Using femtosecond laser pulses, coherent GHz acoustic phonons are efficiently photogenerated and photodetected in BiFeO3 (BFO) multiferroic single crystal. Due to the crystal lattice symmetry, longitudinal as well as two transverse acoustic modes are generated and detected, and the corresponding sound velocities are determined. This provides the opportunity to experimentally evaluate the elastic coefficients of the multiferroic compound BiFeO3 that have been estimated so far only through ab initio calculations. The knowledge of the elastic properties of BFO is highly desired for BFO integration in nanoelectronic devices. Moreover, our findings highlight also that BFO may be a good candidate for light-controlled coherent acoustic phonons sources.

Abstract:The enhanced electric field at plasmonic resonances in nanoscale antennas can lead to efficient harmonic generation, especially when the plasmonic geometry is asymmetric on either inter-particle or intra-particle levels. The planar Archimedean nanospiral offers a unique geometrical asymmetry for second-harmonic generation (SHG) because the SHG results neither from arranging centrosymmetric nanoparticles in asymmetric groupings, nor from non-centrosymmetric nanoparticles that retain a local axis of symmetry. Here, we report forward SHG from planar arrays of Archimedean nanospirals using 15 fs pulses from a Ti:sapphire oscillator tuned to 800 nm wavelength. The measured harmonic-generation efficiencies are 2.6·10 −9 , 8·10 −9 and 1.3·10 −8 for left-handed circular, linear, and right-handed circular polarizations, respectively. The uncoated nanospirals are stable under average power loading of as much as 300 µW per nanoparticle. The nanospirals also exhibit selective conversion between polarization states. These experiments show that the intrinsic asymmetry of the nanospirals results in a highly efficient, two-dimensional harmonic generator that can be incorporated into metasurface optics.Keywords: nonlinear plasmonics, asymmetric nanoparticles, polarization conversion, metasurfaces, near-field enhancement, Archimedean nanospirals The second-order susceptibility governs a host of important nonlinear optical phenomena, including frequency mixing, sum-frequency and harmonic generation, and optical rectification. In crystalline and molecular materials, second-order nonlinearities are nonvanishing only at surfaces or in materials with a noncentrosymmetric crystal structure; moreover, the efficient generation of a second-order nonlinear effect requires that the fundamental incident and the nonlinear outgoing waves be phase matched through a macroscopic volume of material, typically on the order of cubic millimeters [1].Plasmonic nanostructures and nanostructure arrays also exhibit second-order nonlinearities, and can generate forward second harmonics if their geometries are not centrosymmetric. Such structures are inherently planar, and therefore compatible with thin-film optical and optoelectronic technologies and with metasurface optics. With advances in nanofabrication, the symmetry of the structures can be exquisitely controlled at the level of a few nanometers [2]. Combining these effects with ultrafast, high-intensity laser pulses yields massive electricfield enhancements and correspondingly greater secondharmonic yield. The localized surface plasmon resonance enhances efficiency and can be designed by selecting the nanoparticle shape for a given wavelength; selective polarization response can also be designed into the nanoparticle. A number of asymmetric plasmonic geometries have been used for harmonic generation, including L-and V-shaped nanoparticles, nanocups, and asymmetric trimers [3][4][5][6][7]. Larger plasmonic structures-such as the ratchet wheel-have also been shown to affect the polarization o...

Efficient frequency conversion techniques are crucial to the development of plasmonic metasurfaces for information processing and signal modulation. In principle, nanoscale electricfield confinement in nonlinear materials enables higher harmonic conversion efficiencies per unit volume than those attainable in bulk materials. Here we demonstrate efficient secondharmonic generation (SHG) in a serrated nanogap plasmonic geometry that generates steep electric field gradients on a dielectric metasurface. An ultrafast pump is used to control plasmon-induced electric fields in a thin-film material with inversion symmetry that, without plasmonic enhancement, does not exhibit an an even-order nonlinear optical response. The