Modulation Of Valley Polarization In Transition Metal Dichalcogenides

Transition metal dichalcogenides(TMDCs)are potential candidates for twodimensional valleytronic applications.In this dissertation,both experimental and theoretical studies of spin-valley polarized properties are carried out in TMDCs monolayers and twisted bilayers with an independently designed and lab-built polarization-resolved photoluminescence(PL)system,combined with transient absorption spectrum measurements.Correlation between upconversion/downconversion process and spin-valley depolarization mechanisms is revealed in monolayer WS2,while both normal and anomalous interlayer coupling effect are observed in twisted bilayer MoS2.And a WS2 device based on photonic crystal grating is fabricated to explore the potential on-chip optoelectronic application.The main results and conclusion are listed as follow.1.Upconversion PL with energy gain up to 190 me V is observed in monolayer WS2 at room temperature.During photoluminescence excitation measurements,PL intensity of A-exciton approximately varies exponentially with off-resonance energy,and the exponential slop in upconversion process is one magnitude higher than downconversion process.For the first time,the relationship between upconversion/downconversion process and spin-valley depolarization mechanisms is investigated,and the parallel conversion model in both valleys is established and confirmed by the theoretical calculation.2.In twisted bilayer MoS2 with normal interlayer coupling effect,the twisted angle modulates the interlayer distance and coupling strength.With interlayer coupling strength varying,the phonon-induced valley scattering and hole-induced interlayer hopping show a trade-off trend,finally resulting a weak modulation of valley polarization with different twisted angles.3.Anomalous interlayer coupling effect is firstly observed in twisted bilayer MoS2,where PL enhancement occurs with obviously blue-shifted exciton peak.Corresponding delayed intervalley scattering model of electron is established from transient absorption spectrum results.Moreover,the secondary rise of B-exciton transient absorption signal,resulting from the valley depolarization process of A-excitons,is illustrated and revealed.4.PL enhancement over 300 folds is achieved at room temperature by designing and integrating monolayer WS2 with photonic crystal grating into a device.The whole PL linear polarization of the device is dominated by the TE-like mode of photonic crystal grating,which is as high as 0.6 and shows no dependence on excitation polarization.And finally,a sole linearly polarized emission at directional angle is achieved in this monolayer WS2/photonic crystal grating device.