12 research outputs found
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 \ (~3 cm/GW) and
nonlinear refraction coefficient \ (-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
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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