Optical radiation force points to the same direction as the photon flux, while the direction reversal is quite challenging and necessitates special efforts. Herein, we present the observation of a photonic nanojet-mediated backaction of dielectric particles owing to the local heating of solvent molecules inside the nanojet in a purely dielectric system. Such backaction has been verified to be photothermal, without the influence from thermally induced turbulence. Our findings have been theoretically corroborated by statistical analysis on the size-dependent force and speed. In addition to the increase with laser power, the backaction force is a competing effect between the absorption and thermal conductivity of the immersion medium and is also affected by the photothermally raised background temperature. The backaction force exerts on dielectric particles with a broad size range. Since most biological particles are dielectric, with a refractive index greater than that for the surrounding medium, this work could inspire applications in biophotonics, such as cell sorting and classification.
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