Laser-induced creation of coherent V2 centers in bulk-grown silicon carbide

Abstract

Solid-state spin defects are promising qubits for quantum network nodes. A key challenge towards larger networks is creating defects with high yield into nanophotonic devices, while maintaining good optical and spin properties. Here, we demonstrate the creation of V2 centers in nanopillars fabricated from commercial bulk-grown 4H-silicon carbide using a pulsed above-bandgap (UV) laser. We observe an eleven-fold increase in the V2 center occurrence after UV laser illumination. These laser-induced V2 centers exhibit narrow optical linewidths and spectral diffusion rates comparable to naturally occurring V2 centers in nanopillars of the same material. Furthermore, we measure a spin coherence time of $T_{2}^{\mathrm{DD}} = 3.6 \pm 0.3~\text{ms}$ under dynamical decoupling, consistent with dephasing by the nuclear-spin bath. This demonstration of the in-situ, post-fabrication generation of coherent V2 centers in nanostructures in widely available bulk-grown 4H-SiC, shows the potential for above-bandgap laser illumination for scalable defect creation in integrated photonic devices.

Figures (28)

• Figure 1: Laser-induced defect creation using an above-bandgap pulsed laser.a) Schematic of the laser-induced creation and detection of defects in HPSI 4H-SiC. A 0.7 NA objective focuses an above-bandgap (337nm) pulsed (3ns) laser onto the center of the nanopillar to create defects. For imaging, we use a 0.9 NA objective and a $785nm$ laser for off-resonant excitation. b) Scanning electron microscopy image of the HPSI amorphized nanopillars that were exposed to a single UV pulse with energies higher then the LIAT ($0.28µJ$). The nanopillars are $\sim$$1.2µm$ in diameter and $\sim$$4µm$ spaced from center to center. c) Amorphisation statistics of nanopillars using a single pulse. For each pulse energy, one hundred nanopillars were each illuminated with a single pulse, and the percentage of nanopillars that showed visible damage is indicated on the y-axis. Blue dots indicate $0.18µJ$ and $0.24µJ$, which are used for \ref{['fig:amound_defects']} and \ref{['fig:diffusion_ple']}. See \ref{['fig:supp_damage_np_bulk']} for SEM pictures of amorphisation. d) 2D PL scans before and after a single UV pulse of $0.24µJ$ at each pillar. The counts for both measurements were corrected to have the same background counts in bulk close to the nanopillars (before counts $\times 1.524$, and after counts $\times 0.656$, see \ref{['subsec:app_before_after_2dpl']} for correction method). e) Fluorescence saturation measurements before and after a single UV pulse of $0.24µJ$ on the same pillar.
• Figure 2: Photoluminescence Excitation spectroscopy (PLE) and number of $\mathrm{V2}$ centers.a) Single PLE of a nanopillar with three V2 centers passing a threshold (dashed grey line) at 0.3kHz. Laser frequency is offset from 327.112THz. Inset shows the experimental sequence, green indicates an off-resonant (repump) pulse (10µs, 10µW), red indicates a single resonant laser (2ms, 100nW). Grey indicates microwaves (MW) at (70MHz) to counteract spin pumping. b) Percentage of $\mathrm{V2}$ centers whose fitted peak amplitude exceeds the count-rate threshold, shown separately for nanopillars exposed to a UV pulse (0.18µJ and 0.24µJ) and those left unexposed; in each condition, $n=64$ nanopillars are examined. We find a maximum count rate for the non-exposed $\mathrm{V2}$ center of 0.76kHz. For this threshold, we also observe three and eleven times more $\mathrm{V2}$ centers with the same or higher count rate at UV laser powers of 0.18µJ and 0.24µJ, respectively. c) Ensemble inhomogeneous distribution of all $\mathrm{V2}$ centers in unexposed nanopillars (bottom grey bars). A Gaussian fit yields a FWHM of the inhomogeneous distribution of 22±1GHz. The blue bars indicate the frequencies of the $\mathrm{V2}$ centers in the UV-exposed nanopillars that pass the threshold of 0.3kHz (this threshold sets a cut-off to suppress bulk-V2 contributions, also indicated by the dotted line in b)). The black dashed line shows the (scaled) Gaussian fit of the unexposed inhomogeneous distribution as a guide to the eye. d) Number of nanopillars that contain either 0, 1, 2, or 3 $\mathrm{V2}$ centers that pass the 0.3kHz threshold. Nanopillars exposed to the UV laser are shown in shades of blue, while natural (unexposed) $\mathrm{V2}$ centers are shown in shades of grey. The numbers inside the bars show how many pillars contain that specific number of $\mathrm{V2}$ centers.
• Figure 3: $\mathrm{V2}$ optical properties and spectral diffusion.a) Fitted spectral diffusion constant under off-resonant excitation for natural $\mathrm{V2}$ centers (circles) and laser-induced $\mathrm{V2}$ centers (squares and triangles, 0.18µJ and 0.24µJ respectively). We extract $\gamma_\mathrm{d}$ using the methodology described in van de Stolpe et al.vandestolpeCheckprobeSpectroscopyLifetimelimited2025, assuming a homogeneous linewidth (FWHM) of 36MHz. Because the fitted $\gamma_\mathrm{d}$ depends on this homogeneous linewidth (see \ref{['subsec:supp_spectral_diffusion']}), and this linewidth was not measured for each individual $\mathrm{V2}$ center, we indicate the spread in $\gamma_\mathrm{d}$ for a plausible range of linewidths ($1\leq\Gamma/\Gamma_{\mathrm{lifetime}}\leq3$, see \ref{['subsec:app_cp_ple_sweep']}). $\gamma_\mathrm{d}$ of the red square indicates the same $\mathrm{V2}$ center as the green square in b) and data in c) and d). b) Fitted spectral diffusion constants under resonant excitation. c) Scanning laser PLE over 5 minutes of the $\mathrm{V2}$ center highlighted in a) and b) (Pillar 14, \ref{['fig:supp_ple_location_018']}). The laser is on for a total of 334ms per scan with a power at the objective of 20nW. d) Check-Probe PLE (see van de Stolpe et al.vandestolpeCheckprobeSpectroscopyLifetimelimited2025 for methodology) with FWHM = $52\pm4MHz$ indicating similar optical linewidth ($\Gamma /\Gamma_{\mathrm{lifetime}} = 2.21\pm0.04$, see \ref{['fig:supp_cp_ple_sweep_018']}) as natural $\mathrm{V2}$ centers in this material (see \ref{['fig:supp_cp_ple_sweep_0']}). Check-probe PLE was performed at a magnetic field of $\sim$20 aligned along the crystal c-axis.
• Figure 4: Electron spin properties.a) Rabi chevron pattern from the $\mathrm{V2}$ center in pillar 9. Details on the measurement sequence and normalisation procedure are elaborated in \ref{['subsec:app_spin_stuff']}. b) Electron-spin-resonance of the same $\mathrm{V2}$ center revealing a strongly coupled nuclear spin with hyperfine splitting of 2.19±0.05MHz (red line). The two solid vertical lines indicate the fitted frequencies from the Ramsey measurement, matching the ESR frequencies. c) Detuned Ramsey measurement on the same $\mathrm{V2}$ center with $f_{MW} = 181.8MHz$. Red line indicates a fit to an oscillation with two frequency components and a Gaussian decay, which translates to a hyperfine splitting of $f_{HF} = 2.08\pm0.07MHz$, and $T_{2}^{*}$$\,=\,$0.9±0.1µs. d) Scaled Hahn-echo ($N=1$) and dynamical decoupling ($N =2$ to $N =16$) measurements on the $\mathrm{V2}$ center in pillar 14 (\ref{['fig:supp_ple_location_018']}) with no (resolvable) strongly coupled nuclear spin (see \ref{['fig:supp_spin_4_32']} for Chevron, ESR and Ramsey) at a magnetic field of $\sim$1300. We find a $T_{2}^\mathrm{Hahn}$ = 0.49±0.02ms and extend the coherence time to $T_{2}^\mathrm{DD}$ = 3.6±0.3ms with two $\mathrm{XY8}$ sequences. The inset shows $T_{2}$ as a function of the number of $\pi$-pulses N, fitted to a power function $T_{2}=\mathrm{\beta\cdot N^{\alpha}}$(excluding Hahn-echo) from which we extract $\alpha = 0.73\pm0.04$ and $\beta = 0.46\pm0.04ms$.
• Figure S1: SEM of LIAT for bulk and nanopillars.a) Bulk experiment with single pulse energy of 0.20µJ, of a grid of 100 spots with no visible amorphisation. b) Bulk experiment with single pulse energy of 0.38µJ with probabilistic (80%) amorphisation. c) Bulk experiment with single pulse energy of 0.40µJ with 100% amorphisation. d) Nanopillar experiment with single pulse energy of 0.24µJ of a grid of 100 spots with no visible amorphisation. e) Nanopillar experiment with single pulse energy of 0.3µJ with probabilistic (46%) amorphisation. f) Nanopillar experiment with single pulse energy of 0.34µJ with 100% amorphisation. g) The number of damaged sites for each pulse energy, for both bulk and nanopillars. h) Example of the focusing procedure of the UV laser. For each row, the z-position is slightly adjusted to compare the surface-damage diameter. In all the SEM images, the white bar indicates 5µm.