Modular Assembly of Metamaterial Using Light Gradient Forces

This work studies the feasibility of assembling metamaterials using the light gradient force in a Standing Wave Optical Trap (SWOT). When a tightly focused laser beam is reflected from its focal plane, the intensity gradient produced in the standing wave pattern creates a 1D array of traps. When the 1D array is time-shared across a 2D lattice, it creates a 3D array of traps. Traps like these formed in a microfluidic device were subsequently populated with commercially available monodispersed dielectric and metallic nanoparticles (NPs). Hundreds of NPs can be manipulated concurrently into a complex heterogeneous voxel this way The NPs were then anchored in position by photopolymerizing a hydrogel scaffold to create a voxel. Voxels formed this way can be stitched together using step-and-repeat method to produce metamaterials of any size, shape and constituency although imperfectly. To be practical for handling, the matrix can be stiffened by vitrifying the hydrogel scaffold using Tetraethyl Orthosilicate. The NP size and position in the array was estimated using fluorescent confocal microscopy along with iterative deconvolution. The results show that the mean separation of NPs along the optical axis is 322 nm, for 860 nm trapping laser, which is in line with the separation between successive antinodes of the standing wave in the water (<i>λ</i>_trap/2<i>n</i>_water). Compared to a Gaussian beam, a pseudo-Bessel beam produced a larger and more regular array along the optical axis due to its longer focal length and shorter healing distance. The minimum registration error within a voxel (σ = 55 nm) was limited by the Brownian motion, while the minimum error between the voxels (σ = 88 nm) was likely affected by Brownian motion and repeatability of the microscope stage. Finally, the optical performance of the metamaterials was tested using dark-field, cross-polarized reflection spectroscopy, and compared with the finite element simulations accomplished with COMSOL. The cross-polarized spectra showed evidence of a resonance peak. Interestingly, whereas the line-shape from an array of polystyrene NPs was symmetric, an array of rutile NPs was not, which may be indicative of Fano resonance. So, despite the structural defects, reflection spectroscopy revealed a resonance.