ABSTRACT
An optical device configuration allowing efficient electrical tuning of near total optical absorption in monolayer graphene is reported. This is achieved by combining a two-dimensional gold coated diffraction grating with a transparent spacer and a suspended graphene layer to form a doubly resonant plasmonic structure. Electrical tuneability is achieved with the inclusion of an ionic gel layer which plays the role of the gate dielectric. The underlying grating comprises a 2-dimensional array of inverted pyramids with a triple layer coating consisting of a reflective gold layer and two transparent dielectric spacers, also forming a vertical micro-cavity known as a Salisbury screen. Resonant coupling of plasmons between the gold grating and graphene result in strong enhancement of plasmon excitations in the atomic monolayer. Plasmon excitations can be dynamically switched off by lowering the chemical potential of graphene. Very high absorption values for an atomic monolayer and large tuning range, extremely large electrostatically induced changes in absorption over very small shifts in chemical potential are possible thus allowing for very sharp transitions in the optical behavior of the device. Overall this leads to the possibility of making electrically tunable plasmonic switches and optical memory elements by exploiting slow modes.
ABSTRACT
An optical device configuration allowing efficient electrical tuning of surface plasmon wavelength and absorption in a suspended/conformal graphene film is reported. An underlying 2-dimensional array of inverted rectangular pyramids greatly enhances optical coupling to the graphene film. In contrast to devices utilising 1D grating or Kretchman prism coupling configurations, both s and p polarization can excite plasmons due to symmetry of the grating structure. Additionally, the excited high frequency plasmon mode has a wavelength independent of incident photon angle allowing multidirectional coupling. By combining analytical methods with Rigorous Coupled-Wave Analysis, absorption of plasmons is mapped over near infrared spectral range as a function of chemical potential. Strong control over both plasmon wavelength and strength is provided by an ionic gel gate configuration. 0.04eV change in chemical potential increases plasmon energy by 0.05 eV shifting plasmon wavelength towards the visible, and providing enhancement in plasmon absorption. Most importantly, plasmon excitation can be dynamically switched off by lowering the chemical potential and moving from the intra-band to the inter-band transition region. Ability to electrically tune plasmon properties can be utilized in applications such as on-chip light modulation, photonic logic gates, optical interconnect and sensing applications.
