Probing ultrafast carrier dynamics and nonlinear absorption and refraction in core-shell silicon nanowires

We investigate the relaxation dynamics of photogenerated carriers in silicon nanowires consisting of a crystalline core and a surrounding amorphous shell, using femtosecond time-resolved differential reflectivity and transmission spectroscopy at photon energies of 3.15 eV and 1.57 eV. The complex behavior of the differential transmission and reflectivity transients is the mixed contributions from the crystalline core and the amorphous silicon on the nanowire surface and the substrate where competing effects of state filling and photoinduced absorption govern the carrier dynamics. Faster relaxation rates are observed on increasing the photo-generated carrier density. Independent experimental results on crystalline silicon-on-sapphire help us in separating the contributions from the carrier dynamics in crystalline core and the amorphous regions in the nanowire samples. Further, single beam z-scan nonlinear transmission experiments at 1.57 eV in both open and close aperture configurations yield two-photon absorption coefficient \$beta$ (~3 cm/GW) and nonlinear refraction coefficient \$gamma$ (-2.5x10^-4 cm2/GW).Comment: 6 pages, 6 figure

Probing the Interplay between Quantum Charge Fluctuations and Magnetic Ordering in LuFe2O4

Ferroelectric and ferromagnetic materials possess spontaneous electric and magnetic order, respectively, which can be switched by the corresponding applied electric and magnetic fields. Multiferroics combine these properties in a single material, providing an avenue for controlling electric polarization with a magnetic field and magnetism with an electric field. These materials have been intensively studied in recent years, both for their fundamental scientific interest as well as their potential applications in a broad range of magnetoelectric devices [1, 2, 3, 4]. However, the microscopic origins of magnetism and ferroelectricity are quite different, and the mechanisms producing strong coupling between them are not always well understood. Hence, gaining a deeper understanding of magnetoelectric coupling in these materials is the key to their rational design. Here, we use ultrafast optical spectroscopy to show that quantum charge fluctuations can govern the interplay between electric polarization and magnetic ordering in the charge-ordered multiferroic LuFe2O4

Enhanced photosusceptibility near T(c) for the light-induced insulator-to-metal phase transition in vanadium dioxide (vol 99, art no 226401, 2007)

We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity

Enhanced photosuseeptibility in the insulatorto-metal phase transition in vanadium dioxide

We use optical-pump terahertz-probe spectroscopy to measure the photoinduced phase transition in vanadium dioxide thin films. Our measurements reveal a fluence threshold needed to drive this transition that decreases with increasing temperature

Photocurrent-driven transient symmetry breaking in the Weyl semimetal TaAs

Symmetry plays a central role in conventional and topological phases of matter, making the ability to optically drive symmetry changes a critical step in developing future technologies that rely on such control. Topological materials, like topological semimetals, are particularly sensitive to a breaking or restoring of time-reversal and crystalline symmetries, which affect both bulk and surface electronic states. While previous studies have focused on controlling symmetry via coupling to the crystal lattice, we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second harmonic generation spectroscopy as a sensitive probe of symmetry changes, we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs. Our results show that optically driven photocurrents can be tailored to explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast timescales