Abstract
When two different chromophores placed close to each other in water are optically excited by continuous light beams with intensities modulated at two different frequencies, the temperature change from heat diffusing from one chromophore affects the magnitude of the thermal expansion coefficient in the region of the other chromophore, owing to the fact that the thermal expansion coefficient of water increases with temperature. Therefore, in addition to two separate linear photoacoustic emission at the sites of each chromophore, the oscillating variations in the thermal expansion coefficient of water means that a heterodyned photoacoustic effect will be generated at the sum and difference of the two modulation frequencies. It is shown here theoretically that the destructive photothermal interference between two chromophores, occurring at certain frequencies for a specific inter-chromophore distance, will give rise to amplitude nulls and thus drastic phase changes in the heterodyned photoacoustic waves, which hints a method that is capable of determining distances between the two chromophores and acts as a photoacoustic analog of Förster resonance energy transfer.
Published Version
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