Metamaterials for biosensing

The "pi" structure (shown in the figure below) has two main resonances. A dipole and a quadrupole resonances. As the pi structure has no mirror symmetry in xz or yz plane, the quadrupole mode and the dipole mode are no longer orthogonal.

Which means that driving one drives the other. That gives us a Fano resonant metamaterial. The Fano resonant spectrum is the blue curve in the figure above. This can be very useful because unlike a "normal" (Lorentzian) resonance, peaks in the reflectivity curve do not correspond to the actual resonances, especially not for the narrower of the two peaks. There the Fano resonance is where the slope is very high (shown by the black dotted vertical line in the plot.) This means that slight movement of the curve results in significant changes in the value of reflectivity. This slight change can come from a slight change in the environment. That means that these metamaterials are very good as sensors. We used them as sensors to detect minute amounts of protein. The results are shown below.

As you can see, the relative change of reflectivity is quite significant (~15%) even though we only have a monolayer of protein. This is very good news for, for example, disease detection.

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