Photonics and polaritonics with van der Waals heterostructures

11/12/2017 @ 11:00 AM – 12:30 PM Asia/Singapore Timezone
Physics Conference Room (S11-02-07)

Speaker: Dr. Alexander Tartakovskii
Affiliation: University of Sheffield, UK
Host: Assistant Professor Goki Eda
Abstract Details: Monolayer films of van der Waals crystals of transition metal dichalcogenides (TMDs) are direct band gap semiconductors exhibiting excitons with very large binding energies and small Bohr radii, leading to a high oscillator strength of the exciton optical transition. Together with graphene as transparent electrode and hexagonal boron nitride (hBN) as an insulator, TMD monolayers can be used to produce so-called van der Waals heterostructures. Here we use this approach to make electrically pumped light-emitting quantum wells (LEQWs) [1,2] and single-photon emitters [3]. We combine this new technology with optical microcavities to demonstrate control of the emitter spectral properties and directionality, making first steps towards electrically injected TMD lasers [4]. Furthermore, by embedding MoSe2/hBN structures in tuneable microcavities, we enter the regime of the strong light-matter interaction and observe formation of exciton-polaritons [5]. Here we demonstrate that the magnitude of the characteristic anti-crossing between the cavity modes and the MoSe2 excitons (a Rabi splitting) can be enhanced by embedding a multiple-QW structure, containing two MoSe2 monolayers separated by an hBN barrier. We extend this work to demonstrate valley addressable polaritons in both MoSe2 and WSe2, the property inherited from valley excitons, but strongly modified through changes in exciton relaxation in the strong-coupling regime [6]. As the next step towards strongly interacting polaritons, we explore type-II semiconducting TMD heterostructures [7], where we observe Moire excitons and unusual optical selection rules.

[1] F. Withers et al., NATURE MATERIALS, 14, 301 (2015).
[2] F. Withers et al., NANO LETTERS, 15, 8223 (2015).
[3] S. Schwarz et al., 2D Materials, 3 (2016).
[4] S. Schwarz et al., NANO LETTERS, 14, 7003 (2014).
[5] S. Dufferwiel et al., NATURE COMMUNICATIONS, 6, 8579 (2015).
[6] S. Dufferwiel et al. , NATURE PHOTONICS 11, 497 (2017).
[7] E. M. Alexeev et al., NANO LETTERS, 17, 5342 (2017).

About the Speaker:  Alexander Tartakovskii is a Professor of Solid State Physics at the Department of Physics and Astronomy of the University of Sheffield. He graduated with a degree in Applied Physics and Math from Moscow Institute of Physics and Technology (Russia), and obtained his PhD in solid state physics from the Institute of Solid State Physics in Chernogolovka (Russia). His initial contributions to the field were in optical studies of non-linear exciton-polariton phenomena in III-V semiconductor microcavities comprising quantum wells. He moved to the University of Sheffield (UK) in 2001 as a postdoctoral researcher and worked on spin physics in semiconductor quantum dots, with particular emphasis on nuclear magnetism in nano-structures and novel solid state NMR techniques applied to extremely small nuclear spin ensembles in strained semiconductors. In 2005 he was awarded a prestigious EPSRC Advanced Research Fellowship, and in 2007 became a permanent faculty member. In the last few years he started working on optical studies of novel two-dimensional materials, reporting on some of the first realisations of light-emitting devices with electrical injection as well as exciton-polariton phenomena in monolayer semiconductors.