Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called 'flat' optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength. Although high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a threefold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of PhP-based nanophotonic devices.

Time-resolved Raman spectroscopy has been applied to probe the anharmonic coupling and electron-phonon interaction of optical phonons in graphite. From the decay of the transient anti-Stokes scattering of the G-mode following ultrafast excitation, we measured a lifetime of 2.2 ± 0.1ps for zone-center optical phonons. We also observed a transient stiffening of G-mode phonons, an effect attributed to the reduction of the electron-phonon coupling for high electronic temperatures.Because of its intimate relation with nanotubes and graphene, graphite has recently attracted renewed attention. The interactions between its fundamental excitations --electron-electron, electron-phonon and phonon-phonon --play crucial roles in determining the basic physical properties of graphite. Ultrafast pump-probe spectroscopy, by permitting us to achieve non-equilibrium conditions, provides a powerful probe of these interactions [1][2][3][4][5]. Analysis of the dynamics indicates that optical phonons play an important or even dominant role in the relaxation of the excited system [3]. Further, an understanding of phonon dynamics and phonon-phonon interactions is crucial in defining high-field transport properties of graphitic materials [6,7].In this Letter, we present measurements of the ultrafast dynamics of phonons in graphite.Through application of femtosecond time-resolved Raman scattering [8,9], we trace the generation of non-equilibrium optical phonons by carrier cooling, their subsequent interaction with electronic excitations, and their decay through anharmonic coupling to lower-energy phonons. The experimental approach permits a direct determination of the absolute phonon mode population and its temporal evolution following femtosecond laser excitation. It complements a recent independent study by Ishioka et al. [10] in which ultrafast reflectivity measurements were used to trace the dynamics of coherent phonons in graphite. In our investigation we have established a broad understanding of the role of phonons in the ultrafast dynamics in graphite. We find that: (1) photo-excited carriers transfer most of their energy to a set of strongly-coupled optical phonons (SCOPs), including the zone-center (G-mode) phonons, 3 within a few hundred femtoseconds. This produces a significant non-equilibrium phonon population. The electronic excitations retain only a minor fraction of the initial excitation energy.(2) The optical phonons cool with a time constant of 2.2 ± 0.1 ps. Energy flows from the SCOPs to lower-energy phonons by anharmonic coupling. This process also cools the coupled electronic excitations in graphite. (3) In the transient regime, the non-equilibrium G-mode

The terahertz spectral regime, ranging from about 0.1-15 THz, is one of the least explored yet most technologically transformative spectral regions. One current challenge is to develop efficient and compact terahertz emitters/detectors with a broadband and gapless spectrum that can be tailored for various pump photon energies. Here we demonstrate efficient single-cycle broadband THz generation, ranging from about 0.1-4 THz, from a thin layer of split-ring resonators with few tens of nanometers thickness by pumping at the telecommunications wavelength of 1.5 mm (200 THz). The terahertz emission arises from exciting the magnetic-dipole resonance of the split-ring resonators and quickly decreases under off-resonance pumping. This, together with pump polarization dependence and power scaling of the terahertz emission, identifies the role of optically induced nonlinear currents in split-ring resonators. We also reveal a giant sheet nonlinear susceptibility B10 À 16 m 2 V À 1 that far exceeds thin films and bulk non-centrosymmetric materials.