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Fluorescence Colour on Demand: Control by Optical Force.

The ability to modulate, tune, and control fluorescence colour has attracted much attention in photonics-related research fields. The most examined approach has been the design and synthesis of luminescent materials such as luminescent nanocrystals (i.e., quantum dots). However, thus far, it has been impossible to achieve fluorescence colour control (FCC) for material with a fixed structure, size, surrounding medium, and concentration. Here, we propose a novel approach to FCC using optical tweezers. We demonstrate an optical trapping technique using nanotextured Si (black-Si) that can efficiently trap polymer chains. By increasing the laser intensity, the local concentration of perylene-labelled water-soluble polymer chains increased inside the trapping potential of the laser tweezers. Accordingly, the excimer fluorescence of perylene increased while the monomer fluorescence decreased, evidenced by a fluorescence signal colour change from blue to orange. Using nanostructure assisted optical tweezing, we demonstrate control of the relative intensity ratio of fluorescence of the two fluorophores, thus showing remote and reversible FCC of the polymer assembly.