Infrared crystal film breakthrough

The Tokyo Institute of Technology and Fujitsu Laboratories have developed a technology that enables the propagation of infrared light through an optical crystal film on a silicon substrate.

The technology is a result of collaborative development by the laboratory of Associate Prof. Kasuo Shinosaki of the Tokyo Institute of Technology and Fujitsu Laboratories. It paves the way for compact optical communication devices featuring the integration of silicon large-scale integrated circuits (LSIs) with optical devices such as modulators and switches.

Network applications increasingly demand smaller sizes and lower costs for high-speed large capacity optical communications systems. Currently available optical communications systems are comprised of devices for processing of optical signals, and silicon LSI devices for processing electrical signals, which are manufactured and assembled separately. The realisation of a compact optical transmission device in which the two separate types of devices are integrated would enable further downscaling and lower costs for optical communication systems.

In order to enable the use of communications devices such as optical switches and modulators on a silicon substrate, a material with electro-optic effect must be formed on the substrate and light must be propagated through that material. One material known to have excellent electro-optic coefficient is the ferroelectric material, lead zirconate titanate (PZT). However, due to the fact that significant loss of propagation is incurred due to disruption of crystals when single crystal film is formed on top of silicon substrate, it had thus far been difficult to propagate light successfully.

To overcome the aforementioned technical issue, a buffer layer with a three-layer structure was used on the surface of a silicon substrate, and a PZT single crystal film was formed over the layer. This enabled minimisation of the disruption of the atomic alignment, thereby resulting in a high-quality ferroelectric PZT single-crystal-film with proper atomic alignment, and also prevented reaction between the PZT and silicon.

With the same wavelength of infrared light typically used in optical communications (1.55 microns), Tokyo Institute of Technology and Fujitsu Laboratories successfully minimised the PZT propagation loss to less than one decibel per centimetre which is approximately one-tenth the loss that had been incurred with existing technologies.

The researchers also succeeded in demonstrating the world’s first successful propagation of infrared light through an optical crystalline film formed on a silicon substrate.

The electro-optic coefficient, a figure representing the level of change in the refractive index, was verified as being 76 picometres per volt with infrared light. This is approximately three times the value of lithium niobate single crystals, which are widely used as optical modulators.

Fujitsu Laboratories and the Tokyo Institute of Technology will use this new crystal-forming technology for the development of technologies to form various optical devices on silicon substrates.