Broader view for adaptive optics

Adaptive optics has become standard on large ground-based telescopes because it offers far sharper images than otherwise obtainable. However, standard adaptive optics can compensate atmospheric turbulence only over small areas, so they don't let ground-based telescopes match the celestial panoramas imaged by the Hubble Space Telescope. Now a new generation of adaptive optics has demonstrated high-resolution imaging over a larger field of view with the Gemini South telescope in Chile.

Proposed more than a decade ago by François Rigaut, now at Australian National University (Canberra, Australia), the Gemini Multi-conjugate adaptive optics System (GEMS) uses five laser guide stars and three deformable mirror to measure atmospheric distortion and compensate for its affects. Sampling at 500 to 1000 Hz, GEMS can compensate for turbulence over an area of sky 16 times larger than previously possible.

The picture below tells the story, alternating images of the "Orion Bullets" region in the Orion Nebula taken with GEMS in December 28, 2012 and of the same region taken in 2007 with the previous-generation ALTAIR adaptive-optics system, which uses a single laser guide star. The larger field of view is 85 arcsec across. Without the adaptive optics, the telescope's resolution at the observation time was 0.8 to 1.1 arcsec. Adding GEMS improved resolution by a factor of ten to 0.084 to 0.103 arcsec.  The bright spots are "bullets" of gas ejected from the core of the nebula that are ripping through molecular hydrogen at speeds to 400 km/s, leaving behind wakes of hot hydrogen.

GEMS also benefits from processing enhancements, which use tomographic techniques to map air turbulence in three dimensions, and correct uniformly across the entire field of view. "This is huge when it's time for astronomers to reduce their data," says Adam Ginsburg, a graduate student at the University of Colorado (Boulder, CO), because observers often need to compare objects in the same field.

Field size has long been a crucial limitation on adaptive optics. The 85-arcsec width of the GEMS image still falls well short of the more than nearly 200-arcsec width of the Ultra Deep Field image taken by the Hubble Space Telescope, but it's an important step. With Hubble now well into its third decade in orbit, astronomers need new ways to study the depths of the sky from the ground.

Comparison of images of the same field in the Orion nebula recorded with GEMS and ALTAIR. The white "Orion Bullets" are fast-moving gas clouds leaving hot hydrogen in their wake. Their motion is fast enough to detect in the five years between the 2007 ALTAIR and the 2012 GEMS images.

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.