Low Power Optical Phase Array Using Graphene on Silicon Photonics

Electrostatic doping of 2D materials embedded in waveguides could enable ultrafast devices with unprecedented power.

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

Despite enormous advances in integrated photonics over the last decade, an efficient integrated phase delay remains to be demonstrated. This problem is fundamental - most monolithic thin film deposition relies on centro symmetric materials (such as silicon, silicon dioxide, silicon nitride), which by definition do not have an electro-optic effect. Such materials have been shown to be excellent transparent materials, however they are either optically passive, or rely on very small plasma dispersion effect or power-hungry thermo-optic effect for tunability. These phase change materials have losses associated due to heating or carrier injection in the waveguides. This research shows that graphene can be used to provide electro-optic properties to traditionally passive optical materials.

Graphene is a versatile 2D material with wavelength-insensitive electrical tunability of its optical absorption and refractive index (Figure 1). As seen in Figure 1, theory predicts a strong tenability of the graphene's optical properties with tuning of the Fermi level. This tuning is achieved here via electrostatic doping by embedding the graphene in a capacitor (as shown in Figure 1c). As the Fermi level is tuned, it is predicted that the absorption decreases (region I). As the tuning is further increased, the absorption becomes negligible, while the index of refraction changes drastically (region II and III). The ease of integration with silicon photonics and the capacitive nature of graphene electro-optic devices renders graphene an attractive choice for photonics. The electrostatic tunability of the optical properties of graphene lends graphene the novel capability of transcending any passive platform to an active device.