Surface interaction, polarization and molecular weight effects for a whispering gallery mode sensor.

摘要

In this thesis, we investigate the use of an optical resonance of a microsphere in measuring the thickness and dielectric constant of a bio-nano-layer and in determining the molecular orientation of an adsorbate at the sphere surface. We also establish the molecular weight sensitivity of the resonance frequency shift.; The resonances of silica microspheres can be very narrow (i.e. high Quality Factor, Q and as a result are extremely sensitive to dielectric perturbations at the sphere surface. In particular, one may use the shift of the resonant frequency to detect the adsorption of bio-molecules with unprecedented sensitivity.; This work goes beyond the detection of nanoscopic bio-molecules to the characterization of polymer layers ∼100 nm in thickness. The possibility of enlarging the scope of the sensor is inspired by first order perturbation theory. The theory reveals that the evanescent field decay length influences resonance shifts and may be used as a nanoscopic ruler. Through this theoretical discussion, the idea of using the two light sources to stimulate resonances in an individual microsphere precipitated. Microsphere Wavelength Multiplexing (MWM) experiments proved the theory to be sound. The measurement of simultaneous shifts of two resonances at widely separated wavelengths allowed us to determine the thickness and the dielectric excess of an important bionanolayer (poly-L-lysine) even though its refractive index only exceeded that of the aqueous buffer solution by 0.0012.; We revisited protein adsorption and investigated the molecular weight MW sensitivity of the sensor. We found that the resonance shift is proportional to MW1/3. A new theoretical model explains the MW1/3 dependence and concludes that spherical protein covers 34% of the microsphere surface and the effective layer refractive index is 1.39.; We demonstrate polarization specific stimulation of resonances and measure TE and TM mode shifts by inducing a refractive index change in the medium. NaCl addition experiment confirms TM to TE shift ratio predicted by perturbation theory. The polarization specific measurement is applied to investigate the orientation of phenol molecule at the silica surface.