Terahertz Chiral SSPPs Mode on the Helically Grooved Metal Wire

Abstract

Terahertz wave (λ = 30 μm–3 mm@f = 0.1–10 THz) is such a band of electromagnetic wave that locates between light and microwave. Many chiral biomolecules, such as protein and amino acid, have their own unique structure vibrating frequencies at Terahertz region, therefore they have the “fingerprint” terahertz absorption spectrum. It is meaningful to study the chirality of chiral macromolecules through their terahertz circular dichroism (CD) response which is defined as the differential absorption of two opposite chiral terahertz wave. In the present work, we used the Finite-element-method (FEM) to investigate terahertz spoof surface plasmon polariton (SSPP) wave propagating on the helically grooved metal wire. Different from the metal wire decorated by the periodic grooves the helically grooved metal wire can support a kind of chiral SSPP surface mode. Due to the chirality of helical grooves, the normal degenerate HE1 SSPP mode on helically grooved metal wire decompose into two opposite chiral modes, i.e., HE−1 and HE+1 SSPP surface modes. Mode analysis showed that the dispersion curves of HE+1 mode deviated from that of HE−1 modes, and thus form a broad frequency band where the chiral HE+1 mode could exists exclusively. Super-chiral terahertz field concentration that may be achieved on the helically grooved metal wire through chiral HE+1 mode can find important applications in near-field circular dichroism spectroscopy in terahertz frequencies.