Relative merits of Phononics vs. Plasmonics: the energy balance approach

TL;DR

This paper compares phononic and plasmonic systems, highlighting that phononics offers narrower resonances and better energy confinement but less electric field enhancement, making it suitable for frequency-selective applications.

Contribution

It provides a nuanced analysis of the energy balance between phononics and plasmonics, emphasizing the advantages and limitations of each in optical confinement.

Findings

Phononic devices have narrower resonances than plasmonic ones.

Most energy in phononics is stored in lattice vibrations, limiting electric field enhancement.

Phononics is promising for frequency-selective applications.

Abstract

The common feature of various plasmonic schemes is their ability to confine optical fields of surface plasmon polaritons (SPPs) into sub-wavelength volumes and thus achieve a large enhancement of linear and nonlinear optical properties. This ability, however, is severely limited by the large ohmic loss inherent to even the best of metals. However, in the mid and far infrared ranges of the spectrum there exists a viable alternative to metals, polar dielectrics and semiconductors in which dielectric permittivity (the real part) turns negative in the Reststrahlen region. This feature engenders the so-called surface phonon polaritons (SPhPs) capable of confining the field in a way akin to their plasmonic analogues, the SPPs. Since the damping rate of polar phonons is substantially less than that of free electrons, it is not unreasonable to expect that phononic devices may outperform their plasmonic counterparts. Yet a more rigorous analysis of the comparative merits of phononics and plasmonics reveals a more nuanced answer, namely that while phononic schemes do exhibit narrower resonances and can achieve a very high degree of energy concentration, most of the energy is contained in the form of lattice vibrations so that enhancement of the electric field, and hence the Purcell factor, is rather small compared to what can be achieved with metal nanoantennas. Still, the sheer narrowness of phononic resonances is expected to make phononics viable in applications where frequency selectivity is important.