Visible-to-infrared quantum cutting by phonon-assisted energy transfer in YPO4:Tm(3+), Yb(3+) phosphors.

Quantum cutting (QC) of one visible photon into two infrared ones has been reported for the lanthanide ion couple (Tm(3+), Yb(3+)) in a variety of host lattices. The mechanism responsible for QC was assumed to be a cooperative energy transfer (ET) process from Tm(3+) to two Yb(3+) ions, however, no solid evidence was presented. Herein we report visible-to-infrared QC for (Tm(3+), Yb(3+)) in YPO4 phosphors. The ET process from the excited (1)G4 level of Tm(3+) to Yb(3+) was investigated in detail by means of optical spectroscopy. By monitoring the steady-state photoluminescence (PL) and PL decay of the intermediate (3)F4 level of Tm(3+) as a function of the Yb(3+) concentration, we demonstrated the QC of one incident blue photon into one near-infrared emitting photon at 1004 nm from Yb(3+) and simultaneously into one mid-infrared emitting photon at 1791 nm from Tm(3+), rather than two emitting photons from Yb(3+). It was revealed that such visible-to-infrared QC was induced by phonon-assisted ET instead of cooperative ET as previously reported. This kind of QC phosphors may have potential as solar spectral converters to enhance the external quantum efficiency in multi-junction solar cells based on narrow band-gap semiconductors such as Ge, PbS or In1-xGaxN.