Cavity quantum electrodynamics (cQED), one of the most curious and fundamental aspects of optical physics, explores quantum dynamical processes for individual quantum objects strongly coupled to an electromagnetic resonator making two objects into one composite quantum system. One experimental bottleneck for cQED experiments using atoms is atomic motion. Atomic motion was recently reduced, but not stopped, by using lasers and magnets to cool and trap the atoms. In this project a semiconductor quantum system (quantum dot) will be coupled to a resonator. This system will explore different physics than atomic counterparts because the quantum dot does not move. Added features of the proposed experimental system are that the quantum dots will be coupled to an electromagnetic resonator constructed of a structured material called a photonic crystal. The photonic crystal resonator volume is near the minimum possible and confinement of the electromagnetic field is very high, so the coupling between the quantum dot and the electromagnetic field is very strong. Moreover, the structure is stable and totally integrated so it can be used over and over again. This research project will focus on the group's emerging capability to investigate a semiconductor nanosystem where individual quanta play a decisive role. The first specific goal of this project is to observe laser behavior from a single quantum dot. Other goals are to demonstrate a "photon blockade," which turns a incident laser beam into sequence of photons exhibiting quantum mechanical statistics, and to see the multiphoton coupling between the cavity and the quantum dot.
Two graduate students will gain broad experience with semiconductor sample manipulation, nanopositioning, resolution-limited optics, continuous-wave and ultrafast-laser spectroscopy, and photon statistics measurements. The nanophotonics industry is also impacted by the education of undergraduate and graduate students trained in the basic physics and experimental skills of nanodevices. The PI of this project is the senior Faculty Associate for the College of Optical Sciences as a part of the NSF ADVANCE Institutional Transformation award to create a multi-tiered strategy for improving the representation and advancement of women in science, technology, engineering, and math fields. Cavity quantum electrodynamics with strong coupling enables nanoscience to go beyond traditional nonlinear optics and laser physics into a new regime with dynamical processes and active devices now involving quantum dots and photons taken one by one. This research will enable links between the atomic and semiconductor materials communities.
The goal of this semiconductor cavity quantum electrodynamics (QED) project is to study and exploit the interaction or coupling between light and matter at the quantum level, meaning one photon at a time. Light coupling between a single quantum dot and a photonic crystal slab nanocavity can be seen because the nanocavity’s tiny mode volume makes the electric field of a single photon extremely large. In a source of single photons on demand, this coupling (often called Purcell enhancement) can be used to stimulate the photon into the desired spatial and spectral mode for efficient extraction. It also accelerates the emission process making the decays more nearly identical and the emitted single photons more nearly indistinguishable. In the regime of strong coupling, the light-coupling interaction strength is larger than the decay mechanisms of photon loss from the cavity and exciton polarization decay in the medium. As a result the absorption of a single photon can alter the transmission and reflection of the nanocavity, making it possible to build some day an optical switch actuated by a single photon (energy of only 1.8 X 10-19 J at 1500 nm telecom wavelength). During this grant the rate of loss of photons from the nanocavity was reduced by improving the fabrication of gallium arsenide cavities and investigating better materials such as silicon. The lowest photon loss was obtained in a wire-like silicon nanobeam cavity that uses a one-dimensional photonic crystal to confine the light along the beam. Bringing telecom-wavelength light to these silicon nanobeams is being attempted by wafer bonding of quantum wells and quantum dots. Although silicon gives the lowest loss rate, measured values always exceed theoretical ones because of surface roughness introduced during the etching of the photonic crystal holes. An important step forward for semiconductor cavity QED during this grant was the discovery that atomic layer deposition of a thin layer (20 nm) of aluminum oxide smoothens the surface and decreases the loss rate on average by about 50% and sometimes as much as a factor of two. Also reported is evanescent coupling between a semiconductor quantum well grown very close to the top surface by molecular beam epitaxy and metal (silver) U-shaped resonators fabricated on top; this is a significant step toward understanding and utilizing metal-semiconductor hybrid systems. This basic physics research project enhances the impact of women in science and engineering, the scientific education of undergraduate and graduate students, and the link between atomic and nanophotonic researchers.
