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19 Jul 2002
A tabletop source of spatially coherent, extreme ultraviolet light is described in today's issue of Science.
Researchers from the US and Bulgaria have developed a spatially coherent laser-like beam of extreme ultraviolet (EUV) light on a tabletop for the first time. Using optical components occupying only 100 cm x 300 cm, the team says its compact set-up will enable EUV microscopy and holography with high temporal resolution. (Science 297 376).
The team makes use of a process called high-harmonic generation (HHG). "Pulses of short wavelength light with extremely short duration can be produced by focusing a high-intensity femtosecond (fs) laser into a gas," explained researcher Randy Bartels from the department of physics and JILA, University of Colorado. "The HHG process up-shifts the femtosecond pulses from the visible into the EUV."
Previous research by the JILA group has shown that HHG can be phased-matched over a long interaction region using a hollow-core fiber. Now, Bartels and colleagues say that this extended phase matching improves both the beam mode quality and its spatial coherence while retaining the conversion efficiency.
To generate the EUV light, the researchers fire 25 fs pulses from a Ti:sapphire laser operating at 760 nm into a 10-cm-long, 150-µm-diameter, hollow-core fiber. The laser pulses carry 0.8 mJ/pulse and are produced at a repetition rate of 5kHz.
The fiber contains argon gas at a pressure of 29 torr. Interaction between the laser pulses and the argon produces a fully coherent, forward directed, beam of light centered on 40 nm. The HHG pulses have durations less than 10 fs. The team says that its fiber-based approach doubles the EUV conversion efficiency compared with similar free-space focus configurations.
The JILA team measured the spatial coherence using a double-pinhole interference technique. An EUV-sensitive camera captured diffraction images over 240 seconds to confirm the high spatial coherence. To demonstrate the precision of the highly focused beam, the team has also recorded detailed holograms of small objects.
"Further improvements should allow this source to be used for applications in high-precision metrology and inspection, as well as metrology for EUV lithography masks, coatings and microscopy with nanometer resolution" said Bartels.
Author
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.
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