Deep-learning-augmented microscopy for super-resolution imaging of nanoparticles.

Conventional optical microscopes generally provide blurry and indistinguishable images for subwavelength nanostructures. However, a wealth of intensity and phase information is hidden in the corresponding diffraction-limited optical patterns and can be used for the recognition of structural features, such as size, shape, and spatial arrangement. Here, we apply a deep-learning framework to improve the spatial resolution of optical imaging for metal nanostructures with regular shapes yet varied arrangement. A convolutional neural network (CNN) is constructed and pre-trained by the optical images of randomly distributed gold nanoparticles as input and the corresponding scanning-electron microscopy images as ground truth. The CNN is then learned to recover reversely the non-diffracted super-resolution images of both regularly arranged nanoparticle dimers and randomly clustered nanoparticle multimers from their blurry optical images. The profiles and orientations of these structures can also be reconstructed accurately. Moreover, the same network is extended to deblur the optical images of randomly cross-linked silver nanowires. Most sections of these intricate nanowire nets are recovered well with a slight discrepancy near their intersections. This deep-learning augmented framework opens new opportunities for computational super-resolution optical microscopy with many potential applications in the fields of bioimaging and nanoscale fabrication and characterization. It could also be applied to significantly enhance the resolving capability of low-magnification scanning-electron microscopy.

Optical Monitoring of In Situ Iron Loading into Single, Native Ferritin Proteins.

Ferritin is a protein that stores and releases iron to prevent diseases associated with iron dysregulation in plants, animals, and bacteria. The conversion between iron-loaded holo-ferritin and empty apo-ferritin is an important process for iron regulation. To date, studies of ferritin have used either ensemble measurements to quantify the characteristics of a large number of proteins or single-molecule approaches to interrogate labeled or modified proteins. Here we demonstrate the first real-time study of the dynamics of iron ion loading and biomineralization within a single, unlabeled ferritin protein. Using optical nanotweezers, we trapped single apo- and holo-ferritins indefinitely, distinguished one from the other, and monitored their structural dynamics in real time. The study presented here deepens the understanding of the iron uptake mechanism of ferritin proteins, which may lead to new therapeutics for iron-related diseases.

Sum-Frequency Generation in High-Q GaP Metasurfaces Driven by Leaky-Wave Guided Modes.

Resonant metasurfaces provide a unique platform for enhancing multiwave nonlinear interactions. However, the difficulties over mode matching and material transparency place significant challenges in the enhancement of these multiwave processes. Here we demonstrate efficient nonlinear sum-frequency generation (SFG) in multiresonant GaP metasurfaces based on guided-wave bound-state in the continuum resonances. The excitation of the metasurface by two near-infrared input beams generates strong SFG in the visible spectrum with a conversion efficiency of 2.5 × 10-4 W-1, 2 orders of magnitude higher than the one reported in Mie-type resonant metasurfaces. In addition, we demonstrate the nontrivial polarization dependence on the SFG process. In contrast to harmonic generation, the SFG process is enhanced when using nonparallel polarized input-beams. Importantly, by varying the input pump beam polarization it is possible to direct the SFG emission to different diffraction orders, thereby opening up new opportunities for nonlinear light sources and infrared to visible light conversion.

Visible-Band Chiroptical Meta-devices with Phase-Change Adjusted Optical Chirality.

Low-cost large-area chirality meta-devices (CMDs) with adjustable optical chirality are of great interest for polarization-sensitive imaging, stereoscopic display, enantioselectivity analysis, and catalysis. Currently, CMDs with adjusted chiroptical responses in the mid-infrared to terahertz band have been demonstrated by exploiting photocarriers of silicon, pressure, and phase-change of GSTs but are still absent in the visible band, which in turn limits the development of chiral nanophotonic devices. Herein, by employing a phase-change material (Sb2S3), we present a protocol for the fabrication of wafer-scale visible-band enantiomeric CMDs with handedness, spectral, and polarization adjustability. As measured by circular dichroism, the chirality signs of CMDs enantiomers can be adjusted with Sb2S3 from amorphous to crystalline, and the chirality resonance wavelength can also be adjusted. Our results suggest a new type of meta-devices with adjustable chiroptical responses that may potentially enable a wide range of chirality nanophotonic applications including highly sensitive sensing and surface-enhanced nanospectroscopy.

Flatband mode in photonic moiré superlattice for boosting second-harmonic generation with monolayer van der Waals crystals.

We theoretically investigate boosting second-harmonic generation (SHG) of monolayer van der Waals crystals by employing flatband modes hosted by photonic moiré superlattices. Such a system with high quality factor and a monolayer crystal accommodated on the top of it, provides a unique opportunity to enhance and manipulate SHG emission. We show that employing a doubly resonant diagram on such a moiré superlattice system not only boosts the SHG, but also tunes the directional emission of the second-harmonic wave. Moreover, we demonstrate that a structured beam illumination could further boost SHG, with the phase structure retrieved through a two-beam second-harmonic interference configuration. These results suggest the flatband modes in moiré superlattice as a promising platform for boosting SHG with monolayer van der Waals crystals, offering new possibilities for developing compact nonlinear photonic devices.

The effect of green coffee and/or endurance exercise on sperm function in pre-diabetic mice.

This study aimed to investigate the effect of green coffee (GC), chlorogenic acid (CA) as an active ingredient of GC and exercise, alone or in a combination with exercise, on spermatogenesis and sperm function in pre-diabetic mice. Results revealed that pre-diabetic status can have a significant adverse effect on spermatogenesis (Johnson score), and sperm concentration, motility, DNA damage and persistent histone in compared to the control group. Although lipid peroxidation, intracellular ROS production, and persistent histones in sperm were high in pre-diabetic mice, exercise only can improve sperm motility. GC alone only improved sperm motility in pre-diabetic mice while CA alone, even did not have this beneficial effect. However, GC along with exercise, did not improve motility but reduce DNA damage, while CA with exercise, significantly improved motility compared to pre-diabetic stage and to the level comparable to control. Therefore, based on this result in individuals with high DNA damage, GC supplementation and exercise could be useful approach while in asthenozoospermia, CA supplementation and exercise should be considered as an alternative approach. However, such an interpretation awaits validation.

Capsaicin improves sperm quality in rats with experimental varicocele.

Capsaicin is the main capsaicinoid in chilli peppers that have numerous biological and pharmaceutical roles in the body such as antioxidant, anti-inflammatory, anticarcinogenic, analgesic, counterirritant and antiarthritic properties. Numerous studies have shown increased oxidative stress in men with varicocele that is caused by dilation of the spermatic vein and increase of testicular temperature. Therefore, we aimed to assess the effect of Capsaicin on sperm parameters in rats with experimental varicocele. At first, we induced varicocele in 30 Wistar rats and, verify varicocele model only in 10 rats by assessment of sperm parameters, oxidative stress, DNA damage and persistent histone after 2 months. Of the remaining 20 varicocelised rats, half of them were treated with 2.5 mg/kg Capsaicin for two months and the other half served as control. Then, sperm tests were assessed, and the results showed that Capsaicin can restore the mean of sperm oxidative stress (38.78 ± 3.75 versus 58.37 ± 4.34; p < .05), sperm concentration (60.14 ± 7.66 versus 34.87 ± 5.78; p < .05) and motility (62.43 ± 3.10 versus 41.22 ± 5.11; p < .05) in varicocelised rats treated with Capsaicin compared to varicocelised rats that were not treat. Therefore, Capsaicin possibly with reduction of oxidative stress level could improve mean of sperm concentration and motility in varicocele condition.

Tailoring Second-Harmonic Emission from (111)-GaAs Nanoantennas.

Second-harmonic generation (SHG) in resonant dielectric Mie-scattering nanoparticles has been hailed as a powerful platform for nonlinear light sources. While bulk-SHG is suppressed in elemental semiconductors, for example, silicon and germanium due to their centrosymmetry, the group of zincblende III-V compound semiconductors, especially (100)-grown AlGaAs and GaAs, have recently been presented as promising alternatives. However, major obstacles to push the technology toward practical applications are the limited control over directionality of the SH emission and especially zero forward/backward radiation, resulting from the peculiar nature of the second-order nonlinear susceptibility of this otherwise highly promising group of semiconductors. Furthermore, the generated SH signal for (100)-GaAs nanoparticles depends strongly on the polarization of the pump. In this work, we provide both theoretically and experimentally a solution to these problems by presenting the first SHG nanoantennas made from (111)-GaAs embedded in a low index material. These nanoantennas show superior forward directionality compared to their (100)-counterparts. Most importantly, based on the special symmetry of the crystalline structure, it is possible to manipulate the SHG radiation pattern of the nanoantennas by changing the pump polarization without affecting the linear properties and the total nonlinear conversion efficiency, hence paving the way for efficient and flexible nonlinear beam-shaping devices.

Reversible Image Contrast Manipulation with Thermally Tunable Dielectric Metasurfaces.

Increasing demand for higher resolution of miniaturized displays requires techniques achieving high contrast tunability of the images. Employing metasurfaces for image contrast manipulation is a new and rapidly growing field of research aiming to address this need. Here, a new technique to achieve image tuning in a reversible fashion is demonstrated by dielectric metasurfaces composed of subwavelength resonators. It is demonstrated that by controlling the temperature of a metasurface the encoded transmission pattern can be tuned. To this end, two sets of nanoresonators composed of nonconcentric silicon disks with a hole that exhibit spectrally sharp Fano resonances and forming a Yin-Yang pattern are designed and fabricated. Through exploitation of the thermo-optical properties of silicon, full control of the contrast of the Yin-Yang image is demonstrated by altering the metasurface temperature by ΔT ≈ 100 °C. This is the first demonstrated technique to control an image contrast by temperature. Importantly, the turning technique does not require manipulating the external stimulus, such as polarization or angle of the illumination and/or the refractive index of this environment. These results open many opportunities for transparent displays, optical switches, and tunable illumination systems.

Nonlinear Optical Magnetism Revealed by Second-Harmonic Generation in Nanoantennas.

Nonlinear effects at the nanoscale are usually associated with the enhancement of electric fields in plasmonic structures. Recently emerged new platform for nanophotonics based on high-index dielectric nanoparticles utilizes optically induced magnetic response via multipolar Mie resonances and provides novel opportunities for nanoscale nonlinear optics. Here, we observe strong second-harmonic generation from AlGaAs nanoantennas driven by both electric and magnetic resonances. We distinguish experimentally the contribution of electric and magnetic nonlinear response by analyzing the structure of polarization states of vector beams in the second-harmonic radiation. We control continuously the transition between electric and magnetic nonlinearities by tuning polarization of the optical pump. Our results provide a direct observation of nonlinear optical magnetism through selective excitation of multipolar nonlinear modes in nanoantennas.

Adiabatic Nanofocusing in Hybrid Gap Plasmon Waveguides on the Silicon-on-Insulator Platform.

We present an experimental demonstration of a new class of hybrid gap plasmon waveguides on the silicon-on-insulator (SOI) platform. Created by the hybridization of the plasmonic mode of a gap in a thin metal sheet and the transverse-electric (TE) photonic mode of an SOI slab, this waveguide is designed for efficient adiabatic nanofocusing simply by varying the gap width. For gap widths greater than 100 nm, the mode is primarily photonic in character and propagation lengths can be many tens of micrometers. For gap widths below 100 nm, the mode becomes plasmonic in character with field confinement predominantly within the gap region and with propagation lengths of a few microns. We estimate the electric field intensity enhancement in hybrid gap plasmon waveguide tapers at 1550 nm by three-photon absorption of selectively deposited CdSe/ZnS quantum dots within the gap. Here, we show electric field intensity enhancements of up to 167 ± 26 for a 24 nm gap, proving the viability of low loss adiabatic nanofocusing on a commercially relevant photonics platform.

The Interplay of Symmetry and Scattering Phase in Second Harmonic Generation from Gold Nanoantennas.

Nonlinear phenomena are central to modern photonics but, being inherently weak, typically require gradual accumulation over several millimeters. For example, second harmonic generation (SHG) is typically achieved in thick transparent nonlinear crystals by phase-matching energy exchange between light at initial, ω, and final, 2ω, frequencies. Recently, metamaterials imbued with artificial nonlinearity from their constituent nanoantennas have generated excitement by opening the possibility of wavelength-scale nonlinear optics. However, the selection rules of SHG typically prevent dipole emission from simple nanoantennas, which has led to much discussion concerning the best geometries, for example, those breaking centro-symmetry or incorporating resonances at multiple harmonics. In this work, we explore the use of both nanoantenna symmetry and multiple harmonics to control the strength, polarization and radiation pattern of SHG from a variety of antenna configurations incorporating simple resonant elements tuned to light at both ω and 2ω. We use a microscopic description of the scattering strength and phases of these constituent particles, determined by their relative positions, to accurately predict the SHG radiation observed in our experiments. We find that the 2ω particles radiate dipolar SHG by near-field coupling to the ω particle, which radiates SHG as a quadrupole. Consequently, strong linearly polarized dipolar SHG is only possible for noncentro-symmetric antennas that also minimize interference between their dipolar and quadrupolar responses. Metamaterials with such intra-antenna phase and polarization control could enable compact nonlinear photonic nanotechnologies.

Tailored Hypersound Generation in Single Plasmonic Nanoantennas.

Ultrashort laser pulses impinging on a plasmonic nanostructure trigger a highly dynamic scenario in the interplay of electronic relaxation with lattice vibrations, which can be experimentally probed via the generation of coherent phonons. In this Letter, we present studies of hypersound generation in the range of a few to tens of gigahertz on single gold plasmonic nanoantennas, which have additionally been subjected to predesigned mechanical constraints via silica bridges. Using these hybrid gold/silica nanoantennas, we demonstrate experimentally and via numerical simulations how mechanical constraints allow control over their vibrational mode spectrum. Degenerate pump-probe techniques with double modulation are performed in order to detect the small changes produced in the probe transmission by the mechanical oscillations of these single nanoantennas.

Nonlinear Generation of Vector Beams From AlGaAs Nanoantennas.

The quest for nanoscale light sources with designer radiation patterns and polarization has motivated the development of nanoantennas that interact strongly with the incoming light and are able to transform its frequency, radiation, and polarization patterns. Here, we demonstrate dielectric AlGaAs nanoantennas for efficient second harmonic generation, enabling the control of both directionality and polarization of nonlinear emission. This is enabled by specialized III-V semiconductor nanofabrication of high-quality AlGaAs nanostructures embedded in optically transparent low-index material, thus allowing for simultaneous forward and backward nonlinear emission. We show that the nanodisk AlGaAs antennas can emit second harmonic in preferential direction with a backward-to-forward ratio of up to five and can also generate complex vector polarization beams, including beams with radial polarization.

High-efficiency second harmonic generation from a single hybrid ZnO nanowire/Au plasmonic nano-oligomer.

We introduce a plasmonic-semiconductor hybrid nanosystem, consisting of a ZnO nanowire coupled to a gold pentamer oligomer by crossing the hot-spot. It is demonstrated that the hybrid system exhibits a second harmonic (SH) conversion efficiency of ∼3 × 10(-5)%, which is among the highest values for a nanoscale object at optical frequencies reported so far. The SH intensity was found to be ∼1700 times larger than that from the same nanowire excited outside the hot-spot. Placing high nonlinear susceptibility materials precisely in plasmonic confined-field regions to enhance SH generation opens new perspectives for highly efficient light frequency up-conversion on the nanoscale.

Beyond the hybridization effects in plasmonic nanoclusters: diffraction-induced enhanced absorption and scattering.

It is demonstrated herein both theoretically and experimentally that Young's interference can be observed in plasmonic structures when two or three nanoparticles with separation on the order of the wavelength are illuminated simultaneously by a plane wave. This effect leads to the formation of intermediate-field hybridized modes with a character distinct of those mediated by near-field and/or far-field radiative effects. The physical mechanism for the enhancement of absorption and scattering of light due to plasmonic Young's interference is revealed, which we explain through a redistribution of the Poynting vector field and the formation of near-field subwavelength optical vortices.

Structural Flexibility and Disassembly Kinetics of Single Ferritin Molecules Using Optical Nanotweezers.

Ferritin, a spherical protein shell assembled from 24 subunits, functions as an efficient iron storage and release system through its channels. Understanding how various chemicals affect the structural behavior of ferritin is crucial for unravelling the origins of iron-related diseases in living organisms including humans. In particular, the influence of chemicals on ferritin's dynamics and iron release is barely explored at the single-protein level. Here, by employing optical nanotweezers using double-nanohole (DNH) structures, we examined the effect of ascorbic acid (reducing reagent) and pH on individual ferritin's conformational dynamics. The dynamics of ferritin increased as the concentration of ascorbic acid approached saturation. At pH 2.0, ferritin exhibited significant structural fluctuations and eventually underwent a stepwise disassembly into fragments. This work demonstrated the disassembly pathway and kinetics of a single ferritin molecule in solution. We identified four critical fragments during its disassembly pathway, which are 22-mer, 12-mer, tetramer, and dimer subunits. Moreover, we present single-molecule evidence of the cooperative disassembly of ferritin. Interrogating ferritin's structural change in response to different chemicals holds importance for understanding their roles in iron metabolism, hence facilitating further development of medical treatments for its associated diseases.

Use of a gold reflecting-layer in optical antenna substrates for increase of photoluminescence enhancement.

We report on a straightforward way to increase the photoluminescence enhancement of nanoemitters induced by optical nanotantennas. The nanoantennas are placed above a gold film-silica bilayer, which produces a drastic increase of the scattered radiation power and near field enhancement. We demonstrate this increase via photoluminescence enhancement using an organic emitter of low quantum efficiency, Tetraphenylporphyrin (TPP). An increase of the photoluminescence enhancement by a factor larger than three is observed compared to antennas without the reflecting-layer. In addition, we study the possibility of influencing the polarization of the light emitted by utilizing asymmetry of dimer antennas.

Probing the dielectric response of graphene via dual-band plasmonic nanoresonators.

In this article, we use optical transmission spectroscopy to measure the changes in the resonance features of a Au plasmonic nanoresonator array consisting of concentric ring/disc cavity elements, when graphene is introduced as an encapsulating medium. We show that by using finite element modelling to best reproduce our experimental results the dielectric response of the graphene film can be determined. We discuss the potential of such structures for chemical sensing applications.

Subgroup decomposition of plasmonic resonances in hybrid oligomers: modeling the resonance lineshape.

Plasmonic resonances with a Fano lineshape observed in metallic nanoclusters often arise from the destructive interference between a dark, subradiant mode and a bright, super-radiant one. A flexible control over the Fano profile characterized by its linewidth and spectral contrast is crucial for many potential applications such as slowing light and biosensing. In this work, we show how one can easily but significantly tailor the overall spectral profile in plasmonic nanocluster systems, for example, quadrumers and pentamers, by selectively altering the particle shape without a need to change the particle size, interparticle distance, or the number of elements of the oligomers. This is achieved through decomposing the whole spectrum into two separate contributions from subgroups, which are efficiently excited at their spectral peak positions. We further show that different strengths of interference between the two subgroups must be considered for a full understanding of the resulting spectral lineshape. In some cases, each subgroup is separately active in distinct frequency windows with only small overlap, leading to a simple convolution of the subspectra. Variation in particle shape of either subgroup results in the tuning of the overall spectral lineshape, which opens a novel pathway for shaping the plasmonic response in small nanoclusters.