Fig. 8.

Near-Infrared light incident from the surface of the tissue interacts with the implanted MLBs and upconverts to locally-emitted visible light photons. The overall efficiency of this process depends on the propagation loss of the pump intensity, coupling efficiency of light to the MLBs, conversion efficiency of UCNPs, the number of illuminated UCNP particles, and different enhancement mechanisms (field enhancements and Purcell factor) inside the MLB. The directionality of the emission is also dictated by the shape of the MLB.