Chemical Structure, Photo Physics and Emission Control of Single-Photon Emitters in Two-Dimensional Materials

Project Information

2D-QuEST

Grant agreement ID: 844591

DOI

10.3030/844591

Project closed

EC signature date 10 April 2019

Start date 16 July 2019

End date 27 July 2021

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 224 933,76

EU contribution

€ 224 933,76

Coordinated by

IMPERIAL COLLEGE OF SCIENCE TECHNOLOGY AND MEDICINE
United Kingdom

Periodic Reporting for period 1 - 2D-QuEST (Chemical Structure, Photo Physics and Emission Control of Single-Photon Emitters in Two-Dimensional Materials)

Reporting period: 2019-07-16 to 2021-07-15

Summary of the context and overall objectives of the project

Single-photon sources are the foundation of quantum optical technologies. Since the first demonstration of single-photon emission from sodium atoms in 1977, this nonclassical phenomenon has been observed in various types of solid-state zero-dimensional and one-dimensional materials. Recently, a new class of single-photon emitter has emerged based on atomically thin two-dimensional (2D) materials, such as semiconducting transition metal dichalcogenides and hexagonal boron nitride (hBN) monolayers. These novel single-photon emitters are due to the generation and recombination of electron-hole pairs (excitons) that are spatially localized by natural defects in 2D materials. Thosee defects can be located at desired positions with atomic precision suggesting the potential to build extended quantum emitter networks. The promising properties offer a new path to the scalable integration of high-quality quantum emitters in quantum optical technologies. However, the research of 2D quantum emitters (2DQEs) is just at an early stage with many open questions about their fundamental properties. The overall objectives of the project comprise two parts: Firstly, to develop a better understanding of the chemical and electronic structure of the defects in 2D materials. Secondly, to explore controlling the single photon emission for quantum optics applications.

Our results suggests at least two types of hBN defects with different chemical structures and different bandgap, can be selectively excited with different laser frequency and emits single photons. To explore the control of single photon emission, this project has developed a compact solid immersion metalens to collect emission from dipole emission centers, with high collection efficiency which is independent on the dipole orientations. In additions, this project has developed an efficient way to largely enhance the fluorescence of quantum emitter with broadband response by coupling the quantum emitters to plasmonic waveguide. Finally, the project has developed a novel efficient frequency domain technique to measure the spectrally resolved fluorescence lifetime of quantum emitters in the microsecond to millisecond time range, which is hardly achieved by the existing fluorescence lifetime methods.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

To reveal the chemical structure of single photon emission defects in hBN, different types of atoms (carbon, argon, germanium) were implanted into pure hBN flakes. More emission centers could be observed in carbon and germanium doped hBN than the argon doped hBN. It indicates the carbon and germanium related defects are more likely the emission centers in hBN. However, some emission defects can still be observed in hBN without any implantation. To find more conclusive evidence on the chemical structure of single photon emitters, hBN with less natural defects are required for the atom implantation, for instance, grown by metal organic vapor phase epitaxy or molecular beam epitaxy. To investigate the photophysics of defects by single molecule spectroscopy, I modified the existing confocal scanning photoluminescence microscope. Our results suggest there are at least two different types of defects with different phonon energies and at different positions of hBN. They can be selectively excited with different laser frequencies and both emit single photons.

We designed and fabricated plasmonic waveguides that can strongly enhance the emission rate and guide the emission direction of quantum emitters. Quantum emitters (molecules, ions) were coupled to the plasmonic waveguides, their optical properties, such as Raman scattering and flurescence have been characterised using the home-built microscope. We proposed a compact solid immersion metalens to collect the emission of single photon emitter with high collection efficiency. We developed an effective frequency domain technique to measure the lifetime of quantum emitters with high spectral resolution, in the regime of microseconds to milliseconds regime, which is hardly achieved by the existing techniques.

The result about the solid immersion metalens for directional single molecule emission has been submitted to peer-reviewed journal and is under review. One work about the broadband nano-de-focusing of plasmonically enhanced quantum emitter fluorescence is nearly ready to be submitted. Another paper about directional enhanced Raman scattering into plasmonic waveguide is in preparation and will be submitted soon. In addition, the paper about the novel effective frequency domain technique for spectrally resolved fluorescence lifetime measurement is currently in preparation.

We disseminated the results from this project to international conference. I gave an oral presentation about directional Raman scattering on International Conference of Nano-photonics and Nano-opteelectronics 2021. At London Plasmonic Forum 2021, my poster presentation on directional Raman scattering received the best poster award. I organised the first postdoc day of EXSS in Imperial in 2020 to invite the previous postdoc and Marie Curie fellows of Imperial to share their successful experience on scientific research and career development in academic and industry.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We developed a new waveguide Quantum Electrodynamic (QED) approach for efficient generation, propagation, as well as collection of quantum emitters emission. Our non-resonant plasmonic waveguide enable strong emission enhancement of quantum emitters across the telecommunication emission band, with more than 300 times emission rate enhancement. Apart from the fluorescence control, we develop a novel waveguide QED approach for directional broadband Raman scattering of molecules which were bonded onto plasmonic slot waveguide. We experimentally proved more than 99% of the enhanced Raman scattering can be coupled into the waveguide and coupled out through the antenna pairs of the waveguides.

We designed an efficient and compact solid immersion metalens to collect emission from dipole emission centers. The collection efficiency of the device is > 85%, for both horizontal and vertical dipole orientations. We thus achieved a high numerical aperture (NA = 1.65) 100 times magnification, solid immersion singlet metalens system with a low aberration.

Those results of fundamental science will bring new knowledge on controlling not only fluorescence but also Raman scattering of quantum emitters through the QED waveguide and metamaterial approaches. They will attract broad attention of quantum optics, metamaterials, fluorescence imaging and single molecule optics communities.

Lastly, we developed an effective, efficient, and low-cost frequency domain technique for fluorescence lifetime measurement. To the best of our knowledge, it is the unique way to measure the lifetime in the microseconds to milli seconds regime with spectral resolution, which can be used to spectrally resolve the dynamics of slow emission process. One of the potential applications of the technique is to spectrally resolve the decay dynamics of molecules triplet states, which is essential to the performance of Organic Light-Emitting Diode (OLED). The other application of this technique is to spectrally resolve and enhance the fluorescence lifetime of Er ions which play important role on the telecommunications. I am going to work closely with the Imperial Innovations which is a leading technology transfer and commercialization body in the UK, to patent this technology.

Schematic illustration of a hBN monolayer. Random photon (blue) and single photon (red) generation.