Researchers report a microscope that follows ultrafast optical changes in molybdenum disulfide (MoS₂), with a pump-defined spatial resolution as fine as 0.51 micrometres. In a monolayer MoS₂ flake, long-lived signals were present in the centre but largely suppressed near the edge, revealing a local difference in how the material relaxed after excitation.
A broadband probe for a fast signal
The system combined femtosecond pump–probe excitation with broadband supercontinuum probe pulses generated in a photonic crystal fibre. It operated at 80 MHz without pulse amplification.
In the measurement, the pump excited the sample and the broadband probe recorded how its optical signal changed over time.
With PCF lengths of 50 millimetres or less, the paper reports an estimated whole-spectrum time resolution below 100 femtoseconds. A wavelength-dependent time-origin correction applied to HOPG data compensated for dispersion-induced chirp, a timing distortion that varied with wavelength.
Cleaner signals through averaging
Repeated measurements lowered root-mean-square noise roughly as the inverse square root of the number of accumulations. At 10,000 accumulations, the reported sensitivity was approximately 0.021%.
Extrapolating the same trend to 30,000 accumulations put the sensitivity below 10⁻⁴, or 0.009%, but that lower figure was projected rather than directly demonstrated.
Reading signals from MoS₂ layers
The pump-defined spatial resolution was 1.95 micrometres with a ×40 objective and 0.51 micrometres with a ×100 objective.
The first material test used a mechanically exfoliated multilayer MoS₂ flake estimated at 20–50 nanometres thick, or roughly 30–80 MoS₂ layers.
Transient reflectance showed negative features near 670 and 610 nanometres. The authors assigned them to A- and B-exciton ground-state bleaching, meaning a reduced ground-state optical signal after excitation. Positive features across 580–760 nanometres were mainly attributed to excited-state absorption from photoexcited carriers.
Selected fits to the excited-state-absorption signal produced C-ESA τ₁ values of 198.1 picoseconds at 588 nanometres and 61.1 picoseconds at 700 nanometres. The fitted B- and A-exciton bleaching components, labelled B-GSB and A-GSB, had τ₂ values of 9.61 and 14.7 picoseconds.
One flake, different relaxation
The monolayer MoS₂ was synthesized by CVD on a sapphire substrate. Its C-ESA feature shifted toward shorter wavelengths and nearly saturated at 20 picoseconds. The authors interpreted the pattern as showing that cooling of the excited carriers was mostly complete within a sub-100-picosecond timescale.
In the centre of the flake, the long-lived A- and C-exciton components were 75.2 and 45.6 picoseconds, respectively, and were largely suppressed at the edge.
What the measurements leave open
The centre–edge comparison came from positions within the reported flake, leaving open how reproducible the pattern is across other flakes. It is a local observation, not a demonstrated cause of faster edge relaxation.
Underlying data were not publicly available at the reported time, although the authors say they may be obtained on reasonable request.
The authors declare no conflicts of interest. The funding statement lists support from Japanese foundations, MEXT and JSPS programmes, as well as Spanish CM and AEI programmes for one author.
Paper data and sources
Original title: Oscillator-driven broadband femtosecond transient microspectroscopy with supercontinuum probes generated in a photonic crystal fiber
Authors: Rikuto Fukuda, Kyosuke Kishihata, Ryo Man-nami et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-20
DOI: Not available
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