In this work, we report the multifunctional character of neodymium-doped LaF₃ core/shell nanoparticles. Because of the spectral overlap of the neodymium emission bands with the transparency windows of human tissues, these nanoparticles emerge as relevant subtissue optical probes. For neodymium contents optimizing the luminescence brightness of Nd³⁺:LaF₃ nanoparticles, subtissue penetration depths of several millimeters have been demonstrated. At the same time, it has been found that the infrared emission bands of Nd³⁺:LaF₃ nanoparticles show a remarkable thermal sensitivity, so that they can be advantageously used as luminescent nanothermometers for subtissue thermal sensing. This possibility has been demonstrated in this work: Nd³⁺:LaF₃ nanoparticles have been used to provide optical control over subtissue temperature in a single-beam plasmonic-mediated heating experiment. In this experiment, gold nanorods are used as nanoheaters while thermal reading is performed by the Nd³⁺:LaF₃ nanoparticles. The possibility of a real single-beam-controlled subtissue hyperthermia process is, therefore, pointed out.

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In recent years, an unconventional excitation of trivalent neodymium ions ( N d 3 + ) at 1064 nm, not resonant with ground-state transitions, has been investigated with the unprecedented demonstration of a photon-avalanche-like (PA-like) mechanism, in which the temperature increase plays a fundamental role. As a proof-of-concept, N d A l 3 ( B O 3 ) 4 particles were used. A consequence of the PA-like mechanism is the absorption enhancement of excitation photons providing light emission at a broad range covering the visible and near-infrared spectra. In the first study, the temperature increase was due to intrinsic nonradiative relaxations from the N d 3 + and the PA-like mechanism ensued at a given excitation power threshold ( P t h ). Subsequently, an external heating source was used to trigger the PA-like mechanism while keeping the excitation power below P t h at room temperature. Here, we demonstrate the switching on of the PA-like mechanism by an auxiliary beam at 808 nm, which is in resonance with the N d 3 + ground-state transition 4 I 9 / 2 → { 4 F 5 / 2 , 2 H 9 / 2 } . It comprises the first, to the best of our knowledge, demonstration of an optical switched PA, and the underlying physical mechanism is the additional heating of the particles due to the phonon emissions from the N d 3 + relaxation pathways when exciting at 808 nm. The present results have potential applications in controlled heating and remote temperature sensing.

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