Hexagonal boron nitride is a good ultrathin dielectric insulator for the production of transistors made of 2-dimensional materials such as transition metal dichalcogenides or graphene as well as a passivation layer in heterostructures made from 2-dimensional materials. This is a van der Waals material with a large band gap of about 6 eV. As compared to silicon carbide and diamond, the hexagonal boron nitride crystal lattice also encompasses color centers, which provide electronic states within the band gap. These color centers are attractive for the development of on-chip room temperature quantum light sources and spin-photon interfaces.
There have been recent demonstrations in 2-dimensional hexagonal boron nitride such as room temperature single-photon emission in the visible region as well as electrically driven single photon emission in the UV spectral range at cryogenic temperatures. Activation analyses on color center formation under chemical etching and thermal gas annealing show that the origin of the defect center is in the native vacancies. A rather broad zero-phono line and dominant optical phonon sidebands demonstrate the defect center. This defect is also known as ‘type I’ emitter.
Coupling color centers to dielectric microcavity resonators helps to address the shortcomings of the type I emitter to some level. However, it is still necessary to investigate alternative color centers in hexagonal boron nitride with adequately small phonon sidebands contribution.
Magneto-optical attributes of color centers in hexagonal boron nitride are critical for the development of spin-photon interfaces in 2-dimensional materials. However, these properties are still unknown. Therefore, researchers led by Professor Stefan Strauf at the Stevens Institute of Technology, United States, demonstrated that the lesser studied so called “type II” quantum emitters in hexagonal boron nitride are spatially correlated with structural defects and display ultra-narrow zero-phonon line widths. The narrow linewidth is only accessible if spectral diffusion and blinking are minimized with a surface passivation method. They also indicated that type II emitter appeared independent of type I emitter. Their research work is published in journal, ACS Nano.
The researchers realized their results by using a passivation method based on a high quality atomic layer epitaxy grown aluminum oxide capping layer that reduced the detrimental spectral diffusion from the substrate. “Without the aluminum oxide passivation layer the type II emitters are very unstable and dim and thus almost impossible to characterize in the experiments”, explains PhD student Xiangzhi Li from the Nanophotonics Lab at Stevens. They demonstrated that optical emission from individual quantum emitters in hexagonal boron nitride was spatially correlated with structural defects and could show ultra-narrow zero-phonon line width down to 45µeV when the spectral diffusion was eliminated by the surface passivation trick.
The authors further showed that undesired emission into phonon sidebands was majorly absent for this form of emitter, making it an attractive candidate for an on-chip single photon source. Moreover, magneto-optical characterization revealed cyclic optical transitions with an upper bound for the g-factor in the range of 0.2±0.2. Through the spin polarized density functional theory computations, carried out in collaboration with the team lead by Prof. Vincent Meunier at Rensselaer Polytechnic Institute, the authors were able to forecast possible commensurate transitions between like-spin electron states. This was in agreement with the experimental non-magnetic response of the type II defect center emission.
“Creating a reliable single photon source is essential for developing scalable quantum information technologies”, explains Strauf, “so it’s very exciting to investigate this new material that offers such a promising platform for quantum photonics. While the search for a magnetic center in boron nitride remains elusive, the bright and narrowband single photon emission from these quantum emitters embedded in monolayer materials already open countless avenues for engineering the light-matter interaction for example by coupling to on-chip nanocavities.” The outcomes of their study constitute a large step towards the realization of narrowband quantum light sources as well as the development of spin-photon interfaces within 2D materials for the yet-to-be-developed chip-scale quantum networks.
Xiangzhi Li, Gabriella D. Shepard, Andrew Cupo, Nicolas Camporeale, Kamran Shayan, Yue Luo, Vincent Meunier, and Stefan Strauf. Nonmagnetic Quantum Emitters in Boron Nitride with Ultranarrow and Sideband-Free Emission Spectra. ACS Nano, volume 11 (2017), pages 6652−6660.
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