image: Figure | The spin complex in hBN and excitation-dependent spin dynamics: (a) Illustration of the spin complex wavefunctions in hBN represented by a localised, strongly coupled spin pair (orange), and a delocalised weakly coupled spin pair (blue). (b) Optically detected magnetic resonance (ODMR) spectra of the emitter excited with 532 nm (green) and 633 nm (red) showing the relative contrast for the -½ ↔ +½, and 0 ↔ +1 transitions.
Credit: Nicholas Sloane et al.
Optically addressable spin defects in solid-state materials have rapidly emerged as promising candidates for quantum sensing applications. Defects in diamond have been the most widely studied, demonstrating the ability to sense magnetic and electric fields, temperature, and pressure. However, diamond has a fundamental limitation due to its rigid, three-dimensional structure which restricts the proximity of nitrogen vacancy defects to sensing targets.
Hexagonal boron nitride (hBN) has emerged as a promising alternative. Being a two-dimensional material, hBN can be easily peeled into atomically thin, flexible sheets and placed directly onto sensing targets. Recently, researchers have also shown that hBN hosts a unique class of spin defects exhibiting both spin-1 and spin-½ behaviour, dubbed the “spin complex”. This makes it an exciting new platform for quantum sensing, capable of both measuring the magnitude of fields and determining their direction.
In a new paper published in Light: Science & Applications, a team from the ARC Centre of Excellence for Transformative Meta-Optical Systems has discovered that the energy of light used to excite these defects dramatically affects their performance. When using red (633 nm) light instead of green (532 nm) light, the team observed a threefold boost in both the spin dependent readout with the measured contrast nearing 100% under red excitation. This increase in contrast subsequently resulted in a threefold improvement in calculated DC magnetic field sensitivity.
The team also found that excitation wavelength strongly influences the stability of light emission. Red light caused pronounced flickering, or “blinking”, in the emitter, while green light produced stable, consistent emission. To explain this, the researchers proposed a model in which red light more efficiently drives the defect into the "dark" metastable states, accounting for both the increased blinking and the stronger contrast signal. Notably, this behaviour varied between individual emitters, opening the door to defect-by-defect optimisation for quantum sensing applications.
Trying to find a balance between stable emission and high contrast, the team experimented with exciting the emitter using both colours of light simultaneously. They found that adding just a small amount of green light was enough to stabilise the blinking caused by red excitation, though this came at the cost of a slight reduction in contrast. “This points to an inherent trade-off between emission stability and signal strength, a key consideration for future sensor design” says co-first author Ivan Zhigulin.
These findings open up two particularly exciting directions. The first is optimising magnetic field sensitivity by fine-tuning the excitation wavelength to maximise both emission intensity and contrast. The second is the potential for super-resolution imaging: by exploiting the increased blinking under red excitation to push beyond the usual limits of optical microscopy, researchers could achieve finer spatial resolution of both light emission and the spin-dependent contrast.
Co-first author Nicholas Sloane says: “What originally started as a straightforward question about excitation wavelength quickly turned into something much bigger. The behaviour we uncovered raises fundamental questions about the spin dynamics of the spin complex in hBN and already points to concrete ways we can optimise these defects for better, two-dimensional quantum sensors. This is a really exciting step forward for hBN as an architecture for next-generation quantum sensing technologies and the fact that we can tune the defect's behaviour simply by changing the excitation wavelength is a powerful new tool for optimising these defects for quantum sensors.”
Article Title
Multi-wavelength spin dynamics of defects in hexagonal boron nitride