Tunable Multi-Port Surface Plasmon Polariton Excitation with Nanostructures

Surface plasmon polaritons (SPPs) have appealing features such as tighter spatial confinement and higher local field intensity. Manipulation of surface plasmon polaritons on metal/dielectric interface is an important aspect in the achievement of integrated plasmonic circuit beyond the diffraction limit. Here, we introduce a design of pin cushion structure and a holographic groove pattern structure for tunable multi-port SPPs excitation and focusing. Free space light is coupled into SPPs through momentum matching conditions. Both nanostructures are capable of tunably controlling of SPPs depending on the incident polarizations, while the holographic method provides more flexibility of wavelength-dependent excitations. Furthermore, a quantitative method is applied to calculate the efficiencies of excitation for both nanostructures under different conditions, including radially polarized incident beams. These results can work as a guidance and be helpful to further choice of the suitable design strategies for variable plasmonic applications such as beam splitter, on-chip spectroscopy, and plasmonic detectors.     

An Integrated Multistage Nanofocusing System

We demonstrate an integrated multistage nanofocusing system which combines a conventional objective, a surface plasmonic lens, and a center-positioned rounded-tip cone nanoparticle. The surface plasmonic lens, fabricated on the cover glass which has been mounted on the biological microscopic objective, is composed of several concentric annular slits for exciting propagating surface plasmonic wave. The rounded-tip cone nanoparticle is for further generating non-propagating localized surface plasmonic wave. It is revealed that the enhancement of the nanoscale optical field can be improved by carefully choosing the appropriate numerical aperture of the objective to match the specific nanostructure of the surface plasmonic lens and choosing the relatively big cone angle of the nanoparticle. The investigation shows that a highly confined electric field as small as 20 nm and an enhancement factor of 5 orders of magnitude can be achieved through this multistage nanofocusing system when the system is illuminated with a uniform radially polarized beam.     

Influence of an Electron Beam Exposure on the Surface Plasmon Resonance of Gold Nanoparticles

Electron beam imaging is a common technique used for characterizing the morphology of plasmonic nanostructures. During the imaging process, the electron beam interacts with traces of organic material in the chamber and produces a well-know layer of amorphous carbon over the specimen under investigation. In this paper, we investigate the effect of this carbon adsorbate on the spectral position of the surface plasmon in individual gold nanoparticles as a function of electron exposure dose. We find an optimum dose for which the plasmonic response of the nanoparticle is not affected by the imaging process.     

Complicated Wavefront Shaping of Surface Plasmon Polaritons on Metal Surface by Holographic Groove Patterns

Surface plasmon polaritons (SPPs) manipulation on metal surfaces is important for constructing ultracompact integrated micro/nano-optical devices and systems. We employ the methodology of surface electromagnetic wave holography (SWH) to design holographic groove patterns for controlling SPPs with complicated wavefronts traveling on metal surface. SPPs are scattered by these deli groove patterns and interfere with each other to form desired SPP wavefronts. Several devices are demonstrated to control the intensities and phases of SPPs, such as focusing a plane SPP or diverging SPPs to two points with different phases, and focusing SPPs with complicated beam profile to a point. The finite-difference time-domain simulations show that in all cases, the predesignated functionalities are fully achieved by the designed plasmonic holographic structures. The results strongly support the power of SWH for shaping the complicated wavefront of in-plane transporting SPPs.     

A Design Method for a Micron-Focusing Plasmonic Lens Based on Phase Modulation

A design method of a micron-focusing plasmonic lens is proposed, which consists of a nanoaperture surrounded by concentric annular grooves with fixed width and depth. The phase modulation of the radiation lights decoupled from surface plasmon polariton waves by the annular grooves is realized by altering the radii of the grooves. Based on the principle of the constructive interference, a design formula of a micron-focusing plasmonic lens is deduced. The transmitted fields through the designed plasmonic lenses are numerically simulated with finite-difference time-domain method, and the results show that a circular focusing spot is generated where the focal length can be controlled in several micrometers, which agree with our theoretical analysis.     

Beam Manipulating via an Array of Nanoslits Modified by Perpendicular Cuts and Bumps

We report the observation of focusing and deflection phenomena by employing a novel technique to perform phase front profile design in nanoslit-based planar plasmonic lenses and beam deflectors. Introducing perpendicular cuts and bumps to the perforated nanoslits on a thin metallic film is utilized to change the effective depth of the nanoslits which provide the possibility of manipulating the phase front profile based on the propagation property of the surface plasmon polaritons in the metal–insulator–metal waveguides. Using the dispersive finite-difference time-domain numerical method, simulations are conducted to explore the beam focusing and deflection phenomena, and the performance parameters of the lens and beam deflector include the focal length, full-width half-maximum, depth of focus, the efficiency of focusing, and the deflection angle. The whole structure is formed on a planar thin film which is convenient for miniaturization and high density integration besides that it can be fabricated by well-known techniques such as focused ion beam milling.     

Gain-Assisted Magneto-Optical Rotation in a Four-Level Quantum System Near a Plasmonic Nanostructure

We propose and theoretically demonstrate a mechanism to achieve a gain-assisted magneto optical rotation (MOR) of a linearly polarized probe beam in a double V–type closed-loop atomic system. The quantum system is considered to be placed in the proximity of a plasmonic nanostructure which can produce quantum interference between decay channels of the quantum system. We also apply a linearly polarized control beam and a microwave beam to the system. It is shown that manipulating the intensity of the microwave beam and relative phase of the applied beams results in well-optimizing optical properties of the system where by proper choice of these parameters the atomic medium becomes birefringent gain media. Induced birefringence can be reinforced by increasing the intensity of the magnetic field and quantum interference coefficient. It is found, compared with the absence of the plasmonic nanostructure, the presence of the plasmonic nanostructure causes the gain-assisted MOR to occur at much smaller magnetic field. Hence, we propose that such a gain-assisted MOR can have potential application in detecting quantum interference effect.     

Investigation of Physical Limits of Plasmonic Focusing Lens in Different Regions

We present theoretical studies of three regions for plasmonic focusing, which are surface plasmon-dominating, Fresnel, and Fraunhoffer regions. The boundaries of the three regions are defined and the physical behaviors of plasmonic lenses in terms of focal length and focus size in these regions are investigated. A plasmonic lens that renders a subdiffraction-limit focus in the Fresnel region is presented and the lens performance with respect to the design parameters is studied by using finite-difference time-domain simulations. This work can serve as a basis for understanding plasmonic-focusing phenomenon and designing plasmonic lenses for various applications.     

Direct Growth of Optical Antennas Using E-Beam-Induced Gold Deposition

Electron beam induced deposition (EBID) is used to grow on a transparent substrate plasmonic antennas formed by gold nanorods. We first discuss the influence of the growth parameters on the geometrical homogeneity of the structures. The optical response of optimized rods with different aspect ratios are measured using scattering spectroscopy. The optical data show antenna resonances in good agreement with 3D numerical simulations for pure gold antennas, validating EBID as a novel relevant technique for the fabrication of plasmonic nanostructures.     

Tailoring Plasmon Lifetime in Suspended Nanoantenna Arrays for High-Performance Plasmon Sensing

We demonstrate significantly longer plasmon lifetime and stronger electric field enhancement by lifting the nanoantenna arrays above the substrate by dielectric nanopillars. The role of the pillar is to offer a more homogeneous dielectric background allowing stronger diffraction coupling among plasmonic nanoantennas leading to a Fanolike asymmetric lineshape. It is found that the electric fields around the nanoantennas can be greatly enhanced when the Fanolike resonance is excited, and a 4.2 times enhancement is achieved compared with the pure resonance in individual nanoantennas. Furthermore, only a collective surface mode with its electric fields of the same direction as the induced electric moment in the nanoantennas could mediate the excitation of such a Fanolike resonance. More importantly, the sensitivity and the figure of merit (FOM) of this plasmonic structure can reach as high as 900 nm/RIU and 53, respectively. Our study offers a new, simple, and efficient way to design the plasmonic systems with desired electric field enhancement and spectral lineshape for different applications.     

Ferroelectric Hybrid Plasmonic Waveguide for All-Optical Logic Gate Applications

A ferroelectric hybrid plasmonic waveguide, made of a polycrystal lithium niobate waveguide separated from a gold film by a silicon dioxide isolation layer, is fabricated by use of laser molecular beam epitaxy growth, electron beam evaporation, and focused ion beam etching. Strong subwavelength mode confinement and excellent long-range propagation are achieved simultaneously for the hybrid plasmonic mode. An all-optical logic OR gate is also realized based on the ferroelectric hybrid plasmonic waveguide. This may provide a way for the study of all-optical logic gates and integrated photonic circuits.     

Influence of V-Shaped Metallic Subwavelength Slits with Variant Periods for Superfocusing

In this paper, we discussed the influence of a plasmonic lens with V-shaped metallic subwavelength slits and variant periods on transmission properties. In order to analyze the influence, a finite-difference time-domain numerical algorithm was adopted for computational numerical simulation of the plasmonic structures. The structures are flanked with the penetrated slits through a metal (Ag) film which is coated on a quartz substrate. Our simulation results demonstrated that different cone angles originated from the V-shaped slits generate different influences on the beam propagation. The width variation affects the intensity significantly. The cone angles formed by the V-shaped slits can change the focusing performance. These results are very encouraging for future study of the plasmonic lens-based applications.     

Simulation and Analytical Study of Optical Complex Field in Nano-corral Slits Plasmonic Lens

Although spiral plasmonic lens has been proposed as circular polarization analyzer, there is no such plasmonic nanostructure available for linear polarization. In the current work, we have designed nano-corral slits (NCS) plasmonic lens, which focuses the x- and y-polarized light into spatially distinguished plasmonic fields. We have calculated analytically and numerically the electric field intensity and phase of the emission from nano-corral slits plasmonic lens with different pitch lengths under various polarizations of the illumination. It has been shown that one can control the wave front of the output beam of these plasmonic lenses by manipulating the illumination of both circular and linear polarization. Our theoretical study in correlation with FDTD simulation has shown that NCS plasmonic lens with pitch length equal to λ_spp produces scalar vortex beam having optical complex fields with helical wave front and optical singularity at the center under circular polarization of light. When NCS lens (pitch = λ_spp) is illuminated with linearly polarized light, it exhibits binary distribution of phase with same electric field intensity around the center. However, with pitch length of 0.5λ_spp, NCS shows linear dichroism under linearly polarized illumination unlike spiral plasmonic lens (SPL) eliminating the use of circularly polarized light. Optical complex fields produced by these NCS plasmonic lenses may find applications for faster quantum computing, data storage, and telecommunications.

     

High-extinction-ratio and low-insertion-loss Plasmonic Filter with Coherent Coupled Nano-cavity Array in a MIM Waveguide

In this paper, a novel plasmonic filter with very high extinction ratio and low insertion loss is proposed based on the coherent coupled nano-cavity array in a metal–insulator–metal (MIM) waveguide. The coherent coupling interactions among nano-cavities are investigated with an analytical model which is derived based on the temporal coupled-mode theory and transfer-matrix method. The destructive interference of the surface plasmon polaritons coupled from the nano-cavities at the resonant wavelength is achieved by suitably designing the period of the cavity array, which may be used for increasing the extinction ratio of the filter based on the nano-cavity array in the MIM waveguide. A plasmonic filter with an extinction ratio higher than 60 dB and an insertion loss less than 1.0 dB is obtained by applying the destructive interference in the design of a six-rectangular-cavity array in an Ag–air–Ag waveguide. And the correctness of the design for the filter is verified by the results obtained with the finite-difference time-domain simulation technique. This work may provide useful schemes and approaches for realization of various wavelength-sensitive devices in plasmonic integrated circuits.     

Plasmonic Lens with Multiple-Turn Spiral Nano-Structures

In this paper, we investigate the focusing properties of a plasmonic lens with multiple-turn spiral nano-structures, and analyze its field enhancement effect based on the phase matching theory and finite-difference time-domain simulation. The simulation result demonstrates that a left-hand spiral plasmonic lens can concentrate an incident right-hand circular polarization light into a focal spot with a high focal depth. The intensity of the focal spot could be controlled by altering the number of turns, the radius and the width of the spiral slot. And the focal spot is smaller and has a higher intensity compared to the incident linearly polarized light. This design can also eliminate the requirement of centering the incident beam to the plasmonic lens, making it possible to be used in plasmonic lens array, optical data storage, detection, and other applications.     

A CMOS Image Sensor Integrated with Plasmonic Colour Filters

Multi-pixel, 4.5?×?9???m, plasmonic colour filters, consisting of periodic subwavelength holes in an aluminium film, were directly integrated on the top surface of a complementary metal oxide semiconductor (CMOS) image sensor (CIS) using electron beam lithography and dry etch. The 100?×?100-pixel plasmonic CIS showed full colour sensitivities across the visible range determined by a photocurrent measurement. The filters were fabricated in a simple process utilising a single lithography step. This is to be compared with the traditional multi-step processing when using dye-doped polymers. The intrinsic compatibility of these plasmonic components with a standard CMOS process allows them to be manufactured in a metal layer close to the photodiodes. The incorporation of such plasmonic components may in the future enable the development of advanced CIS with low cost, low cross-talk and increased functionality.     

Optimization of Optoelectronic Plasmonic Structures

We discuss the interplay between surface plasmon polaritons (SPPs) and localized shape resonances (LSRs) in a plasmonic structure working as a photo-coupler for a GaAs quantum well photodetector. For a targeted electronic inter-subband transition inside the quantum well, maximum photon absorption is found by compromising two effects: the mode overlapping with incident light and the lifetime of the resonant photons. Under the optimal conditions, the LSR mediates the coupling between the incident light and plasmonic structure while the SPP provides long-lived resonance which is limited ultimately by metal loss. The present work provides insight to the design of plasmonic photo-couplers in semiconductor optoelectronic applications.     

Plasmonic Lens Based on Rectangular Holes

A compact plasmonic lens is proposed in this paper. This plasmonic lens consists of rectangular holes etched on the silver film and arranged on one straight line and possesses the characteristics of short focus length, ultrathin thickness, and strong focus ability. The theoretical design for the plasmonic lens abides by the constructive interference theorem, and the surface plasmon polaritons excited by the holes with linearly polarized light illumination focuses effectively. The plasmonic lenses with single and double focus spots are provided, and the simulation experiment gives the powerful verification. The distinct structure feature and the excellent focusing characteristic of this plasmonic lens are benefit for its applications in optical integration.     

Formation of a Plasmonic Surface Optical Vortex by Evanescent Bessel Light

The total internal reflection of an optical mode with a phase singularity, such as a Bessel beam, can generate evanescent light that displays a rotational property. Notably, using a metallic layer surface, field components extending into the vacuum region have vortex properties besides surface plasmonic features. This vortex retains the phase singularity of the original light, and also maps its associated orbital angular momentum of incident Bessel light of the order ?? >?0. Additionally to a two-dimensional patterning on the metallic surface, the strongly restricted intensity distribution decays with distance vertical to the metallic surface. The detailed characteristics of this vortex structure depend on the input light parameters and the dielectric mismatch of the media. As well as this, they can be controlled by varying the incident angle and the order of Bessel light.     

Polarization Effect on Superfocusing of a Plasmonic Lens Structured with Radialized and Chirped Elliptical Nanopinholes

Tuning effect of different polarization states was presented in this paper. It can be realized by a plasmonic lens constructed with elliptical pinholes ranging from submicron to nanoscales distributed in variant period along radial direction. Propagation properties of the lens illuminated under four different polarization states: linear, elliptical, radial, and cylindrical vector beam, were calculated and analyzed combining with finite-difference time-domain algorithm. Different focusing performances of the lens were illustrated while the polarized light passes through the pinholes. Our calculation results demonstrate that polarization effect of the elliptical pinholes-based plasmonic lens can generate high transmission intensity and sharp focusing for our proposed specific structures. Beam focal region, position, and transmission intensity distribution can be tailored by the four polarization states.