Nonlinear optical response of upconverting nanoparticles and its versatile applications with inherent three-dimensional capabilities: from microscopy to therapy

Upconverting nanoparticles (UCNPs) exhibit a unique nonlinear optical response, where the emission intensity in the UV/blue range increases non-linearly with the excitation intensity of a continuous-wave (CW) laser in the NIR range. This property can provide inherent three-dimensional (3D) capabilities for various applications. As a demonstration, we illustrate that 3D fluorescence imaging is achievable without the need for a pinhole or ultrafast pulsed lasers, allowing us to image mouse cerebrovascular networks up to the depth of around 700 μm through opaque brain tissues. Additionally, we demonstrate that co-dispersing UCNPs with photosensitizers enables depth-targeted photodynamic therapy with reduced damage to superficial cells. The nonlinear optical properties of UCNPs hold promise for providing 3D capabilities across a wide range of applications.