Active plasmonic devices have recently emerged as plasmonic/nanophotonic technologies with tunable optical characteristics. However, electro-optic effects in plasmonic metals are weaker than their photonic counterpart since plasmonic materials such as Au and Ag have extremely high electron densities, resulting in the effective screening of the externally applied electrical fields. In this thesis, to overcome these limitations, I introduce electric field tunable loading of plasmonic nanostructures and demonstrate wireless electric-field sensors for electrophysiology and flat optic modulators.

First, I introduce an ultrasensitive and extremely bright nanoscale electric-field probe overcoming the low photon count limitations of existing optical field reporters. I demonstrate that electrochromic loading of plasmonic nanoantenna allows us to realize optical field probes with 10 − 100 million times larger cross sections than fluorescence molecules and ∼ 3000- fold enhanced sensitivities than conventional plasmonic nanoantennas. Using our nanoprobes, I realize optical detection of electric-field dynamics from diffraction limited spots and high- speed recordings with sub-millisecond temporal response times (∼ 191 μs). Furthermore, I demonstrate label-free optical recording of electrogenic activity of cardiomyocyte cells with low-intensity light. Our nanoprobes offering high spatiotemporal resolution measurement ca- pability opens the door to label-free electrophysiological studies with photons.

Finally, I introduce a subwavelength-thick (< 250 nm) nano-electrochromic flat optic modulator that utilizes the electrochromic modulation mechanism in combination with highly dispersive Fano resonances in extraordinary light transmission (EOT) effect. We demonstrate electro-optic switching capability that can simultaneously deliver remarkably high modulation depth (∼ 17.6 dB) and high speed ( ∼ 500 μs, ∼ 2 kHz) switching capability beyond the video rates. The field-effect flat optic modulator shown here paves the way to the advancement of technologies based on electrochromic soft materials.