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PhD. Jakob Wierzbowski, Walter Schottky Institut und Physik Department, Technische Universität München, Germany

Electrical control of optical processes in atomically thin semiconductors

Time: Thu 2016-04-28 09.00 - 11.00

Location: FB54

Participating: Jakob Wierzbowski

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Abstract: We present electrical control of different optical processes in mono- and few-layer 2H stacked MoS2 crystals driven by an externally applied perpendicular electric field. This is established in a novel lithographically designed Si(n)-SiO2-STMD-Al2O3-metal micro-capacitor device with optical access. By means of electric field dependent low temperature confocal PL we control the direct gap A exciton emission via the quantum confined Stark effect1. We observe a layer-independent exciton polarizability which we attribute to the strong localization of the electron and hole wave function of the A exciton within each individual layer in good accord with reported large exciton binding energies. Selecting the special case of inversion symmetric 2H stacked bilayer MoS2, we continuously control the valley optical selection rules arising from symmetry considerations, by breaking and restoring the inversion symmetry of the crystal. We demonstrate strong tunability of the degree of circular polarization of the emission from η = 20 % up to 58 %. Based on the same control parameter, intense tunability of second-harmonic generation in naturally inversion symmetric 2H stacked bilayer MoS2 is demonstrated. We excite bilayer MoS2 crystals with pulses of ~ 70 fs within a spectral window of 840 – 1000 nm (1.24 – 1.47 eV) and observe a significant tunability throughout the probed region with a ~ 60 fold conversion amplification at its optimum.

Our results demonstrate the potential for electric field controlled devices for control of excitonic emission energies, emergent valley photo-physics and efficient electrically driven broadbandfrequency doubling by external control of the symmetry properties of 2H bilayer MoS2.

1. Klein, J. and Wierzbowski, J. et al. Stark effect spectroscopy of mono- and few-layer. Nano Lett. 16, 1554–1559 (2016).

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