Guest editors Young Min Song, Cunjiang Yu, and Gil Ju Lee introduce the Special Section on Biomimetic Optical Microsystems.

Biological vision offers intriguing inspiration for functional features in imaging systems with small form factors. We report biologically inspired intraoral camera (BIOC) for assorted dental imaging. This fully packaged BIOC features a convex-concave lens, inverted microlens arrays (iMLAs), LED module, and a single CMOS image sensor on a flexible printed circuit board in a handpiece holder. The iMLAs also collect light from wide angles by mounting the convex-concave lens to increase the viewing angle. The clinical trials have been successfully conducted for real-time and multifunctional intraoral monitoring of human teeth, including infinite depth of field, close-up, wide field-of-view, three-dimensional, and autofluorescence imaging. This biomedical camera provides insights for functional imaging not only in dental applications but also in surgical robots and endoscopy applications.

We report on the successful use of three-dimensional (3D) printing by two-photon polymerization to engineer optimized hierarchically composed surface structures at the micro- and nanoscale. The hierarchical composition of the printed structures was inspired by those found on the upper wing surface of blue-winged butterflies from the genus Morpho. In this way, the nanostructures and blue coloration of the organisms was mimicked, but less iridescence was achieved for biomimetic surfaces. Like the biological surface structures, the ones printed exhibited disorders characteristics. As a result, the blue colors generated by biomimetic structures displayed angle-insensitive optical properties similar to those of the Morpho wings. In addition, the great design freedom and simple workflow of the 3D printing technique enabled the fabrication of different structures at the microscale without modifying the dimension of the substructures at the nanoscale. Thus, it was possible to set the direction in which angle-insensitive coloration appeared to an observer. The morphology of biological and biomimetic surface structures was analyzed and compared using scanning electron microscopy. The optical properties of biological and engineered specimens were determined using angle-resolved spectroscopy. Furthermore, the coloration of biomimetic surfaces and the upper wing surface of Morpho butterfly was studied in different liquids. The results were compared, and potential application for biomimetic surfaces was discussed.

Biological eyes in nature have strongly inspired novel optical systems. In this regard, imaging systems mimicking fish eyes and human eyes have been reported for having a wide field-of-view (FoV) and relatively higher magnification properties, respectively. However, most of these systems have complex lens configurations because of their flat image sensors. As these optical systems are bulky, heavy, and expensive, they have limited application in small devices (e.g., drones and mobile phones). In addition to a simplistic design, multi-functionality is essential for broad applications. Therefore, this study proposes a compact zooming optical system (CZOS) that combines the properties of natural vision systems (i.e., human and fish eyes) using a curved focal plane. The CZOS controls the zoom range through the modification of the distances between the single front lens (i.e., negative meniscus) and dual rear lens (i.e., bi-convex/positive meniscus lenses) groups. In the proposed system, the panoramic mode had an FoV of 200 deg and a magnification of 0.23, whereas the conversion of the system to the high magnification mode increased the magnification over two times with an FoV of 70 deg. These promising results demonstrate that the proposed simple imaging system is applicable to small-scale electronics.

Additive manufacturing using two-photon polymerization (TPP) lithography is increasingly used in industry and research. Parameter sweeps of cuboid structures fabricated using TPP lithography were investigated across the parameters of the laser power and scan speed to find dependent mechanical material properties. The employed photoresists were examined using Raman spectroscopy to find the degree of conversion (DC) of monomer to polymer, and subsequently, micro- or nanoindentation was used to find Young’s modulus (E). For the photoresist IP-Dip, the attained DC and E ranged from 20% to 45% and 1 to 2.1 GPa, respectively. The results were compared with reports found in the literature. For IP-Q, the attained DC and E ranged from 53% to 80% and 0.5 to 1.3 GPa, respectively. The characterized properties of IP-Q manifest as the current state of knowledge of the material.

Free-space coupling of Gaussian light beams using flat and curved photonic microelectromechanical systems mirrors was analyzed in detail. The theoretical background and the non-ideal effects, such as limited micromirror extent, asymmetry in the curvature of spherical micromirrors, misaligned axes, and micromirror surface irregularities, were analyzed. The derived formulas were used to study and compare theoretically and experimentally the behavior of flat (one-dimensional), cylindrical (two-dimensional), and spherical (three-dimensional) micromirrors. The analysis focused on the regime of dimensions in which the curved micromirrors radius of curvature is comparable to the incident beam Rayleigh range, also corresponding to a reference spot size. A transfer matrix-based field and power coupling coefficients were derived for general micro-optical systems accounting for different matrix parameters in the tangential and sagittal planes of the microsystem taking into account the possible non-idealities. The results were presented in terms of normalized quantities such that the findings are general and can be applied to different situations. In addition, silicon micromirrors were fabricated with controlled shapes and used to experimentally analyze the coupling efficiency at the visible and near-infrared wavelengths.