Notched microring resonator senses individual nanoparticles and their properties

Nanoparticles such as quantum dots, nanoshells, and "functionalized" polymer nanoparticles (which means they have active biological molecules attached to them) have become important in biophotonics research and medical therapies. Photonically detecting and characterizing a single nanoparticle is thus a key goal in optical sensing. One present-day technique, sensing the evanescently induced changes of a nanoparticle on the optical resonance of a microsphere cavity, produces spectral shifts only in the picometer range (determined using an expensive narrow-linewidth tunable laser), and is thus difficult to perform.

Scientists at the Massachusetts Institute of Technology (Cambridge, MA), New York University, and CUNY (both of New York, NY) are instead using a 4 μm diameter on-chip microring resonator as the sensor; they create a 100 nm notch in the ring so that the particle couples strongly to the ring's resonance mode. As a result, spectral shifts (determined using an ordinary non-narrow-linewidth tunable laser) are in the nanometer range and thus easily detectable. In a first test, the presence of a 20 nm silicon nanoparticle (placed on an atomic force microscope tip for easier positioning) was easily distinguished from the presence of a separately tested 20 nm gold nanoparticle; the spectral shifts induced on the ring resonance by the two nanoparticles differed by 2.2 nm. Contact Yasha Yi at [email protected].