Abstract
The light–matter interaction in materials is of remarkable interest for various photonic and optoelectronic applications, which is intrinsically determined by the bandgap of the materials involved. To extend the applications beyond the bandgap limit, it is of great significance to study the light–matter interaction below the material bandgap. Here, we report the ultrafast transient absorption of monolayer molybdenum disulfide in its sub-bandgap region from ~0.86 µm to 1.4 µm. Even though this spectral range is below the bandgap, we observe a significant absorbance enhancement up to ~4.2% in the monolayer molybdenum disulfide (comparable to its absorption within the bandgap region) due to pump-induced absorption by the excited carrier states. The different rise times of the transient absorption at different wavelengths indicate the various contributions of the different carrier states (i.e., real carrier states in the short-wavelength region of ~<1 µm, and exciton states in the long wavelength region of ~>1 µm). Our results elucidate the fundamental understanding regarding the optical properties, excited carrier states, and carrier dynamics in the technologically important near-infrared region, which potentially leads to various photonic and optoelectronic applications (e.g., excited-state-based photodetectors and modulators) of two-dimensional materials and their heterostructures beyond their intrinsic bandgap limitations.
Highlights
The light–matter interaction in materials is fundamental to various photonic and optoelectronic applications, such as lasers, modulators and solar cells, which are intrinsically determined by the bandgap of the materials
The absorption modulation at the probe wavelength from the reflection change is estimated by deploying the transfer matrix method[29,30] based on our sample structure
From the power-dependent study of transient absorption (TA) dynamics at a few selective probe wavelengths, we reveal that the dynamics are independent of both the pump and probe power, the details of which are given in the Supplementary Information
Summary
The light–matter interaction in materials is fundamental to various photonic and optoelectronic applications, such as lasers, modulators and solar cells, which are intrinsically determined by the bandgap of the materials. We study the ultrafast transient absorption (TA) properties of ML-MoS2 flakes in the NIR region (i.e., from ~0.86 to 1.4 μm) and investigate their ultrafast carrier dynamics when excited by pump light at 400 nm in a reflection geometry.
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