arXiv subjects

Linfang Shen

Publications and source records attributed to Linfang Shen.

11 recordsLinked to original sources

arXiv2026

Full-channel wavefront manipulation of surface waves with chirality-assisted geometric-phase metasurface

Owing to their localized field enhancement and subwavelength resolution, surface waves (SWs) offer broad application potential in communications, sensing, and photonics via on-chip wavefront manipulation. This makes multi-channel SW wavefront manipulation highly desirable. However, conventional metasurfaces for SW wavefront shaping, relying on geometric and propagation phase mechanisms, typically exhibit similar functionalities for co- or cross-polarized output channels under different circularly polarized (CP) incidences, thereby limiting the development of high-capacity on-chip integrated devices. Here, by introducing the chirality-assisted phase as an additional phase control mechanism, we effectively decouple both co- and cross-polarized output channels, enabling independent SW wavefront shaping in four distinct channels. We numerically and experimentally demonstrate two metasurfaces in the microwave range: a four-channel SW meta-deflector and a four-channel SW metadevice that simultaneously produces a focused SW beam, a SW Bessel beam, and two deflected SW beams in different directions. Therefore, chirality-assisted geometric-phase metasurfaces provide a versatile platform for multi-channel SW wavefront engineering, offering significant potential for high-capacity on-chip communication and integrated photonic systems.

physics.optics↗

arXiv2023

Unidirectional guided-wave-driven metasurfaces for arbitrary wavefront control

Metasurfaces, composed of subwavelength electromagnetic microstructures, known as meta-atoms, are capable of reshaping the wavefronts of incident beams in desired manners, making them great candidates for revolutionizing conventional optics. However, the requirement for external light excitation and the resonant nature of meta-atoms make it difficult to fully integrate metasurfaces on-chip or to control wavefronts at deep-subwavelength scales. Here, we introduce the concept and design of a new class of metasurfaces, driven by unidirectional guided waves, and being capable of arbitrary wavefront control based on the unique dispersion properties of unidirectional guided waves rather than resonant meta-atoms. Upon experimentally demonstrating the feasibility and practicality of the unidirectional nature of our designs in the microwave regime, we numerically validate this new principle through the design of several microwave meta-devices using metal-air-gyromagnetic unidirectional surface magnetoplamons, agilely converting unidirectional guided modes into the wavefronts of 3D Bessel beams, focused waves, and controllable vortex beams. We also numerically demonstrate sub-diffraction focusing, which is currently beyond the capability of conventional metasurfaces. Furthermore, we directly show how these concepts can be transferred to the terahertz regime, and discuss their feasibility in the optical domain, too. Based on this nonresonant (that is, broadband) mechanism and on standard plasmonic platforms, our metasurfaces can be integrated on-chip, enabling the manipulation of electromagnetic waves on deep subwavelength scales and over wide frequency ranges, thereby opening up new opportunities for applications in communications, remote sensing, displays, and so forth.

physics.optics↗

arXiv2020

Comment on "Nonreciprocal cavities and the time-bandwidth limit"

In their paper in Optica 6, 104 (2019), Mann et al. claim that linear, time-invariant nonreciprocal structures cannot overcome the time-bandwidth limit, and do not exhibit an advantage over their reciprocal counterparts, specifically with regard to their time-bandwidth performance. In this Comment [Optica 7(9), 1097-1101 (2020)], we argue that these conclusions are unfounded. On the basis of, both, rigorous full-wave simulations and insightful physical justifications, we explain that the temporal coupled-mode theory, on which Mann et al. base their main conclusions, is not suited for the study of nonreciprocal trapped states, and instead direct numerical solutions of Maxwell's equations are required. Based on such an analysis, we show that a nonreciprocal terminated waveguide, resulting in a trapped state, clearly outperforms its reciprocal counterpart, i.e. both the extraordinary time-bandwidth performance and the large field enhancements observed in such modes are a direct consequence of nonreciprocity. Additionally, herein, on the arXiv, we provide further results and explanations on the key points made in the main Comment, as well as further elucidating comments on the Reply to the Comment.

cond-mat.mtrl-sci↗

arXiv2019

Slow light,double directional rainbow trapping and releasing at terahertz frequencies

Slow light and rainbow trapping attract many attentions in last twenty years, and in most of the previous works, the researchers achieve the slow light and rainbow trapping with complicate configurations or techniques, for example, metamaterial techniques. In this paper, we propose a simple waveguide consisted with semiconductor and dielectric layers, and we find that the slow-light peaks appear in the dispersion curves. Besides, the cutoff frequency with $v_g = 0$ ($v_g$ is the group velocity) in the slow-light peaks is depending on the thicknesses of the semiconductor and dielectric. Then, we design a tapered, horizontally symmetric structure. By using software COMSOL and finite difference time domain method as well, we achieve double directional rainbow trapping and more importantly, releasing the rainbow in such structure. This approach is promising for optical isolator, optical buffer, optical switch and other optical functional devices in miniaturization optical integrated circuit.

physics.optics↗

arXiv2019

Magnetoplasmons in monolayer black phosphorus structures

Two-dimensional materials supporting deep-subwavelength plasmonic modes can also exhibit strong magneto-optical responses. Here, we theoretically investigate magnetoplasmons (MPs) in monolayer black phosphorus (BP) structures under moderate static magnetic fields. We consider three different structures, namely, a continuous BP monolayer, an edge formed by a semi-infinite sheet, and finally, a triangular wedge configuration. Each of these structures shows strongly anisotropic magneto-optical responses induced both by the external magnetic field and by the intrinsic anisotropy of the BP lattice. Starting from the magneto-optical conductivity of a single-layer of BP, we derive the dispersion relation of the MPs in the considered geometries, using a combination of analytical, semi-analytical, and numerical methods. We fully characterize the MP dispersions and the properties of the corresponding field distributions, and we show that these structures sustain strongly anisotropic subwavelength modes that are highly tunable. Our results demonstrate that MPs in monolayer BP, with its inherent lattice anisotropy as well as magnetically induced anisotropy, hold potential for tunable anisotropic materials operating below the diffraction limit, thereby paving the way for tailored nanophotonic devices at the nanoscale.

physics.optics↗

arXiv2019

Ultra-subwavelength focusing and giant magnetic-field enhancement in a low-loss one-way waveguide based on remanence

The subwavelength focusing based on surface plasmon polaritons (SPP) has been widely explored in tapered metallic structures. However, the efficiency of energy localization is relatively weak, largely due to high propagation loss and strong back reflection. Here, we propose a straight-tapered 3-dimensional (3D) one-way surface magnetoplasmon (SMP) waveguide with the ending surface of $\sim 10^{-4}λ_0 \times 10^{-4}λ_0$ to achieve energy focusing in the microwave regime. Due to low propagation loss of SMP, we achieve huge magnetic field enhancement in such an ultra-subwavelength area, by five orders of magnitude. Instead of using an external static magnetic field, our proposed SMP waveguide relies on remanence, which is very convenient for operating practical 3D applications. These results show promising applications in magnetic-field enhancing or quenching fluorescence, luminescence or nonlinearity of 2D materials, novel scanning near-field microwave microscopy and energy storage.

physics.optics↗

arXiv2008

Interaction between negative and positive index medium waveguides

The coupling between negative and positive index medium waveguides is investigated theoretically in this paper. A coupled mode theory is developed for such a waveguide system and its validity is verified. Interesting phenomena in the coupled waveguides are demonstrated, which occur in the case when the negative index medium waveguide in isolation guides its mode backward. A new type of coupled mode solution that varies exponentially with the coupling length is found in the special case when the propagation constants of two individual waveguides are nearly the same. A coupler operating in this case is insensitive to the coupling length, and its coupling efficiency can reach 100% as long as the coupling length is long enough. However, when the propagation constants of the two individual waveguides differ greatly, the coupled mode solution is still a periodic function of the coupling length, but the coupled power is output backward. In addition, the modes in the composite waveguide system are also studied using the coupled mode theory, and their fundamental properties are revealed.

physics.optics↗

arXiv2005

A novel directional coupler utilizing a left-handed material

A novel directional coupler with a left-handed material (LHM) layer between two single-mode waveguides of usual material is introduced. The coupling system is analyzed with the supermode theory. It is shown that such a LHM layer of finite length can shorten significantly the coupling length for the two single-mode waveguides. A LHM layer with two slowly tapered ends is used to avoid the reflection loss at the ends.

physics.class-ph↗

arXiv2004

Finite-size effects of a left-handed material slab on the image quality

The characteristics of an imaging system formed by a left-handed material (LHM) slab of finite length are studied, and the influence of the finite length of the slab on the image quality is analyzed. Unusual phenomena such as surface bright spots and negative energy stream at the image side are observed and explained as the cavity effects of surface plasmons excited by the evanescent components of the incident field. For a thin LHM slab, the cavity effects are found rather sensitive to the length of the slab, and the bright spots on the bottom surface of the slab may stretch to the image plane and degrade the image quality.

cond-mat.other↗

arXiv2003

Imaging Studies of Characteristics for a Slab of a Lossy Left-handed Material

The characteristics of an imaging system formed by a slab of a lossy left-handed material (LHM) are studied. The transfer function of the LHM imaging system is written in an appropriate product form with each term having a clear physical interpretation. A tiny loss of the LHM may suppress the transmission of evanescent waves through the LHM slab and this is explained physically. An analytical expression for the resolution of the imaging system is derived. It is shown that it is impossible to make a subwavelength imaging by using a realistic LHM imaging system unless the LHM slab is much thinner than the wavelength.

physics.optics↗

arXiv2002

Abnormal phenomena in a one-dimensional periodic structure containing left-handed materials

The explicit dispersion equation for a one-dimensional periodic structure with alternative layers of left-handed material (LHM) and right-handed material (RHM) is given and analyzed. Some abnormal phenomena such as spurious modes with complex frequencies, discrete modes and photon tunnelling modes are observed in the band structure. The existence of spurious modes with complex frequencies is a common problem in the calculation of the band structure for such a photonic crystal. Physical explanation and significance are given for the discrete modes (with real values of wave number) and photon tunnelling propagation modes (with imaginary wave numbers in a limited region).

cond-mat.mtrl-sci↗