Techniques and Devices
Work towards the goals of SPANGL4Q explores three main topics, and is supported by theoretical work. All of our work is based on using semiconductor quantum dots: artificial atomic-like nanostructures encased in semiconductor material, capable of trapping single electrons and interacting with specific frequencies of light in a controlled manner.
Photonic Crystal Waveguides and Cavities
Photonic crystals are structures that trap and control light by modulating the refractive index over a wavelength scale, for example, by creating holes in a high refractive index semiconductor. By spacing the holes by approximately a quarter wavelength one can show that it is impossible for the light to propagate through the material. One can then leave out holes to trap light in a very small volume. “Lines” of missing holes create waveguides to direct light, and just a few isolated missing holes create a light trap, or “cavity”. We explore how the design of waveguides and cavities affect how the angular momentum of light (polarization) interacts with the angular momentum in a quantum dot electron spin, that is contained within the cavity.
Antennas, such as those used for radio and television, collect signal (electromagnetic radiation) and focus it to a small area. Nanoantennas operate in a similar way, but in scaling them down, the operate at lengths close to the wavelength of light. Due to the optical properties of metals, they are capable of focussing light to a few tens of nanometres, and enhance light-matter interactions very strongly. The strong interaction over a small volume will allow us to control the properties of the quantum dot with light very fast (femto to picosecond timescales). And just as with metamaterials, the antennas may be shaped to manipulate the polarization of the light into novel “orbital” angular momentum states. We will explore how the shape affects the transfer of angular momentum to a quantum dot spin.
Nuclear Spins as Memories
The motivation for this work is to achieve long storage of a photon polarization using a spin in a semiconductor. While nanophotonic structures allow one to trap single photons and transfer their polarization to a quantum dot spin, this is usually contained on the electron, and is therefore subject to the vibrations of a typical semiconductor (phonons, or heat). These randomise the orientation of the electron spin in a way that is almost unavoidable. We will look at using instead the spins of the nuclei within the quantum dot. It is known that nuclei may retain their spin for hours. In this project we will explore ways of addressing single spins within the quantum dot using NMR techniques, and use ultra-sensitive methods to probe the effect of the nuclei on the electron spin. This will finally allow us to transfer the information to a photon, where it may propagate over long distances.
Theory, and future spin-optics devices
While quantum dots and electrons spins are beginning to be well-understood, the role of nuclear interactions is still being explored. We will find ways to describe the electron-nuclear system theoretically. In addition, while polarization of light propagating over long distances is better understood, a theory which describes how they interact in nanocavities is not as well-developed. We will look at theories to describe angular momentum in the cavities, and as a result, develop novel photonic devices with completely new spin characteristics resulting from the photonic design.