A theoretical study of differences in broadband high-indexcontrast grating (HCG) reflectors for TM and TE polarizations is presented, covering various grating parameters and properties of HCGs. It is shown that the HCG reflectors for TM polarization (TM HCG reflectors) have much thicker grating thicknesses and smaller grating periods than the TE HCG reflectors. This difference is found to originate from the different boundary conditions met for the electric field of each polarization. Due to this difference, the TM HCG reflectors have much shorter evanescent extension of HCG modes into low-refractive-index media surrounding the HCG. This enables to achieve a very short effective cavity length for VC-SELs, which is essential for ultrahigh speed VCSELs and MEMS-tunable VCSELs. The obtained understandings on polarization dependences will be able to serve as important design guidelines for various HCG-based devices.

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We report a novel high-quality (Q) factor optical resonator using a subwavelength high-contrast grating (HCG) with in-plane resonance and surface-normal emission. We show that the in-plane resonance is manifested is by a sharp, asymmetric lineshape in the surface-normal reflectivity spectrum. The simulated Q factor of the resonator is shown to be as high as 500,000. A HCG-resonator was fabricated with an InGaAs quantum well active region sandwiched in-between AlGaAs layers and a Q factor of >14,000 was inferred from the photoluminescence linewidth of 0.07 nm, which is currently limited by instrumentation. The novel HCG resonator design will serve as a potential platform for many devices including surface emitting lasers, optical filters, and biological or chemical sensors.

Subwavelength High-Contrast Grating (HCG) and its Applications in Optoelectronic Devices Optical grating is a research topic with a long history. It has been extensively studied over the years due to its various applications in holography, spectroscopy, lasers, and many other optoelectronic devices. In this dissertation, we present a novel single-layer subwavelength high-index-contrast grating (HCG) which opens a new era in the study of grating. HCGs can serve as surface normal broadband (∆λ/λ ~35%), high-reflectivity (>99%) mirrors, which can be used to replace conventional distributed Bragg reflectors (DBRs) in optical devices. Different designs of HCGs can also serve as narrow band, surface emitting, high-quality (Q) factor optical resonators or shallow angle reflectors. In this dissertation, we will review the recent advances in high-index-contrast grating and its applications in optoelectronic devices, including vertical-cavity surface-emitting lasers (VCSELs), high-Q optical resonators, and hollow-core waveguides. We first present a novel HCG-based VCSEL where the conventional DBR mirror is replaced with a HCG-based mirror. A systematic and comprehensive review of the experimental and numerical simulation results is presented to demonstrate many desirable 2 attributes of HCG-based VCSELs, including polarization selection, transverse mode control and a large fabrication tolerance. Next, we present an ultra-fast tuning, HCG-based tunable VCSEL. By integrating a mechanically movable actuator with a single-layer HCG as the VCSEL top mirror, precise, wide continuous wavelength tuning (~18 nm) was achieved at room temperature. The small footprint of the HCG enables each of the mechanical actuator dimensions to be scaled down by at least a factor of 10, resulting in a greater than 1000 times reduction in mass, and an increase in the mechanical resonant frequency. It also allows for a record-fast, HCG-based tunable VCSEL with a tuning time in the ~10 ns range to be obtained. Besides the HCG-based VCSELs/tunable VCSELs, we also present a HCG-based surface normal high-Q resonator with a simulated Q-factor as large as 500,000 and an experimentally measured Q-factor of ~14,000. The unique feature of a high-Q with surface normal emission is highly desirable, as the topology facilitates a convenient and high output coupling with free-space or fiber optics. This feature is promising for array fabrication of lasers and filters, as well as high throughput sensor arrays. In addition, we propose a HCG-based hollow-core waveguide design with an ultralow propagation loss of <0.01dB/m, three orders of magnitude lower than the lowest loss of the state-of-art chip-scale hollow waveguides. This novel HCG hollow-core waveguide design will serve as a basic building block in many chip-scale integrated photonic circuits, enabling system-level applications including optical interconnects, optical delay lines, and optical sensors. ____________________________________ Professor Constance J. Chang-Hasnain Dissertation Committee Chair

Peripheral nerve injuries represent one of the most common causes of permanent disabilities. Therapeutic electrical stimulation has been widely used in neural regeneration for decades. Combined with the implantation of a nerve cuff, several outcomes have proven effectiveness and feasibility in neuroprosthetic applications. However, the current electrical stimulation strategy fails to complete nerve repair. There is a lack of research on long‐term implantable nanogenerators in the neurostimulation scenario. Especially considering many disease models, those devices may not reach the in vitro simulative working setting. Thus, an implanted sciatic nerve stimulation system that spontaneously generates biphasic electric pulses in response to rats’ movement is developed. The electric signals generated by this device could stimulate injured sciatic nerve by cuff electrode. This work introduces an implantable self‐regulated neural electrical stimulation system generated by a contact separation triboelectric nanogenerator with a nerve cuff electrode and compares it with chronic therapeutic electrical stimulation for sciatic nerve restoration effect. Neural function restoration is observed in gait and histological analysis. Moreover, the upregulation of growth associated protein 43 can be a protentional target. This could have potential clinical application in facilitating closed‐loop energy harvesting for long‐term electrical stimulation.

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