  Nanoparticles with anti-Stokes emissions enable many sensing applications,
but their efficiencies are considerably low. The key to enable the process of
anti-Stokes emissions is to create phonons and assist the excited photons to be
pumped from a lower energy state onto a higher one. Increasing the temperature
will generate more phonons, but it unavoidably quenches the luminescence. Here
by quantifying the number of phonons being generated from the host crystal and
at the surface of Yb3+/Nd3+ co-doped nanoparticles, we systematically
investigated mechanisms towards the large enhancements of the phonon-assisted
anti-Stokes emissions from 980 nm to 750 nm and 803 nm. Moreover, we provided
direct evidence that moisture release from the nanoparticle surface at high
temperature was not a main reason. We further demonstrated that the brightness
of 10 nm nanoparticles were enhanced by more than two orders of magnitude,
standing in stark contrast to the thermal quenching effect.

Optics;Applied Physics