Annemarie Exarhos

Research Interests: experimental condensed matter, optical spectroscopy of atom-scale defects in crystals, confocal fluorescence microscopy, single-photon spectroscopy

Defects in crystals can cause drastic changes to the optical, electronic, and magnetic properties of the material and understanding how these changes affect material properties is important for applications in devices. An example of this is yellow diamond, where a small percentage of carbon atoms in a diamond lattice are replaced with nitrogen causing a change in the color of the crystal from clear or colorless to yellow. If instead of nitrogen, boron atoms replace some of the carbon atoms, the result is a blue diamond. By manipulating the type and density of defects as well as the host crystal in which they are embedded, we can take advantage of the new properties that manifest for new technological applications.

The experiments we perform at Lafayette focus on characterizing the optical properties of semiconductors when defects consisting of just one or a few atomic sites exist. Such defects, also known as quantum emitters or artificial atoms, have electronic structures largely isolated from their host crystal, resulting in properties that can make a particular type of defect very attractive for use in quantum technologies, such as qubits for quantum computation, quantum communication, atom-scale sensing, and nanophotonics. The prototypical example of such a defect is the nitrogen-vacancy center in diamond, but the presence of quantum emitters in semiconductors is not unique to diamond. There are many types of crystals which can host quantum emitters and current research has barely scratched the surface of all the chemical and electronic structures which are possible.

Our lab uses a variety of optical spectroscopic techniques to study defects in a variety of wide-bandgap semiconducting crystals which emit single-photons in the visible part of the spectrum. Some of quantum emitters can have an electron spin associated with them, creating an atomic scale magnetic sensor that is incredibly sensitive to the presence of other magnetic or electric fields, as well as to temperature or chemical changes. Using confocal fluorescence spectroscopy, we observe the way in which light is emitted from these defects in response to different wavelengths and powers of the incident laser, the polarization of the incident light, temperature, atmosphere, magnetic fields, etc. By designing experiments to better understand the optical and electronic properties of a particular type of quantum emitter in a particular crystal, we characterize the chemical and electronic properties of the emitter for implementation in future quantum technologies.

Nia Burrell, ’19 worked with Professor Exarhos for her senior thesis to design and build a confocal fluorescence microscope for characterization of quantum emitters.