Near-Field Enhancement Through a Single Subwavelength Aperture with Gaps Inside

Structured subwavelength apertures have been studied widely to enhance the transmission or obtain super resolution, such as bull’s eye hole, H-shaped aperture, and bowtie aperture. In this letter, we present another structured aperture with two gaps inside, which can also lead to near-field enhancement. And the resonance wavelength can be tuned by the geometric parameters of the gaps. The enhancement of this gap-aperture depends strongly on the polarization of the incident wave. Only the polarization perpendicular to the connecting line of the two gaps can realize the enhancement. We attribute the near-field enhancement of this structured aperture to the resonance excitation of the localized surface plasmon. We combine the tooth-aperture and our gap-aperture together and this special aperture can achieve stronger enhancement and super resolution simultaneously.     

Time-of-Flight Model for the Extraordinary Transmission Through Periodic Arrays of Subwavelength Apertures at THz Frequencies

A qualitative model explaining the extraordinary optical transmission of terahertz (THz) radiation through two-dimensional periodic arrays of subwavelength apertures is presented. Systematic terahertz time-domain spectroscopy studies have been undertaken to investigate the combined effects of the lattice arrangement, aperture shape, area and aspect ratio on the transmission properties of electroformed copper arrays. The extensive results presented provide a unified example of how aperture geometry dictates SPP activity. The novel fabrication method creates exemplary peak resonances, allowing the onset of surface plasmon polariton (SPP) decoupling to be distinguished from direct transmission. Furthermore, we provide the first evidence as to how the temporal properties of SPPs are governed by the single-cycle THz pulse. The time-of-flight model presented can not only be used to explain the results observed in both the presented and previously published experiments but serves as a method to engineer specific resonances for sensor applications.     

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.     

Optical Near-Field Enhancement with Graphene Bowtie Antennas

In this paper, the optical near-field enhancement of graphene bowtie antennas is numerically investigated at terahertz frequencies using boundary element method. The enhanced field intensity at the gap region is a result of the mutual coupling between two triangular elements upon the excitation of graphene plasmons. Firstly, wide plasmon frequency tunability is demonstrated by changing the chemical potential of graphene without the need to alter the antenna geometry. Secondly, by varying the tip angle and radius of curvature of the graphene antennas, the field intensity enhancement at the gap center of the two-element antennas is systematically studied. It is found that graphene bowtie antennas with two round-cornered equilateral triangles have superior performance to other two-element antennas, such as ribbon pair, sharp-cornered bowtie, and disk pair antennas. Last but not least, by applying a moderate chemical potential of 0.4 eV to graphene bowtie antennas, we found that the field intensity enhancement at gap center is about 220 times as much as using gold of comparable sizes. In short, graphene bowtie antennas of rounded corners give rise to considerable near-field enhancement and are promising for a wide range of applications such as molecular sensing at terahertz frequencies.

     

Mode Controlling of Surface Plasmon Polaritons by Geometric Phases

Metasurfaces are made of two-dimensional arrays of subwavelength nanostructures that form a spatially varying optical response, to control the wave fronts of optical waves. As the feature size of its constituent materials is nanoscale, investigation of the light-nanostructure interactions in the near field is critical for understanding the novel properties of metasurfaces. Here, we used a scanning near-field optical microscope (SNOM) to observe the near-field distribution of surface plasmon polaritons (SPPs) from a ring-shaped metasurface under illumination of circularly polarized light. It was found that with an additional degree of freedom of the geometric phase provided by the regularly arranged metamolecules, control over the near-field interference of the SPPs can be achieved, which is governed by the metasurface geometric symmetry that can be tuned by its topological charge. Meanwhile, the planar chiral character of the metamolecules exerts a deep influence on the near-field interference patterns. Our results can pave the way for active control of SPP propagation in near fields and have potential applications in highly integrated optical communication systems.

     

Polarization Management of Terahertz Extraordinary Optical Transmission through Ultracompact L-Shaped Subwavelength Patterns on Metal Films

Compact and efficient terahertz (THz) polarization conversion components are of importance for applications where the small dimension of the laser device/system is critical. Here, we propose an ultracompact L-shaped subwavelength patterns on metal films to realize the THz polarization management. By optimizing the geometric parameters of single-layered and double-layered patterns, the linear-polarized THz incidence can be converted to elliptical polarized output or rotated by 90° efficiently due to the THz extraordinary optical transmission phenomenon. The physical mechanism is explored by mode analysis using numerical and analytical modeling.     

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.     

Direct Coupling Strategy in Plasmonic Nanocircuits for Low Loss and Easy Fabrication

Plasmonic nanocircuits can deliver light in subwavelength scale, however, require state-of-the-art fabrication process due to the ultra-small footprints. Here, we introduce direct coupling strategy based on metal-insulator-metal (MIM) waveguide systems to reduce the system loss as well as the fabrication difficulty and increase the structural stability. Following this strategy, the coupling between the input waveguide and square ring resonator (SRR) can be realized via an aperture, and for the coupling between SRRs, the metal gap can be removed. The numerical results show that such direct coupling can produce similar effects with conventional indirect coupling in MIM waveguide systems, and the physics mechanism behind as well as influences of geometric parameters on transmission spectrum is also investigated. This work provides a simpler approach to realize on-chip plasmonic nanodevices, such as filters, sensors, and optical delay lines, in practice.

     

Fishbone-Like High-Efficiency Low-Pass Plasmonic Filter Based on Double-Layered Conformal Surface Plasmons

In this work, we report a fishbone-like high-efficiency low-pass plasmonic filter based on a double-layered conformal surface plasmon waveguide (CSPW) which consists of double-layered symmetrical metal gratings (SMGs) of fishbone shape. Efficient mode conversion between the quasi-transverse electromagnetic (TEM) waves in the microstrip line and the conformal surface plasmons (CSPs) on the double-layered CSPW is realized by using gradient double-layered SMGs and impedance matching technique. Experimental results of the transmission and reflection coefficients of the straight sample show excellent loss-pass performance and agree well with the numerical simulations. The curved samples exhibit low radiation loss when the double-layered CSPW is conformal or even bent thanks to the high confinement of CSPs. The proposed structure can find potential applications in integrating conventional circuits with CSPs devices at microwave and terahertz frequencies.

     

Electric Near-field Modulations of Charged Deoxyribonucleic Acid Nucleobases

Deoxyribonucleic acid (DNA) has been recently recognized as a promising material for nanophotonics due to its outstanding electro-optical tuning. Here using the realistic state-of-the-art quantum mechanical calculations, we carried out a systematic theoretical study on the electric near-field modulations of charged DNA nucleobases. Our results underline that electrical doping (the addition or removal of an electron) produces dramatic modulations to the electric near-field enhancements in the visible spectral range. Interestingly, electrical doping causes high-intensity electric near-field hotspot regions to emerge in the technologically relevant visible spectral range. Our results unveil electric near-field manipulation of DNA nucleobases, which might find applications in novel nanophotonic devices.

     

Defining a Superlens Operating Regime for Imaging Fluorescent Molecules

It has been shown that thin metal-based films can at certain frequencies act as planar near-field lenses for certain polarization components. A desirable property of such “lenses” is that they can also enhance and focus some large transverse spatial frequency components which contain sub-diffraction limit details. Over the last decade there has been much work in optimizing designs to reduce effects (such as material losses and surface roughness) that are detrimental to image reconstruction. One design that can reduce some of these undesirable effects, and which has received a fair amount of attention recently, is the stacked metal-dielectric superlens. Here we theoretically explore the imaging ability of such a design for the specific purpose of imaging a fluorescent dye (the common bio-marker GFP) in the vicinity of the superlens surface. Our calculations take into consideration the interaction (damping) of an oscillating electric dipole with the metallic layers in the superlens. We also assume a Gaussian frequency distribution spectrum for the dipole. We treat the metallic-alloy and dielectric-alloy layers separately using an appropriate effective medium theory. The transmission properties are evaluated via Transfer matrix (-matrix) calculations that were performed in the MatLab and MathCad environments. Our study shows that it is in principle possible to image fluorescent molecules using a simple bilayer planar superlens. We find that optimal parameters for such a superlens occur when the peak dipole emission-frequency is slightly offset from the Surface Plasmon resonance frequency of the metal-dielectric interfaces. The best resolution is obtained when the fluorescent molecules are not too close ( nm) or too far ( nm) from the superlens surface. The realization and application of a superlens with the specified design is possible using current nanofabrication techniques. When combined with e.g. a sub-wavelength grating structure (such as in the far-field superlens design previously proposed [1]) or a fast near-field scanning probe, it could provide a means for fast fluorescent imaging with sub-diffraction limit resolution.     

Multi-mode Hybrid Plasmonic Waveguides with Enhanced Confinement and Propagation

A hybrid waveguide, which consists of a dielectric wire above a dielectric-metal interface, has been previously proposed to achieve high confinement with low loss. By exciting this geometry with an aperture in the metal that takes advantage of the extraordinary transmission through subwavelength apertures, it is possible to strongly couple to multiple modes. The real part of the fundamental mode is in fact capable of exceeding the index of refraction of all the materials used while maintaining a manageable imaginary part, as a result of appropriate choice of materials for the dielectric wire and the metal. In addition, as the confinement of the second mode is comparable to that of the fundamental mode but has a much longer propagation length, this mode can be utilized in light-guiding applications where enhanced confinement and propagation is desired.     

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.     

Imaging brain tissue slices with terahertz near-field microscopy

Photoconductive antenna microprobe (PCAM)-based terahertz (THz) near-field imaging technique is promising for biomedical detection due to its excellent biocompatibility and high resolution; yet it is limited by its imaging speed and the difficulty in the control of the PCAM tip-sample separation. In this work, we successfully realized imaging of mouse brain tissue slices using an improved home-built PCAM-based THz near-field microscope. In this system, the imaging speed was enhanced by designing and applying a voice coil motor-based delay-line. The tip-sample separation control was implemented by developing an image analysis-based technique. Compared with conventional PCAM-based THz near-field systems, our improved system is 100 times faster in imaging speed and the tip-sample separation can be controlled to a few micrometers (e.g., 3 μm), satisfying the requirements of THz near-field imaging of biological samples. It took about ~30 min (not the tens of hours it took to acquire the same kind of image previously) to collect a THz near-field image of brain tissue slices of BALb/c mice (500 μm × 500 μm) with pixel size of 20 μm × 20 μm. The results show that the mouse brain slices can be properly imaged and different regions in the slices (i.e., the corpus callosum region and the cerebrum region) can be identified unambiguously. Evidently, the work demonstrated here provides not only a convincing example but a useful technique for imaging biological samples with THz near-field microscopy. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2741, 2019.     

Ultra-Broadband Linear Polarization Conversion via Diode-Like Asymmetric Transmission with Composite Metamaterial for Terahertz Waves

In this paper, a tri-layer metamaterial composed of a split-disk structure array sandwiched with two layers of twisted sub-wavelength metal grating is proposed and investigated numerically in terahertz region. The numerical results exhibit that linear polarization conversion via diode-like asymmetric transmission for terahertz waves within ultra-broadband frequency range is achieved due to Fabry-Perot-like resonance. In our design, the conversion polarization transmission coefficient for normal incidence is greater than 90 % in the range of 0.23–1.17 THz, equivalent to 134.3 % relative bandwidth. The physical mechanism of the broadband linear polarization conversion effect is further illustrated by simulated electrical field distributions.

     

Subwavelength-Sized Plasmonic Structures for Wide-Field Optical Microscopic Imaging with Super-Resolution

We propose a wide-field super-resolved optical microscopic imaging technique based on subwavelength slit arrays embedded in a thin silver film to generate surface plasmon (SP) standing wave interference patterns. These fringes carrying high spatial frequency information serve as excitation profiles to excite the nanoscale fluorescence objects. The super-resolved fluorescence density distribution is reconstructed from a weight sum of a series of fluorescence images with differently phase-shifted SP standing wave illumination. Simulation and experimental results show that the lateral resolution of the reconstructed fluorescence density image is enhanced by 0.28?λ _SP in two dimensions, which is twofold better than that of conventional high numerical aperture fluorescence microscopy. This technique benefits from a grating coupler to offer a simple way for the generation and phase shift of SP standing wave excitation profiles in two dimensions. The flat configuration, wide field, and noninvasive nature make this approach suitable for real-time analyzing the fine details of bio-samples in biochip applications.     

Near-Field Scanning Optical Microscopy, a Siren Call to Biology

Near-field illumination of a sample with visible light can resolve features well beyond the resolution of conventional, far-field microscopes. Near-field scanning optical microscopy (NSOM) then has the potential of extending the resolution of techniques such as fluorescent labeling, yielding images of cell structures and molecules on the nanoscale. However, major problems remain to be solved before NSOM can be easily used for wet biological samples. The most significant of these is control of the distance between near-field aperture and the sample surface. Hence, while NSOM promises much, its application to biology is about where electron microscopy was 40 or 50 years ago.     

Reflection‐mode switchable subwavelength Bessel‐beam and Gaussian‐beam photoacoustic microscopy in vivo

We have developed a reflection‐mode switchable subwavelength Bessel‐beam (BB) and Gaussian‐beam (GB) photoacoustic microscopy (PAM) system. To achieve both reflection‐mode and high resolution, we tightly attached a very small ultrasound transducer to an optical objective lens with numerical aperture of 1.0 and working distance of 2.5 mm. We used axicon and an achromatic doublet in our system to obtain the extended depth of field (DOF) of the BB. To compare the DOF performance achieved with our BB‐PAM system against GB‐PAM system, we designed our system so that the GB can be easily generated by simply removing the lenses. Using a 532 nm pulse laser, we achieved the lateral resolutions of 300 and 270 nm for BB‐PAM and GB‐PAM, respectively. The measured DOF of BB‐PAM was approximately 229 μm, which was about 7× better than that of GB‐PAM. We imaged the vasculature of a mouse ear using BB‐PAM and GB‐PAM and confirmed that the DOF of BB‐PAM is much better than the DOF of GB‐PAM. Thus, we believe that the high resolution achieved at the extended DOF by our system is very practical for wide range of biomedical research including red blood cell (RBC) migration in blood vessels at various depths and observation of cell migration or cell culture.      

Extraction of near-field fluorescence from composite signals to provide high resolution images of glial cells

The subdiffraction optical resolution that can be achieved using near-field optical microscopy has the potential to permit new approaches and insights into subcellular function and molecular dynamics. Despite the potential of this technology, it has been difficult to apply to cellular samples. One significant problem is that sample thickness causes the optical information to be comprised of a composite signal containing both near- and far-field fluorescence. To overcome this issue we have developed an approach in which a near-field optical fiber is translated toward the cell surface. The increase in fluorescence intensity during z-translation contains two components: a far-field fluorescence signal when the tip of the fiber is distant from the labeled cell, and combined near- and far-field fluorescence when the tip interacts with the cell surface. By fitting a regression curve to the far-field fluorescence intensity as the illumination aperture approaches the cell, it is possible to isolate near-field from far-field fluorescent signals. We demonstrate the ability to resolve actin filaments in chemically fixed, hydrated glial cells. A comparison of composite fluorescence signals with extracted near-field fluorescence demonstrates that this approach significantly increases the ability to detect subcellular structures at subdiffraction resolution.