Microsphere-Based Super-Resolution Imaging for Visualized Nanomanipulation

Advances in Microsphere-Based Super-Resolution Imaging

Techniques to resolve images beyond the diffraction limit of light with a large field of view (FOV) are necessary to foster progress in various fields such as cell and molecular biology, biophysics, and nanotechnology, where nanoscale resolution is crucial for understanding the intricate details of large-scale molecular interactions. Although several means of achieving super-resolutions exist, they are often hindered by factors such as high costs, significant complexity, lengthy processing times, and the classical tradeoff between image resolution and FOV. Microsphere-based super-resolution imaging has emerged as a promising approach to address these limitations. In this review, we delve into the theoretical underpinnings of microsphere-based imaging and the associated photonic nanojet. This is followed by a comprehensive exploration of various microsphere-based imaging techniques, encompassing static imaging, mechanical scanning, optical scanning, and acoustofluidic scanning methodologies. This review concludes with a forward-looking perspective on the potential applications and future scientific directions of this innovative technology.

Metasurface-Coated Liquid Microlens for Super Resolution Imaging

Inspired by metasurfaces' control over light fields, this study created a liquid microlens coated with a layer of Au@TiO2, Core-Shell nanospheres. Utilizing the surface plasmon resonance (SPR) effect of Au@TiO2, Core-Shell nanospheres, and the formation of photonic nanojets (PNJs), this study aimed to extend the imaging system's cutoff frequency, improve microlens focusing, enhance the capture capability of evanescent waves, and utilize nanospheres to improve the conversion of evanescent waves into propagating waves, thus boosting the liquid microlens's super-resolution capabilities. The finite difference time domain (FDTD) method analyzed the impact of parameters including nanosphere size, microlens sample contact width, and droplet's initial contact angle on super-resolution imaging. The results indicate that the full width at half maximum (FWHM) of the field distribution produced by the uncoated microlens is 1.083 times that of the field distribution produced by the Au@TiO2, Core-Shell nanospheres coated microlens. As the nanosphere radius, droplet contact angle, and droplet base diameter increased, the microlens's light intensity correspondingly increased. These findings confirm that metasurface coating enhances the super-resolution capabilities of the microlens.

Acoustic Assembly and Scanning of Superlens Arrays for High-Resolution and Large Field-of-View Bioimaging

High-resolution and dynamic bioimaging is essential in life sciences and biomedical applications. In recent years, microspheres combined with optical microscopes have offered a low cost but promising solution for super-resolution imaging, by breaking the diffraction barrier. However, challenges still exist in precisely and parallelly superlens controlling using a noncontact manner, to meet the demands of large-area scanning imaging for desired targets. This study proposes an acoustic wavefield-based strategy for assembling and manipulating micrometer-scale superlens arrays, in addition to achieving on-demand scanning imaging through phase modulation. In experiments, acoustic pressure nodes are designed to be comparable in size to microspheres, allowing spatially dispersed microspheres to be arranged into arrays with one unit per node. Droplet microlenses with various diameters can be adapted in the array, allowing for a wide range of spacing periods by applying different frequencies. In addition, through the continuous phase shifting in the x and y directions, this acoustic superlens array achieves on-demand moving for the parallel high-resolution virtual image capturing and scanning of nanostructures and biological cell samples. As a comparison, this noncontact and cost-effective acoustic manner can obtain more than ∼100 times the acquisition efficiency of a single lens, holding promise in advancing super-resolution microscopy and subcellular-level bioimaging.

Acoustofluidic scanning fluorescence nanoscopy with a large field of view

Large-field nanoscale fluorescence imaging is invaluable for many applications, such as imaging subcellular structures, visualizing protein interactions, and high-resolution tissue imaging. Unfortunately, conventional fluorescence microscopy requires a trade-off between resolution and field of view due to the nature of the optics used to form the image. To overcome this barrier, we developed an acoustofluidic scanning fluorescence nanoscope that simultaneously achieves superior resolution, a large field of view, and strong fluorescent signals. The acoustofluidic scanning fluorescence nanoscope utilizes the superresolution capabilities of microspheres that are controlled by a programmable acoustofluidic device for rapid fluorescence enhancement and imaging. The acoustofluidic scanning fluorescence nanoscope resolves structures that cannot be resolved with conventional fluorescence microscopes with the same objective lens and enhances the fluorescent signal by a factor of ~5 without altering the field of view of the image. The improved resolution realized with enhanced fluorescent signals and the large field of view achieved via acoustofluidic scanning fluorescence nanoscopy provides a powerful tool for versatile nanoscale fluorescence imaging for researchers in the fields of medicine, biology, biophysics, and biomedical engineering.

Super-resolution imaging based on cascaded microsphere compound lenses

In this paper, a cascaded microsphere compound lens (CMCL) is introduced, in which a 20-µm-diameter barium titanate glass (BTG) primary microsphere and a 250-nm-diameter or 200-nm-diameter polystyrene (PS) secondary microsphere array constitute CMCL1 and CMCL2, respectively. The field of view (FOV) depends on the size of the BTG microsphere, while the waist of the photon nanojet (PNJ) can be adjusted by the size of the PS microsphere. The narrower the waist of the PNJ, the higher the imaging resolution. In the experiment, a 200-nm-diameter hexagonally close-packed PS nanoparticle array is successfully observed by the CMCL with a high magnification of  ∼11.6× and a FOV of  ∼14µm, while the single BTG microsphere is incapable of observing the array. The point spread function is used to quantify the resolution of the CMCL. A well-designed CMCL can improve the imaging performances of a microsphere-assisted microscope.

Lateral Bending of Ag Nanowires toward Controllable Manipulation

Nanowires (NWs) provide opportunities for building high-performance sensors and devices at micro-/nanoscales. Directional movement and assembly of NWs have attracted extensive attention; however, controllable manipulation remains challenging partly due to the lack of understanding on interfacial interactions between NWs and substrates (or contacting probes). In the present study, lateral bending of Ag NWs was investigated under various bending angles and pushing velocities, and the mechanical performance corresponding to microstructures was clarified based on high-resolution transmission electron microscope (HRTRM) detections. The bending-angle-dependent fractures of Ag NWs were detected by an atomic force microscope (AFM) and a scanning electron microscope (SEM), and the fractures occurred when the bending angle was larger than 80°. Compared with an Ag substrate, Ag NWs exhibited a lower system stiffness according to the nanoindentation with an AFM probe. HRTRM observations indicated that there were grain boundaries inside Ag NWs, which would be contributors to the generation of fractures and cracks on Ag NWs during lateral bending and nanoindentation. This study provides a guide to controllably manipulate NWs and fabricate high-performance micro-/nanodevices.

Super-Resolution Fluorescence Imaging for Semiconductor Nanoscale Metrology and Inspection

The increase in the number and complexity of process levels in semiconductor production has driven the need for the development of new measurement methods that can evaluate semiconductor devices at the critical dimensions of fine patterns and simultaneously inspect nanoscale contaminants or defects. However, conventional optical inspection methods often fail to resolve device patterns or defects at the level of tens of nanometers required for device development owing to their diffraction-limited resolutions. In this study, we used the stochastic optical reconstruction microscopy (STORM) technique to image semiconductor nanostructures with feature sizes as small as 30 nm and detect individual 20 nm-diameter contaminants. STORM imaging of semiconductor nanopatterns is based on the development of a selective labeling method of fluorophores for a negative silicon oxide surface using the charge interaction of positive polyethylenimine molecules. This study demonstrates the potential of STORM for nanoscale metrology and in-line defect inspection of semiconductor integrated circuits.

Addressing the imaging limitations of a microsphere-assisted nanoscope

In the past decade, microsphere-assisted nanoscopy has been developed rapidly to overcome the diffraction limit. However, due to the limited size and high surface curvature of microspheres, the magnified imaging still suffers from problems like limited view scope, imaging distortion, and low contrast. In this paper, we specialize in the imaging mechanism of microspheres and find irradiance as the key factor for microsphere imaging quality. Utilizing a modified optical tweezer system, we achieve precise manipulation of microspheres and further propose a high-quality large-field magnified imaging scheme. The results show that the imaging area of 5 µm microspheres can reach 16×12 µm² with the minimum identifiable feature of 137 nm. This scheme provides a new solution for extending the measuring scope of microsphere-assisted nanoscope, and will certainly promote the application of this technology in practice.

Microlens-assisted microscopy for biology and medicine

The addition of dielectric transparent microlens in the optical scheme is an effective and at the same time simple and inexpensive way to increase the resolution of a light microscope. For these purposes, spherical and cylindrical microlenses with a diameter of 1-100 μm are usually used. The microlens focuses the light into a narrow beam called a photonic nanojet. An enlarged virtual image is formed, which is captured by the objective of the light microscope. In addition to microscopy, the microlenses are successfully applied to amplify optical signals, increase the trapping force of optical tweezers and are used in microsurgery. This review considers the design and principle of microlens-assisted microscopes. Taking into account the advantages of the super-resolution optical methods for research in life science, the examples of the use of the microlenses in biomedical practice are discussed in detail.

Bilayer-film-decorated microsphere with suppressed interface reflection for enhanced nano-imaging

Microspheres as special optical lenses have extensive applications due to their super-focusing ability and outstanding resolving power on imaging. The interface reflection between the microsphere and sample surface significantly affects nano-imaging as exhibited in the form of the Newton's rings pattern in virtual images. In this work, a new scheme of decorating the microsphere with a dielectric bilayer thin film is proposed to suppress the interface reflection and thus enhance the imaging performance. The particle swarm optimization algorithm is performed with a full-wave simulation to refine the bilayer thin film decorated microsphere design, which is successfully realized via a novel fabrication strategy. Experimental imaging results demonstrate that the Newton's rings pattern in virtual images is substantially diminished. Both the imaging contrast and effective field-of-view of the microsphere nano-imaging are improved via this effective light manipulation scheme, which is also applicable to promoting the performance of the microsphere in other optical applications.

Microsphere Photonic Superlens for a Highly Emissive Flexible Upconversion-Nanoparticle-Embedded Film

Increasing upconversion luminescence (UCL) to overcome the intrinsically low conversion efficiency of upconversion nanoparticles (UCNPs) poses a fundamental challenge. Photonic nanostructures are the efficient approaches for UCL enhancement by tailoring the local electromagnetic fields. Unfortunately, such nanostructures are sensitive to environmental conditions, and the regulation strength is varied in flexible applications. Here, we report giant UCL enhancement from a flexible UCNP-embedded film coupled with a microsphere photonic superlens (MPS), by which the enhancement ratio of UCL is over 10⁴-fold under 808 nm excitation down to 0.72 mW. The enhancement pathways of MPS-enhanced UCL are attributed to Mie-resonant nanofocusing for high excitation-photon density, optical whispering-gallery modes (WGMs) for fast radiative decay, and the directional antenna effect for far-field emission confinement. The contribution of optical resonance in the MPS to suppressing the phonon-induced nonradiative transition and thermal quenching is experimentally validated. The UCL quantum yield is therefore improved by 3-fold to 4.20% under 120 mW/cm² near-infrared excitation, consistent with the enhancement ratio via the Purcell effect of WGMs. Furthermore, the MPS demonstrates the robust optical regulation capability toward flexible applications, opening up new opportunities for facilitating multiphoton upconversion in wearable optoelectrical devices for nanoimaging, biosensing, and energy conversion in the future.

Correlative AFM and Scanning Microlens Microscopy for Time-Efficient Multiscale Imaging

With the rapid evolution of microelectronics and nanofabrication technologies, the feature sizes of large-scale integrated circuits continue to move toward the nanoscale. There is a strong need to improve the quality and efficiency of integrated circuit inspection, but it remains a great challenge to provide both rapid imaging and circuit node-level high-resolution images simultaneously using a conventional microscope. This paper proposes a nondestructive, high-throughput, multiscale correlation imaging method that combines atomic force microscopy (AFM) with microlens-based scanning optical microscopy. In this method, a microlens is coupled to the end of the AFM cantilever and the sample-facing side of the microlens contains a focused ion beam deposited tip which serves as the AFM scanning probe. The introduction of a microlens improves the imaging resolution of the AFM optical system, providing a 3-4× increase in optical imaging magnification while the scanning imaging throughput is improved ≈8×. The proposed method bridges the resolution gap between traditional optical imaging and AFM, achieves cross-scale rapid imaging with micrometer to nanometer resolution, and improves the efficiency of AFM-based large-scale imaging and detection. Simultaneously, nanoscale-level correlation between the acquired optical image and structure information is enabled by the method, providing a powerful tool for semiconductor device inspection.

Simulation and experimental observations of axial position control of a photonic nanojet by a dielectric cube with a metal screen

In this Letter, we report on a numerical study, fabrication, and experimental observations of photonic nanojet (PNJ) shaping by control of a tangential electric field component. Here the PNJs are generated by a single mesoscale micro-cube that is fabricated from polydimethylsiloxane, deposited on a silicon substrate and placed on thick metal screen at illuminating wavelengths of 405, 532, and 671 nm. It is shown that the length, focal length, and width of the PNJ can be significantly reduced in the presence of the metal masks along the side faces of the micro-cube. Experimental measurements of the PNJ imaging are performed by a scanning optical microscope with laser sources. Our experimental results are in reasonable agreement with simulation predictions of the finite-difference time-domain method. Due to the appearance of the metal masks, the PNJ focal length decreases 1.5 times, the PNJ decay length decreases 1.7 times, and the PNJ resolution increases 1.2 times. Such PNJs possess great potential in complex manipulation, including integrated plasmonic circuits, biosensing, and optical tweezers.

In Situ Electrohydrodynamic Jet Printing-Based Fabrication of Tunable Microlens Arrays

Tunable microlens arrays (MLAs) with controllable focal lengths have been extensively used in optical sensors, biochips, and electronic devices. The commonly used method is electrowetting on dielectric (EWOD) that controls the contact angle of the microlens to adjust the focal length. However, the fabrication of tunable MLAs at the microscale remains a challenge because the size of MLAs is limited by the external electrodes of EWOD. In this study, a highly integrated planar annular microelectrode array was proposed to achieve an electrowetting tunable MLA. The planar microelectrode was fabricated by electrohydrodynamic jet (E-jet) printing and the liquid microlens was then deposited in situ on the microelectrode. This method could realize 36 tunable liquid microlenses with an average diameter of 24 μm in a 320 × 320 μm² plane. The fabricated tunable MLAs with higher integration levels and smaller sizes can be beneficial for cell imaging, optofluidic systems, and microfluidic chips.

Direct observation Brownian motion of individual nanoparticles in water using microsphere-assisted microscopy

Observing Brownian motion of nanoscale objects through a traditional optical microscope is still a challenge. Here, we present a method to overcome this challenge by using a traditional optical microscope assisted with a removable microsphere-embedded thin film. The diffusion coefficient of individual unconstrained polystyrene (PS) nanoparticles with a diameter of 300 nm in water is calculated from their respective mean-square displacement versus time curves, and the measured diffusion coefficient shows good agreement with the theoretical Stokes-Einstein one, proving the feasibility of our method. In addition, the experimental results show that the movement of the PS nanoparticles is slowed down near a plane wall, and the diffusion coefficient is consistent with the theoretical constrained diffusion coefficient, which shows that our method can also study the constrained Brownian motion of nanoparticles constrained near a plane wall. Our research results are helpful for the application of microsphere-assisted microscopy in new fields and also provide a new method for nanoparticle tracking.

A Closer Look at Photonic Nanojets in Reflection Mode: Control of Standing Wave Modulation

The photonic nanojet phenomenon is commonly used both to increase the resolution of optical microscopes and to trap nanoparticles. However, such photonic nanojets are not applicable to an entire class of objects. Here we present a new type of photonic nanojet in reflection mode with the possibility to control the modulation of the photonic nanojet by a standing wave. In contrast to the known kinds of reflective photonic nanojets, the reported one occurs when the aluminum oxide hemisphere is located at a certain distance from the substrate. Under illumination, the hemisphere generates a primary photonic nanojet directed to the substrate. After reflection, the primary nanojet acts as an illumination source for the hemisphere, leading to the formation of a new reflective photonic nanojet. We show that the distance between the hemisphere and substrate affects the phase of both incident and reflected radiation, and due to constructive interference, the modulation of the reflective photonic nanojet by a standing wave can be significantly reduced. The results obtained contribute to the understanding of the processes of photonic nanojet formation in reflection mode and open new pathways for designing functional optical devices.