Microsphere photolithography (MPL) is a fabrication technique that combines the ability to self-assemble arrays of microspheres with the ability of a microsphere to focus light to a photonic jet, in order to create highly ordered nanoscale features in photoresist. This paper presents a model of photoresist exposure with the photonic jet, combining a full-wave electromagnetic model of the microsphere/photoresist interaction with the sequential removal of exposed photoresist by the developer. The model is used to predict the dose curves for the MPL process based on the photoresist thickness, illumination conditions, and development time. After experimental validation, the model provides insight into the process including the resolution, sensitivity, and effects of off-normal illumination. This guides the fabrication of sub-100 nm hole/disk arrays using lift-off, and superposition is shown to predict the geometry for split-ring resonators created using multiple exposures. This model will assist synthesizing fabrication parameters to create large area scalable metasurfaces with sensing and energy management applications.
- Award ID(s):
- Publication Date:
- NSF-PAR ID:
- Journal Name:
- IEEE Access
- Page Range or eLocation-ID:
- Sponsoring Org:
- National Science Foundation
More Like this
Intravital microscopy using multiphoton processes is the standard tool for deep tissue imaging inside of biological specimens. Usually, near-infrared and infrared light is used to excite the sample, which enables imaging several mean free path inside a scattering tissues. Using longer wavelengths, however, increases the width of the effective multiphoton Point Spread Function (PSF). Many features inside of cells and tissues are smaller than the diffraction limit, and therefore not possible to distinguish using a large PSF. Microscopy using high refractive index microspheres has shown promise to increase the numerical aperture of an imaging system and enhance the resolution. It has been shown that microspheres can image features ~λ/7 using single photon process fluorescence. In this work, we investigate resolution enhancement for Second Harmonic Generation (SHG) and 2-photon fluorescence microscopy. We used Barium Titanate glass microspheres with diameters ∼20–30 μm and refractive index ∼1.9–2.1. We show microsphere-assisted SHG imaging in bone collagen fibers. Since bone is a very dense tissue constructed of bundles of collagen fibers, it is nontrivial to image individual fibers. We placed microspheres on a dense area of the mouse cranial bone, and achieved imaging of individual fibers. We found that microsphere assisted SHG imaging resolves featuresmore »
A direct-write configuration of microsphere photolithography (MPL) is investigated for the patterning of IR metasurfaces at large scales. MPL uses a self-assembled hexagonal close-packed array of microspheres as an optical element to generate photonic nanojets within a photoresist layer. The photonic jets can be positioned within the microsphere-defined unit cells by controlling the illumination’s angle of incidence (AOI). This allows the definition of complex antenna elements. A digital micromirror device is used to provide spatial modulation across the microsphere arrays and coordinated with a set of stages providing AOI control. This provides hierarchical patterning at the sub- and super-unit cell levels and is suitable for a range of metasurfaces. The constraints of this approach are analyzed and demonstrated with a polarization-dependent infrared perfect absorber/emitter, which agrees well with modeling.
An optofluidic metasurface for lateral flow-through detection of cancer biomarker (Conference Presentation)The rapid growth of point-of-care tests demands for biomolecule sensors with higher sensitivity and smaller size. We developed an optofluidic metasurface that combined silicon photonics and nanofluidics to achieve a lateral flow-through biosensor to fulfill the needs. The metasurface consists of a 2D array of silicon nanoposts fabricated on a silicon-on-insulator substrate. The device takes advantage of the high-Q resonant modes associated with the optical bound state and the nanofluidic delivery of analyte to overcome the problem of diffusion-limited detection that occurs in almost all conventional biosensors and offer a high refractive index sensitivity. We used rigorous coupled wave analysis and finite element analysis to design and optimize the device. We will present its photonic band diagram to identify the optical bound state and high-Q resonance modes near 1550 nm. The device was fabricated using e-beam lithography followed by a lift-off and reactive ion etching process. Reflectance of the sensor was measured using a tunable laser and a photodetector. The preliminary result shows a refractive index sensitivity of 720 nm/RIU. Furthermore, we implemented the optical metasurface as a lateral flow-through biosensor by covering the nanoposts using a PDMS cover. The nanofluidic channels are formed between the nanoposts for the flowmore »
Residual thickness enhanced core-removed D-shaped single-mode fiber and its application for VOC evaporation monitoring
A core-removed D-shaped structure with different residual thickness (RT) was manufactured on a single mode silica fiber (SMF) to enhance the sensitivity by using of ultra-precise polishing technology. With six different RTs ranging from ∼55 µm to ∼28 µm, the RT enhancement effect in a D-shaped SMF was researched in detail. The influence of the RT on its transmission spectra was investigated both theoretically and experimentally. Considering a compromise between the multimode interference efficiency and optical power loss, an optimum RT value of 34.09 µm was achieved. The obtained refractive index (RI) sensitivity was 10243 nm/RIU in the RI range of 1.430-1.444, corresponding to a RI resolution of 1.9×10−6RIU. A high-performance all-fiber sensor was developed to monitor the evaporation process volatile organic compounds (VOCs) based on the RT-enhanced D-shaped SMF. As proof of concept, a 2-hour continuous monitoring was carried to monitor the chloroform and alcohol mixture. As a result, the evaporation of alcohol and chloroform were clearly identified and monitored. The developed RT-enhanced D-shaped fiber sensor provides an alternative way for chemical process monitoring and industrial applications.