Fig. 1: Near-infrared photoacoustic multiplexed differential imaging (NIR-PAMDi) through the use of sharp-peaked lanthanide nanocrystals.

Photoexcitation of Nd³⁺ phonon sensitizers in nanocrystal host lattices enables efficient photon absorption and PA generation. The presence of Nd³⁺ in high concentration quenches excitation energy and prevents photon emission through non-radiative cross-relaxation. This non-radiative decay pathway can be further enhanced by energy transfer to Sm³⁺ dopants, whose dense energy states match the intermediate energy level ⁴F_2/3 of long-lifetime Nd³⁺. Based on first principles phonon density of states (PDOS) calculations, we hypothesize that host lattices with higher phonon energies exhibit higher multiphonon relaxation rates (k_mp), which promote non-radiative relaxation, leading to more pronounced phonon cascade events. Manipulating the host material from fluoride (with a slow multiphonon relaxation rate, k_mp) to orthovanadate and hydroxide (with faster k_mp) facilitates stronger host-lattice vibrations, further promoting non-radiative energy dissipation from excited ions. The increase in non-radiative decay of energy states consequently induces thermal expansion at sharp wavelengths.