Nanostructures Improve Raman-Silicon Interaction for Lasers, Sensors
The ability to develop low-cost, monolithic, silicon-based integrated optoelectronic systems for applications from the medical to the security industry has attracted a great deal of attention, although light generation in bulk silicon through band-to-band radiative electron-hole recombination is very inefficient. Much of the focus for light emission using silicon has been on the use of nanocrystals, Si/SiO2 superlattices, erbium-doped silicon-rich oxides, Si/SiGe quantum cascade lasers, and surface-textured bulk silicon, while recent experiments have exploited stimulated raman scattering for lasing (see oemagazine, January 2005, p. 9). Researchers led by Haisheng Rong at Intel in Santa Clara, CA, and in Jerusalem, Israel, have shown how structural improvements to the silicon waveguide can result in lower thresholds for silicon lasers.
Their experiment used an S-shaped silicon waveguide pumped with pulses from an external-cavity, continuous-wave diode laser operating at 1.536 µm. An erbium-doped fiber amplifier produced a pump beam of up to 2 W peak power (previous efforts required up 9 W to induce lasing). The Raman laser optical cavity was formed by coating one of the waveguide facets with a multilayer coating, while leaving the other facet uncoated. The coated and uncoated facets had 90% and 30% reflectivity, respectively. By coupling the pump beam into the cavity through the uncoated facet, researchers achieved lasing at 1.67 µm. A lasing threshold of ~0.4 mW and a slope efficiency of 9.4% from the uncoated facet was achieved. The laser spectral width is much narrower than that of spontaneous Raman emission on an identical waveguide.
To reduce the problems associated with the free carrier effect at high power excitation, a p-i-n diode structure was designed along the rib waveguide. A reverse bias applied to the diode enabled electron-hole pairs to be swept out of the silicon waveguide, which allowed the carrier transit time or effective carrier lifetime to be modified by the applied electric field.
"Intel's all-silicon Raman laser is a major achievement as it opens up a whole range of new and very exciting applications in the near term," says Philippe Mattelaer, business development manager at BTG International Inc., a technology development company. Mattelaer expects more complementary photonic device technology for integration with silicon in the future.
Raman scattering is more typically used in chemical sensing, particularly in medical and pharmaceutical applications. The enormous limitation of the technique is its extremely weak Raman signal. The ability to enhance Raman scattering by factors of 1 million using roughened metallic surfaces, which provide "surface enhancement," has been known for decades. The effect, however, was previously difficult to control, predict, and reproduce for practical sensing applications.
Researchers at Rice University (Houston, TX) collabor-ated with Nanospectra Biosciences Inc. (Houston, TX) to demonstrate the ability to reproducibly enhance the sensitivity of Raman scattering by a factor of 10 billion using nanoshells. Consisting of a silica core covered by a metallic shell, the nanoshells capture and focus the passing light, enhancing the Raman effect. The nanoshells can be tuned to interact with specific wavelengths of light by varying the thickness of their shells, which allows for Raman enhancement at specific wavelengths. "The work . . . at Rice University demonstrates that each individual nanoshell works as a chemical sensor, magnifying the Raman signal to levels that single molecule detection is not only feasible, but now practical, even in biological solutions," says J. Donald Payne, president of Nanospectra.
The stages of growth of the nanoshell show how the silver aggregates on the non-conducting core. (Photo courtesy Rice University).
In the UK, Mesophotonics Ltd. (Southampton, UK) has shown how nanostructured surfaces based on ordered arrays of metal-coated holes can enhance the Raman effect. By designing the hole array to form a photonic crystal, Mesophotonics was able to control how individual holes interact, leading to additional resonances for Raman enhancement. Highly reproducible photonic crystal en-hancing surfaces are made on silicon wafers using standard processes, allowing access to volume manufacture with all the process control of the silicon semiconductor industry.
In the future, the use of standard silicon techniques for enhanced Raman scattering means such surfaces can be simply integrated with additional optical or electronic components for the ultimate chip-level chemical detector systems at low cost.
