Fig. 5. Anti-Stokes excitation scheme of V_Si defects.

a Energy diagram of the quartet (red horizontal lines) and doublets. The position of the doublets (blue and gray horizontal lines) is not exactly known. The energy differences are not linearly scaled for the sake of clarity, which includes the ionic relaxations in each multiplet (ZPL energies). The shaded area is the phonon sideband of states 1 and 2. The lowest energy doublet (²E) is shown among the other doublets. Σ is the energy difference between the ground state manifold and the ²E level (state 5’). A predominant spin-selective intersystem crossing (ISC) occurs between a resonant doublet state and 4’ (m_s = ±3/2 manifolds). b The optical excitation (red arrow) between the doublets ²E (state 5’) and the highest doublet close to resonance with the quartet levels (state 6’) followed by a fast ISC (shaded green arrow) toward the m_s = ±3/2 states due to spin–orbit coupling leads to an effective spin-flipping optical excitation (purple arrow). There might be a weaker ISC toward the m_s = ±1/2 states (green dotted arrow) too. c The nature of 3’ and 4’ states is depicted, which are the phonon coupled m_s = ±3/2 (3 and 7) and m_s = ±1/2 states (4 and 8) from the ⁴A₂ and ⁴E quartet excited states, respectively. d The high energy vibronic states 1’ and 2’ of states 1 and 2, respectively. ISC with enhanced rates may occur from states 1’ and 2’ to state 5’ beside the optical excitation toward states 3’ and 4’, respectively.