Observing distant galaxies, stars and cosmic events is the closest thing we have to time travel. Because light can take millions or even billions of years to get to us, by looking into deep space we can see moments of cosmic history that tell us how the universe formed and evolved.

Looking into deep space also plays a part in the search for extraterrestrial life, as we examine potentially habitable exoplanets and look for signs of microbial life. By studying the atmospheres of planets outside of our solar system, we can look for biosignatures, like specific gases, that could indicate life.

However, it is our own atmosphere that makes this difficult, as the light that travels unimpeded from planets and stars quadrillions of kilometers away is distorted at the last moment by the layers of gas that envelope the Earth.

Researchers from TMOS, the ARC Centre of Excellence for Transformative Meta-Optical Systems, are working to counter this interference using the new field of meta-optics alongside an optical technique popularized in the 1950s when 20^th Century Fox devised a way to capture a wide-screen film image onto a standard 35mm lens, and then reverse the process as the movie was projected to deliver wide-screen images to audiences.

Anamorphic compression used two cylindrical lenses, and it allowed cinematographers to capture a wider field of view than had been possible. Filmmakers could now show more scenery, bigger crowds, and elaborate action sequences.

Researchers from the Centre’s team at the Australian National University are employing this same concept of anamorphic compression to the adaptive optics systems that are currently used by large telescopes to counter the effect of the atmosphere when looking into space. However, instead of using an array of double-layer traditional lenses, which are too bulky for adaptive optics and too complicated to manufacture with the required variation of focal distances, the team has designed metasurface-based anamorphic lens arrays.

Meta-lenslet arrays are made from surfaces thinner than a piece of clingwrap and have nanostructured patterns on them that are smaller than a wave a light. These surfaces can manipulate light as if it had interacted with traditional lenses, but with a much, much smaller footprint. The two-layer design of this array creates the anamorphic compression that sets this technology apart from other attempts to improve adaptive optics. 

Adaptive optics changes the shape of a telescope mirror to adjust for atmospheric turbulence, resulting in a clearer, more detailed image of deep space and providing astronomers with more information. A laser guide star is projected into the sodium layer of the atmosphere and measuring the distortion of the laser light with a wavefront sensor enables the control system to adjust the shaped the telescope’s mirror to compensate for how the various gases refract the light.

Traditional wavefront sensors use traditional spherical lenses, but like early films, their field of view is limited, so many lenses are needed to capture the full image. Each lens increases the level of noise in the image, obscuring a clear picture. Because the meta-lenslet array uses anamorphic compression and captures more of the picture, it requires fewer lenses, reducing the amount of noise and delivering a clear image of galaxies far from ours.

First author, TMOS post-doctoral researcher Josephine Munro says, “This lenslet array has been designed for the Giant Magellan Telescope, one of three next-generation extremely large telescopes that are scheduled to begin operation in the next decade. Its mirror will be twenty-four meters in diameter and, with adaptive optics, its resolution will be ten times sharper than the Hubble Telescope. With the significant amount of funding that has gone into the development of this telescope by governments, universities, private donors and philanthropic organisations, it is essential that the images produced are the highest possible quality, hence our research into improving adaptive optics with cutting edge meta-optics technology.”

The primary challenge of using traditional lenses in an adaptive optics system is that because the laser is a column of light and not a point of light like a star, lenses farther from the laser experience elongated focal spots. Elongated focal spots have a detrimental effect on the computer algorithm that determines how the mirror needs to adapt. This is particularly true when the elongation extends beyond the boundaries of a size pixel on the detector. The researcher’s anamorphic compression meta-lenslet array corrects the focal spot distortion.

Munro says, “This study presents the first design of a metasurface-based array to directly and uniquely minimize the effects of laser guide-star elongation for Slack-Hartmann wavefront sensors. In modelling, the meta-lenslets achieved an image size reduction of 50 – 100% compared with traditional spherical lenses.”

“Overall, the anamorphic compression does not affect the sensitivity achieveable with the meta-lenslet array. This is because the anamorphic compression has two effects on the wavefront sensor behaviour that cancel each other out. The first relates to the decrease in effective focal length and the second relates to the decrease in the full-width-half-maximum of the spot. There is some residual laser guide star image elongation, but is offers a significant improvement.” 

TMOS Chief Investigator Andrey Sukhorukov says, “The next steps for this research will be to fabricate a prototype made with silicon to verify the performance of the meta-lenslets with a range of anamorphic compression ratios. Once the design is validated, further samples can be made with titanium dioxide, which will gain a higher throughput efficiency. It is our hope that this work will integrate with existing plans for the Giant Magellan Telescope and that the resulting deep-space images that come from it provide insights that change our understanding of the universe.”

For more information about this research, please contact connect@tmos.org.au