Nonlocal PL emission near monolayer to bilayer transitions in the two-dimensional material WS2 is an important indicator of dynamics of the system. For example, we find excitons excited in a bilayer can emit microns away at such a transition. We find that spectral shift of the emission is also important, as it indicates the bandgap in the emission region, and reduced bandgap regions can trap the excitons. We use nonlocal fluorescence measurements in conjunction with position-correlated 2nd harmonic microscopy, which is always local, AFM and Raman spectroscopy to understand the dynamic processes of the carriers. Two complementary nonlocal measurement approaches are applied to detect the Photo Luminescent (PL) emission region around the excitation spot. Quantitative analysis of the spectral and spatial dynamics is discussed.
Non-linear optical processes are effective label-free methods for molecular sensing and imaging. An essential consideration for implementing nonlinear optical spectroscopy and imaging is the ultrafast laser source. Applications, such as coherent Raman spectroscopy and imaging, also require wavelength tunability and multiline outputs. Existing solid state systems are complex, costly and bulky. Fiber-based systems, on the other hand, are cost-effective and easier to use. Here we present the development of a divided pulse soliton self-frequency shift source capable of generating multi-line, ultrashort pulses with broadband tunability, while keeping a compact footprint. Recent experimental progress will be discussed.
Nonlocal exciton emission in the two-dimensional material WS2 is an important indicator of dynamics of the system. Spectral shift of the emission is also important, as it indicates regions with different bandgaps, and a smaller bandgap region can 'trap' excitons. We observe both at once using a CCD readout on a spectrometer in which 1D spectral together with 1D spatial measurements are performed. Back-propagation of the slit to the sample indicates the Photo Luminescent (PL) emission region around the excitation spot. A dove prism allows effective rotation of the slit. Quantitative analysis of the spectral and spatial dynamics is discussed.
KEYWORDS: Luminescence, Near field scanning optical microscopy, Transition metals, Signal generators, Signal detection, Second-harmonic generation, Near field, Mode locking, Harmonic generation, Femtosecond phenomena
The combination of far-field and near-field scanning optical microscopy (NSOM) with a position (line) and wavelength dependent detection allow us to observe the fluorescence form points away from the excitation spot. This contrasts with confocal illumination/detection and with fluorescent imaging of uniform illumination, the two commonly used fluorescence measurement modes. We discuss the origins of the nonlocal emission and argue that the results can be used to measure the exciton diffusion length in these two-dimensional materials. In particular, we study transition metal dichalcogenides. We use near-field second harmonic generation (SHG) with NSOM detection of the mode-locked femtosecond laser pulse generated signals.
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