Experimental Demonstration of Controllable PT and Anti-PT Coupling in a Non-Hermitian Metamaterial.

Non-Hermiticity has recently emerged as a rapidly developing field due to its exotic characteristics related to open systems, where the dissipation plays a critical role. In the presence of balanced energy gain and loss with environment, the system exhibits parity-time (PT) symmetry, meanwhile as the conjugate counterpart, anti-PT symmetry can be achieved with dissipative coupling within the system. Here, we demonstrate the coherence of complex dissipative coupling can control the transition between PT and anti-PT symmetry in an electromagnetic metamaterial. Notably, the achievement of the anti-PT symmetric phase is independent of variations in dissipation. Furthermore, we observe phase transitions as the system crosses exceptional points in both anti-PT and PT symmetric metamaterial configurations, achieved by manipulating the frequency and dissipation of resonators. This work provides a promising metamaterial design for broader exploration of non-Hermitian physics and practical application with a controllable Hamiltonian.

First Observation of New Flat Line Fano Profile via an X-Ray Planar Cavity.

A new Fano profile of a flat line is achieved experimentally by manipulating the relative amplitude of the continuum path, when q takes the pure imaginary number of -i in the x-ray regime. The underlying mechanism is that the interference term in the scattering will cancel the discrete term exactly. This new Fano profile renders only an observable continuum along with an invisible response to the discrete state of atomic resonance. The results suggest not only a different strategy to invisibility studies which provides a possible tool to identify weaker structures hidden by the strong white line, but also a new scenario to enrich the manipulations of two-path interference and nonlinear Fano resonance.

EIA metamaterials based on hybrid metal/dielectric structures with dark-mode-enhanced absorption.

Metamaterial analogue of electromagnetically induced absorption (EIA) has promising applications in spectroscopy and sensing. Here we propose an EIA metamaterial based on hybrid metal/dielectric structures, which are composed of a metallic wire and a dielectric block, and investigate the EIA-like effect by simulations, experiments, and the two-oscillator model. An EIA-like effect emerges in virtue of the near-field coupling between metallic wire and dielectric block, and the dielectric block exhibiting magnetic dipolar resonance makes a major contribution to the resonance absorption. The magnetic flux through the dielectric block engendered by the near filed of the metallic wire determines the coupling between dielectric block and metallic wire. With the variation of the separation between dielectric block and metallic wire, the EIA-like effect is preserved and does not convert into the EIT-like effect although the coupling and consequently the absorbance are altered. Based on the two-oscillator model, the absorption spectrum of the EIA metamaterial is quantitatively analyzed and the parameters of the oscillator system are retrieved.

An investigation of the anomalous asymptotic behavior of elastic electron scattering of helium.

For the inelastic electron scattering of atoms and molecules, a consensus has been reached that the first Born approximation is easily approached by decreasing the momentum transfer at the same impact electron energy or increasing the impact electron energy at the same momentum transfer. Although this consensus is applicable for the elastic electron scattering of most atoms and molecules, it is violated for helium where the experimental differential cross sections deviate from the first Born approximation prediction gradually with the decrease of squared momentum transfer at the same impact electron energy. Since this anomalous phenomenon was observed more than 40 years ago, the intrinsic mechanism is not explicit. In the present work, using the high-resolution x-ray scattering, we isolate the scattering contribution from the nucleus and directly obtain the pure electronic structure of helium. Then, the anomalous asymptotic behavior of the elastic electron scattering of helium has been elucidated, i.e., in the small squared momentum transfer region, the scattering contribution from the target's electrons is counteracted by the one from the atomic nucleus, which results in the residual contribution beyond the first Born approximation being drastically enlarged.

Generalized oscillator strengths of the low-lying valence-shell excitations of N2, O2, and C2H2 studied by fast electron and inelastic x-ray scattering.

The generalized oscillator strengths of the low-lying valence-shell excitations of N2, O2, and C2H2 have been studied by the high-energy electron scattering, the high-resolution inelastic X-ray scattering, and the multireference single- and double-excitation configuration-interaction methods. Good agreement between the present electron-scattering results and the X-ray-scattering ones for the a''1Σg +v'=0 and a''1Σg +v'=1+b1Πuv'=0 excitations of N2 and the A'3Δu excitation of O2 is achieved in the small squared momentum transfer region, while obvious discrepancies among them are observed in the large squared momentum transfer region. This phenomenon indicates that the first Born approximation is satisfied in the small squared momentum transfer region, while it does not hold in the large squared momentum transfer region at an incident electron energy of 1500 eV, in view of the fact that the first Born approximation is satisfied in the X-ray scattering. In addition, the present calculation for the a''1Σg + excitation shows that the traditional assigned v' = 0 and 1 of the a″1Σg + excitation correspond to v' = 9 and 13 of the 21Σg + excitation and reproduces the X-ray-scattering results of the a''1Σg +v'=0 excitation very well except the ones in the small squared momentum transfer region. We also report the generalized oscillator strengths of the à + B̃ excitations of C2H2, and its profile shows that the bending geometry has great influence on the transition feature.

A 1-m non-resonant inelastic x-ray scattering spectrometer at BL15U, Shanghai Synchrotron Radiation Facility.

We report the design, construction, and commissioning of a spectrometer for non-resonant inelastic x-ray scattering study installed at BL15U, Shanghai Synchrotron Radiation Facility. It features a 1-m vertical scattering arm. An energy resolution of 1.3 eV is achieved based on the 1 m Rowland circle and the diced Si(555) crystal analyzer with a fixed Bragg angle of about 88.8°. The inelastic squared form factors of 21S + 21P of helium with respect to the momentum transfer were measured and compared with the accurate and reliable theoretical calculations in order to verify the spectrometer. Furthermore, the spectrometer is designed to work in the momentum transfer region of 0 Å-1 < q < 8.68 Å-1 and to initially focus on the non-resonant inelastic x-ray scattering studies on gaseous samples.

Field redistribution inside an X-ray cavity-QED setup.

The field redistribution inside an X-ray cavity-QED setup with an embedded 57Fe layer is calculated and studied in detail. The destructive interference between two transitions from the ground state to the two upper dressed states causes that the cavity mode can not be driven. So the field intensity is very weak when the nuclear ensemble is resonant. Moreover, It is found that the resonant nuclear layer can play a role of reflective layer like a mirror and cut the size of the cavity, which will destroy the guided mode. To support this idea, we employ the 57Fe film as the bottom mirror layer of the cavity where a guided mode can only be formed at the resonant energy. Following this perspective, the electromagnetically induced transparency structure based on X-ray cavity-QED setup with nuclear ensemble is reviewed and a phenomenologically self-consistent analysis for the field redistribution is presented.