Fig. 4: Widefield video-rate distance sensing using lateral PSFs of upconversion emission self-interference.

a The schematic shows the single UCNPs on a fabricated step structure using different thickness of SiO₂ on an Ag mirror substrate. b Atomic force microscopy (AFM) images show the measured heights of the three steps as 88.2 ± 0.8 nm, 105.4 ± 1.7 nm, and 115.4 ± 1.8 nm, respectively. The heights are measured by averaging the multiple positions within the region. c Widefield images of UCNPs on Step 1(ii), Step 2(i), and Step 3(iii) within the colored square area in b. A defocused 980 nm laser creates a widefield excitation, and the fluorescence image is collected by a CMOS camera through a blue color filter (475 ± 12.5 nm). d–g The distance sensing results by measuring the four typical characteristic parameters of the lateral PSFs. The imaging/sensing frame rate is 50 Hz. The spacings in d–g are the sum of step thickness with UCNPs’ radius (16.5 nm). Hence the AFM characterized spacing for Step 1, 2, and 3 are 104.7, 121.9, and 131.9 nm, respectively. h 3D trajectories of a UCNP in glycerol solution, observed for 13 s (see Supplementary Movie 1). The refractive index of the glycerol solution is 1.48. The color shows the observation time. The imaging/sensing frame rate is 10 Hz for compensating the reduced intensity from the UCNP by glycerol solution. i Cumulative displacement and j mean-square displacement (MSD) analysis of the UCNPs. The time lag is the time required for the particle to be displaced by a certain amount through diffusion. The fabrication, preparation, and composition of the samples are described in Supplementary Information Note 2. The method to calculate viscosity is shown in Supplementary Information Note 6. Error bars are based on standard deviation.