248 research outputs found
Atomistic defect states as quantum emitters in monolayer MoS
Quantum light sources in solid-state systems are of major interest as a basic
ingredient for integrated quantum device technologies. The ability to tailor
quantum emission through deterministic defect engineering is of growing
importance for realizing scalable quantum architectures. However, a major
difficulty is that defects need to be positioned site-selectively within the
solid. Here, we overcome this challenge by controllably irradiating
single-layer MoS using a sub-nm focused helium ion beam to
deterministically create defects. Subsequent encapsulation of the ion bombarded
MoS flake with high-quality hBN reveals spectrally narrow emission lines
that produce photons at optical wavelengths in an energy window of one to two
hundred meV below the neutral 2D exciton of MoS. Based on ab-initio
calculations we interpret these emission lines as stemming from the
recombination of highly localized electron-hole complexes at defect states
generated by the helium ion bombardment. Our approach to deterministically
write optically active defect states in a single transition metal
dichalcogenide layer provides a platform for realizing exotic many-body
systems, including coupled single-photon sources and exotic Hubbard systems.Comment: Main: 9 pages, 3 figures + SI: 19 pages, 10 figure
Time- and spatially-resolved magnetization dynamics driven by spin-orbit torques
Current-induced spin-orbit torques (SOTs) represent one of the most effective
ways to manipulate the magnetization in spintronic devices. The orthogonal
torque-magnetization geometry, the strong damping, and the large domain wall
velocities inherent to materials with strong spin-orbit coupling make SOTs
especially appealing for fast switching applications in nonvolatile memory and
logic units. So far, however, the timescale and evolution of the magnetization
during the switching process have remained undetected. Here, we report the
direct observation of SOT-driven magnetization dynamics in Pt/Co/AlO dots
during current pulse injection. Time-resolved x-ray images with 25 nm spatial
and 100 ps temporal resolution reveal that switching is achieved within the
duration of a sub-ns current pulse by the fast nucleation of an inverted domain
at the edge of the dot and propagation of a tilted domain wall across the dot.
The nucleation point is deterministic and alternates between the four dot
quadrants depending on the sign of the magnetization, current, and external
field. Our measurements reveal how the magnetic symmetry is broken by the
concerted action of both damping-like and field-like SOT and show that
reproducible switching events can be obtained for over reversal
cycles
Time- and spatially-resolved magnetization dynamics driven by spin-orbit torques
Current-induced spin-orbit torques (SOTs) represent one of the most effective
ways to manipulate the magnetization in spintronic devices. The orthogonal
torque-magnetization geometry, the strong damping, and the large domain wall
velocities inherent to materials with strong spin-orbit coupling make SOTs
especially appealing for fast switching applications in nonvolatile memory and
logic units. So far, however, the timescale and evolution of the magnetization
during the switching process have remained undetected. Here, we report the
direct observation of SOT-driven magnetization dynamics in Pt/Co/AlO dots
during current pulse injection. Time-resolved x-ray images with 25 nm spatial
and 100 ps temporal resolution reveal that switching is achieved within the
duration of a sub-ns current pulse by the fast nucleation of an inverted domain
at the edge of the dot and propagation of a tilted domain wall across the dot.
The nucleation point is deterministic and alternates between the four dot
quadrants depending on the sign of the magnetization, current, and external
field. Our measurements reveal how the magnetic symmetry is broken by the
concerted action of both damping-like and field-like SOT and show that
reproducible switching events can be obtained for over reversal
cycles
Midinfrared Surface Waves on a High Aspect Ratio Nanotrench Platform
Optical
surface waves, highly localized modes bound to the
surface of media, enable manipulation of light at nanoscale, thus
impacting a wide range of areas in nanoscience. By applying metamaterials,
artificially designed optical materials, as contacting media at the
interface, we can significantly ameliorate surface wave propagation
and even generate new types of waves. Here, we demonstrate that high
aspect ratio (1:20) grating structures with plasmonic lamellas in
deep nanoscale trenches, whose pitch is 1/10–1/35 of a wavelength,
function as a versatile platform supporting both surface and guided
bulk infrared waves. The surface waves exhibit a unique combination
of properties: directionality, broadband existence (from 4 μm
to at least 14 μm and beyond) and high localization, making
them an attractive tool for effective control of light in an extended
range of infrared frequencies
Engineering nanoscale hypersonic phonon transport
Controlling the vibrations in solids is crucial to tailor their mechanical
properties and their interaction with light. Thermal vibrations represent a
source of noise and dephasing for many physical processes at the quantum level.
One strategy to avoid these vibrations is to structure a solid such that it
possesses a phononic stop band, i.e., a frequency range over which there are no
available mechanical modes. Here, we demonstrate the complete absence of
mechanical vibrations at room temperature over a broad spectral window, with a
5.3 GHz wide band gap centered at 8.4 GHz in a patterned silicon nanostructure
membrane measured using Brillouin light scattering spectroscopy. By
constructing a line-defect waveguide, we directly measure GHz localized modes
at room temperature. Our experimental results of thermally excited guided
mechanical modes at GHz frequencies provides an eficient platform for
photon-phonon integration with applications in optomechanics and signal
processing transduction
Macroscopic Zeno effect in Su-Schrieffer-Heeger photonic topological insulator
The quantum Zeno effect refers to slowing down of the decay of a quantum
system that is affected by frequent measurements. Nowadays, the significance of
this paradigm is extended far beyond quantum systems, where it was introduced,
finding physical and mathematical analogies in such phenomena as the
suppression of output beam decay by sufficiently strong absorption introduced
in guiding optical systems. In the latter case, the effect is often termed as
macroscopic Zeno effect. Recent studies in optics, where enhanced transparency
of the entire system was observed upon the increase of the absorption, were
largely focused on the systems obeying parity-time symmetry, hence, the
observed effect was attributed to the symmetry breaking. While manifesting
certain similarities in the behavior of the transparency of the system with the
mentioned studies, the macroscopic Zeno phenomenon reported here in topological
photonic system is far more general in nature. In particular, we show that it
does not require the existence of exceptional points, and that it is based on
the suppression of decay for only a subspace of modes that can propagate in the
system, alike the quantum Zeno dynamics. By introducing controlled losses in
one of the arms of a topological insulator comprising two closely positioned
Su-Schrieffer-Heeger arrays, we demonstrate the macroscopic Zeno effect, which
manifests itself in an increase of the transparency of the system with respect
to the topological modes created at the interface between two arrays. The
phenomenon remains robust against disorder in the non-Hermitian topological
regime. In contrast, coupling a topological array with a non-topological one
results in a monotonic decrease in output power with increasing absorption
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