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A nonlinear shift in the dispersion relation of a surface phonon-polariton (SPhP) is observed with grating-coupled pump–probe reflection spectroscopy. Upon excitation of an SPhP on a 4H-SiC surface, an instantaneous frequency shift of the SPhP mode at a constant wavevector is observed. This pump-induced frequency shift is equivalent to a nonlinear dispersion shift and to a Kerr-like nonlinear phase shift. The effective nonlinear index is evaluated to be orders of magnitude larger than the typical values of nonresonant dielectric responses. A nonlinear forced oscillator model aided by the first-principles calculations reproduce our observation and, furthermore, indicates that the primary origin is either the Born effective charge or the phonon anharmonicity depending on the frequency within the Reststrahlen band. The instantaneous shift is followed by a picosecond recovery, reflecting the energy relaxation and dissipation of the excited SPhP. This observed nonlinearity forms the basis of the self-phase modulation and four-wave mixing of SPhPs and paves the way toward nonlinear phonon-polaritonics.

Plasmonic nanoantennas provide powerful tools in enhancing light−matter interactions by linking propagating far fields with localized near fields. Microscopic understanding on the enhancements of light−matter interactions requires precise knowledges on the near-field distributions upon nanoantenna excitations. In this article, we study nearfield distributions of nanoantennas under oblique illumination. We acquire the amplitude-and phase-resolved images of the near fields on Au nanorods in the infrared range, by use of the scattering-type scanning nearfield optical microscopy and the interferometric homodyne detection. Asymmetric near-field distributions are experimentally visualized and theoretically reproduced. The asymmetry in the near-field distributions is attributed to the preferential excitation of the surface plasmon polariton in one direction and the finite loss of the surface plasmon polariton inside the cavity. Furthermore, we organize the excitation conditions of the nanoantenna modes under oblique illumination as two formulas, each of which illustrates the cavity resonance condition and the phase matching condition for surface plasmon polariton excitation, respectively. The findings will form the basis for novel near-field engineering with inhomogeneous or spatially structured illumination.

An yttrium-aluminum-garnet (YAG) Thomson scattering (TS) system was constructed and applied to the tandem mirror GAMMA 10 device to measure the electron temperature and density. A large solid-angle TS light-

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