Mayumi Noto

Molecular weight dependence of a whispering gallery mode biosensor

We report on molecular weight dependence measurements for an optical resonance biosensor. A dielectric microparticle is evanescently coupled with an optical fiber for the resonance stimulation, and a shift of the resonance wavelength is measured to monitor protein monolayer formation on the microparticle surface. Wavelength shifts for proteins over two orders of magnitude in molecular weight are measured. We show that the shift is proportional to molecular weight to the one-third power. Our result demonstrates that the optical resonance biosensor provides protein size information upon detection. This molecular weight dependency differentiates optical resonance sensing from electrical detection using field-effect transistors.

Nanolayer characterization through wavelength multiplexing a microsphere resonator

Hydrogel nanolayer (<150 nm) formation on a silica microsphere is optically characterized while forming in situ by simultaneously following the shifts of whispering gallery modes at two wavelengths.

Consequences of Extreme Photon Confinement in Micro- Cavities: I. Ultra-Sensitive Dedection of Perturbations by Bio-Molecules

It is becoming possible to confine optical photons within a dielectric microparticle (radius∼100µm) for microseconds. This lifetime would allow a free-ranging photon to travel ∼300 m in vacuum. In the frequency domain such a mode resonates with a ratio of frequency to line-width Q }109. With such a narrow line-width, the sensitivity to size and refractive index perturbations is extreme. An average size change of less than 1 picometer shifts the resonance line through its complete width. These resonances may be stimulated evanescently by coupling to the guided wave in an optical fiber core. Researchers recently used this fiber-microsphere system to detect hybridization of DNA on a microsphere surface, and found that a single nucleotide polymorphism (SNP, one base mismatch) in a long DNA target could be detected with a signal to noise ratio of 54.