Atomically Smooth Epitaxial Metallic Films on Silicon as a Platform for Plasmonics

Angle-resolved transmission spectra for a  perforated epitaxial Ag films on Si.

The ability to create atomically smooth metallic films on semiconductor surfaces allows us to venture into another current topics of condesed matter physics:  plasmonics
Surface plasmon polaritons (SPPs) are spatially-confined electromagnetic waves propagating along a metal/dielectric interface.

Because of their ability to carry optical signals at subwavelength scales, SPPs have been proposed as a solution to the important problem of integrating nanoscale electronics with wavelength-scale photonics. However, despite an enormous amount of fascinating plasmonics research, technological applications critically depend on increasing SPP propagation length and on integrating plasmonic structures with electronic components that are mostly Si-based.

The Shih Group and its collaborators have recently demonstrated that atomically smooth, epitaxial Ag films on Si support extremely low-damped SPPs on both sides of the metal film. Using angle-resolved infrared spectroscopy, we have probed the coherent propagation of long-range SPPs by employing the phenomenon of extraordinary optical transmission (EOT), and have discovered detailed SPP band structures (see figure).  By comparing EOT data from epitaxial and conventional polycrystalline films, we have revealed the dramatic advantages of the former platform for plasmonics studies and technological application.

Shown in Fig. (a) are experimentally measured angle-dependent mid-infrared transmission spectra of air/Ag/Si under TM-polarized illumination for a perforated epitaxial Ag film.  Fig. (b) shows a theoretical simulation of the same transimission spectra, carried out by Professor Gennady Shvet’s group.  In analogy to the electronic band structures in a periodic lattice, here the SPPs undergo coherent Bragg scattering due to the periodic hole array, resulting in rich plasmonic band structures.  The most signficant result is that high-Q SPP propagation is observed both at the Ag/Si interface and at the Ag/Air interface.  These results open a new door toward monolithic integration of plasmonics and electronics on the same platform, using atomically smooth epitaxial Ag films on Si.