User Highlight: Optical Devices
by Richard Osgood
Richard
Osgood
Our project at the CFN has been to show how nanoscale dielectric materials can be used to “squeeze” light into a volume much smaller than it occupies in bulk materials and then to use that “squeezing” or confinement to affect secondary but important properties of the light confined in the structure.
Currently, we are working on a project to examine how a tightly confined light beam propagates with different velocity and dispersive properties than the same light beam in an unconfined medium. To accomplish these studies, which involve IBM’s T.J. Watson Research Center, we work with tiny linear structures known as silicon-photonic wires. These wires are made of single-crystal silicon, formed on a layer of silicon dioxide, sometimes called “SOI” for silicon-on-insulator. Our wires typically have a highly confined optical waveguide mode, which allows a variety of new optical properties to be realized. One of the most important properties is the control of optical dispersion; this quantity controls the propagation speed of different spectral components of the light pulses in the wire as well as the temporal spreading of pulses and the pulse asymmetry. Control of these quantities is realized by carefully adjusting the dimensions and shape of the wire. Note that the optical response of our wires must be numerically calculated via several simulation packages, which we have obtained via collaboration with RSoft, a New York-based software company.
Making such high-confinement structures is a major challenge. The walls of the photonic wire must be exceedingly smooth so as not to scatter light out of the waveguide mode. To accomplish this, we need a suite of fabrication tools that are seldom seen outside of multibillion-dollar advanced-fabrication facilities. At the CFN, we use the clean room to carry out the exacting nanofabrication needed for wires having predetermined optical size and hence dispersion, as well low optical loss. The JEOL 30-keV electron-beam tool has provided us a quick way to bracket the processing conditions needed to fabricate high-performance wires. This device has a writing “field” that enables wire lengths that can be used for optical characterization. On the other hand, fabricating optical nanocircuits or wires requires much a larger writing field. In this case, the JEOL 100-keV electron-beam tool at Lucent, with operation time rented to the CFN, is essential since it allows “stitching” of the fields over long distances. Thus, this tool allows us to define lithographically practical nanoscale device patterns having the millimeter-long lengths needed for optical circuits or even large devices.
We use a very high-quality etching tool, the Oxford Plasmalab DRIE, to transfer the pattern smoothly into the silicon. Soon, we will have the Trion Orion III PECVD for depositing an oxide layer and the Trion Phantom III RIE oxide etcher for that step. In the longer term, we will use these systems in combination with other tools at the CFN clean room to extend further the functionalities of waveguide devices such as growing silicon-oxide and nitride thin films on waveguides, and making any needed electronic contacts.
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