SITIS Archives - Topic Details
Program:  SBIR
Topic Num:  A11-134 (Army)
Title:  Nanostructured High Performance, High Angle of Incidence Anti-Reflection Coatings
Research & Technical Areas:  Materials/Processes

Acquisition Program:  PEO Missiles and Space
 The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), which controls the export and import of defense-related material and services. Offerors must disclose any proposed use of foreign nationals, their country of origin, and what tasks each would accomplish in the statement of work in accordance with section 3.5.b.(7) of the solicitation.
  Objective:  Design, develop and demonstrate robust nanostructured high performance anti-reflection coatings that allow over 98.5% transmission over a broad visible spectral range with cone angles up to 120 degrees.
  Description:  Soldier warfare requires the accurate detection, recognition and identification of possible targets for engagement without giving away one’s position to the enemy. As such, soldiers rely on optical sighting systems to identify targets while maintaining stealth movement. Anti-reflection coatings are applied to these optical systems to serve a dual purpose. First, reflections from the front lens of the optical train can alert enemy troops to the soldier’s position. An example of this phenomenon can be witnessed when the sun glints off the windows of a distant mountain home to reveal its presence in an otherwise unnoticeable location. A famous example of a glint-induced casualty is exemplified by Gunnery Sergeant Carlos Hathcock, who shot a Viet-Cong sniper after locating the glint from his scope. Secondly, anti-reflection coatings are applied to maximize the transmission throughput of the optical device. By maximizing the transmission throughput, lens apertures can be minimized and overall system sizes can be kept small. Small arms weapon systems are already heavy in nature, therefore it is important to resist adding additional weight to these weapons in the form of large optical sighting systems, so that there is no reduction in the soldier’s combat ability. Current anti-reflection coatings suffer from transmission losses over broad fields-of-view. As the light source moves from being directly in front of the system to off to the side of the system, the coating spectral properties tend to “blue shift.” In other words, the light transmitted through the coating will shift towards lower wavelengths. If this shift is large enough, critical spectral transmission may be lost, causing either unwanted reflections from the front lens or loss of spectral resolution through the device. In either case, soldier lethality is compromised. To minimize the off-angle glint potential, a honeycomb-type anti-reflection device (ARD) is installed in front of the optic, to eliminate off-angle reflections. However, these ARDs reduce the amount of incident light by approximately 15% and reduce the overall viewing angle of the optic. Both of the aforementioned drawbacks limit the utility of the optic, and in some operational environments may require a larger optic to accommodate the reduced light transmission due to the ARD. Increasing the size of the optic directly correlates to an increase in weight, further burdening the warfighter. In more extreme instances, the ARD is completely removed from the optic to maximize the light throughput and viewing angle, thus exposing the warfighter to a potential glint-induced positional compromise. Nanostructured antireflective coatings can eliminate the aforementioned high angle of incidence issues and alleviate the need for an ARD. Nanostructured coatings have been proposed for solar cell applications to eliminate the need for sun tracking while maintaining high quantum efficiencies. In other words, these nanostructured coatings allow for high light transmission by eliminating the reflections at most angles of incidence, including the reflections at large angles. It is envisioned that the nanostructured technology can be utilized in optical sighting systems to decrease both the off-angle reflections and overall optical system sizes, while reducing the soldier’s signature on the battlefield without the use of ARDs.

  PHASE I: Using N-BK7 or equivalent glass substrate, identify materials and methods for preparing nanostructured anti-reflection coatings. Spectral properties shall be modeled and simulated for angular response. Small-scale proof-of-concept samples shall be produced with identified materials and methods.
  PHASE II: Develop prototype anti-reflection coatings with broadband visible transmission that demonstrate minimal angular shift. Coatings will be spectrally analyzed for reflection properties from 0 degrees to 80 degrees. Cross-section imaging will be performed to show microstructure of coating. Coatings shall be developed to meet the durability requirements of MIL-C-675 for immersion, adhesion and abrasion. Coatings are expected to operate from -65 degrees F to +160 degrees F with no degradation. Coatings should withstand high relative humidity at elevated temperatures. Delivery of prototype parts on minimum 45 mm diameter substrates required. Investigation of transmission properties into the short-wavelength infrared (1.4 to 3 µm) coatings shall be evaluated.

  PHASE III: Develop large scale, sustainable processing capabilities for anti-reflection coatings for a wide variation of substrate materials and substrate sizes. Dual use capabilities include digital camera lens coatings, solar panel coatings, glasses/spectacle coatings, and optical lens coatings for use with CCD and FPA sensors.

  References:   1. P. Yu, C.-H. Chang, C.-H. Chiu, C.-S. Yang, J.-C. Yu, H.-C. Kuo, S.-H. Hsu, and Y.-C. Chang, “Efficiency Enhancement of GaAs Photovoltaics Employing Antireflective Indium Tin Oxide Nanocolumns”, Advanced Materials, Vol. 21, pg. 1618 – 1621. 2. http://www.physorg.com/news146926366.html 3. http://www.physorg.com/news144940463.html 4. MIL-C-675C, Military Specification Coating of Glass Optical Elements (Anti-Reflection). http://www.everyspec.com/MIL-SPECS/MIL+SPECS+(MIL-C)/download.php?spec=MIL-C-675C.010362.PDF

Keywords:  Anti-Reflection Coating, Nanostructure, Metamaterial

Questions and Answers:
Q: Is the presence of Indium tin oxide on the target BK7 or equivalent glass substrate an acceptable coating or film, in addition to the nanostructured anti-reflective coating being requested? This question is inspired by the call, and some of the references in the call.
A: The presence of Indium Tin Oxide (ITO) is an acceptable coating in addition to the nanostructured anti-reflective coating.

Notwithstanding, if the nanostructured anti-reflective coating/structure is sufficient in itself to meet the requirements of the SBIR (98.5% transmission, 120 degree cone angle), an ITO coating or any other secondary coating is not necessarily needed or required.

Q: Is it possible for the Army to provide us with samples of target N-BK7 or equivalent glass substrates? If this is not possible, could you tell us an acceptable source at which we can acquire such samples?
A: It will not be possible for the Army to provide samples of target N-BK7 or equivalent glass substrates. Also, it is inappropriate for the Army to identify potential suppliers of this glass. N-BK7 or equivalent glass (borosilicate crown glass) is the most readily available glass that is on the market. A simple internet search will identify multiple suppliers for these target substrates.

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