Zhixuan Xia

High resolution on-chip spectroscopy based on miniaturized microdonut resonators.

We experimentally demonstrate a high resolution integrated spectrometer on silicon on insulator (SOI) substrate using a large-scale array of microdonut resonators. Through top-view imaging and processing, the measured spectral response of the spectrometer shows a linewidth of ~0.6 nm with an operating bandwidth of ~50 nm. This high resolution and bandwidth is achieved in a compact size using miniaturized microdonut resonators (radius ~2 μm) with a high quality factor, single-mode operation, and a large free spectral range. The microspectrometer is realized using silicon process compatible fabrication and has a great potential as a high-resolution, large dynamic range, light-weight, compact, high-speed, and versatile microspectrometer.

Vertical integration of high-Q silicon nitride microresonators into silicon-on-insulator platform

We demonstrate a vertical integration of high-Q silicon nitride microresonators into the silicon-on-insulator platform for applications at the telecommunication wavelengths. Low-loss silicon nitride films with a thickness of 400 nm are successfully grown, enabling compact silicon nitride microresonators with ultra-high intrinsic Qs (~ 6 × 10(6) for 60 μm radius and ~ 2 × 10(7) for 240 μm radius). The coupling between the silicon nitride microresonator and the underneath silicon waveguide is based on evanescent coupling with silicon dioxide as buffer. Selective coupling to a desired radial mode of the silicon nitride microresonator is also achievable using a pulley coupling scheme. In this work, a 60-μm-radius silicon nitride microresonator has been successfully integrated into the silicon-on-insulator platform, showing a single-mode operation with an intrinsic Q of 2 × 10(6).

Hybrid integrated plasmonic-photonic waveguides for on-chip localized surface plasmon resonance (LSPR) sensing and spectroscopy

We experimentally demonstrate efficient extinction spectroscopy of single plasmonic gold nanorods with exquisite fidelity (SNR > 20dB) and high efficiency light coupling (e. g., 9.7%) to individual plasmonic nanoparticles in an integrated platform. We demonstrate chip-scale integration of lithographically defined plasmonic nanoparticles on silicon nitride (Si3N4) ridge waveguides for on-chip localized surface plasmon resonance (LSPR) sensing. The integration of this hybrid plasmonic-photonic platform with microfluidic sample delivery system is also discussed for on-chip LSPR sensing of D-glucose with a large sensitivity of ∼ 250 nm/RIU. The proposed architecture provides an efficient means of interrogating individual plasmonic nanoparticles with large SNR in an integrated alignment-insensitive platform, suitable for high-density on-chip sensing and spectroscopy applications.

Azimuthal-order variations of surface-roughness-induced mode splitting and scattering loss in high-Q microdisk resonators

We report an experimental observation of strong variations of quality factor and mode splitting among whispering-gallery modes with the same radial order and different azimuthal orders in a scattering-limited microdisk resonator. A theoretical analysis based on the statistical properties of the surface roughness reveals that mode splittings for different azimuthal orders are uncorrelated, and variations of mode splitting and quality factor among the same radial mode family are possible. Simulation results agree well with the experimental observations.

Unified approach to mode splitting and scattering loss in high-Qwhispering-gallery-mode microresonators

Current theoretical treatment of mode splitting and scattering loss resulting from sub-wavelength scatterers attached to the surface of high-quality-factor whispering-gallery-mode microresonators is not satisfactory. Different models have been proposed for two distinct scatterer regimes, i.e., a-few- and many-scatterers. In addition, many experimental results seem difficult to understand within the existing theoretical framework. Here we develop a unified approach that applies to an arbitrary number of scatterers, which reveals the applicable conditions and the limits of the existing theoretical models. Moreover, many new understandings on mode splitting and scattering loss have been achieved, which are supported by numerical and experimental evidences. Such a unified approach is essential for the fundamental studies as well as the practical applications of mode splitting and scattering loss in high-quality-factor whispering-gallery-mode microresonators.

Silicon microring resonator sensor with integrated PC spectrometer for sharp spectral features detection

We demonstrate a novel sensing platform using microring resonator sensors integrated with superprism based photonic crystal spectrometers for sharp spectral features detection. Using this platform a 20 pm sensor resonance wavelength shift was experimentally resolved.

Modeling and Optimization of Magnesiothermically-formed Porous Silicon in Silicon-on-insulator Microresonator Sensors

We develop a model for silicon-on-insulator microresonators with magnesiothermically-formed porous silicon cladding possessing three-dimensional interconnected pores. Investigation of waveguide design and geometrical parameters indicates an optimized areal mass sensitivity of ~ 0.2 pm/(pg/mm2).

Compact on-chip multiplexed photonic gas sensors

A compact resonator-based integrated photonic sensor for the detection of multiple gas analytes with high resolution is demonstrated. On-chip referencing is employed to compensate for environmental effects and laser instabilities to achieve high accuracy.

Multiplexed Gas Sensing with arrayed SOI microring resonators

We report a compact resonator-based integrated optical sensor for the detection of volatile organic compounds (VOC) with high resolution. On-chip reference is used to compensate for environmental effects and laser instabilities for high accuracy.

Highly Sensitive SOI Optical Sensors with Porous Si

We demonstrate microring resonators using a thin layer of porous silicon (pore size ~30 nm) as the cladding. With a loaded Q factor of 25,000, this new type of resonators is promising for bio/chemical sensing.