Asymmetric split ring resonators for organic sensing

We have previously reported the use of asymmetric split ring resonators for sensing organic material in the infra red [1]. Arrays of circular split ring resonators can be tuned to particular wavelengths in the mid infra red. By producing asymmetry in the structures, the two sides of the ring produce plasmon resonances that are reflection peaks and a trapped mode corresponding to a reflection minimum. The combination leads to a steep slope between the reflection peaks and can increase the sensitivity to the addition of different molecular species. In addition to the shift induced, geometrical tuning allows matching of the ASSRs to molecular resonances. Thin films of, for example, PMMA shift the resonances by several hundred nanometres. Further enhancements to sensitivity are to be expected from localization of the organic material.