Plasmonic Nanostructured Metal–Oxide–Semiconductor Reflection Modulators

Abstract

We propose a plasmonic surface that produces an electrically controlled reflectance as a high-speed intensity modulator. The device is conceived as a metal-oxide-semiconductor capacitor on silicon with its metal structured as a thin patch bearing a contiguous nanoscale grating. The metal structure serves multiple functions as a driving electrode and as a grating coupler for perpendicularly incident p-polarized light to surface plasmons supported by the patch. Modulation is produced by charging and discharging the capacitor and exploiting the carrier refraction effect in silicon along with the high sensitivity of strongly confined surface plasmons to index perturbations. The area of the modulator is set by the area of the incident beam, leading to a very compact device for a strongly focused beam (∼2.5 μm in diameter). Theoretically, the modulator can operate over a broad electrical bandwidth (tens of gigahertz) with a modulation depth of 3 to 6%, a loss of 3 to 4 dB, and an optical bandwidth of about 50 nm. About 1000 modulators can be integrated over a 50 mm(2) area producing an aggregate electro-optic modulation rate in excess of 1 Tb/s. We demonstrate experimentally modulators operating at telecommunications wavelengths, fabricated as nanostructured Au/HfO2/p-Si capacitors. The modulators break conceptually from waveguide-based devices and belong to the same class of devices as surface photodetectors and vertical cavity surface-emitting lasers.

Gate-Tunable Conducting Oxide Metasurfaces

“…An as-yet unrealized milestone in the field is to achieve an actively tunable metasurface with arbitrary control of phase and amplitude of individual antenna elements, by post-fabrication electrical modulation which would enable dynamical wavefront control in thin flat optical devices, such as dynamical beam steering, reconfigurable imaging, tunable ultrathin lens, and high capacity data storage. There have been number of attempts to actively control the overall response of the metasurface by using various physical mechanisms [13][14][15][16][17][18][19][20][21][22] . It has been shown that one can actively control transmittance or reflectance of the impinging light.…”

Tunable Metasurfaces Based on Active Materials

“…b) An SEM image of an electrically tunable metasurface by electrostatic gating of a silicon substrate. Reproduced with permission . Copyright 2015, American Chemical Society.…”

“…However, in many applications, electrostatic gating is a superior method to optical pumping, since it is a more integrated solution, and typically consumes less power. Metal‐oxide‐semiconductor (MOS) capacitor structures are exploited to modulate the response of metasurfaces with electrostatic gating, as depicted in Figure b . Via carrier depletion or accumulation in the silicon substrate, the relative reflectance of a plasmonic grating is modulated with a relative reflectance change of 3% to 6% and a modulation rate over 1 GHz with a bias voltage under 2 V.…”

“…Reproduced with permission. [51] Copyright 2015, American Chemical Society. c) Schematic of an active THz metasurface driven by carrier depletion in the GaAs substrate.…”

Multiple p-n junction subwavelength gratings for transmission-mode electro-optic modulators

“…In particular, ENZ nanofilms incorporated in metal-oxide-semiconductor (MOS) capacitor arrays have showed remarkable intensity and phase modulation properties driven by field-effect free-carrier accumulation and depletion78910. In this approach, highly dissipative, deep-subwavelength plasmonic resonances are necessary to induce significant optical interaction with sub-10-nm-thick EO-active layers.…”