Optical Transitions from Hexavalent Chromium in Lithium-Borate Glasses

The melt-quenching technique was used to prepare a series of chromium-doped borate glasses with the composition xCr2O3–(70 − x) B2O3–30 Li2O–(x = 0, 0.1, 0.2, 0.3 and 0.5 mol %). The low-doping level here employed allowed to unambiguously identify well-defined near-edge Cr6+ optical transitions, and to precisely determine the optical band gap of the borate glass host. Additional Cr3+ transitions were observed in the visible regime, rendering a strong modulation of the glass color, from colorless to dark greenish, with Cr content. Both Cr6+ (after the charge transfer transformation into Cr5+) and Cr3+oxidation states and their variations with Cr doping were identified from electron spin resonance spectroscopy. All samples exhibit similar vibrational spectra dominated by BO3 and BO4 structural units, with the development of weak Cr6+ vibration with Cr doping. The present study provides structurally similar but optically active and tunable glass hosts suitable for various optical applications.