Fig. 1. (a) Schematic of the MPA on the GaAs substrate; (b) simulated transmission (T), reflection (R), and absorption (A) spectra; (c) microscope image of the ESRR array of the MPA (inset: close-up view of a unit cell); (d) simulated time domain field strength of the incident THz pulse (10×) and the electric field in the middle of the capacitive gap [inset: the 2D map of the electric field enhancement factor (fE)].

Fig. 2. Simulated time-domain response of (a) the MPA and (b) the reference demonstrates the multi-reflection effect of the GaAs substrate. The second pulse (gray shaded area) is the internal absorption response that is considered in our analysis.

Fig. 3. (a) Measured reflection spectra of the nonlinear MPA under different THz field strengths, (b) calculated nonlinear absorbance spectra, (c) the resonance frequency versus the incident THz peak field, (d) the absorbance at 0.68 THz versus the incident THz peak field.

Fig. 4. (a) Simulated spatial distribution of the field enhancement across the gap at depths of 0.1 and 0.5 μm (inset: the cross section of the MPA), (b) simulated reflection spectra with different carrier densities and mobilities.

Fig. 5. (a) Illustration of the flexible MPA, (b) the fabricated MPA wrapped on a plastic vein [top inset: pristine unit cell; bottom inset: the unit cell damaged by the high field (the white circle highlights the damaged area)], (c) the field enhancement spectra of the flexible and solid MPAs, (d) measured absorbance at different field strengths.