Strain-Dependent Fluorescence Spectroscopy of Nanocrystals and Nanoclusters in Cr:YAG Crystalline-Core Fibers and Its Impact on Lasing Behavior

Abstract

We report the results of our experimental investigation on the strain field distribution in Cr:Y 3 Al 5 O 12 (Cr:YAG) double-clad crystal fiber (DCF) via spatially resolved near-field imaging in the visible (Cr 3+ ) and nearinfrared (Cr 4+ ) spectral regions. The efficient and lowthreshold lasing from Cr 4+ :YAG DCF is well-described by a minimization of the localized strain field in the core due to the thermal expansion coefficient mismatch between YAG crystalline core and polycrystalline inner cladding (mixture of γ-Al 2 O 3 nanocrystal and Y 2 O 3 −Al 2 O 3 −SiO 2 glass). The Cr 4+ fluorescence is found to be at a maximum for the DCF with a core diameter around 20 μm where the strain field in the Cr 4+ :YAG DCF laser is near zero. The results are presented and discussed.

Ligand-Driven and Full-Color-Tunable Fiber Source: Toward Next-Generation Clinic Fiber-Endoscope Tomography with Cellular Resolution

“…To our knowledge, these high conversion efficiencies are the highest among conventional glass fibers. − The high optical-to-optical conversion efficiency can be ascribed to the large emission cross-sections of the color centers produced by the odd-parity phonon vibrations . The emission cross-sections of the F- and F 2 -type color emitters in the sapphire crystal are 1–2 orders of magnitude higher than those found in typical transition-metal-doped sapphire − and YAG ,, wideband gain media (10 –23 m 2 vs 10 –21 –10 –22 m 2 ). The resulting high conversion efficiency is also the highest reported to date among the existing waveguide-type white light-production techniques, including that obtained using supercontinuum lasers.…”

“…Considering the average reported value of the strain-induced Raman shift of ∼1.700 cm –1 /GPa in bulk sapphire, 77 this difference in the Raman shift of ∼−1.256 cm –1 (from 755.630 to 754.374 cm –1 ) corresponds to a tensile stress change of ∼−0.74 GPa, as found for typical luminescent Nd:YVO4, Nd:YAG, and Cr:YAG analogs. 34,82,83 In contrast, one can reasonably estimate the thermally induced residual stress, σ, in the sapphire core in terms of Young’s modulus ν, Poisson’s ratio E , core-to-clad volume fraction f , TEC α, and temperature change Δ T using the following relation 84 whereAssuming that the physical properties of the micrometer-sized fiber are similar to those of the millimeter-sized bulk sample, the values of ν, E , and α for the c axis sapphire core are 0.23, 400 GPa, and 4.5 × 10 –6 °C –1 , 85,86 respectively. Furthermore, f = (20 μm) 2 /(160 μm) 2 = 1/64, and typically, Δ T = 525–25 °C = 500 K (i.e., the transition temperature for borosilicate glass is 525 °C).…”

“…In fact, the rapid heating and cooling that occur during such an LHPG process could cause considerable residual stress. The employed CO 2 laser heating may result in a temperature of over 2000 °C, as evidenced by the change of the sapphire crystal into a melt, observed using yttrium–aluminum–garnet (YAG) crystal fibers . Thus, defect formation under the influence of laser heating during LHPG growth, as occurs during pulsed laser ablation, can result in faults, dislocations, and twins. , Nevertheless, because of the transformed sapphire crystalline core, which has a defective structure and ionic vacancies, these defect centers could act as color emitters for broadband generation, as has been discussed later.…”

“…Considering the average reported value of the strain-induced Raman shift of ∼1.700 cm −1 /GPa in bulk sapphire, 77 this difference in the Raman shift of ∼−1.256 cm −1 (from 755.630 to 754.374 cm −1 ) corresponds to a tensile stress change of ∼−0.74 GPa, as found for typical luminescent Nd:YVO4, Nd:YAG, and Cr:YAG analogs. 34,82,83 In contrast, one can reasonably estimate the thermally induced residual stress, σ, in the sapphire core in terms of Young's modulus ν, Poisson's ratio E, core-to-clad volume fraction f, TEC α, and temperature change ΔT using the following relation 84…”

Asymmetric Line Shape of Near-field Luminescence Spectrum Induced by Stress in Al2O3

“…Near-field-derived asymmetric peaks are also observed in luminescence spectra reported in other earlier works; however, the physics behind these results has not been deeply investigated. (7,8) In this study, we perform the spectral analysis of the obtained asymmetric spectra to explore near-field luminescence origins. We introduce the effect of vibronic bands, as well as that of stress-gradient-induced broadening, in the R-line spectrum analysis.…”