IMEC recently reported a method to integrate high-speed CMOS electronics and nanophotonic circuitry based on plasmonic technology, which has the potential to be used in future applications such as nanoscale optical interconnects for high-performance computer chips, extremely sensitive (bio)molecular sensors -- and highly efficient thin-film solar cells.

by Debra Vogler, senior technical editor, Solid State Technology

May 7, 2009 - Plasmonic technology, today still in an experimental stage, has the potential to be used in future applications such as nanoscale optical interconnects for high-performance computer chips, extremely sensitive (bio)molecular sensors, and highly efficient thin-film solar cells. IMEC recently reported a method to integrate high-speed CMOS electronics and nanophotonic circuitry based on plasmonic effects.

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Top: Schematic overview of the device, showing focused illumination of a slit in the waveguide using polarized light. This results in plasmon excitation of the waveguide for the red polarization and the generation of electron/hole pairs in the semiconductor. Middle: SEM picture of a typical device. Bottom: Photocurrent scans for the "red" (bottom) and "blue" (top) polarization indicate a strong polarization dependence of the photoresponse. (Source: IMEC)

There are a number of interesting features concerning surface plasmons, noes Pol Van Dorpe, senior researcher in plasmonics at IMEC. First of all, their wavelength can be reduced significantly below the free space wavelength (which has the same frequency). "The reduction of the wavelength can result in a strongly improved degree of confinement, which allows deep sub-wavelength waveguides whose sizes can match state-of-the-art transistors," explained Van Dorpe. "A number of designs allow such strong confinement." One of the more advanced designs, which is explained in IMEC's paper in the May issue of Nature Photonics, consists of a metal/dielectric/metal layered structure (a MIM waveguide).

Second, surface plasmons can be focused in small holes or slits, giving rise to extraordinary transmission through deep sub-wavelength holes. "This property is also particularly interesting to reduce the noise and the capacitance of photodetectors, as light can be captured on a metal film (i.e., converted to surface plasmons) by the appropriate gratings and focused in a deep sub-wavelength aperture of slit that connects to a semiconductor," noted Van Dorpe. Ultrafast photodetectors could thus be constructed, with a small semiconductor active area resulting in limited capacitance and low noise, "without sacrificing the total signal to noise."

Moreover, when patterned into deep sub-wavelength nanostructures (or nanoparticles), noble metals respond in-phase to the exciting electromagnetic field. Depending on the shape, the specific metal used (Au, Ag, Cu, Al, etc.) and the dielectric surrounding the metal, the polarizability of the metal nanostructures shows a resonant behavior in the visible. "This goes hand in hand with strongly enhanced local electric fields, absorption and/or scattering," said Van Dorpe. Local surface plasmon resonance (LSPR) properties of metal nanoparticles have applications in several areas, he pointed out, including biosensing (shifts in the resonant wavelengths upon molecular binding events); surface enhanced Raman scattering (utilizing the enhanced local electric fields); and cancer treatment (local heating of cancer cells by labeling them with metal nanoparticles, and irradiating with near-infrared light).

The technology also has application in solar cell enhancements. Reducing the thickness of solar cells not only promises lower material costs and therefore the intrinsic cost of solar cells, but it also results in efficiency reductions.

"There are a number of ways that plasmonic effects can be used to boost the efficiencies of thin-film solar cells," notes Van Dorpe. "Most importantly, the strong scattering of metal nanoparticles can result in a significant enhancement of the optical path inside the photo-absorbing material, allowing a strong absorption enhancement for near-bandgap photons that don't get completely absorbed otherwise."

This situation is typically realized by surface texturing for thicker solar cells, he noted, but as the texture features are in the range of several micrometers, this strategy obviously breaks down for thin-film solar cells whose thickness is in the same range. "Secondly, the enhanced local fields of plasmon supporting metal nanoparticles can result in an enhanced optical absorption of light in organic semiconductors," he added.


This article was originally published by Solid State Technology.