New visions for space telescopes

NASA stumbled into a rare bit of good luck recently when the National Reconnaissance Office did some housecleaning. NRO decided that a pair of space-qualified 2.4 m telescopes dating from the late 1990s were no longer suitable for their original mission in spy satellites. So NRO offered the surplus optics to one of its poorer relations, NASA, for use in new space-based instruments. 

Unexpected hand-me-downs can bring opportunity, like the the piles of Scientific American and Sky & Telescope that came with a house my family rented when I was in high school. Astronomers and NASA scientists are pondering what to do with the windfall. The f/8 Cassegrain telescopes lack instruments, electronics, or spacecraft, but NRO long ago paid for the optics, saving NASA serious money. The Study on Applications for Large Space Optics workshop held February 5 and 6, 2013, in Huntsville, AL heard and discussed 34 proposals for building new instruments around the mirrors. They will be narrowed to six proposals and submitted to NASA management in May.

The range of ideas is impressive. Adaptive optics can do wonders on the ground, but ultraviolet astronomy must remain above the atmosphere, so three proposals call for studying the ultraviolet sky. Other common themes are spectroscopy, planetary science inside the solar system, and attempts to image challenging targets including extrasolar planets.

Bare-bones surplus telescopes inherited by NASA (Government work not subject to copyright)

Some proposals are intriguing. Alfred McEwen of the University of Arizona (Tucson, AZ) envisions the Mars Orbiting Space Telescope, and  Zachary Bailey of the Jet Propulsion Laboratory (Pasadena, CA) proposes "high-resolution surface science at Mars."  Rebecca Farr of the NASA Marshall Space Flight Center (Huntsville, AL) proposes using both mirrors as a deep-space binocular telescope stationed at the Lunar L2 Lagrange point.

Not everything is exactly a telescope. Abhijit Biswas of JPL wants to use a mirror as an optical communications node in space. J. H. Clemmons of the Aerospace Corp. (El Segundo, CA) wants to use one in a lidar to explore the Earth's thermosphere.  Richard Eastes of the University of Central Florida (Orlando, FL) has a plan for "Atmospheric TeleConnections on Earth."

There are plenty more listed on the program, and NASA will be recording the proceedings for later viewing. The ideas are not fully formed, of course, and some seem to duplicate others. But there are enough bright ideas to make one hope that NRO can find more goodies sitting in storage for its needy relatives.

Source:  http://science.nasa.gov/salso/telescope-characteristics-and-capabilities/

Zero refractive index

The latest example of the amazing versatility of metamaterials is the demonstration of one that has a refractive index of zero, just reported in Physical Review Letters. Theorists had predicted the possibility of zero-refractive-index materials, and some similar effects have been reported, but the metal-clad glass waveguide developed by Albert Polman's group at the Center for Nanophotonics of the FOM Institute AMOLF (Amsterdam, Netherlands) with Nader Engheta of the University of Pennsylvania (Philadelphia, PA) is the first to have a near-zero index throughout.

Zero-index materials, like negative-index materials, do not occur in nature, but can be built by assembling subwavelength elements into a structure designed to have the desired characteristics. The left part of the figure shows an electron microscope image of the metamaterial, a small slab of glass encased in silver forming a waveguide 200 nm wide and 2 µm long. The strong interaction between the metal and the glass on that scale gives an entire waveguide an effective refractive index of 0 at 770 nm.

Electron microscope image of a zero-index waveguide, showing a silver-coated nanoscale glass slab 200 nm wide and 2 µm long. The images at right compare the standing-wave pattern visible in a 400-nm-wide tube which disappeared in a 190-nm-wide tube, showing the material has a refractive index of zero at 770 nm. (Courtesy of Albert Polman)

The phase velocity of light is the speed of light divided by the refractive index of the medium, so phase velocity should be infinite for a zero-index material. Similarly, wavelength in a zero-index material should be infinite because it equals the wavelength in vacuum divided by refractive index. To study how the light behaved, Polman and colleagues used a technique they had developed earlier called "cathodoluminescence spectroscopy" to examine light waves in waveguides at various widths. When the index was above zero in a 400 nm waveguide, the light formed standing waves showing normal light propagation, as shown in the figure. But for a 190 nm waveguide the index was near zero, and the standing waves disappeared, as shown at right in the figure, indicating nearly constant phase and nearly infinite phase velocity and wavelength through the waveguide.

Infinite phase velocity does not violate Einstein's cosmic speed limit because phase velocity cannot carry information. Group velocity, the speed of a modulated optical signal, decreases with the refractive index below one, eventually reaching zero for a zero-index material.

That's not all that happens. "As the index approaches n=0 the losses increase, damping out the waves. The index then becomes a complex number of which the real part is 0," Polman told me in an email. That means no light is left to travel at infinite speed after a short distance. Wenshan Cai of Georgia Tech, who wrote a Viewpoint for the online publication Physics, told me the light should travel about 50 to 100 µm--far enough to be useful in integrated optics, but not over macroscopic distances.

A 2011 report of zero refractive index was based on different physics, combining two photonic-crystal materials, one with positive index and the other with negative index, so the net phase advance through the entire structure is zero. A key difference is that the building blocks of photonic-crystal materials are large enough to be seen by the wave, typically half a wavelength, but those of metamaterials are much smaller, so the incident wave responds to it as if it was a bulk material.