The Photon Sieve Could Revolutionize Optics

By Matthew Williams - July 27, 2016 at 3:10 PM UTC | Telescopes

Ever since astronomers first began using telescopes to get a better look at the heavens, they have struggled with a basic conundrum. In addition to magnification, telescopes also need to be able to resolve the small details of an object in order to help us get a better understanding of them. Doing this requires building larger and larger light-collecting mirrors, which requires instruments of greater size, cost and complexity.

However, scientists working at NASA Goddard's Space Flight Center are working on an inexpensive alternative. Instead of relying on big and impractical large-aperture telescopes, they have proposed a device that could resolve tiny details while being a fraction of the size. It's known as the

photon sieve

, and it is being specifically developed to study the Sun's corona in the ultraviolet.

Basically, the photon sieve is a variation on the Fresnel zone plate, a form of optics that consist of tightly spaced sets of rings that alternate between the transparent and the opaque. Unlike telescopes which focus light through refraction or reflection, these plates cause light to diffract through transparent openings. On the other side, the light overlaps and is then focused onto a specific point - creating an image that can be recorded.

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This image shows how the photon sieve brings red laser light to a pinpoint focus on its optical axis, but produces exotic diffraction patterns when viewed from the side. Credits: NASA/W. Hrybyk

Image showing the photon sieve bringing red laser light to a pinpoint focus on its optical axis, and producing exotic diffraction patterns. Credits: NASA/W. Hrybyk

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The photon sieve operates on the same basic principles, but with a slightly more sophisticated twist. Instead of thin openings (i.e. Fresnel zones), the sieve consists of a circular silicon lens that is dotted with millions of tiny holes. Although such a device would be potentially useful at all wavelengths, the Goddard team is specifically developing the photon sieve to answer a 50-year-old question about the Sun.

Essentially, they hope to study the Sun's corona to see what mechanism is heating it. For some time, scientists have known that the corona and other layers of the Sun's atmosphere (the chromosphere, the transition region, and the heliosphere) are significantly hotter than its surface. Why this is has remained a mystery. But perhaps, not for much longer.

As Doug Rabin, the leader of the Goddard team, said in a NASA

press release

:

With support from Goddard's Research and Development program, the team has already fabricated three sieves, all of which measure 7.62 cm (3 inches) in diameter. Each device contains a silicon wafer with 16 million holes, the sizes and locations of which were determined using a fabrication technique called photolithography - where l

ight is used to transfer a geometric pattern from a photomask to a surface.

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Doug Rabin, Adrian Daw, John O’Neill, Anne-Marie Novo-Gradac, and Kevin Denis are developing an unconventional optic that could give scientists the resolution they need to see finer details of the physics powering the sun’s corona. Other team members include Joe Davila, Tom Widmyer, and Greg Woytko, who are not pictured. Credits: NASA/W. Hrybyk

The Goddard team led by Doug Rabin (left) is working on a new optic device that will drastically reduce the size of telescopes. Credits: NASA/W. Hrybyk

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However, in the long-run, they hope to create a sieve that will measure 1 meter (3 feet) in diameter. With an instrument of this size, they believe they will be able to achieve up to 100 times better angular resolution in the ultraviolet than NASA's high-resolution space telescope - the

Solar Dynamics Observatory

. This would be just enough to start getting some answers from the Sun's corona.

In the meantime, the team plans to begin testing to see if the sieve can operate in space, a process which should take less than a year. This will include whether or not it can survive the intense g-forces of a space launch, as well as the extreme environment of space. Other plans include marrying the technology to a series of CubeSats so a two-spacecraft formation-flying mission could be mounted to study the Sun's corona.

In addition to shedding light on the mysteries of the Sun, a successful photon sieve could revolution optics as we know it. Rather than being forced to send massive and expensive apparatus' into space (like the

Hubble Space Telescope

or the

James Webb Telescope

), astronomers could get all the high-resolution images they need from devices small enough to stick aboard a satellite measuring no more than a few square meters.

This would open up new venues for space research, allowing private companies and research institutions the ability to take detailed photos of distant stars, planets, and other celestial objects. It would also constitute another crucial step towards making space exploration affordable and accessible.

Further Reading: NASA