Plasmonics

The absorption of light by a molecule depends on the intensity of the local electric field. Therefore, in order to increase their sensitivity, infrared (IR) sensors can be engineered to display surface enhancement of the electromagnetic field in close proximity to the molecule. In this frame, plasmonics is the most promising approach to achieve sub-wavelength concentration of optical fields. This is obtained thanks to resonant coupling between electromagnetic waves and collective oscillations of free electrons. The term “optical antennas” is used in this context to address those nanostructures that, in analogy with their radio-frequency counterparts, are specifically designed to convert propagating radiation to intense localized energy, and vice versa.

A new generation of materials for surface-enhanced mid-infrared spectroscopy (SEIRA) is under development in the last years: there the metal is substituted by highly-doped semiconductors, graphene or conducting oxides. Electron-doped germanium thin films epitaxially grown on silicon wafers feature excellent crystal quality providing superior optical and mechanical properties, possibility for quantum design of heterostructures [3] and full compatibility with microelectronic integrated circuit fabrication.

In the past years we have focused on highly electron doped Germanium on Silicon as a platform for obtaining CMOS-compatible on-chip sensors. We have in particular demonstrated up to 2 orders of magnitude of field enhancement form Ge nanoantennas on Si,  benchmarked the use of highly doped Ge against Au for plasmonic and sensing in the mid-IR, and fabricated scanning-probe three-dimensional nGe antennas, by modifying AFM scanning probes are manufactured by our group in collaboration with Politecnico di Milano, Institute for Photonics and Nanotechnology and the Molecular Foundry of the Lawrence Berkeley National Labs, California.

More details on our work can be found in the FP7 GEMINI FET project website (now ended). 

We have used metal patch antennas to demonstrate strong coupling between the antenna modes and intersubband transitions in a single quantum well, and within a collaborative PRIN project with IFN-CNR and PoliMi we have devised plasmonic structures able to enhance enantiospecific interactions between light and chiral molecules.