Nature provides many examples of optical structures whose properties arise from an intricate morphology. The brilliant colors seen in butterfly wings, beetle carapaces, and peacock feathers, for example, are due in large part to their complex structure, says DARPA. These hierarchical structures, scaling from micron to nanometer level, achieve remarkable optical functionality through a complex combination of scattering, reflection, and absorption phenomena. Fabrication of advanced photonic devices can take inspiration from the fault tolerance of these structures, nature’s ability to manufacture at the nanoscale, and the hierarchical structurally dependant optical properties.
DARPA invited research proposals for the Bioinspired Photonics program in 2009. The program’s goal was to harness the best of nature’s photonic structures and utilize advances in materials technology to create controllable photonic devices at visible and near infrared wavelengths.
DARPA’s Bioinspired Photonics program
The Bioinspired Photonics program seeks to enable advanced functionality in two technical areas of interest related to potential modes of activation: Physical and Chemical Activation. Physical Activation refers to directly controlling the optical properties through incorporation of some actuating material into the structure. Chemical Activation refers to optical property (reflectance) changes being driven by chemical molecules being incident on the structure. Demonstration of these activation modalities will be through dynamically tunable reflectors and volatile organic vapor sensors, respectively.
DARPA has identified three main technical challenges that must be addressed to achieve the goals of the Bioinspired Nanophotonics program: 1) fabrication; 2) activation; and 3) scalability.
Proposed research should investigate innovative bio-inspired synthetic organic and inorganic approaches that enable revolutionary advances in 3-D dynamically tunable photonic band-gap structures and volatile organic vapor sensors at visible and NIR wavelengths.
Technical Area One: Physical Activation
Today’s military demands improved performance from its electronic systems with decreasing system size, weight and power (SWaP). Existing technology in the optical components of these electronic systems will not be able to keep pace with the increasing demands. For example, today’s tunable filters are often polarization and angle dependent and require stacks of devices to achieve a complex optical response.
A new design paradigm is needed which enables both added functionality with small form factors for utility in portable electronic devices, and next generation imaging systems. This technical area of interest aims to harness nature’s ability to create structures at the nanoscale to provide complex dynamic structures in small form factors with superior optical characteristics through intricate hierarchical design and new materials synthesis.
Materials capable of actuation should be included in the structure to controllably alter the structural parameters, resulting in dynamically tunable reflectors operating across the visible and/or NIR wavelength range. Topics of interest in this technical area include novel methods to tune the wavelength of the reflector at visible and NIR wavelengths using nature as a guide through the exploitation of material properties such as refractive index, periodic spacing, layer thickness, angle of reflection. Activation of the bio-inspired photonic structures includes, but is not limited to, the following methods: microfluidics, magnetostrictives, temperature gradients, piezoelectrics, and pressure gradients.
Specific areas of interest include, but are not limited to, the following:
- Bio-inspired designs that couple more than one method of light manipulation (e.g. Bragg reflection) by exploiting nature’s intricate 3-D structures through synthetic organic and inorganic processing methods
- Bio-inspired designs that incorporate active materials to emulate biological structure/function relationships
Technical Area Two: Chemical Activation
Chemical explosives and improvised explosive devices are an understood threat for today’s military. This technical area of interest will utilize lessons from nature to create highly selective, lightweight, reusable gas sensors with sensitivities rivaling state of the art for use by forward soldiers.
It has been demonstrated that biophotonic structures, such as butterfly wings, provide highly selective responses to similar molecules (methanol, ethanol, and water) based upon their vapor pressures. The intricate hierarchical structures allow for selectivity of different gases by detecting the difference in vapor pressures through physisorptive processes to provide an easily detectable optical response in the visible spectrum. Thorough understanding of the interaction of incident molecules on these structures will enable design of optimized structures for high confidence sensitivity and selectivity.
This technical area of interest will exploit bioinspired materials to mimic those structures found in nature to enable highly sensitive, high selectivity reusable gas sensors. It is envisioned that both chemisorptive and physisorptive interactions may be utilized in a 3-D nanostructure to provide parts-per-trillion chemical sensitivity with reversible performance.
A focus should include, but is not limited to, utilizing the structure and materials to provide various reflective responses, such as scattering and Bragg diffraction effects, from different molecules on one photonic structure. It is envisioned that molecules will react to different surfaces based upon their differences in molecular weight, chemical structure, and vapor pressure. The intricacy of the 3-D photonic structure, and the optimization of material choices, will provide selectivity and predictable optical responses. A design scheme must be proposed where one chemical is to be detected amongst a group of similar chemical components (interferent molecules) of military interest. For example, the detection of TNT amongst nitroglycerine and similar nitroamines.
Bio-Inspired Chemical and Explosive Sensors Nab $6M DARPA Grant
Scientists at GE Global Research, GE’s technology development arm, in collaboration with Air Force Research Laboratory, State University at Albany, and University of Exeter, have received a four-year, $6.3 million award from the Defense Advanced Research Projects Agency (DARPA) to develop new bio-inspired nanostructured sensors that would enable faster, more selective detection of dangerous warfare agents and explosives.
Earlier, GE scientists discovered that nanostructures from wing scales of butterflies exhibited acute chemical sensing properties. Since then, GE scientists have been developing a dynamic, new sensing platform that replicates these unique properties. Recognizing the potential of GE’s sensing technologies for improving homeland protection, DARPA is supporting further research.
Radislav Potyrailo, a principal scientist at GE Global Research and principal investigator, said, “GE’s bio-inspired sensing platform could dramatically increase sensitivity, speed and accuracy for detecting dangerous chemical threats. All of these factors are critical, not only from the standpoint of preventing exposure, but in monitoring an effective medical response if necessary to deal with such threats.”
Potyrailo noted that GE’s sensors can be made in very small sizes, with low production costs. This would allow large volumes of these sensors to be readily produced and deployed wherever needed. Unique sensing properties, combined with the size and production advantages offered by GE’s bio-inspired sensors, could enable an array of other important industrial and healthcare applications, including:
- Emissions monitoring at power plants
- Food and beverage safety monitoring
- Water purification testing for home, environmental and industrial applications
- Breath analysis for disease detection
- Wound healing assessment
Potyrailo said, “Now, more than ever, sensors are being used to collect data on gas concentrations and to deliver important information about air conditions in localized regions or over large distributed areas. This information can range from warning of impending chemical or health threats to more precisely measuring air quality at a power plant. The unique sensing properties of GE’s bio-inspired sensors provide an opportunity to improve the quality of this sensing data and the ability to collect this data at previously unavailable levels of detail.”
DARPA Program Manager Viktoria Greanya, Ph.D., said: “We have been greatly inspired by examples of naturally occurring optical structures whose properties arise from an intricate morphology. For example, the brilliant colors seen in butterfly wings, beetle carapaces, and peacock feathers are due in large part to their complex structure, not simply their color. DARPA’s goal in this program is to harness the best of nature’s own photonic structures and use advances in materials technology to create controllable photonic devices at visible and near-infrared wavelengths.”
For the DARPA project, GE has assembled a world-class team of collaborators who are recognized experts in their fields. They include: Dr. Helen Ghiradella, from State University at Albany, an expert on the biology of structural color; Dr. Peter Vukusic, from the University of Exeter, an expert on the physics of structural color; Dr. Rajesh Naik, from the Air Force Research Laboratory, with a strong background in bio-inspired functional materials and surface functionalization; and Dr. John Hartley, also from State University at Albany, specializing in advanced lithographic nanofabrication. These team members will complement GE’s strong multidisciplinary team of analytical chemists, material scientists, polymer chemists, optical engineers and nanofabrication engineers who are contributing to development of this new platform.