PhD in physics at KU Leuven on “Quantum color centers in diamond: unraveling the link between the atomic-scale structure and functionality”

During the last two decades, point defects in diamond – also referred to as color centers– have arisen as excellent building blocks for a wide variety of solid state quantum devices and quantum technologies. Indeed, it has been shown that various defects can be introduced, which show bright single-photon emission at room temperature. So far, the negatively charged NV- defect,consisting of a single nitrogen impurity neighbored by a vacancy, has been the most widely studied candidate. Despite a number of particular advantages of the NV- center, the long fluorescent lifetime and a weak emission of theNV- into the zero-phonon line (ZPL) set an upper limit to the achievable photon rates of the quantum devices.

As a result, there has recently been a major effort in identifying alternative color centers and investigating their optical properties for applications where high photon counts are required. A specific class of centers that show very promising properties are the so-called group IV defects, i.e., the silicon-vacancy (SiV), germanium-vacancy (GeV), tin-vacancy (SnV) andlead-vacancy (PbV). Unlike the NV centers, they exhibit strong, narrow band emission into the ZPL and limited spectral diffusion, which has been assigned to the D3d symmetric configuration of the defect structure.

In nearly all cases, ion implantation is the key methodology either to introduce the impurity ina very controlled way, or to activate existing impurities via the introduction of vacancies. The advantages of implantation are multiple, e.g., excellent control of the impurity position and of the concentration, fully compatible with technological processing, etc. At the same time, the collision cascade resulting from the implantation process delivers the required vacancies to formthe color centers. Whereas the NV center consists of a substitutional nitrogenatom with a neighboring carbon vacancy, the group IV vacancy centers are generally believed to exhibit a “split-vacancy”configuration, where the impurity resides at the middle of two C positions (bond-centeredsite), with the vacancy “split” over the two adjacent C sites. This configuration was initially suggested from abinitio calculations and confirmed indirectly from (magneto-)optical spectroscopic measurements, although thus far not experimentally proven. 

Our unique approach based on emission channeling (alattice location technique of unprecedented sensitivity) combined with photo luminescence enables us to unambiguouslyidentify the atomic defect structure and provide a direct link to specific PLlines via radio tracer photo luminescence experiments. The lattice location studies will be performed at the ISOLDE facility at CERN, making use of radioactive probe atoms. On the one hand, based on the an isotropic emission of b particles upon decay of these implanted radioactive isotopes, we can identify and quantify the lattice site, even for extremely low fluences. Using this approach, we recently demonstrated a proof-of-principle of identifying and quantifying the split-vacancy SnV configuration in diamond. On the other hand,during the decay, the specific isotope transforms to a daughter nucleus.Consequently, the decay or growth of the corresponding PL lines enables to distinguish between luminescence originating from specific elements or specific defect configurations. These experiments using radioactive isotopes will be complemented with detailed investigations on stable isotopes, using the ion implantation and PL setups in Leuven.

Based on the outcome of this PhD project, we expect to be able to provide the fundamental understanding in these color centers, required to further enhance their efficiency.

Specifically,the aim of this PhD research is to:

• Determine the exact lattice site(s) of a series of impurities (group IV and others) in diamond, both after implantation and upon thermal annealing. Using emission channeling, you will be able to investigate the low concentration regime that is relevant for single photon emission.
• Investigate the photo luminescence (position, intensity, line width…) of the color centers,both on stable ions and on radio tracers.
• Intimately link the specific PL lines to the respective configurations, and study how these can be tuned by optimizing the implantation and annealing conditions.This will allow you to unravel the formation efficiency of color centers in diamond, as well as the conditions required for narrow line emission.
• Investigate correlations between particular defect configurations and the spin dephasing and decoherence time, using Ramsey and Hahn echo pulse sequencing.