Ultrafast Nonlinear Optical Response in Plasmonic 2D Molybdenum Oxide Nanosheets for Mode-Locked Pulse Generation

摘要

2D plasmons are of particular interest in the exploration of light–matterrninteractions in 2D materials. In 2D plasmonic materials, response timern(in sub-picosecond scale) of free carriers is order of magnitude faster thanrnexcitonic recombination in semiconductors, making them highly attractivernfor realizing faster optical switching and modulation in nanoscale devices.rnHowever, the small carrier density in gapless 2D plasmonic materials likerngraphene has strongly limited the strength of light–matter interaction andrnthe operative spectral range. Here, it is shown that in optically activatedrnplasmonic molybdenum oxide (MoO_3) sheets of atomic thickness, thernnonlinear optical (NLO) absorption, characterized by a saturable behaviorrnin the near-infrared region, is notably enhanced by the plasmon resonance,rngiving a modulation depth of 34.96%. Ultrafast pump-probe spectroscopyrnreveals that the transient photobleaching in plasmonic MoO_3 nanosheetsrnassociated with the relaxation of hot electrons recovers in a timescalernless than 200 femtoseconds (fs). This ultrafast NLO response allows therndevelopment of an optical switch based on saturable absorption that enablesrnthe generation of mode-locked laser pulses from a fiber laser operating atrn1.0 μm region. The results may have strong implications for the application ofrn2D plasmonics based on 2D MoO_3 in nanophotonics.