Employing nitrogen-vacancy centers in diamond for scanning probe microscopy from cryogenic to room temperature

Abstract

The negatively charged nitrogen-vacancy (NV) center is a crystal lattice defect in diamond. It exhibits a spin-1 ground state that is widely used in quantum metrology and quantum information. This prominence stems from its favorable spin and optical properties, which span a wide temperature range from zero to over 600K. In the field of scanning probe microscopy, for example, the NV center is used as a quantum sensor that is raster-scanned in nanoscopic steps over a sample surface. This technique is versatile since it is applicable at all temperatures, and thus, a wide variety of samples and temperature-dependent phenomena can be studied. In this thesis, the engineering of such a variable-temperature (7K to 350K) scanning NV microscope is described. The base temperature of 7K is achieved with a very compact cryostat design. The involved challenges in temperature management are addressed. Furthermore, a critical step in accomplishing uninterrupted operation was the design of an improved vacuum system, which avoids contamination aggregation on the sample surface under the scanning tip. Besides the experimental challenges, a fundamental lack of understanding impeded scanning NV experiments at variable temperature: despite the NV center's role in quantum applications, its photo-physics were incompletely understood, especially at intermediate temperatures between 10K to 100K where phonons become activated. This poses a key problem as the NV center's quantum state is initialized and read out via its photo-physical properties. Crucially, a prominent dip in the sensing performance is observed between 30K to 60K. The core subject of this thesis is to present a rate model that can describe the cross-over from the low-temperature to the high-temperature regime, including the dip at intermediate temperatures. Key to the model is a phonon-driven hopping between the two orbital branches in the electronic excited state, which causes a spin-lattice relaxation via an interplay with the spin precession. The model is extended to include magnetic and electric fields as well as crystal strain, allowing simulation of the population dynamics over a wide range of experimental conditions. The model recovers existing descriptions for the low- and high-temperature limits and successfully reproduces observations from previous literature. Additionally, a large and diverse set of data is used to deduce system parameters by fits with the model. Based on these parameters, the model allows for predicting various experimental observables relevant to quantum applications --- in particular, the photoluminescence intensity, spin contrast, and spin initialization fidelity. The new understanding allows for probing of the electron-phonon interaction of the NV center and reveals a gap between the current understanding and recent experimental findings. Finally, with the required simulation tools at hand, improvements to the common optical initialization and readout scheme are found and compared with experiments. The model is made openly accessible as a Python library and can be used as a predictive tool for optimizing experimental conditions in NV center applications.