Taking Our Cue From Nature:Bio-inspired Shutters and Aperatures for Infrared Imaging Applications
Currently, applications for infrared (IR) imaging devices are limited by the need for costly, slow, bulky components that require cooling. These components include high-magnification sensors that require large lenses and telescope configurations, noisy mechanical shutters that limit the usefulness of many imaging systems in covert applications, electronics required to mitigate focal plane array (FPA) saturation effects (i.e., blooming) in all-weather applications, and filters that improve imaging contrast at dusk and in haze.
The drive to reduce system size, weight and power consumption, while increasing the resolution and format of IR imaging systems, presents many new challenges. Current efforts to develop lightweight, fast components have been focused on established micro-electromechanical systems (MEMS) approaches; however, reliable MEMS-based technologies are processing intensive and still not cost effective. Raytheon, in collaboration with the University of California at Santa Barbara (UCSB), has developed new polymers to address these challenges. Polymers, with their infinite customizability and low production costs, offer a novel solution.
While investigating the use of polymers for IR imaging, Raytheon looked to nature and the world’s oceans for inspiration. Cephalopods (e.g., octopus, squid and cuttlefish) manipulate light for camouflage and inter-individual signaling using their ability to selectively scatter light with both pigmentation and reflection. The reflectors responsible for the dynamic iridescence of the cephalopod’s skin (Figure 1A) are composed of proteins called reflectins. These are organized within stacks of intracellular, membrane-enclosed Bragg reflectors that in some contexts act as filters but in other contexts act as reflectors (Figure 1B & C). Professor Dan Morse’s laboratory at UCSB has recently discovered that the catalytic phosphorylation* of specific amino acids in the reflectin protein drives a conformational change in the protein that activates its hierarchical assembly, simultaneously tuning the spacing, thickness and density (refractive index) of the thin protein layers and quickly promoting change in the transmission of light across the entire visible range (Figure 1D).
The biopolymer’s inherent elasticity and conformability; its quick, reversible assembly; and the synergistic effects it provides by being able to alter both density and thickness are remarkable and unique. Biology shows us that these exquisitely finely tuned lightweight polymers (proteins), when spatially constrained as single thin layers, can tune and deliver all the optical functionalities conventionally provided by heavy, bulky, noisy and power-hungry devices. Inspired by this biological effect, Raytheon and UCSB have begun a close collaboration to develop thin films of synthetic polymeric materials that exhibit electrically driven simultaneous changes in morphology and refractive index. This has led to further development with the Army Research Labs to incorporate tunable organic layers into optical devices.
Optical Device Development
Conjugated polymers are solution processable semiconducting materials that, upon electrochemical activation, exhibit changes in refractive index coupled with changes in morphology (due to ion diffusion). This results in distinct transformations in absorption and reflection over the entire electromagnetic spectrum. As the polymer-based materials transition from semiconducting to conducting, free carriers and conformational changes absorb and scatter wide bands of infrared radiation. The resulting change in refractive index from the simultaneous production of absorbing species and their increased density closely parallels the synergistic simultaneous changes in the reflectin-based Bragg layers that provide the high gain exhibited by the biological system. The basic device is a uniform layer that functions as a neutral density filter (NDF) or shutter, and that can be extended with spatial patterning to act as a solid-state dynamic shutter or as a coded aperture (Figure 2).
Initial designs based on poly (3-hexyl thiophene) (P3HT) exhibit a sharp transition from highly transparent to opaque in the medium wave infrared (MWIR) band (Figure 3-1), with the difference in transmission (on versus off) at a specified wavelength being a key metric for denoting optical contrast. Improved designs leveraging customized polymer design and fabrication increased switching speed, as illustrated in Figure 3-2. Further polymer customization led to a polymer chain that contained more complex elements, promoting ion diffusion and leading to faster switching with better device stability (Figure 3-3). This is illustrated by the repeatability of device absorption versus time with no change between cycles as the device is switched on and off.
Device geometries have been investigated, and separations between active areas as small as 25 μm have been demonstrated, allowing for the very close mating of optical elements (e.g., rings in an aperture). Aperture devices have been fabricated from these polymer layers in an array of 1,000 μm active regions with 25 μm spacing (Figure 4). These devices can be actuated with a 0–3V signal. The transmission has been captured as a function of time using a Raytheon short wave infrared (SWIR) camera system.
Two frames are presented in Figure 4: one with 0V bias applied (left image) and one with 3V bias applied (right image). The device consists of a 3x3 array of pixels, with one pixel fixed at 3V bias (left column, center row) and one pixel unbiased (center of array). The change in contrast between the two frames is apparent with the darkening of the remaining pixels between the two frames, demonstrating that a shutter and a coded aperture are feasible.
Raytheon and UCSB are collaborating to leverage research in cephalopod biology and the mechanisms by which cephalopods change skin color with the objective of reducing the size, weight and power consumption of infrared imaging systems. Aperture devices with dimensions of 25 μm and with sub-second switching times have been demonstrated, showing a viable path to bio-inspired shutters and coded apertures.
* The introduction of a phosphate group into an organic molecule by means of a catalyst.
Andreas Hampp, Amanda Holt, Dan Morse, Justin Wehner