Dresden 2017 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 78: Optics and Light-Matter Interaction with Excitons in 2D Materials (SYLM) (joined session DS, DY, HL, TT, organized by HL)

HL 78.5: Invited Talk

Thursday, March 23, 2017, 17:30–18:00, HSZ 02

Exciton spectroscopy in transition metal dichalcogenide monolayers and van der Waals heterostructures — •Bernhard Urbaszek — CNRS - Toulouse University, France

Excitons in transition metal dichalcogenide monolayers (MLs) provide exciting opportunities for applications and new frontiers in physics: (i) with binding energies of several hundred meV, excitons dominate optical properties even at room temperature, (ii) strong exciton oscillator strength leads to absorption of up to 20 % per ML, and (iii) the interband selection rules are valley selective. In combination with strong spin-orbit splittings this allows studying spin-valley physics. Although ML samples on Si/SiO2 substrates are widely studied in the literature, conclusive measurements on the excited exciton states and fine structure (2s/2p) are still missing. In hBN / ML WSe2 / hBN samples we measure for the linewidth of the neutral and charged exciton emission values down to 1.6 meV at T=4K, close to the homogenous limit. This allows us to perform 1 and 2-photon spectroscopy which reveal details previously masked by inhomogeneous broadening. Also, we demonstrate control of the exciton valley coherence in ML WSe2 on SiO2 by tuning the applied magnetic field B perpendicular to the ML plane. Linearly polarized laser excitation prepares a coherent superposition of valley states and the induced valley Zeeman splitting between K+ and K- results in a change of the oscillation frequency of the superposition of valley states. This corresponds to a rotation of the exciton valley pseudo-spin by angles as large as 30 degrees for fields up to B=9T.