Photo-excitation dynamics of nitrogen-vacancy centers in diamond

The negatively-charged nitrogen-vacancy defect (NV–) possesses an interesting combination of spin and optical properties that lead to remarkable properties that can potentially be exploited in applications such as solid-state qubits, highly sensitive electric and magnetic field probes and single-photon emitters1.The negatively-charged nitrogen-vacancy defect (NV–) possesses an interesting combination of spin and optical properties that lead to remarkable properties that can potentially be exploited in applications such as solid-state qubits, highly sensitive electric and magnetic field probes and single-photon emitters1.Within the diamond bad gap, the NV– center forms a two-level quantum system which is comprised of a spin-triplet ground state (GS) of 3A2 symmetry and a spin-triplet excited state (ES) of 3E symmetry (see Fig. 1). The transition between both states can be excited by light of 1.95-eV photon energy (637-nm wavelength). Vibronic coupling to localized vibrational modes gives rise to a broad (~300meV) phonon sideband in the optical absorption and photoluminescence (PL) spectrum, permitting excitation with blue-shifted lasers (most commonly at 532 nm). Two more electronic levels, both being spin-singlet states, are situated within the bandgap, having 1E and 1A1 symmetry. Both singlet states are connected through a transition of 1.19-eV photon energy (1042-nm wavelength).The predominantly utilized feature of the NV– center is the spin-triplet 3A2 state that can be manipulated with microwave radiation and its spin state read out via the PL of the triplet transition as optically detected magnetic resonance (ODMR). The ability of the NV– center to perform ODMR, which requires establishing spin polarization in 3A2, relies on a series of transitions between the NV– electronic states. The non-equilibrium dynamics of NV–, shown schematically in Fig. 1, are thus of crucial importance to the utility of the defect.We present results using femtosecond transient absorption spectroscopy to investigate the dynamics of ensembles of NV– centers in bulk diamond after photoexcitation of the triplet transition by probing the transient dynamics of its optical signatures2.We probe the timescale of the vibrational relaxation in the 3E state, i.e. how fast does an excitation into the phonon sideband relax to the 3E vibrational ground state. We find a vibrational relaxation time of about 50 fs from the first and second phonon line3. This is exceptionally fast, occurring faster than the vibrational period of 60 fs of the involved vibrational mode (~65 meV).We also monitor the population dynamics of the 1A1 state, which is populated via intersystem crossing (ISC) from the excited 3E state, by probing stimulated emission and absorption from the singlet transition 1E - 1A1 at 1042nm. We find a lifetime of 100 ps for the 1A1 state4.Finally, by combining femtosecond laser photoexcitation with CW laser probing, we are able to probe the whole equilibration cycle over timescales from nanoseconds to seconds with nanosecond time resolution by monitoring the bleaching of the NV– triplet absorption, its stimulated emission, as well as the absorption of the singlet transition.

References

1 Doherty, M. W. et al. Phys. Rep. 528, 1-45 (2013).

2 Ulbricht, R. et al. Nat. Commun. 7 (2016).

3 Ulbricht, R. et al. Phys. Rev. B 97, 220302(R) (2018).

4 Ulbricht, R. and Loh, Z.-H. Phys. Rev. B 98, 094309 (2018).