negative refraction in the optical

experimental results show strong negative refraction in the optical
this could be very useful for astronomical imaging

Lezec et al report n ~ -5 negative refraction synthetic material that works in the optical (~ 500 nm) over a respectable bandwidth (~ 50 nm) (website and link here).

Negative Refractive Index materials were pointed out by Veselago (1968) and made concrete theoretically by Pendry (2000).

Shortly after synthetic metamaterials were constructed with the requisite properties at microwave wavelengths, demonstrating the reality of some of the more interesting properties such as "cloaking" and "superlensing".

Theoretically, and as it turns out in practise (up to imperfections and limitations in geometry of construction), these materials can both guide electromagnetic radiation perfectly around a solid body (ie an "invisibility cloak"), and image to better than the diffraction limit - in fact in the physical optics limit the sharpness of focus can approach the perfect geometric limit.

There is considerable interest in the microwave materials, they are surprisingly straightforward to make, have practical applications, and are proof of concept. Limitations include the difficulty of making full 3D shapes (as opposed to 2D proof of concept optical elements), imperfections in construction (practise!) and higher order effects, like polarization of the wave.

Optical metamaterials are harder to construct, but it is possible to "cheat" by using "plasmon" propagation - and Atwater's group has constructed 2D optical elements (ie thin prisms and lenses, with length but negligible height) from Au/Si₃N₄/Ag (gold/silcon nitride/silver) layers.

The material shows strong broadband negative refraction in the optical, including superlensing.

Reduced version from figure 3 from Lezec et al, Science 316, 430, 2007 (fair use)

This is going to be very interesting for astronomical optics.
If we can get secondary optical elements with high throughput, broadband or narrowband transmission and robust high negative refractive index, then we can seriously think about sub-diffraction limited imaging - ie small mirrors could deliver arbitarily sharp images, limited by the detector sampling, not the optics.
Doesn't help so much with spectroscopy, we'll still need lightbuckets for that, but it is very intriguing.

A secondary issue is whether this would be useful for high contrast imaging, such as planetary searches. I have not seen any studies on what could be done with that.
Worth thinking about.