Light scattering by optical nanoantennas

This study is conducted in collaboration with Maciej Klemm, Department of Electrical and Electric Engineering, University of Bristol, UK.

Optical antennas and antenna arrays offer possibilities in manipulating light at sub-wavelength scale. Similar to their counterparts at microwave frequencies, optical antenna arrays offer the capability of beam shaping and steering through controlling the phase of each antenna element. The phase control of optical antenna arrays can be implemented by either delay lines or antenna geometry and location. Identical antennas with uniform lattice are usually utilized when delay lines are employed to adjust the phase of antennas. Although sophisticated antenna design and arrangement can be avoided, delay lines are space consuming and difficult to integrate. Without using delay lines, antenna elements have to be carefully designed to obtain the required phase by taking into account the geometry, arrangement, coupling between elements and micro/nano fabrication tolerance.

In the currently realized designs, optical antenna arrays are based on conventional metal or low loss dielectric materials. Utilizing phase change material (PCM), dynamic beam shaping and steering can be achieved with array structures. Antenna arrays with dynamic high-speed beam shaping and steering will have a great impact on future micro/nano devices. University of Bristol has recently proposed a tunable reflectarray as a pathway to PCM based optical antenna arrays for free-space inter/intra chip interconnects. The tunable operation is realized by switching between two states of PCM. The difference in refractive index of the amorphous and crystalline state of PCM will change the behavior of the reflectarray.

The reflectarray consists of an array of equally separated antenna elements. In one state of PCM, antenna elements are designed to provide a constant phase gradient along the interface. It modifies the wavefront of incident light by altering its phase in a desired manner. The gradient of phase shift generates an effective wavevector along the interface, which is able to deflect light from specular reflection, described by the generalized Snell's law. In another state of PCM, the gradient phase shift is disturbed due to the significant chance of material’s permittivity.

Design of a single antenna element: influence of the curvature

We study the influence of the shape on the behavior of a core nanoantenna element in order to anticipate the consequences of the mismatch between the theoretical and effective (i.e. resulting from the fabrication process) geometrical designs.

DGTD simulation of an antenna array: scattered field pattern