Large near-to-far field spectral shifts for terahertz resonances

Abstract

We have performed far-field extinction measurements and near electric field measurements on gold bowtie antennas resonant at THz frequencies. These measurements show a very large shift between the resonant frequencies of the near-field and the far-field spectra. We use the established damped-driven harmonic oscillator model for resonators to model the far-field response of the antennas from the near-field spectrum and show that there is a large discrepancy between the predicted and measured far-field response. We were able to explain this discrepancy by improving the oscillator model with a Fano model. This large shift makes the prediction of the near-field response of resonant structures at THz frequencies very imprecise, provided that only information of the far-field response is available and establishes the necessity of measuring near fields for a correct and accurate characterization of these structures.

Terahertz diffraction enhanced transparency probed in the near field

“…Near-field THz microscopy has seen tremendous progress over recent years towards very high spatial resolutions [21][22][23][24], and has previously been employed in many successful implementations to explore near-field dynamics in both resonant structures [18,25,26] and apertures [27][28][29][30][31]. In the work of Bitzer et al [25], lattice modes were investigated for the case of split ring resonators (SRR), where the array was shown to hybridize lattice modes with magnetic dipole resonances.…”

“…The transmission measurements are referenced to a quartz substrate. The THz near-field microscope used in these measurements has been described previously [18], where THz microprobes [19] (Protemics GmbH, Germany) are used to measure the near field at a distance of 0.5 μm above the sample. The THz microprobes used in these experiments allow isolation of a specific polarization of the electric near field.…”

Visualizing near-field coupling in terahertz dolmens

“…[22][23][24] Over recent years, a number of innovative advances have emerged in near-field THz microscopy resulting in diverse implementations, ranging from techniques based on sharp tips 25,26 or electron pulses 27 for ultrahigh spatial resolution, to others harnessing electro-optic sampling in nonlinear crystals, 28,29 to schemes based on miniaturized photoconductive probes. 13,30,31 We employ the latter approach to demonstrate the hybridization of modes that follows the near-field coupling between two resonant structures, as well as the excitation of the dark resonance which is responsible for the transparency window in the far-field.…”

“…10,11 Near-field coupling presents the benefit of allowing further manipulation of the scatterers spectral behaviour, but demands near-field microscopy for full device characterization since the link from near-field properties to far-fields is not always direct. 12,13 In this manuscript, we present a study on a dolmen structure, consisting of a horizontal rod and two orthogonal vertical rods, where the near-field coupling leads to electromagnetically induced transparency (EIT). 14,15 The horizontal rod is oriented along the polarization of the incident field and supports a bright mode, while the vertical rods support a dark mode.…”

“…1(c), we probe an isolated dolmen in a nearfield microscope. Here a separate, previously described 13,38 experimental setup is used, based on a THz near-field probe (TD-800-Z-WT, Protemics GmbH, Germany) with a lateral spatial resolution in the order of 10 lm. 30 The electric field E z t ð Þ is measured across the sample by raster-scanning the near-field probe at a height of 1 lm above the sample.…”