Tuning Valley Polarization in aWSe2Monolayer with a Tiny Magnetic Field

Abstract

In monolayers of semiconducting transition metal dichalcogenides, the light helicity (σ þ or σ − ) is locked to the valley degree of freedom, leading to the possibility of optical initialization of distinct valley populations. However, an extremely rapid valley pseudospin relaxation (at the time scale of picoseconds) occurring for optically bright (electric-dipole active) excitons imposes some limitations on the development of opto-valleytronics. Here, we show that valley pseudospin relaxation of excitons can be significantly suppressed in a WSe 2 monolayer, a direct-gap two-dimensional semiconductor with the exciton ground state being optically dark. We demonstrate that the already inefficient relaxation of the exciton pseudospin in such a system can be suppressed even further by the application of a tiny magnetic field of about 100 mT. Time-resolved spectroscopy reveals the pseudospin dynamics to be a two-step relaxation process. An initial decay of the pseudospin occurs at the level of dark excitons on a time scale of 100 ps, which is tunable with a magnetic field. This decay is followed by even longer decay (>1 ns), once the dark excitons form more complex pseudo-particles allowing for their radiative recombination. Our findings of slow valley pseudospin relaxation easily manipulated by the magnetic field open new prospects for engineering the dynamics of the valley pseudospin in transition metal dichalcogenides.

Magnetic field induced polarization enhancement in monolayers of tungsten dichalcogenides: effects of temperature

“…So far the FIPE e ect has been only observed in WSe 2 monolayers [16]. In the reported experiments a rather weak magnetic eld of 20 mT was shown to signi cantly enhance the e ciency of the optical orientation, η(B), of the localized excitons, leading to a dip-like feature in the η(B) dependence at B = 0. is observation was interpreted as related to the eld-dependent inter-valley relaxation of the dark excitons.…”

“…Earlier investigations [16] established a link between the occurence of the FIPE for monolayer WSe 2 and the inter-valley relaxation of dark excitons, which could precede the localized excitons in the relaxation path (see Fig. 1).…”

Exciton and trion dynamics in atomically thinMoSe2andWSe2: Effect of localization

“…Despite significant differences in the band-structure leading to the opposing tempera- The impact of localization requires further study, as it fundamentally defines the character of electronic excitations in the two-dimensional system, which in addition to trions in the low electron density and/or strong localization limit can also exhibit Fermi polarons 33 in the opposite limit. In the context of TMDCs, the ability to uncover this character will help to reveal the origin of the unexpected values for exciton and trion g-factors observed recently, [34][35][36][37][38] …”

Ultrafast dynamics in monolayer transition metal dichalcogenides: Interplay of dark excitons, phonons, and intervalley exchange

“…the K (K ) point can be excited with left handed (right handed) polarized light σ + (σ − ) allowing to selectively excite the opposite valleys at the corners of the Brillouin zone [10]. On short time scales, polarization (σ + , σ − ) resolved pump-probe signals revealed an ultrafast intervalley coupling finding a sub-picosecond timescale for the intervalley transfer between the pumped and unpumped valley [11][12][13][14][15][16][17].…”