Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂

Abstract

Many of the fundamental optical and electronic properties of atomically thin transition metal dichalcogenides are dominated by strong Coulomb interactions between electrons and holes, forming tightly bound atom-like states called excitons. Here, we directly trace the ultrafast formation of excitons by monitoring the absolute densities of bound and unbound electron−hole pairs in single monolayers of WSe 2 on a diamond substrate following femtosecond nonresonant optical excitation. To this end, phaselocked mid-infrared probe pulses and field-sensitive electro-optic sampling are used to map out the full complex-valued optical conductivity of the nonequilibrium system and to discern the hallmark low-energy responses of bound and unbound pairs. While the spectral shape of the infrared response immediately after above-bandgap injection is dominated by free charge carriers, up to 60% of the electron−hole pairs are bound into excitons already on a subpicosecond time scale, evidencing extremely fast and efficient exciton formation. During the subsequent recombination phase, we still find a large density of free carriers in addition to excitons, indicating a nonequilibrium state of the photoexcited electron−hole system. KEYWORDS: Dichalcogenides, atomically thin 2D crystals, exciton formation, ultrafast dynamics A tomically thin transition metal dichalcogenides (TMDCs) have attracted tremendous attention due to their direct bandgaps in the visible spectral range, 1,2 strong interband optical absorption, 3,4 intriguing spin-valley physics, 5−7 and applications as optoelectronic devices. 8−11 The physics of twodimensional (2D) TMDCs are governed by strong Coulomb interactions owing to the strict quantum confinement in the out-of-plane direction and the weak dielectric screening of the environment. 12,13 Electrons and holes in these materials can form excitons with unusually large binding energies of many hundreds of millielectronvolts, 14−19 making these quasiparticles stable even at elevated temperatures and high carrier densities. 20,21 The properties of excitons in 2D TMDCs are a topic of intense research, investigating, for example, rapid exciton−exciton scattering, 22 interlayer excitons, 23 charged excitons and excitonic molecules, 24,25 ultrafast recombination dynamics, 19,26−28 or efficient coupling to light and lattice vibrations. 4,19,29,30 In many experiments, excitons are created indirectly through nonresonant optical excitation or electronic injection, which may prepare unbound charge carriers with energies far above the exciton resonance. 8,18 Subsequently, the electrons and holes are expected to relax toward their respective band minima and form excitons in the vicinity of the fundamental energy gap. In principle, strong Coulomb attraction in 2D TMDCs should foster rapid exciton formation. Recent optical pump−probe studies relying on interband transitions have reported characteristic formation times on subpicosecond time-scales. 31 The relaxation of large excess energies, however, requires many sca...

Dielectric Engineering of Electronic Correlations in a van der Waals Heterostructure

“…These features are characteristic of a resonant absorption and can be assigned to the 1s−2p transition of excitons in this system. 12,13 In the case of the heterostructure (Figure 2a,b, yellow shading), we find a similar spectral shape. However, the characteristic peak in Δσ 1 and the zero crossing in Δε 1 are now located at a distinctly lower energy of 147 meV while the resonance is notably narrower.…”

“…Δ is expected to increase with n X . 12,13 In our experiment, the line width Δ hetero is found to be smaller than Δ bare by 23%, even though the density n X hetero slightly exceeds n X bare by 8%. Thus, the hBN cover layer clearly reduces the width of the intraexcitonic 1s−2p transition line.…”

“…There is no measurable contribution from unbound electron−hole pairs in the investigated spectral region, in agreement with previous works. 12,13 In contrast, adding the hBN cover layer red shifts the resonance E res to 147 meV, whereas the line width Δ decreases to 90 meV at t PP = 0 fs (Figure 2a,b, red spheres). Furthermore, the fit of the heterostructure data reveals a nonvanishing density of unbound electron−hole pairs n FC .…”

Towards properties on demand in quantum materials

“…The strength of the perturbing electric field is a key enabler in dynamic materials control research. For example, strong, high-frequency fields can initiate phase transitions, including the insulator-to-metal transition 45 (discussed in 'Revealing hidden phases and new states of matter') or drive nonlinear effects such as on-resonance parametric amplification and high harmonic generation 46,47 . Sub-gap electromagnetic The tenet of dressed states is central to the physics of quantum materials.…”

Coherent Light-Matter Interactions in Monolayer Transition-Metal Dichalcogenides

“…eV photons (hv > EA), we create free electron-hole pairs that immediately form excitons within the excitation pulse duration [137,138]. This is due to the strong Coulomb attraction in monolayer WS 2 that leads to a very rapid exciton formation.…”