A Device of Tracking a Single Nanometer-Sized Particle in 3D with Nanometer Resolution and Millisecond Response Time.

We demonstrate a microscope system based on a confocal setup that can `track' a moving particle in three dimensions (3D) and `trap' it at a target position with nanometer spatial and millisecond time resolution. This is achieved by moving a 3D piezoelectric translation stage controlled by a feedback circuit to `cancel' the displacement of the particle. By keeping the target particle inside the confocal volume, it is now possible to perform other spectroscopic experiments on the particle simultaneously for a long time while the particle is moving freely in solutions. Our device overcomes the difficulty of single molecule spectroscopy (SMS) experiment on free moving samples.