[Paper Review] Optically-active spin defects in few-layer thick hexagonal boron nitride

Alrik Durand, T. Clua-Provost+12 more|PubMed|Apr 24, 2023

Diamond and Carbon-based Materials Research10 citations

TL;DR

The paper demonstrates optically detected electron spin resonance of V_B^- centers in few-layer hBN and analyzes how zero-field splitting, optical spin polarization, and longitudinal relaxation depend on hBN thickness, revealing 2D-limit modifications and enhanced photonic effects.

ABSTRACT

Optically active spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of two-dimensional quantum sensing units offering optimal proximity to the sample being probed. In this Letter, we first demonstrate that the electron spin resonance frequencies of boron vacancy centers (V_{B}^{-}) can be detected optically in the limit of few-atomic-layer thick hBN flakes despite the nanoscale proximity of the crystal surface that often leads to a degradation of the stability of solid-state spin defects. We then analyze the variations of the electronic spin properties of V_{B}^{-} centers with the hBN thickness with a focus on (i) the zero-field splitting parameters, (ii) the optically induced spin polarization rate and (iii) the longitudinal spin relaxation time. This Letter provides important insights into the properties of V_{B}^{-} centers embedded in ultrathin hBN flakes, which are valuable for future developments of foil-based quantum sensing technologies.

Motivation & Objective

• Investigate whether V_B^- centers retain optically detectable spin resonances in atomically thin hBN flakes.
• Characterize how electronic spin properties (zero-field splitting and strain-electric interactions) vary with hBN thickness.
• Measure how optical pumping influences spin polarization as a function of thickness.
• Determine how longitudinal spin relaxation time T1 changes from bulk-like to few-layer hBN and at cryogenic temperatures.

Proposed method

• Use neutron-irradiated isotopically pure hBN to create V_B^- centers.
• Exfoliate to obtain flakes of varying thickness and transfer onto SiO2/Si substrates.
• Record optically detected ESR spectra under zero and applied magnetic fields with MW excitation while reading PL.
• Model ESR via a microscopic charge distribution around V_B^- to account for E-splitting through transverse electric fields.
• Compute thickness-dependent optical absorption using a transfer-matrix approach to explain polarization-rate variations.
• Measure T1 by a laser-pump, dark-relax, readout sequence across thicknesses and temperatures.

Experimental results

Research questions

• RQ1 Do V_B^- centers in ultrathin hBN exhibit optically detectable ESR despite proximity to the surface?
• RQ2 How does the axial (D) and transverse (E) zero-field splitting evolve as hBN thickness approaches the 2D limit?
• RQ3 How does the optically induced spin polarization rate depend on thickness and optical absorption?
• RQ4 How is the longitudinal spin relaxation time T1 affected by reduced thickness and by temperature?

Key findings

• ESR frequencies of V_B^- centers remain optically detectable in few-layer hBN, with D unchanged and E reduced, yielding a single resonance in ultrathin flakes.
• A local electric-field model explains the reduced E-splitting in thin flakes via fewer surrounding charges, aligning with observed ESR spectra.
• The optically induced spin polarization rate is enhanced in few-layer hBN due to increased absorption near the air/hBN interface, as shown by thickness-dependent S_p.
• T1 decreases to ~1 μs in few-layer hBN at room temperature, but increases to the millisecond range at 4 K, indicating suppression of surface-related magnetic noise at low temperature and possible modified spin-phonon coupling in 2D.