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We theoretically and numerically reveal that under a given level of extinction cross section and with definite angular momentum channels dominant, there exists a physical limitation for absorption cross section being maximum and scattering cross section being minimum. In addition, any scattering systems operated at this condition would be accompanied by a needle Dirac-delta-like far-field radiation pattern, reducing to perturb the background field except in the forward direction. We therefore refer to this outcome as dark superabsorbers. Moreover, by considering the mathematical Gibbs phenomenon, we find that a completely equivalent Dirac-delta far-field radiation is excluded even we could properly design the scatterers operated at such conditions. We believe this finding has potential applications in design of dark energy harvesting, lower-visibility receivers, superdirective light-matter interaction, and Fresnel diffractive imaging.

We present a theoretical study of a novel polarization beam splitter (PBS), different to conventional time-reversal symmetry one, where can be totally reflected at two opposite sides with one specific linearly polarized light incident and can be transparent at only one side with its orthogonal linearly polarized light incident. %In addition, intensity of totally reflected beam would suffer from different The mechanism we employ is by both an exceptional point and finite periodicity of Non-Hermitian PT-symmetry. To be more specific, we design such PBS made of a finite periodic structure in which each unit cell has a delicate balance gain and loss in spatial distribution. In order to have one linearly polarized light totally reflected, the corresponding polarized unit cell has to be operated at some PT-symmetry phase of reflection band. To have single-sided transparent for its orthogonal polarized light, the corresponding polarized unit cell should be designed at an exceptional point as well as has asymmetric reflectance. Interestingly, such single-sided transparent phenomenon is independent of total number of unit cell. We believe this asymmetry PBS may excite a new route to polarization control.

We find that there exists a universal law of coiling not only for a long elastic strip contacting within a tube but also for a short one. Here the elastic strip we consider has the ratio of $2 < L/R \le 2\pi$ for its length $L$ to the tube radius $R$. By varying the ratio of $L/R$, we identify four types of deformation for such a short elastic strip, namely, two point-contact, three point-contact, continuous-contact, and self-contact. With theoretical formulas in closed forms and experimental demonstration, these four types are verified for any elastic strips contacting within a tube, irrespective of elastic properties, strip lengths, and tube radius. Our results on coiling can be readily applied to a variety of physical systems, including thin flexible electronic devices, van der Waals materials in scroll shape, and DNA packaging into viral capsids.

We theoretically and numerically investigate the scattering behavior of a periodic parity-time (PT)-symmetric waveguide network composed of a finite number of unit cells. Specifically, we put forward rigorous and formally exact expressions for wave propagation, bi-directional reflectionless, and coherent perfect absorption and lasing (CPAL) occuring in a finite periodic optical waveguide network. Through the use of the generalized parametric space derived from observation of PT-symmetric transfer matrix, Lorentz reciprocity theorem and non-imaginary Bloch phase, we observe that when the unit cell is operated at the PT broken phase or exceptional point, the system can always have propagating modes, independent of the number and transmission phase of the unit cell. On the other hand, when the unit cell is operated at the exact PT-symmetric phase, the formation of propagating waves would depend on the transmission phase of the unit cell. More interestingly, we find that even though the unit cell is not operated at the exceptional point, reflectionless with bi-directionality as well as unity transmittance can be achieved by choosing appropriate number of unite cells and specific PT phases. We also find two approaches to implement CPAL. One is to exploit odd number of the unit cell operated at the CPAL point. Another way is to manipulate specific broken phase with an appropriate number of the unit cells, while making transmission phase to be null. We believe this work may offer a theoretical underpinnings for studying extraordinary wave phenomena of PT-symmetric photonics and may open avenues for manipulation of light.

We discuss the optical forces exerted on parity-time (PT) symmetric heterostructures under normal incidence of a single and two counter-propagating plane waves. The underlying strategy is through generalized parametric space, stemming from consideration of PT-symmetry condition and Lorentz reciprocity theorem. In such a generalized parametric space, we are able to not only exhaustively indicate various PT phases and extraordinary wave phenomena, but also deduce the directionality and magnitudes of optical forces. We find that when the system is illuminated by a normally incident wave, it can exhibit the symmetric pushing effect in the exact symmetry phase, uni-directional null (UNF) and bi-directional null force (BNF) at the exceptional point (EP), and pulling-pushing flipped forces in the broken symmetry phase, with BNF found at the pushing-pulling turning point. In two counter-propagating plane waves interference, the magnitudes as well as the directionality of resultant optical force can be tuned by relative phase of incident waves. More interestingly, we observe that there has a null force independent of the relative phase occurred in a specific region of broken symmetry phase and exceptional point. In addition, we offer several PT-symmetric heterostructures to support our findings. Our results may be beneficial for applications in PT optomechanics and force rectifiers.

Based on the reciprocity theorem, we put forward a generalized parametric space for an arbitrary transfer matrix with parity time (PT) symmetry. Through this space, one can extract complete information involving PT phases, reflectances, transmittance and known extraordinary scattering phenomena. We demonstrate a PT heterostructure with coherent perfect absorption-lasing, anisotropic transmission resonance, and parity symmetry of reflection coefficients at the frequencies of interest. In addition, with the parametric space and the analytical formula, the corresponding complex dielectric permittivities for a simple PT system made of a gain, a gap, and a loss media in deeply subwavelength is derived to achieve various exotic PT functionality. This work could offer an alternative route to design versatile optical and photonic PT devices.

We theoretically and numerically prove that under an electromagnetic plane wave with linear polarization incident normally to cylindrical passive scatterers, a single energy diagram can integrate absorption, scattering, and extinction cross sections for arbitrary scattering systems, irrespective of internal configuration, material parameters, and its sizes. For a system with definite resonant orders, along the corresponding boundary of the energy diagram, not only the magnitudes of scattering coefficients, but also its phases, are required the same, corresponding to superabsorption or superscattering. For systems composed by larger resonant orders, the domain of the energy diagram can completely cover systems with lower one. Hence, systems with different resonant orders can provide the same energy characteristics, reflecting the energy degenerate property in light scattering. This degeneracy may relax more degrees of freedom in functional designs, such as energy harvesting, imaging, sensing devices. We demonstrate various systems based on real materials to support this finding.

We propose a universal strategy to realize a broadband control on arbitrary scatterers, through multiple coherent beams. By engineering the phases and amplitudes of incident beams, one can suppress the dominant scattering partial waves, making the obstacle lose its intrinsic responses in a broadband spectrum. The associated coherent beams generate a finite and static region, inside which the corresponding electric field intensity and Poynting vector vanish. As a solution to go beyond the sum-rule limit, our methodology is also irrespective of inherent system properties, as well as extrinsic operating wavelength, providing a non-invasive control on the wave-obstacles interaction for any kinds of shape.

With the wave interferometric approach, we study how extrinsically multiple coherent waves excitation can dramatically alter the overall scattering states, resulting in tailoring the energy assignment among radiation and dissipation. To explore the concept, we derive the corresponding formulas for dissipation and scattering powers for cylindrical passive systems encountered by general configurations of incident waves with various illuminating directions, phases, and intensities. We demonstrate that a linear superposition of incident waves extrinsically interferes the target channels in a desirable way. Moreover, the interferometric results can be irrespective to the inherent system configurations like size, materials, and structures. The extrinsic interfering waves pave a non-invasive solution to manipulate light and matter interaction, with potential applications in metasurfaces, nanophotonics, and metadevices.

With theoretical analyses and numerical calculations, we show that a passive scatterer at the sub-wavelength scale can simultaneously exhibit both nearly zero forward scattering (NZFS) and nearly zero backward scattering (NZBF). It is related to the interference of dipolar quadrupole modes of different origin, leading to coexistence of Kerker's first and second conditions at the same time. For optical frequencies, we propose two different sets of composited materials in multi-layered nano-structures, i.e., CdTe/Si/TiO2 and TiO2/Au/Si, for the experimental realization.

To explore resonance phenomena in the nonlinear region, we show by experimental measurements and theoretical analyses that resonance happens in modulation instability (MI) from non-instantaneous nonlinearities in photorefractive crystals. With a temporally periodic modulation in the external bias voltage, corresponding to a modulation in the nonlinear strength, an enhancement in the visibility of MI at resonant frequency is reported through spontaneous optical pattern formations. Modeled by such temporally periodic nonlinear driving force to the system, theoretical curves obtained from a nonlinear non-instantaneous Schr\"{o}dinger equation give good agreement to experimental data. As MI is a universal signature of symmetry-breaking phenomena, our observation on the resonance in MI may provide a control on chaotic, solitary, and turbulence waves., Comment: 5 pages, 4 figures

For matter wave scattering from passive quantum obstacles, we propose a phase diagram in terms of phase and modulus of scattering coefficients to explore all possible directional scattering patterns. In the phase diagram, we can not only have the physical bounds on scattering coefficients for all channels, but also indicate the competitions among absorption, extinction, and scattering cross sessions. With help of this phase diagram, we discuss different scenarios to steer scattering probability distribution, through the interference between $s$- and $p$-channels. In particular, we reveal the required conditions to implement a quantum scatterer, i.e., a quantum dot in semiconductor matrix, with a minimum (or zero) value in the scattering probability toward any direction. Our results provide a guideline in designing quantum scatterers with controlling and sensing matter waves., Comment: 6 pages, 3 figures

For passive electromagnetic scatterers, we explore a variety of extreme limits on directional scattering patterns in phase diagram, regardless of details on the geometric configurations and material properties. By demonstrating the extinction cross-sections with the power conservation intrinsically embedded in phase diagram, we give an alternative interpretation for Kerker first and second conditions, associated with zero backward scattering (ZBS) and nearly zero forward scattering (NZFS). The physical boundary and limitation for these directional radiations are illustrated, along with a generalized Kerker condition with implicit parameters. By taking the dispersion relations of gold-silicon core-shell nanoparticles into account, based on the of phase diagram, we reveal the realistic parameters to experimentally implement ZBS and NZFS at optical frequencies., Comment: 8 pages, 4 figures

A phase diagram, defined by the amplitude square and phase of scattering coefficients for absorption cross-section in each individual channel, is introduced as a universal map on the electromagnetic properties for passive scatterers. General physical bounds are naturally revealed based on the intrinsic power conservation in a passive scattering system, entailing power competitions among scattering, absorption, and extinction. Exotic scattering and absorption phenomena, from resonant scattering, invisible cloaking, coherent perfect absorber, and subwavelength superscattering can all be illustrated in this phase diagram. With electrically small core-shell scatterers as an example, we demonstrate a systematic method to design field-controllable structures based on the allowed trajectories in the phase diagram. The proposed phase diagram not only provides a simple tool to design optical devices but also promotes a deep understanding on Mie's scattering theory., Comment: 5 pages, 4 figures

By applying the interplay among the nodal points of partial waves, along with the concept of streamline in fluid dynamics for the probability flux, a quantum invisible cloak to the electron transport in a host semiconductor is demonstrated by simultaneously guiding the probability flux outside the core region and keeping the total scattering cross section negligible. As the probability flux vanishes in the interior region, one can embed any material inside a multiple core-shell sphere without affecting physical observables from the outside. Our results reveal the possibility to design a protection shield layer for fragile interior parts from the impact of transports of electrons., Comment: 5 pages, 4 figures