Wavelength-scale lens microscopy via thermal reshaping of colloidal particles

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.

Curvature-Adjustable Polymeric Nanolens Fabrication Using UV-Controlled Nanoimprint Lithography

Nanolenses are gaining importance in nanotechnology, but their challenging fabrication is thwarting their wider adoption. Of particular challenge is facile control of the lens' curvature. In this work, we demonstrate a new nanoimprinting technique capable of realizing polymeric nanolenses in which the nanolens' curvature is optically controlled by the ultraviolet (UV) dose at the pre-curing step. Our results reveal a regime in which the nanolens' height changes linearly with the UV dose. Computational modeling further uncovers that the polymer undergoes highly nonlinear dynamics during the UV-controlled nanoimprinting process. Both the technique and the process model will greatly advance nanoscale science and manufacturing technology.

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.

Colloidal Lithography for Photovoltaics: An Attractive Route for Light Management

The pursuit of ever-more efficient, reliable, and affordable solar cells has pushed the development of nano/micro-technological solutions capable of boosting photovoltaic (PV) performance without significantly increasing costs. One of the most relevant solutions is based on light management via photonic wavelength-sized structures, as these enable pronounced efficiency improvements by reducing reflection and by trapping the light inside the devices. Furthermore, optimized microstructured coatings allow self-cleaning functionality via effective water repulsion, which reduces the accumulation of dust and particles that cause shading. Nevertheless, when it comes to market deployment, nano/micro-patterning strategies can only find application in the PV industry if their integration does not require high additional costs or delays in high-throughput solar cell manufacturing. As such, colloidal lithography (CL) is considered the preferential structuring method for PV, as it is an inexpensive and highly scalable soft-patterning technique allowing nanoscopic precision over indefinitely large areas. Tuning specific parameters, such as the size of colloids, shape, monodispersity, and final arrangement, CL enables the production of various templates/masks for different purposes and applications. This review intends to compile several recent high-profile works on this subject and how they can influence the future of solar electricity.

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.

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.

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.

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.

Unusual imaging properties of superresolution microspheres

We employ a self-assembly method to fabricate dielectric microsphere arrays that can be transferred to any desired positions. The arrays not only enable far-field, broad-band, high-speed, large-area, and wide-angle field of views but also achieve superresolution reaching λ/6.4. We also find that many proposed theories are insufficient to explain the imaging properties; including the achieved superresolution, effects of immersion, and unusual size-dependent magnification. The half-immersed microspheres certainly do not behave like any ordinary solid immersion lenses and new mechanisms must be incorporated to explain their unusual imaging properties.

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

Frequently-used subsurface nano-imaging techniques have limitations in interference, stability, complexity, timeliness and cost reduction on account of the combination of excited ultrasound signal or probed cantilever tip. Though some improved optical methods can directly and visually obtain subsurface nanofeatures, the high refractive index difference (RID) between introduced superlens and subsurface object will inevitably degenerate the image quality. In this paper, a simple and reliable experimental technique is presented to self-assemble spherical cap optical nanoscopy (SCON) subsurface nano-imaging system (SNIS) with two low RID materials. By using SCON-SNIS, subsurface objects with a spacing as small as 0.16 times of illumination wavelength, and involving wider field of views (nearly one-half of SCON's great-circle diameter in the direction of the equator) and deeper depth (several micrometers) can be imaged. In order to get insights into the imaging mechanism, a finite element simulation and a ray-optics analytical study are performed, in which the imaging process is elucidated both theoretically and experimentally. This non-invasive, label-free and real-time subsurface nano-imaging paradigm could be a promising tool in life, material, biology and engineering sciences.

Mechanically modulated dewetting by atomic force microscope for micro- and nano- droplet array fabrication

Organizing a material into well-defined patterns during the dewetting process provides an attractive micro-/nano-fabrication method without using a conventional lithographic process, and hence, offers potential applications in organic electronics, optics systems, and memory devices. We report here how the mechanical modification of polymer surface by an Atomic Force Microscope (AFM) can be used to guide thin film dewetting evolution and break the intrinsic spatial correlation of spontaneous instability. An AFM is used to implement the mechanical modification of progressively narrow grids to investigate the influence of pattern size on the modulation of ultrathin polystyrene films dewetting evolution. For films with different initial thicknesses, when grid size is close to or below the characteristic wavelength of instability, the spinodal dewetting is suppressed, and film rupture is restricted to the cutting trench. We will show in this paper it is possible to generate only one droplet per gridded area on a thin film subsequent to nucleation dominated dewetting on a non-patterned substrate. Furthermore, when the grid periodicity exceeds the spinodal length, the number of droplets in predefined areas gradually approaches that associated with unconfined dewetting.

Nano-imaging enabled via self-assembly

Imaging object details with length scales below approximately 200 nm has been historically difficult for conventional microscope objective lenses because of their inability to resolve features smaller than one-half the optical wavelength. Here we review some of the recent approaches to surpass this limit by harnessing self-assembly as a fabrication mechanism. Self-assembly can be used to form individual nano- and micro-lenses, as well as to form extended arrays of such lenses. These lenses have been shown to enable imaging with resolutions as small as 50 nm half-pitch using visible light, which is well below the Abbe diffraction limit. Furthermore, self-assembled nano-lenses can be used to boost contrast and signal levels from small nano-particles, enabling them to be detected relative to background noise. Finally, alternative nano-imaging applications of self-assembly are discussed, including three-dimensional imaging, enhanced coupling from light-emitting diodes, and the fabrication of contrast agents such as quantum dots and nanoparticles.

Experimental imaging properties of immersion microscale spherical lenses

Using the immersion lensing technique, the resolution of a conventional spherical lens can be improved by a factor of 1/n over its value in air (n, the refractive index of the immersion medium). Depending on the relative position between an object and a lens, either a real or a virtual image is formed. Here we report a new physical phenomenon experimentally observed in the microscale lens imaging. We find that when a microscale spherical lens is semi-immersed in a medium, the resolution of the lens is improved as it can intercept more fine details of the object. However, the microscale lens has two image channels for the fine and coarse details and two images corresponding to the two components can be formed simultaneously. Our findings will advance the understanding of the super-resolution imaging mechanisms in microscale lenses.

Rigorous analytical modeling of high-aperture focusing through a spherical interface

High-aperture focusing through a spherical interface has been employed in optical data storage, photolithography, and especially microscopy. This paper first forms an approximate model, based on geometrical optics and Fourier optics, for evaluating focal fields of the focusing systems. This approximate model helps to clarify some doubts existing in literature. We then propose a rigorous model that is applicable to more general systems. Our model is based on multipole theory, which expands the electromagnetic fields into spherical harmonics.