EIT produces some interesting effects like dramatically slowed-down light pulses. Indeed, the effect has already been used to slow down pulses of light so they can effectively be "stored" in a medium – for example, in an ultracold cloud of atoms. The phenomenon requires the atoms to have a specific configuration of three energy levels in which transitions between one specific pair of levels are forbidden.

EIT comes about from the coupling of a dark (forbidden) and a bright (allowed) state. As an analogy, metamaterials can also be thought of as artificial atoms with states that have different characteristics like these. Until now, scientists engineered metamaterials to mimic the single dark and bright states of EIT media. But now, Hatice Altug and colleagues Alp Artar and Ahmet Ali Yanik have come up with a new strategy: by tailoring the interactions in a 3D metamaterial, the researchers show that it is possible to achieve even higher numbers of states that provide EIT-like effects over multiple frequency windows.

"It is important to note that our approach does not just involve merging uncoupled fundamental units but forming a homogeneous medium consisting of complex artificial atoms that provide coupled multiple resonance states," explained Altug.

The number of EIT-like spectral windows can be increased by simply stacking more metamaterial layers onto the existing structure.

Lift-off free
The new technique employs free-standing silicon nitride membranes as a building block to make 3D structures layered horizontally. Metal is then deposited onto both the top and bottom surfaces of the ensemble. This fabrication scheme does not require any lift-off processes, allowing for better structures with sharper edges. More layers can be added using additional dielectric and metal depositions atop the existing three-layer medium.

The new structure could simultaneously slow down multiple light pulses of different carrier frequencies, Artar told nanotechweb.org. "In addition, non-linear phenomena could be enhanced in such materials through increased interaction times between the pulses – a tool that could be important for realizing multicolour nonlinear interactions, such as all-optical switching at very low power levels."

Typical EIT designs are not usually scalable but the new fabrication technique allows the researchers to easily increase the number of layers in their structure. "Such scalability is important for slow-light experiments since the medium's volume should be large enough to accommodate the whole incident light pulse," said Yanik.

The Boston University team's approach makes use of multiple layers of the same fundamental unit cell but the researchers now plan to investigate more composite, asymmetrical designs. "We also hope to thoroughly analyse the spatial properties of these designs as well as their spectral response," added Artar.

The work was detailed in Nano Letters.