OBJECTIVE: Develop, optimize, and demonstrate fast, frequency-agile, stimuli-responsive, and tunable optical filters that autonomously protect sensors from damaging optical beams, while allowing unobstructed transmission of non-damaging wavelengths and intensities. DESCRIPTION: The Department of Defense (DoD) increasingly relies on advanced imaging technologies to perform critical functions like intelligence, surveillance, and reconnaissance (ISR) and positioning, navigation, and timing (PNT). Advanced imaging sensors like electro-optical/infrared (EO/IR) cameras and micro-bolometers will dramatically proliferate as DoD continues to modernize with autonomous platforms such as the Army's Next Generation Combat Vehicle. However, these sensors are vulnerable to inadvertent and adversarial electromagnetic (EM) attack: future operating environments will be replete with a wide range of EM waves of varied frequency and intensity, many of which could temporarily or permanently blind or damage imaging systems. The ability to control strong light-matter interaction in liquid crystals[1], metamaterials [2-5], epsilon-near-zero (ENZ) materials [6,7], phase change materials (PCMs) [8,9], micro-electromechanical systems (MEMS) [10],and soft materials [11-14] suggest that these state-of-the-art materials systems can be leveraged to create tunable filters that autonomously respond to EM attack. For example, spatial light modulation (SLM) by metamaterials, holography, and liquid crystals enables selective-area light attenuation [1,2]. Digital metamaterials offer lenses and phase modulators capable of light redirection and beam steering [3,4]. Non-linear optical responses in Bragg reflector stacks and ENZ materials provide another potential route to autonomous light attenuation [5-7]. Integrating these concepts with PCMs, MEMS, and micro-mirrors may reveal new opportunities and platforms for programmable SLM and beam steering [8-10]. Finally, soft materials like liquid crystal elastomers and photo-responsive hydrogels have recently emerged as new platforms for autonomous manipulation of light [11-14], offering new abilities to create nano/microstructures that move in response to light and platforms for trapping and guiding laser beams. New capabilities in nano-/microfabrication may enable new, hierarchical approaches that combine multiple stimuli-responsive materials and architectures to further enhance adaptability. If under EM attack, an imaging system must incorporate a filter that rapidly senses an EM wave, determines its wavelength, and autonomously responds to attenuate or re-direct the wave if necessary. This capability is necessary over broad spectral ranges, including ultraviolet, visible, short-wave infrared (SWIR), and long-wave infrared (LWIR). An ideal intelligent filter mechanism would be (1) Operable in typical battlefield conditions; (2) Compatible with dynamically reconfigurable and/or multi-spectral imaging systems; (3) Low-cost, to enable wide adaptation across the DoD; (4) Adaptable to different imaging architectures; and (5) Non-disruptive to imaging performance (e.g., range, resolution, frame-rate, etc.). This topic seeks innovative materials approaches to creating such filters, based on passive material-based intrinsic responses and/or active feedback circuits incorporating tunable materials. Adaptive filters should autonomously detect changes in operating conditions and/or EM-based insults (e.g., a change in lighting conditions vs. incident laser beams) and respond appropriately by physically dimming, shuttering, switching, filtering, blocking, and/or rejecting the dynamic light conditions. The response should further be localized to the incident beam or spot where appropriate, to maintain an unobstructed field-of-view for the rest of the imaging system. Finally, there is also a significant need for scaled-up manufacturing capacity and yield the desired adaptive filters should be amenable to integration with imaging sensors with minimum added size, weight, power, and cost (SWaP-C). PHASE I: Design a concept for candidate FAST filters and describe the proposed materials systems, architectures, and control schemes that will be employed. Perform ab initio atomistic modeling, full wave EM simulations, quasi-static simulations, finite element analysis, and/or technology computer-aided design (TCAD) as needed to demonstrate the feasibility of the proposed approach. Outline the techniques and procedures that will be used to fabricate the proposed design and characterize its dynamic filtering performance. Develop an approach to integrate the proposed FAST filter with a desired sensor technology, imaging platform, or form factor, selected from the following: focal-plane array (flat or curved), optical windows and lenses, glasses, contact lenses, goggles, or visors. As appropriate, create a partial prototype that demonstrates the functionality of one or more of the proposed design elements: the dynamic materials system, the filter architecture, the control scheme (if applicable), and/or the integration scheme. FAST filter designs should inherently address or be easily adaptable to operate in the visible (380 780 nm) and the short-wave infrared (SWIR, 780 3000 nm) portions of the EM spectrum. Ultraviolet (UV, 100 380 nm) and mid-/long-wave infrared (MWIR/LWIR, 3000 14000 nm) light are also of (secondary) interest. The filtering mechanism should be dormant under normal imaging conditions, with optical transmission of 70 80% or greater at the nominal imaging wavelengths. The filter should autonomously respond to respond to high-fluence light (e.g., a high-power laser with a fluence on the order of 1 J/cm2) above the damage threshold of the imaging system should be interdicted by the filter changing its optical density (OD) by 3 or more (i.e., less than 0.1% of the incident laser energy should reach the sensor). The filter responses should be as localized to the incident spot size, in order to allow uninterrupted imaging while obscuring the sensing element(s) beneath the incident spot. These filter responses should be fully reversible, and the response time, including recovery to normal imaging conditions, should be no longer than 50 milliseconds. Articulate feasible pathways to response times of 1 nanosecond or less. Secondary considerations for FAST filter design include: 1) Low fluence light (e.g., changes in lighting conditions caused by shadows or moving between indoors and outdoors) should cause the filter to remain passive so that the imaging system can automatically adjust to lighting conditions; 2) Medium fluence light (e.g., an adversarial laser-based sensor or probe) should be fully absorbed, redirected, or otherwise mitigated to cloak the imaging system. PHASE II: Based on Phase I modeling and proofs of concept, fabricate, test, and demonstrate at least one operational FAST filter prototype. The prototype should be capable of autonomous optical responses with sub-ns response times. The FAST filters should reversibly cycle over 105 times without suffering more than 2% degradation in response time, OD change, reflection, transmission, dormant state/position, etc. Using a detailed analysis of system trades and input from appropriate stakeholders, propose a pathway to refine and integrate the FAST filter prototype with a candidate imaging system of interest to or used by the Army. Depending on the target imaging system, the FAST filters should increase the total SWaP-C burden by 0.1% or less, should not adversely impact imaging performance, and should allow normal imaging modality over typical ranges of brightness/lighting conditions; more specifically, FAST filters under normal imaging conditions should not change the system's modulation transfer function by more than 10%, and should not change transmission of imaging wavelengths by more than 20%. PHASE III DUAL USE APPLICATIONS: Phase II should demonstrate a FAST filter that is appropriate for implementation with existing and/or future Army imaging systems. Phase III will transition the newly developed FAST filter technology to commercial availability through the prime contractors that build these imaging systems, the original equipment manufacturers that manufacture sensing components, other relevant suppliers, and/or other partnering agreement(s), as appropriate. Commercialization of this technology may occur via the incorporation of one or more FAST filters anywhere in an imaging system (e.g., windows, lenses, shutters, FPA pixels, etc.). Ideally, a successful effort will deliver a capability upgrade for a relevant Army Program of Record at the end of Phase III, in the form of an imaging system that autonomously responds to EM attack with no added cognitive burden to the user, and a minimum added SWaP-C burden. Expected dual-use applications include autonomous vehicles, LiDAR, border security, and protecting civilian optical imaging systems (e.g., thermal imaging of the sun). REFERENCES: 1. Forbes, A., Dudley, A., & McLaren, M. (2016). Creation and detection of optical modes with spatial light modulators. Advances in Optics and Photonics, 8(2), 200-227; 2. Fan, K., Suen, J. Y., & Padilla, W. J. (2017). Graphene metamaterial spatial light modulator for infrared single pixel imaging. Optics Express, 25(21), 25318-25325; 3. Della Giovampaola, C., Engheta, N. Digital metamaterials. Nature Mater 13, 1115 1121 (2014); 4. Cui, T. J., Qi, M. Q., Wan, X., Zhao, J., & Cheng, Q. (2014). Coding metamaterials, digital metamaterials and programmable metamaterials. Light: Science & Applications, 3(10), e218-e218; 5. Vella, J. H., Goldsmith, J. H., Browning, A. T., Limberopoulos, N. I., Vitebskiy, I., Makri, E., & Kottos, T. (2016). Experimental realization of a reflective optical limiter. 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