New strategy to enhance the broadband near‐infrared emission of bismuth‐doped laser glasses

Bismuth‐doped glasses and fibers with broadband near‐infrared (NIR) emission have garnered much attention on account of their potential applications in new fiber lasers and broadband amplifiers. Yet the realization of high gain from Bi‐doped fibers and highly efficient NIR emission from Bi‐doped glasses are still a stubborn puzzle. The enhancement of Bi NIR emission is normally based on modifying the glass composition and topology, which will change the structure of the glass over a wide range and alter the thermal or mechanical properties of the glass simultaneously, making it more complicated for the designing and fabricating of Bi fibers with good performance. Here, we find that a trace addition of Si₃N₄ can efficiently enhance the Bi NIR emission without changing the glass structure significantly, right followed by spectral and structural analysis. ²⁷Al NMR measurement reveals that the short‐ to medium‐range order of this glass is unchanged. The EPMA measurement confirms the homogeneity of fabricated glass. The great enhancement and red‐shift under blue light excitation may originate from the conversion of Bi active centers to low valence. Our results indicate that the trace addition of nitride could be a facile and maneuverable way to control the valence of active ions in glasses, which may contribute to improving the performance of photonic glasses.     

Manipulating Bi NIR emission by adjusting optical basicity,boron and aluminum coordination in borate laser glasses

Bismuth‐doped (Bi) glasses and fibers have raised considerable attention for broadband emission and tunable fiber lasers in the near infrared (NIR) region. However, they suffer from low efficiency and it remains challenging to enhance Bi NIR emission. Here, we propose a facile way to enhance and tune the Bi NIR emission by adjusting optical basicity and modulating the boron and aluminum coordination in borate glasses. We find that BO₄ and AlO₅ species favor Bi NIR emission, right followed by the analyses of static emission spectra, the Fourier transform infrared (FT‐IR), and nuclear magnetic resonance (NMR) spectroscopy. Furthermore, abnormal Bi NIR luminescence phenomenon and boron anomaly were observed, which are attributed to the synthetic effect of B and Al coordination transformation. Besides, we find that BO₄ tetrahedron plays a major role in enhancing Bi NIR emission at low Al content, while AlO₅ hexahedron group will dominate at high Al concentration. Our investigation may give an insight into the luminescent behaviors of Bi in borate glasses and contribute to improving the performance of Bi‐doped fiber and fiber lasers in future.     

Enhancement of ultrabroadband Bi NIR emission via fluorination for all wavelength amplification of optical communication

Bismuth (Bi)-doped photonic glasses and fibers with broadband near-infrared (NIR) photoemission have potential applications in tunable lasers and broadband amplifiers. Yet, when it comes to all wavelength amplification of optical communication, it remains challenging to achieve efficient Bi NIR emission in the technically relevant C- and L- bands (1530-1625 nm). Here, we propose a scheme by fluorination triggered enhancement of ultra-broadband Bi NIR emission in nitrided germanate glasses. Besides, compared to previous research, a unique and efficient Bi-activated ultra-wideband NIR emission with new emission bands peaked at ~924 and ~1520 nm under excitation of 450 nm are obtained in nitrided germanate glasses after fluorination. Moreover, the fluorination can modulate the local chemical environment by forcing the conversion of aluminum species from AlO₄ to AlO₅ and AlO₆ and consequently increase the flexibility of the glass network structure, which finally induces the conversion of Bi species and then manipulates the relative emission intensity of different Bi NIR centers. Thus, a flat and tunable emission spectrum covering the entire optical communication band is obtained by optimizing the fluoride amount. We believe this work is helpful to design the Bi-doped tunable fiber lasers and ultra-broadband amplifiers for all wavelength amplification of optical communication.     

Crystallization kinetics and enhanced Bi NIR luminescence of transparent silicate glass‐ceramics containing Sr2YbF7 nanocrystals

Bismuth‐doped glasses and crystals have been widely investigated due to their intriguing potential applications in superbroadband fiber amplifier and lasers in new NIR spectral range. However, few reports have been devoted so far to bismuth‐doped transparent glass‐ceramics. Here, this work reports on bismuth‐doped silicate glasses and glass‐ceramics, which were prepared by melt‐quenching and consequent annealing processes, respectively. On the basis of the analyses on crystallization kinetics, nucleation and growth rate of crystalline phase can be modulated and Sr₂YbF₇ nanophase can, therefore, be precipitated uniformly inside the glass matrix in a controlled way to maintain proper transparence especially in optical telecommunication windows. Once the nanophase comes into being, enhanced bismuth NIR luminescence can be observed by more than 40 times upon excitation of 470 nm. Similar enhancement can appear upon different excitation schemes and the mechanism is discussed accordingly. Such Bi doped transparent glass‐ceramics with improved luminescence efficiency might find application in fiber lasers for future optical fiber communication.     

Predictable tendency of Bi NIR emission in Bi‐doped magnesium aluminosilicate laser glasses

Because of superbroad luminescence in the range of near infrared (NIR), Bi‐doped glasses and fibers have received more attentions recently for the applications in super broadband optical fiber amplifiers or new wavelength lasers. As the luminescence comes from the transitions between naked 6p orbitals of bismuth, it is very susceptible to slight changes of local field around Bi. Therefore, it is always very challenging to predict NIR emission of bismuth in advance. Here, we found bismuth NIR emission shows predictable tendency in ternary glass system of MgO–Al₂O₃–SiO₂. The emission peak shifts red along the content of magnesium upon the excitation of 484 nm, which follows a single exponential growth equation. In the meantime, the full width at half maximum (FWHM) is broadened while the lifetime keeps decreasing. Glass structure analysis on basis of FTIR, ²⁷Al NMR, ²⁹Si NMR spectra reveals that these changes correlate to integrity of glass network, the increased disorder of local field around bismuth and the enhanced interaction between bismuth and host, which are perhaps due to the linear increase of nonbridging oxygen, and the enhanced Si–O asymmetric stretching vibrations along with magnesium, respectively. Electron probe microanalysis shows good homogeneity of Si, Al, Mg, Bi, and O distribution within the samples, and yoyo experiments of heating and cooling between 30°C and 300°C reveal the good resistance of such doped glasses to thermal degradation. This makes the glasses promising in applications of fiber devices even under extreme condition such as at higher temperature. The finding in this work should be helpful for the design of Bi‐doped laser glasses in future.     

The origin of the heterogeneous distribution of bismuth in aluminosilicate laser glasses

As one kind of novel and burgeoning laser materials, bismuth‐doped silicate glasses have aroused increasing attention for the super broadband near‐infrared (NIR) emission. However, the large optical scattering loss, resulting from optical heterogeneity in glass color and refractive index, limits their further applications in telecommunication system. Thus, it is urgent to uncover the essence of heterogeneity in Bi‐doped silicate glasses and subsequently improve glass optical performance. It will give us some hint to homogenize the glass component and Bi active centers so as to boost the development of Bi‐based glass materials. Here, taking 1 typical Bi‐doped calcium aluminosilicate glass as an example, we revealed the origin of the optical heterogeneities in glass color and refractive index through the NIR emission spectra, electron probe microanalyzer (EPMA) of elements and X‐ray photoelectron spectroscopy (XPS) of Bi 4f_5/2, Bi 4f_7/2, and Al 2p. The inhomogeneous distribution of Bi and aluminum components is responsible for the heterogeneity in this glass system. In addition, we found that tetrahedral coordinated aluminum favors the existence of Bi NIR centers, consequently resulting in enhanced Bi NIR emissions. Furthermore, based on our results and the role of Al³⁺ in glass network, we demonstrate the homogenizing of glass component by finely tuning glass composition. This work will enrich the understanding of Bi‐doped laser glass and provide a guideline for the design of component‐derived Bi‐doped silicate glasses and fibers with efficient NIR emission and high optical quality.     

Ultra-broadband and thermally stable NIR emission in Bi-doped glasses and fibers enabled by a metal reduction strategy

Bismuth (Bi)-doped glasses with broadband near-infrared (NIR) emission have been drawing increasing interest due to their potential applications in tunable fiber lasers and broadband optical amplifiers. Yet, the implementation of highly efficient and ultra-broadband Bi NIR emission covering the whole telecommunication window remains a daunting challenge. Here, via a metal reduction strategy to simultaneously create a chemically reductive environment during glass melting and enhance the local network rigidity, a super broadband (FWHM ≈ 600 nm) NIR emission covering the entire telecommunications window with greatly enhanced intensity was achieved in Bi-doped germanate glasses. More importantly, due to the excellent thermal stability, the super broadband Bi NIR emission can be well retained after the glass was drawn into an optical fiber. Furthermore, the transmission loss of 0.066 dB/cm at 1310 nm and an obvious broadband amplified spontaneous emission spectrum spanning a range of 1000–1600 nm were observed in this fiber. This work can strengthen our comprehension of the complicated Bi NIR luminescence behaviors and offer a feasible and universal way to fabricate tunable fiber lasers and broadband optical amplifiers based on Bi-doped multicomponent glasses.     

Glass-forming region and enhanced Bi NIR emission in sodium tantalum silicate laser glass

Bismuth (Bi)-doped glasses and fibers are of current interest as promising active media for new fiber lasers and amplifiers due to their 800-1700 nm near-infrared (NIR) emission. However, the optically active Bi centers in silica are easily volatilized during high-temperature fiber drawing, which results in low Bi doping concentration and low gain NIR luminescence. Here, we explored the glass-forming region in a model glass system of sodium tantalum silicate (Na₂O-Ta₂O₅-SiO₂) glass and attained suitable glass host for enhancing Bi NIR emission, right followed by detailed analysis on optical and structural characterization. Glass-forming region roughly lies in where Ta₂O₅ ≤ 30 mol%, SiO₂ ≥ 40 mol%, and Na₂O ≤ 40 mol%. Not only is glass-forming ability improved but also Bi NIR emission is enhanced (~60 times) by the introduction of Ta into glass network. Dissociated Na cations are restricted beside Ta, the high-field-strength element, so that the negative impacts of Na cations on glass formation and Bi NIR emission are weakened, which is responsible for the highly enhanced Bi NIR emission. This work helps us understand the glass-forming of tantalum silicate glass systems and luminescent behaviors of Bi. Hopefully, it could contribute to designing the Bi-doped laser glasses and high gain fibers with stable luminescent properties in future.     

Near‐Infrared Broadband Photoluminescence of Bismuth‐Doped Zeolite‐Derived Silica Glass Prepared by SPS

Through the order–disorder transition process of zeolites, bismuth‐doped zeolite‐derived silica glasses with broadband near‐infrared (NIR) photoluminescence have been successfully prepared by spark plasma sintering (SPS). The samples were characterized by X‐ray diffraction, UV‐vis, photoluminescence, and fluorescence lifetime. The results showed that as‐prepared samples possessed favorable broadband NIR luminescence. The NIR emission (peaked at ~1140 nm) intensity decreased with increasing the bismuth doping concentration when excited by 500 and 700 nm. The tendency was different from the emissions (peaked at ~1240 nm) excited by 800 nm. In addition, the NIR fluorescence peaks of the fixed Bi concentration sample can be observed almost around 1140 or 1240 nm when excited by different wavelengths from 500 to 950 nm. These phenomena implied that the NIR emission peaked at different wavelengths may originate from different bismuth species. Three kinds of Bi active centers Bi⁺, Bi⁰, and (Bi₂)^2? were proposed to contribute to the NIR emission peaks at ~1140, 1240, and 1440 nm, respectively. Interestingly, a broadband NIR emission peaked at 1207 nm with a full‐width at half maximum of 273 nm was observed when excited by 600 nm, whose intensity was stronger than that excited by 800 nm. This property might be useful for broadband fiber amplifiers and tunable lasers.     

Enhanced NIR photoemission from Bi-doped aluminoborate glasses via topological tailoring of glass structure

Bismuth (Bi)-doped laser glasses with broadband emission are of current interest in the fields of sensing, bio-imaging, and photonics. For practical applications, it must be considered how to improve the emission efficiency, in particular, for borate glasses with wide glass-forming range, low melting point, and excellent fiberizing ability. Herein, we experimentally demonstrate that addition of GeO₂ to aluminoborate glasses can effectively enhance Bi NIR emission by more than 300 times with prolonged decay time (~500 μs) and good homogeneity, which is, to our best knowledge, seldom achieved in Bi-doped borate multi-component glasses. The addition of second glass-former GeO₂, as revealed by detailed optical and structural analysis, leads to the facile regulation on local glass structure, forcing the conversion of aluminum species from AlO₅ and AlO₆ to AlO₄ and consequently pushes the conversion of Bi³⁺ to Bi⁺ and Bi⁰ and stabilizes Bi NIR centers, which finally results in highly enhanced Bi NIR emission. We believe these results could contribute to designing Bi-activated multi-component laser glass and fibers with efficient NIR photoemission.     

Improving luminescence behavior and glass stability of tellurium-doped germanate glasses by modifying network topology

Broadband near-infrared (NIR) luminescent materials are of great interest for their potential application in optical communication, remote sensing, imaging, and homeland security. Tellurium (Te) doped glasses were recently recognized as such a promising candidate due to their broadband NIR emission (700–1700 nm). However, the achievement of Te-doped glasses with high luminescence efficiency and glass stability (GS) remains a daunting challenge. Here, the luminescence behavior and GS of Te-doped germanate glasses are manipulated by tailoring the glass network topology. Te NIR luminescence is enhanced by tailoring topological cages in germanate glass network structure through varying glass network modifiers. Meanwhile, the GS of potassium germanate glass is significantly enhanced due to increased network connectivity caused by the co-introduction of alkaline earth oxides. Finally, NIR luminescence intensity of the glass was further enhanced by optimizing the doping concentration of TeO₂. The findings here could contribute to designing Te-activated glasses with improved performance for application in optical amplifiers and tunable fiber lasers.     

Quantum-dots-precipitated rare-earth-doped glass for ultra-broadband mid-infrared emissions

Mid-infrared (MIR) fiber lasers have wide application prospects and great commercial value in the fields of medical operation, remote sensing and military weapon, etc. At present, Tm³⁺-doped glass can obtain broadband luminescence at 2 μm, the introduction of Ho³⁺ or Er³⁺ ions also shows a tunable MIR emission but with limited success. Herein, the rare-earth (RE) doped glass with quantum dots (QDs) precipitation is proposed for achieving ultra-broadband MIR emissions. The types and sizes of QDs are determined by the XRD and TEM, and their optical properties are further characterized by the absorption and emission spectra as well as the lifetime decay curves. It is found that the diameter of the QDs is gradually increased from 1.7 to 5.1 nm by increasing the heat-treated temperature from 490°C to 530°C, respectively. Interestingly, an ultra-broadband emission covering 1400-2600 nm is achieved from the heat-treated glass upon the excitation of 808 nm laser diode as a result of an overlapped emission from Tm³⁺ and PbS. All results suggest that these QDs-precipitated RE-doped glasses have important application prospects in ultra-broadband MIR laser glass, glass fiber, and fiber lasers.     

Efficient Enhancement of Bismuth NIR Luminescence by Aluminum and Its Mechanism in Bismuth‐Doped Germanate Laser Glass

As a new member of laser glass family, bismuth‐doped glasses have received rising interests due to the application of fiber amplifiers and laser sources in the new spectral range for the next‐generation optical communication system. For practical application of the glasses, it must be considered on how to improve the luminescence efficiency. Here, we demonstrate that addition of aluminum can enhance the bismuth near‐infrared luminescence by more than 10 000 times, which is right followed by the discussion on the mechanism on why this can happen. We believe this work can be helpful for designing bismuth‐doped multiple component laser glasses with high efficiency. In addition, because of high susceptibility of bismuth to local field change, it can be used as probe ion to envision glass structures. Using bismuth as a luminescent structural probe, we can see the modifier ions of Bi⁺ are not completely randomly distributed inside germanate glass and they prefer the residence around tetrahedral AlO₄ sites.     

Tunable broadband photoluminescence from bismuth-doped calcium aluminum germanate glasses prepared in oxidizing atmosphere

Tunable photoluminescence (PL) from transparent inorganic glass matrices is of interest for applications demanding a semitransparent photoconverter that does not elastically scatter incoming light. For this purpose, bismuth (Bi)-doped optical materials exhibit unique spectral characteristics in terms of bandwidth and emission tunability. Here, we demonstrate a facile route for preparing such converters from Bi-doped calcium-aluminate and calcium-aluminogermanate glasses. These glasses offer tunable PL across the near violet and visible-to-near-infrared (NIR) spectral range, with an emission lifetime in the range of 300 μs. The addition of GeO₂ exerts a decrease in optical basicity, which in turn enables the stabilization of NIR-active low-valence Bi species for broadband NIR PL.     

Regulation of bismuth valence in nano-porous silica glass for near infrared ultra-wideband optical amplification

The ultra-wideband near-infrared (NIR) emission of Bi-doped glass and fiber makes it play an increasingly important role in laser technology and optical communication system. However, challenge remains to grasp the original of NIR emission and manipulate the valence state of bismuth for efficient Bi-doped fiber lasers and amplifiers. In this article, we present a facile method to control the valence of bismuth by using one kind of porous host, the nano-porous silica glass (NPSG). Bi-doped silicate glass with novel NIR emission was obtained in NPSG based on glass phase separation technology (GPST). The valence state and NIR luminescence of bismuth can be controlled by sintering of NPSGs impregnated with ${\text{Al}}^{3+}$ and ${\text{P}}^{5+}$ ions in inert atmosphere. Three kinds of bismuth active centers (BACs) with their own fluorescence characteristics are formed in NPSG. $\text{Al}$ can disperse bismuth active centers (BACs) to enhance O band NIR emission; $\text{P}$ can promote the reduction of bismuth in glass to produce up-conversion and S+C+L+U band NIR emission. This work pushes us to take a big step towards understanding the truth of NIR emission of bismuth. The GPST may serve as key technology for efficient Bi-doped fiber lasers and amplifiers.     

Effect of topological structure on photoluminescence of PbSe quantum dot‐doped borosilicate glasses

Borosilicate glasses doped with PbSe quantum dots (QDs) were prepared by a conventional melt‐quenching process followed by heat treatment, which exhibit good thermal, chemical, and mechanical stabilities, and are amenable to fiber‐drawing. A broad near infrared (NIR) photoluminescence (PL) emission (1070‐1330 nm) band with large full‐width at half‐maximum (FWHM) values (189‐266 nm) and notable Stokes shift (100‐210 nm) was observed, which depended on the B₂O₃ concentration. The PL lifetime was about 1.42‐2.44 μs, and it showed a clear decrease with increasing the QDs size. The planar [BO₃] triangle units forming the two‐dimensional (2D) glass network structure clearly increased with increasing B₂O₃ concentration, which could accelerate the movement of Pb²⁺ and Se^2? ions and facilitate the growth of PbSe QDs. The tunable broadband NIR PL emission of the PbSe QD‐doped borosilicate glass may find potential application in ultra‐wideband fiber amplifiers.     

Enhanced near-infrared to green upconversion from Er3+-doped oxyfluoride glass and glass ceramics containing BaGdF5 nanocrystals

In this work, effect of glass composition as well as ceramization on visible and near-infrared (NIR) luminescence properties along with their decay dynamics of Er³⁺ ions has been compared considering two different oxyfluoride glasses yielding BaF₂ and BaGdF₅ nanocrystals. Both the glass systems have exhibited an intense normal and upconversion green emission under ultraviolet (378 nm) and NIR (978 nm) excitations, respectively. A remarkable enhancement of these emission intensities is observed for gadolinium-(Gd) containing glasses. Interestingly, NIR fluorescence intensity from Er³⁺ ions at 1540 nm has showed marginal decrease in gadolinium-containing glass which is attributed to occurrence of strong excited-state absorption (ESA) due to higher fluorine content ensuing an augmentation of upconversion green emission with a concomitant decrease in NIR emission. The quadratic dependence of upconversion green emission intensity on its pump power for all the samples revealed biphotonic absorption process from ground-state ⁴I_15/2 to the excited-state ⁴I_11/2 followed by ESA of second photon to the ⁴F_7/2 level. The intense green upconversion emission as well as enhanced NIR fluorescence lifetimes indicate the suitability of these glass/glass ceramics for upconversion lasers and amplification in the third telecom window.     

Green to red and NIR tunable luminescence in heavily Er3+-doped tellurite glasses via selective energy transfer mechanism

Here, we select simple Er³⁺-doped tellurite glass as model system to systematically explore the up-conversion, down-shifting mechanisms with different excitations (980 and 447 nm), respectively. We observe for the first time, to the best of our knowledge that tunable photo-luminescence occurs from green to red and NIR region, rather than merely from the long-accepted green to red region. Direct evidence of selective energy transfer mechanism is expounded in detail, and its potential applications are demonstrated. In addition, we provide evidence that the cross-relaxation process between dopant ions can enhance photo-luminescence in Er³⁺ doped tellurite glasses with high dopant concentrations, whereas the crucial reason for emission decrease is the energy loss long-distance energy migration. These fundamental insights into the photophysical processes in heavily doped photonic glasses will broaden the applications of rare-earth-doped materials ranging from optical communications to medical imaging.     

Photoemission from Bi-doped calcium aluminate glasses similar to sunlight

Light sources as the substitution for sunlight play an important role due to their practical applications in aircraft, medicine, and agriculture. Bi³⁺ ions show tunable emission from ultraviolet to deep red depending on local crystal field, presenting great potential in mimicking sunlight. However, these emissions are mainly limited to crystals. To stabilize Bi³⁺ ions inside photonic glasses with desirable emission and excellent physical properties remains challenging. Here, we managed to stabilize Bi³⁺ with ultrabroad and tunable emission inside glasses at air atmosphere. This emission spans the whole visible range of 350-780 nm and matches well with the sunlight spectra. For the first time, an external quantum efficiency of 34% was obtained from Bi-doped photonic glasses without optimizing glass composition. We find that the local oxidation atmosphere from super-oxide ions and proper crystal field formed by AlO₄ are responsible for this unique luminescence, which is revealed by the detailed comparison on the optical and structural properties of calcium aluminate glasses and Ca₁₂Al₁₄O₃₃ crystals with the same composition. Moreover, glasses with distortion structure further results in multiluminescent centers of Bi³⁺ so as to broaden the emission band. This work provides new insights into the luminescent behaviors of Bi ions in luminescent materials, and should contribute to designing new photonic glasses in future.     

Controllable ultra-broadband visible and near-infrared photoemissions in Bi-doped germanium-borate glasses

The design of functional materials with tunable broadband luminescence performance is still of great interest in the fields of lighting, solar cells, tunable lasers, and optical amplifiers. Here, via a melt-quenching method, a series of bismuth (Bi)-doped germanium-borate glasses with composition of 40GeO₂–25B₂O₃–25Gd₂O₃–10La₂O₃–xBi₂O₃ have been prepared, in which multiple Bi active centers can be stabilized simultaneously. Dual-modulating modes of visible (380-750 nm) and near-infrared (NIR) (1000-1600 nm) broadband photoemissions were effectively controlled under flexible excitation scheme. Photoluminescence (PL) spectra at low temperature 10-298 K were appropriately employed to interpret such an unusual wide visible emission band. To further illustrate the origin of NIR component, transmission electron microscopy (TEM) measurement was carried out. It is demonstrated experimentally that the visible emission mainly originates from the collective contribution of the ³P₁/³P₀¹S₀ transitions of Bi³⁺, while the broadband NIR luminescence should be related to the formation of low valent Bi⁺ and (or) Bi⁰ centers. This work may help to enhance the knowledge of the complex luminescence mechanism for the Bi species and it also enables such transparent glass materials to be a promising candidate for the multifunctional tunable light source.