A notch above for single nanoparticle detection

Caren B. Les, caren.les@photonics.com

Working in the field of single nanoparticle detection, researchers have demonstrated strong optical coupling between an on-chip notched microring resonator and a nanoparticle in the notch.

“Recently, it was found that the electromagnetic modes of certain photonic devices are very similar to the electronic wave functions of molecules,” said Yasha Yi, who led the team that performed the work. “One of the most interesting examples is microring resonators, especially when we arrange two or more microring resonators together within optical coupling regime. The electromagnetic modes of the whole structure are very similar to the bonding symmetric or antibonding antisymmetric electronic wave function modes formed in molecules.”

The technique is very different from previous studies, which used evanescent coupling to the surface of the microsphere, he said.

Yi is a professor at the Integrated Nano-photonics Lab, New York University and City University of New York Graduate Center, and an adjunct professor at the Microsystems Technology Laboratories at MIT in Cambridge and at the Shanghai Institute of Microsystems and Information Technology, Academia Sinica, in China.

He and his colleagues believe that their work provides a unique way to achieve single nanoparticle detection and sorting with thousands of times better signal enhancement and that the nature of the on-chip microresonators will make large-scale integration on a single chip possible.

Scientists have shown strong coupling between an on-chip nanoscale notched ring resonator and a nanoparticle. Shown (left) is the image of the 100-nm-size notch in the ring resonator taken using portable scanning electron microscopy and (right) the image with a clear 100-nm coupling gap between the notched ring resonator and the single-mode bus waveguide. Reprinted with permission from S. Wang et al (2010). Applied Physics Letters, Vol. 97, 051102. Copyright 2010, American Institute of Physics.

Single nanoparticle detection is one of the ultimate goals for a sensing device, representing sensing at the extreme, according to the scientific report, which was published online Aug. 2, 2010, in the journal Applied Physics Letters. The article notes that many nanoscale photonic devices hold promise in the manipulation of photons at the chip scale and in applications in areas including photovoltaic cells, solid-state lighting, telecommunications and biomedicine.

“Placing a nanoparticle in the notch produces a much stronger response than simply placing it in contact with the exterior of the core,” Yi said. “In the exemplary case of a dielectric silicon and metallic gold nanoparticle placed in a notch, we have demonstrated that a nanoparticle induces a large wavelength splitting (~nm) and very different shift in the resonant modes of the resonator.

“This is a significant improvement over the smaller wavelength shifts and splitting (~pm) observed in earlier experiments – where the nanoparticle was placed outside the core of a conventional microsphere resonator – and lowers the requirement for very high Q resonator devices.”

He noted that the usefulness of this approach is not limited to the ring resonators used as examples here but can be extended to other types of resonator geometries, such as racetracks and polygons.

The researchers used a CMOS-compatible process to fabricate the on-chip photonic device configuration of a silicon microring resonator with a notch, a nano-particle and a two-bus waveguide. They fabricated a 100 x 100-nm notch in the ring using electron beam lithography. With a portable atomic force microscope, they analyzed the case of the notch with a 20-nm-diameter dielectric nanoparticle silicon tip and a 20-nm-diameter metallic gold particle inside the notch.

They used a laser that was tunable from 1480 to 1580 nm to couple the light from a tapered optical fiber to the silicon waveguide and a germanium detector to collect the through port signal at the other end.

“The flexibility to intentionally fabricate the defects for on-chip microresonators will bring us more exciting opportunities in the future, not only for single nanoparticle sensing but also for a deeper understanding of photonic molecules,” Yi said.

An example of this investigative work applied to the medical field, titled “Metallic Nanoparticle on Micro Ring Resonator for Bio Optical Detection and Sensing,” appears in the Sept. 1, 2010, issue of Biomedical Optics Express.