Fabrication of a material assembly of silver nanoparticles using the phase gradients of optical tweezers.

Optical matter can be created using the intensity gradient and electrodynamic (e.g., optical binding) forces that nano- and microparticles experience in focused optical beams. Here we show that the force associated with phase gradient is also important. In fact, in optical line traps the phase gradient force is crucial in determining the structure and stability of optical matter arrays consisting of Ag nanoparticles (NPs). NP lattices can be repeatedly assembled and disassembled simply by changing the sign of the phase gradient. The phase gradient creates a compressive force (and thus a stress) in the optically bound Ag NP lattices, causing structural transitions (a stress response) from 1D "chains" to 2D lattices, and even to amorphous structures. The structural transitions and dynamics of driven transport are well described by electrodynamics simulations and modeling using a drift-diffusion Langevin equation.