Light-amplifying resin boosts invisibility cloaks - New Scientist - New Scientist

Light-amplifying resin boosts invisibility cloaks: The team realised that they could compensate for those severe losses by replacing the aluminium oxide with a "gain" material – resin doped with a dye called rhodamine 800. When hit by an infrared laser pulse, the dye's electrons are excited and respond to visible light passing through the metamaterial by creating more photons – a process called stimulated emission…

"We can't use too much of the gain material because it has a positive refractive index which could 'kill' the negative index of the original sample," says Shalaev. That would mean losing the very light-bending properties they are seeking to preserve and improve.

Fortunately, because the production process involves adding the dye between the metal layers, where the electromagnetic fields are stronger than in free space, the gain process is roughly 50 times more efficient and so less gain material is required, he says.