skip to main content

Explore Research Products in the NSF-PAR

Title: Probing excitons in transition metal dichalcogenides by Drude-like exciton intraband absorption
Understanding excitonic dynamics in two-dimensional semiconducting transition metal dichalcogenides is important for developing their optoelectronic applications. Recently, transient absorption techniques based on resonant excitonic absorption have been used to study various aspects of excitonic dynamics in these materials. The transient absorption in such measurements originates from phase-space state filling, bandgap renormalization, or screening effects. Here we report a new method to probe excitonic dynamics based on exciton intraband absorption. In this Drude-like process, probe photons are absorbed by excitons in their intraband excitation to higher energy states, causing a transient absorption signal. Although the magnitude of the transient absorption is lower than that of the resonant techniques, the new method is less restrictive on the selection of probe wavelength, has a larger linear range, and can provide complementary information on photocarrier dynamics. Using the WS 2 monolayer and bulk samples as examples, we show that the new method can probe exciton–exciton annihilation at high densities and reveal exciton formation processes. We also found that the exciton intraband absorption cross section of the WS 2 monolayer is on the order of 10 −18 cm 2 .  more » « less
Award ID(s):
1505852
NSF-PAR ID:
10058204
Author(s) / Creator(s):
; ; ; ; ; ;
Date Published:
Journal Name:
Nanoscale
ISSN:
2040-3364
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ; ( , Physical Chemistry Chemical Physics)
    null (Ed.)
    Excitons in two-dimensional transition metal dichalcogenide monolayers (2D-TMDs) are of essential importance due to their key involvement in 2D-TMD-based applications. For instance, exciton dissociation and exciton radiative recombination are indispensible processes in photovoltaic and light-emitting devices, respectively. These two processes depend drastically on the photogeneration efficiency and lifetime of excitons. Here, we incorporate femtosecond pump–probe spectroscopy to investigate the ultrafast dynamics of exciton formation and decay in a single crystal of monolayer 2D tungsten disulfide (WS 2 ). Investigation of the formation dynamics of the lowest exciton (X A ) indicated that the formation time linearly increases from ∼150 fs upon resonant excitation, to ∼500 fs following excitation that is ∼1.1 eV above the band-gap. This dependence is attributed to the time it takes highly excited electrons in the conduction band (CB) to relax to the CB minimum (CBM) and contribute to the formation of X A . This is confirmed by infrared measurements of electron intraband relaxation dynamics. Furthermore, pump–probe experiments suggested that the X A ground state depletion recovery dynamics depend on the excitation energy as well. The average recovery time increased from ∼10 ps in the case of resonant excitation to ∼50 ps following excitation well above the band-gap. Having the ability to control whether generating short-lived or long-lived electron–hole pairs in 2D-TMD monolayers opens a new horizon for the application of these materials. For instance, long-lived electron–hole pairs are appropriate for photovoltaic devices, but short-lived excitons are more beneficial for lasers with ultrashort pulses. 
    more » « less
  2. ; ; ; ; ; ; ; ( , Optics Express)

    Transient absorption spectroscopy is a powerful tool to monitor the out-of-equilibrium optical response of photoexcited semiconductors. When this method is applied to two-dimensional semiconductors deposited on different substrates, the excited state optical properties are inferred from the pump-induced changes in the transmission/reflection of the probe,i.e., ΔT/Tor ΔR/R. Transient optical spectra are often interpreted as the manifestation of the intrinsic optical response of the monolayer, including effects such as the reduction of the exciton oscillator strength, electron-phonon coupling or many-body interactions like bandgap renormalization, trion or biexciton formation. Here we scrutinize the assumption that one can determine the non-equilibrium optical response of the TMD without accounting for the substrate used in the experiment. We systematically investigate the effect of the substrate on the broadband transient optical response of monolayer MoS2(1L-MoS2) by measuring ΔT/Tand ΔR/Rwith different excitation photon energies. Employing the boundary conditions given by the Fresnel equations, we analyze the transient transmission/reflection spectra across the main excitonic resonances of 1L-MoS2. We show that pure interference effects induced by the different substrates explain the substantial differences (i.e., intensity, peak energy and exciton linewidth) observed in the transient spectra of the same monolayer. We thus demonstrate that the substrate strongly affects the magnitude of the exciton energy shift and the change of the oscillator strength in the transient optical spectra. By highlighting the key role played by the substrate, our results set the stage for a unified interpretation of the transient response of optoelectronic devices based on a broad class of TMDs.

     
    more » « less
  3. ; ; ; ; ; ; ; ; ; ; et al ( , Laser & Photonics Reviews)
    Abstract

    Quantum interactions between transition metal dichalcogenides (TMDs) and optical cavities are rapidly becoming an appealing research topic since these interactions underly a broad spectrum of optical phenomena. Here, we fabricate a simple device in which coherent strong coupling interactions occur between a photonic crystal (PhC) slab and monolayer tungsten disulfide (WS2). Both steady‐state angle‐resolved spectroscopy and transient absorption microscopy (TAM) are employed to explore the coupling behavior of this device. Specifically, anticrossing dispersions are observed in the hybrid device, indicating a Rabi splitting of 40.2 meV. A newly formed spectral feature emerges in the transient absorption (TA) spectrum of this polariton device under near‐resonant excitation, which is subsequently evidenced to be a signature of the upper hybrid exciton–polariton state. Moreover, by carefully analyzing the ultrafast responses of both bare WS2and the WS2‐PhC polariton device excited both off resonance and near resonance, it is found that nonequilibrium thermal decay induces Coulombic screening in the monolayer WS2, which has a major impact on the formation of exciton–polariton. The results of this work could not only improve the current understanding of photophysics in the strong light–matter coupling regime but also lay the foundation for tailoring the development of TMD‐based coherent devices.

     
    more » « less
  4. ; ; ; ; ; ; ; ; ; ( , Advanced Materials)
    Abstract

    The homogeneous exciton linewidth, which captures the coherent quantum dynamics of an excitonic state, is a vital parameter in exploring light–matter interactions in 2D transition metal dichalcogenides (TMDs). An efficient control of the exciton linewidth is of great significance, and in particular of its intrinsic linewidth, which determines the minimum timescale for the coherent manipulation of excitons. However, such a control is rarely achieved in TMDs at room temperature (RT). While the intrinsic A exciton linewidth is down to 7 meV in monolayer WS2, the reported RT linewidth is typically a few tens of meV due to inevitable homogeneous and inhomogeneous broadening effects. Here, it is shown that a 7.18 meV near‐intrinsic linewidth can be observed at RT when monolayer WS2is coupled with a moderate‐refractive‐index hydrogenated silicon nanosphere in water. By boosting the dynamic competition between exciton and trion decay channels in WS2through the nanosphere‐supported Mie resonances, the coherent linewidth can be tuned from 35 down to 7.18 meV. Such modulation of exciton linewidth and its associated mechanism are robust even in presence of defects, easing the sample quality requirement and providing new opportunities for TMD‐based nanophotonics and optoelectronics.

     
    more » « less
  5. ; ; ; ; ; ; ; ; ; ; et al ( , Nature Communications)
    null (Ed.)
    Abstract The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS 2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email