1,720,999 research outputs found
Capturing Coherent Magnons by Tip-Assisted Terahertz Spectroscopy
No full textOur study highlights the versatility of tip-assisted terahertz spectroscopy in probing coherent magnons, the elementary quanta of spin waves in magnetic materials. We identify two distinct coherent magnon types in canted antiferromagnet YFeO3. The remarkable consistency with far-field terahertz spectroscopy in crucial magnon parameters, such as coherence time and resonance frequency, firmly establishes the credibility of tip-assisted terahertz spectroscopy. Notably, we capture more coherent ferromagnetic magnons near the sample surface, underscoring the strength of the technique. This approach paves the way for local, free-standing, and real-space investigations of spin waves in solid magnets.11Nsciescopu
Switching to Hidden Metallic Crystal Phase in Phase-Change Materials by Photoenhanced Metavalent Bonding
No full textCopyright © 2022 American Chemical Society©. Metavalent bonding is crucial for the determination of phase transition and improvement of device performance in phase-change materials, which are attracting interest for use in memory devices. Although monitoring dielectric and phononic parameters provides a direct measure of the metavalent bonding, the control of phase-change phenomena and metavalent bonding in the dynamical regime has yet to be demonstrated. This study reports the photoenhanced metavalent bonding and resulting hidden metallic crystalline state of Ti-doped Sb2Te3, a representative phase-change material with ultralong sustainability. Using ultrafast terahertz spectroscopy, Ti0.4Sb2Te3 was discovered to possess ultralong pump-probe dynamics, which is retained over hundreds of picoseconds, unlike the short-lived state of undoped Sb2Te3. Moreover, for Ti0.4Sb2Te3 during the long-lived transmission change, the infrared-active phonon is highly softened, even more than the amount of a thermal phonon shift, indicating the photoenhancement of lattice anharmonicity. Such a long-lived relaxation implies photoinduced transition into a crystalline state of ultrastrong metavalent bonding in Ti0.4Sb2Te3, on the basis of comparisons of the dynamical dielectric constant and temporal phonon shift. Our results show the realization of photoengineering of phase-change materials by tuning electron sharing or transferring.11Nsciescopu
Electrically tunable THz graphene metasurface wave retarders
No full text© 2023 the author(s), published by De Gruyter, Berlin/Boston. Anisotropic materials with chirality or birefringence can be used to manipulate the polarization states of electromagnetic waves. However, the comparatively low anisotropy of natural materials hinders the miniaturization of optical components and devices at terahertz frequencies. In this study, we experimentally demonstrate that the relative phase retardation of a THz wave can be electrically controlled by integrating patterned mono- and bilayer graphene onto an otherwise isotropic metasurface. Specifically, we show that a refractive index for one of the orthogonal polarization states can be electrically controlled by modulating graphene's conductivity, thereby weakening the capacitive coupling between adjacent meta-atoms in an anisotropic manner. With monolayer graphene, phase retardation of 15 degrees to 81 degrees between two orthogonal polarization states can be achieved. Maximum phase retardation of 90 degrees through a metasurface with bilayer graphene suggests its use as a tunable quarter-wave plate. Continuous control from linear- to circular-polarization states may provide a wide range of opportunities for the development of compact THz polarization devices and polarization-sensitive THz technology.11Nsciescopu
Terahertz spectroscopy of antiferromagnetic resonances in YFe1-xMnxO3(0 ≤ x ≤ 0:4) across a spin reorientation transition
No full text© 2022 AIP Publishing LLC. We have conducted a terahertz spectroscopic study of antiferromagnetic resonances in bulk orthoferrite YFe1-xMnxO3 0 ≤ x ≤ 0.4. Both the quasi-ferromagnetic resonance mode and the quasi-antiferromagnetic resonance mode in the weak ferromagnetic Γ4 phase disappear near the spin reorientation temperature, TSR, for the onset of the collinear antiferromagnetic Γ1 phase (x ≥ 0.1). Below TSR, an antiferromagnetic resonance mode emerges and exhibits a large blueshift with decreasing temperature. However, below 50 K, this mode softens considerably, and this tendency becomes stronger with Mn doping. We provide a deeper understanding of such behaviors of the antiferromagnetic resonance modes in terms of the influence of the Mn3+ ions on the magnetocrystalline anisotropy. Our results show that terahertz time-domain spectroscopy is a useful, complementary tool in tracking magnetic transitions and probing the interaction between disparate magnetic subsystems in antiferromagnetic materials with multiple ionic species.11Nsciescopu
Extremely Broadband Topological Surface States in a Photonic Topological Metamaterial
Full text link© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimMetamaterials, artificially engineered materials consisting of subwavelength unit cells, have shown potentials in light manipulation with their extraordinary optical properties. Especially, photonic topological metamaterials possessing topologically protected surface states enable extremely robust control of light. Here, an extremely broadband topological phase in photonic topological metamaterials is demonstrated. In particular, topological surface states are observed for all the frequencies below a certain cut-off, originating from a double Weyl point at zero frequency. The extreme bandwidth and robustness of the photonic topological metamaterial are beneficial for practical applications such as one-way waveguide and photonic integrated systems but also advantageous in design and fabrication since the only necessary condition is to satisfy the effective hyperbolic and chiral properties, without entailing strict periodic arrangement11sciescopu
Single-crystalline Cu2O thin films of optical quality as obtained by the oxidation of single-crystal Cu thin films at low temperature
Full text linkHigh-quality, single-crystal-like Cu2O thin films of various thicknesses (10 nm–45 nm) were prepared at a low temperature (150 °C) by controlling layer-by-layer oxidation of wafer-scale Cu thin films sputtered along the (111) direction using a pure single-crystal Cu target. The cross-sectional images of the thin films reveal high crystallinity of Cu2O layers except for 60° twinning in the sequential stacking order as evidenced by high-resolution transmission electron microscopy, which is consistent with the absence of the photoluminescence (PL) signals arising from atomic-scale vacancies. The optical properties of our Cu2O films were investigated using temperature-dependent PL and Raman spectroscopy. All of the Cu2O thin films exhibit characteristic band-to-band transitions together with the series of yellow excitonic transitions slightly below the fundamental bandgap. The spectral locations for the PL are approximately consistent with those for the bulk counterpart. The excellent optical quality of our Cu2O was further demonstrated by significantly reduced quasi-direct transition that occurs at symmetry-breaking crystal imperfection, which relaxes the stringent momentum conservation rule. We identified the three main Raman scattering modes of the Cu2O thin films, where the two forbidden modes of Γ15(1) and Γ12−+Γ25− are resonantly allowed by the proximity of the incident photon energy to the green bandgap. We believe that our synthesis technique can be utilized for the preparation of single-crystal-like metal oxide thin films at low production temperatures with precise thickness control for the development of novel optoelectronic devices and for the exploration of the nanoscale light-matter interaction as well
Directed self-assembly of a helical nanofilament liquid crystal phase for use as structural color reflectors
Full text linkThe fabrication of molecular structures with a desired morphology, e.g., nanotubes, nanoribbons, nanosprings, and sponges, is essential for the advancement of nanotechnology. Unfortunately, realization of this objective is expensive and complicated. Here, we report that irradiating a film comprising azobenzene derivatives with UV light produces oriented arrays of helical nanofilaments via the photoisomerization-induced Weigert effect. As a result, structural colors are observed due to the extrinsic chiral reflection in the visible wavelength range, and the reflected color can be tuned by adjusting the molecular length of the azobenzene derivative. This simple fabrication method can be used for fabricating large, reversible, and patternable color reflectors, providing a new platform for interference-based structural coloration as it exists in nature, such as morpho butterflies, green-winged teal, and various beetles
High-temperature differences in plasmonic broadband absorber on PET and Si substrates
No full text© 2020 Springer Nature Limited The characteristics of a plasmonic resonator with a metal–dielectric–metal structure is influenced by the size, shape, and spacing of the nanostructure. The plasmonic resonators can be used in various applications such as color filters, light emitting diodes, photodetectors, and broadband absorbers. In particular, broadband absorbers are widely used in thermophotovoltaics and thermoelectrics. To achieve a higher photothermal conversion efficiency, it is important to provoke a larger temperature difference in the absorber. The absorption and thermal conductance of the absorber has a great impact on the temperature difference, but in order to further improve the temperature difference of the absorber, the thermal conductivity of the substrate should be considered carefully. In this study, we designed Cr/SiO2/Cr absorbers on different substrates, i.e., polyethylene terephthalate (PET) and silicon. Although their optical properties do not change significantly, the temperature difference of the absorber on the PET substrate is considerably higher than that on the Si substrate under laser illumination, i.e., 164 K for ΔTPET and 3.7 K for ΔTSi, respectively. This is attributed to the thermal conductance of the substrate materials, which is confirmed by the thermal relaxation time. Moreover, the Seebeck coefficient of graphene on the absorber, 9.8 μV/K, is obtained by photothermoelectrics. The proposed Cr/SiO2/Cr structure provides a clear scheme to achieve high performance in photothermoelectric devices.11sci
Electrically Tunable Slow Light Using Graphene Metamaterials
Full text linkMetamaterials with classical analogues
of electromagnetically induced
transparency open new avenues in photonics for realizing smaller,
more efficient slow light devices without quantum approaches. However,
most of the metamaterial-based slow light devices are passive, which
limits their practical applications. Here, by combining diatomic metamaterials
with a gated single-layer graphene, we demonstrate that the group
delay of terahertz light can be dynamically controlled under a small
gate voltage. Using a two coupled harmonic oscillators model, we show
that this active control of group delay is made possible by an effective
control of the dissipative loss of the radiative dark resonator by
varying the graphene’s optical conductivity. Our work may provide
opportunities in the design of various applications such as compact
slow light devices and ultrasensitive sensors and switches
Probing Interfacial Charge Transfer between Amyloid‑β and Graphene during Amyloid Fibrillization Using Raman Spectroscopy
No full textCharge
transfer plays a key role in the structural transformation
of amyloid-β proteins (Aβs), as it fibrillizes from small
monomers to intermediate oligomers and to ordered fibrils. While the
protein fibrillization states have been identified using cryo-electron
microscopy, X-ray diffraction, Raman, infrared, terahertz spectroscopies, etc., there is little known about the electronic states
during the fibrilization of Aβ protein. Here, we probe the charge
transfer of Aβ42 proteins at different aggregation
stages adsorbed on monolayer graphene (Gr) and molybdenum disulfide
(MoS2) using Raman spectroscopy. Monomers, oligomers, and
fibrils prepared in buffer solutions were deposited and dried separately
on Gr and MoS2 where well-established characteristic Raman
modes (G, 2D for Gr and E2g, A1g for MoS2) were monitored. The shifts in Raman parameters showed that
the small Aβ monomers withdraw electrons, whereas fibrils donate
electrons to Gr and MoS2. Oligomers undergo transient charge
states near the neutrality point. This is explained in terms of modulated
carrier concentration in Gr and MoS2. This finding provides
insight into the electronic properties of Aβs that could be
essential to identifying the onset of toxic fibril forms and developing
a straightforward, label-free diagnosis using Gr and MoS2
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