Artificial structuring of materials can lead to enhanced and even novel optical properties. Materials that are structured on the sub-wavelength scale, so-called metamaterials, can be engineered to exhibit properties such as a negative, zero or very large refractive index and to control amplitude and phase of light. Importantly, the optical properties of metamaterials are very sensitive to the geometry of the nanostructure. Therefore mechanical rearrangement of the metamaterial’s nanoscale building blocks allows the metamaterial’s optical properties to be changed.

The simplest realization of such a mechanically reconfigurable metamaterial is an array of nanowires that may be actuated by thermal, electric or magnetic forces. The main challenge is to achieve random access, i.e. independent selective actuation of each individual nanowire. Pablo Cencillo-Abad and co-authors from the Nanophotonics and Metamaterials Group at Southampton’s Optoelectronics Research Centre created an array of nanowires of about 18 micron length and only 100 nm thickness by focused ion beam milling on a gold-coated silicon nitride membrane. Separate electrical contacts allow a different electrical current to be applied to each individual nanowire. The current heats the nanowire, which then bends due to different rates of thermal expansion of gold and silicon nitride. Chip integration and computer control provide a convenient solution for addressing any chosen nanowire or nanowire combination by different distances of up to 100 nm.

The spatial resolution of the metadevice is given by the nanowire spacing of 620 nm, providing an opportunity to control its optical properties with sub-wavelength resolution, and thus without unwanted diffracted beams, in the red to infrared part of the spectrum. Thus, it offers more than an order of magnitude higher resolution than established spatial light modulation technologies based on digital micromirrors or liquid crystal cells that have a characteristic size of about 10 microns. The one-dimensional nanowire metadevice design could be used to provide diffraction gratings and cylindrical lenses with dynamic focusing and beam steering on demand. Challenges include extending the concept to two and ultimately three dimensions to provide arbitrary dynamic control over electromagnetic space.

The research is reported in Nanotechnology.