xLa2O3-(100 − x)Ga2O3 binary glasses were synthesized by an aerodynamic levitation technique. The glass-forming region was found to be 20 ≤ x ≤ 57. The refractive indices were greater than 1.92 and increased linearly with increasing x. The polarizabilities of oxide ions were estimated to be 2.16–2.41 Å3, indicating that the glasses were highly ionic. The glasses were transparent over a very wide range from the ultraviolet to the mid-infrared region. The widest transparent window among the oxide glasses was from 270 nm to 10 μm at x = 55. From the Raman scattering spectra, a decrease in bridging oxide ions and an increase in non-bridging oxide ions were confirmed to occur with increasing La2O3 content. The maximum phonon energy was found to be approximately 650 cm−1, being one of the lowest among oxide glasses. These results show that La2O3-Ga2O3 binary glasses should be promising host materials for optical applications such as lenses, windows, and filters over a very wide wavelength range.

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with regions associated with strong water absorption. [1][2][3][4] However, because the energy gap between the 4 I 13/2 and the 4 I 11/2 levels is as small as ≈3700 cm −1 , the 2.7 µm emission has rarely been attained from Er 3+ -doped oxide glasses because of their large multiphonon relaxation rates, short intrinsic IR cut-off wavelengths, and strong absorptions of OH − groups. In this regard, fluoride glasses are candidate host materials for MIR emission from Er 3+ because of their low-phonon energy, good solubility of rare-earth ions, and low OH − absorptions. [5][6][7][8] However, their troublesome fabrication process, poor chemical durability, and low mechanical and thermal resistivity are serious problems for practical applications as a bulk form and for scaling up the output power. Furthermore, because the upper 4 I 11/2 level has a shorter lifetime than the lower 4 I 13/2 level, the 4 I 11/2 → 4 I 13/2 transition in Er 3+ is a self-terminating process; consequently, achieving population inversion between these two levels for laser operation is generally difficult. Utilizing a cooperative energy-transfer upconversion (ETU) among the 4 I 13/2 level by increasing the Er 3+ concentration is a well-known approach to promoting population inversion between the 4 I 11/2 and the 4 I 13/2 levels. [9] Upon excitation at ≈980 nm, the following ground-state absorption and excited-state absorption processes are possible within a single Er 3+ ion: 4 I 15/2 + hν (≈980 nm) → 4 I 11/2 and 4 I 11/2 + hν (≈980 nm) → 4 F 7/2 . In addition, at high Er 3+ concentrations and high excitation power density, two or more neighboring Er 3+ ions are excited simultaneously by pump photons and the following ETU processes can occur (Figure 1): 4 I 13/2 + 4 I 13/2 → 4 I 9/2 + 4 I 15/2 and 4 I 11/2 + 4 I 11/2 → 4 F 7/2 + 4 I 15/2 (hereafter denoted as ETU1 and ETU2, respectively). ETU1 depopulates the 4 I 13/2 level and further repopulates the 4 I 11/2 level through fast multiphonon relaxation from the 4 I 9/2 level; this process results in energy reutilization from 4 I 13/2 to 4 I 11/2 and promotes population inversion between these levels.Although several oxide glasses with relatively low-phonon energies, including tellurite, [10] germinate, [11,12] and heavy-metal oxide glasses, [13,14] have been investigated as hosts for 2.7 µm emission, their doping concentrations of Er 2 O 3 are limited to a few mol%, which is inadequate for efficient 2.7 µm operation. In oxide glasses, achieving efficient MIR emission by increasing the Er 3+ concentration has proven difficult because, Highly Er 3+ -doped La 2 O 3 -Ga 2 O 3 glasses up to ≈5.85 × 10 21 cm −3 in Er 3+ concentration are synthesized by an aerodynamic levitation technique. The glasses are characterized by high glass-transition temperatures, low OH − absorptions, and long infrared cut-off wavelengths. Judd-Ofelt analysis reveals a large radiative transition rate and a high branching ratio of the 4 I 11/2 → 4 I 13/2 transition, e.g., 46 s −1 and 21%, respectively, at 10 mol% Er 2 O 3 . The ...

La 2 O 3 -Ga 2 O 3 -M 2 O 5 (M = Nb or Ta) ternary glasses were fabricated using an aerodynamic levitation technique, and their glass-forming regions and thermal and optical properties were investigated. Incorporation of adequate amounts of Nb 2 O 5 and Ta 2 O 5 drastically improved the thermal stabilities of the glasses against crystallization. Optical transmittance measurements revealed that all the glasses were transparent over a wide wavelength range from the ultraviolet to the mid-infrared. The refractive indices of the glasses increased and the Abbe number decreased upon substituting Ga 2 O 3 with Nb 2 O 5 , and the decrease in the Abbe number was significantly suppressed when Ta 2 O 5 was incorporated into the glass. As a result, excellent compatibility between high refractive index and lower wavelength dispersion was realized in La 2 O 3 -Ga 2 O 3 -Ta 2 O 5 glasses. Analysis based on the single-oscillator Drude-Voigt model provided more systematical information and revealed that this compatibility was due to an increase in the electron density of the glass. K E Y W O R D S glass, optical materials/properties, thermal properties

La 2 O 3 −Ga 2 O 3 binary glass exhibits unusual optical properties owing to its high oxygen polarizability and low vibration energy. These optical properties include high refractive indices and a wide transmittance range. In this study, we performed classical molecular dynamics simulations on La 2 O 3 −Ga 2 O 3 glass synthesized by an aerodynamic levitation technique. We have obtained structural models that reproduce experimental results, such as NMR, high-energy X-ray diffraction, and neutron diffraction. Based on our analysis, the structural features were clarified: 5-, 6-coordinated Ga, edge-sharing GaO x −GaO x polyhedral linkages, and oxygen triclusters. Additionally, the vibrational density of states was calculated by diagonalization of the dynamical matrix derived from the structural models and the results were compared with Raman scattering spectra. The mode analysis of the Raman spectra revealed that the principal bands at 650 cm −1 were mainly attributed to the stretching modes of the bridging and nonbridging oxygens. Meanwhile, the shoulder bands at the highest frequency of 750 cm −1 were mainly attributed to the stretching modes of the bridging oxygens and oxygen triclusters. The structural models obtained in this study well describe the characteristic local structures and vibrational properties of the La 2 O 3 −Ga 2 O 3 glass.