1. Introduction

Due to the promising applications in remote sensing, “eye-safe” laser surgery, etc [1,2], significant efforts have been devoted to the development of 2.0 μm solid state laser activated by Ho³⁺ ions. However, owing to the lack of well-matched absorption bands, Ho³⁺ can’t be pumped by the commercially available 808 and 980 nm laser diodes (LDs). Accordingly, rare-earth (RE) ions characterized by settled photo-excitation and emission bands in narrow bandwidth have been widely utilized as sensitizers to improve the optical pumping efficiency of LDs [3–5]. On the contrary to RE ions, the d-d transitions of Cr³⁺ are very sensitive to the change of local environment, and thus typically feature broad photo-excitation and emission bands [6]. Meanwhile, it is noted that the broad near-infrared (NIR) emission of Cr³⁺-doped glass also overlaps well with the NIR absorption of Ho³⁺. Therefore, in principle, Ho³⁺ might be also efficiently activated by the direct energy transfer (ET) from Cr³⁺. Nonetheless, up to now, the ET mechanism of Cr³⁺ → Ho³⁺ is not clear at all.

As an excellent sensitizer, Cr³⁺ is well known for its broadband absorption in the ultraviolet (UV)-to-visible region that can efficiently couple to the conventional flashlamp pumping sources. However, in the Cr³⁺-doped glasses, non-radiative transitions usually dominate the relaxations of excited states, thus easily leading to the optical inactivity of amorphous hosts. In practice, the majority of previous researches about Cr³⁺ in amorphous materials are realized in silicate glass ceramics and glasses [7]. Considering the similarity in the network structure with silicate glass, germanate glasses might likewise be available Cr³⁺-activated hosts, which provide extra advantages like low phonon energy, excellent draw ability and good infrared transmissivity for 2 μm laser.

In the present paper, the spectroscopic properties of Cr³⁺-doped and Cr³⁺/Ho³⁺ co-doped fluorogermanate glasses are studied in detail. The underlying Cr³⁺ → Ho³⁺ ET mechanisms are rationally demonstrated according to the static-to-dynamic luminescence spectra. The intense sensitization of Cr³⁺ to Ho³⁺ provides an efficient pumping approach with the flashlamp and commercial LDs, which would greatly boost the potential application of Cr³⁺/Ho³⁺ co-doped fluorogermanate glasses for fiber laser.

2. Experimental

Glass samples with nominal molar composition of 50GeO₂-20Al₂O₃-15LaF₃-15LiF-xHoF₃-yCr₂O₃ (x = 0, 1; y = 0, 0.1) were prepared by the conventional melt-quenching method. Analytical reagents of Al₂O₃ and LiF, and LaF₃, GeO₂, HoF₃ and Cr₂O₃ (all 99.99%) were applied as raw materials. The well ground stoichiometric chemicals were put into a covered corundum crucible and melted at 1350 °C for 1 h with the controlled prepared condition (covered corundum crucibles and a mount of NH₃HF₃ were used). Subsequently, the melts were casted onto a stainless steel plate, and further annealed at 500 °C for 2 h. Then the prepared glass samples were cut into small pieces and polished for the optical measurements. To check the composition of the glass samples, energy dispersive spectra (EDS) were taken with a Philips XL-30FEG scanning electron microscope equipped with an EDAX DX4i EDS. The results reveal that a very limited loss (< 3 mol.%) due to evaporation of fluorine while melting, which might be due to the low glass melting temperature and duration with the controlled prepared condition.

Absorption spectra were measured on Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer. Excitation and emission spectra were performed on an Edinburgh FLS920 spectrophotometer equipped with a continuous wave 450 W Xe lamp as excitation source, a liquid nitrogen cooled R5509-72 photomultiplier (PMT) for NIR emission, and an InSb detector for mid-infrared (MIR) emission. Fluorescence decay curves were further recorded on FLS920 system with μF900 microsecond Xe lamp excitation source. Additionally, the photoemission spectra pumped by an 808 nm LD were carried out on a Triax 320 type spectrofluorometer (Jobin-Yvon Corp.) fitted with an InGaAs detector (830-1450nm) and an InAs detector (1800-2250nm).

3. Results and discussions

Figure 1 shows the absorption spectra of Cr³⁺-doped, Ho³⁺-doped and Cr³⁺/Ho³⁺ co-doped fluorogermanate glasses in the wavelength range of 250-2200 nm, respectively. For Ho³⁺ singly doped glass, a series of sharp absorption peaks are observed clearly because of the typical electronic transitions from the ⁵I₈ ground state to the corresponding excited states of Ho³⁺. For Cr³⁺-doped sample (Fig. 1), three broad absorption bands centered at 360, 430 and 630 nm can be obtained definitely, where the UV absorption band around 360 nm should be caused by the strong charge transfer of Cr⁶⁺ [8], but the bands around 430 and 630 nm are readily attributed to the d-d electronic transitions from the ⁴A₂ ground state to the ⁴T₁ and ⁴T₂ excited states of Cr³⁺ [9], respectively. Moreover, for Cr³⁺/Ho³⁺ codoped glass sample, the various absorption bands of Ho³⁺ and Cr³⁺ all can be detected obviously, and the sharp absorption bands of Ho³⁺ are well superimposed on the broad absorption bands of Cr³⁺, as depicted in Fig. 1. These results indicate that Ho³⁺ and Cr³⁺ ions have been well incorporated into the as-prepared fluorogermanate glasses. It should be noted that, the intense broadband absorption of Cr³⁺ nearly covers the near UV-to-NIR wavelength range likely of 300-850 nm both in Cr³⁺-doped and Cr³⁺/Ho³⁺ co-doped glass samples, which perhaps benefits the feasible pumping of a Xe lamp greatly if the ET process of Cr³⁺ → Ho³⁺ occurs efficiently. Additionally, the daylight photographs of all glass samples are shown in the inset of Fig. 1, all featuring good transparency even with Cr³⁺ or/and Ho³⁺ dopants.

 

Fig. 1 Absorption spectra of Ho³⁺-doped, Cr³⁺-doped and Cr³⁺/Ho³⁺ co-doped fluorogermanate glasses. The inset shows the corresponding photographs of all samples.

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According to the optical absorption transitions [10], the ligand parameter D_q/B here is calculated to be 2.02, which means that the ⁴T₂ level lies below the ²E level on account of the Tanabe-Sugano diagram for Cr³⁺ in the octahedral symmetry. Related parameters in various Cr³⁺-doped hosts are enumerated and compared in Table 1. It is obvious that Cr³⁺ ions experience a typical strong crystal field (D_q/B>2.3) in the ruby, where the first laser in the world was obtained. As for the glass hosts except for lead borate glass, they usually exhibits a weak crystal field (D_q/B≤2.3), which might be ascribed to the loosely packed glass network.

Table 1. Spectroscopic parameters in various Cr³⁺-doped hosts

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To investigate the effect of Cr³⁺ ions on spectral properties of Ho³⁺ ions, NIR emission spectra are measured for all samples excited at 600 nm that is just located in the absorption band range of Cr³⁺ but outside that of Ho³⁺, as comparatively shown in Fig. 2(a).It can be seen that the Ho³⁺-doped glass sample does not yield any fluorescence, while the Cr³⁺-doped glass sample exhibit an intense broad emission band centered at 850 nm, readily ascribed to the Cr³⁺: ⁴T₂ → ⁴A₂ transition [15]. Interestingly, the 850 nm emission band of Cr³⁺ in 700-1300 nm well overlaps with the absorption band of ⁵I₈ → ⁵I₅ at about 890 nm and that of ⁵I₈ → ⁵I₆ at 1150 nm of Ho³⁺, as pictured in Figs. 1 and 2(a), which suggests there might exist feasible ET from the excited Cr³⁺ to Ho³⁺ ions. Correspondingly, as Ho³⁺ codoped into Cr³⁺-doped sample, the peak intensity of 850 nm emission decreases greatly, and meanwhile another typical NIR emission of Ho³⁺ at 1200 nm obviously emerges as a shoulder due to the ⁵I₆ → ⁵I₈ electronic transition, as shown in Fig. 2(a). This observation directly confirms that the ET from Cr³⁺ to Ho³⁺ does take place in Cr³⁺/Ho³⁺ co-doped fluorogermanate glass. Meanwhile, among various speciations of chromium in glass hosts, tetrahedrally coordinated Cr⁴⁺ ions usually show an emission band centered around 1400 nm in the glass host, while the Cr³⁺ emission band is observed at around 900 nm in the silicate glass [15]. Hence, it is deduced that chromium ions might exist mainly as Cr³⁺ in this fluorogermanate glass.

 

Fig. 2 (a) NIR emission spectra of all glass samples excited at 600 nm; (b) MIR emission spectra of all samples excited at 600 nm; (c) Excitation spectra of Ho³⁺-doped and Cr³⁺/Ho³⁺ co-doped samples monitored at 1200 nm; (d) Fluorescence decay curves of Cr³⁺: ⁴T₂→⁴A₂ at 850 nm in fluorogermanate glasses without/with Ho³⁺ dopant under pulsed light excitation of 600 nm.

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On the other hand, MIR emission spectra of Cr³⁺-doped, Ho³⁺-doped and Cr³⁺/Ho³⁺ co-doped glass samples are further detected under excitation of 600 nm, as shown in Fig. 2(b). As expected, the Cr³⁺-doped and Ho³⁺-doped sample both cannot give rise to the 2.0 μm fluorescence, but only the Cr³⁺/Ho³⁺ co-doped glass can efficiently yield the broad 2.0 μm emission with a full width at half maximum of 175 nm due to Ho³⁺: ⁵I₇ → ⁵I₈ electronic transition. This result additionally demonstrates that, upon excitation of Cr³⁺ at 600 nm, the ET process from Cr³⁺ to Ho³⁺ does take place effectively in the Cr³⁺/Ho³⁺ co-doped fluorogermanate glass.

Figure 2(c) depicts the excitation spectra of Ho³⁺-doped and Cr³⁺/Ho³⁺ co-doped glasses monitored at 1200 nm (Ho³⁺: ⁵I₆ → ⁵I₈). For Ho³⁺-doped sample, a series of excitation bands are gained typically due to the electronic transitions from the ⁵I₈ ground state to the corresponding excited states of Ho³⁺, but no photo-absorption excitation band appears at about 600 nm. Whereas, for Cr³⁺/Ho³⁺ co-doped sample, the broad excitation bands due to ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ of Cr³⁺ are strongly detected except the typical excitation bands of Ho³⁺, which just provides another evident proof for the presence of ET from the excited Cr³⁺ ions to Ho³⁺ ions in Cr³⁺/Ho³⁺ co-doping system. Noted that, as shown in Fig. 2(c), the resulted excitation bands of Cr³⁺/Ho³⁺ co-doped glass nearly covers the complete near-UV-to-NIR region. It also means that the intriguing 2.0 μm fluorescence of Ho³⁺ can be efficiently obtained by using excitation source with wavelength output roughly in 325-830 nm (Fig. 1 and Fig. 2(c)).

To further confirm the existing Cr³⁺ → Ho³⁺ ET, and calculate the related ET efficiency, luminescence decay curves of 850 nm (Cr³⁺: ⁴T₂ → ⁴A₂) are measured in Cr³⁺-doped and Cr³⁺/Ho³⁺ co-doped samples under pulsed light excitation of 600 nm with μF900 microsecond Xe lamp, as pictured in Fig. 2(d). Herein, lifetimes of excited Cr³⁺: ⁴T₂ were determined from the first e-folding time of emission intensities in the decay curves. It can be found that the lifetime of ⁴T₂ level is 23.7 μs in Cr³⁺ singly doped glass, while it rapidly reduces to 17.5 μs with the introduction of Ho³⁺, readily caused by the efficient ET from Cr³⁺ to Ho³⁺ ions. Generally, the total ET efficiency (η) of Cr³⁺ → Ho³⁺ can be evaluated as following:

From the absorption (Fig. 1) and excitation (Fig. 2(c)) spectra of Cr³⁺/Ho³⁺ codoped glass, we can observe that the excitation intensity starts to decrease rapidly at wavelength beyond 750 nm, and even become weakest as wavelength exceeds 800 nm in the whole excitation region. To check the availability of this full near-UV-to-NIR excitation (325-830 nm) in Cr³⁺/Ho³⁺ co-doping system, a cheap commercial 808 nm semiconductor laser is utilized as excitation source. As comparatively shown in Fig. 3(a), no NIR emission is obtained in Ho³⁺-doped glass due to the lack of intra-4f absorption transition associated with an 808 nm LD. However, similar to the phenomena in Figs. 2(a)-2(b), the broad luminescence band of Cr³⁺: ⁴T₂ → ⁴A₂ is intensely recorded in 850-1400 nm for Cr³⁺-doped sample pumped by the 808 nm LD, but the intensity of Cr³⁺: ⁴T₂ → ⁴A₂ decreases clearly as Ho³⁺ codoped into the Cr³⁺-doped sample, which also accompanies with a superimposed fluorescence peak of Ho³⁺: ⁵I₆ → ⁵I at 1200 nm. It is of great interest that, in comparison with Fig. 2(a), the emission peak shifts from 850 to 1000 nm. This apparent shift could be ascribed to the various excitation wavelengths for different powers [16], which favor different ions within the crystal-field distribution in the glass matrix, thereby leading to the distinct luminescence characteristics. Moreover, the different detectors of R5509-72 PMT and InGaAs photoconductor also make a varied contribution because of their distinct photoresponse performance. On the other hand, Fig. 3(b) shows the MIR spectra of Ho³⁺-doped, Cr³⁺-doped, and Cr³⁺/Ho³⁺ co-doped glasses. It is found that, under excitation of an 808 nm LD, Cr³⁺-doped and Ho³⁺-doped samples do not generate any apparent fluorescence, but the Cr³⁺/Ho³⁺ co-doped sample features a typical 2.0 μm emission with a full width at half maximum of 165 nm because of Ho³⁺: ⁵I₇ → ⁵I₈ transition. These results confidently reveal that, even for the rather weak excitation intensity in 325-830 nm, the available 2.0 μm emission can be easily obtained through the Cr³⁺ → Ho³⁺ ET in Cr³⁺/Ho³⁺ co-doped fluorogermanate glass.

 

Fig. 3 (a) NIR emission spectra in 830-1450 nm and (b) MIR fluorescence spectra in 1800-2200 nm of Ho³⁺-doped, Cr³⁺-doped and Cr³⁺/Ho³⁺ co-doped glass samples pumped by an 808nm LD.

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According to the aforementioned results and Tanabe-Sugano diagram given for d³ electron configuration, the possible ET mechanism between Cr³⁺ and Ho³⁺ are proposed in Fig. 4. In glass hosts, the effects of spin-orbit coupling and electron-vibrational interaction would admix ⁴T₂ and ²E levels together, just as shown in Fig. 4. Addition of Cr₂O₃ into Ho³⁺-doped glass results in effective absorption around 600 and 808 nm, readily corresponding to the broad absorption band from ⁴A₂ to ⁴T₂ level. After fast non-radiative relaxations to lower ⁴T₂ levels, ET processes of ⁴T₂ (Cr³⁺) → ⁵I₅ (Ho³⁺) and ⁴T₂ (Cr³⁺) → ⁵I₆ (Ho³⁺) can efficiently take place except for that partial Cr³⁺ ions de-excite to the ⁴A₂ ground state by emitting NIR photons. The ET brings about Ho³⁺ ions being excited from the ⁵I₈ ground state to the ⁵I₅ and ⁵I₆ excited states and subsequently populating the ⁵I₆ and ⁵I₇ states with the assistance of multiphonon relaxations. Eventually, 1.2 and 2.0 μm emissions are efficiently induced from the ⁵I₆ → ⁵I₈ and ⁵I₇ → ⁵I₈ transitions of Ho³⁺, respectively.

 

Fig. 4 Simplified energy level schemes illustrating the possible ET mechanism of Cr³⁺ → Ho³⁺. The ²E energy level of Cr³⁺ is embedded into the ⁴T₂ energy level.

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4. Conclusion

Under excitation of a conventional Xe lamp or an 808 nm LD, we successfully obtain the intense MIR 2.0 μm luminescence of Ho³⁺ in Cr³⁺/Ho³⁺ codoped fluorogermanate glass. Through the efficient sensitization of Cr³⁺, the excitation wavelength range for getting 2.0 μm luminescence of Ho³⁺ is greatly enlarged for the whole near-UV-to-NIR area of about 325-830 nm. In terms of absorption, excitation and photoemission spectra, and time-resolved spectra, the resonant ET mechanisms of ⁴T₂ (Cr³⁺) → ⁵I₅ (Ho³⁺) and ⁴T₂ (Cr³⁺) → ⁵I₆ (Ho³⁺) are rationally demonstrated. Furthermore, the ET efficiency of Cr³⁺ → Ho³⁺ is appropriately calculated to be 26.2%. The further development of this Cr³⁺/Ho³⁺ system would provide the available 2.0 μm laser material efficiently excited by flashlamps or various commercial lasers.