Transition metal dichalcogenides (TMDs) have garnered considerable interest in recent years owing to their layer thickness-dependent optoelectronic properties. In monolayer TMDs, the large carrier effective masses, strong quantum confinement, and reduced dielectric screening lead to pronounced exciton resonances with remarkably large binding energies and coupled spin and valley degrees of freedom (valley excitons). Coherent control of valley excitons for atomically thin optoelectronics and valleytronics requires understanding and quantifying sources of exciton decoherence. In this work, we reveal how exciton-exciton and exciton-phonon scattering influence the coherent quantum dynamics of valley excitons in monolayer TMDs, specifically tungsten diselenide (WSe<sub>2</sub>), using two-dimensional coherent spectroscopy. Excitation-density and temperature dependent measurements of the homogeneous linewidth (inversely proportional to the optical coherence time) reveal that exciton-exciton and exciton-phonon interactions are significantly stronger compared to quasi-2D quantum wells and 3D bulk materials. The residual homogeneous linewidth extrapolated to zero excitation density and temperature is ~1:6 meV (equivalent to a coherence time of 0.4 ps), which is limited only by the population recombination lifetime in this sample.
We have designed, fabricated, and tested a novel three-dimensional (3-D) flexible microprobe used for recording the neural signals of lateral giant (LG) on the escape system of American crayfish. We report an electrostatic actuation process to fold the planar probes to be the arbitrary orientations of 3-D probes for neuroscience application. The batch assembly method based on electrostatic force techniques gave more simple fabrication compared to others. A flexible probe could reduce both the chronic inflammation response and material fracture when animal breathes or moves. Furthermore, the cortex corresponds to hypothetical cortical modules with mostly vertically organized layers of neurons. Therefore, the 3-D flexible probe suits to understand how the cooperative activity for different layers of neurons. Advisedly, we present a novel fabrication for the 3-D flexible probe by using Parylene technology. The mechanical strength of the neural probe is strong enough to penetrate into a biogel. At the end, the flexible probe was used to record neural signals of the LG cell from American crayfish.