One major technical barrier in working with both NIR and visible light on an optical microscope is obtaining their precise coalignment at the imaging plane position. Current techniques require complex setups and software. Photon upconverting particles (UCPs) can bridge this gap as they are excited by NIR light but emit in the visible range. Here, two different UCPs have been identified, high-efficiency micro540-UCPs and lower efficiency nano545-UCPs are compared, and it is found that the lower efficiency nano-UCPs were superior for precise coalignment of the NIR beam with the visible light path consistent with limited particle-to-particle energy transfer, superlinear power dependence for emission, and much smaller particle size
