Nanoscopic Photomechanical Motion of Small Particles Induced by Optical Force due to Positive and Negative Photoabsorption by Excited States of Molecules

In the present study, we have successfully induced microscopic mechanical motions of single small particles by using optical force due to the transient absorption (TA) of organic dyes in the excited state. Single polymer microparticles containing a perylenediimide derivative (BPPDI) were optically trapped in water with a focused near-infrared (NIR) pulsed laser light. The trapped microparticle was photoexcited with visible (VIS) laser pulses to pump the dye to the excited state. Since the dye in the excited state has strong absorption band in NIR wavelength region, the trapped particle additionally experienced optical force due to the TA. The particle was thus pushed, and the trapping point moved toward the propagation direction along the optical axis of the NIR laser. Furthermore, we demonstrated optical pulling force by using "negative" TA, which is due to stimulated emission. A single polymer microparticle with Styryl 9M (S9M), showing stimulated emission in the NIR region, was optically trapped and photoexcited in a similar manner with microparticles containing BPPDI. The trapping point moved in the opposite direction of the incident NIR pulses because of the stimulated emission. These optical trapping behaviors were analyzed, and the positive (pushing) and negative (pulling) TA forces were evaluated on the basis of detailed theoretical calculation.