Many optical metrology applications require light that is both coherent and broadband. Supercontinuum (SC) spanning several wavelength octaves is an obvious candidate for such applications. Optical fibers are a natural platform for SC generation due to the long interaction length of light within the fiber which allows for broad SC which can ultimately be used as a tunable narrowband source. For tunability in the mid-IR regime, YAG fibers are an excellent candidate due to their high transparency, Kerr nonlinearity, and damage threshold. In our work, we study SC in undoped crystalline YAG fibers produced via laser-heated pedestal growth. We use femtosecond pulses to generate SC in fiber, pumping at several wavelengths ranging out to the mid-IR. Studying the power-dependence of SC generation, we use SC width and shape as indicators of mechanisms that generate SC at each pump wavelength.
Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band
structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley-dependent carrier spin
polarization, which is the basis for recently proposed valleytronic applications. These systems also exhibit
unusually strong many body effects, such as strong exciton and trion binding, due to reduced dielectric
screening of Coulomb interactions. Not much is known about the impact of strong many particle correlations on
spin and valley polarization dynamics. Here we report direct measurements of ultrafast valley specific relaxation
dynamics in single layer MoS2 and WS2. We found that excitonic many body interactions significantly
contribute to the relaxation process. Biexciton formation reveals hole valley/spin relaxation time in MoS2. Our
results suggest that initial fast intervalley electron scattering and electron spin relaxation leads to loss of valley
polarization for holes through an electron-hole spin exchange mechanism in both MoS2 and WS2.
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