Highly efficient, tunable, ultrabroadband NIR photoemission from Bi-doped nitridated germanate glasses toward all-band amplification in optical communication

Fuguang Chen; Zhi Chen; Jianrong Qiu; Shuai Zhang; Zhijun Ma

doi:10.3788/COL202321.051601

Abstract

Bismuth (Bi)-doped near-infared (NIR) glass that can cover the entire optical communication window (850, 1310, and 1550 nm) has become the subject of extensive research for developing photonic devices, particularly, tunable fiber lasers and ultrabroadband optical amplifiers. However, the realization of highly efficient NIR luminescence from Bi-doped glass is still full of challenges. Notably, due to the co-existence of multiple Bi NIR centers in the glass, the origin of newly generated Bi NIR emission peaks at ∼930 and ∼1520 nm is still controversial. Here, we report a new Bi-doped nitridated germanate glass with tunable ultrabroadband NIR emission (850–1700 nm) and high external quantum efficiency (EQE) of

\sim 50 %

. A series of studies, including spectral analysis, nuclear magnetic resonance (NMR), and others, provide powerful evidence for the mechanism of luminescence enhancement and tunability, and make reasonable inferences about the origin of the new emission bands at ∼930 and ∼1520 nm. We believe that the results discussed above would enrich our understanding about multiple Bi NIR emission behaviors and contribute to the design and fabrication of highly efficient Bi-doped ultrabroadband wavelength-tunable optical glass fiber amplifiers and lasers in the future.

Keywords

bismuth germanate glass near-infrared luminescence

1. Introduction

With the arrival of a new era of cloud storage, the Internet of things, and 5G/6G, the demand for improving information transmission capacity in optical communication systems, which is limited by the gain spectra of fiber amplifiers, is even more urgent^[1–3]. To satisfy this need, novel photonic materials with highly efficient and ultrabroadband near-infrared (NIR) emission performance have been pursued for a long time. Traditionally, rare earth (RE)- or transition metal (TM)-doped glasses, glass ceramics, or glass fibers can exhibit higher NIR emission efficiency; however, the narrow gain bandwidth (not exceeding 100 nm) of RE ions^[4–6] and the requirement of suitable crystal field for TM ion-doped glass^[7,8], have resulted in limited success.

Alternatively, Bi-doped glasses and fibers, found by Y. Fujimoto in 1999^[9,10], presenting ultrabroadband NIR luminescence features from 1000 to 1600 nm, have drawn extensive attention, which enables ultrabroadband gain material for full-wavelength amplification covering the entire optical communication window^[11–14]. However, the insufficient efficiency and low gain are the critical defects limiting the application of Bi-based fiber. Therefore, how to improve the NIR luminescence performance of Bi-doped glasses is of great importance to promote the development of the Bi-doped fibers toward highly efficient optical amplifiers and tunable lasers. Over the past two decades, various strategies have been attempted, ranging from modulating the glass networks, optimizing the melting regime (such as melting or annealing temperature, time, and atmosphere), using high-energy radiation or crystallization, co-doping other active NIR centers, to improving the NIR emission intensity and bandwidth of Bi-doped glasses.

To enhance the NIR emission intensity by modulating the glass networks will result in worse thermal properties of Bi-doped glasses (such as soft point and refractive index) at the same time, which will make it complicated to prepare the desired fibers^[15–18]. For optimizing the melting regime, it usually generates little effect when adjusting the melting or annealing temperature and time^[19,20]. However, melting in the $H_{2}$ atmosphere will greatly enhance the emission intensity of Bi-doped glasses, but it is difficult and dangerous to control the reduction progress due to the rapid reaction at high temperatures^[21]. The use of high-energy radiation or crystallization to enhance the NIR emission intensity of Bi-doped glasses both bring challenges to fabricating fibers^[22,23]. Moreover, co-doping rare earth (RE) or TM active centers is a simple and commonly used method to broaden the NIR emission range to cover the whole optical communication window, but it is also faced with the problem of requiring coupling of multiple pump sources^[24,25]. Hence, it remains challenging to intensify and broaden Bi NIR emission in a proper way.

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In this work, we developed a new Bi-doped nitridated germanate glass with tunable ultrabroadband photoemission (850–1700 nm), an enhancement of $\sim 70$ times for the integrated intensity of Bi-related four emission bands, and a high external quantum efficiency (EQE) of $\sim 50 %$ by regulating the short-to medium-range order of the glass via manipulation of ${Al}_{2} O_{3}$ , as sketched in Fig. 1. To elucidate how the multiple Bi NIR centers respond to variations of ${Al}_{2} O_{3}$ introduced into the glass matrices, a series of microstructure analyses, including Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, ${}^{27}{Al}$ nuclear magnetic resonance (NMR), have been leveraged. Here, the results show that the conversion among ${AlO}_{6}$ , ${AlO}_{5}$ , and ${AlO}_{4}$ leads to the enhancement and different behaviors of Bi NIR emission. Nevertheless, introducing excessive ${Al}_{2} O_{3}$ into the glass will contribute to formation of local excess negative charges, resulting in low-valence Bi being oxidized into high-valence. Thus, a Bi-doped nitridated germanate glass with highly efficient, tunable, and ultrabroadband NIR luminescence, covering the entire optical communication window, can be achieved by reasonable design and manipulation of ${Al}_{2} O_{3}$ . We believe that this work will contribute to the design and fabrication of Bi-doped glass fiber with highly efficient, tunable, and ultrabroadband NIR emission and provide deep insight into the mechanism of the multiple Bi NIR centers.

Figure 1.Evolution of the microscopic structure of Bi-doped nitridated germanate glass with the introduction of Al₂O₃.

2. Experimental Setup

2.1. Sample synthesis

Glass samples with molar composition of $(87 - x) {GeO}_{2} - {x Al}_{2} O_{3} - 10 BaO - 3 Al N - {0.02 Bi}_{2} O_{3}$ ( $x = 0$ , 3, 6, 8, 9, 10, 11, 12, 13), where x denotes the content of ${Al}_{2} O_{3}$ in the glass sample, were prepared by the melting and quenching method. Analytical grade ${Al}_{2} O_{3}$ , ${BaCO}_{3}$ , and high-purity ${GeO}_{2}$ , AlN, and ${Bi}_{2} O_{3}$ (99.99%) were used as starting materials. Batches of raw materials weighting 20 g were mixed homogeneously in an agate mortar. Each batch of the mixture was melted in an alumina crucible at a temperature of 1520°C in air for 30 min. Before the glass was formed, a quartz glass rod was used to stir the glass liquid evenly to achieve uniform mixing to a certain degree. Afterwards, the melt was cast onto a stainless-steel plate and pressed by another stainless-steel plate simultaneously so as to speed up the cooling process and avoid crystallization, and eventually form solid-state glass (labeled as xAl, according to the values of x). Finally, the glasses were cut and polished for subsequent optical analysis.

2.2. Characterization

Optical absorption spectra of the samples were measured by using a Perkin Elmer Lambda 900 UV/VIS spectrophotometer covering the spectral range of 300–1200 nm. Static excitation and emission spectra, dynamic emission spectra excited at 460 nm, and luminescence lifetime were recorded by using an Edinburgh FLS920 spectrofluorometer equipped with a continuous-wave 450 W Xe lamp and a microsecond flashlamp ( $μF 900$ ) as excitation sources, respectively. The EQE of the samples was measured by an absolute photoluminescence (PL) quantum yield spectrometer (Quantaurus-QY Plus C13534-11, Hamamatsu Photonics). X-ray diffraction (XRD) patterns were characterized by using an X-ray diffractometer (Rigaku D/max-IIIA) with $Cu K α 1$ radiation (1.5405 Å; cathode voltage, 40 kV; current, 40 mA) in the $2 θ$ range of 10°–80°. FTIR profiles were measured in transmission mode with a Bruker Vector 33 spectrometer by dispersing glass powders into KBr pellets. Raman spectra were obtained via a Raman spectrometer (Renishaw inVia) excited by a Nd:YAG laser at 532 nm with a power of 25 mW. A Bruker AVANCE III HD 400 instrument equipped with high-speed magic angle spinning (MAS) probe heads was used to measure the ${}^{27}{Al}$ NMR spectra. If not being specially stated, all measurements were conducted at room temperature.

3. Experimental Results and Discussion

The NIR PL emission spectra of the glass samples containing different contents of ${Al}_{2} O_{3}$ over 0–13% (molar fraction), upon 460 nm excitation, are depicted in Fig. 2(a). It can clearly be observed that all the samples present ultrabroadband emission from 850 to 1700 nm and consist of four emission peaks located at $\sim 930$ , $\sim 1160$ , $\sim 1260$ , and $\sim 1520 nm$ , respectively. The emission peak at $\sim 1160 nm$ originates mainly from the typical transition of ${{}^{3}P}_{1} \to {{}^{3}P}_{0}$ from ${Bi}^{+}$ , while the emission band at $\sim 1260 nm$ is assigned to ${{}^{4}S}_{3 / 2} \to {{}^{2}P}_{1 / 2}$ of ${Bi}^{0}$ ^[16,26–28], and the origin of newly generated Bi NIR emission centered at $\sim 930$ and $\sim 1520 nm$ is still controversial^[29,30], the nature of which will be explored and interpreted in the following. Moreover, the intensity of the ultrabroadband NIR emission can dramatically be enhanced (that is, the integrated intensity is enhanced by $\sim 77$ times for 11Al sample compared to that of 0Al sample) by regulating the content of ${Al}_{2} O_{3}$ , and, the EQE of the glass without Al doping is only 2.52%, while that of 11Al is up to $\sim 50 %$ [Fig. 2(c)], which has never been reported in Bi-doped NIR glasses^[31], and shows great potential for an ultrabroadband NIR light source. Interestingly, the relative intensity ( $I_{1260 nm} / I_{1160 nm}$ ) of emission peaked at 1160 and 1260 nm can also be modulated by tuning the concentration of ${Al}_{2} O_{3}$ , showing a monotonically decreasing trend with an increase of ${Al}_{2} O_{3}$ content, as plotted in Fig. 2(b) and Fig. S1 (see Supplementary Material for more information). Moreover, the emission intensities at $\sim 930$ and $\sim 1520 nm$ share a similar decreasing trend with that of $\sim 1260 nm$ , which provides important clues to exploring the origin of the newly generated emission bands at $\sim 930$ and $\sim 1520 nm$ .

Figure 2.(a) NIR PL emission spectra (λ_ex = 460 nm) of Bi-doped nitridated germanate glasses containing xAl₂O₃ (x = 0–13%); (b) corresponding emission spectra normalized to the emission peak at 1160 nm; (c) emission spectra inside the integrating sphere without and with the sample 0Al or 11Al upon 460 nm excitation; inset shows the magnified spectra in the wavelength range of 800 to 1600 nm.

The UV–VIS–NIR absorption feature of Bi-doped nitridated germanate glass with various ${Al}_{2} O_{3}$ doping was examined. As presented in Fig. 3(a), the glass free of ${Al}_{2} O_{3}$ shows no visible absorption, indicating that there are few activate NIR centers in the sample. Nevertheless, a small amount of alumina leads to fantastic changes of characteristic absorption of Bi NIR centers in the range of 300–1200 nm. By increasing the ${Al}_{2} O_{3}$ , three obvious absorption peaks located at $\sim 460$ , $\sim 570$ , and $\sim 710 nm$ appear, in accordance with the typical transition of ${{}^{4}S}_{3 / 2} \to {{}^{2}P}_{1 / 2}$ from ${Bi}^{0}$ , newly generated Bi NIR center, and ${{}^{3}P}_{0} \to {{}^{3}P}_{2}$ from ${Bi}^{+}$ , respectively. Moreover, the absorption intensity increases further as ${Al}_{2} O_{3}$ varies from 0 to 11%, and reaches the optimum for the 11Al glass sample, but an opposite phenomenon occurs when further increasing ${Al}_{2} O_{3}$ content. The parallel variation between NIR PL and absorption spectra evidences that the number of multiple Bi NIR activation centers can be adjusted by controlling the ${Al}_{2} O_{3}$ concentration.

Figure 3.(a) Optical absorption spectra of the glass samples containing xAl₂O₃ (x = 0–13%); (b) contour plot of the emission-excitation map from the 6Al glass sample; corresponding PL excitation spectra monitored at (c) 930 nm, (d) 1160 nm, (e) 1260 nm, and (f) 1520 nm of the glass samples containing xAl₂O₃ (x = 0–13%).

To reveal the deeper relationship between emission peaks and excitation wavelength in more detail, the dependence of emission peaks on excitation wavelength has been studied (Fig. S2; see Supplementary Material), and the contour plot of NIR emission-excitation map from 6Al glass sample was collected as shown in Fig. 3(b). Clearly, the Bi NIR centers show different responses when varying the excitation wavelength from 420 to 750 nm. When monitoring at $\sim 1260 nm$ ( ${Bi}^{0}$ ), a wide absorption band from 454 to 500 nm was observed. Similarly, the centers at $\sim 930$ and 1520 nm can only be excited at around 460 nm. This similar feature gives another clue that the root of the newly generated NIR centers at $\sim 930$ and 1520 nm may be related to $\sim 1260 nm$ ( ${Bi}^{0}$ ). The emission at $\sim 1160 nm$ ( ${Bi}^{+}$ ), however, can simultaneously be excited by blue light (480–520 nm) and red light ( $\sim 700 nm$ ). The result discussed above further confirms that at least three types of Bi NIR centers coexist in the germanate glass.

To obtain more clues on active Bi NIR centers, the excitation spectra of the four emission bands were collected. As shown in Figs. 3(c)–3(f), the active Bi NIR centers share diverse excitation dependent features, which is consistent with the above discussion. On one hand, all the observed active Bi NIR centers absorb UV–VIS light and can be well excited by blue light, which matches well with commercial blue LEDs and shows great potential for blue LED-excited ultrabroadband NIR light sources. In addition, all the excitation intensities reach the optimum state at $x = 11$ , and decrease with further increasing ${Al}_{2} O_{3}$ content with respect to the absorption and emission spectra. On the other hand, the four excitation spectra are different in some details. The emission band at 1160 nm shares three excitation bands at $\sim 340$ , $\sim 508$ , and $\sim 718 nm$ , respectively. However, the emission band at 1260 nm has two weak and broad excitation bands at $\sim 346$ and $\sim 736 nm$ and a strong excitation band at $\sim 502 nm$ . Five excitation bands at $\sim 336$ , $\sim 388$ , $\sim 456$ , $\sim 570$ , and $\sim 682 nm$ with various intensities are observed for the emission band at 930 nm; these complicated excitation features are different from the excitation bands at $\sim 354$ , $\sim 400$ , and $\sim 456 nm$ for the emission band at 1520 nm. The complicated features for excitation spectra at 930, 1160, 1260, and 1520 nm discussed above indicate that the newly generated Bi NIR centers at $\sim 930$ and $\sim 1520 nm$ are quite different from the NIR centers at $\sim 1160$ and $\sim 1260 nm$ .

PL lifetime is another vital dynamic clue to illustrate the difference among the active Bi NIR centers. The decay curves of multiple active Bi NIR centers from 11Al glass sample are plotted in Figs. 4(a)–4(d). All the decay curves follow a double-exponential decay equation, $I (t) = A_{1} \exp (- t / τ_{1}) + A_{2} \exp (- t / τ_{2}),$ (1)where $I (t)$ is the intensity at time $t$ ; $τ_{1}$ and $τ_{2}$ are short and long decay components, respectively; and $A_{1}$ and $A_{2}$ are fitting constants. Fitting each curve to the equation produces $τ_{1}$ and $τ_{2}$ , and the correlation coefficients always lie between 99.94% and 99.96%. The average fluorescence lifetimes $\bar{τ}$ were calculated by the following formula: $\bar{τ} = (A_{1} τ_{1}^{2} + A_{2} τ_{2}^{2}) / (A_{1} τ_{1} + A_{2} τ_{2}) .$ (2)

Figure 4.Decay curves of (a) 930 nm, (b) 1160 nm, (c) 1260 nm, and (d) 1520 nm emission peaks of 11Al glass sample (λ_ex = 460 nm).

The obtained individual fluorescence lifetimes of 11Al glass sample at 930, 1160, 1260, and 1520 nm are 28.74, 482.84, 404.94, and 304.37 µs, respectively. Compared to that of ${Bi}_{5}^{3 +}$ ( $\sim 5 μs$ )^[32,33], ${Bi}^{+}$ ( $\sim 460 μs$ )^[30], and ${Bi}^{0}$ ( $\sim 400 μs$ )^[34], the great difference in fluorescence lifetime indicates that $\sim 930$ and $\sim 1520 nm$ emission bands are truly rooted in other types of Bi species. Based on the results discussed above and previous study on the origin of Bi NIR centers, a reasonable conclusion can be drawn that the newly generated Bi NIR centers peaked at $\sim 930$ and $\sim 1520 nm$ are formed by the interaction between interstitial ${Bi}^{0}$ and inherent defects generated by introducing AlN in germanate glass^[35,36].

It is widely accepted that the valence states of Bi show hypersensitization to the variation of the glass local environment around them^[22,37,38]. The details of the variation of the glass microstructure with the substitution of germanium by aluminum can contribute to the valence state change of Bi in the glasses. Hence, the local microstructure of the glass can help to extend our comprehension of the great enhancement and tunability of the Bi NIR emission. As shown in Fig. 5(a), the XRD patterns of xAl (x = 0–13) samples present no distinct diffraction peaks, confirming that the glass samples are in an amorphous state. In addition, the shape of the humps remains almost constant, which rules out the effect of nanocrystal formation on the Bi NIR emission with introduction of ${Al}_{2} O_{3}$ .

Figure 5.(a) Normalized XRD spectra; (b) Raman spectra; (c) FTIR spectra; and (d) ²⁷Al 1D MAS NMR spectra (400 MHz) of the glass samples containing xAl₂O₃ (x = 0–13%), showing a gradual variation with increasing Al₂O₃ content, with the vertical scales (not shown) representing the intensity in arbitrary units, normalized to the maximum peak height in each spectrum; (e) percentage distribution of AlO₄, AlO₅, and AlO₆ by integrating area calculation of Gaussian multipeak fitting; the purple, green, and blue polyhedra beside the NMR spectra stand for Al tetrahedra, hexahedra, and octahedra, respectively. (f) Visible PL emission spectra (λ_ex = 350 nm) of Bi-doped nitridated germanate glasses containing xAl₂O₃ (x = 0–13%).

To gain deep insight into the local microstructure feature of the glass, FTIR and Raman spectra of the glass samples are illustrated in Figs. 5(b) and 5(c), respectively. The strongest peak in the Raman spectra located at $446 {cm}^{- 1}$ is assigned to the symmetric stretching vibration of Ge–O–Ge in ${[{GeO}_{4}]}_{6}$ rings, while the latter peak lies at $524 {cm}^{- 1}$ can be attributed to the symmetric breathing mode of Ge–O–Ge in ${[{GeO}_{4}]}_{3}$ or ${[{GeO}_{4}]}_{4}$ rings^[39,40]. Although ${Al}_{2} O_{3}$ is introduced into the glasses, there is no significant shift of the Raman peaks, indicating that no transformation of ${[{GeO}_{4}]}_{n}$ (n = 3, 4, and 6) rings happens. Furthermore, two main absorption bands at 593 and $848 {cm}^{- 1}$ and a wide absorption at $1036 {cm}^{- 1}$ are observed in the FTIR spectra. The absorption band at $593 {cm}^{- 1}$ is assigned to the symmetric stretching mode of Ge–O–Ge in the ${GeO}_{4}$ tetrahedra, and the absorption bands at 848 and $1036 {cm}^{- 1}$ are due to the asymmetric stretching vibrations of Ge–O–Ge and the Ge–O stretching vibrations in the ${GeO}_{4}$ tetrahedra^[41,42], respectively. Similarly, no distinct shift of the absorption bands is observed for the glass with various ${Al}_{2} O_{3}$ , implying that the introduction of ${Al}_{2} O_{3}$ has little effect on the overall glass network.

To further probe the glass topology network, in particular of the short- to medium-range order of the glass samples with ${Al}_{2} O_{3}$ doping, the normalized ${}^{27}{Al}$ NMR spectra of glass samples are depicted in Fig. 5(d). There are three main types of aluminum coordination polyhedra, including octahedra ( $\approx 0 ppm$ ), hexahedra ( $\approx 25 ppm$ ), and tetrahedra ( $\approx 50 ppm$ ) in the glass matrix^[43,44]. As expected, a vast amount of the alumina in the glasses exits in tetrahedral ( ${AlO}_{4}$ ) site, which acts as the network formerly in the glasses. To obtain visual conversion of the coordination polyhedra [ ${AlO}_{m}$ ] (m = 4, 5, and 6), Fig. 5(e) clearly presents the percentage of ${AlO}_{4}$ , ${AlO}_{5}$ , and ${AlO}_{6}$ in each glass sample. With the increase of ${Al}_{2} O_{3}$ , the percentage of ${AlO}_{4}$ increased at the expense of ${AlO}_{5}$ and ${AlO}_{6}$ polyhedra. The variation of the NIR emission intensity almost coincides with that of ${AlO}_{4}$ species, indicating that the ${AlO}_{4}$ species play a vital role in affecting the multiple Bi NIR centers.

To the best of our knowledge, ${AlO}_{4}$ species usually act as network formers to connect the destroyed glass network, while [ ${AlO}_{5}$ ] and [ ${AlO}_{6}$ ] species commonly serve as network intermediates and modifiers, respectively. On one hand, the conversion from ${AlO}_{5}$ and ${AlO}_{6}$ species to ${AlO}_{4}$ species will strengthen the glass networks, thus triggering the chain reaction ${Bi}^{3 +} \to {Bi}^{+} \to {Bi}^{0}$ . Meanwhile, the interaction between ${Bi}^{0}$ and defects in the glass gets strengthened, which brings about the improvement of the newly generated emission bands at $\sim 930$ and $\sim 1520 nm$ . On the other hand, the excessive ${AlO}_{4}$ bonding with the glass structure will form more Ge–O–Al bonds, resulting in local excess negative charges. In order to maintain charge balance, the low-valence Bi will be oxidized to a high-valence state, resulting in ${Bi}^{0}$ being oxidized as ${Bi}^{+}$ and ${Bi}^{3 +}$ , which exhibits the ultrabroadband tunable performance of Bi NIR emission and results in the decline of NIR emission intensity. To get deeper insight into this, the visible PL spectra from ${Bi}^{3 +}$ upon 350 nm excitation can be leveraged to verify it, as shown in Fig. 5(f). The intensity of the characteristic blue light emission from ${Bi}^{3 +}$ decreases monotonically upon the addition of ${Al}_{2} O_{3}$ , and then an anomalous phenomenon occurs when the content of ${Al}_{2} O_{3}$ exceeding 11%, namely, no red emission from ${Bi}^{2 +}$ is observed^[45,46], which shows exactly the opposite trend in comparison with that of the NIR emission intensity. The results show clearly that the interaction between the two mechanisms discussed above contributes to the enhancement and ultrabroadband tunability of the multiple Bi NIR emissions, which predominantly depends on the concentration of ${Al}_{2} O_{3}$ .

4. Conclusion

In conclusion, we successfully designed and fabricated a novel Bi–N co-doped germanate photonic glass, with highly efficient NIR emission (EQE of $\sim 50 %$ ) and ultrabroadband tunable performance (850–1700 nm), via modulation of the short- to medium-range topological order of the glasses by regulating ${Al}_{2} O_{3}$ amounts. As revealed by optical and microstructure characterization, with the introduction of ${Al}_{2} O_{3}$ , aluminum tetrahedra ${AlO}_{4}$ increased at the expense of ${AlO}_{5}$ and ${AlO}_{6}$ polyhedra, which is favorable for the multiple Bi NIR emissions. Moreover, the parallel variation trend of bands at $\sim 930$ , 1520, and $\sim 1260 nm$ ( ${Bi}^{0}$ ) implies that the newly generated NIR center is related to the ${Bi}^{0}$ center in the nitridated germanate glasses. Meanwhile, due to the replacement of ${Al}_{2} O_{3}$ with ${GeO}_{2}$ , the formation of local excess negative charges promotes ${Bi}^{0}$ to be oxidized as ${Bi}^{+}$ and ${Bi}^{3 +}$ , which exhibits the ultrabroadband tunable performance of the multiple Bi NIR emissions. Importantly, this work opens an avenue for enhancing and regulating the multiple Bi NIR emissions and helps us to understand multiple Bi NIR emission behaviors in multicomponent glass, which shows great potential application in ultrabroadband wavelength-tunable glass fiber amplifier in optical communication.

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