Fabrication process may enable creation of invisible materials

A team from Harvard University has developed a fabrication process that could be used to make materials with invisibility applications.

According to a statement, the team from Harvard’s School of Engineering and Applied Sciences has cleared an important hurdle in the development of advanced materials that bend light in unconventional ways, known as metamaterials.

The group used extremely short and powerful laser pulses to create three-dimensional patterns of tiny silver dots within a material that are considered essential to making futuristic optical devices that appear to be invisible.

The fabrication process, described in the journal Applied Physics Letters, advances nanoscale metal lithography into three dimensions — and does it at a resolution high enough to be practical for metamaterials.

‘If you want a bulk metamaterial for visible and infrared light, you need to embed particles of silver or gold inside a dielectric and you need to do it in 3D with high resolution,’ said lead author Kevin Vora, a graduate student at the Harvard School of Engineering and Applied Sciences.

‘This work demonstrates that we can create silver dots that are disconnected in X, Y and Z,’ said Vora. ‘There’s no other technique that feasibly allows you to do that. Being able to make patterns of nanostructures in 3D is a very big step towards the goal of making bulk metamaterials.’

The team used a femtosecond laser to investigate how very tightly focused, powerful bursts of light can change the electrical, optical, and physical properties of a material.

When a conventional laser shines on a transparent material, the light passes straight through, with slight refraction. The femtosecond laser is special because it emits a burst of photons as bright as the surface of the sun in a flash lasting only 50 quadrillionths of a second. Instead of shining through the material, the energy gets trapped within it, exciting the electrons within the material and achieving a phenomenon known as nonlinear absorption.

Inside the pocket where that energy is trapped, a chemical reaction takes place, permanently altering the internal structure of the material.

In the process, the team combined silver nitrate, water, and a polymer called PVP into a solution, which was baked onto a glass slide. The solid polymer then contains ions of silver, which are photoreduced by the tightly focused laser pulses to form nanocrystals of silver metal, supported by the polymer matrix.

The work was supported by the US Air Force Office of Scientific Research.