Optically Detected Magnetic Resonance of a Cerium-Doped Nanocrystal: Toward the Generality of a Direct Spin-Optical...

The spin-optical interface, enabling active control of spin states in materials, has attracted widespread interest. Circularly polarized light can be used to directly inject angular momentum into spin carriers by optical excitation, providing a robust means of spin polarization and readout. This approach, although featuring direct mapping between the photonic and spin quantum states, is yet underexplored among chemical systems. In this work, CaS:Ce³⁺ nanocrystals were used to demonstrate the chemical degrees of freedom in the realization of such a strategy. CaS host possesses high symmetry, low magnetic noise, red-shifted photoluminescence spectra, and openness to further functionalization. An analytical derivation of the exact mechanism underlying the spin selectivity in this system is presented, which is numerically verified using effective Hamiltonian parameters extracted from low-temperature photoluminescence spectra. We built a cryogenic confocal optically detected magnetic resonance (ODMR) spectrometer to realize the optical spin polarization in CaS:Ce³⁺ nanocrystals and to conduct pulsed ODMR experiments, and the results of which are compared with X-band EPR for further insights. To the best of our knowledge, this is the first realization of ODMR on chemically synthesized nanocrystals. With the current setup at 2.9 K and 350 G, we achieved spin polarization exceeding 12% and a detection limit of 10⁵ spins. Spin-lattice relaxation time T₁ = 0.65(2) ms was measured by the recovery of spin polarization. Spin coherence time T₂ = 2.5(7) μs and dephasing time $T_2^{*}=45\left(7\right)\mathrm{ns}$ were measured by pulsed ODMR. Based on the resonance spectrum, the CaS:Ce³⁺ nanocrystals can function as a magnetic field sensor with an estimated sensitivity normalized to 55 fT·mol^1/2·Hz^-1/2.