Multistable elastic pixels (MEPs) for reversible programming of optical properties

Two-dimensional materials patterned with colloidal pixels that can be actuated by external forces (such as electromagnetic fields) for stable switching of optical properties.

Problem:
Current reversible metasurfaces either cannot be programmed for specific functionalities or are difficult to control. Ideal metamaterials comprising discrete, patterned units have found applications in altering incident electromagnetic (EM), thermal, or acoustic waves. These reversible metamaterials are used as optical filters and light polarizers. Ideal metamaterials should stably alter wave properties and provide for reversibility. However, current technologies do not provide for programmability or easy manipulation to achieve desired effects.

Solution:
The Penn developed technology is a 2-D material embedded with colloidal suspensions that can be physically moved between two or more stable states by an external field. The optical properties of these colloidal multi-elastic pixels (MEPs) persist once the energy source is removed, reducing energy consumption. Unlike current methods, MEPs can be patterned in diverse configurations, allowing for multiple applications including tunable windows and displays.

Technology:
This metamaterial relies on MEPs which are composed of conductive colloids confined in nematic liquid crystals (NLC). Each MEP is designed with unique surface features, and colloids are physically moved between these features by an external EM field. The colloids remain locked in these stable positions even after the applied energy field is removed. The switching times of the MEPs can be tuned by adjusting the viscosity of the NLC system or by changing the surface shape of NLCs.

(a) Multistable elastic pixels are comprised of a colloidal particle confined in nematic liquid crystals. The system has two configurations, and application of an external EM field shifts colloid positions between these two configurations. (b) An ordered array of MEPs (red squares) and fixed colloids (blue squares) provides a mechanism to change the optical properties of metasurfaces. (c) MEP-based metasurfaces can be reversibly induced to alter incident light. A simple EM field is used to toggle behavior of the metamaterial.

Advantages: