Fig. 1. Schematics of two gap structures used in SRRs: (a) 1D slot-like gap; (b) 2D interdigitated gap structure. The areas plotted in pink color represent the regions with high electric field. If G=g and if the same voltages are applied, the electric fields in the gap regions in (a) and (b) are of similar magnitude.

Fig. 2. Proposed interdigitated electric split-ring-resonator (ID-eSRR) MMs for THz sensing applications. In the unit cell structure, n is the size of the square ID-eSRR, h the width of the metal stripes, G the gap width, l the finger length, w the finger width, and g the width of the minigap between adjacent fingers.

Fig. 3. (a) Transmittance spectra of the bare ID-eSRR and eSRR MMs. (b) Refractive index sensing: frequency shift as a function of analyte thickness (d). Inset shows the location of the analyte in all simulations. The analyte material is SiO2 (ϵr=3.75+j·0.0004).

Fig. 4. Two top rows of panels: Electric field distribution in the plane of the metal surface at the respective resonance frequency for the three structures: (a) ID-eSRR; (b) eSRR (G=0.6  μm); and (c) eSRR (G=12  μm). Bottom row of panels: Cuts through the 3D electric field distribution along the red dashed lines shown in the middle row of panels. The amplitude of the incident electric field is 8.09×104  V/m for all structures.

Fig. 5. (a) SEM image of an ID-eSRR structure fabricated by electron beam lithography. (b) Simulated transmittance spectra of the ID-eSRR MM (G=12  μm; w=g=0.6  μm) and the eSRR MM (G=12  μm) without analyte. (c) Measured transmittance spectra of the ID-eSRR and eSRR MMs without analyte.

Fig. 6. (a) Measured transmittance spectra of the ID-eSRR structure loaded with a SiO2 layer of varying thickness (from 23 to 150 nm). (b) Likewise, but for the eSRR structure (SiO2 layer thickness varying from 20 to 155 nm). (c) Simulated and measured resonance frequency shift in dependence of the SiO2 layer thickness.

Fig. 7. Radiation power flow on a sphere with a radius of 1 m for a single resonator in air: (a) ID-eSRR; (b) eSRR (G=0.6  μm); and (c) eSRR (G=12  μm).

Fig. 8. 2D projection of the radiation power flow of a single resonator in air: (a) x-y plane; (b) y-z plane; and (c) x-z plane.

Fig. 9. Electric field enhancement factor (Emax/Ein) along the z axis around the metal corner where the electric field is maximal at resonance frequencies for the three structures: z<0 represents the substrate side; and z>0 represents the air side.

Fig. 10. Influences of minigap width (g) on the transmittance spectra and resonance frequencies. (a) Transmittance spectra with the variation of g from 0.2 to 1.2 μm, while the finger width kept constant (w=0.6  μm). (b) Resonance frequency and frequency shift when 100 nm SiO2 is deposited on the unit cell structure.

Fig. 11. Transmittance spectra for the ID-eSRR MMs sensor at different incident angle of THz radiation.

Fig. 12. Transmittance spectra for the ID-eSRR MMs sensor with the variation of analyte (SiO2) thickness from 0 (no analyte) to 20 μm.