We describe and demonstrate a novel method to measure directly excitation-energy migration distances in fluorescing materials. In any desired direction, migration distances corresponding to 1/10 to 3 times the fluorescence wavelength can be determined with an accuracy of ~ 10%. Fluorescing materials in the form of typically 10-µm-thick samples are coated with a mirror on one surface. A coherent excitation beam is used to create a standing-wave pattern in the sample. The fluorescence radiation is shown to possess an interference peak in one particular direction. In the case of energy diffusion, this peak becomes indistinct. From the amplitude of the peak, the energy-diffusion distance is directly obtained. The existence of the peak, its position and amplitude are experimentally verified by use of a fluorescing film of rhodamine 6G doped polyurethane.
H. P. Weber and P. F. Liao, "Optical diffusiometer for submicrometer-distance energy-diffusion determination," J. Opt. Soc. Am. 64, 1337-1342 (1974)