Guoxiong Cai

Super-long photonic nanojet generated from liquid-filled hollow microcylinder

Photonic nanojet (PNJ) from liquid-filled hollow microcylinder (LFHM) under a liquid immersion condition is numerically investigated based on the finite element method and physically analyzed with ray optics. Simulation and analysis results show that, by simultaneously introducing the immersed liquid and filled liquid, the propagation beam is greatly flattened, and super-long PNJs with decay length more than 100 times the illumination wavelengths are obtained in the outer near-field region of the LFHM. With the variation of the refractive index contrast between the filled and immersed-liquids, the properties of the PNJs, such as the focal distance, decay length, full width at half-maximum, and maximum light intensity can be flexibly tuned.

Broadband absorber with periodically sinusoidally-patterned graphene layer in terahertz range

We demonstrate that a broadband terahertz absorber with near-unity absorption can be realized using a net-shaped periodically sinusoidally-patterned graphene sheet, placed on a dielectric spacer supported on a metallic reflecting plate. Because of the gradient width modulation of the unit graphene sheet, continuous plasmon resonances can be excited, and therefore broadband terahertz absorption can be achieved. The results show that the absorbers normalized bandwidth of 90% terahertz absorbance is over 65% under normal incidence for both TE and TM polarizations when the graphene chemical potential is set as 0.7 eV. And the broadband absorption is insensitive to the incident angles and the polarizations. The peak absorbance remains more than 70% over a wide range of the incident angles up to 60° for both polarizations. Furthermore, this absorber also has the advantage of flexible tunability via electrostatic doping of graphene sheet, which peak absorbance can be continuously tuned from 14% to 100% by controlling the chemical potential from 0 eV to 0.8 eV. The design scheme is scalable to develop various graphene-based tunable broadband absorbers at other terahertz, infrared, and visible frequencies, which may have promising applications in sensing, detecting, and optoelectronic devices.

A Slot-Based Surface Plasmon-Polariton Waveguide With Long-Range Propagation and Superconfinement

A full-vector spectral element method (SEM) is applied to model and simulate surface plasmon-polariton (SPP) waveguides. Gauss-Lobatto-Legendre (GLL) polynomials are used to construct higher-order basis functions to achieve spectral accuracy. A discretization scheme featuring a nonuniform mesh with extra elements near the metal-dielectric interface is proposed to capture the waveguide configuration and dramatical mode field variations of the SPP waveguide. The studies on the accuracy and mode field distribution show that SEM is highly efficient and accurate. Using SEM simulation, a slot-based SPP waveguide operating at telecom wavelengths is proposed. Numerical results show that the proposed structure can simultaneously achieve millimeter-scale propagation distance (Lp ~2.6 mm) and below-diffraction-limited effective mode area (Aeff/A0 ~0.3) . Parametric plots illustrate a significant improvement when compared to conventional SPP waveguides. Investigation of the mode width and crosstalk also demonstrates the excellent 3-D integration performance of the structure. The proposed slot-based SPP waveguide thus can become a potential candidate for highly integrated photonic circuits.

Subsurface nano-imaging with self-assembled spherical cap optical nanoscopy

Frequently-used subsurface nano-imaging techniques have limitations in interference, stability, complexity, timeliness and cost reduction on account of the combination of excited ultrasound signal or probed cantilever tip. Though some improved optical methods can directly and visually obtain subsurface nanofeatures, the high refractive index difference (RID) between introduced superlens and subsurface object will inevitably degenerate the image quality. In this paper, a simple and reliable experimental technique is presented to self-assemble spherical cap optical nanoscopy (SCON) subsurface nano-imaging system (SNIS) with two low RID materials. By using SCON-SNIS, subsurface objects with a spacing as small as 0.16 times of illumination wavelength, and involving wider field of views (nearly one-half of SCONs great-circle diameter in the direction of the equator) and deeper depth (several micrometers) can be imaged. In order to get insights into the imaging mechanism, a finite element simulation and a ray-optics analytical study are performed, in which the imaging process is elucidated both theoretically and experimentally. This non-invasive, label-free and real-time subsurface nano-imaging paradigm could be a promising tool in life, material, biology and engineering sciences.

Investigation of Optical Spectrum Properties of Hexagonal Boron Nitride from Metal to Dielectric Transition

Hexagonal boron nitride as a natural hyperbolic material has attracted lots of attention recently. Here, we investigate numerically the optical spectrum properties of hexagonal boron nitride from the perspective of optical transition. After careful data analysis, hexagonal boron nitride at the epsilon-near-zero point of permittivity either turns from a hyperbolic material to an effective dielectric for transverse magnetic-polarized wave or from an effective metal to an effective dielectric for transverse electric-polarized wave. The results in this work may pave the way for potential applications of hexagonal boron nitride in the field of metamaterials.

Strongly Confined Spoof Surface Plasmon Polaritons Waveguiding Enabled by Planar Staggered Plasmonic Waveguides

We demonstrate a novel route to achieving highly efficient and strongly confined spoof surface plasmon polaritons (SPPs) waveguides at subwavelength scale enabled by planar staggered plasmonic waveguides (PSPWs). The structure of these new waveguides consists of an ultrathin metallic strip with periodic subwavelength staggered double groove arrays supported by a flexible dielectric substrate, leading to unique staggered EM coupling and waveguiding phenomenon. The spoof SPP propagation properties, including dispersion relations and near field distributions, are numerically investigated. Furthermore, broadband coplanar waveguide (CPW) to planar staggered plasmonic waveguide (PSPW) transitions are designed to achieve smooth momentum matching and highly efficient spoof SPP mode conversion. By applying these transitions, a CPW-PSPW-CPW structure is designed, fabricated and measured to verify the PSPW’s propagation performance at microwave frequencies. The investigation results show the proposed PSPWs have excellent performance of deep subwavelength spoof SPPs confinement, long propagation length and low bend loss, as well as great design flexibility to engineer the propagation properties by adjusting their geometry dimensions and material parameters. Our work opens up a new avenue for development of various advanced planar integrated plasmonic devices and circuits in microwave and terahertz regimes.

Electrically Tunable Broadband Terahertz Absorption with Hybrid-Patterned Graphene Metasurfaces

We numerically demonstrate a broadband terahertz (THz) absorber that is based on a hybrid-patterned graphene metasurface with excellent properties of polarization insensitivity, wide-angle, and active tunability. Our design is made up of a single-layer graphene with periodically arranged hybrid square/disk/loop patterns on a multilayer structure. We find that broadband absorption with 90% terahertz absorbance and the fractional bandwidth of 84.5% from 1.38 THz to 3.4 THz can be achieved. Because of the axisymmetric configuration, the absorber demonstrates absolute polarization independence for both transverse electric (TE) and transverse magnetic (TM) polarized terahertz waves under normal incidence. We also show that a bandwidth of 60% absorbance still remains 2.7 THz, ranging from 1.3 THz to 4 THz, for a wide incident angle ranging from 0° to 60°. Finally, we find that by changing the graphene Fermi energy from 0.7 eV to 0 eV, the absorbance of the absorbers can be easily tuned from more than 90% to lower than 20%. The proposed absorber may have promising applications in terahertz sensing, detecting, imaging, and cloaking.

Multiple resonant excitations of surface plasmons in a graphene stratified slab by Otto configuration and their independent tuning

Multiple resonant excitations of surface plasmons in a graphene stratified slab are realized by Otto configuration at terahertz frequencies. The proposed graphene stratified slab consists of alternating dielectric layers and graphene sheets, and is sandwiched between a prism and another semi-infinite medium. Optical response and field distribution are determined by the transfer matrix method with the surface current density boundary condition. Multiple resonant excitations appear on the angular reflection spectrum, and are analyzed theoretically via the phase-matching condition. Furthermore, the effects of the system parameters are investigated. Among them, the Fermi levels can tune the corresponding resonances independently. The proposed concept can be engineered for promising applications, including angular selective or multiplex filters, multiple channel sensors, and directional delivery of energy.

High transmission in a metal-based photonic crystal

We propose metal-based photonic crystals (PCs) with annular air cavities. The unit cell could be analogue to a two-dimensional finite quantum well, which makes the PC system closely describe the similar physics of atomic crystals. By tuning the filling ratio of air annuluses, we discover a band inversion between monopolar and dipolar states (or similarly, the s and p states in quantum mechanics). There is a transition system of accidental degeneracy, where a Dirac-like cone could be achieved. Such design could be used to implement high transmission in a bulk metal near the frequency of the Dirac-like point. Numerical simulations are performed to investigate the wave transport behaviors of such a metallic system.

Broadband terahertz reflector based on dielectric metamaterials

An isotropic reflector with near-unity reflectivity is shown based on the three-dimensional monolayer microparticles consisting of ceramic cube arrays. Numerical results show that a broad reflective band with high performance can be excited in an array of dielectric cubes. Meanwhile, from the calculated data, it is observed that the designed reflector is independent of the incidence angle for the transverse electric and transverse magnetic polarizations. In principle, high reflectivity can be realized at arbitrary wavelengths of interest where only a single dielectric layer is required. This work may provide a convenient route to design adaptive metamaterials.