Electron–phonon coupling mechanisms of broadband near-infrared emissions from Cr³⁺ in the Ca₃Sc₂Si₃O₁₂ garnet

Abstract

Cr³⁺ in the Ca₃Sc₂Si₃O₁₂ garnet (CSSG) has the ability to convert blue light to broadband near-infrared (NIR) emissions, which is a promising strategy for next-generation smart NIR light sources based on blue LEDs. The Cr³⁺ luminescence strongly depends on temperature due to electron-phonon coupling (EPC). We reveal the EPC mechanism of Cr³⁺ in CSSG for the first time by temperature-dependent photoluminescence measurement from 77 to 573 K and cathodoluminescence using a scanning electron microscope. Cr³⁺ occupies the Sc³⁺ site and experiences a weak crystal field in CSSG, manifesting a broad NIR emission in the 700-900 nm range that originates from the ⁴T_2g→⁴A_2g transition. The zero phonon line (ZPL) of the ⁴T₂ state is observed at ∼713 nm with a vibrational energy of ∼310 cm^-1. A strong EPC leads to a large Stokes shift (∼2900 cm^-1). The Huang-Rhys parameter (S = 4), crystal field strength (D_q/B), and Racah parameters (B and C) are estimated.