-, Sb-, Sn-, Te-, and In-doped germinate glasses were also reported [23] and similar spectroscopic properties to those in Bi- doped glasses were observed. The point defect optical centers caused by the presence of 6p (Bi, Pb) and 5p (Sn, Sb) ions were suggested to explain the mechanism of the observed NIR emission in these laser materials. More details on the correlation between the NIR emission and the proposed defects described in this paper and Ref. 23 is under study to support above assumption.

4. Conclusions

We have presented the NIR luminescence properties of BFB crystal. The observed broadband NIR emission peaking at 1070 and 1500 nm were found to be analogous to those of Pb⁺(1) and Pb⁰(2) related centers, respectively. We propose the electron transition of Bi²⁺ or Bi⁺ centers adjacent to vacancy defects is the origin of the observed NIR luminescence.