Luminescent Color Controller-Based Bilayer PC-UCNPs-PC Dynamically Modulate Photon Propagation

Lanthanide-doped upconversion nanoparticles (UCNPs) exhibit abundant emission wavelengths under near infrared (NIR) excitation. This property enables UCNPs to have potential for multicolor output. However, the realization of multicolor-tunable emission output in UCNPs at a fixed composition has remained challenging. Here, a luminescent color controller that can dynamically control the upconversion luminescence (UCL) colors from UCNPs is reported. By constructing bilayer photonic crystals (PCs) in which the UCNPs are distributed between the two-layer PCs, the propagation of green and red emission can be modulated by the top and bottom PCs, respectively. Moreover, the photonic bandgap (PBG) could be varied by changing the reflected angles, according to Bragg diffraction. Thus, the variation in the viewing angles also results in the yellow to blue and red to cyan structural color changes on the top and bottom bilayer PC-UCNPs-PC films. Correspondingly, the emission color outputs are changed from red to yellow-green and green to yellow-green, respectively. These findings provide a general method to achieve multicolor-tunable UCL by a bilayer PC-UCNPs-PC film. Moreover, the UCNPs with multicolor emissions in this work enrich the upconversion system and should have potential applications in display and anticounterfeiting.