Intervalley biexcitons and many-body effects in monolayerMoS2

Abstract

Interactions between two excitons can result in the formation of bound quasiparticles, known as biexcitons. Their properties are determined by the constituent excitons, with orbital and spin states resembling those of atoms. Monolayer transition metal dichalcogenides (TMDs) present a unique system where excitons acquire a new degree of freedom, the valley pseudospin, from which a novel intervalley biexciton can be created. These biexcitons comprise two excitons from different valleys, which are distinct from biexcitons in conventional semiconductors and have no direct analogue in atomic and molecular systems. However, their valley properties are not accessible to traditional transport and optical measurements. Here, we report the observation of intervalley biexcitons in the monolayer TMD MoS2 using ultrafast pump-probe spectroscopy. By applying broadband probe pulses with different helicities, we identify two species of intervalley biexcitons with large binding energies of 60 meV and 40 meV. In addition, we also reveal effects beyond biexcitonic pairwise interactions in which the exciton energy redshifts at increasing exciton densities, indicating the presence of many-body interactions among them.

Coherent Light-Matter Interactions in Monolayer Transition-Metal Dichalcogenides

“…For the pump arm, the pulses were sent to an optical parametric amplifier to generate tunable photon energy below the exciton absorption (hv < E), while for the probe arm the pulses were sent through a delay stage and a white-light continuum generator (hv = 1.78-2.48 eV, chirp-corrected). The two beams were focused at the sample, and the probe beam was reflected to a monochromator and a photodiode for lockin detection [26]. By scanning the grating and the delay stage, we were able to measure AR/R (and hence Aa) as a function of energy and time delay.…”

“…This method, however, is insensitive to distinguishing a peak shift from a peak broadening. Thus, it is crucial to measure the full spectrum of a, which can be performed in transient absorption spectroscopy [26], and verify if the measured Aa spectrum profile results from a peak shift (Fig 4.11b).…”

Observation of Intervalley Biexcitonic Optical Stark Effect in Monolayer WS2

“…The sample consists of high-quality monolayers of WS 2 that were grown by chemical vapor deposition on sapphire substrates [13][14][15]. For such an atomically-thin layer on a transparent substrate, the change of absorption (∆α) can be directly extracted from the change of reflection [10]. The resulting ∆α spectrum allows us to determine the direction and magnitude of the exciton energy shift (∆E), which typically shows a single-cycle waveform, as depicted in Fig 1c-d.…”

“…The coherent signal arises from the optical Stark effect. The incoherent signal arises from the created exciton population, which is unavoidable using the above-resonance photoexcitation, causing band renormalization, biexciton absorption and Pauli blocking [9,10,[17][18][19][20][21][22][23][24]. These two types of processes evolve differently with the pump-probe time delay.…”

“…Recent research has revealed significant interactions between individual excitons in monolayer TMDs. In particular, two excitons at different valleys can be bound to form an excitonic molecule, the intervalley biexciton, with unusually large binding energies (40−70 meV) [9,10,[26][27][28]. These intervalley biexcitons offer an effective channel to couple the two valleys, with selection rules different from those for single excitons.…”