UC Berkeley

The previous century was defined to a major extent by the digital revolution, which ushered in the information age. This rapid proliferation and improvement of electronic technology continues to have far-reaching effects on human existence. Similarly, advances in photonics have the potential to lead to technology that drastically changes our lives. So far, the vast potential of photonics, and especially nanophotonics, has not resulted in such sweeping societal change. While the laser, LED, and photovoltaic cell serve important, and increasing, functions, technological advances in photon control at the nanoscale are required for the full potential of photonics for computing, medicine, and energy to be realized. In this dissertation, we examine some methods and devices that can address these needs in photonics, particularly by exploiting the refractive index of materials, the phase of light, and the relation between the two.

The first chapter focuses on controlling the refractive index of silicon by inducing spatially varying porosity. This technique enables the fabrication of gradient index devices that can control light’s propagation through the device to a degree that is not possible with traditional optical elements. By using the transformation optics design technique, it is possible to design a device with complete control over the path of light in the device, limited only by the material properties available. Using photoelectrochemical etching, we demonstrate the ability to control the refractive index of light in a range from 1.2 to 2.1 in devices that can be as large as centimeters, a combination not possible with other methods. This method is used to fabricate gradient index waveguides and Min˜ano concentrators, ideal optical concentrators, which are of particular interest in photovoltaics.

The next two chapters focus on using nonlinearity to manipulate light. First, the thermo- optic effect is used to design a dynamically self-assembled band structure in a distributed Bragg reflector, which exhibits reconfigurability of its transmission and reflection properties as well as self-healing to mitigate the effects of defects in the device structure. Next, a metasurface, a subwavelength array of resonant antennas is used to generate third harmonic frequency light and directly impart phase delays to it to control its reflection and refraction. Generalized laws of reflection and refraction for nonlinear harmonic generation at an interface with phase discontinuities are developed and experimentally confirmed, and asymmetric transport, which is predicted by these laws, is demonstrated. Asymmetric transport is critical for the development of integrated photonics for computing applications.

In the final section, a technique for integrating quantum dots, nanoscale light sources, with other photonic components is demonstrated. The quantum dots are incorporated into electron beam resist materials in controlled concentrations and patterned with electron beam lithography, which can precisely control their location. Single quantum dots serve as non- classical emitters, capable of generating single photons necessary for quantum computing and many other emerging fields in quantum optics.