  In this study, a novel electric field compressor is proposed by doping a
metal-air-metal pillar in epsilon-near-zero medium within a metallic waveguide,
which effectively enhances the background electric fields in a sub-wavelength
air pillar with high uniformity. The field enhancement factor can be
analytically determined through theoretical derivations from Maxwell's
equations, reaching its maximum value by appropriately selecting the size of
the air pillar. Furthermore, the proposed compressor can achieve a tunable
electric field enhancement effect within the deep sub-wavelength air pillar by
adjusting the height of the air pillar through movement of two metal pillars
inserted into the waveguide. Both theoretical analysis and numerical
simulations are employed to validate the performance of the electric field
compressor, which exhibits a wide range of tunable field enhancement effect
(i.e., continuously adjustable enhancement factor between 20 and 800) with
uniformity below 1.5 within the deep sub-wavelength air pillar (wherein the air
volume is smaller than la0/10 * la0/10 * la0/16). Finally, a practical model is
proposed for the realization of the electric field compressor in the microwave
range, where specially placed metal pillars and wires are incorporated into a
rectangular metallic waveguide structure. The field enhancement and tunable
effects of this practical model have been verified through simulations.
