The Interuniversity Microelectronics Centre (IMEC) near Leuven, Belgium, has reported a plasmonic method of integrating high-speed CMOS electronics with nanophotonic circuitry. The work is part of IMEC's 'More Moore' program, exploring ways of scaling electronics to 22nm and beyond. Plasmonic technology is still in an experimental stage, but it has the potential to be used in future applications such as nanoscale optical interconnects for high performance computer chips, extremely sensitive biomolecular sensors, and highly efficient thin-film solar cells.
Interlinking of photonic and electronic circuitry has hitherto been difficult because of the difference in the scales of the two circuits: a single CMOS electronic transistor on an integrated circuit may be only a few tens of nanometres across, whereas the dimensions of the conventional dielectric waveguides used in optical components have been 400-500nm - the minimum size dictated by the diffraction limit of the wavelength of light used in the photonic circuit.
The optical properties of nanostructured noble metals show promise for use in nanophotonic applications. When such nanostructures are illuminated with visible to near-infrared light, the excitation of collective oscillations of conduction electrons – called surface plasmons – generates strong optical resonances. Moreover, surface plasmons are capable of capturing, guiding, and focusing electromagnetic energy in deep-subwavelength length-scales, i.e. smaller than the diffraction limit of the light. This is unlike conventional dielectric optical waveguides, which are limited by the wavelength of the light, and which therefore cannot be scaled down to tens of nanometers, which is the dimension of the components on today's nanoelectronic ICs.
Nanoscale plasmonic circuits would allow massive parallel routing of optical information on ICs. But eventually that high-bandwidth optical information has to be converted to electrical signals. To make such ICs that combine high-speed CMOS electronics and plasmonic circuitry, efficient and fast interfacing components are needed that couple the signals from plasmon waveguides to electrical devices.
As an important stepping stone to such components, IMEC has now demonstrated integrated electrical detection of highly confined short-wavelength surface plasmons in metal-dielectric-metal plasmon waveguides. The detection was done by embedding a photodetector in a metal plasmon waveguide. Because the waveguide and the photodetector have the same nanoscale dimensions, there is an efficient coupling of the surface plasmons into the photodetector and an ultrafast response.
Pol van Dorpe, a post-doctoral researcher at IMEC and author of the recent paper published in Nature , told electrooptics.com: 'It turns out that using the metal-inset waveguides with slits that are coupled to a semiconductor detector, one can very efficiently couple the energy from the plasmonic waveguide into a photo-detector. Furthermore, because of the detector's small size, you can also make the process very fast - it has to be fast otherwise one loses the advantage of the bandwidth of photonic waveguides.'
The process can be fast because all of the energy of the light is coupled inside a small slit (20-30nm), allowing near-total absorption at the detector. This efficiency allows small detector, which intrinsically means a very fast detector, because drawbacks due to capacitance are avoided. IMEC believes that these results will pave the way for the integration of nanoscale plasmonic circuitry and high-speed electronics.
Van Dorpe believes that it may be some time before plasmonic waveguides are adopted into electronics, as many supporting technologies must be developed alongside them. He says 'I hope that these findings actually trigger some more work, as there hasn't been much research into the interface of plasmonics and electronics. The next step is to measure the limits of the speed of the devices, and work on the reverse effect, i.e. connecting small light sources to plasmonic wave guides.'
