Silicon has been the material of choice of the photonics industry over the last decade due to its easy integration with silicon electronics as well as its optical transparency in the near-infrared telecom wavelengths. Besides these, it has very high refractive index, and also a broad optical transparency window over the entire mid-IR till about 8[Mu]m. Photonic crystal is well known that it can slow down the speed of light. It also can provide a universal platform for microcavity optical resonators with high quality factor Q and small modal volumes. The slow light effect, high Q and small modal volumes enhance light-matter interaction, together with high refractive index of silicon can be utilized to build a highly sensitive, high throughput sensor with small footprint. In this research, we have demonstrated highly compact and sensitive silicon based photonic crystal biosensor by engineering the photonic crystal microcavity in both cavity size and cavity-waveguide coupling condition. We have developed solutions to increase biosensor throughput by integrating multimode interference device and improving the coupling efficiency to a slow light photonic crystal waveguides. We have also performed detailed investigations on silicon based photonic devices at mid-infrared region to develop an ideal platform for highly sensitive optical absorption spectroscopy on chip. The studies have led to the demonstration of the first slot waveguide, the first photonic crystal waveguide, and the first holey photonic crystal waveguide and first slotted photonic crystal waveguide in silicon-on-sapphire at mid-infrared. The solutions and devices we developed in our research could be very useful for people to realize an integrated photonic circuit for biological and chemical sensing in the future.

In a recent decade, miniaturized optical sensor devices based on photonic integrated circuits (PICs) have been investigated vigorously due to their exceptional advantages such as high speed, high sensitivity, high-density integration, low cost, compactness, and robustness against the environmental fluctuation by eliminating the assembly and alignment of the free-space optical components. Especially, near- and mid-infrared photonics have intrigued a great interest in various sensing applications including the physical object/environment imaging and mapping, material identification, and chemical/biomolecule sensing due to the strong fundamental vibrational-rotational signatures of the distinct chemical bonds and high transmittance in an ambient atmospheric environment or in-vivo mammalian bio-cells. The basic components constituting the PICs are the micro/nano waveguides to guide the light by the total internal reflection, and the most matured PIC material platforms are typically based on silicon platforms. However, depending on the targeting application's operation scheme and wavelengths, appropriate material platforms and structures should be designed and optimized to minimize material loss and maximize performance. This Ph.D. dissertation focuses on the design, fabrication, and experimental characterization of the various passive photonic components in different material platforms and photonic integrated sensor devices for the mid-infrared (MIR) absorption trace gas sensors, near-infrared (NIR) on-chip spectrometers, and biosensing applications. The first chapter covers the MIR trace gas sensor devices based on the suspended InGaAs membrane waveguides. We designed and compared two different waveguide structures on the III-V material platform considering the monolithic integration of the sensing waveguides with the light sources and detectors in MIR wavelengths. The second chapter demonstrates the NIR spatial heterodyne Fourier transform spectrometer (SHFTS) integrated with a subwavelength grating coupler (SWGC) for the dual-polarization bandpass sampling on the Si3N4 platform to solve the intrinsic trade-off limitation between the resolution and bandwidth. After that, the micro-ring resonator (MRR) biosensor devices integrated with SHFTS for the lab-on-a-chip optical biosensor platform on the silicon-on-insulator wafer were presented which can substitute the external optical spectrum analyzers for detecting the resonance wavelength shift. Finally, a thorough study on the point-of-care SARS-CoV-2 lab-on-a-chip (LoC) optical biosensor platforms is presented, and the recommendations for future research will be discussed

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