Tag Archives: #telescopes

“Spaceplate”, A New Avenue For Manipulating Light That Could Lead to Paper Thin Cameras, Telescopes (Physics)

Can you imagine one day using a telescope as thin as a sheet of paper, or a much smaller and lighter high-performance camera? Or no longer having that camera bump behind your smartphone?

In a paper published in Nature Communications, researchers from the University of Ottawa have proposed a new optical element that could turn these ideas into reality by dramatically miniaturizing optical devices, potentially impacting many of the applications in our lives.

To learn more about this project, we talked to lead author Dr. Orad Reshef, a senior postdoctoral fellow in the Robert Boyd Group, and research lead Dr. Jeff Lundeen, who is the Canada Research Chair in Quantum Photonics, Associate Professor in the Department of Physics at the University of Ottawa, and head of the Lundeen Lab.

Can you describe the new optical element your team developed, the spaceplate?

Orad Reshef: “Light naturally “spreads out” when it is travelling and every optical device we know of relies on this spread; we wouldn’t know how to design cameras without it. For example, in every telescope there is a large gap between the eyepiece and the objective lens to give light room to spread.

“A spaceplate simulates the same spreading that light would experience travelling a large distance in a small device. To light, a spaceplate looks like “more space” than it occupies. In a way, the spaceplate is a counterpart to the lens, doing things the lens can’t do to shrink down entire imaging systems.

“We introduced the idea of a spaceplate in our paper, experimentally demonstrating it and showing it is compatible with broadband light in the visible spectrum that we use to see.”

Jeff Lundeen: “We considered what would happen if you manipulated light based on the angle rather than the position of a light ray. Lenses act via the position of the ray. Angle is a completely novel domain, and no one had shown that it could be used to make something particularly useful. We identified a useful application, compressing space. And then we showed that we could actually design and experimentally demonstrate plates that do exactly that.”

Orad Reshef: “This is exciting because this device will let us shrink down all sorts of very large devices that we thought were impossible to miniaturize in optics. In order to design it, we need to come up with a new set of rules that is incompatible with that used in lens design. Nobody knows what they are, it’s like the wild west.”

How did you come up with this idea?

Jeff Lundeen: “Orad Reshef is an expert in using nanotechnology for manipulating a ray based on its position (e.g. meta-lenses or, more generally, meta-surfaces). We were casually discussing the limitations of manipulating light with these meta-surfaces and I said it would be cool to instead manipulate light based on its angle.”

“Dr. Reshef was immediately confident that he could design and fabricate something that could do that and I subsequently concluded the easiest goal would be to replace the space needed for spread (i.e. propagation).”

“Over the course of the next few months, in discussions with Dr. Boyd and Dr. Reshef, we slowly realized how amazing and useful such a device would be. Both Dr. Reshef and I came up with viable and completely different designs, which showed there were many ways to create such a device. We studied three in our paper but there are more coming.”

How could this technology be used? What are the applications of the spaceplate in our daily lives?

Orad Reshef: “A spaceplate can be used to miniaturize many optical systems, be it a display or a sensor. For example, an advanced spaceplate can enable paper-thin telescopes or cameras; it could be used to remove the “camera bump” on the back of your smartphone.”

Jeff Lundeen: “People lug around large cameras with huge telephoto lenses. If we can sufficiently improve the spaceplate’s performance, I envision the possibility of building smaller, lighter cameras with much better performance. In particular, the spaceplate combined with metalenses would allow us to make the entire back surface of, say, an iPhone Max, into a flat and thin camera. It would have as much as 14 times better resolution and low-light performance than those large and heavy cameras.

“Thin and small cameras would be useful in a wide variety of applications, including in health care where camera pills or endoscopes could look inside arteries or the digestive system.”

What are the next steps?

Orad Reshef: “We are hard at work developing the next generation of this technology. We want to try and increase the compression factor and to improve the overall performance. We already have some designs to increase the compression factor from 5 to over 100 times, and to increase the total transmission. To continue doing this, we need to come up with a completely new design paradigm.”

Any final thoughts?

Orad Reshef: “It’s surprising that optical elements like lenses have been around for a millennium and their design rules have been well understood for over 400 years, and yet we’re still discovering such fundamental new optical elements for imaging.”

This research is a collaboration between two research groups of two physics professors at uOttawa, Robert Boyd, Canada Research Chair in Quantum Nonlinear Optics, and Jeff Lundeen, Canada Research Chair in Quantum Photonics. Both groups work closely together as part of the Canada Excellence Research Chair Group in Quantum Photonics (CERC) assembled by Robert Boyd (coauthor), CERC Laureate in Quantum Nonlinear Optics.

The article “An optic to replace space and its application towards ultra-thin imaging systems” is published in Nature Communications.

Featured image: Operating principle of a spaceplate. a, A spaceplate can compress a propagation length of deff into a thickness d. For example, a beam incident on the spaceplate at angle θ will emerge at that same angle and be transversely translated by length w (resulting in a lateral beam shift ?x), just as it would for deff of free space. b, Adding a spaceplate to an imaging system such as a standard camera (top) will shorten the camera (center). An ultrathin monolithic imaging system can be formed by integrating a metalens and a spaceplate directly on a sensor (bottom). © Orad Reshef and Jeff Lundeen

Provided by University of Ottawa

What Degrades the Performance Of Silver Thin Film Mirrors Use In Telescopes? (Instrumentation / Astronomy)

High-performance astronomical telescopes benefit from durable broadband mirrors based on silver (Ag). Advantages of Ag include higher reflectivity and lower emissivity in the thermal infrared spectrum when compared to aluminum mirrors currently in prevalent use. There have been some successful implementations of Ag mirrors; notably those used on the Gemini telescopes. However, few ground-based telescopes choose to utilize Ag mirrors in actual observatory environments, which reveals the elusiveness of Ag mirrors. The deep blue and UV spectra are very important for many astronomical research programs; however, even the Ag mirrors used in the Gemini telescope suffer from compromised deep blue and UV reflectivity due to the presence of an optically absorbing NiCrN adhesion film placed on a Ag film. In addition, bare Ag tarnishes easily in the presence of oxygen and especially sulfur in Earth’s atmosphere, and it experiences rapid corrosion via salt formation with halides. Therefore, reflective surfaces of Ag films must be immediately covered by an optically transparent protection coating to avoid tarnish and maintain original reflectivity for years.

Both the reflective Ag film and the subsequent protection coating are routinely deposited at room temperature by various physical vapor deposition (PVD) techniques including sputtering, evaporation, and cathodic arc deposition. However, extensive literature indicates that PVD techniques produce protection coatings with superior durability against corrosion when carried out above room temperature. Apart from PVD, atomic layer deposition (ALD) offers protection coatings with performance often superior to those deposited by PVD techniques. For instance, in our previous work, the highly uniform, conformal, and virtually pinhole-free nature of Al2O3 protection coatings deposited by ALD demonstrated superior durability over comparable Al2O3 deposited by electron-beam evaporation. In our other previous work, it was found that ALD based Al2O3 protection coatings on Ag exhibited higher durability with higher ALD processing temperature. However, before further work can be done to maximize durability of Ag mirrors covered with protection coatings by ALD done at an optimal elevated processing temperature, the effects of such elevated processing temperatures on properties of Ag films themselves and of the entire Ag mirror structure must be thoroughly studied.

Fig. 1 Illustration of the three types of Ag mirror samples showing details of layered structures with nominal thicknesses. (a) UCO Ag 2018, (b) Bare-Ag, and (c) Al2O3/Ag. © Kobayashi et al.

Thus, in their recent study, Kobayashi and colleagues designed and fabricated high-performance Ag mirrors with a new benchmark. The resulting Ag mirrors were annealed (i.e., post-fabrication annealing) at various temperatures to investigate the viability of introducing thermal processes during and/or after fabrication in improving overall optical performance and durability of protected silver mirrors. In their experiments, Ag mirror samples were deposited by electron-beam evaporation and subsequently annealed at various temperatures in the range from 60 °C to 300 °C, and then the mirror samples underwent an environmental stress test at 80 °C and 80% humidity for 10 days.

They found that, while all the mirror samples annealed below 200 °C showed negligible corrosion after undergoing the stress testing, those annealed below 160 °C presented spectral reflectivity comparable to or higher than that of as-deposited reference samples. In contrast, the mirror samples annealed above 200 °C exhibited significant degradation after the stress testing. The comprehensive analysis indicated that delamination and voids caused by the growth of Ag grains during the annealing are the primary mechanisms of the degradation.

“Formation of void defects and delamination between the Ag thin film and adjacent barrier film, caused by Ag grain growth during annealing at temperatures higher than 160 °C, are suggested as the major failure mechanisms. Our experiment suggests that the common practice, for Ag-based protected mirrors, of not using post-fabrication processing at elevated temperature should be re-evaluated for the sake of improving performance and durability.”

— concluded authors of the study

Featured image: SEM images of as-deposited and 500 °C-annealed Bare-Ag and Al2O3/Ag samples. Scale bar represents 500 nm. © Kobayashi et al.

Reference: David M. Fryauf, Andrew C. Phillips, Nobuhiko P. Kobayashi, “Critical processing temperature for high performance protected silver thin film mirrors”, ArXiv, pp. 1-18, 2021. https://arxiv.org/abs/2104.08233

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Small Telescopes, Big Science (Astronomy)

A quest to open up the dynamic, infrared sky begins

Our Milky Way galaxy is chock-full of dust. Stars are essentially dust-making factories that infuse the galaxy with a haze of dusty elements required for making planets and even life. But all that dust can make viewing the cosmos difficult. Telescopes that detect visible, or optical, light cannot see through the murkiness, and thus some of what goes on in the universe remains enshrouded.

Luckily for astronomers, infrared light, which has longer wavelengths than optical light, can sneak past dust. Several infrared-sensing telescopes, such as NASA’s Spitzer Space Telescope, have taken advantage of this fact and revealed much of the so-called infrared sky, including hidden planets, stars, supermassive black holes, and more. The next frontier for infrared astronomy involves watching how the infrared sky changes over time, an effort that Mansi Kasliwal (MS ’07, PhD ’11), an assistant professor of astronomy at Caltech, refers to as opening up the dynamic infrared sky.

To that end, Kasliwal has planned a series of four small ground-based infrared telescopes that will reveal everything from never-before-seen star explosions, asteroids, and even infrared counterparts to stellar collisions that send ripples through space and time known as gravitational waves.

The ambitious plan begins with Palomar Gattini-IR, a robotic instrument now in operation at Palomar Observatory, and will eventually include two additional instruments, called WINTER (Wide-field Infrared Transient Explorer) and DREAMS (Dynamic Red All-Sky Monitoring Survey), both of which are under construction. The final step in the plan is to build an instrument destined for Antarctica, where the chilly temperatures lead to even crisper views of the infrared sky.

“We are changing the game,” says Kasliwal. “We are building little telescopes that do big science.”

The little structure at Palomar Observatory where Gattini is housed.Credit: Caltech

Palomar Gattini-IR, or just Gattini for short, refers to the Italian word for kittens, gattini, and came from Kasliwal’s collaborator Anna Moore, a professor of astronomy at Australian National University, who used the term to casually refer to her own fleet of small telescopes in the Antarctic. “The name just stuck,” explains Kasliwal. Palomar Gattini-IR has been busy robotically scanning the skies from its perch in a small dome at Palomar Observatory since 2019 and has already produced some interesting results.

Stars That Go Bang

One recent paper accepted in The Astrophysical Journal reports the first real estimate of the number of nova explosions, or novae, that go off in our Milky Way galaxy per year (the answer is about 46). Novae are not as bright as supernovae, but powerful nonetheless and can briefly shine brighter than one million suns. They occur when a white dwarf, the burned-out core of a star, siphons enough material off a companion star to cause an explosion. These bursts are thought to seed our universe with many of the elements that make up our periodic table; in fact, novae are thought to be the main producers of lithium in our galaxy.

But novae can be hard to find because they often lie within the thick and dusty band of our Milky Way. Previous estimates of the rate of novae in our galaxy were wildly uncertain, with only about a dozen novae discovered each year.

“There was little consensus before now on the rate of novae in our galaxy,” says Kishalay De (MS ’18), a graduate student at Caltech and lead author of the Gattini study on novae. “The novae can be hidden behind huge columns of dust, so optical surveys could not find them.”

The novae results demonstrate the power of an infrared survey like Gattini, which scans the whole Northern sky every two nights. The newfound novae were “insanely easy to pick out,” according to Kasliwal, because they glow brightly when viewed in infrared light.

Mansi Kasliwal with members of her team who helped build the Gattini telescope. From left to right: Kishalay De, Scott Adams, Alex Delacroix, and Timothee Greffe of Caltech; Jamie Soon of Australian National University, and Kasliwal. Credit: Caltech

“This is truly a ground-breaking study,” says Allen Shafter, a nova expert at San Diego State University. “Dust limits the reach of optical nova surveys to a relatively small volume of space near the sun. As a result, optical estimates of the Galactic nova rate require a large and uncertain extrapolation of the nova rate in the solar neighborhood to the full extent of our Milky Way galaxy. The new Gattini infrared nova study has greatly increased the volume of space that can be directly surveyed, thereby reducing the extent of the required extrapolation and resulting in a more accurate estimate of the Galactic nova rate than has been hitherto possible.”

An Infrared Legacy

Caltech is a pioneer in the field of infrared astronomy. The late astronomy professors Gerry Neugebauer (PhD ’60) and Robert Leighton (BS ’41 and PhD ’47) designed and built one of the world’s first infrared telescopes. Later, Neugebauer and Tom Soifer (BS ’68), the Harold Brown Professor of Physics, Emeritus, helped create the first space mission to perform an all-sky infrared survey mission, called IRAS (Infrared Astronomical Satellite), which launched in 1983 and led to the creation of Caltech’s Infrared Processing and Analysis Center, now called simply IPAC.

Other IPAC infrared projects include the ground-based 2MASS (Two Micron All-Sky Survey), which scanned the entire sky from 1997 to 2001; NASA’s Spitzer Space Telescope, a sister telescope to the Hubble Space Telescope that ceased operations in 2020; and NASA’s WISE (Wide-field Infrared Survey Explorer), now called NEOWISE (Near-Earth Object WISE) and dedicated primarily to the search for asteroids.

These previous infrared surveys catalogued millions of never-before-seen asteroids, stars, galaxies, and other objects, and had better resolutions than Gattini, but they did not scan the whole sky as quickly.

“We’re doing a large chunk of what 2MASS did every night,” says De. “Gattini is the first-ever survey of the dynamic, or changing, infrared sky. We have traded in resolution for a wide field of view to enable us to regularly capture the whole night sky.” Gattini’s telescope is only 30 centimeters in size but its field of view is a whopping 25 square degrees, 40 times larger than any past or current infrared telescopes.

“Caltech is a pioneer for both infrared astronomy and time-domain astronomy, so it only makes sense that we would combine the two in the first dynamic infrared sky survey,” says Kasliwal. Time-domain astronomy refers to nightly surveys of the changing sky; Caltech’s Zwicky Transient Facility (ZTF) is a key instrument in this growing field, but unlike Gattini, it detects optical light.

From the Ground Up

The Gattini instrument was built at Caltech by Kasliwal and her team, including both graduate and undergraduate students. It was first installed at Palomar in 2018 and took some time to calibrate and set up to work automatically. “We left the telescope on its own to operate robotically,” says De. “Then the data came pouring down from the sky to our computers thanks to our data pipeline.”

One of the challenges in designing a survey instrument like Gattini is the development of software. Gattini’s software automatically sifts through enormous amounts of data to detect changes in the sky every night. De spent six months developing the software and data pipeline for the project as part of his PhD thesis.

“These software techniques are of prime importance to future space-based telescopes as well,” says De, “because they remove the blurring caused by the earth’s atmosphere and hence can in principle get extremely sharp images.”

Now that Gattini is up and running, astronomers have been mining its data for use in various projects. For instance, Caltech professor of astronomy Lynne Hillenbrand and her team used the instrument’s data to help discover a rare bursting young star hidden by clouds of dust. Hillenbrand’s group had previously discovered a similar star with the help of NEOWISE.

“Gattini can uniquely detect objects that are so buried in dust to not be seen in visible light, and which brighten so rapidly that only Gattini scans the sky fast enough to pick them out,” says Hillenbrand.

Next-Generation “Kittens”

Next up in Kasliwal’s plan to open the dynamic infrared skies are WINTER and DREAMS. WINTER, which is currently being built at MIT under the leadership of Kasliwal’s collaborator Rob Simcoe (PhD ’04), a professor of physics, is scheduled to begin operations at Palomar in the fall of 2021. DREAMS is being built by a team led by Moore in Australia and is scheduled to begin operations at Siding Springs Observatory in 2022. Both telescopes will use next-generation infrared detectors that are more efficient than those on Gattini.

The final step is to build an infrared survey telescope in Antarctica that will take advantage of the frigid air. “The night sky is blindingly bright in infrared light, but it’s 40 times darker in Antarctica at infrared wavelengths, which is partly due to the cold temperatures,” explains Kasliwal.

Another reason for building a survey telescope at the South Pole is because, together with those in the North, they will cover the entire sky. “It’s always nighttime somewhere,” she says.

A Goldmine of a Find

One of Kasliwal’s dreams is to be able to identify cataclysmic mergers of neutron stars, dramatic events that produce what astronomers call kilonovas. These explosions are even more powerful than novae, and are thought to generate a significant amount of the universe’s heaviest elements, including gold and platinum. Kasliwal’s team identified one such explosion along with other groups back in 2017, when LIGO (Laser Interferometer Gravitational-wave Observatory) first identified the gravitational waves produced by the collision. The occasion marked the first time that both gravitational waves and light were detected from the same event, and helped usher in the field of multi-messenger astronomy (where gravitational waves, light, and neutrinos are the messengers).

Since that time, LIGO has detected dozens of additional gravitational-wave events, but none have been seen simultaneously in light. Kasliwal suspects this may be due to the fact that kilonovas inherently produce much more infrared than optical light and are thus being missed by optical telescopes. Each step in Kasliwal’s plan—Gattini, WINTER, DREAMS, and a future instrument in Antarctica—has the ability to sleuth out the hidden kilonovas with increasing sensitivities. It is also possible that one of the telescopes may even catch a long-sought neutron star and black hole merger, which could be even more luminous in infrared light than neutron star collisions.

“There is a lot you can do with small ground-based telescopes,” she says. “Our small teams are very agile, and enable us to have some fun, take risks, and try something new. We have the freedom to dream big.”

Palomar Gattini-IR is funded by Caltech, Australian National University, the Mt. Cuba Foundation, the Heising-Simons Foundation, and the US-Israel Binational Science Foundation. The instrument is a collaborative project among Caltech, Australian National University, University of New South Wales, Columbia University, University of Chinese Academy of Sciences, and the Weizmann Institute of Science.

Featured image: The Gattini telescope. Credit: Caltech

Reference: De, Kishalay and Kasliwal, Mansi M. and Hankins, Matthew J. and Sokoloski, Jennifer L. and Adams, Scott M. and Ashley, Michael C. B. and Babul, Aliya-Nur and Bagdasaryan, Ashot and Delacroix, Alexandre and Dekany, Richard and Greffe, Timothee and Hale, David and Jencson, Jacob E. and Karambelkar, Viraj R. and Lau, Ryan M. and Mahabal, Ashish and McKenna, Daniel and Moore, Anna M. and Ofek, Eran O. and Sharma, Manasi and Smith, Roger M. and Soon, Jamie and Soria, Roberto and Srinivasaragavan, Gokul and Tinyanont, Samaporn and Travouillon, Tony D. and Tzanidakis, Anastasios and Yao, Yuhan (2021) A population of heavily reddened, optically missed novae from Palomar Gattini-IR: Constraints on the Galactic nova rate., ArXiv, pp. 1-25, 2021. https://arxiv.org/abs/2101.04045

Provided by Caltech