Incident Wave Vector Research Articles

Magneto-chiral backscatterings by rotationally symmetric nonreciprocal structures

It was proved that the joint operation of electromagnetic reciprocity and n-fold (n ≥ 3) rotational symmetry would secure arbitrary polarization-independent backscattering efficiency [Phys. Rev. B 103(4), 045422 (2021)10.1103/PhysRevB.103.045422]. Here we remove the restriction of reciprocity and study the backscatterings of plane waves by rotationally symmetric magneto-optical structures, with collinear incident wavevector, rotational axis and externally applied magnetic field. It is revealed that though nonreciprocity removes the degeneracy of backscattering efficiencies for circularly-polarized incident waves of opposite handedness, the remaining rotational symmetry is sufficient to guarantee that the efficiency is related to the polarization ellipticity only, having nothing to do with the orientations of the polarization ellipses. Moreover, the backscattering efficiency reaches its extremes (maximum or minimum values) always for circularly-polarized incident waves, and for other polarizations the efficiency is their ellipticity-weighted arithmetic average. The principles we have revealed are dictated by rotational symmetries only, which are irrelevant to specific geometric or optical parameters and are intrinsically robust against any rotational-symmetry preserving perturbations. The correlations we have discovered could be further exploited for fundamental explorations in nonreciprocal photonics and practical applications including polarimetry and ellipsometry.

Analytical study of linear optical properties of metal nanoparticle trimers: the impact of dipole-dipole interaction on different modes of configuration

In this study, we theoretically investigate the linear properties of a metal nanoparticle (MNP) trimer. Three identical spherical nanoparticles (NPs) whose centers are equidistantly oriented on a same straight line are considered. Using the solid core approximation for NPs and considering interaction between particles through induced electric dipoles, the motion equation of each NP conduction electrons is analytically solved. Some appropriately approximated expressions are derived for the permittivity of each NP based on a Drude-like model, allowing the clear observation of the contribution of inter-particle interaction. Depending on the orientation of the trimer axis and incident laser beam geometry (i.e., orientation of electromagnetic (EM) fields and wave vector of laser beam), three different configurations or modes are considered. The extinction efficiency of each NP as a function of wavelength is plotted, revealing that when the laser electric field is perpendicular to the symmetry axis of the trimer, it increases compared to the case of non-interactional single NP, and its plasmon resonance peak experiences a red shift. For other cases where the electric field of the laser beam is parallel to the trimer axis, the extinction efficiency of each NP decreases, and its peak shifts to the blue. In all cases, the effect of interaction on the optical properties of the middle NP is greater than on the other NPs.

Optimized Excitation in Microwave-induced Thermoacoustic Imaging for Artifact Suppression.

Microwave-induced thermoacoustic imaging (M-TAI) allows the visualization of macroscopic and microscopic structures of bio-tissues. However, it suffers from severe inherent artifacts that might misguide the subsequent diagnostics and treatments of diseases. To overcome this limitation, we propose an optimized excitation strategy. In detail, the strategy integrates dynamically compound specific absorption rate (SAR) and co-planar configuration of polarization state, incident wave vector and imaging plane. Starting from the theoretical analysis, we interpret the underlying mechanism supporting the superiority of the optimized excitation strategy to achieve an effect equivalent to homogenizing the deposited electromagnetic energy in bio-tissues. The following numerical simulations demonstrate that the strategy enables better preservation of the conductivity weighting of samples while increasing Pearson correlation coefficient. Furthermore, the in vitro and in vivo M-TAI experiments validate the effectiveness and robustness of this optimized excitation strategy in artifact suppression, allowing the simultaneous identification of both boundary and inside fine structures within bio-tissues. All the results suggest that the optimized excitation strategy can be expanded to diverse scenarios, inspiring more suitable strategies that remarkably suppress the inherent artifacts in M-TAI.

Multi-Dimensional Light-Emitting Meta-Display: Photoluminescence and Pumping Light Multiplexing.

The advent of intelligent display devices has given rise to diverse and complex demands for miniature light-emitting devices. Light-emitting metasurfaces have emerged as a practical and efficient means of achieving precise light modulation. However, their practicality is limited by certain constraints. First, there is a need for further exploration of the ability to manipulate both pumping and emitting light simultaneously. Second, there is currently no encoding freedom in multi-dimensional emitting light. To address these concerns, using meta-atoms is proposed to encode both fluorescence and pumping light independently, and expand the encoding freedom with different incident wavevector directions. A light-emitting metasurface with quad-fold multiplex encoding meta-displays, including dual scattering images and dual fluorescence images, is further demonstrated. This design strategy not only manipulates both pumping and fluorescence light but also broadens encoding freedom for comprehensive multi-functionality. This can pave the way for multiplexing optical displays, information storage, and next-generation wearable displays.

The Electron–Phonon Interaction at Vicinal Metal Surfaces Measured with Helium Atom Scattering

Recently, it was demonstrated that inelastic helium atom scattering from conducting surfaces provides a direct measurement of the surface electron–phonon coupling constant (mass enhancement factor λ) via the temperature or the incident wave vector dependence of the Debye–Waller exponent. Here, previous published as well as unpublished helium atom scattering diffraction data from the vicinal surfaces of copper (Cu(11α), with α = 3, 5, 7) and aluminum (Al(221) and Al(332)) were analyzed to determine λ. The results suggested an enhancement with respect to the corresponding data for the low-index surfaces (111) and (001) above the roughening transition temperature. The specific role of steps compared to that of terraces is briefly discussed.

Broadband directional invisibility

The discovery of unidirectional invisibility and its broadband realization in optical media satisfying spatial Kramers–Kronig relations are important landmarks of non-Hermitian photonics. We offer a precise characterization of a higher-dimensional generalization of this effect and find sufficient conditions for its realization in the scattering of scalar waves in two and three dimensions and electromagnetic waves in three dimensions. More specifically, given a positive real number α and a continuum of unit vectors Ω, we provide explicit conditions on the interaction potential (or the permittivity and permeability tensors of the scattering medium in the case of electromagnetic scattering) under which it displays perfect (non-approximate) invisibility whenever the incident wavenumber k does not exceed α (i.e., k∈(0,α]) and the direction of the incident wave vector ranges over Ω. A distinctive feature of our approach is that it allows for the construction of potentials and linear dielectric media that display perfect directional invisibility in a finite frequency domain.

Pseudospin collapse, multidirectional supercollimations, and all-electrons transmission and reflection in irradiated 8-Pmmn borophene

Propagation of ballistic electrons shows various optical-like phenomena. Here, we demonstrate a flexible method to modulate the band structure and manipulate the electron beams propagation in 8-Pmmn borophene by an off-resonant linearly polarized light. It is proposed to form fully tunable anisotropic dispersion by changing the polarization direction of the off-resonant light in an experimentally controllable way. Accompanied with it, the pseudospin symmetry of the electronic state in 8-Pmmn borophene collapses from a helical form into x or y direction, which undergoes a dramatic alteration. As a result of the wedge-shaped dispersions, the electron wave packet can be guided to propagate with undistorted shape along different directions, multidirectional electron supercollimations are exhibited in the system. Moreover, by constructing the optical sensing n–p and n–p–n junctions, interesting transport phenomena such as all-electrons Klein tunneling and omnidirectional reflection are realized by modulating the illumination parameters of the off-resonant light, both of them are independent of the incident energy and wave vector. It is expected that the peculiar transport properties in 8-Pmmn borophene modified by the off-resonant light field can offer more opportunities for device applications in valleytronics and electron-optics.

Fourier-space generalized magneto-optical ellipsometry

The magneto-optical Kerr effect (MOKE) is widely exploited in laboratory-based setups for the study of thin films and nanostructures, providing magnetic characterization with good spatial and temporal resolutions. Due to the complex coupling of light with a magnetic sample, conventional MOKE magnetometers normally work by selecting a small range of incident wave-vector values, focusing the incident light beam to a small spot, and recording the reflected intensity at that angular range by means of photodetectors. Using this approach, additional methodologies and measurements are required for full vectorial magnetic characterization. Here, we computationally investigate a Fourier-space MOKE setup, where a focused beam ellipsometer using high numerical aperture optics and a camera detector is employed to simultaneously map the intensity distribution for a wide range of incident and reflected wave vectors. We employ circularly incident polarized light and no analyzing optics, in combination with a fitting procedure of the light intensity maps to the analytical expression of the Kerr effect under linear approximation. In this way, we are able to retrieve the three unknown components of the magnetization vector as well as the material' s optical and magneto-optical constants with high accuracy and short acquisition times, with the possibility of single-shot measurements. Fourier MOKE is thus proposed as a powerful method to perform generalized magneto-optical ellipsometry for a wide range of magnetic materials and devices.

In-Plane and Out-of-Plane Investigation of Resonant Tunneling Polaritons in Metal-Dielectric-Metal Cavities.

Polaritons can be generated by tuning the optical transitions of a light emitter to the resonances of a photonic cavity. We show that a dye-doped cavity generates resonant tunneling polaritons with Epsilon-Near-Zero (ENZ) effective permittivity. We studied the polariton spectral dispersion in dye-doped metal-dielectric-metal (MDM) cavities as a function of the in-plane (k||) and out-of-plane (k⊥) components of the incident wavevector. The dependence on k|| was investigated through ellipsometry, revealing the ENZ modes. The k⊥ dependence was measured by varying the cavity thickness under normal incidence using a Surface Force Apparatus (SFA). Both methods revealed a large Rabi splitting well exceeding 100 meV. The SFA-based investigation highlighted the collective nature of strong coupling by producing a splitting proportional to the square root of the involved photons. This study demonstrates the possibility of generating ENZ polaritons and introduces the SFA as a powerful tool for the characterization of strong light-matter interactions.

Anomalous Reflection From a Phase Gradient Metasurface With Arbitrary Incident Angle

A planar phase gradient metasurface (PGM) with phase gradients in two in-plane directions will introduce two additional wave vectors to the reflected wave vector. For small incident angles, close to the normal vector, the magnitude of the reflected wave vector is smaller than that of the incident wave vector and the direction of the anomalous reflected wave can be determined from the incident wave vector components and additional phase gradient components. The expanded generalized Snell’s law, which includes diffraction order modes, is combined with array theory to accurately predict the directions of the reflected waves from a planar PGM for incident angles where the magnitude of the reflected wave vector is larger than that of the incident wave vector. The predicted directions of the reflected waves from a planar PGM are compared with simulated RCS results obtained with CST Studio Suite and measured results obtained in a compact range.

Transmission properties of grating-gate GaN-based HEMTs under different incident angles in the mid-infrared region.

The absorption tunability of grating-gate GaN-based HEMTs in the mid-infrared region has been confirmed in wide frequency regions. However, the application potential of grating-gate GaN-based HEMTs is limited due to a lack of study on transmission properties under different incident angles. Therefore, this paper studied the transmission characteristics of grating-gate GaN-based HEMTs under different incident angles in the mid-infrared region. By using the optical transfer matrix approach to model the dispersion characteristics in the structure, we found that the stronger plasmon polaritons and phonon polaritons occur in conductive channel and GaN layer. The variation of different incident wave vectors with incident angle affects the plasmon polaritons and phonon polaritons excitation intensities, resulting in the angular tunability transmission properties of grating-gate GaN-based HEMTs. After simulating the electric field distribution in COMSOL, the different transmission properties of grating-gate GaN-based HEMTs occur under different incident angles. Simulated results reveal the excellent angle-selectivity in grating-gate GaN-based HEMTs. The research into these characteristics shows that the structure has a lot of promise for designing mid-infrared angle selection filters, sensors, and other subwavelength devices in the future.

Polarization-Resolved and Helicity-Resolved Raman Intensity of Monolayer and Bilayer β-InSe

The structural and vibrational properties along with the polarization-resolved and helicity-resolved Raman spectra of monolayer and bilayer β-InSe excited by two commonly used laser lines (532 and 633 nm) in experiments are investigated by first-principles calculations so that we can establish the correlation between layer stacks and optical features in two-dimensional InSe. The stability of both monolayer and bilayer InSe are examined and confirmed by phonon dispersion. The monolayer and bilayer InSe are found to exhibit different point group symmetries and thereby the Raman properties due to the symmetry breaking of bilayer structure caused by interlayer van der Waals interaction. All Raman-active modes for monolayer (bilayer) InSe are listed, and the polarization-angle-dependent Raman intensities are accordingly discussed separately in terms of the configuration of the incident light (parallel or perpendicular to certain planes). By calculating the Raman tensor of each Raman-active mode, we obtained polarization-angle-dependent Raman intensities and discussed the location of each maximum under different polarization configurations, as well as the nonpolarized Raman intensity. The helicity-resolved Raman intensity is also obtained. It is found that more Raman-active modes can be observed under helical excitation when the incident light wave vector is in the y-direction. Among all of the Raman-active modes, the Raman intensity of A′1(A1g) is the strongest. Our work might help material scientists to characterize monolayer and bilayer InSe in experiments.

Two band tunneling for a pnp junction in tetralayer graphene

We investigate the band structure and the tunneling of ABCA-tetralayer graphene (ABCA-TTLG) subjected to an external potential U 0 applied between top and bottom layers. Using the tight-binding model, including the nearest t and next-nearest-neighbor t ′ hopping, low-energy model and two-band approximation model we study the band structure variation along the lines Γ − M − K − Γ in the first Brillouin zone , electronic band gap near Dirac point K , transmission properties and conductance, respectively. Our results reveal that ABCA-TTLG exhibits markedly different properties as functions of t ′ and U 0 . We show that the hopping parameter t ′ changes the energy dispersion, the position of K and breaks sublattice symmetries. A sizable band gap is created at K , which could be opened and controlled by the applied potential U 0 . This gives rise to 1D-like van Hove singularities (VHS) in the density of states (DOS). Resonant electronic transmission through graphene-based a p n p junction is studied as a function of the incident wave vector, the width and height of the barrier with and without U 0 . The resonant features in the transmission result from resonant hole states in the barrier and strongly influence the conductance. Our results are numerically discussed and compared with the literature. • Quantum tunneling of ABCA-tetralayer graphene in an potential applied between top and bottom layers is investigated. • The hopping parameter acts by changing the energy dispersion and the position of Dirac point K, also breaks the sublattice symmetries. • A sizable band gap is created at K, which could be opened and controlled by the potential giving rise to 1D-like van Hove singularities (VHS) in the density of states. • The resonant features in the transmission result from resonant hole states in the barrier and strongly influence the conductance.

Terahertz metasurfaces based on bound states in the continuum (BIC) for high-sensitivity refractive index sensing

Terahertz metasurface has attracted much attention in the field of label-free refractive index sensing due to its special fingerprint spectrum. Combining the special electromagnetic response of the metasurface and the ultra-high Q property of the quasi-BIC, a novel high-sensitivity THz sensor is proposed based on symmetry-protected BIC. The breaking of the mirror symmetry means that the resulting dipole displacement excites the surface resonance, and the BIC state is verified by satisfying the quality factor of the inverse square law. It is verified that the oblique incident wave vector can excite the BIC to the quasi-BIC. And the two different asymmetric parameters (displacement S and incident angle θ) of generating quasi-BIC can influence or even counteract each other. In order to increase the accuracy of the detection results, two different sensing parameters are used to detect the same analyte at the same time. The sensitivity of frequency shift difference and amplitude difference is as high as 11.17 GHz/RIU and 0.1766 /RIU, respectively. This work not only provides new ideas for the design of high-sensitivity sensors, but also has potential applications in the field of low-concentration sensing.

Angular‐Encrypted Quad‐Fold Display of Nanoprinting and Meta‐Holography for Optical Information Storage

AbstractThe compactness and programmable versatility make metasurface a promising candidate for optical information encryption and storage. The typical strategy for storage capacity enhancement is essential to expand the metasurface multiplexing channels. Various controllable optical parameters are progressively realized for optical encryption and multiplexing, including wavelength, polarization, orbital angular momentum (OAM), forward/backward illumination, etc. However, as one of the most critical parameters, the incident wavevector (k) direction, namely, the illumination angle, has not been fully explored due to the lack of angular‐encoding capability. Herein, the angular‐encrypted freedom for both amplitude and phase manipulation is theoretically and experimentally achieved by strategically screening the architectural blocks to fulfill the complex‐amplitude requirement. Utilizing the angular‐induced alteration from surface plasmonic resonances (SPRs) and Fabry–Perot (FP) nanocavity resonances, a single angular‐encrypted metasurface has been successfully realized to simultaneously enable quad‐fold independent‐encoding channels, including dual‐holography and dual‐nanoprinting images. Compared to the spatial multiplexing scheme in many previous works, the angular‐encrypted strategy drastically boosts the information storage density and capacity. It is believed that the proposed angular‐encryption finds a new path to achieve multiplexing meta‐devices with fully controlled angular‐resolution and promisingly expands the information capacity for optical encryption with a brand‐new degree of freedom.

Microsphere-Assisted Imaging of Periodic and Non-Periodic Structures

In this work, the difference in imaging small clusters of nanoparticles and hexagonally close packed nanoparticle arrays by microsphere-assisted microscopy is studied. For both high-contrast samples and low-contrast ones, compared with small cluster samples, a better image quality and a larger lateral magnification are observed when imaging periodic array samples. We propose that in imaging periodic samples, the evanescent waves carrying high-frequency information can be converted into propagating waves by shifting the incident wavevectors into different diffraction orders. Therefore, more high-frequency information of the samples can be collected by the imaging system, which results in an improved image quality and a larger magnification when observing periodic array samples.

Sparsity of Radiating Characteristic Modes on Infinite Periodic Structures

Characteristic modes on infinite periodic structures are studied using spectral dyadic Green's functions. This formulation demonstrates that, in contrast to the modal analysis of finite structures, the number of radiating characteristic modes is limited by unit cell size and incident wavevector (i.e., scan angle or phase shift per unit cell). The reflection tensor is decomposed into modal contributions from radiating modes, indicating that characteristic modes are a predictably sparse basis in which to study reflection phenomena.

Critical-point behaviors of gratings in Littrow mountings.

By using the recent theory of elemental scattering matrix, I study the critical-point behaviors of conically mounted and crossed gratings in Littrow mountings. As functions of the projected incident wave vector in the grating plane, the (m, n)th-order reflected principal diffraction efficiencies (the maximum and minimum diffraction efficiencies of a diffraction order for all possible incident polarizations) of a crossed grating have C2 symmetry with respect to the (m, n)th-order Littrow point, and for a conically mounted grating, the symmetry type is increased to C2v. In a Littrow mounting, each one of the two principal diffraction efficiencies, independent of the other, can only be at one of the three possible states: a local maximum, a local minimum, or a saddle point of an even (most likely the second) degree. If the grating is mirror-symmetric with respect to a plane parallel to the grating's mean plane, the results hold for a transmitted diffraction order as well.

Angular‐Multiplexing Metasurface: Building Up Independent‐Encoded Amplitude/Phase Dictionary for Angular Illumination

AbstractAs metasurfaces have shown great potential for light manipulation, the majority of optical source parameters have been extensively explored and progressively realized for new degrees of freedom in optical control and multiplexing, including wavelength, polarization, and forward/backward illumination, etc. However, the incident wave vector (k) direction, namely the illumination angle, as one of the critical parameters, has not been fully explored for optical multiplexing due to the lack of angular‐encoding freedom. Here, a general strategy is proposed to realize angular‐multiplexing metasurface based on building up an independent‐encoded amplitude/phase dictionary, in which one can retrieve any arbitrary combination of optical response with different angular illuminations. A series of angular‐multiplexing designs are theoretically and experimentally demonstrated, including angular‐multiplexed meta‐holographs, nanoprintings as well as a hybridized nanoprinting with holography. These studies’ numerical calculations and experiments confirm the new degree of freedom for arbitrary encoded amplitude and phase at different illumination angles. The findings establish a new platform for achieving a complexity of functions for angular resolved illumination, and expanding the manipulative capabilities for optical multiplexing.

Three-dimensional transfer function of optical microscopes in reflection mode.

Three-dimensional (3D) transfer functions build the basis for a comprehensive characterization of optical imaging systems in the spatial frequency domain. Utilizing the projection-slice theorem, the 2D modulation transfer function of an incoherent imaging system can be derived from a 3D transfer function by integration with respect to the axial spatial frequency. For a diffraction limited microscope with homogeneous incoherent pupil illumination, the modulation transfer function equals the 2D autocorrelation function of a circular disc. However, until now to the best of our knowledge no 3D transfer function has been published, which exactly leads to the 2D modulation transfer function of a diffraction limited microscope in reflection mode. In this article, we derive a formula, which after integration with respect to the axial spatial frequency coordinate perfectly fits to the diffraction limited 2D modulation transfer function. The inverse three-dimensional Fourier transform of the 3D transfer function results in a complex-valued 3D point spread function, from which the depth of field, the lateral resolution and, in addition, the corresponding 3D point spread function of both, a conventional and an interference microscope, can beobtained.