Broadband Tip-Enhanced Nonlinear Optical Response in a Plasmonic Nanocavity.

We report a significantly broad nonlinear optical response enhanced in a tip-substrate plasmonic nanocavity. Focusing on the near-field second harmonics of the wavelength-tunable femtosecond laser, we demonstrate that the tip-enhancement of nonlinear optical effects efficiently works over the broad wavelength range through the visible to infrared region. We also found that this broadband nonlinear optical property is directly affected not only by the nanometer-scale sharpness of the tip apexes but also by the micrometer-scale surface geometry of the tip shafts. While spatially nonlocal plasmonic modes excited throughout the micrometer-scale tip shafts enhance near-to-mid-infrared incoming light, the radiation of visible-to-near-infrared second harmonics is boosted by localized plasmons at the nanogap. These two plasmonic modes simultaneously affect the excitation and emission processes, realizing the strong and broad enhancement of second harmonic generation. Our results provide a new basis for the physical understanding and fine manipulation of nonlinear optical phenomena enhanced in plasmonic nanocavities.