The impact of high pressure on the number of photons involved in upconversion luminescence of NaYF4:Er3+@NaYF4 and NaYF4:Yb3+,Er3+@NaYF4 core@shell nanoparticles and application as contactless pressure sensor

• UC emission of NaYF 4 :Er 3+ and NaYF 4 :Yb 3+ ,Er 3+ were investigated under pressure. • Laser power density dependencies under pressure were analyzed for the first time. • The number of photons involved in population of the excited state was calculated. • Changes in emission bands in the compression cycle were compared. • Pressure sensing performance - relative sensitivity was estimated as ≈20 % GP −1 . Inorganic nanomaterials doped with lanthanide ions and exhibiting upconversion (UC) luminescence are leading in research of nonlinear optical materials. Extreme conditions, such as high-pressure compression (in the GPa range), may influence the physicochemical properties of various compounds. In this work, we investigated the optical properties of upconverting NaYF 4 :Er 3+ @NaYF 4 and NaYF 4 :Yb 3+ ,Er 3+ @NaYF 4 core@shell nanoparticles (NPs), focusing on the impact of high pressure on the number of photons involved in the UC mechanism. We measured the UC emission spectra of the synthesized NPs under high-pressure conditions, which allowed us to observe the pressure-induced intensity changes, band centroids spectral shifts, and band intensity ratios alterations. Importantly, by measuring the power-dependent UC emission spectra as a function of pressure, we determined and analyzed the number of photons ( n ) involved in the UC processes for the first time. It turned out that the n value changes with pressure as a result of diverse multi-phonon relaxation processes enhanced under high-pressure conditions. The results show potential adjustment of UC mechanisms with the use of high-pressure. The calculated band intensity ratios were also used for pressure sensing applications, resulting in the relative sensitivity reaching almost ≈20 % GPa −1 for the Er 3+ -doped core@shell NPs.