Research round-up

13 Jul 2005

A look at some of the breakthroughs that have been made this month, including a high repetition rate EUV source.

EUV sources

A team of researchers in Germany led by Ferenc Krausz and Theodor Hänsch has generated pulses of coherent extreme ultraviolet (EUV) radiation at a repetition rate of more than 100 MHz. This is said to be a 1000-fold improvement over previous experiments and could open up a "joint frontier in precision spectroscopy and ultrafast science". (Nature 436 234)

"We have shown that efficient high-order harmonic generation directly from a Ti:Sapphire oscillator is possible using a high finesse resonator," say the authors. "This reduces the complexity of the set-up and increases the repetition rate. At such a high repetition rate, the frequency comb from the fundamental laser will also be usable in the XUV bringing high resolution XUV spectroscopy into reach."

Retinal imaging

Using adaptive optics (AO) in tandem with a scanning laser ophthalmoscope (SLO) permits accurate optical sectioning of the human retina, say a team from Houston University in the US. According to the team, the axial resolution of its adaptive optics SLO (AOSLO) is three times better than conventional instruments. (Applied Optics 44 4032)

Adaptive optics improve the axial resolution by reducing blur caused by aberrations and in turn this increases the contrast of all resolved features in the focal plane. "The axial resolution of a standard SLO is generally in the range of 300 microns," explain the authors. "The axial resolution of the AOSLO, with an 80 micron confocal pinhole, is in the range from 204 to 110 microns. Axial resolutions as low as 71 microns are possible with smaller pinholes."

Photonic crystals

Jian Zi and colleagues at Fudan University in China have discovered that the brown colors seen in peacock feathers are a result of 2D photonic crystal structures and not pigments in the feather. Because brown is mixture of different colors and not a pure color such as blue or yellow, the researchers believe this work will help others fabricate artificial nanostructures that produce mixed colors. (Physical Review E 72 010902).

The researchers studied the different colors emitted from different regions of the feather. "The lattice constant, the number of periods and even the inter-distance and missing holes between two melanin layers in the feather are important in the production of structural brown colors," conclude the authors.