Aspect-ratio driven evolution of high-order resonant modes and near-field distributions in localized surface phonon polariton nanostructures.

Chase T Ellis,Joshua D Caldwell,Francisco J Bezares,Orest J Glembocki,Dmitry N Chigrin,Joseph G Tischler,Loretta Shirey,Jeffrey C Owrutsky,Richard Kasica,Alexander J Giles

doi:10.1038/srep32959

Abstract

Polar dielectrics have garnered much attention as an alternative to plasmonic metals in the mid- to long-wave infrared spectral regime due to their low optical losses. As such, nanoscale resonators composed of these materials demonstrate figures of merit beyond those achievable in plasmonic equivalents. However, until now, only low-order, phonon-mediated, localized polariton resonances, known as surface phonon polaritons (SPhPs), have been observed in polar dielectric optical resonators. In the present work, we investigate the excitation of 16 distinct high-order, multipolar, localized surface phonon polariton resonances that are optically excited in rectangular pillars etched into a semi-insulating silicon carbide substrate. By elongating a single pillar axis we are able to significantly modify the far- and near-field properties of localized SPhP resonances, opening the door to realizing narrow-band infrared sources with tailored radiation patterns. Such control of the near-field behavior of resonances can also impact surface enhanced infrared optical sensing, which is mediated by polarization selection rules, as well as the morphology and strength of resonator hot spots. Furthermore, through the careful choice of polar dielectric material, these results can also serve as the guiding principles for the generalized design of optical devices that operate from the mid- to far-infrared.

Highlights

Infrared spectral range bounded by the longitudinal (LO) and transverse (TO) optic phonon frequencies, known as the Reststrahlen band[20,21,22,23]
The influence of the pillars upon the resonant spectra is clearly observed in Fig. 2a,b, which show the results of polarized FTIR reflectance measurements performed on un-patterned 4H-SiC and the aspect ratio (AR) = 4 pillar array, with a 22° weighted average angle of incidence and polarization nominally oriented parallel (R||) and perpendicular (R⊥) to the length of the pillars, respectively
The existence of coupling effects between the P1 phonon and localized surface phonon polaritons (SPhPs) resonances is further corroborated by the AR dependence of reflectance measurements that are shown in Figure S2 of the Supporting Information, where the P1 mode clearly strengthens as the localized SPhP resonances spectrally approach and cross through this phonon mode, resulting in a Fano-like interference during the crossing

Summary

Introduction

Infrared spectral range bounded by the longitudinal (LO) and transverse (TO) optic phonon frequencies (ωLO and ωTO), known as the Reststrahlen band[20,21,22,23]. We experimentally and theoretically explore the far- and near-field behavior and aspect-ratio (shape) driven evolution of both transverse and longitudinal, high-order localized SPhP resonances that occur in arrays of three-dimensional rectangular pillars These structures are etched into semi-insulating, 4H silicon carbide substrates, similar to efforts previously reported from our group[21,41]. Despite the degree of control over both the resonance energies and near-field profiles that varying AR can provide in three-dimensional rectangular structures, such an investigation has remained elusive in literature, even for metal-based SPP systems This is largely due to the difficulty in producing three-dimensional, rectangular plasmonic nanoparticles with well-controlled dimensions, a high degree of monodispersity, and low optical losses, where the latter two properties significantly broaden any resonances, resulting in strongly overlapping modes that are difficult to separate and/or distinguish. By selecting a single polarization orientation for each measurement, we are able to effectively isolate localized SPhP resonances that are excited along the length and width of the pillar, since, in general, the incoming light couples to resonant modes that have a net dipole moment component that is oriented parallel to the incoming polarization

Methods

Results

Conclusion