Nanoparticle Spectroscopy:  Birefringence in Two-Dimensional Arrays of L-Shaped Silver Nanoparticles

We present theoretical models and experiments demonstrating nanoparticle optical birefringence. The experiments use polarization optical methods to describe the birefringence properties of two-dimensional arrays of L-shaped silver nanoparticles. These particles have two major resonances with perpendicular polarization directions. The beam depolarization at incident angles intermediate to the resonance polarization directions is explained with a model based on a finite-difference time-domain (FDTD) calculation for both arrays and single particles. The maximum relative phase retardation is observed between the two overlapping dipole resonance wavelengths, and experimentally it is about 30°. While the FDTD models predict a larger effect of up to 105°, this might be due to the statistical variation of nanoparticle shapes in the experimental arrays. The arrays were fabricated by electron beam lithography, and the size of particles was ∼145 and ∼155 nm in nominal total edge length, 63 nm arm width, and 30 nm height. The observed phase difference is large compared to the conventional birefringence materials with the same thickness. This study suggests the possible application of two-dimensional nanoparticle arrays or single particles as wavelength-tunable, extremely thin birefringence materials.