Controlling Casimir force using optical metamaterials

It has been well accepted that a gecko’s ability to stick to walls or ceilings is due to the Van de Waals force between their feet and the surface. Van de Waals force, or Casimir force in a more general sense, is an attractive force due to the electric dipole interactions between two electrically neutral surfaces. Interestingly, the attractive Casimir force between gecko’s feet and the surface has a quantum origin, as it arises from the quantum fluctuations of the zero energy states – or vacuum states. While the attractive Casimir force can be useful in some cases, it could lead to problems in other situations. For example, the ongoing effort in miniaturisation of Micro-Electro-Mechanical Systems (MEMS) devices encounters serious stiction issues due to the Casimir force.

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Foot of a Tokay Gecko, showing adhesive pads (source Wikimedia Commons).

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The SEM image of a terahertz metamaterial exhibiting extremely strong chirality.

Thus, it is quite important to be able to control Casimir force. It has recently been proposed that Casimir force may be manipulated by engineering the texture or structure of surfaces. In particular, there has been growing interest to reduce or even reverse Casimir force to make it repulsive – something called quantum levitation, by using metamaterials (artificially engineered photonic materials exhibiting unnatural optical properties and functionalities), there have been debates regarding whether metamaterials can lead to repulsive Casimir force. However, very few experiments have been done.

This project aims to investigate experimentally the impact of metamaterials on Casimir force between two flat surfaces, by combinng large-scale nanofabrication techniques for the manufacturing of optical metamaterials, and precise measurement of the Casimir force between two large-area nano-structured flat surfaces at separations from several to tens of micrometres. Theory has predicted that a significant reduction in Casimir force can be achieved by introducing strong chirality. Strong chirality does not exist in natural materials, but it can be obtained with metamaterials. One example is the chiral negative index metamaterials at the terahertz frequencies that we have developed recently. The successful accomplishment of this project will dramatically enhance our capability to control Casimir force, and will serve as a benchmark for the theories on Casimir force between nano-structured metamaterial surfaces.

Dr Shuang Zhang
University of Birmingham

Shuang was awarded a Research Project Grant in December 2012.