Near-Field Spectroscopy of Broadband Emissions from γ-Al2O3 Nanocrystals in Cr-Doped Double-Clad Fibers

Abstract

Embedded nanocrystals (NCs) of γ-Al2O3 in SiO2 matrixes of Cr-doped double-clad fibers (CDFs) were fabricated by the codrawing laser-heated pedestal growth technique. The spatially resolved broadband emissions of these NCs were examined as a function of NC morphology and distribution by near-field scanning optical microscopy. Comparison of the measured emissions with crystal-field ligands together with careful examination of the NC morphologies by high-resolution transmission electron microscopy confirmed this broadband emission as being an effect of nanotwinning and the Cr3+–Cr6+ redox reaction. The impact of nanotwinning on the relative strength and emission wavelength of Cr3+ and Cr6+ is potentially beneficial for broadband emission at visible wavelengths to achieve ultra-high-resolution optical coherence tomography (OCT). As is evident from the near-field spectrum, a 1.2-μm axial resolution in air for OCT can be achieved with a CDF with a 165-nm bandwidth centered at 690 nm.