Spin-Optical Dynamics and Quantum Efficiency of a Single V1 Center in Silicon Carbide

Color centers in silicon carbide are emerging candidates for distributed spin-based quantum applications due to the scalability of host materials and the demonstration of integration into nanophotonic resonators. Recently, silicon vacancy centers in silicon carbide have been identified as a promising system with excellent spin and optical properties. Here, we fully study the spin-optical dynamics of the single silicon vacancy center at hexagonal lattice sites, namely V1, in 4H-polytype silicon carbide. By utilizing resonant and above-resonant sublifetime pulsed excitation, we determine spin-dependent excited-state lifetimes and intersystem-crossing rates. Our approach to inferring the intersystem-crossing rates is based on all-optical pulsed initialization and readout scheme, and is applicable to spin-active color centers with similar dynamics models. In addition, the optical transition dipole strength and the quantum efficiency of V1 defect are evaluated based on coherent optical Rabi measurement and local-field calibration employing electric field simulation. The measured rates well explain the results of spin-state polarization dynamics, and we further discuss the altered photoemission dynamics in resonant enhancement structures such as radiative lifetime shortening and Purcell enhancement. By providing a thorough description of the V1 center’s spin-optical dynamics, our work provides deep understanding of the system, which guides implementations of scalable quantum applications based on silicon vacancy centers in silicon carbide.

Figure 1

(a) General five-level rate model applicable for many optically active solid-state spin defects. (b) Spin-quartet systems with $|\mathit{S}|=3/2$ are further described by a six-level energy level scheme. Dashed arrows indicate all nonradiative processes while solid lines denote radiative transitions. The factor of $1/2$ for ${\gamma }_{3,4}$ in (a) ensures equivalence to the six-level model in (b).

Figure 2

Confocal setup with main instruments sketched in this work (for details see the main text).

Figure 3

Excited-state lifetime and optical Rabi measurements. (a) Measurement sequence for the spin-selective excited-state lifetimes. (b) Fluorescence emission from excited states with single exponential fits. (c) Measurement sequence for optical Rabi oscillations. The ground-state spin is initialized by the above-resonant laser. (d) Fluorescence emission as a function of the square root of the calibrated pulse energy ($E_p^{1/2}$). Black curves are from simulation based on quantum master equations [25].

Figure 4

Deterministic spin initialization with pulse-train scheme and pump-probe measurement results. (a) The pulse sequence of the measurement. The system is initialized by pulses with long intervals and relaxed into the ground states. Then, the system is excited with two consecutive pulses for “pump” and “probe” to extract the rates. (b) The pulsed excitation probability with its saturation fit. (c) The fluorescence ratio data $1-N_2/N_1$ are shown for three different excitation probabilities after the background subtraction and pulse energy correction (see Appendix pp4) with their single exponential fits. (d) The prefactor of the exponential decay $\alpha$ obtained from fitting in (c) depicts its linear dependence of the excitation probability with the slope determined by the rates.

Figure 5

(a) Resonant depletion measurement sequence (see the main text for detailed explanation). (b) The remaining ground-state population after depletion into the dark state as a function of the initialization time at different initialization powers. The solid lines denote the simulated curves using the experimental obtained rates in Table 1.

Figure 6

Full electronic fine structure of V1 (left). Simplified six-level energy scheme (right).

Figure 7

Simulation of the initialization of the ground-state spin population using a pulse train of short pulses. (a) Spin population prepared with various pulse interval $t_p$ with a large number of initialization laser pulses $N_p=100$. (b) Ground-state spin population in $m_s\pm 1/2$ prepared with a various number of initialization laser pulses at fixed $t_p=1\mu \mathrm{s}$. The initial state (before applying any laser pulse) is in the completely depolarized state. (c) Simulated spin population in $m_s\pm 1/2$ with nine initialization pulses at each interpulse delay in the pulse-train experiment at first, second, and the third round of the measurement sequence. The spin polarization can be well prepared from the second round of the sequence. For all plots, $P_e=0.608$ is used, and the vertical axis is a relative error of the spin population in $m_s\pm 1/2$ from the ideal theoretical value of $n_{g_1}^{\mathrm{\infty }}(\mathrm{\infty })$ given in Eq. (C1).

Figure 8

Pulse energy fluctuation of the “pump” and “probe” pulses in the pulse-train measurement is shown for different excitation probabilities in (a),(b),(c).

Figure 9

Measurement sequence for resonant spin-polarization readout.

Figure 10

Parameter fine-tuning-based simulated resonant depletion curves denoted by solid lines.

Figure 11

Photoluminescence as a function of the laser power (cw 730 nm) measured before the objective for the defect in SIL (on which all the measurements mentioned in the main text are conducted) and a V1 defect in the bulk area.

Figure 12

Electric field strength at focus as a function of the focal depth, i.e., the distance from the focus to the air-SiC (bulk) interface simulated from FDFD method using the diffraction limited beam waist ($\mathrm{NA}=0.9$) and the measured $\pi$-pulse power as input parameters.