Non-Resonant and Resonant Surface Plasmon Polariton Modulators for Optical Communications

Abstract

The ongoing technical revolution demands for a continuous growth of computational power. This demand is addressed by reducing the size of electronic circuits in order to increase their computational power, as predicted by Moore’s law. However, the communication speed of electronic circuits is limited to data rates of several Gbit/s and cannot be increased due to energy dissipation of electrons. This bottleneck is the major cause for slowing down the growth of computational power. Photonic communication is envisioned to overcome this limitation. Photons can transmit data at rates of Tbit/s with negligible energy dissipation, which is not possible for electrons. Thus, integrating photonics within electronic circuits on a single platform is the scope of current research. Such a platform uses photonics for communication and electronics for local computation. A fundamental component of this platform is the electro-optic (EO) modulator, which encodes the computed electrical signal onto an optical carrier. Ideally, EO-modulators should encode data at highest speed, with lowest energy consumption and on the most compact footprint. These characteristics are improved when photons and electronic signals are confined to smallest area. Plasmonics achieve the tightest confinement among all photonic technologies and promises high-speed and energy efficient EO-modulators. The tight confinement is enabled by coupling photons to the electron gas of a metal to form surface-plasmon polartion (SPP). This coupling forces electrons to oscillate with the frequency of light. However, moving electrons experience Ohmic losses due to scattering processes. This attenuates the optical carrier and diminishes the performance of the optical link. For instance, current state-of-the-art high-speed plasmonic modulators feature insertion loss (IL) in excess of 10 dB. This hampers practical implementation of plasmonics, despite the promises of high-speed and compactness. In the course of this thesis low-loss plasmonic EO-modulators were developed. These devices feature low IL (~2.5 dB), a high-speed modulation capability (>>100GHz), a compact footprint (several square microns) and a low electrical energy consumption (~10fJ/bit). The following outstanding results have been achieved. First, the modulation efficiency scales more strongly with the confinement of SPP than the Ohmic losses increase. This allows one to reduce the device length, and thus, IL as they scale with length. This is contrary to other approaches which try to minimize device losses by reducing the confinement. Based on this approach, the first plasmonic high-speed Mach-Zehnder modulator (MZM) was realized with a record voltage-length product (modulation efficiency) of 40 Vmicrons, as well as reduced IL of 8 dB. Second, the modulation efficiency is increased two fold when utilizing the EO-material’s resonances. These are harnessed by operating the modulator in close proximity to the absorption peak of the EO-material. Normally, this is avoided for photonic approaches as losses increase too fast when approaching the material resonances, however, these losses are negligible in comparison to Ohmic losses in plasmonic devices. Third, employing a hybrid resonant modulator schema comprising of a plasmonic cavity and a photonic bus waveguide. This allows one to bypass light with a photonic waveguide in the on-state (max. transmission desired), while in the off-state (min. transmission desired) light is converted to SPPs confined to a closed loop plasmonic ring. All the other demonstrated high-speed plasmonic EO-modulator utilize non-resonant schemes. While exploring this route, three first-of-their-kind high-speed modulator types have been realized, namely a plasmonic MZM, a plasmonic IQ-modulator and a plasmonic ring-modulator. These realizations demonstrate the unique integration density enabled by plasmonics. For instance, the footprint of the MZM and IQ-modulator presented here is two and three orders of magnitude smaller, respectively, compared to their photonic counterparts. All devices are capable of generating high-speed data streams of 72 Gbit/s (MZM, ring modulator) to 144 Gbit/s (IQ-modulator). The experimental results prove that losses of high-speed devices can be reduced from 10 dB down to 2.5 dB by utilizing stronger plasmonic confinement and most importantly bypassing Ohmic losses with the help of a resonant approach. Furthermore, the experiments show that material resonances can be harnessed to reduce device losses, and thus, ultimately achieving loss of approximately 1dB. The results of this thesis provide an outlook on the potential of plasmonics to generate unique data-rates on smallest footprint.