Pinholes produce sharp focus

17 Jun 2002

An optical element with thousands of pinholes focuses light with unprecedented sharpness.

A team of German researchers has developed a "photon sieve" containing an array of pinholes that can focus light with unrivalled sharpness. This device looks set to improve high-resolution microscopy in physical and life sciences (Nature 414 184).

As well as producing a sharper focus, the photon sieve significantly reduces the presence of secondary maxima resulting in images with better spatial resolution.

Lutz Kipp and colleagues from the universities of Kiel and Hamburg have developed a prototype sieve that operates in the visible spectrum at 632 nm, the wavelength of a helium-neon laser. This was printed on a 35 mm slide with 4000 lines.

The diameter of the sieve is 10 mm. It contains 5 646 pinholes, the smallest having a diameter of 100 µm. The size of the focal spot produced was below that of the smallest pinhole used and secondary maxima were significantly reduced.

"We are mainly developing the optics to focus coherent X-rays because of the absorption from conventional lenses but there are certainly applications in the optical regime," said Kipp.

Conventional optics, which are based on refraction, cannot focus soft X-rays because of the amount they absorb. To overcome this problem, the researchers have based their devices on diffractive elements known as Fresnel zone plates.

A Fresnel zone plate consists of concentric rings that are finely spaced close to the outer edge (producing a large diffraction angle) and coarsely spaced at the center (producing a small diffracting angle), giving a point focus. Kipp's modified zone plate contains pinholes that produce a focal point smaller than the size of the pinholes themselves.

Kipp explained: "We have used a large number of appropriately distributed pinholes instead of rings as the basic diffractive elements. To obtain a distinct first order focus, the pinholes have to be positioned such that the optical path length from the source via the center is an integral number of wavelengths."

Kipp told Optics.Org that the team plans to "produce sieves 100 times smaller to account for the wavelength of an X-ray free-electron laser (6 nm) being developed at the Hamburg synchotron, DESY. This should have a diameter of about 100 µm with 30 nm pinholes."