Formation and characterization of stochastic subwavelength structures on polymer surfaces

When PMMA is exposed to plasma, a stochastic surface morphology forms under certain conditions. The surface shows a notable transmittance increase in the visible spectral region. Moreover, the technology is an inexpensive alternative to the deposition of interference coatings to increase the light transmission of polymeric optical elements. The principle of antireflective subwavelength structures is also known from the "moth's eye". This work is driven by the need to fabricate such broadband antireflective morphologies on arbitrary shaped surfaces of different kinds of polymers. Beside PMMA, three transparent thermoplasts were chosen to analyze the physical processes in order to understand the structure formation. The plasma-polymer interaction was supported by Monte Carlo method to investigate the energy transfer of the ions on the substrate. The optical effect of the structured surfaces has been optimized by the use of spectrophotometry. The structure formation has been analyzed by AFM and SEM. The revealed effective pin shape was correlated with the results from spectral reverse engineering. The structure formation has been established on the surface of the polymers and shows a self-organized nature, since the topography was not prescribed from outside before or during the process. The differences in size, shape, and antireflective effect are specific for the particular polymer. The structure formation results from a combination of physical sputter-erosion and chemical etching. Aspect ratios of the pin structure above 1 can only be explained by anisotropic etching due to impact of high energetic plasma-ions. The structure has been modeled as effective medium, which allows for the subsequent description by means of graded-index layers. The occurrence of scatter losses can be kept low and might be negligible for many applications. Particular PET surfaces show an industrially appropriate transmittance increase in the visible spectral region after plasma treatment

Formation and characterization of stochastic subwavelength structures on polymer surfaces

When PMMA is exposed to plasma, a stochastic surface morphology forms under certain conditions. The surface shows a notable transmittance increase in the visible spectral region. Moreover, the technology is an inexpensive alternative to the deposition of interference coatings to increase the light transmission of polymeric optical elements. The principle of antireflective subwavelength structures is also known from the "moth's eye". This work is driven by the need to fabricate such broadband antireflective morphologies on arbitrary shaped surfaces of different kinds of polymers. Beside PMMA, three transparent thermoplasts were chosen to analyze the physical processes in order to understand the structure formation. The plasma-polymer interaction was supported by Monte Carlo method to investigate the energy transfer of the ions on the substrate. The optical effect of the structured surfaces has been optimized by the use of spectrophotometry. The structure formation has been analyzed by AFM and SEM. The revealed effective pin shape was correlated with the results from spectral reverse engineering. The structure formation has been established on the surface of the polymers and shows a self-organized nature, since the topography was not prescribed from outside before or during the process. The differences in size, shape, and antireflective effect are specific for the particular polymer. The structure formation results from a combination of physical sputter-erosion and chemical etching. Aspect ratios of the pin structure above 1 can only be explained by anisotropic etching due to impact of high energetic plasma-ions. The structure has been modeled as effective medium, which allows for the subsequent description by means of graded-index layers. The occurrence of scatter losses can be kept low and might be negligible for many applications. Particular PET surfaces show an industrially appropriate transmittance increase in the visible spectral region after plasma treatment

Miniature curved artificial compound eyes.

International audienceIn most animal species, vision is mediated by compound eyes, which offer lower resolution than vertebrate single-lens eyes, but significantly larger fields of view with negligible distortion and spherical aberration, as well as high temporal resolution in a tiny package. Compound eyes are ideally suited for fast panoramic motion perception. Engineering a miniature artificial compound eye is challenging because it requires accurate alignment of photoreceptive and optical components on a curved surface. Here, we describe a unique design method for biomimetic compound eyes featuring a panoramic, undistorted field of view in a very thin package. The design consists of three planar layers of separately produced arrays, namely, a microlens array, a neuromorphic photodetector array, and a flexible printed circuit board that are stacked, cut, and curved to produce a mechanically flexible imager. Following this method, we have prototyped and characterized an artificial compound eye bearing a hemispherical field of view with embedded and programmable low-power signal processing, high temporal resolution, and local adaptation to illumination. The prototyped artificial compound eye possesses several characteristics similar to the eye of the fruit fly Drosophila and other arthropod species. This design method opens up additional vistas for a broad range of applications in which wide field motion detection is at a premium, such as collision-free navigation of terrestrial and aerospace vehicles, and for the experimental testing of insect vision theories

A lightweight, inexpensive robotic system for insect vision

Designing hardware for miniaturized robotics which mimics the capabilities of flying insects is of interest, because they share similar constraints (i.e. small size, low weight, and low energy consumption). Research in this area aims to enable robots with similarly efficient flight and cognitive abilities. Visual processing is important to flying insects' impressive flight capabilities, but currently, embodiment of insect-like visual systems is limited by the hardware systems available. Suitable hardware is either prohibitively expensive, difficult to reproduce, cannot accurately simulate insect vision characteristics, and/or is too heavy for small robotic platforms. These limitations hamper the development of platforms for embodiment which in turn hampers the progress on understanding of how biological systems fundamentally works. To address this gap, this paper proposes an inexpensive, lightweight robotic system for modelling insect vision. The system is mounted and tested on a robotic platform for mobile applications, and then the camera and insect vision models are evaluated. We analyse the potential of the system for use in embodiment of higher-level visual processes (i.e. motion detection) and also for development of navigation based on vision for robotics in general. Optic flow from sample camera data is calculated and compared to a perfect, simulated bee world showing an excellent resemblance

Concept, manufacturing and challenges of ultra-compact snapshot multi-spectral multi-aperture imaging systems

Snapshot multispectral imaging is a rising non-invasive and contact-free analysis method and technology to discriminate or identify objects based on their spectral characteristics. We demonstrate a versatile system approach for compact and real-time capable snapshot cameras for the visible (VIS) and the near-infrared (NIR) or the short-wave infrared (SWIR) wavelength range based on a micro-optical multi-aperture system and various spectral filter approaches. In addition, the manufacturing, the calibration, and the limitations of the demonstration systems are described

Optical and non-optical characterization of Nb2O5-SiO2 compositional graded-index layers and rugate structures

The deposition of graded-index layers and rugate structures was performed by coevaporation of silicon dioxide as the low index material and niobium pentoxide as the high index material. To obtain information about the composition depth profile of the films, we used cross-sectional transmission electron microscopy to supplement deposition rate data recorded by two indipendent crystal quartz monitors during film preparation. The concentration depth profile was transformed to a refractive index profile using the effective medium approximation. The thus obtained refractive index profiles turned out to represent efficient initial approximations for re-engineering purposes

A small-scale hyperacute compound eye featuring active eye tremor: application to visual stabilization, target tracking, and short-range odometry

International audienceIn this study, a miniature artificial compound eye (15 mm in diameter) called the curved artificial compound eye (CurvACE) was endowed for the first time with hyperacuity, using similar micro-movements to those occurring in the fly's compound eye. A periodic micro-scanning movement of only a few degrees enables the vibrating compound eye to locate contrasting objects with a 40-fold greater resolution than that imposed by the interommatidial angle. In this study, we developed a new algorithm merging the output of 35 local processing units consisting of adjacent pairs of artificial ommatidia. The local measurements performed by each pair are processed in parallel with very few computational resources, which makes it possible to reach a high refresh rate of 500 Hz. An aerial robotic platform with two degrees of freedom equipped with the active CurvACE placed over naturally textured panels was able to assess its linear position accurately with respect to the environment thanks to its efficient gaze stabilization system. The algorithm was found to perform robustly at different light conditions as well as distance variations relative to the ground and featured small closed-loop positioning errors of the robot in the range of 45 mm. In addition, three tasks of interest were performed without having to change the algorithm: short-range odometry, visual stabilization, and tracking contrasting objects (hands) moving over a textured background

Curved artificial compound-eyes for autonomous navigation

Natural compound-eyes consist of a large number of ommatidia that are arranged on curved surfaces and thus are able to detect signals from a wide field of view. We present an integrated artificial compound-eye sensor system with enhanced field of view of 180° × 60° due to the introduction of curvature. The system bases on an array of adaptive logarithmic wide-dynamic-range photoreceptors for optical flow detection and compound-eye optics for increasing sensitivity and expanding the field of view. Its assembling is mainly done in planar geometry on a flexible printed circuit board. The separation into smaller ommatidia blocks by dicing enables flexibility and finally allows for mounting on curved surfaces. The signal processing electronics of the presented system is placed together with further sensors into the concavity of the photoreceptor array, and facilitates optical flow computation for navigation purposes

An artificial elementary eye with optic flow detection and compositional properties

International audienceWe describe a 2 mg artificial elementary eye whose structure and functionality is inspired by compound eye ommatidia. Its optical sensitivity and electronic architecture are sufficient to generate the required signals for the measurement of local optic flow vectors in multiple directions. Multiple elementary eyes can be assembled to create a compound vision system of desired shape and curvature spanning large fields of view. The system configurability is validated with the fabrication of a flexible linear array of artificial elementary eyes capable of extracting optic flow over multiple visual directions