Visible to near-infrared single pixel microspectrometer using electrothermal MEMS grating.

Compact spectrometers facilitate non-destructive and point-of-care spectral analysis. Here we report a single-pixel microspectrometer (SPM) for visible to near-infrared (VIS-NIR) spectroscopy using MEMS diffraction grating. The SPM consists of slits, electrothermally rotating diffraction grating, spherical mirror, and photodiode. The spherical mirror collimates an incident beam and focuses the beam on the exit slit. The photodiode detects spectral signals dispersed by electrothermally rotating diffraction grating. The SPM was fully packaged within 1.7 cm3 and provides a spectral response range of 405 nm to 810 nm with an average 2.2 nm spectral resolution. This optical module provides an opportunity for diverse mobile spectroscopic applications such as healthcare monitoring, product screening, or non-destructive inspection.

Microlens array camera with variable apertures for single-shot high dynamic range (HDR) imaging.

We report a microlens array camera with variable apertures (MACVA) for high dynamic range (HDR) imaging by using microlens arrays with various sizes of apertures. The MACVA comprises variable apertures, microlens arrays, gap spacers, and a CMOS image sensor. The microlenses with variable apertures capture low dynamic range (LDR) images with different f-stops under single-shot exposure. The reconstructed HDR images clearly exhibit expanded dynamic ranges surpassing LDR images as well as high resolution without motion artifacts, comparable to the maximum MTF50 value observed among the LDR images. This compact camera provides, what we believe to be, a new perspective for various machine vision or mobile devices applications.

Ultrathin arrayed camera for high-contrast near-infrared imaging.

We report an ultrathin arrayed camera (UAC) for high-contrast near infrared (NIR) imaging by using microlens arrays with a multilayered light absorber. The UAC consists of a multilayered composite light absorber, inverted microlenses, gap-alumina spacers and a planar CMOS image sensor. The multilayered light absorber was fabricated through lift-off and repeated photolithography processes. The experimental results demonstrate that the image contrast is increased by 4.48 times and the MTF 50 is increased by 2.03 times by eliminating optical noise between microlenses through the light absorber. The NIR imaging of UAC successfully allows distinguishing the security strip of authentic bill and the blood vessel of finger. The ultrathin camera offers a new route for diverse applications in biometric, surveillance, and biomedical imaging.

Lissajous scanned variable structured illumination for dynamic stereo depth map.

Structured illumination plays an important role in advanced photographic and microscopic imaging applications. Here we report variable structured illumination (VSI) using Lissajous scanning techniques. The variable structured illumination module comprises Lissajous scanning micromirror and fiber-based diode pumped solid state (DPSS) laser with intensity modulation, combined with a stereo camera for dynamic stereo depth map. The micromirror projects static and discrete patterns by modulating the intensity of a laser beam at the least common multiple (LCM) of two scanning frequencies. The pattern density is increased by either decreasing the greatest common divisor (GCD) of scanning frequencies or decreasing the duty rate of the laser modulation. The scanning amplitude also controls the field-of-view (FOV) for the exact illumination of a target object for dynamic stereo depth map. The variable structured illumination module provides a new route for advanced imaging applications such as high-quality depth map, super-resolution, or motion recognition.

Multifocal microlens arrays using multilayer photolithography.

We report a new microfabrication method of multifocal microlens arrays (MF-MLAs) for extended depth-of-field (DoF) using multilayer photolithography and thermal reflow. Microlenses of different focal lengths were simultaneously fabricated on a single glass wafer by using repeated photolithography with multiple photomasks to define microposts of different thicknesses and concurrent thermal reflow of multi-stacked microposts. The diverse lens curvatures of MF-MLAs are precisely controlled by the thickness of the micropost. Hexagonally packaged MF-MLAs clearly show three different focal lengths of 249 µm, 310 µm, and 460 µm for 200 µm in lens diameter and result in multifocal images on a single image sensor. This method provides a new route for developing various three-dimensional (3D) imaging applications such as light-field cameras or 3D medical endoscopes.

Solvent-Free, Single Lithium-Ion Conducting Covalent Organic Frameworks.

Porous crystalline materials such as covalent organic frameworks and metal-organic frameworks have garnered considerable attention as promising ion conducting media. However, most of them additionally incorporate lithium salts and/or solvents inside the pores of frameworks, thus failing to realize solid-state single lithium-ion conduction behavior. Herein, we demonstrate a lithium sulfonated covalent organic framework (denoted as TpPa-SO3Li) as a new class of solvent-free, single lithium-ion conductors. Benefiting from well-designed directional ion channels, a high number density of lithium-ions, and covalently tethered anion groups, TpPa-SO3Li exhibits an ionic conductivity of 2.7 × 10-5 S cm-1 with a lithium-ion transference number of 0.9 at room temperature and an activation energy of 0.18 eV without additionally incorporating lithium salts and organic solvents. Such unusual ion transport phenomena of TpPa-SO3Li allow reversible and stable lithium plating/stripping on lithium metal electrodes, demonstrating its potential use for lithium metal electrodes.

1.65 mm diameter forward-viewing confocal endomicroscopic catheter using a flip-chip bonded electrothermal MEMS fiber scanner.

We report a 1.65 mm diameter forward-viewing confocal endomicroscopic catheter using a flip-chip bonded electrothermal MEMS fiber scanner. Lissajous scanning was implemented by the electrothermal MEMS fiber scanner. The Lissajous scanned MEMS fiber scanner was precisely fabricated to facilitate flip-chip connection, and bonded with a printed circuit board. The scanner was successfully combined with a fiber-based confocal imaging system. A two-dimensional reflectance image of the metal pattern 'OPTICS' was successfully obtained with the scanner. The flip-chip bonded scanner minimizes electrical packaging dimensions. The inner diameter of the flip-chip bonded MEMS fiber scanner is 1.3 mm. The flip-chip bonded MEMS fiber scanner is fully packaged with a 1.65 mm diameter housing tube, 1 mm diameter GRIN lens, and a single mode optical fiber. The packaged confocal endomicroscopic catheter can provide a new breakthrough for diverse in-vivo endomicroscopic applications.

Antireflective glass nanoholes on optical lenses.

Antireflective structures, inspired from moth eyes, are still reserved for practical use due to their large-area nanofabrication and mechanical stability. Here we report an antireflective optical lens with large-area glass nanoholes. The nanoholes increase light transmission due to the antireflective effect, depending on geometric parameters such as fill factor and height. The glass nanoholes of low effective refractive index are achieved by using solid-state dewetting of ultrathin silver film, reactive ion etching, and wet etching. An ultrathin silver film is transformed into nanoholes for an etch mask in reactive ion etching after thermal annealing at a low temperature. Unlike conventional nanopillars, nanoholes exhibit high light transmittance with enhancement of ~4% over the full visible range as well as high mechanical hardness. Also, an antireflective glass lens is achieved by directly employing nanoholes on the lens surface. Glass nanoholes of highly enhanced optical and mechanical performance can be directly utilized for commercial glass lenses in various imaging and lighting applications.

Batch fabrication of functional optical elements on a fiber facet using DMD based maskless lithography.

We report a facile and direct fabrication method for integrating functional optical microstructures on the top surface of an optical fiber. A programmable maskless fabrication system was developed by using digital micromirror device (DMD), which allows rapid prototyping and low-cost fabrication without physical photomask. This maskless UV exposure system has the spatial resolution of 2.2 µm for an exposed area of 245 µm x 185 µm. Diverse optical microstructures were photolithographically defined on multimode fibers and a single mode optical fiber serially spliced with a coreless silica fiber segment. This method provides a new route for developing compact functional fiber-optic applications such as laser scanning, biosensing, or laser endomicroscopy.

Biologically Inspired Organic Light-Emitting Diodes.

Many animal species employ highly conspicuous traits as courtship signals for successful mating. Fireflies utilize their bioluminescent light as visual courtship signals. In addition to efficient bioluminescent light emission, the structural components of the firefly lantern also contribute to the enhancement of conspicuous optical signaling. Recently, these firefly lantern ultrastructures have attracted much interest and inspired highly efficient light management approaches. Here we report on the unique optical function of the hierarchical ultrastructures found in a firefly (Pyrocoelia rufa) and their biological inspiration of highly efficient organic light-emitting diode (OLED) applications. The hierarchical structures are comprised of longitudinal nanostructures and asymmetric microstructures, which were successfully replicated using geometry-guided resist reflow, replica molding, and polydimethylsiloxane (PDMS) oxidation. The external quantum efficiency (EQE) of the bioinspired OLEDs was enhanced by up to 61%. The bioinspired OLEDs clearly showed side-enhanced super-Lambertian emission with a wide-viewing angle. The highly efficient light extraction and wide-angle illumination suggest how the hierarchical structures likely improve the recognition of firefly optical courtship signals over a wide-angle range. At the same time, the biologically inspired designs provide a new paradigm for designing functional optical surfaces for lighting or display applications.

Electrothermal MEMS fiber scanner for optical endomicroscopy.

We report a novel MEMS fiber scanner with an electrothermal silicon microactuator and a directly mounted optical fiber. The microactuator comprises double hot arm and cold arm structures with a linking bridge and an optical fiber is aligned along a silicon fiber groove. The unique feature induces separation of resonant scanning frequencies of a single optical fiber in lateral and vertical directions, which realizes Lissajous scanning during the resonant motion. The footprint dimension of microactuator is 1.28 x 7 x 0.44 mm3. The resonant scanning frequencies of a 20 mm long optical fiber are 239.4 Hz and 218.4 Hz in lateral and vertical directions, respectively. The full scanned area indicates 451 µm x 558 µm under a 16 Vpp pulse train. This novel laser scanner can provide many opportunities for laser scanning endomicroscopic applications.

Electrothermal MEMS parallel plate rotation for single-imager stereoscopic endoscopes.

This work reports electrothermal MEMS parallel plate-rotation (PPR) for a single-imager based stereoscopic endoscope. A thin optical plate was directly connected to an electrothermal MEMS microactuator with bimorph structures of thin silicon and aluminum layers. The fabricated MEMS PPR device precisely rotates an transparent optical plate up to 37° prior to an endoscopic camera and creates the binocular disparities, comparable to those from binocular cameras with a baseline distance over 100 µm. The anaglyph 3D images and disparity maps were successfully achieved by extracting the local binocular disparities from two optical images captured at the relative positions. The physical volume of MEMS PPR is well fit in 3.4 mm x 3.3 mm x 1 mm. This method provides a new direction for compact stereoscopic 3D endoscopic imaging systems.

Compact stereo endoscopic camera using microprism arrays.

This work reports a microprism array (MPA) based compact stereo endoscopic camera with a single image sensor. The MPAs were monolithically fabricated by using two-step photolithography and geometry-guided resist reflow to form an appropriate prism angle for stereo image pair formation. The fabricated MPAs were transferred onto a glass substrate with a UV curable resin replica by using polydimethylsiloxane (PDMS) replica molding and then successfully integrated in front of a single camera module. The stereo endoscopic camera with MPA splits an image into two stereo images and successfully demonstrates the binocular disparities between the stereo image pairs for objects with different distances. This stereo endoscopic camera can serve as a compact and 3D imaging platform for medical, industrial, or military uses.

Electrokinetic preconcentration of small molecules within volumetric electromagnetic hotspots in surface enhanced Raman scattering.

The on-chip integration of a preconcentration chamber for ultrasensitive surface-enhanced Raman scattering (SERS) is shown. Small molecules are preconcentrated using 3D volumetric electromagnetic hotspots. The experimental results demonstrate an enhancement of the SERS signals of over two orders of magnitude, which allows the fingerprinting of neurotransmitter molecules at the nanomolar level and furthers the selective detection of oppositely charged molecules. This on-chip integration will provide new directions for ultrasensitive SERS applications.

Biologically inspired LED lens from cuticular nanostructures of firefly lantern.

Cuticular nanostructures found in insects effectively manage light for light polarization, structural color, or optical index matching within an ultrathin natural scale. These nanostructures are mainly dedicated to manage incoming light and recently inspired many imaging and display applications. A bioluminescent organ, such as a firefly lantern, helps to out-couple light from the body in a highly efficient fashion for delivering strong optical signals in sexual communication. However, the cuticular nanostructures, except the light-producing reactions, have not been well investigated for physical principles and engineering biomimetics. Here we report a unique observation of high-transmission nanostructures on a firefly lantern and its biological inspiration for highly efficient LED illumination. Both numerical and experimental results clearly reveal high transmission through the nanostructures inspired from the lantern cuticle. The nanostructures on an LED lens surface were fabricated by using a large-area nanotemplating and reconfigurable nanomolding with heat-induced shear thinning. The biologically inspired LED lens, distinct from a smooth surface lens, substantially increases light transmission over visible ranges, comparable to conventional antireflection coating. This biological inspiration can offer new opportunities for increasing the light extraction efficiency of high-power LED packages.

Lissajous fiber scanning for forward viewing optical endomicroscopy using asymmetric stiffness modulation.

We report a fully packaged and compact forward viewing endomicroscope by using a resonant fiber scanner with two dimensional Lissajous trajectories. The fiber scanner comprises a single mode fiber with additional microstructures mounted inside a piezoelectric tube with quartered electrodes. The mechanical cross-coupling between the transverse axes of a resonant fiber with a circular cross-section was completely eliminated by asymmetrically modulating the stiffness of the fiber cantilever with silicon microstructures and an off-set fiber fragment. The Lissajous fiber scanner was fully packaged as endomicroscopic catheter passing through the accessory channel of a clinical endoscope and combined with spectral domain optical coherence tomography (SD-OCT). Ex-vivo 3D OCT images were successfully reconstructed along Lissajous trajectory. The preview imaging capability of the Lissajous scanning enables rapid 3D imaging with high temporal resolution. This endoscopic catheter provides many opportunities for on-demand and non-invasive optical biopsy inside a gastrointestinal endoscope.

Asymmetric optical microstructures driven by geometry-guided resist reflow.

We report a new method, termed geometry-guided resist reflow, for the batch fabrication of asymmetric optical microstructures. Thermoplastic microstructures reflow along the geometric boundaries of the adjacent thermoset microstructures above the glass transition temperature of thermoplastic resin. The shape profiles can be freely formed as a concave, convex, or linear shape and the slope angle can also be tuned from 7 to 68 degrees, depending on the geometric parameters. This new method provides a new route for developing functional optical elements.

Micromachined tethered silicon oscillator for an endomicroscopic Lissajous fiber scanner.

This work reports micromachined tethered silicon oscillators (MTSOs) for endoscopic Lissajous fiber scanners. An MTSO comprises an offset silicon spring for stiffness modulation of a scanning fiber and additional mass for modulation of resonant scanning frequency in one body. MTSOs were assembled with a resonant fiber scanner and enhanced scanning reliability of the scanner by eliminating mechanical cross coupling. The fiber scanner with MTSOs was fully packaged as an endomicroscopic catheter and coupled with a conventional laparoscope and spectral domain OCT system. The endomicroscope was maneuvered with the integrated laparoscope and in vivo swine tissue OCT imaging was successfully demonstrated during open surgery. This new component serves as an important element inside an endoscopic Lissajous fiber scanner for early cancer detection or on-demand minimum lesional margin decision during noninvasive endoscopic biopsy.

Corrigendum: Clinical feasibility of miniaturized Lissajous scanning confocal laser endomicroscopy for indocyanine green-enhanced brain tumor diagnosis.

[This corrects the article DOI: 10.3389/fonc.2022.994054.].

Plasmon-enhanced optoacoustic transducer with Ecoflex thin film for broadband ultrasound generation using overdriven pulsed laser diode.

Significance: Ultrasonic transducers facilitate noninvasive biomedical imaging and therapeutic applications. Optoacoustic generation using nanoplasmonic structures provides a technical solution for highly efficient broadband ultrasonic transducer. However, bulky and high-cost nanosecond lasers as conventional excitation sources hinder a compact configuration of transducer. Aim: Here, we report a plasmon-enhanced optoacoustic transducer (PEAT) for broadband ultrasound generation, featuring an overdriven pulsed laser diode (LD) and an Ecoflex thin film. The PEAT module consists of an LD, a collimating lens, a focusing lens, and an Ecoflex-coated 3D nanoplasmonic substrate (NPS). Approach: The LD is overdriven above its nominal current and precisely modulated to achieve nanosecond pulsed beam with high optical peak power. The focused laser beam is injected on the NPS with high-density electromagnetic hotspots, which allows for the efficient plasmonic photothermal effect. The thermal expansion of Ecoflex finally generates broadband ultrasound. Results: The overdriven pulsed LD achieves a maximum optical peak power of 40 W, exceeding the average optical power of 3 W. The 22 µm thick Ecoflex-coated NPS exhibits an eightfold optoacoustic enhancement with a fractional -6 dB bandwidth higher than 160% and a peak frequency of 2.5 MHz. In addition, the optoacoustic amplitude is precisely controlled by the optical peak power or the laser pulse width. The PEAT-integrated microfluidic chip clearly demonstrates acoustic atomization by generating aerosol droplets at the air-liquid interface. Conclusions: Plasmon-enhanced optoacoustic generation using PEAT can provide an approach for compact and on-demand biomedical applications, such as ultrasound imaging and lab-on-a-chip technologies.