The next frontier: meta-optics in space
09 Oct, 2023
Of the many challenges the space industry faces, size and weight constraints when sending payloads into space is one of the most keenly felt pain points. The limited capacity of launch vehicles means every kilogram and every cubic centimetre of room matters. This makes improving the functionality of space technology difficult, as increased functionality often leads to increased size, weight and power requirements. Traditional optics has often been this choke point with lens systems adding significant bulk. However, with advancements in technology, we may finally break through this bottleneck.
The relatively new field of meta-optics is the relief the industry has been waiting for, allowing for the simultaneous advancement and miniaturization of devices as it works in concert with existing optical elements to generate, manipulate and detect light.
Meta-optics uses sub-wavelength patterned surfaces—metasurfaces—to mimic the effects of traditional optical components such as lenses and mirrors. These artificially engineered materials manipulate light by scattering it within nanoparticles rather than refracting it through a traditional lens. Metasurfaces can engineer the light wavefront, emission and absorption in a way that demonstrates improved performance over traditional optics with a vastly smaller footprint, and they offer functionalities that cannot be achieved conventionally.
If you’re looking for wavelength or polarization selectivity from multiple angles without using multiple systems, meta-optics can help with that. In addition, metasurfaces can also perform other functions, such as beam steering, beam focusing, and light modulation, by incorporating elements such as phase shifters, reflectors, and transmissive elements.
Unlike some other fields of fundamental science, research into meta-optics is largely driven by industry needs with desired applications defining research problems in university systems. Prof. Dragomir Neshev, Centre Director of TMOS, the Australian Research Council Centre of Excellence for Transformative Meta-Optical Systems, says “We’re working closely with industry partners to identify the research pathway that is going to have the greatest real-world impact. Already, some of our projects have been taken in new directions as engineers in private space, defence and medtech companies have asked ‘can meta-optics do this…?’”
“We offer meta-optics discovery calls to help businesses identify how meta-optics might contribute towards their objectives, but these calls also give us crucial insight that helps to guide our work.”
Meta-optics has a part to play in almost all aspects of space technology. In Australia, the focus is on space communications, space navigation, earth observation, and the development of new lightweight optical instruments.
Secure quantum communications has been a key objective for many industries over the past few decades. While significant progress has been made, much of the technology requires clean rooms and cryogenic cooling, which are impractical for widespread use in space. There is a lack of affordable, reliable quantum light sources that can encode and transmit the information without these tools. Low light intensity also poses an additional challenge.
Researchers are currently working on incorporating metamaterials into devices that generate and/or detect photons as part of a quantum communication platform. They have developed new technology that integrates quantum sources and waveguides on chip in a manner that is both affordable and scalable, paving the way for lighter satellite payloads and more accessible use. These quantum source nanostructures are made of easily sourced and cost-effective hexagonal boron nitride, also known as white graphene. These new quantum emitters can be created using $20 worth white graphene pressed onto an adhesive and exfoliated. The process of exfoliation, or peeling off the top layer, allows for the creation of a flexible 2D material that can be stacked and assembled like building blocks, providing a novel bottom-up approach to replace traditional 3D systems.
In addition to this evolution in photon sources, the team has developed a high efficiency on-chip waveguide, a vital component for on-chip optical processing, and is currently working on developing ultra-high quantum efficiency detectors suitable for quantum key distribution
Prof. Neshev says “This combination of innovative quantum emitters and efficient waveguides sets the stage for exciting advancements in the field of optical technology. Low signal levels and bulky devices have been a significant barrier preventing quantum communications from becoming a typical inclusion in space technology.”
While GPS has proven the most cost-effective system for space navigation, sometimes high accuracy Light Detection and Ranging (LiDAR) systems are preferable, such as during rendezvous and docking operations or mapping the surface of celestial bodies in order to aid with landing. Single photon detection at near-infrared is critical for this, but current high performance, portable LiDAR technologies generally rely on silicon-based single-photon avalanche diodes, which are limited in their operational wavelengths to 905nm. InP diodes can extend the operational wavelengths of a detector, however, in their current bulk form, they produce too much dark current to be effective.
Metamaterials made of low-dimensional III-V compound semi-conductor nanowires aim to increase the functionality of LiDAR detection systems by improving sensitivity and decreasing the dark count rate while simultaneously minimizing the device size to lower production costs and the costs associating with putting LiDAR systems into space.
Prof. Neshev says “Using nanowires enables high-quality materials at a reduced cost. Furthermore, nanowires can decrease the operating temperature of the device, which contributes to thermal noise while maintaining sensitivity to light. Nanowire metasurfaces have demonstrated good absorption of photons despite their ultra-thin form factor.”
Earth Observation and Situational Awareness
Small sized satellites, like nanosats, are an important tool for Earth observation as they are more cost effective than traditional large satellites, but the size and weight of conventional optics restricts their capability to execute complex imaging, such as polarimetry and/or multi-spectral imaging. Multiple polarimetric and/or spectral sensors must be able to accurately measure intensity and polarization state over a wide spectral range and multiple angles of incidence. A metasurface-based alternative can analyse the polarization state of light at different angles by combining polarization rotators and splitters in an engineered pattern that measures the intensity of the multiple polarization components to determine the degree and orientation of the polarization.
In addition to its flexibility to measure light of different wavelengths and angles of incidence, sensors made from metasurfaces have increased sensitivity and, when used as a diffractive metasurface, can form polarisation measurements without losing light to filtering. This is essential as dynamic earth observation requires low-light imaging.
Polarisation imaging has a wide range of satellite-based remote sensing applications, such as detecting organic aerosols in the atmosphere or removing sun glint from ocean surface images. Following successful simulations, researchers at ANU are currently fabricating a metasurface made of a 1 micrometer-thick patterned surface on a sapphire substrate that simulates and optimises polarimetric behaviour.
Prof. Neshev says “Polarisation and spectral imaging offers the opportunity to see the information that is invisible to our eyes. Metasurface-based polarisation and spectral imaging allows us to extract this information without the need for bulky optical components and it can be directly implemented in small satellites.”
Challenges facing the field
Metasurfaces have tremendous potential in the field of optics, and the recent advancements in this technology have been nothing short of remarkable. However, it is not without its challenges. In particular, the scalability of meta-surfaces for commercial use has significant hurdles. Metasurfaces are fabricated using planar fabrication technology, effectively merging optics and chip-making technologies.
Researchers at the University of Western Australia have taken a major step forward in this area, demonstrating this sovereign capability in the laboratory. They recently fabricated Australia’s largest semiconductor infrared imaging array that measures 10mm x 8mm and containing more than 80,000 individual photovoltaic detectors. However, incorporating meta-optics with imaging arrays is much more challenging, because the metasurface layer typically includes structures with dimensions that are sub-wavelength, thus requiring advanced photolithography capabilities such as electron beam lithography. In addition, most electron-beam lithography tools used in the laboratory cannot pattern large areas, and advanced photomasks are required for deep-UV lithography.
Businesses currently manufacturing chips are the best positioned to manufacture metasurfaces at scale. This presents a significant opportunity for those willing to invest in fabrication tool upgrades. The Australian Government has classified metasurfaces as a technology of interest and, as such, funding to assist with the commercialisation of metasurfaces may be available through the Modern Manufacturing Initiative.
Another challenge is the need for non-standard optical thin-films and substrates. Some headway has been made towards this by depositing amorphous silicon on glass wafers, however the technology still faces obstacles, including robustness in the face of environmental changes, a point of concern for all metasurface materials. Having already conducted rigorous laboratory testing and evaluation of several metasurfaces, TMOS is in negotiations to fully space qualify the materials by sending a payload into orbit in 2023.
The applications of meta-optics explained above are just a fraction of the many ways this exciting field is going to change the space industry as we know it. TMOS, the Australian Research Council Centre of Excellence for Transformative Meta-Optical Systems, offers discovery calls to help businesses understand how meta-optics can give added functionality to their products. The Centre is also keen to explore new research pathways based on industry needs. To explore a potential collaboration, please contact firstname.lastname@example.org