Abstract
We use computational approaches to explore the role of a high-refractive-index dielectric TiO2 grating with deep subwavelength thickness on InSb as a tunable coupler for THz surface plasmons. We find a series of resonances as the grating couples a normally-incident THz wave to standing surface plasmon waves on both thin and thick InSb layers. In a marked contrast with previously-explored metallic gratings, we observe the emergence of a much stronger additional resonance. The mechanism of this giant plasmonic resonance is well interpreted by the dispersion of surface plasmon excited in the air\\TiO2\\InSb trilayer system. We demonstrate that both the frequency and the intensity of the giant resonance can be tuned by varying dielectric grating parameters, providing more flexible tunability than metallic gratings. The phase and amplitude of the normally-incident THz wave are spatially modulated by the dielectric grating to optimize the surface plasmon excitation. The giant surface plasmon resonance gives rise to strong enhancement of the electric field above the grating structure, which can be useful in sensing and spectroscopy applications.
Highlights
Terahertz (THz) frequency range of electromagnetic spectrum has attracted significant fundamental research interest and technological development due to various potential applications, such as spectroscopy[1, 2], bio sensing[3, 4], high speed communication[5], and subwavelength imaging[6, 7]
We explore the role of a dielectric grating as a tunable coupler between a normally-incident free-space wave and THz surface plasmon polaritons (SPPs) on thin and thick InSb layers
These two resonance modes correspond to the odd and even SPP modes on air/InSb/air trilayers and we have studied them in detail in a previous article[17]
Summary
Terahertz (THz) frequency range of electromagnetic spectrum has attracted significant fundamental research interest and technological development due to various potential applications, such as spectroscopy[1, 2], bio sensing[3, 4], high speed communication[5], and subwavelength imaging[6, 7]. We explore the role of a dielectric grating as a tunable coupler between a normally-incident free-space wave and THz SPPs on thin and thick InSb layers. We established micrometer-thin InSb layers with metallic gratings as a platform for THz plasmonic devices; the metal grating couples the free-space wave to SPPs17. A dielectric grating allows spatial modulation of both the phase and amplitude of the incident THz wave to manipulate the excitation of SPPs. By varying the thickness, period, and refractive index of the grating, we are able to control both the strength and frequency of the SPP resonance in THz transmission and reflection of our structure. The dielectric grating structure exhibits strong surface electric field enhancement typical of SPPs. Figure 1 shows the geometric configuration of our THz plasmonic structure, which consists of a grating on InSb layer. The geometric parameters of this structure allow fabrication and processing using established photolithography and microfabrication methods
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