We demonstrate, with both numerical simulation and experiment, that of all the waveforms that can exist in a band-limited system, superoscillation provides the best contrast for distinguishing optically similar substances.
Nanocomposite (YBa2Cu3O7−x)0.5:(BaZrO3)0.5 thin films were fabricated on (001) oriented SrTiO3 substrates by pulsed laser deposition using a single uniformly mixed target. Both x-ray diffraction and transmission electron microscopy revealed remarkable, spontaneous formation of YBa2Cu3O7−x (YBCO) and BaZrO3 (BZO) multilayers. The high integrity and continuity of the multilayer made it possible to achieve a critical temperature of 88 K, given that the BaZrO3 fraction in the films is 50 mol %. The unique self-assembled microstructure led to a surprising field dependent critical current density along the ab plane.
Spatial symmetries and the time-reversal symmetry determine how natural and artificial materials interact with light. The time-reversal symmetry can be broken in magnetic materials, which leads to polarization rotation via the Faraday effect. A new effect known as nonreciprocal directional anisotropy emerges when the magnetic material also lacks the spatial inversion symmetry, which results in the difference of transmitted light intensity in the forward and backward directions as measured with unpolarized light. We will consider several cases studies, including a polar magnet and artificial magneto-chiral metamaterials for the THz frequency range that exhibit this emerging phenomenology and also allow new ways of polarization control.
The mechanisms producing strong coupling between electric and magnetic order in multiferroics are not always well understood, since their microscopic origins can be quite different. 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 the influence of magnetic ordering on quantum charge fluctuations via the double-exchange mechanism can govern the interplay between electric polarization and magnetism in the charge-ordered multiferroic LuFe2O4.
Detection and identification of molecular materials based on their THz frequency vibrational resonances remains an open technological challenge. The need for such technology is illustrated by its potential uses in explosives detection (e.g., RDX) or identification of large biomolecules based on their THz-frequency vibrational fingerprints. The prevailing approaches to THz sensing often rely on a form of waveguide spectroscopy, either utilizing geometric waveguides, such as metallic parallel plate, or plasmonic waveguides made of structured metallic surfaces with sub-wavelength corrugation. The sensitivity of waveguide-based sensing devices is derived from the long (1 cm or longer) propagation and interaction distance of the THz wave with the analyte. We have demonstrated that thin InSb layers with metallic gratings can support high quality factor "true" surface plasmon (SP) resonances that can be used for THz plasmonic sensing. We find two strong SP absorption resonances in normal-incidence transmission and investigate their dispersion relations, dependence on InSb thickness, and the spatial distribution of the electric field. The sensitivity of this approach relies on the frequency shift of the SP resonance when the dielectric function changes in the immediate vicinity of the sensor, in the region of deeply sub-wavelength thickness. Our computational modeling indicates that the sensor sensitivity can exceed 0.25 THz per refractive index unit. One of the SP resonances also exhibits a splitting when tuned in resonance with a vibrational mode of an analyte, which could lead to new sensing modalities for the detection of THz vibrational features of the analyte.
We present a femtosecond optical pump-probe study of the multiferroic manganite Eu$_{0.75}$Y$_{0.25}$MnO$_3$. The optical response of the material at pump energies of 1.55 and 3.1 eV is dominated by the $d$-$d$ and $p$-$d$ transitions of the Mn$^{3+}$ ions. The relaxation of photoexcited electrons includes the relaxation of the Jahn-Teller distortion and polaron trapping at Mn$^{2+}$ and Mn$^{4+}$ sites. Ultrafast switching of superexchange interactions due to modulated $e_g$ orbital occupancy creates a localized spin excitation, which then decays on a time scale of tens of picoseconds at low temperatures. The localized spin state decay appears as a tremendous increase in the amplitude of the photoinduced reflectance, due to the strong coupling of optical transitions to the spin-spin correlations in the crystalline $a$-$b$ plane.
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