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Optical Design & Engineering

Focusing on Fresnel lenses

Eye on Technology - diffractive optics

From oemagazine October 2001
30 October 2001, SPIE Newsroom. DOI: 10.1117/2.5200110.0002

Fresnel lenses are garnering attention at all ends of the spectrum. Whether the focus is on broadband x-ray Fresnel lenses as reported by researchers at the recent SPIE annual meeting (29 July–3 August; San Diego, CA) or diamond-based Fresnel lenses for infrared applications, the technology offers advantages over conventional refractive designs for a variety of applications.

A Fresnel lens consists of concentric, optically opaque annular zones of decreasing width, interspersed with annular zones transparent to the wavelength of interest (see figure 1). Fresnel lenses are often less than a micron in thickness. Unlike conventional bulk glass lenses, which control light by refraction, Fresnel lenses control light by diffraction.

Figure 1. Consisting of concentric, annular rings of optically transparent and opaque regions, Fresnel zone plates control electromagnetic waves by diffraction.

This is a particularly useful property for x-ray optics. The index of refraction for most materials at x-ray wavelengths is almost unity and the materials are highly absorptive, so a conventional refractive design is simply not feasible. A system based on diffractive Fresnel lenses, however, is.

Christian David of the Paul Scherrer Institute (Switzerland) garnered attention in the x-ray micro- and nano-focusing session with his work on broadband x-ray Fresnel lenses (paper #4499-16).

Typically, Fresnel lenses focus light over a narrow band of soft x-ray wavelengths. Using semiconductor fabrication methods in conjunction with materials such as silicon, silicon nitride, germanium, chromium, and tantalum, David has developed Fresnel lenses that cover the entire x-ray spectral band, including hard x-rays with wavelengths so short they are instead defined by kilo-electron volt energies.

To focus hard x-rays (energies above 6 keV), he uses Bragg-Fresnel lenses (BFLs), which combine Bragg reflection from a crystal with the focusing properties of a Fresnel zone plate pattern etched into the crystal surface. Using reactive ion etching, he carved 1 mm-deep linear Fresnel structures with outer zone widths of 100 nm into a silicon substrate (see figure 2). "This is significantly smaller than any other silicon BFL reported so far," David says. The linear BFLs range from 125 mm to 1000 mm in width and 5 mm to 10 mm in length and yield efficiency values of up to 26% at 13.25 keV photon energies.

Figure 2. This cross-section through a Bragg-Fresnel lens with 100-nm outmost zones was etched into a <111> oriented silicon substrate using reactive ion etching.

Three points particularly impressed session chair Ian McNulty of the Advanced Photon Source (Chicago, IL) concerning David's work. "Traditionally, people have only been able to fabricate these plates in a few materials such as gold and nickel, which limits what wavelengths they can be used with," McNulty says. David makes zone plates in six or seven different kinds of materials, however, increasing their utility over a broader range of energies.

David's lenses have been tested at x-ray energies up to 30 keV, but he hopes to move even further into the hard x-ray range. "There are no other competitors in that range above 10 keV," he says. "Fresnel zone plates have been considered unsuitable for that range, and I consider that untrue."

research from Russia

Meanwhile, researchers at the Russian Academy of Sciences (Moscow, Russia) have developed chemical-vapor-deposited diamond (CVDD) lenses for use with high-power (about 4 kW CW) or high-intensity (about 10 kW/cm2 CW) carbon-dioxide laser beams. The diamond material offers a thermal conductivity about five times better than that of copper, together with low thermal expansion.

The Russian group has focused on a 10-mm diameter design with a theoretical efficiency of more than 90% and a focal length of 127 mm (with a convergent input beam). De Beers Industrial Diamonds (Johannesburg, South Africa) grows and polishes the substrates. The Fresnel structure is then ablated pixel by pixel using an excimer laser. The lenses are robust, says says Manfred Berger, technical managing director of L.O.T.-Oriel GmbH (Darmstadt, Germany). "Of the more than approximately 1000 windows we have supplied over the years for high-intensity laser beams, we never got a specimen back for rework or refurbishing," Berger says. So it's true what they say: Diamonds are forever.