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.

September 1, 2018

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.

Figure 1. Graphene’s Electro-Optic Properties. (a) Theoretical absorption and refractive index as a function of Fermi level for intrinsic graphene (region I - high absorption, region II - low absorption). (b) Optical micrograph of the fabricated device (interferometer arms false colored). (c) Device cross section showing graphene-HfO₂-graphene capacitor on Si₃N₄ waveguide.

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.

To date, the state-of-the-art graphene electro-optic modulators level has been mostly designed using the voltage tunable absorption of graphene. The potential of utilizing the voltage dependent refractive index tunability of graphene has been recently demonstrated. In these devices, however, the phase modulation is accompanied by loss modulation at low voltages of operation, where absorption and refractive index gets tuned simultaneously.

The regime of low loss and high refractive index of graphene has also been electrostatically tuned using electrolytic ion gels, which render these devices low speed. Using the current silicon-insulator- graphene configuration, the regime where the absorption of graphene is low and the phase change is high has not yet been achieved. Those devices are limited within the fermi energy level of 0.5 eV (region I of Figure 1a), where loss and phase change proceed simultaneously. Recently, the devices demonstrated have limited speed of operation since they use electrolyte, whereas the silicon-insula-tor-graphene configuration are limited to the high loss and phase change regime (region I of Figure 1a).

This work was done by Ipshita Datta, Brian Lee, and Michael Lipson, Columbia University, for the Air Force Research laboratory. AFRL-0265