Plasmonic Luneburg lens characterization with phase detection

Luneburg lenses are optical elements with a gradient refractive index that focus plane waves at the outer surface of the lens without aberration. In this work, we designed, fabricated and characterized a plasmonic Luneburg... [ view full abstract ]

Luneburg lenses are optical elements with a gradient refractive index that focus plane waves at the outer surface of the lens without aberration. In this work, we designed, fabricated and characterized a plasmonic Luneburg lens which focuses a surface-plasmon polariton (SPP) beam at the boundary of the structure [Fig. 1]. The gradient in the effective refractive index was produced by a periodic array of holes with different radii in a nano-patterned PMMA structure, and a grating is used to couple the incident light into SPPs. An almost plane SPP beam is incident onto the two-dimensional (2D) Luneburg lens and it focuses at the close vicinity of its surface. The characterization was performed using leakage radiation microscopy (LRM), for usual intensity distribution imaging [Fig. 2], and phase detection was achieved using a slight modification of the illumination properties, which allowed to directly observe the SPP wavefront distribution as the beam propagates through the lens and focuses [Fig. 3]. Phase detection is obtained by imaging the interference pattern produced by the incident light and the leakage radiation of the SPPs, which are out of phase. The results are in good agreement with the 2D numerical simulations. The proposed design is efficient to focus SPPs at the surface of the structure, and opens the possibility to be operated inversely, i.e., to generate a plane SPP beam if excited with a point SPP source positioned at the outer surface. Moreover, Fourier imaging, available in most LRM setups, provides a new analysis approach for plasmonic Luneburg lenses, as it is possible to obtain the effective index of the SPP mode as it propagates through the lens. Such Fourier analysis can help verify the values of the designed gradient-index lens.