The Optical Coatings Behind JWST's Out-Of-This-World Pictures
By John Oncea, Editor
Optical coatings enhance aerospace performance, from reducing satellite reflectivity to ultrablack films for light absorption. NASA and JWST use advanced coatings for durability and efficiency.
Optical coatings are incredibly important in the aerospace industry, improving the performance, safety, and efficiency of the spacecraft and satellites sent to circle the Earth, the Sun, the Moon, and many other destinations within our Solar System. Aerospace applications include anti-reflective coatings on aircraft windows, sensor and imaging system enhancements, radar and stealth technology, and more.
Starting in late 2023, in response to years of complaints about the high reflectivity of their constellation of internet satellites, SpaceX started using a mirror film that scatters light away from Earth, reports ExtremeTech. The film, which acts as a Bragg mirror, “is composed of multiple layers of plastic with varying refractive indices. This produces interference patterns that scatter light but allow radio waves to pass through. SpaceX also coated parts of the satellites with low-reflectivity black paint to absorb light.”
More recently, researchers from the University of Shanghai for Science and Technology and the Chinese Academy of Sciences announced the development of a thin, ultrablack film coating for aerospace-grade magnesium alloys they say can absorb a whopping 99.3% of light in even the harshest of conditions, writes Space.com.
“Existing black coatings like vertically aligned carbon nanotubes or black silicon are limited by fragility,” Yunzhen Cao, study co-author and professor at the Shanghai Institute of Ceramics, Chinese Academy of Sciences said. “It is also difficult for many other coating methods to apply coatings inside a tube or on other complicated structures. This is important for their application in optical devices as they often have significant curvature or intricate shapes.”
To accomplish this, the research team utilized atomic layer deposition (ALD), a manufacturing technique that occurs in a vacuum chamber and exposes a target to specific types of gas. The ultrablack coating is formed with alternating layers of aluminum-doped titanium carbide (TiAlC) and silicon nitride (SiO2). When combined, these layers act as a barrier to nearly all light.
“One big advantage of the ALD method lies in its excellent step-coverage ability, which means we can obtain uniform film coverage on very complex surfaces, such as cylinders, pillars, and trenches,” Cao said. “TiAlC acted as an absorbing layer, and SiO2 was employed to create an anti-reflection structure, and as a result, nearly all of the incident light is trapped in the multilayer film, achieving efficient light absorption.”
NASA, too, utilizes optical coatings to enhance the performance and durability of its space missions. These coatings are crucial for protecting and optimizing the function of optical components in the harsh conditions of space, as well as improving the efficiency of solar panels and protecting astronauts from radiation.
Applications Of Optical Coatings By NASA
NASA continues to innovate optical coatings, with research focusing on enhancing the durability and performance of these coatings under space conditions. Recent advancements are pushing the boundaries of technology, focusing on enhancing performance and functionality. These advancements include:
- Nanostructured Coatings: Nanostructured coatings, engineered at the nanoscale, allow precise control over optical properties. These coatings enhance light absorption, reduce reflections, and improve spectral selectivity, particularly useful in optical filters, solar cells, and anti-reflective applications. Their ability to optimize light trapping makes them crucial for high-efficiency solar cells and other advanced optical systems.
- Magnetron Sputtering: This thin-film deposition technique offers high precision and uniformity, essential for creating coatings with enhanced optical properties and durability. Magnetron sputtering is widely used to produce coatings for various optical applications, ensuring improved performance and reduced defects.
- Anti-Reflective (AR) Coatings: AR coatings are vital for space applications due to the harsh environmental conditions. These coatings minimize reflection losses and enhance light transmission, critical for LiDAR systems and other optical instruments used in space missions.
NASA’s collaboration with research institutions and private companies is accelerating the development of new coating technologies. These partnerships are essential for tackling the unique challenges posed by space exploration and ensuring the success of future missions.
According to UQG Optics, nanotechnology is expected to play a significant role in the future of optical coatings, as is a growing emphasis on developing environmentally friendly coatings. And, as technologies like AR and VR continue to evolve, there will be a need for specialized coatings including the integration of smart coatings with adaptive capabilities.
The Unique Coatings Used By The James Webb Space Telescope
The James Webb Space Telescope (JWST) uses advanced and highly specialized optical coatings to enhance its performance in various ways. These coatings are tailored to enhance performance in the infrared spectrum, protect the mirrors, and improve overall efficiency. The key optical coatings used in JWST include:
- Gold Coating on Mirrors: JWST's primary, secondary, and tertiary mirrors are coated with a very thin layer of gold.
- Uniqueness: Gold is highly reflective in the infrared spectrum, which is essential for JWST as it primarily observes in the near-infrared to mid-infrared wavelengths (0.6 to 28.5 microns). The gold layer is only about 100 nanometers thick, ensuring minimal weight while providing excellent reflectivity.
- Silicon Dioxide (SiO2) and Hafnium Oxide (HfO2) Overcoat: The gold-coated mirrors are further protected by a combination of silicon dioxide and hafnium oxide.
- Uniqueness: This overcoat serves multiple purposes: it protects the delicate gold layer from physical and chemical damage and helps to maintain the mirror's reflective properties over the telescope's operational lifetime. The combination of SiO2 and HfO2 provides durability and a hard, protective surface.
- Dielectric Anti-Reflective Coatings: These coatings are applied to various lenses and optical elements within JWST's instruments.
- Uniqueness: The anti-reflective coatings are tailored to specific wavelengths to minimize reflection losses and maximize the transmission of light through the optical elements. This ensures that as much light as possible reaches the scientific instruments, improving sensitivity and data quality.
- High-Efficiency Dichroic Coatings: Dichroic filters are used to split light into different wavelengths, directing specific bands to various instruments.
- Uniqueness: These coatings are designed to reflect certain wavelengths while transmitting others with very high efficiency. This selective reflection and transmission are crucial for the simultaneous operation of multiple instruments on JWST, allowing it to gather a wide range of data.
- Multi-Layer Interference Filters: These filters are used in scientific instruments to isolate specific spectral lines and bands.
- Uniqueness: Multi-layer interference filters consist of multiple thin layers of materials with different refractive indices, engineered to create constructive and destructive interference at specific wavelengths. This allows for precise control over the wavelength range that is transmitted or reflected, enabling detailed spectroscopic studies.
These unique optical coatings are integral to the success of JWST’s mission, allowing it to capture high-resolution, high-sensitivity infrared images and spectra, as well as explore the formation of stars and galaxies, study the atmospheres of exoplanets, and explore the early universe.