Super-Resolution Imaging with Direct Laser Writing-Printed Microstructures

Edge smoothing optimization method in DMD digital lithography system based on dynamic blur matching pixel overlap technique

Due to digital micromirrors device (DMD) digital lithography limited by non-integer pixel errors, the edge smoothness of the exposed image is low and the sawtooth defects are obvious. To improve the image edge smoothness, an optimized pixel overlay method was proposed, which called the DMD digital lithography based on dynamic blur effect matching pixel overlay technology. The core of this method is that motion blur effect is cleverly introduced in the process of pixel overlap to carry out the lithography optimization experiment. The simulation and experimental results showed that the sawtooth edge was reduced from 1.666 µm to 0.27 µm by adopting the 1/2 dynamic blur effect to match pixel displacement superposition, which is far less than half of the sawtooth edge before optimization. The results indicated that the proposed method can efficiently improve the edge smoothness of lithographic patterns. We believe that the proposed optimization method can provide great help for high fidelity and efficient DMD digital lithography microfabrication.

Maskless lithography for large area patterning of three-dimensional microstructures with application on a light guiding plate

This paper presents a maskless lithography system that can perform three-dimensional (3D) ultraviolet (UV) patterning on a photoresist (PR) layer. After PR developing processes, patterned 3D PR microstructures over a large area are obtained. This maskless lithography system utilizes an UV light source, a digital micromirror device (DMD), and an image projection lens to project a digital UV image on the PR layer. The projected UV image is then mechanically scanned over the PR layer. An UV patterning scheme based on the idea of obliquely scanning and step strobe lighting (OS³L) is developed to precisely control the spatial distribution of projected UV dose, such that desired 3D PR microstructures can be obtained after PR development. Two types of concave microstructures with truncated conical and nuzzle-shaped cross-sectional profiles are experimentally obtained over a patterning area of 160 ×115 mm². These patterned microstructures are then used for replicating nickel molds and for mass-production of light-guiding plates used in back-lighting and display industry. Potential improvements and advancements of the proposed 3D maskless lithography technique for future applications will be addressed.

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.

Efficient fabrication method for non-periodic microstructures using one-step two-photon lithography and a metal lift-off process

This paper presents a mask-less, flexible, efficient, and high-resolution fabrication method for non-periodic microstructures. Sub-wavelength micro-polarizer arrays, (MPAs) which are the most essential part of the focal plane polarimeters, are typical non-periodic structures. The grating ridges of each polarizer were oriented in four different directions offset by 45°, corresponding to different polarization directions. The finite element method was introduced to optimize the structural parameters of the MPA in the far-infrared region. The numerical results demonstrated that the designed MPA had a TM transmittance of more than 55% and an extinction ratio no less than 7 dB. An aluminum MPA that operates in the 8-14 µm infrared region was prepared by one-step two-photon lithography (TPL) and the metal lift-off process. The femtosecond laser exposed the photoresist with only a single scan, making TPL very efficient. The fabricated single-layer sub-wavelength MPAs with a period of 3 µm, a duty cycle of 0.35-0.5, and a height of 150 nm, were analyzed by an optical microscope and an atomic force microscope. The successful fabrication of the MPA indicated that one-step TPL could be a viable and efficient method for pattern preparation in the fabrication of non-periodic microstructures.

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.

Photonic Nanojet Generation Using Integrated Silicon Photonic Chip with Hemispherical Structures

Photonic nanojet (PNJ) is a tightly focused diffractionless travelling beam generated by dielectric microparticles. The location of the PNJ depends on the refractive index of the material and it usually recedes to the interior of the microparticle when the refractive index is higher than 2, making high index materials unsuitable to produce useful PNJs while high index favours narrower PNJs. Here we demonstrate a design of CMOS compatible high index on-chip photonic nanojet based on silicon. The proposed design consists of a silicon hemisphere on a silicon substrate. The PNJs generated can be tuned by changing the radius and sphericity of the hemisphere. Oblate spheroids generate PNJs further away from the refracting surface and the PNJ length exceeds 17λ when the sphericity of the spheroid is 2.25 The proposed device can have potential applications in focal plane arrays, enhanced Raman spectroscopy, and optofluidic chips.