[Paper Review] Stark effect of quantum blue emitters in hBN

Ivan Zhigulin, Jake Horder+8 more|arXiv (Cornell University)|Aug 1, 2022

Diamond and Carbon-based Materials Research4 citations

TL;DR

This study investigates the Stark effect of blue quantum emitters in hexagonal boron nitride (hBN) with a 436 nm emission wavelength, revealing a weak, predominantly quadratic Stark shift indicating negligible transition dipole moment. The results suggest these emitters have D3h symmetry and are likely due to a carbon-based split interstitial defect (C₂^N), making them highly resistant to spectral diffusion and ideal for integrated quantum photonics.

ABSTRACT

Inhomogeneous broadening is a major limitation for the application of quantum emitters in hBN to integrated quantum photonics. Here we demonstrate that blue emitters with an emission wavelength of 436 nm are less sensitive to electric fields than other quantum emitter species in hBN. Our measurements of Stark shifts indicate negligible transition dipole moments for these centers with dominant quadratic stark effect. Using these results, we employed DFT calculations to identify possible point defects with small transition dipole moments, which may be the source of blue emitters in hBN.

Motivation & Objective

• To understand the electric field sensitivity of blue quantum emitters in hBN with 436 nm emission.
• To identify the atomic structure of these emitters by analyzing their Stark shift response.
• To reduce inhomogeneous broadening caused by spectral diffusion due to fluctuating electric fields.
• To identify defects with minimal permanent dipole moments for improved spectral stability in quantum devices.
• To link experimental Stark shift measurements with density functional theory (DFT) simulations for defect structure assignment.

Proposed method

• Performed coherent excitation to measure Stark shifts under applied in-plane and out-of-plane electric fields.
• Measured zero-phonon line (ZPL) energy shifts as a function of electric field strength to determine linear and quadratic contributions.
• Used symmetry analysis to infer the point group of the defect based on the observed nonlinear Stark response.
• Conducted DFT-PBE calculations to model the electronic structure and ZPL energy dependence on electric field for candidate defects.
• Evaluated candidate defect structures including nitrogen and carbon split interstitials, substitutional, and anti-site defects.
• Proposed a defect model (C₂^N) based on symmetry, energy gain in reconstruction, and compatibility with electron-beam-induced creation.

Experimental results

Research questions

• RQ1 What is the nature of the Stark shift response of blue emitters in hBN under applied electric fields?
• RQ2 Why do blue emitters exhibit negligible spectral diffusion compared to other hBN quantum emitters?
• RQ3 What atomic defect structure in hBN is responsible for the 436 nm emission with minimal dipole moment?
• RQ4 How does the symmetry of the defect influence its electric field response and spectral stability?
• RQ5 Can DFT simulations reproduce the observed quadratic Stark shift and support the proposed defect model?

Key findings

• Blue emitters in hBN exhibit a weak, predominantly quadratic Stark shift, indicating a negligible transition dipole moment.
• The measured linear Stark shift is small relative to the linewidth, with a magnitude of less than 100 MHz per MV/m.
• The quadratic Stark shift is significantly larger in the in-plane direction, suggesting the dipole is primarily confined within the hBN plane.
• DFT-PBE calculations show that the C₂^N defect model reproduces the nonlinear ZPL energy shift under electric fields, in qualitative agreement with experiment.
• The C₂^N defect, formed via electron irradiation-induced reconstruction of a 4.1 eV emitter, has D3h symmetry and no permanent dipole in ground and excited states.
• The defect is energetically stable and can form spontaneously under electron irradiation without annealing, consistent with experimental creation conditions.