Strain-engineered nanoscale spin polarization reversal in diamond nitrogen-vacancy centers

TL;DR

The work addresses spin control in solid-state quantum emitters by introducing strain engineering as an in situ tool to tailor spin-dependent photodynamics in NV centers. By applying high-pressure, symmetry-breaking strain gradients in a diamond-anvil cell, it reveals complete reversal of optical spin polarization from $|0\rangle$ to $|\pm 1\rangle$ through strain-induced excited-state mixing and altered intersystem crossing, with a nanoscale transition region of about $120~\mathrm{nm}$. A quantitative Hamiltonian framework with $D$, $E_1$, and $E_2$ terms, coupled with ODMR and time-resolved spectroscopy, extracts the strain magnitudes and spin-dynamics rates, and demonstrates magnetic-field tuning to suppress the reversal. This strain-driven control opens routes to programmable quantum light sources, high-density spin memories, and hybrid photonic–mechanical devices, extending emitter engineering beyond conventional cavity-based approaches.

Abstract

The ability to control solid-state quantum emitters is fundamental to advancing quantum technologies. The performance of these systems is fundamentally governed by their spin-dependent photodynamics, yet conventional control methods using cavities offer limited access to key non-radiative processes. Here we demonstrate that anisotropic lattice strain serves as a powerful tool for manipulating spin dynamics in solid-state systems. Under high pressure, giant shear strain gradients trigger a complete reversal of the intrinsic spin polarization, redirecting ground-state population from $|0\rangle$ to $|\pm 1\rangle$ manifold. We show that this reprogramming arises from strain-induced mixing of the NV center's excited states and dramatic alteration of intersystem crossing, which we quantify through a combination of opto-magnetic spectroscopy and a theoretical model that disentangles symmetry-preserving and symmetry-breaking strain contributions. Furthermore, the polarization reversal is spatially mapped with a transition region below 120 nm, illustrating sub-diffraction-limit control. Our work establishes strain engineering as a powerful tool for tailoring quantum emitter properties, opening avenues for programmable quantum light sources, high-density spin-based memory, and hybrid quantum photonic devices.

Figures (5)

• Figure 1: Strain-dependent spin polarization in NV centers under high pressure. (a) Schematic cross-section of the diamond anvil cell. The laboratory coordinate frame is aligned with the NV symmetry axes: $z\parallel[111], x\parallel[\bar{1}10], y\parallel[\bar{1}\bar{1}2]$. (b) Finite-element simulation of the symmetry-breaking stress value $\sigma_{\perp}$ distribution in the anvil (SI.I(c)). The grey and white spin arrows denote the corresponding local spin polarization of NV centers, indicating the transition from the conventional down-polarized state ($\ket{m_s=0}$) to the reversed up-polarized state ($\ket{m_s=\pm 1}$). (c) Energy level diagram of the NV center ground-state triplet. The $\ket{m_s=0}$ state exhibits bright photoluminescence (PL), while the $\ket{m_s=\pm 1}$ states are dark. These spin states can be coherently manipulated using microwaves (MW) via electron spin resonance. (d)-(g) Optically detected magnetic resonance (ODMR) spectra from Site I to Site IV under approximately 129 GPa. The color gradient (blue to red) tracks the transition from negative to positive ODMR contrast, correlating with increasing symmetry-breaking stress. (h) ODMR spectra from Site I under increasing pressure (31 - 116 GPa). The dashed lines mark the blue shift of the $\ket{0}\rightarrow\ket{+1}$ ($f^+$) and $\ket{0}\rightarrow\ket{-1}$ resonance frequencies ($f^-$).
• Figure 2: Quantifying excited-state spin polarization dynamics. (a) Schematic of the time-resolved fluorescence measurement sequence. A picosecond-laser pulse excitation (green triangle) probes the excited-state spin population following continuous-wave laser initialization (green rectangle). (b-c) Fluorescence decay curve for NV centers at Site I (blue curve) and Site IV (red curve). Solid lines represent tri-exponential fits to the data. A reference decay curve for $P_4$ = 0.5 (black dashed-dotted line) is included for comparison. (d) The multi-pulse polarization measurement Following optical initialization and a polarization-inverting microwave $\pi$ pulse (yellow rectangle), a train of picosecond laser pulses monitors the evolution of the spin polarization. (e-f) Evolution of bright state population $P_4$ as a function of the ps-laser pulse number for Site I (blue) and Site IV (red). Solid lines serve as guidelines highlighting the opposing polarization trends between the two sites.
• Figure 3: Magnetic field control of strain-induced spin reversal. (a-b) Magnetic field dependence of the ODMR spectrum of NV centers at Site I and Site IV, with $B_z$ applied along the NV symmetry axis. The red and blue colors in the heatmap denote positive and negative ODMR contrasts, respectively. Black dashed lines represent eigenfrequency fits based on the excited-state (ES) Hamiltonian (Eq. 1). Unfitted spectra correspond to resonant transitions of the NV ground state (GS). (c) ODMR spectra at Site IV under a magnetic field from 12 G to 152 G, corresponding to the region outlined by the grey dashed-dotted rectangle in (b). (d) Excited-state lifetimes $\tau_j$ at Site IV versus $B_z$. Increasing $B_z$ quenches the strain-induced superposition of excited states (from $\ket{j}(j=4,5,6)$ to $\ket{0},\ket{\pm 1}$), resulting in the observed lifetime variations. Data points represent experimental measurements. Solid curves represent the corresponding model fits in (e). (e) The range of $|E_1|$ and $|E_2|$ terms derives from joint fitting, quantifying the extent of the local symmetry-breaking strain at different sites. The error bars represent the standard errors of the fitting results.
• Figure 4: Nanoscale spin polarization reversal induced by a strain gradient. (a) Fluorescence images of NV centers within DAC. A blue dashed line marks the Metal-Powder interface mentioned in Fig.1(b). Sites I, II, IV and an inversion point are identified along the grey dashed-dotted line from the center to the edge. The reversal point denotes the location where the ODMR contrast transitions from negative to positive. (b) The moduli of $D$, $E_1$ and $E_2$ terms as a fucntion of position. the Metal-Powder interface is marked with a blue dashed line. (c) ODMR contrast as a function of distance in the vicinity of the reversal point (corresponding to blue-red gradient regions in (b)). The contrast reverses from negative to positive over a spatial scale of $\sim$ 120 nm (grey rectangle), demonstrating a nanoscale spin polarization reversal driven by the local strain gradient. The error bars in (b) and (c) represent the standard errors of measured values.
• Figure A5: The energy level structure of NV centers under high magnetic field ($B_z >700$ G). The spin states are purified into $\ket{0}$, $\ket{\pm 1}$ under high $B_z$. The green arrows denote the 532-nm laser-induced excitation from ground states to excited states. The red arrows represent the spontaneous emission Process. The dashed-line arrow represents the spin-dependent ISC process. $\eta$: excitation probability. $k_r$: spontaneous emission rates. $k_{\rm{isc0}}$, $k_{\rm{isc1}}$: up-ISC transition rates from pure excited state $\ket{0}$, $\ket{\pm 1}$ to the metastable singlet state $\ket{7}$. $q_0$: down-ISC transition probability from singlet state to ground state $\ket{0}$.