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The magneto-optical (MO) materials are essential for designing nonreciprocal devices, like isolators and circulators. Even though the study of MO effect has a long history, the recent works of fabricating nonreciprocal nanostructures, novel MO metamaterials, and topological photonics have garnered significant attention in both theoretical and experimental research of MO materials. In this work, we consider the planar MO waveguide mode. By setting the general form of the fields and utilizing the boundary conditions, the analytical solution of MO modes is obtained. We have shown the potential of such effective solution in analyzing the dispersions and transport behaviors of MO modes in the waveguide. Crossings and avoided crossings of modes will happen, which may due to the strong coupling of TE and TM modes in the waveguide. Faraday rotation can be observed during the propagation of MO modes and the energy flow will precess in the waveguide. These results can be applied in predicting the evolution of the modes in MO waveguides, which has potential in designing MO nonreciprocal devices.

Electromagnetic wave transmission in a magneto-optical (MO) medium is a basic and old topic but has raised new interest in recent years, because MO medium plays a vital role in optical isolator, topological optics, electromagnetic field regulation, microwave engineering, and many other technological applications. Here, we describe several fascinating physical images and classical physical variables in MO medium by using a simple and rigorous electromagnetic field solution approach. We can easily obtain explicit formulations for all relevant physical quantities, such as the electromagnetic field distribution, energy flux, reflection/transmission phase, reflection/transmission coefficients, and Goos-Hänchen (GH) shift in MO medium. This theory can help to deepen and broaden our physical understanding of basic electromagnetics, optics, and electrodynamics in application to gyromagnetic and MO homogeneous medium and microstructures, and might help to disclose and develop new ways and routes to high technologies in optics and microwave.

Electromagnetic (EM) metasurface mantles afford an alternative avenue, allowing for the possibility of rendering arbitrary-shape objects unobservable. But the available mechanisms either depend on the states of polarization or the azimuth of wave incidence, or cannot dynamically manipulate cloaking responses without altering the structures. Herein, a three-dimensional closed-ring-based metasurface carpet cloak involving G e 2 S b 2 T e 5 that circumvents current drawbacks of metasurface structures is proposed. By judiciously designing meta-atoms on the external surface of a spherical object, the scattered wavefront, including the distributions of EM fields and polarizations, can be reconstructed, resembling what is deflected from a flat plane. Enabled by the perfectly symmetric distribution of meta-atoms, the carpet cloak is demonstrated to work well under arbitrary states of polarization and arbitrary azimuthal angles of incident light. Meanwhile, by converting G e 2 S b 2 T e 5 from the amorphous to crystalline state, the designed scheme is empowered with the ability to switch "ON" and "OFF" of stealth states. Furthermore, the unique design achieves invisibility over ±20 ∘ angular span in the mid-infrared range from 8800 to 9450 nm. The validated recipe empowers robust steps forward to achieve full-polarization, full-azimuth operation, and switchable cloaking in the real-world, showing great potential applications in stealth, camouflage, and illusion fields.

Being invisible at will has fascinated humanity for centuries and it has become more tangible with the development of metasurfaces, which have demonstrated the extraordinary ability of wavefront manipulation. However, state-of-the-art invisibility cloaks typically work in a deterministic system with a limited bandwidth and small incident angle ranges. Here, by integrating the phase-change material of Ge 2 Sb 2 Te 5 and the wavefront tailoring functionality of a reflective metasurface, we have achieved a unique carpet cloak that is endowed with broadband invisibility from 6920 to 8220 nm, fully concealing objects over a wide angular span of ±25° and a prominent radar cross-section reduction. Furthermore, the central cloaking wavelength can be continuously tuned with Ge 2 Sb 2 Te 5 film under different intermediate phases by precisely controlling external stimuli, which will provide a flexible and encouraging way to achieve active features once fabricated. Simulation results also show that the cloaking bandwidth can be significantly extended by triggering Ge 2 Sb 2 Te 5 from the amorphous to crystalline states. Importantly, the hybrid metasurface can realize switching of "ON" and "OFF" states in terms of cloaking features by converting Ge 2 Sb 2 Te 5 from the amorphous to the crystalline state. To the best of our knowledge, this is the first metasurface carpet cloak that utilizes the phase-change material of Ge 2 Sb 2 Te 5 to achieve ultra-broadband, wide-angle, continuously tunable and switchable cloaking with low profiles, light weights, and easy access. This design of a reconfigurable cloak is expected to find potential applications in various areas such as vehicle cloaking, illusions and so on.

We theoretically study the topological transition of dispersion types and propose a tunable planar lens based on graphene hyperbolic metamaterials (HMMs). By tuning the chemical potential (μ c ) of graphene, the dispersion relation of the HMM is topologically switchable between ellipse (μ c <0.6 eV) and hyperbola (μ c >0.6 eV) where positive and negative refractions occur respectively. Especially, for μ c >0.6 eV, a Gaussian light beam is negatively refracted twice and focuses at a far-field point finally, acting well as a planar lens. Furthermore, its focal length l can be sensitively tuned by controlling μ c , and Δl reaches 260 μm (from 528 to 268 μm) while μ c varies with only 0.05 eV (from 0.65 to 0.7 eV). The physical reason is attributed to the different anisotropy degrees of EFCs for different μ c . Such a compact, high-speed, and sensitively tunable planar lens holds great promise in photonic integration, photonic imaging, and directional coupling applications.

Photonic devices based on a high-Q cavity are commonly constrained by narrow operation bandwidth, slow charging, and releasing speed. Here, we analytically and numerically demonstrate that this limitation can be broken via dynamically tuning the Q factors of a single nanocavity and driving controllable photonic transitions between a low-Q cavity mode and an ultrahigh-Q mode by temporal refractive index modulations inside a multimode cavity. During this dynamic process, the bandwidth of signal light is also reversibly compressed. As a result, the fundamental link between the photon lifetime and the operation bandwidth is broken, and a delay-bandwidth product of ∼76 is achieved, which is far greater than that of the usual optical resonator systems. This approach may pave the way for dynamic control of on-chip all-optical information processing.

Efficient confining of photons into subwavelength scale is of great importance in both fundamental researches and engineering applications, of which one major challenge lies in the lack of effective and reliable on-chip nanofabrication techniques. Here we demonstrate the efficient subwavelength light focusing with carefully engineered pyramidal structures fabricated by direct laser writing and surface metallization. The important effects of the geometry and symmetry are investigated. Apertures with various sizes are flexibly introduced at the apex of the pyramids, the focusing spot size and center-to-sidelobe ratio of which could be improved a factor of ~4 and ~3, respectively, compared with the conical counterparts of identical size. Moreover, two pairs of asymmetric through-nanogratings are conceptually introduced onto the top end of the pyramids, showing significantly improved focusing characteristics. The studies provide a novel methodology for the design and realization of 3D plasmonic focusing with low-noise background and high energy transfer.

We theoretically study the absorption property of graphene manipulated by a dielectric cylinder through an analytical method. The distinctive absorption properties of incident waves with different polarizations (TM and TE) are analyzed and they are strongly correlated with the structure resonance and material dispersion. Besides, the characteristics of graphene absorption tuned by the cylinder radius and refractive index as well as the chemical potential of graphene are systematically investigated. It is found that enhancement and continuous tunability of graphene absorption can be achieved by utilizing the whispering gallery mode produced in the dielectric cylinder and harnessing the graphene optical conductivity via tuning its chemical potential by exterior electrical grating. The theoretical studies open up a simple while efficient means to manipulate the absorption of graphene in a broad frequency range via the geometric and physical configuration of hybrid graphene-microstructures.

We experimentally demonstrate a broadband one-way transmission by merging the operating bands of two types of one-way edge modes that are associated with Bragg scattering and magnetic surface plasmon (MSP) resonance, respectively. By tuning the configuration of gyromagnetic photonic crystals and applied bias magnetic field, the fused bandwidth of unidirectional propagation is up to 2 GHz in microwave frequency range, much larger than either of the individual one-way bandwidth associated with Bragg scattering or MSP resonance. Our scheme for broadband one-way transmission paves the way for the practical applications of one-way transmission.

We have investigated second harmonic generation (SHG) from Ag-coated LiNbO₃(LN) core-shell nanocuboids and found that giant SHG can occur via deliberately designed double plasmonic resonances. By controlling the aspect ratio, we can tune fundamental wave (FW) and SHG signal to match the longitudinal and transverse plasmonic modes simultaneously, and achieve giant enhancement of SHG by 3 × 10(5) in comparison to a bare LN nanocuboid and by about one order of magnitude to the case adopting only single plasmonic resonance. The underlying key physics is that the double-resonance nanoparticle enables greatly enhanced trapping and harvesting of incident FW energy, efficient internal transfer of optical energy from FW to the SHG signal, and much improved power to transport the SHG energy from the nanoparticle to the far-field region. The proposed double-resonance nanostructure can serve as an efficient subwavelength coherent light source through SHG and enable flexible engineering of light-matter interaction at nanoscale.

One of the simplest models involving the atom-field interaction is the coupling of a single two-level atom with single-mode optical field. Under the rotating wave approximation, this problem is reduced to a form that can be solved exactly. But the approximation is only valid when the two levels are resonant or nearly resonant with the applied electromagnetic radiation. Here we present an analytical solution without the rotating wave approximation and applicable to general atom-field interaction far away from the resonance. We find that there exists remarkable influence of the initial phase of optical field on the Rabi oscillations and Rabi splitting, and this issue cannot be explored in the context of the rotating wave approximation. Due to the retention of the counter-rotating terms, higher-order harmonic appears during the Rabi splitting. The analytical solution suggests a way to regulate and control the quantum dynamics of a two-level atom and allows for exploring more essential features of the atom-field interaction.

Manipulation of surface plasmon polaritons (SPPs) on metal surfaces is important for constructing ultracompact integrated micro/nano optical devices and systems. We employ the method of surface electromagnetic wave holography (SWH) to design holographic groove patterns for managing the transport of broadband SPPs on metal surface. Several sets of groove patterns corresponding to different wavelengths are etched on the same region on metal surface to form a broadband SPP hologram. The incident SPPs are scattered by the composite hologram and interfere with each other to focus at different or the same positions for SPPs of different wavelengths. Finite-difference time-domain simulations show that broadband demultiplexing of SPPs is realized by the designed plasmonic holographic structures. In addition, the broadband SPPs can be focused to a pre-designated spot by a designed plasmonic hologram and as a result focusing of an ultrashort plasmonic pulse can be achieved. The results show that the SWH can successfully handle design of plasmonic holographic structures for SPPs wavelength management on metal surface.

It is shown that photonic crystal (PhC) optical reflectors with reflectance in excess of 60% and fractional bandwidths greater than 10% can be fabricated by ion beam milling of fewer than ten periods of rectangular cross section through-holes in micron-scale tapered fibers. The optical characteristics agree well with numerical simulations when allowance is made for fabrication artefacts and we show that the radiation loss, which is partly determined by optical interference, can be suppressed by design. The freely-suspended devices are compact and robust and could form the basic building block of optical cavities and filters.

The manipulation of surface plasmon polaritons (SPPs) on metal surfaces is an important aspect in the design of ultra-compact integrated micro/nano optical systems. We present a direct method for the easy, yet accurate design of complicated groove patterns to control SPPs traveling on metal surfaces, using a surface electromagnetic wave holography method, based on the Huygens-Fresnel principle. SPPs are scattered by these deliberately and appropriately determined groove patterns and interfere with each other to form new paths with desire. Two devices are demonstrated, with predesignated functionalities fully implemented by the designed plasmonic holographic structures, according to the finite-difference time-domain simulations. The results strongly indicate that this direct method is effective, efficient, and user friendly in its application to control SPPs on metal surfaces.

We present a direct-method solution toward the general problem of plasmonic wavefront manipulation and shaping to realize pre-designated functionalities based on the surface-wave holography (SWH) method. We demonstrate theoretically and experimentally the design and fabrication of holographic plasmonic lenses over surface plasmons with complex wavefront profiles. We show that visible light at 632.8 nm transmitting through a high-aspect-ratio slit or a micro-rectangle hole in a silver film can be focused to a preset three-dimensional point spot in free space via appropriately manipulating the interaction of excited surface plasmons with the nanoscale groove pattern of the holographic lens. The experiment results of scanning near-field optical microscopy for measuring the three-dimensional optical field distribution agree well both with designs and with numerical simulations, and this strongly supports the effectiveness and efficiency of the SWH method in the design of plasmonic devices that can fulfill manipulation and transformation of complicated-profile surface plasmons.

We demonstrate trapping and characterization of multiple gold nanospheres with a setup composed of dark field imaging and optical tweezers. The number of trapped nanospheres is quantified by the overall dark-field scattering intensity. The spectra of the scattering intensity show that there is no interparticle coupling among trapped nanospheres when the density of nanospheres in the trap is low enough (less than 10 particles), while the density of nanosphere increases the interparticle coupling of nanospheres becomes obvious. In addition, the trapping potential of a single gold nanosphere is obtained by trapping an ensemble of gold nanospheres.

Interferometric behaviours in radiation generation process of a one-dimensional photonic-crystal (PhC) reflector are investigated on a silica waveguide. An analytical model, which can calculate total radiation losses, is presented after analysing eigen-mode properties and radiation pattern characteristics. Our model takes into account the interference of component radiated waves generated at interfaces between different waveguide sections and is verified by comparing with numerical simulations of periodic and non-periodic one-dimensional PhC reflectors. Its capability to quickly find minimum-radiation tapering geometries helps circumvent time-consuming numerical simulations. Our model negates the conventional knowledge that a gradually tapered transition from uniform waveguide to PhC waveguide yields lower radiation loss. In one example, the gradually tapered transition produces 2.3dB more radiation than the optimum geometry.

We report a two-dimensional silicon photonic crystal structure that supports the self-collimation effect with large incident angles over a broad wavelength range. We theoretically and experimentally show the propagation light in the designed and fabricated structure without diffraction. Broadband large-angle self-collimation is observed in infrared wavelength.

We present a versatile technique based on nano-imprint lithography to fabricate high-quality semiconductor-polymer compound nonlinear photonic crystal (NPC) slabs. The approach allows one to infiltrate uniformly polystyrene materials that possess large Kerr nonlinearity and ultrafast nonlinear response into the cylindrical air holes with diameter of hundred nanometers that are perforated in silicon membranes. Both the structural characterization via the cross-sectional scanning electron microscopy images and the optical characterization via the transmission spectrum measurement undoubtedly show that the fabricated compound NPC samples have uniform and dense polymer infiltration and are of high quality in optical properties. The compound NPC samples exhibit sharp transmission band edges and nondegraded high quality factor of microcavities compared with those in the bare silicon PC. The versatile method can be expanded to make general semiconductor-polymer hybrid optical nanostructures, and thus it may pave the way for reliable and efficient fabrication of ultrafast and ultralow power all-optical tunable integrated photonic devices and circuits.

Optical trapping of gold nanoparticles is experimentally demonstrated using radially and azimuthally polarized beams. The transverse optical trapping stiffness of gold nanoparticles is measured. The radially polarized beam exhibits a higher trapping efficiency than the azimuthally polarized beam and the Gaussian beam. The transverse stiffness of particles with different diameters is measured experimentally and calculated via the discrete-dipole approximation method, and good agreement between theory and experiment is found.

Optical isolation is a long pursued object with fundamental difficulty in integrated photonics. As a step towards this goal, we demonstrate the design, fabrication, and characterization of on-chip wavelength-scale optical diodes that are made from the heterojunction between two different silicon two-dimensional square-lattice photonic crystal slabs with directional bandgap mismatch and different mode transitions. The measured transmission spectra show considerable unidirectional transmission behavior, in good agreement with numerical simulations. The experimental realization of on-chip optical diodes with wavelength-scale size using all-dielectric, passive, and linear silicon photonic crystal structures may help to construct on-chip optical logical devices without nonlinearity or magnetism, and would open up a road towards photonic computers.

We report a method in the framework of surface wave holography to manipulate the electromagnetic wave on the metallic surface for realizing complicated electromagnetic wave transport functionalities in three-dimensional (3D) space. The method allows for direct determination of the metallic surface structure morphology for a given transport functionality, by means of writing desirable object information on the metallic surface via interference with a reference surface wave. We have employed the analytical approach to design and build metallic surface structures that realize arbitrary single-point focusing, arbitrary single-direction beam collimation, and simultaneous two-point focusing of electromagnetic wave in 3D space. Good agreement between numerical simulations and microwave experimental measurements has been found and confirms the power of the method in conceptually understanding and exploiting the surface electromagnetic wave on subwavelength metal structures., (© 2011 Optical Society of America)

We have demonstrated super quasi-phase matching (QPM) in a super periodically poled lithium niobate (PPLN), which is composed of multiple ordinary PPLN cells. When super QPM is achieved, the slight phase mismatch in each PPLN cell can be further compensated for, and the relevant second harmonic generation is facilitated greatly. This mechanism provides an insightful means to relieve the limitation imposed by sample fabrication inaccuracy and will open up a promising avenue toward highly efficient nonlinear interactions.

We investigate the surface plasmon polariton (SPP)-assisted interaction between two dipoles near a metal surface. The radiation energy from a dipole can excite SPPs and transport to another dipole through the channel of the localized SPP modes. This energy transfer can be much more efficient than direct energy transfer via dipole-dipole radiation interaction in free space. A simple analytical model is proposed to describe the underlying physics behind the influence of SPP on the dipole-dipole interaction energy, and it predicts a wide variety of complicated interaction features that agree well with rigorous calculations., (© 2011 Optical Society of America)

We report on the efficient surface plasmon amplification by stimulated emission of radiation (spaser) from a gold nanorod coated with proper gain media. Numerical simulations show that the threshold of the nanorod-based spaser is nearly 1 order of magnitude lower than that of the core-shell nanosphere, which is verified by analysis with electrostatic theory. Furthermore, it is found that the nanorod-based nanosystem possesses unique optical properties such as wavelength tunability and polarization sensitivity.

This article demonstrates theoretical design of ultracompact all-optical AND, NAND, OR, and NOR gates with two-dimensional nonlinear photonic crystal slabs. Compound Ag-polymer film with a low refractive index and large third-order nonlinearity is adopted as our nonlinear material and photonic crystal cavities with a relatively high quality factor of about 2000 is designed on this polymer slab. Numerical simulations show that all-optical logic gates with low pump-power in the order of tens of MW/cm2 can be achieved. These design results may provide very useful schemes and approaches for the realization of all-optical logic gates with low-cost, low-pump-power, high-contrast and ultrafast response-time.

We propose a scheme to enhance near-UV band absorption of a rutile TiO(2) nanoparticle by placing Au nanoparticles in its neighborhood. The discrete-dipole approximation method was employed to calculate the absorption spectrum of pure rutile TiO(2) and that of TiO(2) mixed with Au nanoparticles. The results indicate that pure rutile TiO(2) has its maximum absorption located in the deep-UV band. With the existence of Au nanoparticles, a significant light harvesting effect occurs, and this maximum shifts to the near-UV band, where usual excitation wavelength falls.

Optical bistability at nanoscale is a promising way to realize optical switching, a key component of integrated nanophotonic devices. In this work we present an analytical model for optical bistability in a metal nano-antenna involving Kerr nonlinear medium based on detailed analysis of the correlation between the incident and extinction light intensity under surface plasmon resonance (SPR). The model allows one to construct a clear picture on how the threshold, contrast, and other characteristics of optical bistability are influenced by the nonlinear coefficient, incident light intensity, local field enhancement factor, SPR peak width, and other physical parameters of the nano-antenna. It shows that the key towards low threshold power and high contrast optical bistability in the nanosystem is to reduce the SPR peak width. This can be achieved by reducing the absorption of metal materials or introducing gain media into nanosystems.

Cloaking can prevent a scattering object from detection by electromagnetic waves as it can significantly reduce the scattering cross section of the object. In this work we show that when two cloaks are placed in close neighborhood, the interaction will greatly enhance the perturbation of near field and significantly increase the scattering cross section compared with a single cloak. This means that the cloaking functionality may strongly degrade when two or more cloaks come close to each other. This offers a route to break down electromagnetic cloaking.