Electrical tuning of photonic bandgap structures in silicon
There is growing interest in building electro-optic modulators out of silicon, because they are a basic building block of photonic systems, and a critical component of emerging optical interconnect systems. 1 Recent work on silicon-based modulators has focused on modulating the refractive index of silicon using the free-carrier plasma-dispersion effect. 2–3 Unfortunately, the change in refractive index this effect creates in silicon is relatively small, which can lead to large and/or slow devices.
One alternative is to use silicon as a passive optical host, surrounded by an active electro-optic material. This combines the benefits of microelectronic silicon processing with the optical performance of the materials that are integrated into the silicon structure during fabrication.
The key to the performance of such modulators is confining both the light and electric fields inside the active material. It is straightforward to confine the light using a photonic-bandgap (PBG) structure, whose periodic refractive index concentrates the light in the low-refractive-index active material. In contrast, it has been thought impossible to confine the electric field inside the active material due to the high conductivity of the silicon host. The host deflects the electric field lines away from the active material, an effect known as electric field screening.
We've found a way to suppress this effect. We have shown that the external voltage should be applied to both the active material and the silicon in parallel4 to ensure the electric field is uniformly distributed throughout the structure. The approach has been validated by integrating the silicon host with liquid crystals and measuring the response of the resultant system. In order to observe liquid crystal reorientation along the applied electric field, we use a PBG membrane composed of an array of cylindrical holes or pores. To allow for transmission measurements along the pore axes, the substrate was removed by anisotropic chemical etching, as shown in Figure 1(a). Aluminum contacts were then deposited on the two opposing sides of the silicon host membrane.
Figure 1(b) shows the transmission of polarized light as a function of applied electric field. A significant decrease in light intensity is observed at a field of 0.8V/μm, which corresponds to the reorientation of liquid crystal molecules parallel to the electric field lines. The observed switching field is close to the bulk threshold field of the liquid crystals, indicating that the field is not attenuated inside the pores. This tunable PBG structure not only demonstrates the feasibility of active material switching in silicon-based PBG structures, but can also be used for light modulation
The ability to switch PBG structures electrically may be useful in semiconductor photonics and optical interconnects, but has been handicapped by the electric field screening effect. Our solution to this problem is to provide a parallel current flow through the host silicon. Liquid crystal switching inside the two-dimensional PBG structures is demonstrated at electric fields lower than 1V/μm. Our next step will be to introduce faster active materials, such as electro-optic chromophores, into the silicon PBG structure for electrical light modulation. This approach could also be generalized to other pairs of materials.
This work was supported by the Air Force Office of Scientific Research (Grant F 49620-02-1-0376) and Intel Corporation.
