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.     

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.     

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.     

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.     

A Novel Plasmonic Zone Plate Lens Based on Nano-Slits with Refractive Index Modulation

Conventionally, plasmonic lenses introduce a phase delay distribution across their surfaces by modulating the dimensions of nanostructures within a metal film. However, there is very limited modulation of the phase delay due to the small dependence of the mode propagation constant on the structure dimensions. In this paper, a novel design of plasmonic zone plate lenses (PZPL) with both slit width and refractive index modulation is proposed to enable integrating more slits in a fixed lens aperture with the extended phase delay range and, therefore, greatly enhance the performance of the devices. More than three-time enhancement of the light intensity at the focus is achieved compared to the structure with only slit width modulation. Like a conventional immersion system, a PZPL embedded in a dielectric is found to have a further improved focusing performance, where light is focused down to a 0.44λ spot using a PZPL with an aperture of 12λ and a focal length of 6λ. Dispersive light-focusing behaviour is also analysed and the modulation of the focal length by colour has a potential application in stacked image sensors and multi-dimensional optical data storage.     

Robustly Efficient Superfocusing of Immersion Plasmonic Lenses Based on Coupled Nanoslits

We report plasmonic lenses consisting of coupled nanoslits immersed in a high-index medium to obtain the robustly efficient superfocusing. Based on the geometrical optics and the wavefront reconstruction theory, an array of nanoslits perforated in a gold film and a series of spacings between adjacent nanoslits are optimally designed to realize the desired phase modulation for light focusing. The numerical results verify the design of each plasmonic lens in excellent agreement. For the given total phase difference of 2π, the immersion plasmonic lenses with smaller lens aperture can have much better focusing performance than the non-immersion one. A superfocusing spot of λ/4.39 is achieved using an oil immersion plasmonic lens with an aperture size of 4.97λ, resulting in a resolution improvement of 68.9 % compared with the non-immersion lens. Moreover, such superfocusing performance can be still well kept when the structural parameters of the lens, e.g., nanoslit width and metal film thickness, are deviated from the original design, making the final implementation of the superfocusing lenses much easier.     

Experimental Study of Metallic Elliptical Nano-Pinhole Structure-Based Plasmonic Lenses

An elliptical nano-pinhole structure-based plasmonic lens was designed and investigated experimentally by means of focused ion beam nanofabrication, atomic force microscope imaging, and scanning near-field optical microscope (NSOM). Two scan modes, tip scan and sample scan, were employed, respectively, in our NSOM measurements. Both the scan modes have their characteristics while probing the plasmonic lenses. Our experimental results demonstrated that the lens can realize subwavelength focusing with elongated depth of focus. This type of lens can be used in micro-systems such as micro-opto-electrical–mechanical systems for biosensing, subwavelength imaging, and data storage.     

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.     

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.     

Effect of the Number of Zones in a One-Dimensional Plasmonic Zone Plate Lens: Simulation and Experiment

A 1D plasmonic zone plate lens (PZPL) consisting of nano-slits within a metal film introduces a phase delay distribution across the planar device surface by a modulation of the slit widths and positions to achieve light focusing. Using the finite-difference time-domain method, the number of zones is found to be a crucial factor for a well-controlled focal length, i.e. at least three zones are necessary for a PZPL exhibiting a focal length in agreement with the design. This conclusion is confirmed by confocal scanning optical microscopy on PZPLs patterned in an aluminium film. In addition, subwavelength light focusing is demonstrated both theoretically and experimentally in a PZPL. A larger PZPL, i.e. more zones, shows a higher resolution. A full full-width half-maximum of 0.37λ in the focal plane is shown theoretically in a PZPL with seven zones. A comparison between the PZPL and the plasmonic Fresnel zone plate shows that PZPLs have a higher contrast at the focus.     

Modulating Plasmonic Sensor with Graphene-Based Silicon Grating

We propose a modulating plasmonic structure device which is composed of a single layer graphene above the silicon Bragg grating with the silica spacer layer. This graphene-based plasmonic modulation provides a broad stop-band with high tunability in the mid-infrared region of the transmission spectra achieved by altering the geometrical parameters of the silicon grating and the gate voltage. By engineering, a phase discontinuity into the graphene-based Bragg grating, we can selectively open a transmission window in the previous stop-band spectra. These proposed graphene-based structures are easy to fabricate and operate, which have potential applications as ultra-compact high-sensitivity sensors.     

Numerical Study of a Nonplanar Two-Stage Surface Plasmonic Lens Illuminated by a Radially Polarized Beam

We design and fabricate a nonplanar two-stage surface plasmonic lens composed of concentric circular slits for exciting propagating surface plasmonic wave and a center-positioned cone-like nanoparticle for generating localized surface plasmonic waves. The numerical investigation based on the finite difference in time domain method is performed. It is found that, when a radially polarized beam illumination is applied, a highly confined electric field with full width half maximum of as small as 6 nm and the transmission enhancement factor of six orders higher than the incident beam is achievable. The optimization design is conducted through comparison of different conic angles and different materials of the cone-like nanoparticles.     

Plasmonic Focusing in Nanostructures

In this review, we show that by designing the metallic nanostructures, the surface plasmon (SP) focusing has been achieved, with the focusing spot at a subwavelength scale. The central idea is based on the principle of optical interference that the constructive superposition of SPs with phase matching can result in a considerable electric-field enhancement of SPs in the near field, exhibiting a pronounced focusing spot. We first reviewed several new designs for surface plasmon focusing by controlling the metallic geometry or incident light polarization: We made an in-plane plasmonic Fresnel zone plates, a counterpart in optics, which produces an obvious SP focusing effect; We also fabricated the symmetry broken nanocorrals which can provide the spatial phase difference for SPs, and then we propose another plasmon focusing approach by using semicircular nanoslits, which gives rise to the phase difference through changing refractive index of the medium in the nanoslits. Further, we showed that the spiral metallic nanostructure can be severed as plasmonic lens to control the plasmon focusing under a linearly polarized light with different angles.

     

Plasmonic Lenses: A Review

Four types of plasmonic lenses for the purpose of superfocusing designed on the bases of approximate negative refractive index concept, subwavelength metallic structures, waveguide mode were introduced, and curved chains of nanoparticles, respectively, were introduced. Imaging mechanism, fabrication, and characterization issues were presented. Theoretical analyses of the illumination with different polarization states on focusing performance of the plasmonic lenses were given also. In addition, a hybrid Au-Ag plasmonic lens with chirped slits for the purpose of avoiding oxidation of Ag film was presented.     

Experimental Investigation of Focusing of Gold Planar Plasmonic Lenses

To experimentally demonstrate the subwavelength focusing of depth-tuned or non-depth-tuned plasmonic lenses, we first designed this type of lens using diffraction-coupling-angle based method, then fabricated the structure in gold thin film with focused ion beam, and finally characterized its focusing behavior using near-field scanning optical microscope. It is found that this type of lens has a resolution limit on the focal plane due to the field represented by angular spectrum having a cut-off frequency, while at the near field the lens has sub-diffraction limit focusing capability due to the existence of high-angular-frequency components in the field.     

Study of the Plasmon Talbot Effect of Metallic Nanolenses Induced by Linearly Polarized Illumination

The plasmon Talbot effect of metallic nanolenses was studied theoretically and experimentally for the linearly polarized incident beam case. To demonstrate this self-imaging-based focusing property of the metallic nanolenses, a plasmonic nanolens with five periodic concentric through rings on Al film supported on quartz substrate was numerically studied firstly by the use of rigorous finite-difference and time-domain algorithm. To further demonstrate its working performance experimentally, it was fabricated by means of a focused ion beam direct milling technique. A near-field scanning optical microscope (NSOM) was then employed for the optical characterization of its focusing property. The experimental results indicate that the NSOM probing-based results are in agreement with the theoretical calculation results in general.     

Plasmonic Band Gap: Role of the Slit Width in 1D Metallic Grating on Higher Refractive Index Substrate

This paper reports the excitation of surface plasmon polaritons (SPPs) and associated plasmonic band gap (PBG) while using TM plane wave interacting with 1D metallic grating on higher refractive index GaP substrate. A simple method is introduced to estimate the PBG which is crucial for many plasmonic devices. The PBG is estimated by measuring the transmission spectra obtained through the plasmonic grating structures when slit width is varied while periodicity and the thickness of the gold (Au) film remained fixed. The PBG is observed for the grating devices whose slit width is less than one third of the periodicity which is caused by the presence of a higher plasmonic mode. The PBG is absent for the grating device whose slit width is slightly less than half and greater than one third of the periodicity. Such grating devices support only a fundamental plasmonic mode because the profile/shape of the slit in the grating device is more like a sinusoidal nature. Furthermore, such grating offers intermediate scattering to the incident light and the SPP as well which in turn couple more incident energy to the SPPs. Far-field modelling results also support the results obtained through experiment.

     

Polarization Dependent of Plasmonic Lenses with Variant Periods on Superfocusing

A plasmonic lens with variant periods was investigated for optical behavior at near-field by means of numerical computational method. To study influence of incident light on different polarization modes, we considered linear polarization, circular polarization, elliptical polarization, radial polarization (RP), and azimuthally polarization in our computational analyses. A finite difference and time domain algorithm is employed in the numerical study. Our computational numerical calculation results demonstrate that focusing performance for the plasmonic lens illuminated under radial polarization is best in comparison to that of the illumination with the other four polarization states. The plasmonic lens with RP illumination can realize superfocusing with ultra-long depth of focus. It is possible to be used as an optical probe or a type of plasmonic lens for imaging with high resolution in the near future.     

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.

     

Modulation of Main Lobe for Superfocusing Using Subwavelength Metallic Heterostructures

A subwavelength metallic heterostructure is put forth for the purpose of suppressing sidelobes and improving superfocusing at a quasi-far field region. Improvement has been made by means of optimization of the heterostructure composed of structured Au and Ag thin films. By tuning thicknesses of both the structured Au and Ag films, we can modulate propagation distance of the plasmonic lens and beam width of main lobe for the superfocusing. A finite-difference and time-domain (FDTD) algorithm-based computational numerical calculation was carried out for analyzing the focusing performance and tuning ability of the metal films. Our computational calculation results show that the sidelobes which play negative role for the focusing can be suppressed significantly in the case of the metal film thicknesses of h _Au = 50 nm and h _Ag = 10 nm. Theoretically, the metallic structure with smaller thicknesses of the structured Au and Ag films is helpful for improving the focusing performance. This heterostructure-based device is possible to be used as a superlens or nanoprobe in data storage, nanometrology/inspection, and biosensing etc.