Cherenkov terahertz radiation from Dirac semimetals surface plasmon polaritons excited by an electron beam

We demonstrate a physical mechanism for terahertz(THz) generation from surface plasmon polaritons(SPPs). In a structure with a bulk Dirac semimetals(BDSs) film deposited on a dielectric substrate, the energy of the asymmetric SPP mode can be significantly enhanced to cross the light line of the substrate due to the SPP-coupling between the interfaces of the film. Therefore, the SPPs can be immediately transformed into Cherenkov radiation without removing the wavevector mismatch. Additionally, the symmetric SPP mode can also be dramatically lifted to cross the substrate light line when a buffer layer with low permittivity relative to the substrate is introduced. In this case, dual-frequency THz radiation from the two SPP modes can be generated simultaneously. The radiation intensity is significantly enhanced by over two orders due to the field enhancement of the SPPs. The radiation frequency can be tuned in the THz frequency regime by adjusting the beam energy and the chemical potential of the BDSs. Our results could find potential applications in developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz band.     

Anomalous transmission of terahertz wave through one-dimensional lamellar metallic grating

The anomalous transmission through one-dimensional lamellar metallic gratings was investigated in terahertz (THz) regime. The extraordinary optical transmission (EOT) is identified to originate from two possible ways: coupling of incident light with waveguide resonances and coupling of surface plasmon polaritons (SPPs) at the upper and lower interfaces of metal grating. The dual effects of SPPs have been clarified in this study: (i) the excitation of SPP modes at each individual interface results in the weakness of the THz wave transmission; and (ii) the coupling of SPP modes at two interfaces of metal grating is attributed to enhancement of THz wave transmission. The enhanced transmission is dominated by the coupling of incident light with transverse waveguide resonances. Numerical simulation based on finite-difference time-domain (FDTD) agrees well with experimental results.     

Directional excitation of surface plasmon polaritons in structure of subwavelength metallic holes

In this paper, we propose a structure formed by two subwavelength holes fabricated in a metal film to realize directional excitation of surface plasmon polaritons (SPPs). The holes are employed as SPP sources, and the relative phase of SPPs generated at the hole exit end can be adjusted by changing the dielectric material filled in holes. Using the difference in relative phase values of SPP for two holes filled with different dielectric media, the SPPs can interfere constructively along one direction while destructively along the opposite direction. Our theoretical analysis is verified by the three-dimensional finite-difference time-domain method. Moreover, the directional excitation of SPPs in two-hole array structure is also discussed. It is found that the effect of SPPs directional excitation is improved with the increase of the number of two-hole.     

Surface plasmon polariton at the interface of dielectric and graphene medium using Kerr effect

We theoretically investigate the control of surface plasmon polariton(SPP) generated at the interface of dielectric and graphene medium under Kerr nonlinearity. The controlled Kerr nonlinear signal of probe light beam in a dielectric medium is used to generate SPPs at the interface of dielectric and graphene medium. The positive, negative absorption, and dispersion properties of SPPs are modified and controlled by the control and Kerr fields. A large amplification(negative absorption) is noted for SPPs under the Kerr nonlinearity. The normal/anomalous slope of dispersion and propagation length of SPPs is modified and controlled with Kerr nonlinearity. This leads to significant variation in slow and fast SPP propagation. The controlled slow and fast SPP propagation may predict significant applications in nano-photonics, optical tweezers, photovoltaic devices, plasmonster, and sensing technology.     

Surface enhanced Raman scattering from mildly roughened surfaces: variation of signal with metal grain size

The intensity of surface enhanced Raman scattering from benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal grain size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate grain size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate, it is argued that the contrast in Raman scattering is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities.     

Plasmonic coupler based on the nanoslit with bump

In this paper, we propose a plasmonic coupler which is composed of a nanoslit with a bump. The slit is used to generate surface plasmon polariton (SPP), and the bump is employed as a SPP reflector. It is found that the phase difference between the SPP propagating the opposite direction to the bump and the one reflected by the bump can be periodically adjusted by the distance between the center of slit and the bump. When the constructive interference between the two SPPs occurs, the proposed structure can be regarded as a undirectional plasmonic coupler. Moreover, we also find that the propagation of the interfering SPPs is influenced by the width and length of bump. It is expected that our results may be utilized to control the electromagnetic wave in subwavelength optics.     

Coupling effect of surface plasmon polaritons in single-negative lamellar heterostructure

Propagation characteristics of surface plasmon polaritons (SPPs) in the lamellar heterostructure, which is actually a SPP waveguide array, constructed by two kinds of single negative (SNG) material layers stacked alternatively are investigated. Based on the finite element method (FEM), the negative-refraction (NR) property is demonstrated when the electromagnetic wave penetrates through free space into such SNG lamellar structure. A clear view of the underlying physics of NR is presented qualitatively that is mainly related to the coupled SPPs. The strong coupling effect leads to the novel SPP dispersion curves and then the anomalous propagation characteristics.     

Study of interference in a double-slit without walls by plasmon tomography techniques

We investigate the free propagation of two parallel surface plasmon polaritons (SPP) beams using plasmon tomography. In the Fourier-plane images, we observed interference features that are not in correspondence with the images of SPPs on the sample's surface. We clearly observed that the interference maxima and minima are distributed over an arc of a circle. We explain the characteristics of the observed interference patterns assuming that each SPP beam can be considered as a “slit without walls”. We discuss important implications of this work for SPP tomography and interferometric plasmonic sensors.     

Dynamic generation of plasmonic Moiré fringes using phase-engineered optical vortex beam

Dynamic generation of plasmonic Moiré fringes using a phase engineered optical vortex (OV) beam is experimentally demonstrated. Owing to the unique helical phase carried by an OV beam, the initial phase of surface plasmon polaritons (SPPs) emanating from a metallic grating can be adjusted dynamically by changing the phase hologram displayed on a spatial light modulator. Plasmonic Moiré fringes are readily achieved by overlapping two SPP standing waves with certain angular misalignment, excited by the positive and negative topological charge components, respectively, of a cogwheel-like OV beam. The near-field scanning optical microscopy measurement result of SPP distributions has shown a good agreement with the numerical predictions.     

Deterministic relief dielectric structures to realize phase modulation of surface-plasmon polaritons

We propose a modified effective-refractive-index model for the design of relief dielectric structures on a metal surface to realize phase modulation of surface-plasmon polaritons (SPPs). In this model, the length of the dielectric structure is optimized to reach phase shifting between the SPP waves transmitted through dielectric structure and those directly propagated with the consideration of SPP losses. Specifically, a one-dimensional dielectric Fresnel zone plate is designed by employing the proposed model to demonstrate phase modulation of SPPs for the highest focusing efficiency.     

Experimental study of unidirectional excitation of SPPs by binary area-coded nanohole arrays

We present an experimental study on a unidirectional surface plasmon polariton （SPP） launcher based on a compact binary area-coded nanohole array, where the symmetry breaking is realized via effective-index modulation in the binary pattern of the gold film, thus avoiding the challenge of modulating nanostructure in its depth. It is shown that SPPs can be unidirectionally and effectively excited at normal incidence. The SPP intensity and asymmetric excitation ratio, which are two key figure-of-merits of SPP launchers, can be improved by increasing the number of array rows. The proposed device is compatible with most mature top-town nanofabrieation techniques and thus is perspective for low-cost mass production.     

Surface plasmon polaritons frequency-blue shift in low confinement factor excitation region

Surface plasmon polaritons' (SPPs') frequency blue shift is observed in finite-difference time-domain (FDTD) simulation of parallel electron excitation Au bulk structure. Comparing with cold dispersion of SPPs, an obvious frequency blue shift is obtained in low confinement region excitation simulation results. Then, according to SPPs' transverse attenuation characteristics, the excited frequency mode instead of cold dispersion corresponding frequency mode matches it. Thence, this excited mode is confirmed to be SPPs' mode. As is well known the lower the frequency, the smaller the confinement factor is and the lower the excitation efficiency, the wider the bandwidth of excited SPPs is. And considering the attenuation in whole structure, the excited surface field contains attenuation signal. In a low confinement factor region, the higher the SPPs' frequency, the higher the excitation efficiency is, while broadband frequency information obtained in attenuation signal provides high frequency information in stimulation signal. Thence, in the beam-wave interaction, as the signal oscillation time increases, the frequency of the oscillation field gradually increases. Thus, compared with cold dispersion, the frequency of excited SPP is blueshifted This hypothesis is verified by monitoring the time domain signal of excited field in low and high confinement factor regions and comparing them. Then, this frequency-blue shift is confirmed to have commonality of SPPs, which is independent of SPPs' material and structure. Finally, this frequency-blue shift is confirmed in an attenuated total reflection (ATR) experiment. Owing to frequency dependence of most of SPPs' devices, such as coherent enhancement radiation and enhancement transmission devices, the frequency-blue shift presented here is of great influence in the SPPs applications.     

Effects of a one-dimensional surface defect on designer surface plasmon polaritons

By using a finite difference time domain(FDTD) method,the effects of a one-dimensional(1D) surface defect on designer surface plasmon polaritons(designer SPPs) supported by a 1D metallic grating in THz domain are investigated.When the size of the defect is in a special range which is not too large,the designer SPPs reflected and scattered by the defect are weak enough to be neglected.The defect only induces a disturbance in the energy distribution of the designer SPP supported by the whole defect grating.If the defect size exceeds the said range,the reflecting and scattering are dominant in the influences of the defect on designer SPPs.Our analysis opens opportunities to control and direct designer SPPs by introducing a 1D defect,especially in low frequency domain.     

Image edge-enhancement in optical microscopy with a phase mismatched spiral phase plate

We present a spiral phase filtering system with a large tolerance for edge enhancement of both phase and amplitude objects in optical microscopy.The method is based on a Fourier 4-f spatial filtering system.A phase mismatched spiral phase plate (SPP) fabricated by electron beam lithography is employed as the radial Hilbert transform for image edge enhancement.Compared with holography,SPP is simple,economical,reliable,and easy to integrate.     

Modeling of a surface plasmon polariton interferometer

We model the operation of a micro-optical interferometer for surface plasmon polaritons (SPPs) that comprises an SPP beam-splitter formed by equivalent scatterers lined up and equally spaced. The numerical calculations are carried out by using a relatively simple vectorial dipolar model for multiple SPP scattering [Phys. Rev. B 67 (2003) 165405]. The SPP beam-splitter is simulated elucidating the influence of system parameters, such as the angle of SPP beam incidence, scattering particle size, and inter-particle distance, on the splitting efficiency and phase difference between the transmitted and reflected beams. It is found that the splitting efficiency is very sensitive to the size of scatterers and angle of incidence. Comparing our simulations with experimental data available in the literature, we conclude that this approach can be used, with certain limitations, for modelling of SPP components assembled of individual scatterers, e.g., beam-splitters and interferometers, and suggest further improvements of the model used.     

Plasmon-Assisted Phase-Matched Second- and Third-Harmonic Generation in Single-Negative Heterostructures

Dispersion relations of surface plasmon polaritons （SPPs） in sandwiched optical systems are studied. The system is actually a kind of SPP waveguides, with two kinds of single negative material （SNG） as core and cladding layers, respectively. Since both TM and TE polarized SPPs can be excited in the structure, the dispersion of SPPs becomes more abundant and leads to colorful nonlinear opticM properties. The authors demonstrate the effective phase-matched second and third-harmonic generation （SHG, THG） assisted by the coupled SPPs. A cascaded second-order nonlinear process can Mso be achieved in the structure when the thickness of the core layer is properly selected, leading to the simultaneous SHG and THG. Further investigations show that much easier phase-matching can be fulfilled in the SNG waveguide array. Our results would be of potential use for surface-enhanced frequency conversion device such as light emitters or lasers.     

Surface-plasmon-polariton-assisted dipole-dipole interaction near metal surfaces

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.     

Complex Modulation in Plasmon-Assisted Transmission Spectra of a Two-Slit Structure

We study the plasmon-assisted transmission of two kinds of slit structures in a 290-nm-thick silver film on a glass substrate. For the two-slit structure, the total transmission intensity spectra displays a complex modulation, which is attributed to different propagation constants of the surface plasmon polaritons （SPPs） on the silver-air and silver-glass interfaces. Replacing one of the two slits by a shallow corrugation results in a simple sinusoidal intensity modulation because of the only effective SPP excitation and propagation on the silver-air interface. These confirm the role of different SPP propagations and interference in the transmission properties of metal nanoslits.     

Theoretical reexamination of the cross conversion between surface plasmon polaritons and quasi-cylindrical waves

The cross conversion between surface plasmon polaritons (SPPs) and quasi-cylindrical waves (CWs) is theoretically reexamined. Except for the CW-to-SPP conversion, we find the SPP-to-CW conversion, as well as the reflection and transmission of the CW, plays an indispensable role in the interaction between SPPs and light via periodic grooves. The completeness of the whole scattering coefficients is emphasized by an SPP-CW model proposed to quantitatively predict the SPP excitation efficiency for any number of periodic grooves.     

SNOM Observations of Surface Plasmon Polaritons on Metal Heterostructures

We observe surface plasmon polariton （SPP） refraction on a metal heterostructured sample with a scattered-type scanning near-field optical microscope （SNOM）. The sample consists of AI and Au in-plane whose boundary is smooth enough with proper etching time. SPPs excited on the AI film travel to the boundary and a portion of SPPs propagates into the Au film. In addition, interference fringes appear in the SNOM image bent at the boundary. The result is analysed with effective index method and the refracted angle is explained by Shell＇s law.