Multi-pulse nonlinear optical spectroscopy and light-matter interactions in layered materials.


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The interaction between light and matter is one that underscores many applications from solar energy conversion to optical sensing of biological materials. The use of multiple pulses to study these interactions are especially useful in understanding the nonlinear optical properties of materials. Transition metal dichalcogenides such as molybdenum disulfide (MoS2) are attractive materials due to the existence of a direct excitonic resonance that can be used to enhance nonlinear optical phenomena, such as Raman spectroscopy. Here, we have investigated four-wave mixing (FWM) processes in bulk MoS2 using a multiplex coherent anti-Stokes Raman spectroscopy setup. The observed FWM signal has a resonance at approximately 680 nm, corresponding to the energy of the A excitonic transition of MoS2. This resonance can be attributed to the increased third-order nonlinear susceptibility near the excitonic transition. This phenomenon shows the potential of MoS2 as a substrate for enhancing FWM processes. Understanding how particles and light interact in a liquid environment is vital for optical and biological applications. The interaction between two femtosecond pulses and MoS2 nanoparticles suspended in liquid is studied. The laser pulses induce bubble formation on the surface of a nanoparticle and a nanoparticle aggregate then forms on the surface of the trapped bubble. Two-dimensional organometallic lead halide perovskites are generating great interest due to their optoelectronic characteristics, such as a direct band gap in the visible regime. However, the presence of defect states within the crystal structure can affect these properties, resulting in changes to their emission and the emergence of nonlinear optical phenomena. Here, we have investigated the effects of the presence of defect states on the nonlinear optical phenomena of the hybrid perovskite (BA)2(MA)2Pb3Br10. When two pulses are incident on a perovskite flake, FWM occurs, with peaks corresponding to the defect energy levels present within the crystal. The longer lifetime of the defect state, in comparison to that of virtual transitions, allows for a larger population of electrons to be excited by the second pump photon, resulting in increased FWM signal at the defect energies. This technique has the potential to detect defect energy levels in bulk crystals using FWM.



Semiconductors. Spectroscopy. Four-wave mixing.