Role of epsilon-near-zero substrates in the optical response of plasmonic antennas

Jongbum Kim,Gururaj V Naik,Joseph G Tischler,Joshua D Caldwell,Chase T Ellis,Francisco J Bezares,Alexander J Giles,Alexandra Boltasseva,Humeyra Caglayan,Nader Engheta,Alexander V Kildishev,Ahmed M Mahmoud,Aveek Dutta,Orest J Glembocki

doi:10.1364/optica.3.000339

Abstract

Radiation patterns and the resonance wavelength of a plasmonic antenna are significantly influenced by its local environment, particularly its substrate. Here, we experimentally explore the role of dispersive substrates, such as aluminum- or gallium-doped zinc oxide in the near infrared and 4H-silicon carbide in the mid-infrared, upon Au plasmonic antennas, extending from dielectric to metal-like regimes, crossing through epsilon-near-zero (ENZ) conditions. We demonstrate that the vanishing index of refraction within this transition induces a “slowing down” of the rate of spectral shift for the antenna resonance frequency, resulting in an eventual “pinning” of the resonance near the ENZ frequency. This condition corresponds to a strong backward emission with near-constant phase. By comparing heavily doped semiconductors and undoped, polar dielectric substrates with ENZ conditions in the near- and mid-infrared, respectively, we also demonstrate the generality of the phenomenon using both surface plasmon and phonon polaritons, respectively. Furthermore, we also show that the redirected antenna radiation induces a Fano-like interference and an apparent stimulation of optic phonons within SiC.

Highlights

The field of nanophotonics was initiated with the predominant focus on plasmonic effects in metals, where due to the negative real part of the dielectric function, electromagnetic fields can be confined to subdiffractional dimensions
We report on the IR-active longitudinal optic (LO) phonon modes of 4H-SiC demonstrating a Fano-like interference with the antenna resonance that is not observed near the transverse optic (TO) phonon nor within the plasmonic response of the transparent conducting oxides (TCOs)
As discussed in the previous section, polar dielectric crystals offer natural materials that exhibit ENZ behavior near the LO phonon as well as ENP behavior near the TO phonon. This behavior is found within the Reststrahlen band of a polar dielectric or semiconductor material that occurs between its LO and TO phonons

Summary

INTRODUCTION

The field of nanophotonics was initiated with the predominant focus on plasmonic effects in metals, where due to the negative real part of the dielectric function, electromagnetic fields can be confined to subdiffractional dimensions. As demonstrated in several experimental studies [28,29,30,31,32,33,34], the ENZ response is observed near the plasma frequency of highly doped semiconductors, as well as near the longitudinal optic (LO) phonon frequency of polar dielectrics [18] Conventional plasmonic materials such as Au and Ag exhibit an ENZ point within the ultraviolet range [18,35,36]; optical losses Im ε for these materials are too high to be useful for ENZrelated studies or applications. Through the patterning of periodic arrays of gold antennas upon these materials, we demonstrate the “pinning” of the antenna resonance frequency as a result of the vanishing index of refraction (reduced real permittivity) of the substrate during the spectral approach to the ENZ condition and observe a drastic modification of the associated radiation pattern. We report on the IR-active LO phonon modes of 4H-SiC demonstrating a Fano-like interference with the antenna resonance that is not observed near the TO phonon nor within the plasmonic response of the TCOs

THEORETICAL STUDY

EXPERIMENT RESULTS

CONCLUSION