2.7 µm Mid‐Infrared Emission in Highly Erbium‐Doped Lanthanum Gallate Glasses Prepared Via an Aerodynamic Levitation Technique

Abstract

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 ...

Instantaneous Color Tuning of Upconversion Emission in a Novel Lanthanide-Doped Monolithic Glass via Excitation Modulation

“…Emission and absorption cross section parameters are crucial for theoretical simulation and design of a laser. Large emission cross section is a favorable aspect to obtain high gain and low threshold values . Peak emission cross sections of the transitions at 475, 545, and 660 nm are found as 0.28 × 10 –20 , 0.54 × 10 –20 , and 0.57 × 10 –20 cm 2 , respectively (Figure a,b).…”

“…Large emission cross section is a favorable aspect to obtain high gain and low threshold values. 47 Peak emission cross sections of the transitions at 475, 545, and 660 nm are found as 0.28 × 10 −20 , 0.54 × 10 −20 , and 0.57 × 10 −20 cm 2 , respectively (Figure 1a,b). As there is no previous report on the emission cross section of a Ho 3+ /Tm 3+ /Yb 3+ triply doped amorphous or crystalline host for the visible region, cross sections are compared to the NIR cross section values of Ho 3+ /Yb 3+ -, Tm 3+ /Yb 3+ -, and Ho 3+ /Tm 3+ /Yb 3 -doped hosts.…”

Principal Vibration Modes of the La2O3–Ga2O3 Binary Glass Originated from Diverse Coordination Environments of Oxygen Atoms

“…Since the La 2 O 3 –Ga 2 O 3 glass has low vibrational energy as oxide glass, it displays certain unique properties, such as good infrared transmittance and efficient mid-infrared emission of Er 3+ ( 4 I 11/2 → 4 I 13/2 ) . Therefore, understanding the vibration spectrum of the glass and their structural origins is important.…”

“…The displays certain unique properties, such as good infrared transmittance 16 and efficient mid-infrared emission of Er 3+ ( 4 I 11/2 → 4 I 13/2 ). 18 Therefore, understanding the vibration spectrum of the glass and their structural origins is important. Figure 6 illustrates the Raman scattering spectra 16 and total VDOS calculated from the structural models using eq 5.…”

“…However, NWF-free materials generally have low glass-forming abilities, making it difficult to obtain glass in a bulk form. A levitation technique is suitable for obtaining bulk glass with unconventional compositions without NWFs − since it efficiently suppresses the heterogeneous nucleation and promotes deep undercooling of the melt. Recently, binary glass systems containing only rare-earth oxides and gallium oxide were synthesized by the levitation technique. , Particularly, La 2 O 3 –Ga 2 O 3 glass exhibited excellent optical properties, such as high refractive index, low dispersion, wideband transmittance, and efficient fluorescent emission by rare-earth activation. − These properties can be attributed to the high oxygen polarizability, low vibrational frequency, and large rare-earth solubility of the La 2 O 3 –Ga 2 O 3 glass.…”