Silicon's Role in Photonics Recast

A discovery about silicon nanowires and nanocones will allow the element to play a larger role in photonics.

Silicon, the element responsible for most of our advances in computer and electronics technology, has played a comparatively lesser role in photonic or optical devices. While large crystals of silicon scatter light, the optical scattering intensity from a nanowire of just the right diameter can be significantly larger -- nearly 1000 times larger.

Nanocones such as the one depicted at left were used as variable-length nanowires in the research. (Image: Jonathan Spanier/Drexel University)

Jonathan Spanier, a professor of materials science and engineering at Drexel, along with Bahram Nabet, a Drexel professor of electrical and computer engineering, and Linyou Cao, a Drexel graduate student in materials science and engineering, made the discovery. They explain their experimental results in terms of a building up of the incident electromagnetic field inside the tiny cross-section. While there is no inherent gain in the system, the incident light rays resonate with the nanowire cross-section at a particular combination of wavelength of light and nanowire diameter, in somewhat the same manner that a guitar string would resonate with a tuning fork.

The physics is not entirely new: The well-known theory for scattering from small spherical particles was first published by the German physicist Gustav Mie in 1908. But the findings open the way for new photonic and optical-based chemical sensing applications involving silicon and other semiconducting or insulating nanowires produced by bottom-up methods.

Most of the reports of enhanced scattering from nanostructures for sensing applications to date have involved a different mechanism involving the peculiar behavior of electrons in metallic films and nanostructures. An emerging trend in electronic and photonic devices is the construction of materials and devices from the "bottom-up" -- starting with chemical precursors in the liquid or vapor phase and seeding the growth of nanoscaled materials as building blocks for nanotechnology. Traditional approaches are "top-down," where a bulk wafer is processed into useful devices by growth or deposition of thin films and by selective patterning and etching of layers.

Though still in the development stage, bottom-up methods have some distinct advantages, the Drexel team said. For example, nanoscaled components produced by bottom-up methods can be more easily transferred to and integrated within nonconventional substrates, such as plastic.

The researchers made their discovery by looking at nanowires with widths just below and above 100 nm, sizes somewhat larger than those many researchers have been investigating. And the use of nanocones made their efforts easier, they said. A nanocone can be thought of as a variable diameter nanowire. As one moves from the tip of the cone to its base, the diameter increases. So instead of making many nanowires each sampling a specific diameter, the nanocone can be used to evaluate the diameter dependence of this effect. Integration of nanowires and nanocones on typical silicon substrates then allows the optical elements to be ingrated with electronic ones.

The research was sponsored by the Army Research Office; by the Nano/Bio Interface Center, an NSF Nanoscale Science and Engineering Center; and by the Commonwealth of Pennsylvania. The work was featured recently in Physical Review Letters.