  The process of self-assembly is guided by the minimization of free energy,
which limits the symmetries available for assembly and ultimately the
usefulness of devices fabricated in this fashion. Here, we demonstrate
experimentally for the first time a technique which we term light-assisted,
templated self assembly. By exciting a guided-resonance mode of a
photonic-crystal slab, an optical potential landscape is created above the slab
which results in stably trapped, periodic patterns of particles at the
potential minima. The underlying symmetry of particle clusters matches that of
the photonic-crystal lattice, thus enabling square, rectangular or other
symmetries not usually available for assembly due to energy concerns. We
anticipate that this technique could be used to tailor the optical
characteristics of nanofabricated devices, and the optical trapping techniques
demonstrated herein may enable a variety of complex nanomanipulation tools and
lab-on-a-chip devices.
