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Femtosecond laser enhances microlens array

01 Feb 2007

Researchers in Austria demonstrate a single-step Ti:Sapphire laser process that creates well-defined, round apertures in metal-coated microspheres.

Confocal microscopy and parallel laser-surface processing are two key areas that could benefit from a laser fabrication process being developed at Johannes Kepler University Linz, Austria. Researchers have used a Ti:Sapphire source to generate well-defined, round apertures in a monolayer of either gold or nickel coated, amorphous quartz (SiO2) microspheres. (Appl. Phys. Lett. 89 261104)

"The apertures significantly improve the imaging properties of the microspheres," Dieter Baeuerle, head of the Institute for Applied Physics told optics.org. "Femtosecond laser-induced forward transfer is the only technique that permits the fabrication of apertures on microlens arrays of this type."

Firstly, the team deposited an aqueous solution of microspheres on a quartz slide and coated the resulting monolayer structure with a film of either gold (40-120 nm) or nickel (60 nm). The researchers then exposed the uncoated side of the sample to a Ti:Sapphire source with a wavelength of 797 nm and a pulse duration of between 100 fs and 10 ps, to form an aperture in each microsphere.

As Baeuerle explained, it is critical to match the laser pulse to the diameter and material properties of the metal-coated microspheres. These spherical lens-like particles can display strong aberrations and the addition of a well-defined, round aperture helps to diminish imaging faults by behaving as an iris.

To better understand the fabrication process, the team modelled the intensity distribution of a plane wave incident on a sphere and used the results to determine suitable operating parameters. Currently, the group can create well-defined apertures as small as 0.1 microns (radius) in a 75 nm thick gold film with a fluence of 40 mJ/cm2 and a pulse duration of around 130 fs.

Experiments with nickel coated microspheres revealed that the metal offered increased protection against laser ablation of the quartz surface. The scientists believe that this is related to the higher absorptivity of the nickel film compared with gold.

Bauerle was unwilling to comment on the next steps for his team, but the group appears to be focused on optimizing its process.

Author

James Tyrrell is News Editor of Optics & Laser Europe magazine and a contributor to optics.org.

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