Topological engineering of doped photonic glasses
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Introduction The optical properties of glass are highly correlated with the incorporated dopants. In the early 1960s,1 scientists tried rational incorporation of Nd3+ dopant as the active ion in glass in order to realize laser operation. In the following decades, several research teams constructed various rare-earth ion-, transition-metal ion-, and other metal ion-activated photonic glasses, with the spectral range covering the ultraviolet, visible, and infrared waveband regions (270–4500 nm).2–12 Since then, impurity doping in glasses has emerged as a topical area across multiple disciplines, including materials science, condensedmatter physics, and photonics. The significant issues lying at the heart of doping science in glass include (1) rational control over the chemical state of the dopant in the target matrix and (2) effective modulation of the ligand field around the dopant to achieve the desired spectroscopic features. Rare-earth ions show a relatively stable oxidation state in glass and their typical f orbital only weakly interacts with the matrix lattice. In this case, the doping methodology mainly involves handling the dopant–dopant and dopant– host interactions to avoid unexpected emission quenching. For example, the stabilization mechanism of rare-earth ions in glass has been clarified based on the local charge neutrality principle.13,14 That is, the positive charges of rare-earth dopant must equal the sum of negative charges around it. Studies have illustrated various ways of enhancing dopant solubility and
radiative transition probability.15 In stark contrast, transitionmetal and other nonconventional metal dopants are more sensitive to the host, primarily because of their own rich chemical states and because their outermost orbital (d orbital for transitionmetal ions and p orbital for p-block metal ions) strongly interacts with neighboring ions or molecules.8,9 Although these active ions show fascinating emission properties and are widely recognized as the key active centers of next-generation photonic glasses, grand challenges remain for better control of these dopants.16 Here, we provide an overview of recent progress on control over the chemical state and ligand fields of dopants in glass through topological engineering.
Topological approach The structure of glass is characterized by the absence of longrange order and the presence of rich short- and medium-range order. In this sense, topological structure plays a critical role in defining the properties of glass. Topological approaches to correlate the thermal, mechanical, and rheological properties of glasses with their structural parameters have been widely studied, and encouragingly, the approaches have been demonstrated to be successful.17–21 Moreover, several typical topological features such as packing density, sublattices, and hybrid networks have also been identified.22 For photonic glasses, especially luminescent glasses, the incorporated dopants are embedded in the host and connect
Shifeng Zhou, State Key Laboratory of Luminescent Material
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