Photo-Excitation Dynamics of Nitrogen-Vacancy Centres in Diamond
Dr. Ronald Ulbricht (Max-Planck-Institut für Polymerforschung)

The negatively-charged nitrogen-vacancy defect (NV–) possesses an interesting combination of spin and optical properties that can potentially be exploited in applications such as solid-state qubits, highly sensitive electric and magnetic field probes and single-photon emitters.
Within the diamond bandgap, the NV– centre forms an optically accessible two-level quantum system which consists of a spin-triplet ground state of 3A2 symmetry and a spin-triplet excited state of 3E symmetry. Two more electronic levels, both being spin-singlet states (1E and 1A1), are situated within the bandgap. NV centres can also exist in the neutral charge state (NV0). The predominantly utilized feature of the NV– centre is the spin-triplet 3A2 state that can be manipulated with microwave radiation and its spin state read out via the PL yield of the triplet transition as optically detected magnetic resonance (ODMR). Recently, photoelectric detection of magnetic resonance (PDMR) has been demonstrated as an alternative that utilizes state-selective ionization of NV– to NV0 and photocurrent detection. Despite being one of the best-studied solid-state defects, the non-equilibrium dynamics of NV centres are not yet fully understood, particularly with respect to charge conversion.
We present results using time-resolved spectroscopic techniques such as transient absorption spectroscopy, photocurrent spectroscopy and THz time-domain spectroscopy to investigate the dynamics of ensembles of NV centers in bulk diamond after photoexcitation by probing the transient response of its optical signatures. Two separately wavelength-tunable femtosecond pulses (450-1040nm) for excitation, combined with broadband spectral probing (400-1650nm) over timescales reaching from fs to ms enable us to probe all relevant optical transitions in a time-resolved fashion, providing a direct measure of complex processes such as photoionization and charge conversion. Variation of the concentration of single substitutional nitrogen (Ns) in different samples permits us to characterize their influence on NV dynamics.
We probe the electronic dynamics of both NV0 and NV– centres. For the latter one, we characterize the whole spin polarization cycle and find two additional localized electronic states. We find that recombination of electrons from the conduction band after photoionization of NV– via 3E proceeds through two distinct relaxation channels. Using photocurrent spectroscopy, we also experimentally determine the photon energy threshold for photoionization from 3E.