Dongmin Keum

Fluorescent microscopy beyond diffraction limits using speckle illumination and joint support recovery

Structured illumination microscopy (SIM) breaks the optical diffraction limit by illuminating a sample with a series of line-patterned light. Recently, in order to alleviate the requirement of precise knowledge of illumination patterns, structured illumination microscopy techniques using speckle patterns have been proposed. However, these methods require stringent assumptions of the speckle statistics: for example, speckle patterns should be nearly incoherent or their temporal average should be roughly homogeneous. Here, we present a novel speckle illumination microscopy technique that overcomes the diffraction limit by exploiting the minimal requirement that is common for all the existing super-resolution microscopy, i.e. that the fluorophore locations do not vary during the acquisition time. Using numerical and real experiments, we demonstrate that the proposed method can improve the resolution up to threefold. Because our proposed method succeeds for standard fluorescence probes and experimental protocols, it can be applied in routine biological experiments.

Planar Emulation of Natural Compound Eyes

Natural compound eyes comprise an array of integrated optical units called ommatidia whose individual component includes a facet lens, a crystalline cone, a light-guiding rhabdom, and photoreceptor cells. The unique schemes exhibit distinguished benefi ts in wide fi eld-of-view, fast motion, or polarization detection. [ 1–3 ] The compound eyes have attracted extensive research interest in photonics because the physiological features and the principles of visual information processing can provide technical solutions for cutting-edge optical systems in medical, industrial, and military fi elds. [ 4–11 ] The vision has been emulated with multiapertures on an image sensor arrays by stitching multiple sets of angular images. [ 4–8 ] The physiological structures were mimicked by spherically arranging artifi cial ommatidia on a hemispherical dome. [ 9 ] The previous works, however, still have some technical limitations in mining smartness from diverse natural compound eyes. Nature provides ten different optical schemes of compound eyes. The distinctive features include seven apposition and three superposition types. The apposition types cover simple apposition, open rhabdom apposition, neural superposition, afocal apposition, transparent apposition lightguide, transparent apposition axial gradient, and transparent apposition radial gradient types. [ 12–16 ] The superposition types can also be classifi ed into refracting, refl ecting, and parabolic types. [ 12 , 17–19 ] The individual scheme was not well researched for engineering applications even though it has some attractive fi gures-of-merit for sustainable life style in visual acuity, photon collection effi ciency, and spectral or polarization sensitivity. In particular, the cross-sectional anatomical structures of ten different types provide a clear indication to understand their functions and even to utilize the optical schemes for advanced photonic sensors. The diverse optical schemes can be predetermined on a planar substrate as illustrated in Figure 1 . A natural ommatidium can be emulated by a cylindrical microlens, a conical structure, a waveguide, and a photodetector. Like the natural one, the ommatidia of the apposition type are optically isolated so that each waveguide

Artificial compound eye with fractal zone plate arrays

This work reports the artificial compound eye using fractal zone plate (FraZP) arrays. Spherically arranged optical elements (ommatidia) of compound eyes facilitate wide field-of-view (FOV) imaging. However, due to the lack of curved image sensors, it was difficult to integrate hemispherical shape of an artificial compound eye with commercially available flat image sensors. FraZP gives sufficient variation of focal length compared to a micro lens fabricated by reflow process. Flat image plane can be easily obtained by controlling the focal length of each FraZP so that 3-dimensional structure of the artificial compound eye is compatible to a conventional image sensor.

Ultrathin camera inspired by visual system of Xenos peckii

This paper reports ultrathin digital camera inspired by the visual system of Xenos peckii, which is an endoparasite of paper wasps. Males of the Xenos peckii have an unusual visual system that consists of a large lens and multiple photoreceptor cells. This optical scheme exhibits distinguished benefits in resolution and sensitivity. Xenos peckii vision inspired camera consists of microprism arrays, microlens arrays, and aperture arrays. The microprism arrays were implemented by using a ball lens and backside exposure with prepatterned metal mask. And the light absorbing structures were formed between the microprism arrays using black polymer. Each channel of the camera detects different part of the total field of view, and captured partial images are reconstructed in the image processing step.

Artificial compound eye inspired by imaging principle of Xenos peckii

This work reports an artificial compound eye inspired by the imaging principle of Xenos peckii, which is an endoparasite of paper wasps. The unique eye design exhibits higher spatial resolution and better sensitivity than conventional compound eyes. The biomimetic compound eye comprises three layers, a microprism arrays, a microlens arrays and an aperture arrays. All of the layers were formed on a planar substrate, the device can be directly integrated with a commercial image sensor. Each channel detects a different part of the whole scene, and the partial images are stitched in the following image processing step. The proposed artificial compound eye can create great opportunities for applications in medical, industrial and military fields.

Xenos peckii vision inspires an ultrathin digital camera

Increased demand for compact devices leads to rapid development of miniaturized digital cameras. However, conventional camera modules contain multiple lenses along the optical axis to compensate for optical aberrations that introduce technical challenges in reducing the total thickness of the camera module. Here, we report an ultrathin digital camera inspired by the vision principle of Xenos peckii, an endoparasite of paper wasps. The male Xenos peckii has an unusual visual system that exhibits distinct benefits for high resolution and high sensitivity, unlike the compound eyes found in most insects and some crustaceans. The biologically inspired camera features a sandwiched configuration of concave microprisms, microlenses, and pinhole arrays on a flat image sensor. The camera shows a field-of-view (FOV) of 68 degrees with a diameter of 3.4 mm and a total track length of 1.4 mm. The biologically inspired camera offers a new opportunity for developing ultrathin cameras in medical, industrial, and military fields.

Asymmetric microstructures for high light extraction and light pattern modulation

This work presents a novel method of fabricating asymmetric microstructures for light extraction enhancement and light pattern modulation. Asymmetric microstructures were fabricated at wafer level by two-step photolithography, surface energy modulation, and thermal reflow. The microstructures improve light extraction efficiency by suppressing total internal reflection and modulate far-field light distribution by changing incident angle of light. Light extraction and light pattern can be changed by design of asymmetric microstructures. These structures can be utilized for display and lighting applications.

Laser induced self-aligned microlens and waveguide arrays using a self-writing process in a photosensitive polymer resin

A microfabricated compound eye lens, compatible to a natural compound eye shows a spherical arrangement of integrated optical units called artificial ommatidia. Each consists of a self-aligned microlens and waveguide. The increase of the waveguide length is an indispensible prerequisite for obtaining high resolution images through an artificial compound eye for wide field-of-view imaging as well as fast motion detection. This work presents an effective method for increasing the waveguide length of artificial ommatidium using a laser inducec self-writing process in a photosensitive polymer resin. The experimental results show the uniform formation of waveguides and the increment of waveguide length over 500µm.

Biologically inspired optical structures for wide field-of-view imaging and wide angle illumination

Two optical schemes inspired by an insect are implemented to novel optical devices for wide field-of-view imaging and wide angular illumination in miniaturized optical systems, using polymer based microfabrication techniques.

Direct visualization of light propagation inside a planar artificial compound eye using a Rhodamine 6G doped photosensitive polymer resin

Light progration inside a planar artificial compound eye has been successfully demonstrated by using a Rhodamine 6G doped SU-8 photoresist. The individual ommatidium comprising a microlens and waveguide shows high angular sensitivity for directional illumination.