Here, we use a novel growth scheme to overcome this roadblock and directly grow on-chip InGaAs nanopillar lasers, demonstrating the potency of bottom-up nano-optoelectronic integration. Unique helically-propagating cavity modes are employed to strongly confine light within subwavelength nanopillars despite low refractive index contrast between InGaAs and silicon. These modes thereby provide an avenue for engineering on-chip nanophotonic devices such as lasers. Nanopillar lasers are as-grown on silicon, offer tiny footprints and scalability, and are thereby particularly suited to high-density optoelectronics. They may ultimately form the basis of the missing monolithic light sources needed to bridge the existing gap between photonic and electronic circuits. 2Since the first laser demonstrated that stimulated emission processes in an optical medium can implement a powerful, coherent light source 1 , the field of photonics has witnessed an explosion of applications in telecommunications, lighting, displays, medicine, and optical physics amongst others. Integration of photonic and electronic devices to leverage the advantages of both has subsequently attracted great interest. In particular, integration of optical interconnects onto silicon (Si) chips has become critical as ongoing miniaturization of Si logic elements has incurred a bottleneck in inter-and intra-chip communications 2,3 . Efforts towards creating on-chip light sources for optical interconnects have included engineering silicon and germanium for optical gain 4-6 and stimulated Raman scattering 7-9 . Concurrently, III-V lasers have been heterogeneously bonded onto silicon substrates [10][11][12] . However, numerous challenges face these approaches. Wafer bonding have low yields because of a stringent surface flatness requirement down to the atomic scale, while group IV emitters must overcome an indirect band gap that offers exceedingly inefficient radiation. Monolithic growth of high-performance III-V lasers on silicon thereby remains a "holy grail" for cost-effective, massively scalable, and streamlined fabrication of on-chip light sources.The fundamental roadblock facing monolithic integration up to now has been a large mismatch of lattice constants and thermal expansion coefficients between III-V materials and The nanopillar-based laser is schematically depicted in Figure 1A. shows the hexagonal cross-section of the nanopillar, which results from its unique single crystal wurtzite structure 15 . As we will later show, the as-grown nanopillar structure provides a natural optical cavity supporting unique resonances that have not been observed before to the best of our knowledge. As such, nanopillars do not require additional top-down processing to form on-chip optical cavities. Instead, they provide a viable bottom-up approach towards integrating light sources and resonators onto a silicon chip.Importantly, nanopillars possess several critical advantages for optoelectronic integration onto silicon. They grow at a low temperature of 400 °C, which is dra...

A simple analytic analysis of the ultra-high reflectivity feature of subwavelength dielectric gratings is developed. The phenomenon of ultra high reflectivity is explained to be a destructive interference effect between the two grating modes. Based on this phenomenon, a design algorithm for broadband grating mirrors is suggested.

We propose planar, high numerical aperture (NA), low loss, focusing reflectors and lenses using subwavelength high contrast gratings (HCGs). By designing the reflectance and the phase of non-periodic HCGs, both focusing reflectors and lenses can be constructed. Numerical aperture values as high as 0.81 and 0.96 are achieved for a reflector and lens with very low losses of 0.3 and 0.2 dB, respectively. The design algorithm is also shown to be readily extended to a 2D lens. Furthermore, HCG optics can simultaneously focus the reflected and transmitted waves, with important technological implications. HCG focusing optics are defined by one-step photolithography and thus can be readily integrated with many devices including VCSELs, saturable absorbers, telescopes, CCDs and solar cells.

We demonstrate slow light via population oscillation in semiconductor quantum-well structures for the first time. A group velocity as low as 9600 m/s is inferred from the experimentally measured dispersive characteristics. The transparency window exhibits a bandwidth as large as 2 GHz.

Monolithic integration of III-V compound semiconductor devices with silicon CMOS integrated circuits has been hindered by large lattice mismatches and incompatible processing due to high III-V epitaxy temperatures. We report the first GaAs-based avalanche photodiodes (APDs) and light emitting diodes, directly grown on silicon at a very low, CMOS-compatible temperature and fabricated using conventional microfabrication techniques. The APDs exhibit an extraordinarily large multiplication factor at low voltage resulting from the unique needle shape and growth mode.

We propose a novel ultra-low loss single-mode hollow-core waveguide using subwavelength high-contrast grating (HCG). We analyzed and simulated the propagation loss of the waveguide and show it can be as low as 0.006 dB/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.

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

We report room temperature demonstration of slow light propagation via coherent population oscillation (CPO) in a GaAs quantum well waveguide. Measurements of the group delay of an amplitude modulated signal resonant with the heavy-hole exciton transition reveal delays as long as 830 ps. The measured bandwidth, which approaches 100 MHz, is related to the lifetime of the photoexcited electron-hole (e-h) plasma as expected for CPO process.