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Hyperspectral Sensor Metamaterial Lens in Imaging Applications


RT&L FOCUS AREA(S): General Warfighting Requirements

TECHNOLOGY AREA(S): Battlespace Environments; Electronics; Materials / Processes

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with section 3.5 of the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Design, build, and demonstrate on-chip hyperspectral focal plane array, with integration of dynamically tunable metamaterial lens to perform and produce 2D spatial images with a single exposure at a few selected wavelength bands instead of 1D spatial and all spectral band images.

DESCRIPTION: Hyperspectral imagery (HSI) provides the means to detect targets smaller than the size of a pixel using spectral unmixing techniques. HSI contains hundreds of bands of spectral information per pixel. HSI is traditionally performed using a dispersive (prism or grating) reimaging system with a slit and focal plane array (FPA) at the conjugate image planes. Current HSI state-of-the-art detectors are based on various photon-to-electron conversion principles that, at best, have quantum efficiencies (QE) of 40 percent in the blue spectrum. Since the conversion process is substantially less than unity, additional laser power is required from the transmitter to make up for the loss of signal on the detector. If a photo detector type material with near unity QE could be used, the HSI system performance would dramatically increase with no additional laser power. The very high efficiency of metamaterial photodetectors will dramatically increase the electrical power available for electric small Unmanned Air Vehicle (UAV). Improvements in photo detection efficiencies are sought to advance the tactical capabilities of HSI systems used on UAVs.

Optical metamaterials with negative refractive index behavior have extraordinary promise in HSI applications. Unlike a conventional lens, a negative refractive index implies that when a material refracts an incoming light ray, the refracted ray will be deviated at a negative angle to the normal according to Snell's law. This seemingly trivial observation has profound consequences: focusing can be accomplished by a slab of material instead of a conventionally-shaped lens. More subtly, lenses made from negative index metamaterials (NIMs) can be much more compact than curved optical lenses such as cylindrical and aspheric lenses, since wave vector components along the optical axis can be used for imaging. In conventional optics, these components typically decay at distances very close to the lens surface (the near field), and account for a loss of imaging information, and ultimately, resolution. For a NIM, that decaying evanescent wave instead grows, allowing near-field resolution to extend into the far field. Furthermore, a negative index implies that the phase of a wave decreases, rather than advances, through the metamaterial NIM. A material with n = -1 can be considered to reverse the effect of propagation through an equivalent thickness of a vacuum. Consequently, NIMs have a potential advantage to form highly efficient low reflectance surfaces by exactly canceling the scattering properties of other materials. If the NIM is isotropic, then these effects occur regardless of the direction of the incident wave. 

NIMs require negative values of both the electrical permittivity (e) and magnetic permeability (µ). Negative permittivity is common in metals at optical frequencies, but negative permeability does not occur naturally; therefore, the construction of metamaterials involves engineering an effective negative permeability using nonmagnetic materials. This can be done by including electromagnetically resonant structures. Optical-frequency resonators are much smaller in scale (< 1 mm), have been recently made with advanced lithographic procedures, and have shown negative index behavior in the visible and near-infrared spectrum. Still, these NIMs are not isotropic as the features are planar, and the index varies with orientation. Additionally, most of them have large optical losses due to the materials that comprise them. 

A quick examination of NIM literature reveals that in most cases where a metamaterial lens would be used to create images, the refractive index should be independent of direction of the incoming radiation. Yet, the properties of most NIMs reported in the literature are not randomly dispersed inclusions, are not dependent on random orientation of crystal grains, and are not inherently isotropic in three dimensions. 

The primary challenge in NIMs is advancing the diffraction limit in Near Field capabilities to identify a threshold to separate targets from clutter in hyperspectral data idiosyncrasies. Hence, NIM designs need to provide larger phase shifts and reduce aberrations to enable tuning the focal length with adjustable sequential metamaterial lens structures, resulting in low far-field resolution of features beyond the diffraction limit in the visible spectrum. Highly viable/manufacturable single and sequential metamaterial lens designs addressing the 3-12 µm spectral range with a focus on 3-5 and 8-12 microns are a possible solution. 

NIM lens system parameters for trade analysis are:

(a) Operational spectral range @ 3-12 microns;

(b) Smallest Spectral sampling step @ 1 nm;

(c) Spectral resolution @ full-width half maximum (FWHM) @ 7–10 nm;

(d) Spectral Stability @ < 1 nm;

(e) Wavelength switching speed @ < 2 ms;

(f ) Incidence angle to the Fabry-Perot Cavity @ < 5° (max < 7°);

(g) Average spectral transmission @ > 0.2;

(h) Image size @ 480 x 750;

(i) Dynamic range @ 10 bit;

(j) F-number range of the optics @ 4.0 – 16.0;

(k) Focal length @ 8–25 mm;

(l) Field of View (FOV) @ 20° x 30°;

(m) Object distance @ 0.05 m – Infinity;

(n) Operational quantum efficiency @ >100% in the 3-12 microns spectral band;

(o) Noise factor @ < 1.1;

(p) Bandwidth @ > 100 MHz;

(q) Fast response speed@ (rise time tr < 68 µs);

(r) Uniform optical quality in terms of refractive index and extinction co-efficient;

(s) Root Mean Square (RMS) errors below 1×10-3 refractive index units (RIU)

(t) Weight @ < 350 g;

(u) Thickness @ 0.2 to 0.5 mm (ultrathin with < wavelength (lambda) divided by 8 surface flatness);

(v) Active areas on the order of 1 to 2 inches in diameter; and

(w) Focusing performance for oblique incidence with an incident angle up to 15 degrees.

PHASE I: Conduct research and experiments to determine potential NIMs for HSI NIM lens and select optimum technical approach using the system parameters for trade. Develop preliminary design and perform detailed analysis for on-chip hyperspectral focal plane arrays, with integration of dynamically tunable NIM lens to allow for spectral reconstruction with a single photodetector; and to be directly integrated with arbitrarily-sized read-out integrated circuits (ROICs) for real-time HSI in-pixel image processing. Preliminary design should also include an integrated/embedded metamaterial structure that can be easily subjected to change in temperature or to stress loads while interrogated by electromagnetic fields. Through experimentation, identify NIM technical risk elements in the HSI metamaterial lens and provide viable risk mitigation strategies. The Phase I effort will include on-chip hyperspectral focal plane arrays, with integration of dynamically tunable NIM lens prototype plans to be developed under Phase II.

PHASE II: Refine the design based on outcomes of simulated data, boot strap error analysis, tests and customer feedback in Phase I. Develop, demonstrate, and validate an HSI metamaterial lens prototype in the lab, chamber, and/or field. Demonstrate and validate the prototype system with all of the parameters identified in Phase I. Prepare a report that summarizes the experimental evaluation and validation of the performance characteristics of the developed system.

PHASE III DUAL USE APPLICATIONS: Complete prototype hardware that will cover operational spectral range@ 300 – 1200 nm. Fully develop and transition the technology and methodology based on the research and development results developed during Phase II for DOD applications in the areas of UAVs detection and identification, and other anomaly surveillance and reconnaissance applications. 

This SBIR topic has direct relevance to commercial private sector airborne remote sensing companies engaged in environmental monitoring, agriculture assessments and exploration of natural resources due to the system’s compact form factor, flexible flight profiles and precision identification, and change/anomaly detection. 

Lower cost hyperspectral sensors for agriculture, land use, search/rescue, and homeland security could employ this technology. The use of low-cost solution-based metamaterials and their ability to be directly integrated with arbitrarily-sized ROICs results in HSI cameras that can be produced at a small fraction of the cost of traditional camera systems.


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