Over the recent years, van der Waals (vdW) materials, a class of materials composed of weakly bound two-dimensional (2D), atomically thin, crystalline layers, have attracted great interest due to their ability to deeply confine light and therefore significantly enhance its interaction with matter. This interaction is embodied in coupled states between light and the polarization of the media, polaritons. The most studied type of polaritons, plasmon polariton, stems from the collective oscillations of conduction electrons. These, however, suffer great losses and therefore offer limited applications. Recently, among the several other types of polaritons supported by vdW materials, the exciton polariton (EP) has stimulated intense research efforts because it can sustain both strong light– matter interactions and long-distance propagation that is necessary for applications associated with energy harvesting or information manipulation and transfer. In this context, WSe2 is of particular interest for integrated applications since it supports EP modes in the Visible- Near Infrared (VIS-NIR) spectral region at room temperature due to its tightly bond excitonic state. In the quest to unravel the underlying physics, scanning near field optical microscope (SNOM) has provided valuable insights on the nature of the steady state EP modes sustained in vdW and in WSe2 in particular. However the dynamics of the EP formation, happening in the first few hundreds of femtoseconds subsequent to light absorption, remains largely unexplored. Here we employ a unique broadband ultrafast near-field pump-probe imaging method and observe for the first time, at femtosecond and nanometric spatiotemporal scale, the dynamics of the EP waves generation and propagation in WSe2 waveguides. Our observations suggest an important interplay between the waveguide EP mode and the tip-supported plasmon. Morever, we observe an intriguing ultrafast change in the EP waveguiding properties of the WSe2 waveguides happening in the first few hundreds of femtoseconds of the EP wave formation. Our method paves the way to in-situ ultrafast coherent control of EPs modes in vdW materials.
In the past two decades, ultrashort pulse lasers oscillators and amplifiers became common equipment in the fundamental scientific exploration as well as in handful of industrial applications. Those sources, which by their nature are broadband and coherent, allow exploring processes and dynamics in nature at ultrafast time scale. Due to the extremely high peak power, nonlinear optics in the ultrashort regime results in efficient frequency conversion generation processes. Among the various nonlinear conversion processes, three wave mixing and especially second harmonic generation (SHG) became widely used. Yet, frequency conversion in the ultrashort regime usually exhibits a tradeoff between the conversion bandwidth and the conversion efficiency due to the phase mismatch between the interacting waves. In the last decade, adiabatic frequency conversion method has overcome the tradeoff between conversion efficiency and bandwidth for sum frequency generation, difference frequency generation, Optical parametric amplification and recently in SHG processes. Here, we experimentally demonstrate that an adiabatic design is capable of extremely robust and efficient SHG at power levels characteristic of high-rep-rate femtosecond oscillators. We show that with pulse peak energies of nJ, one can achieve above 50% of energy conversion efficiency for 70fs Ti-Sapphire pulses. Furthermore, the flat conversion response of the presented design allows performing broadband pulse shaping manipulations before the nonlinear optical conversion. More specifically, using a spatial light modulator in a 4-f configuration, we present a tunable pump-probe based on a varying spectral phase profile in the frequency domain. Additionally, we show that by applying a π-step spectral phase, coherent control of the SHG spectrum can be achieved.