Electron cyclotron motion excited surface plasmon and radiation with orbital angular momentum on a semiconductor thin film.

In this work, surface plasmons (SPs) on a germanium (Ge) thin film in terahertz (THz) region that are excited by electron cyclotron motion (ECM) and the subsequent SP emission (SPE) by adding Ge gratings on the film are explored by finite-difference time-domain (FDTD) and particle-in-cell FDTD (PIC-FDTD) simulations. The optical properties of ECM-excited SPs are the same as those of SPs that are excited by electron straight motion (ESM). For operating at the flat band of SPs' dispersion curve on the Ge film, changing the electron energy will only change the wavevector of SPs and hence the number of periods of SPs on the circular orbital. When the periodic gratings are deposited on the Ge film along the circular orbital of electrons, the emitted SPE contains the orbital angular momentum (OAM). The number of arms and chirality of the spiral patterns in phase map (i.e. the quantum number of OAM) of SPE are determined by the difference between the number of SPs' periods and the number of gratings. Manipulations of the quantum number of OAM by changing the number of gratings for a fixed electron energy and by changing the electron energy for a fixed number of gratings are also demonstrated. This work provides an active OAM source and it is not required to launch circularly polarized beams or pumping beams into the structure.

Surface plasmons manipulated Smith-Purcell radiation on Yagi-Uda nanoantenna arrays.

An electron bunch (e-bunch) passing through an insulator-metal-insulator (IMI) substrate can excite surface plasmons (SPs) on the substrate. Recent studies demonstrate that Smith-Purcell radiation (SPR) from one-dimensional gratings on an IMI substrate can be manipulated and enhanced by e-bunch excited SPs. However, under this configuration, only the emission along the direction of electron moving can be controlled. To steer both the azimuthal and polar angles of the far-field emission pattern requires other mechanisms. In this work, the SP-manipulated SPR with a Yagi-Uda nanoantenna (YUNA) array on an IMI substrate for generation of light beams with designed far-field patterns is proposed and explored by computer simulations. Emission of SPR along and perpendicular to the direction of electron movement can be manipulated by designing grating period and YUNA structure, respectively. Dependence of the azimuthal and polar angles of emitted light beam on geometry parameters of feed and directors of YUNA are elucidated. Furthermore, emission of multiple beams containing a single wavelength and multiple wavelengths with required far-field angles can be achieved using different groups of YUNA arrays on different IMI substrates. The proposed mechanism may have applications for light sources, optical imaging, optical beam steering, holography, microdisplay and cryptography.

Author Correction: Generation of convergent light beams by using surface plasmon locked Smith-Purcell radiation.

A correction to this article has been published and is linked from the HTML version of this paper. The error has not been fixed in the paper.

Generation of convergent light beams by using surface plasmon locked Smith-Purcell radiation.

An electron bunch passing through a periodic metal grating can emit Smith-Purcell radiation (SPR). Recently, it has been found that SPR can be locked and enhanced at some emission wavelength and angle by excitation of surface plasmon (SP) on the metal substrate. In this work, the generation of a convergent light beam via using the SP-locked SPR is proposed and investigated by computer simulations. The proposed structure is composed of an insulator-metal-insulator (IMI) substrate with chirped gratings on the substrate. The chirped gratings are designed such that a convergent beam containing a single wavelength is formed directly above the gratings when an electron bunch passes beneath the substrate. The wavelength of the convergent beam changes with the refractive index of dielectric layer of the IMI structure, which is determined by the frequency of SP on the IMI substrate excited by the electron bunch. Moreover, reversing the direction of electron bunch will make the emitted light from the proposed structure to switch from a convergent beam to a divergent beam. Finally, the formation of a convergent beam containing red, green and blue lights just above the chirped gratings is also demonstrated. This work offers potential applications in the fields of optical imaging, optical beam steering, holography, microdisplay, cryptography and light source.

GaN Metalens for Pixel-Level Full-Color Routing at Visible Light.

Metasurface-based components are known to be one of the promising candidates for developing flat optical systems. However, their low working efficiency highly limits the use of such flat components for feasible applications. Although the introduction of the metallic mirror has been demonstrated to successfully enhance the efficiency, it is still somehow limited for imaging and sensing applications because they are only available for devices operating in a reflection fashion. Here, we demonstrate three individual GaN-based metalenses working in a transmission window with extremely high operation efficiency at visible light (87%, 91.6%, and 50.6% for blue, green, and red light, respectively). For the proof of concept, a multiplex color router with dielectric metalens, which is capable of guiding individual primary colors into different spatial positions, is experimentally verified based on the design of out-of-plane focusing metalens. Our approach with low-cost, semiconductor fabrication compatibility and high working efficiency characteristics offers a way for establishing a complete set of flat optical components for a wide range of applications such as compact imaging sensors, optical spectroscopy, and high-resolution lithography, just named a few.

Temperature tunability of surface plasmon enhanced Smith-Purcell terahertz radiation for semiconductor-based grating.

In this work, the terahertz (THz) Smith-Purcell radiations (SPRs) for the relativistic electron bunch passing over an indium antimonide (InSb)-based substrate with a subwavelength grating under various temperatures of substrate are investigated by FDTD simulations and theoretical analyses. The explored SPR is locked and enhanced at a certain emission wavelength with the emission angle still following the wavelength-angle relation of the traditional SPR. This wavelength agrees with the (vacuum) wavelength of surface plasmons (SPs) at the air-InSb interface excited by the electron bunch. The enhancement of SPR at this wavelength is attributed to the energy from electron concentrated in the excited SPs and then transformed into radiation via the SPR mechanism. When the temperature of InSb increases, the emission wavelength of the enhanced SPR decreases along with the emission angles increasing gradually. This work demonstrates that the emission wavelength and angle of the enhanced SPR from the InSb grating can be manipulated by the temperature of InSb. The temperature tunability of SP-enhanced SPR has potential applications in the fields of optical beam steering and metamaterial light source.

Plasmonic Archimedean spiral modes on concentric metal ring gratings.

Plasmonic Archimedean spiral modes on concentric silver (Ag) ring gratings are investigated by FDTD simulations and theoretical analyses. These modes are generated by placing the ring grating under an Ag nanorod to extract the orbital angular momentum (OAM) of spiral surface plasmon (SSP) modes on the nanorod and transform it into the orbital motion of SP on the grating. The formation of Archimedean spiral patterns is ascribed to two factors: both the r- and θ-directional wavevectors are conserved for SSP on nanorod coupling into SP on ring grating and both the r- and θ-directional velocities of SP keep unchanged when it propagates on the ring grating. The number of strands of Archimedean spiral pattern is determined by the topological charge of SSP mode. The plasmonic Archimedean spiral modes have potential applications in the fields of data storage, dielectric microparticle manipulation, biosensing and directional switching.

Tunable tapered waveguide for efficient compression of light to graphene surface plasmons.

Dielectric-graphene-dielectric (DGD) structure has been widely used to construct optical devices at infrared region with features of small footprint and low-energy dissipation. The optical properties of graphene can be manipulated by changing its chemical potential by applying a biased voltage onto graphene. However, the excitation efficiency of surface wave on graphene by end-fire method is very low because of large wavevector mismatch between infrared light and surface wave. In this paper, a dielectric-semiconductor-dielectric (DSD) tapered waveguide with magnetic tunability for efficient excitation of surface waves on DGD at infrared region is proposed and analyzed. Efficient excitation of surface waves on DGD with various chemical potentials in graphene layer and incident frequencies can be attained by merely changing the external magnetic field applied onto the DSD tapered waveguide. The electromagnetic simulations verify the design of the proposed structure. More importantly, the constituent materials used in the proposed structure are available in nature. This work opens the door toward various applications in the field of using surface waves.

Spiral surface plasmon modes inside metallic nanoholes.

Spiral surface plasmon (SSP) modes that propagate inside a silver (Ag) nanohole are investigated by performing both simulations and theoretical analyses. The SSP modes are formed by a linear combination of two rotating SP eigenmodes of the Ag nanohole in the fast-wave branch. Inside a uniform Ag nanohole, the handedness and the number of strands of the SSP modes are determined by both the component SP eigenmodes and their rotation directions. The spiral pitch of the SSP mode increases with the nanohole radius for a fixed wavelength and is inversely related to the incident wavelength for a fixed nanohole radius. Inside a tapered Ag nanohole, the spiral pitch decreases with the reduction of nanohole radius. However, the azimuth-integrated field energy density increases to a maximum value and then falls. For a tapered Ag-clad fiber capped by a tapered Ag nanorod, the SSP mode reverses its handedness when it passes through the fiber-nanorod interface. Furthermore, using this composite structure, the field energy density of SSP mode that arrives at the tip of the tapered nanorod is largely increased.

Magnetically controlled planar hyperbolic metamaterials for subwavelength resolution.

Breaking diffraction limitation is one of the most important issues and still remains to be solved for the demand of high-density optoelectronic components, especially for the photolithography industry. Since the scattered signals of fine feature (i.e. the size is smaller than half of the illuminating wavelength λ) are evanescent, these signals cannot be captured by using conventional glass- or plastic-based optical lens. Hence the corresponding fine feature is lost. In this work, we propose and analyze a magnetically controlled InSb-dielectric multi-layered structure with ability of subwavelength resolution at THz region. This layered structure can resolve subwavelength structures at different frequencies merely changing the magnitude of external magnetic field. Furthermore, the resolving power for a fixed incident frequency can be increased by only increasing the magnitude of applied external magnetic field. By using transfer matrix method and effective medium approach, the mechanism of achieving super resolution is elucidated. The electromagnetic numerical simulation results also prove the rationality and feasibility of the proposed design. Because the proposed device can be dynamically reconfigured by simply changing the magnitude of external magnetic field, it would provide a practical route for multi-functional material, real-time super-resolution imaging, and photolithography.

Achieving planar plasmonic subwavelength resolution using alternately arranged insulator-metal and insulator-insulator-metal composite structures.

This work develops and analyzes a planar subwavelength device with the ability of one-dimensional resolution at visible frequencies that is based on alternately arranged insulator-metal (IM) and insulator-insulator-metal (IIM) composite structures. The mechanism for the proposed device to accomplish subwavelength resolution is elucidated by analyzing the dispersion relations of the IM-IIM composite structures. Electromagnetic simulations based on the finite element method (FEM) are performed to verify that the design of the device has subwavelength resolution. The ability of subwavelength resolution of the proposed device at various visible frequencies is achieved by slightly varying the constituent materials and geometric parameters. The proposed devices have potential applications in multi-functional material, real-time super-resolution imaging, and high-density photonic components.

Actively controlled super-resolution using graphene-based structure.

A super-resolution (with λ/50 resolution ability at mid-infrared region) device that consists of a monolayer graphene sandwiched between two dielectric materials with two alternate chemical potentials in graphene (which can be obtained by alternately applying two biased voltages to graphene) is proposed and analyzed. When the subwavelength resolution is achieved, the graphene-based device can be viewed as an effective optical medium with alternate arrangement of positive and negative refractive indices. And the isofrequency dispersion curves of the effective optical medium have the hyperbolic form. Furthermore, the super-resolution at different desired frequencies can be reached by merely changing the chemical potentials of graphene. The proposed devices have potential applications in multi-functional material, real-time subwavelength imaging, and high-density optoelectronic components for using the abnormal diffraction feature.

Breaking optical diffraction limitation using optical Hybrid-Super-Hyperlens with radially polarized light.

We propose and analyze an innovative device called "Hybrid-Super-Hyperlens". This lens is made of two hyperbolic metamaterials with different signs in their dielectric tensor and different isofrequency dispersion curves. The ability of the proposed lens to break the optical diffraction limit is demonstrated using numerical simulations (with the resolution power of about λ/6). Both a pair of nano-slits and a nano-ring can be imaged and resolved by the proposed lens using the radially polarized light source. Such a lens has great potential applications in photolithography and real-time nanoscale imaging.

Gain-assisted hybrid-superlens hyperlens for nano imaging.

We propose an innovative active imaging device named gain-assisted hybrid-superlens hyperlens and examine its resolving power theoretically. This semi-cylindrical device consists of a core of semi-cylindrical super-lens and a half cylindrical outer shell of hyperlens. Both the superlens and hyperlens parts of the device are appropriately designed multi-layered metal-dielectric structures having indefinite eigenvalues of dielectric tensors. The dielectric layers of the hyperlens are doped with Coumarin, which play the role of gain medium. The gain medium is analyzed thoroughly using a generic four-level system model, and the permittivity of the gain medium is extracted from this analysis for simulating the imaging characteristics of the device. According to our simulation at wavelength of 365 nm, an excellent resolution power much better than the diffraction limit value can be achieved.

Plasmonic Zener tunneling in metal-dielectric waveguide arrays.

We elucidate in this Letter plasmonic Zener tunneling (PZT) in metal-dielectric waveguide arrays (MDWAs) by using numerical simulations and theoretical analyses. PZT in MDWAs occurs at the waveguide entrance and wherever the beam completes Bloch oscillations, because the bandgap between the first and second bands is minimal at the center of the first Brillouin zone. This feature significantly differs from that of optical Zener tunneling in dielectric waveguide arrays. The dependence of the simulated tunneling rate on the gradient of the relative permittivity of the dielectric layers correlates with the tunneling theory, thus confirming the occurrence of PZT in MDWAs.

Plasmonic Bloch oscillations in cylindrical metal-dielectric waveguide arrays.

This study investigates plasmonic Bloch oscillations (PBOs) in cylindrical metal-dielectric waveguide arrays (MDWAs) by performing numerical simulations and theoretical analyses. Optical conformal mapping is used to transform cylindrical MDWAs into equivalent chirped structures with permittivity and permeability gradients across the waveguide arrays, which is caused by the curvature of the cylindrical waveguide. The PBOs are attributed to the transformed structure. The period of oscillation increases with the wavelength of the incident Gaussian beam. However, the amplitude of oscillation is almost independent of wavelength.

Long-range surface magnetoplasmon on thin plasmon films in the Voigt configuration.

This study elucidates the characteristics of a long-range surface magnetoplasmon (LRSMP) that propagates on a plasmon film with the Voigt configuration. Particle-in-cell (PIC) simulations and theoretical analyses are performed. Simulation results indicate that LRSMP has non-symmetrical fields. The proposed scheme also verifies the non-reciprocal properties of LRSMP as the direction of an applied external magnetic field is reversed. When surface waves propagate on a plasmon film across an interface on one side of which long-range surface plasmon (LRSP) is allowed while on the other side of which LRSMP is allowed, the interface behaves similar to a defect and transforms the surface waves into radiation modes owing to the mismatch between the field patterns of LRSP and LRSMP. Furthermore, PIC simulation results confirm the presence of a new high-frequency LRSMP whose frequency exceeds the plasma frequency and lacks a LRSP counterpart.

Resonant tunneling effects on cavity-embedded metal film caused by surface-plasmon excitation.

We investigate cavity-modulated resonant tunneling through a silver film with periodic grooves on both surfaces. A strip cavity embedded in the film affects tunneling frequencies via a coupling mode and waveguide mode. In the coupling mode, both the resonant tunneling through the gap between the groove and the cavity and the cavity itself form an entire resonant structure. In the waveguide mode, however, the cavity functions as a surface-plasmon waveguide. Hence, tunneling frequencies are close to resonant absorption frequencies of the groove structure and are irrelevant to cavity properties.

Surface plasmon-like modes on structured perfectly conducting surfaces.

Surface plasmon-like (SPL) modes are the electromagnetic surface eigenmodes supported by the structured perfectly conducting surfaces. The standard eigenvalue-solving method is adopted to solve these SPL modes. The field patterns of the SPL modes in the square holes for in-plane wavevectors k(x) = 2pi / 2d and k(x) = 2pi /d are TE(10)-like and TE(11), respectively. However, the field patterns can no longer be identified as any particular waveguide mode for other in-plane wavevectors. The dispersion relations of the SPL modes are obtained numerically. The change in mode character with wavevector prevents the dispersion relation from being derived by assuming only the fundamental mode in the holes. On a thin perfect conductor perforated with structures, the SPL mode splits into a high-frequency anti-symmetric mode and a low-frequency symmetric mode, which is caused by the mutual interaction of the electromagnetic evanescent fields on both sides.