Holographic three-dimensional motion detection of an optically trapped sub-100Â nm gold nanoparticle

We demonstrated, for the first time, three-dimensional (3D) motion detection of a gold nanoparticle held in optical tweezers in water using holographic microscopy. Motion detection was performed with an in-line, low-coherence digital holographic microscope. The nanoparticle diameter was 60 nm, which is the smallest reported particle whose 3D motion can be measured. The motion of the optically-trapped nanoparticle had an axial variation of 7.6 nm (standard deviation) when a 1070 nm laser beam with an intensity of more than 27 MW/cm2 was focused with a 1.25 NA objective lens. From a comparison of a gold nanoparticle fixed on a glass substrate and a gold nanoparticle trapped by optical tweezers with a sufficient light intensity, we found that most of the variation was caused by noise in the experimental setup, and not the motion itself. The lateral variations for the fixed and trapped nanoparticles were the same, but the axial variation for the trapped nanoparticle was slightly larger. This suggested that the optically trapped nanoparticle had a motion with a single-nanometer order in the standard deviation only along the axial direction when the laser intensity was sufficiently large for trapping, which in this experiment was more than 27 MW/cm2.