Subsurface nano-imaging with self-assembled spherical cap optical nanoscopy

Sub-50 nm optical imaging in ambient air with 10× objective lens enabled by hyper-hemi-microsphere

Optical microsphere nanoscope has great potential in the inspection of integrated circuit chips for semiconductor industry and morphological characterization in biology due to its superior resolving power and label-free characteristics. However, its resolution in ambient air is restricted by the magnification and numerical aperture (NA) of microsphere. High magnification objective lens is required to be coupled with microsphere for nano-imaging beyond the diffraction limit. To overcome these challenges, in this work, high refractive index hyper-hemi-microspheres with tunable magnification up to 10× are proposed and realized by accurately tailoring their thickness with focused ion beam (FIB) milling. The effective refractive index is put forward to guide the design of hyper-hemi-microspheres. Experiments demonstrate that the imaging resolution and contrast of a hyper-hemi-microsphere with a higher magnification and larger NA excel those of a microsphere in air. Besides, the hyper-hemi-microsphere could resolve ~50 nm feature with higher image fidelity and contrast compared with liquid immersed high refractive index microspheres. With a hyper-hemi-microsphere composed microscale compound lens configuration, sub-50 nm optical imaging in ambient air is realized by only coupling with a 10× objective lens (NA = 0.3), which enhances a conventional microscope imaging power about an order of magnitude.

Realization of noncontact confocal optical microsphere imaging microscope

A novel noncontact confocal optical microsphere imaging microscope (COMIM) is proposed to achieve high contrast optical nano-imaging at a high scanning speed. The developed setup obtains images with significantly higher contrast when compared with conventional bright-field microsphere nano-imaging, as well as with a commercial confocal laser scanning microscope (CLSM). With the image-by-image scanning scheme, COMIM takes only 11% time to complete a three-dimensional (3D) scanning image than using a commercial CLSM, while still offers an outstanding resolving power for the nano-features on the sample.

Super-Resolution Imaging with Direct Laser Writing-Printed Microstructures

Dielectric microstructures coupled with a conventional optical microscope have been proven to be a successful way to achieve super-resolution imaging. However, a limitation of such super-resolution imaging is the microstructure fabrication ability, which generally provides natural structures (such as spherical, hemispherical, superhemispherical microlenses, and so on). Meanwhile, the influences of microstructures with complex shapes on the super-resolved imaging still remain unknown. In this paper, direct laser writing (DLW) lithography is used to produce a series of complex microstructures, which are capable of achieving super-resolution imaging in the optical far-field region. Cylinder, truncated cone, hemisphere, and protruding hemisphere microstructures are successfully fabricated by this 3D printing technology, allowing us to resolve features as small as 100 nm under classical microscopy. Moreover, different microstructures lead to different photonic nanojet (PNJ) illuminations and collection efficiencies, resulting in a critical role in super-resolved imaging. The microstructures with spherical surfaces can easily collect the light scattered by the object and convert the high-spatial-frequency evanescent waves into propagating waves.

Single nanoparticle detection using a photonic nanojet

A novel method of detecting single nanoparticles (NPs) in a microfluidic channel directly using a photonic nanojet (PNJ) was investigated. The theoretical model comprised a plane wave-illuminated, liquid-filled hollow-microcylinder (LFHM) and a single Au NP. Relevant studies were implemented and demonstrated with a finite element method (FEM)-based numerical simulation and explained physically through a ray-optics theoretical analysis with the assistance of energy flow line shifts. When depicting the optical-field distribution by gradually altered contour lines for LFHMs with or without a single Au NP, the outward distances of the specific points on the right end of each contour line, for a LFHM with a single Au NP relative to a LFHM without a NP, increased exponentially with decreasing contour levels. By dividing the contour levels into ten levels, the detectable NP of size of a few nanometers can be reflected through the outward distance of the contour points. The key parameters of the PNJ (the maximum light intensity, decay length and lateral beam waist), combined with the electric field distribution and focal point offset, can provide information on NP location. This work showed the PNJ itself to be a powerful and promising tool for the detection and identification of single NPs.

Identification of nanoparticles and nanosystems in biological matrices with scanning probe microscopy

Identification of nanoparticles and nanosystems into cells and biological matrices is a hot research topic in nanobiotechnologies. Because of their capability to map physical properties (mechanical, electric, magnetic, chemical, or optical), several scanning probe microscopy based techniques have been proposed for the subsurface detection of nanomaterials in biological systems. In particular, atomic force microscopy (AFM) can be used to reveal stiff nanoparticles in cells and other soft biomaterials by probing the sample mechanical properties through the acquisition of local indentation curves or through the combination of ultrasound-based methods, like contact resonance AFM (CR-AFM) or scanning near field ultrasound holography. Magnetic force microscopy can detect magnetic nanoparticles and other magnetic (bio)materials in nonmagnetic biological samples, while electric force microscopy, conductive AFM, and Kelvin probe force microscopy can reveal buried nanomaterials on the basis of the differences between their electric properties and those of the surrounding matrices. Finally, scanning near field optical microscopy and tip-enhanced Raman spectroscopy can visualize buried nanostructures on the basis of their optical and chemical properties. Despite at a still early stage, these methods are promising for detection of nanomaterials in biological systems as they could be truly noninvasive, would not require destructive and time-consuming specific sample preparation, could be performed in vitro, on alive samples and in water or physiological environment, and by continuously imaging the same sample could be used to dynamically monitor the diffusion paths and interaction mechanisms of nanomaterials into cells and biological systems. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Photonic jets for highly efficient mid-IR focal plane arrays with large angle-of-view

One of the trends in design of mid-wave infrared (MWIR) focal plane arrays (FPAs) consists in reduction of the pixel sizes which allows increasing the resolution and decreasing the dark currents of FPAs. To keep high light collection efficiency and to combine it with large angle-of-view (AOV) of FPAs, in this work we propose to use photonic jets produced by the dielectric microspheres for focusing and highly efficient coupling light into individual photodetector mesas. In this approach, each pixel of FPA is integrated with the appropriately designed, fixed and properly aligned microsphere. The tasks consist in developing technology of integration of microspheres with pixels on a massive scale and in developing designs of corresponding structures. We propose to use air suction through a microhole array for assembling ordered arrays of microspheres. We demonstrate that this technology allows obtaining large-scale arrays containing thousands of microspheres with ~1% defect rate which represents a clear advantage over the best results obtained by the techniques of directed self-assembly. We optimized the designs of such FPAs integrated with microspheres for achieving maximal angle of view (AOV) as a function of the index of refraction and diameter of the microspheres. Using simplified two-dimensional finite difference time domain (FDTD) modeling we designed structures where the microspheres are partly-immersed in a layer of photoresist or slightly truncated by using controllable temperature melting effects. Compared to the standard microlens arrays, our designs provide up to an order of magnitude higher AOVs reaching ~8° for back-illuminated and ~20° for front-illuminated structures.

Nanoscale subsurface imaging

The ability to probe structures and functional properties of complex systems at the nanoscale, both at their surface and in their volume, has drawn substantial attention in recent years. Besides detecting heterogeneities, cracks and defects below the surface, more advanced explorations of chemical or electrical properties are of great interest. In this article, we review some approaches developed to explore heterogeneities below the surface, including recent progress in the different aspects of metrology in optics, electron microscopy, and scanning probe microscopy. We discuss the principle and mechanisms of image formation associated with each technique, including data acquisition, data analysis and modeling for nanoscale structural and functional imaging. We highlight the advances based on atomic force microscopy (AFM). Our discussion first introduces methods providing structural information of the buried structures, such as position in the volume and geometry. Next we present how functional properties including conductivity, capacitance, and composition can be extracted from the modalities available to date and how they could eventually enable tomography reconstructions of systems such as overlay structures in transistors or living systems. Finally we propose a perspective regarding the outstanding challenges and needs to push the field forward.

Superlensing microscope objective lens

Conventional microscope objective lenses are diffraction limited; they cannot resolve subdiffraction features of a size smaller than 250-300 nm under white lighting condition. New innovations are required to overcome this limitation. In this paper, we propose and demonstrate a new superlensing objective lens that possesses a resolution of 100 nm, which is a two-times resolution improvement over conventional objectives. This is accomplished by integrating a conventional microscope objective lens with a superlensing microsphere lens using a customized lens adaptor. The new objective lens was successfully demonstrated for label-free super-resolution imaging of 100 nm features in engineering and biological samples, including a Blu-ray disk sample and adenoviruses. Our work opens a new door to develop a generic optical superlens, which may transform the field of optical microscopy and imaging.

Spatial control of photonic nanojets

For a long time, light focusing from microspheres has been intensively researched. The microsphere has been shown to be capable of generating a high intensity beam with sub-wavelength width, known as a photonic nanojet (PNJ). In this article, we present a detailed report on the properties of a new asymmetrical microstructure, consisting of a supporting stage and a spherical cap, and demonstrate precise engineering of the PNJ characteristics by simply selecting its geometrical dimensions. More importantly, we find that a single asymmetrical microstructure can generate an ultra-elongated PNJ on the shadow side and the cascade of two asymmetrical elements can generate a PNJ with a full width at half maximum (FWHM) waist down to 0.27λ. In addition, because of the presence of energy convergence regions within the second element, an ultra-narrow PNJ can be generated even when the length of the second element in the cascade is many orders of magnitude greater than the wavelength or deviates somewhat from the optimal dimensions. This offers design flexibility and manufacturing tolerance, which has not been demonstrated in the conventional microsphere design or its derivatives. We believe that these remarkable performance features make the asymmetrical structure and its cascade attractive in numerous applications.

Super resolution from pure/hybrid nanoscale solid immersion lenses under dark-field illumination

We report a super-resolution strategy based on pure/hybrid nanoscale solid immersion lenses (n-SILs) under dark-field illumination. Dependences of the focusing performance of pure n-SILs on different diameters, locations, and substrates are investigated. Simulation results demonstrate a higher resolution (up to a 22.8% improvement) of pure n-SILs under dark-field illumination than convergent plane wave illumination. A hybrid n-SIL with a higher-index nanorod embedded into the n-SIL is proposed. Under dark-field illumination, the hybrid n-SIL can generate a near-field focal spot with a much higher resolution (~λ/8) that is beyond the Abbe diffraction limit in the nanorod. These results have potential applications in data storage, near-field mapping and spectra, and nanoscale lithography.

Polymer Colloidal Sphere-Based Hybrid Solid Immersion Lens for Optical Super-resolution Imaging

The optical microscope is a widely used real-time investigation tool, but usually suffers from low resolution due to the Abbe diffraction limit. Herein, we design and successfully synthesize ZrO₂/polymer hybrid colloidal microspheres with as high as 47.5 wt % inorganic nanoparticles by suspension polymerization of 9,9'-bis[4-(2-acryloyloxyethyloxy)phenyl]fluorene (BAEPF). Owing to the homogeneous dispersion, high density, and high refractive index of inorganic nanoparticles and deformability of polymers, the obtained ZrO₂/poly(BAEPF) hybrid colloidal microspheres have a high refractive index, optical transparency, and controllable curvature and thus can be directly used as a hybrid solid immersion lens (hSIL) for the optical microscope, achieving super-resolution imaging of 50 nm and even 45 nm under a standard white light or blue light optical microscope, which is far beyond the diffraction limit for visible light optical microscopes. Our hSIL design concept and strategy demonstrate efficient, fast, and solid practical potentials for optical super-resolution imaging and may also create another application possibility for polymer colloidal spheres.