Dispersion engineering of a microsphere via multi-layer coating

Optical frequency combs in aqueous and air environments at visible to near-IR wavelengths

The ability to detect and identify molecules at high sensitivity without the use of labels or capture agents is important for medical diagnostics, threat identification, environmental monitoring, and basic science. Microtoroid optical resonators, when combined with noise reduction techniques, have been shown capable of label-free single molecule detection; however, they still require a capture agent and prior knowledge of the target molecule. Optical frequency combs can potentially provide high precision spectroscopic information on molecules within the evanescent field of the microresonator; however, this has not yet been demonstrated in air or aqueous biological sensing. For aqueous solutions in particular, impediments include coupling and thermal instabilities, reduced Q factor, and changes to the mode spectrum. Here we overcome a key challenge toward single-molecule spectroscopy using optical microresonators: the generation of a frequency comb at visible to near-IR wavelengths when immersed in either air or aqueous solution. The required dispersion is achieved via intermodal coupling, which we show is attainable using larger microtoroids, but with the same shape and material that has previously been shown ideal for ultra-high sensitivity biosensing. We believe that the continuous evolution of this platform will allow us in the future to simultaneously detect and identify single molecules in both gas and liquid at any wavelength without the use of labels.

Theoretical and experimental investigation of broadband dispersion tailoring of high-order mode in the hybrid microsphere cavity

The large normal dispersion of the fundamental mode (TE_n=1 mode) in the whispering gallery modes (WGM) microsphere is detrimental to the visible comb generation. Herein, we demonstrate that this fundamental limitation can be removed by considering the high-order radial modes (TE_n=2 mode) of the hybrid microsphere cavity (HMC). The studied HMC consists of a high-refractive-index coating (TiO₂ or HfO₂) and silica microsphere. The simulated electric field energy distribution and measured Q value in our experiment show that optical confinement of the coating effectively excites the TE_n=2 mode and reduces the free spectral range (FSR) and modal dispersion. In addition, the observed redshift of WGM and decreased trend of FSR are in accordance with simulations. The zero-dispersion wavelength can be linearly shifted to a shorter wavelength or even into the visible region with the reduction of coating thickness or refractive index and larger microcavity, which advances the visible comb generation.

Compound Glass Microsphere Resonator Devices

In recent years, compound glass microsphere resonator devices have attracted increasing interest and have been widely used in sensing, microsphere lasers, and nonlinear optics. Compared with traditional silica resonators, compound glass microsphere resonators have many significant and attractive properties, such as high-Q factor, an ability to achieve high rare earth ion, wide infrared transmittance, and low phonon energy. This review provides a summary and a critical assessment of the fabrication and the optical characterization of compound glasses and the related fabrication and applications of compound glass microsphere resonators.