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Finite-element simulations of nanoscale molecule interaction with the evanescent radiation field of an optical resonant microcavity are conducted to characterize the detection of single transparent molecules using the microcavity as an extremely sensitive micro/nano-sensor. The model sensor is an integrated device consisting of a dielectric microdisk and a waveguide that can be nanofabricated on Si-based dielectric thin film. When the microdisk is operated at a whispering-gallery mode, a strong evanescent field arises, surrounding the periphery of the cavity. Foreign target molecules such as proteins present in the near-field will interact with the electromagnetic resonant field and induce changes to the resonance. Such induced changes are investigated in this report and their significance in the detection of single molecules for nanotechnology development is discussed.

The near-field gap effects are investigated in planar dielectric microdisc and waveguide coupling structures, emphasizing miniaturization of integrated sensor systems. The simulation results show that the resonance frequency is not obviously affected by the gap dimension when the gap between a microcavity and its coupler is larger than 300 nm. However, the resonance frequency shifts observably with a further decreasing gap to the nanometre level. This shift is generally larger than the cavity resonance linewidth in the 10 µm diameter microdisc system, but is comparable to the cavity resonance linewidth in the 2 µm diameter microdisc system. With increasing gap, the cavity Q increases exponentially until it is saturated at a limit Q factor. An optimal gap dimension exists for maximum light energy transfer and storage. The concept of optimum gap is introduced and defined at the gap dimension where half-maximum energy storage capability is achieved; meanwhile, the cavity Q is high and the resonance frequency remains stable.

A fast 3-D optical imaging method with use of exogenous fluorescence agent is proposed and demonstrated by simulation in a model tissue. After administration of fluorescent agent, ultrashort near-infrared laser pulses are used to illuminate the tissue and excite fluorescence emission. The transient fluorescence signals are detected on the tissue boundaries and employed to reconstruct a 3-D image of relative fluorescence emission distribution inside the tissue. A region with greater fluorescence emission represents a diseased tissue if the fluorescent agent has a close affinity with the disease. We successfully demonstrated the feasibility of this method in the imaging of a 4x4x4mm(3) tumor embedded at the center of a cubical tissue phantom with an uptake distribution of fluorescent indocyanine green dye. The image reconstruction does not involve any inverse problem. It took less than 5 minutes in a PC for the model imaging problem.

We characterize planar microcavities in whispering-gallery mode optical resonances. The microcavity consists of a waveguide and a microdisk, and a nanoscale gap separates the waveguide and the microdisk. The devices can be fabricated on Si-based thin films by using conventional microelectronics techniques. To characterize these types of cavity, we study a broad range of resonator configuration parameters including the size of the microdisk, the width of the gap, and the waveguide dimensions. The finite-element method is used for solving Maxwell's equations. The electric fields and the energy density distributions are obtained and compared between the on-resonance and off-resonance situations. A brilliant ring with a strong electric field and a high-energy density is found inside the periphery of the microdisk under first-order resonance. While under second-order resonance, there are two bright rings, and the light intensity in the inner ring is stronger than that in the outer ring. The resonant frequencies and their free spectral ranges are predominantly determined by the size of the microdisk. The gap effect on the resonant frequencies is observable, although it is minor. The gap strongly affects the full width at half-maximum (FWHM), finesse, and quality factor of the resonances. With an increase in the gap width from 100 to 300 nm, both the Q value and finesse increase substantially, while the FWHM decreases. The waveguide width has a visible influence on the Q value, FWHM, and finesse as well.