  We demonstrate a multi-photon excitation (MPE) scheme for luminescence
thermometry using ScVO$_4$:Bi$^{3+}$. MPE is performed using a 37 fs
Ti:Sapphire laser pulse centred at 800 nm. Log-log plots of the phosphorescence
intensity versus excitation power show that the 800 nm MPE of
ScVO$_4$:Bi$^{3+}$ involves a 2- and 3-photon absorption process in comparison
to a single-photon excitation (SPE) process at 266 nm and 400 nm. Spectroscopic
investigation shows that with the 800 nm MPE and 266 nm SPE schemes, the
emission spectra of ScVO$_4$:Bi$^{3+}$ are similarly characterized by emissions
of the VO$_4^{3-}$ groups and Bi$^{3+}$. The MPE is advantageous to suppress
fluorescence which interfere with the phosphorescence signal. We demonstrate
this aspect for a ScVO$_4$:Bi$^{3+}$ coating applied on an alumina substrate.
The luminescence lifetime is calibrated with temperature over 294-334 K; the
MPE scheme has an equally impressive temperature sensitivity (3.4-1.7% / K) and
precision (0.2-0.7K) compared to the SPE schemes. The MPE scheme can be applied
to a variety of phosphors and is valuable for precise temperature measurements
even in applications where isolating interfering background emissions is
challenging.
