Large-scale Data Centers (DC) and High Performance Computing (HPC) systems demand ever increasing computing power, speed and energy efficiency. In the near future conventional technology will no longer satisfy such requirements. Optical interconnects based on photonic technologies may satisfy the growing needs of DCs and HPCs, but they have downsides: large size and high costs. On the other hand, plasmonic technologies offer the potential for very high operation speeds and small footprint, with low costs.

Plasmonics encopass generation, processing, transmission and detection of signals at optical frequencies along metallic surfaces. Unlike in photonics, plasmonic waves are not just photons. Unlike in electronics, plasmonic waves are not just relying on electrons. Plasmonics deals with electromagnetic waves that excite electrons at a metal surface and oscillate at optical frequencies

Clear market trends indicate the need for higher speed, more energy efficient and lower cost interconnects to be deployed in evolving HPC systems and DC networks. Optical interconnects based on either photonic or plasmonic technologies offer potentially a better alternative to currently used CMOS-based electronic interconnects.

The realization of chip-scale interconnects based on plasmonics technology was the focus of EU-funded project NAVOLCHI, combining the advantages of electronics (such as small footprint and low fabrication cost) and of photonics (such as high speed and low power operation) to realize the first prototype of a new generation of optical interconnects.

Thanks to their fundamental properties, plasmonic devices, may well serve as the missing link between the slow and power consuming electronics and the large and costly photonics technologies.

The NAVOLCHI project designed and developed a full suite of novel nanoscale plasmonic devices (including monolithically integrated plasmonic lasers, modulators, photodetectors and amplifiers) that are the required building blocks of a chip-scale optical interconnect. Such an optical interconnect need to offer highest speed at lowest power consumption on an extremely compact footprint on a densely integrated chip. The three fundamental challenges in realizing such optical interconnects are:

  • the transmitter arrays should be arranged in a highly parallel manner;
  • the fiber-to-chip interfaces  should be able to overcome distances from millimeters up to hundreds of meters
  • the detector solutions have to be highly parallel

An advantage of the so-called surface plasmon polaritons (SPPs), is that they can be confined to an ultra-compact area – an area much smaller than an optical wavelength, thus enabling ultra-compact devices.

On the transmitter side of the NAVOLCHI interconnect system, ultra-compact lasers and modulators convert electrical signals to the optical domain. The transmitter chip consists of a metallo-dielectric nanolaser achieving record low dimensions and a plasmonics based IQ modulator to externally encode data at the rate of up to 100 Gbit/s.

NAVOLCHI developed an all-plasmonic Mach-Zehnder Modulator (MZM) operating at 72 Gbit/s with a length as short as 10 micrometers and a Upp of only 3.5V. The plasmonic MZM was integrated into a standard silicon waveguide and featured a small power consumption of just ~20 fJ/bit.

The small footprints of both the plasmonic laser as well as the modulator make their combination perfect for realization of an optical transmitter chip with dimensions comparable to electronic devices. Thus, it will be possible to build several tens to hundreds of such Si based transmitter modules integrated together using standard high-volume, low-cost silicon CMOS manufacturing technologies in order to produce low-cost, ultra-small size photonic chips.

On the receiver side of the NAVOLCHI interconnect system, compact footprint photodetector arrays for direct or coherent detection were considered. The final testing investigations used integrated silicon-germanium detectors offering operation beyond 56 Gbit/s. However in the framework of the project a variety of different plasmonic detectors were investigated. Since losses are an issue in plasmonics, a plasmonic optical amplifier may be used in front of the photodetector in order to deliver the appropriate signal level.

Plasmonic amplifiers and detectors in NAVOLCHI made use of colloidal quantum dots eventually embedded into conductive polymers to define micrometric photodetectors on Si-substrates or directly into nano-gaps between metal nano-contacts.

The activities of EU project NAVOLCHI showed that plasmonic implementations offer advantages in terms of high bandwidth densities at low power consumption and cost, while showcased experimentally the actual performance of key plasmonic interconnect devices that were fabricated during the project execution. 

Project
Nano Scale Disruptive Silicon-Plasmonic Platform for Chip-to-Chip Interconnection
Project coordinator
Prof. Dr. Manfred Kohl Karlsruhe Institute of Technology
Project Acronym
NAVOLCHI
Project website