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.     

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.

     

Experimental Study of Indirect Phase Tuning-Based Plasmonic Structures for Finely Focusing

Three types of indirect phase tuning-based plasmonic structures with subwavelength circular grooves/slits and/or central apertures corrugated on Au film supported by glass substrate: depth modulation, width modulation, and hybrid depth-width modulation, were put forth in this paper. They were investigated experimentally by means of nanofabrication and near-filed scanning optical microscope characterization. The plasmonic structures were fabricated using the technique of focused ion beam direct milling. Our experimental results demonstrated that all of the phase tuning-based structures have focusing functions. Both the width and depth modulation-based structures can realize beam focusing and produce an elongated depth of focus. Moreover, after comparison among these three structures, we found that the width modulation-based structure has the best focusing performance.     

Beam-Scanning Planar Lens Based on Metal–Dielectric–Metal Waveguide Arrays

Under specific illumination conditions, periodic arrays of metal–dielectric–metal (MDM) waveguides act as uniform optical phased-array antenna where the phase of the radiating optical wave can be controlled by modifying the refractive index distribution of the dielectric material. Based on this property, we propose a planar gradient index MDM-based lens which can transform spherical waves of the transverse-magnetic surface plasmon polariton waves to plane waves with specific beam deflections by adjusting the refractive index configurations. Using numerical simulations based on two-dimensional finite-difference time-domain method, it is confirmed that beam focusing and splitting with multiple dflection angles can also be achieved.     

Optical Absorption Enhancement in Freestanding GaAs Thin Film Nanopyramid Arrays

Although III–V compound semiconductor multi‐junction cells show the highest efficiency among all types of solar cells, their cost is quite high due to expensive substrates, long epitaxial growth and complex balance of system components. To reduce the cost, ultra‐thin films with advanced light management are desired. Here effective light trapping in freestanding thin film nanopyramid arrays is demonstrated and multiple‐times light path enhancement is realized, where only 160 nm thick GaAs with nanopyramid structures is equivalent to a 1 μm thick planar film. The GaAs nanopyramids are fabricated using a combination of nanosphere lithography, nanopyramid metal organic chemical vapor deposition (MOCVD) growth, and gas‐phase substrate removal processes. Excellent optical absorption is demonstrated over a broad range of wavelengths, at various incident angles and at large‐curvature bending. Compared to an equally thick planar control film, the overall number of photons absorbed is increased by about 100% at various incident angles due to significant antireflection and light trapping effects. By implementing these nanopyramid structures, III–V material usage and deposition time can be significantly reduced to produce high‐efficiency, low‐cost thin film III–V solar cells.     

In Vitro Validation of an Artefact Suppression Algorithm in X-Ray Phase-Contrast Computed Tomography

X-ray phase-contrast tomography can significantly increase the contrast-resolution of conventional attenuation-contrast imaging, especially for soft-tissue structures that have very similar attenuation. Just as in attenuation-based tomography, phase contrast tomography requires a linear dependence of aggregate beam direction on the incremental direction alteration caused by individual voxels along the path of the X-ray beam. Dense objects such as calcifications in biological specimens violate this condition. There are extensive beam deflection artefacts in the vicinity of such structures because they result in large distortion of wave front due to the large difference of refractive index; for such large changes in beam direction, the transmittance of the silicon analyzer crystal saturates and is no longer linearly dependent on the angle of refraction. This paper describes a method by which these effects can be overcome and excellent soft-tissue contrast of phase tomography can be preserved in the vicinity of such artefact-producing structures.     

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.     

Targeted irradiation of Mammalian cells using a heavy-ion microprobe

The existing focusing heavy-ion microprobe at the Gesellschaft für Schwerionenforschung in Darmstadt (Germany) has been modified to enable the targeted irradiation of single, selected cells with a defined number of ions. With this setup, ions in the range from helium to uranium with linear energy transfers (LETs) up to approximately 15,000 keV/microm can be positioned with a precision of a few micrometers in the nuclei of single cells that are growing in culture on a thin polypropylene film. To achieve this accuracy, the microbeam traverses a thin vacuum window with minimal scattering. Electron emission from that window is used for particle detection. The cells are kept in a specially designed dish that is mounted directly behind the vacuum window in a setup allowing the precise movement and the imaging of the sample with microscopic methods. The cells are located by an integrated software program that also controls the rapid deflection and switching of the beam. In this paper, the setup is described in detail together with the first experiments showing its performance. We describe the ability of the microprobe to reliably hit randomly positioned etched nuclear tracks in CR-39 with single ions as well as the ability to visualize the ion hits using immunofluorescence staining for 53BP1 as a marker of DNA damage in the targeted cell nuclei.     

Calibration of MTT assay in proton beams using radiochromic films

PurposeThis study provides methodology of calibrating as well as controlling the output for an MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) colorimetric assay irradiated in a low energy proton beam using EBT3-model GAFCHROMIC^TM film, without correcting for quenching effect.MethodsA calibrated Markus ionization chamber was used to measure the depth dose and beam output for 26.5 MeV protons produced by a CS30 cyclotron. A time-controlled aluminum cylinder was added in front of the horizontal beam-exit serving as a radiation shutter. Following the TRS-398 reference dosimetry protocol for proton beams, the output was calibrated in water at a reference depth of 3 mm. EBT3 film was calibrated for doses up to 8 Gy at the same depth. To verify the dose distribution for each 96-well MTT assay plate, EBT3 film was placed at the reference depth during irradiation and cell doses were scaled by measured percent depth dose (PDD) data.ResultsThe radiochromic film dosimetry system in this study provides dose measurements with an uncertainty better than 3.3% for doses higher than 1 Gy. From a single exposure and utilizing the Gaussian shape of the beam, multiple dose points can be obtained within different wells of the same plate ranging from 6.9 Gy (sigma ∼4%) in the central well, and 2 Gy (sigma ∼8%) for wells positioned closer to the periphery.ConclusionsWe described a methodology for radiochromic film-based dose monitoring system, using low-energy protons, which can be used for the MTT assay in any proton beam, except within Bragg peak region.     

A surface-chemical basis for cell motility.

The properties of droplets encapsulated in a thin film of another liquid are discussed and it will be shown that by applying the macrocluster concept, in which surface tension is important, many phenomena resembling biological activity can be explained, the driving force being the loss of chemical potential as a surfactant moves from a lipid to an aqueous phase.     

REDUCTION OF HEATING ARTIFACTS IN THIN SECTIONS EXAMINED IN THE ELECTRON MICROSCOPE

Certain phenomena affecting contrast obtained from tissue sections with the electron microscope have been investigated and a technique is described for reducing destruction by the electron beam of fine details in sections. It has been concluded that loss of embedding material is slightly higher at exposed surfaces of sections than it is at surfaces covered by substrate film. Covering of both surfaces of sections with thin films of formvar, collodion, or carbon materially improves the general appearance, reduces distortion, and sometimes reduces loss of tissue mass from the section as result of exposure to the electron beam. This improvement is considered to result from the relatively high melting-point of the covering films which serve to eliminate or reduce surface-tension or other forces operating in methacrylate softened by the electron beam.     

A Surface Design for Enhancement of Light Trapping Efficiencies in Thin Film Silicon Solar Cells

We report on a surface design of thin film silicon solar cells based on silver nanoparticle arrays and blazed grating arrays. The light transmittance is increased at the front surface of the cells, utilizing the surface plasmon resonance effect induced by silver nanoparticle arrays. As a reflection layer structure, blazed gratings are placed at the rear surface to increase the light reflectance at bottom of the thin film cells. With the combination of the silver nanoparticle arrays and the blazed gratings, the light trapping efficiency of the thin film solar cell is characterized by its light absorptance, which is determined from the transmittance at front surface and the reflectance at bottom, via the finite-difference time-domain (FDTD) numerical simulation method. The results reveal that the light trapping efficiency is enhanced as the structural parameters are optimized. This work also shows that the surface plasmon resonance effect induced by the silver nanoparticles and the grating characteristics of the blazed gratings play crucial roles in the design of the thin film silicon solar cells.     

A lubrication analysis of pharyngeal peristalsis: Application to flavour release

After eating a liquid or a semi-liquid food product, a thin film responsible for the dynamic profile of aroma release coats the pharyngeal mucosa. The aim of this article was to analyse the fluid mechanics of pharyngeal peristalsis and to develop a simple biomechanical model in order to understand the role of saliva and food bolus viscosity on the coating of pharyngeal mucosa. We began by analysing the physiology and the biomechanics of swallowing in order to determine relevant model assumptions. This analysis of the literature clarified the types of mechanical solicitations applied on the food bolus. Moreover, we showed that the pharyngeal peristalsis in the most occluded region is equivalent to a forward roll coating process, the originality of which is lubrication by a film of saliva. A model based on the lubrication theory for Newtonian liquids was developed in dimensionless form. The parametric study showed the strong influence of relative saliva thickness on the food bolus coating. A specific experimental device was designed that confirms the model predictions. Two sets of conditions that depend on the relative thickness of saliva were distinguished. The first is characterised by a relatively thin film of saliva: food bolus viscosity has a strong impact on mucosa coating. These phenomena are well represented by the model developed here. The second is obtained when the saliva film is relatively thick: hydrodynamic mixing with saliva, interdiffusion or instabilities may govern mucosa coating. Finally, these results were extrapolated to determine the influence of food bolus viscosity on the dynamic profile of flavour release according to physiological parameters.     

Efficient All-Optical Molecule-Plasmon Modulation Based on T-shape Single Slit

Efficient all-optical molecule-plasmon modulation is experimentally demonstrated by employing a compact T-shape single slit on a metal film coated with an azopolymer film, in which the azobenzene molecules can be reoriented by a pump beam. In the T-shape single slit, the transmission spectra exhibit periodic behaviors and are quite sensitive to variations of the refractive index of the azopolymer in the groove. Under a pump beam, the azobenzene molecules are reoriented, so the SPPs in the groove feel a refractive index quite different from that of the originally isotropic azopolymer with randomly orientations. This leads to a high modulation depth of about 53 % (3.3 dB) and a phase variation of >π experimentally.     

Ordered Nanoscale Heterojunction Architecture for Enhanced Solution‐Based CuInGaS2 Thin Film Solar Cell Performance

Nanopatterned CuInGaS₂ (CIGS) thin films synthesized by a sol‐gel‐based solution method and a nanoimprint lithography technique to achieve simultaneous photonic and electrical enhancements in thin film solar cell applications are demonstrated. The interdigitated CIGS nanopatterns in adjacent CdS layer form an ordered nanoscale heterojunction of optical contrast to create a light trapping architecture. This architecture concomitantly leads to increased junction area between the p‐CIGS/n‐CdS interface, and thereby influences effective charge transport. The electron beam induced current and capacitance–voltage characterization further supports the large carrier collection area and small depletion region of the nanopatterned CIGS solar cell devices. This strategic geometry affords localization of incident light inside and between the nanopatterns, where created excitons are easily dissociated, and it leads to the enhanced current generation of absorbed light. Ultimately, this approach improves the efficiency of the nanopatterned CIGS solar cell by 55% compared to its planar counterpart, and offers the possibility of simultaneous management for absorption and charge transport through a nanopatterning process.     

Engineering Multiphase Metal Halide Perovskites Thin Films for Stable and Efficient Solar Cells

The intrinsic instability of lead halide perovskite semiconductors in an ambient atmosphere is one of the most critical issues that impedes perovskite solar cell commercialization. To overcome it, the use of bulky organic spacers has emerged as a promising solution. The resulting perovskite thin films present complex morphologies, difficult to predict, which can directly affect the device efficiency. Here, by combining in‐depth morphological and spectroscopic characterization, it is shown that both the ionic size and the relative concentration of the organic cation, drive the integration of bulky organic cations into the crystal unit cell and the thin film, inducing different perovskite phases and different vertical distribution, then causing a significant change in the final thin film morphology. Based on these studies, a fine‐engineered perovskite is constructed by employing two different large cations, namely, ethyl ammonium and butyl ammonium. The first one takes part in the 3D perovskite phase formation, the second one works as a surface modifier by forming a passivating layer on top of the thin film. Together they lead to improved photovoltaic performance and device stability when tested in air under continuous illumination. These findings propose a general approach to achieve reliability in perovskite‐based optoelectronic devices.     

Cation Substitution of Solution‐Processed Cu2ZnSnS4 Thin Film Solar Cell with over 9% Efficiency

To alleviate the limitations of pure sulfide Cu₂ZnSnS₄ (CZTS) thin film, such as band gaps adjustment, antisite defects, secondary phase and microstructure, Cadmium is introduced into CZTS thin film to replace Zn partially to form Cu₂Zn_1?xCd_xSnS₄ (CZCTS) thin film by low‐cost sol–gel method. It is demonstrated that the band gaps and crystal structure of CZCTS thin films are affected by the change in Zn/Cd ratio. In addition, the ZnS secondary phase can be decreased and the grain sizes can be improved to some degree by partial replacement of Zn with Cd in CZCTS thin film. The power conversion efficiency of CZTS solar cell device is enhanced significantly from 5.30% to 9.24% (active area efficiency 9.82%) with appropriate ratio of Zn/Cd. The variation of device parameter as a function of Zn/Cd ratio may be attributed to the change in electronic structure of the bulk CZCTS thin film (i.e., phase change from kesterite to stannite), which in turn affects the band alignment at the CZCTS/buffer interface and the charge separation at this interface.     

Fabrication of Ag Nanoparticles Embedded in Al:ZnO as Potential Light-Trapping Plasmonic Interface for Thin Film Solar Cells

Incident photon conversion efficiency of the absorbing materials at either side of a thin film solar module can be enhanced by integrating a plasmonic interface. Silver nanoparticles represent a good candidate that can be integrated to a thin film solar cell for efficient light-trapping. The aim of this work is to fabricate plasmonically active interface consisting of Ag nanoparticles embedded in Al:ZnO that has the potential to be used at the front surface and at the back reflector of a thin film solar cell to enhance light-trapping and increase the photoconversion efficiency. We show that Ag can readily dewet the Al:ZnO surface when annealed at temperatures significantly lower than the melting temperature of Ag, which is beneficial for lowering the thermal budget and cost in solar cell fabrication. We find that such an interface fabricated by a simple dewetting technique leads to plasmonic resonance in the visible and near infrared regions of the solar spectrum, which is important in enhancing the conversion efficiency of thin film solar cells.     

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.     

Plasmonic Planar Lens Based on Slanted Nanoslit Array

The novel plasmonic lenses based on slanted nanoslits have been proposed theoretically. The slanted nanoslits with different slant angles can provide unequal propagation distances for the surface plasmon polaritons excited by incident light. The phase retardation for wavefront shaping can be obtained to realize constructive interference on a preset single spot. We can actively modulate the position of the optical focus by adjusting the slits slant angles properly. The simulation results of the finite element method are used to verify our proposals.