High Q-Factor, High Contrast, and Multi-Band Optical Sensor Based on Plasmonic Square Bracket Dimer Metasurface

A high-performance resonant metasurface is rather promising for diverse application areas such as optical sensing and filtering. Herein, a metal-insulator-metal (MIM) optical sensor with merits of a high quality-factor (Q-factor), multiple operating bands, and high spectrum contrast is proposed using plasmonic square bracket dimer metasurface. Due to the complex square bracket itself, a dimer structure of two oppositely placed square brackets, and metasurface array configuration, multiple kinds of mode coupling can be devised in the inner and outer elements within the metasurface, enabling four sensing channels with the sensitivities higher than 200 nm/RIU for refractive index sensing. Among them, the special sensing channel based on the reflection-type surface lattice resonance (SLR) mechanism has a full width at half maximum (FWHM) of only 2 nm, a high peak-to-dip signal contrast of 0.82, a high Q-factor of 548, and it can also behave as a good sensing channel for the thickness measurement of the deposition layer. The multi-band sensor can work normally in a large refractive index or thickness range, and the number of resonant channels can be further increased by simply breaking the structural symmetry or changing the polarization angle of incident light. Equipped with unique advantages, the suggested plasmonic metasurface has great potential in sensing, monitoring, filtering, and other applications.

Different Roles of Surface Chemistry and Roughness of Laser-Induced Graphene: Implications for Tunable Wettability

The control of surface wettability is a technological key aspect and usually poses considerable challenges connected to high cost, nanostructure, and durability, especially when aiming at surface patterning with high and extreme wettability contrast. This work shows a simple and scalable approach by using laser-induced graphene (LIG) and a locally inert atmosphere to continuously tune the wettability of a polyimide/LIG surface from hydrophilic to superhydrophobic (Φ ∼ 160°). This is related to the reduced amount of oxygen on the LIG surface, influenced by the local atmosphere. Furthermore, the influence of the roughness pattern of LIG on the wettability is investigated. Both approaches are combined, and the influence of surface chemistry and roughness is discussed. Measurements of the roll-off angle show that LIG scribed in an inert atmosphere with a low roughness has the highest droplet mobility with a roll-off angle of ΦRO = (1.7 ± 0.3)°. The superhydrophobic properties of the samples were maintained for over a year and showed no degradation after multiple uses. Applications of surfaces with extreme wettability contrast in millifluidics and fog basking are demonstrated. Overall, the proposed processing allows for the continuous tuning and patterning of the surface properties of LIG in a very accessible fashion useful for "lab-on-chip" applications.

Hybrid-Type Transparent Organic Light Emitting Diode with High Contrast Using Switchable Windows

Transparent organic light emitting diode (OLED) display is one of the most promising devices among next-generation information displays because of beneficial characteristics, such as self-emissive and optically clear properties. Nevertheless, in conventional transparent OLED display devices, there are serious intrinsic problems in terms of the transmittance in the dark state because of empty windows in the cell, so the contrast ratio of the transparent OLED display would be deteriorated even though it can exhibit excellent bright state. In general, the transparent mode using the OLED device applies an empty area in each pixel because an emitting device could never reveal the background image, so the transparent OLED should contain the empty area in the pixel for transparent images. This may cause the optical degradation in the dark state. To solve this problem, we propose hybrid-type transparent OLED display modes that apply a liquid crystal (LC) to the transparent window part of the empty space. In this paper, we applied two dichroic LC modes- which use an electrically controlled birefringence (ECB) mode (Heilmeier type) for the polarized mode and a cholesteric LC mode (Guest-Host mode) for the non-polarized mode-to the empty area. In each hybrid mode, we have observed optical performance, including the transmittance in the dark/bright state, contrast ratio and response time as a function of cell parameters. As a result, we confirmed that the dark state and the contrast ratio could be improved by applying the proposed modes without serious decay of the transmittance in the bright state.

An apparatus for qualitative assessment of the shading ratio of oblique illumination and real-time high-contrast imaging

Oblique illumination imaging can significantly improve the contrast of transparent thin samples. However, in traditional oblique illumination methods, either the condenser is offset or a block is added to the condenser, which makes it complicated and challenged to build a stable oblique illumination imaging. Herein, we present a method to measure the optimal shading ratio of oblique illumination in an inverted microscope, and develop an apparatus for stable high-speed high-contrast imaging with uniform brightness. At optimal shading ratio, the oblique illumination imaging has better imaging quality than differential interference contrast, which characteristic is independent on sample. In oblique illumination with low magnification objective, the images have uneven brightness. According to target brightness, we have developed a brightness unevenness correction algorithm to form uniform background brightness for oblique illumination. Integrating the algorithm with imaging acquisition, corrected oblique illumination microscopy is appropriate to observe living cells with high contrast.

A 3-D Full Convolution Electromagnetic Reconstruction Neural Network (3-D FCERNN) for Fast Super-Resolution Electromagnetic Inversion of Human Brain

Three-dimensional (3-D) super-resolution microwave imaging of human brain is a typical electromagnetic (EM) inverse scattering problem with high contrast. It is a challenge for the traditional schemes based on deterministic or stochastic inversion methods to obtain high contrast and high resolution, and they require huge computational time. In this work, a dual-module 3-D EM inversion scheme based on deep neural network is proposed. The proposed scheme can solve the inverse scattering problems with high contrast and super-resolution in real time and reduce a huge computational cost. In the EM inversion module, a 3-D full convolution EM reconstruction neural network (3-D FCERNN) is proposed to nonlinearly map the measured scattered field to a preliminary image of 3-D electrical parameter distribution of the human brain. The proposed 3-D FCERNN is completely composed of convolution layers, which can greatly save training cost and improve model generalization compared with fully connected networks. Then, the image enhancement module employs a U-Net to further improve the imaging quality from the results of 3-D FCERNN. In addition, a dataset generation strategy based on the human brain features is proposed, which can solve the difficulty of human brain dataset collection and high training cost. The proposed scheme has been confirmed to be effective and accurate in reconstructing the distribution of 3-D super-resolution electrical parameters distribution of human brain through noise-free and noisy examples, while the traditional EM inversion method is difficult to converge in the case of high contrast and strong scatterers. Compared with our previous work, the training of FCERNN is faster and can significantly decrease computational resources.

Recent Trends in Fluorescent Organic Materials for Latent Fingerprint Imaging

Fingerprints are an important kind of material evidence with the key function in personal identification, which are unique and life-long to everyone. Latent (invisible) fingerprints are common at the crime scene, needing to be visualized with proper methods in order to identify sources of the fingerprints in routine forensic practice. Fluorescent imaging of latent fingerprints has the advantage of high contrast, sensitivity, selectivity, and less dependency on instruments. Taking the environment and users' safety into consideration, organic materials for fluorescent imaging of latent fingerprints are reviewed mainly in recent 5 years. New strategies of fingerprint reagents and improved performances established for fingerprint development based on fluorescent organic materials are discussed in the view of forensic practice. In addition, we briefly highlight current challenges of recent fluorescent imaging works based on organic materials for the latent fingerprints development in forensic practice.

Visualizing the Synaptic and Cellular Ultrastructure in Neurons Differentiated from Human Induced Neural Stem Cells-An Optimized Protocol

The size of the synaptic subcomponents falls below the limits of visible light microscopy. Despite new developments in advanced microscopy techniques, the resolution of transmission electron microscopy (TEM) remains unsurpassed. The requirements of tissue preservation are very high, and human post mortem material often does not offer adequate quality. However, new reprogramming techniques that generate human neurons in vitro provide samples that can easily fulfill these requirements. The objective of this study was to identify the culture technique with the best ultrastructural preservation in combination with the best embedding and contrasting technique for visualizing neuronal elements. Two induced neural stem cell lines derived from healthy control subjects underwent differentiation either adherent on glass coverslips, embedded in a droplet of highly concentrated Matrigel, or as a compact neurosphere. Afterward, they were fixed using a combination of glutaraldehyde (GA) and paraformaldehyde (PFA) followed by three approaches (standard stain, Ruthenium red stain, high contrast en-bloc stain) using different combinations of membrane enhancing and contrasting steps before ultrathin sectioning and imaging by TEM. The compact free-floating neurospheres exhibited the best ultrastructural preservation. High-contrast en-bloc stain offered particularly sharp staining of membrane structures and the highest quality visualization of neuronal structures. In conclusion, compact neurospheres growing under free-floating conditions in combination with a high contrast en-bloc staining protocol, offer the optimal preservation and contrast with a particular focus on visualizing membrane structures as required for analyzing synaptic structures.

Dual-beam confocal light-sheet microscopy via flexible acousto-optic deflector

Confocal detection in digital scanned laser light-sheet fluorescence microscopy (DSLM) has been established as a gold standard method to improve image quality. The selective line detection of a complementary metal–oxide–semiconductor camera (CMOS) working in rolling shutter mode allows the rejection of out-of-focus and scattered light, thus reducing background signal during image formation. Most modern CMOS have two rolling shutters, but usually only a single illuminating beam is used, halving the maximum obtainable frame rate. We report on the capability to recover the full image acquisition rate via dual confocal DSLM by using an acousto-optic deflector. Such a simple solution enables us to independently generate, control and synchronize two beams with the two rolling slits on the camera. We show that the doubling of the imaging speed does not affect the confocal detection high contrast.

Quantitative phase microscopy with enhanced contrast and improved resolution through ultra-oblique illumination (UO-QPM)

Recent developments in phase contrast microscopy have enabled the label-free visualization of certain organelles due to their distinct morphological features, making this method an attractive alternative in the study of cellular dynamics. However tubular structures such as endoplasmic reticulum (ER) networks and complex dynamics such as the fusion and fission of mitochondria, due to their low phase contrast, still need fluorescent labeling to be adequately imaged. In this article, we report a quantitative phase microscope with ultra-oblique illumination that enables us to see those structures and their dynamics with high contrast for the first time without labeling. The imaging capability was validated through comparison to the fluorescence images with the same field-of-view. The high image resolution (~270 nm) was validated using both beads and cellular structures. Furthermore, we were able to record the vibration of ER networks at a frame rate of 250 Hz. We additionally show complex cellular processes such as remodeling of the mitochondria networks through fusion and fission and vesicle transportation along the ER without labels. Our high spatial and temporal resolution allowed us to observe mitochondria "spinning", which has not been reported before, further demonstrating the advantages of the proposed method.

Realizing Large-Area Arrays of Semiconducting Fullerene Nanostructures with Direct Laser Interference Patterning

We present a laser interference patterning method for the facile fabrication of large-area and high-contrast arrays of semiconducting fullerene nanostructures, which does not rely on a tedious application of sacrificial photoresists or photomasks. A solution-deposited phenyl-C₆₁-butyric acid methyl ester (PCBM) fullerene thin film is exposed to a spatially modulated illumination intensity, as realized by a two-beam laser interference. The PCBM molecules exposed to strong intensity are photochemically transformed into a low-solubility dimeric state, so that the nontransformed PCBM molecules can be selectively removed in a subsequent solution-based development step. Following brief exposure to green laser light (λ = 532 nm, t = 5 s, p = 0.17 W cm^-2) in the designed two-beam interference setup, and a 1 min development in a tuned acetone-chloroform solution, we realize well-defined and ordered PCBM nanostripe patterns with a fwhm line width of ∼200 nm and a repetition rate of ∼2.900 lines mm^-1 over a large area of 1 cm². We demonstrate that a desired high contrast is effectuated because the initial PCBM-dimer transformation rate is dependent on the square of the illumination intensity. The semiconducting functionality of the patterned fullerene is verified in a field-effect transistor experiment, where a typical PCBM nanostripe featured an electron mobility of 5.3 × 10^-3 cm² V^-1 s^-1 and an on/off ratio of 3 × 10³.

Stable and Photoswitchable Carbon-Dot Liposome

Carbon-dot (C-dot) liposome consisting of several thousands of C-dots shows interesting photoswitching properties. The water-dispersible C-dot liposome possesses intrinsic photoluminescence (PL) and is stable against salt and photoirradiation. The PL of C-dot liposome can be turned off and then on under photoirradiation over the wavelength regions of 510-540 nm and 365-420 nm, respectively. Like reported C-dots, the C-dot liposome emits various colors when excited at different wavelengths. Having great stability and high contrast, images of individual C-dot liposome have been recorded, showing negligible photoblinking. Through a simple photolithographic approach, micropatterns of C-dot liposomes emitting different colors have been fabricated.

Three-photon luminescence of gold nanorods and its applications for high contrast tissue and deep in vivo brain imaging

Gold nanoparticles can be used as contrast agents for bio-imaging applications. Here we studied multi-photon luminescence (MPL) of gold nanorods (GNRs), under the excitation of femtosecond (fs) lasers. GNRs functionalized with polyethylene glycol (PEG) molecules have high chemical and optical stability, and can be used as multi-photon luminescent nanoprobes for deep in vivo imaging of live animals. We have found that the depth of in vivo imaging is dependent upon the transmission and focal capability of the excitation light interacting with the GNRs. Our study focused on the comparison of MPL from GNRs with two different aspect ratios, as well as their ex vivo and in vivo imaging effects under 760 nm and 1000 nm excitation, respectively. Both of these wavelengths were located at an optically transparent window of biological tissue (700-1000 nm). PEGylated GNRs, which were intravenously injected into mice via the tail vein and accumulated in major organs and tumor tissue, showed high image contrast due to distinct three-photon luminescence (3PL) signals upon irradiation of a 1000 nm fs laser. Concerning in vivo mouse brain imaging, the 3PL imaging depth of GNRs under 1000 nm fs excitation could reach 600 μm, which was approximately 170 μm deeper than the two-photon luminescence (2PL) imaging depth of GNRs with a fs excitation of 760 nm.