Abstract
Highly confined modes in THz plasmonic resonators comprising two metallic elements can enhance light-matter interaction for efficient THz optoelectronic devices. We demonstrate that sub-surface modes in such double-metal resonators can be revealed with an aperture-type near-field probe and THz time-domain spectroscopy despite strong mode confinement in the dielectric spacer. The sub-surface modes couple a fraction of their energy to the resonator surface via surface waves, which we detected with the near-field probe. We investigated two resonator geometries: a λ/2 double-metal patch antenna with a 2 μm thick dielectric spacer, and a three-dimensional meta-atom resonator. THz time-domain spectroscopy analysis of the fields at the resonator surface displays spectral signatures of sub-surface modes. Investigations of strong light-matter coupling in resonators with sub-surface modes therefore can be assisted by the aperture-type THz near-field probes. Furthermore, near-field interaction of the probe with the resonator enables tuning of the resonance frequency for the spacer mode in the antenna geometry from 1.6 to 1.9 THz (~15%).
Highlights
Optoelectronic devices rely on efficient coupling between light and electronic systems, such as semiconductors quantum wells or quantum dots [1]
The fundamental mode is expected at a frequency f1=1.63 THz. This mode was numerically simulated with COMSOL finitedifference time domain (FDTD) solver, and the corresponding electric field distribution in the spacer is shown in Fig. 1(c): the field maxima are located at the edges of the resonator and a node is in the center
We investigated the use of aperture-type THz near-field microscopy for local spectroscopy of double-metal THz resonators in two geometries: the classical double-metal patch antenna with a 2 μm spacer layer and the recently proposed subwavelength-size three-dimensional meta-atom resonator [18,19]
Summary
Optoelectronic devices rely on efficient coupling between light and electronic systems, such as semiconductors quantum wells or quantum dots [1]. The scattering probe tip must be positioned within ~50nm from the surface, creating strong field perturbation to the incident wave, whereas the aperture-type THz nearfield probes operate at variable distances from the surface These probes have already been applied for spectroscopic analysis of modes in plasmonic and dielectric sub-wavelength size THz resonators [25,26,29,30]. The second resonator type considered here is a three-dimensional meta-atom structure consisting of a split-ring resonator and a sub-surface capacitive metallic patch separated by a dielectric spacer layer This structure, named a horse-shoe resonator (HSR) for its shape, was recently proposed for achieving strong electric field confinement [18]. This study validates the strength of the aperture-type THz near-field microscopy for probing the interaction of THz waves with photonic systems, which provide highly subwavelength electromagnetic confinement
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