tEChnology UV lAsERs
➤produces a pulse that is 25 to 30 femtoseconds long and runs at 1kHz with 10mJ pulses. The laser is directed into a noble gas where it kicks the gas’ electrons, out of their nuclear orbits into the vacuum. But half an optical cycle after this ionisation, the electron will reverse direction as the electric field changes, and will accelerate back towards the parent nucleus. Upon returning to the atom, it will emit bremsstrahlung (braking radiation) as this atom returns to its ground state. The result is very short wavelength radiation that the scientists can direct at the samples to be analysed.
For high-resolution imaging of biological
samples using EUV sources, a laser-plasma is used in the 2.3nm to 4.4nm range. This range largely covers the ‘water window’, where the radiation passes through water, but is strongly absorbed in carbon-containing materials such as protein. Thus the wavelength range is ideal for making high-contrast images of cells and other biological samples. This region is where the ultraviolet spectrum at its extreme end meets with what are referred to as soft x-rays. ‘Some interesting biological imaging has been done with water-window x-rays produced by a laser plasma. It’s very early days though,’ says professor Greg Tallents. Tallents works in the physics department of the University of York and studies EUV and x-ray laser development. Tallents explains that free-electron lasers
are another source of EUV, and this time it is not for imaging, but for analysing materials. The Deutsches Elektronen-Synchrotron (DESY) near Hamburg, Germany, has a free- electron laser called Free-Electron Laser in Hamburg, or FLASH. It delivers intense ultra- short femtosecond coherent radiation in the wavelength range between 44nm and 4.1nm without recourse to high harmonic generation. Tallents expects a growing number of applications for EUV: ‘There is going to be an opening up of applications in the EUV spectral region. For example, I think EUV will be useful for cutting solid materials. You don’t heat the material too much and there is high penetration depth into the target, so it is useful if you want to produce deep features.’ Tallents points out that a longer-wavelength laser interacts only with a very thin surface layer of the target material, generating a plasma of the ablated material, with which the laser then interacts. This cloud of material that is evaporating from the lased surface hinders the laser’s ability to get through to the target’s surface. EUV light, however, will go through the plasma
3D nano funnel - the
infrared light (shown in red) is incident at the
entrance of the Xe (green depicted particles) filled nano funnel. Extreme
ultraviolet light (shown in
purple) is generated in the enhanced fields in Xe and exits the funnel.
Picture: christian Hackenberger
cloud and continue altering the surface. XUV Lasers, a spin-out company from
Colorado State Univeristy, is developing applications of these lasers to ablate materials for mass spectroscopy. The two-year old firm is developing compact extreme ultraviolet lasers and optical systems. The product is a desktop- size laser that produces a highly coherent light source at 46.9nm. Its short wavelength is combined with a high energy per pulse, 10µJ, and short pulse duration, 1.5ns. This beam
A longer-wavelength laser
interacts only with a very thin surface layer of the target material
will ablate the surface of a sample and then the plasma will be ionised, stripped of its electrons, for analysis in the mass spectrometer. In the mid-1990s, professor Jorge Rocca at
Colorado State University demonstrated the feasibility of obtaining laser amplification of soft x-ray wavelengths by fast capillary discharges in plasma columns. Subsequent work developed capillary discharge lasers into the tabletop coherent soft x-ray sources with the highest average power presently available. Capillary discharge soft x-ray lasers developed in Colorado
26 ElECtRo optiCs l december 2011/janUary 2012
have been installed and are used in laboratories in the United States and in Europe. The president of XUV Lasers is Carmen
Menoni, a professor of electrical and computer engineering at Colorado State University’s college of engineering. Menoni says: ‘Our laser is the smallest at this wavelength. Applications include microscopic imaging.’ The technology’s properties have made it possible to carry out tabletop experiments in nanoscale imaging, nanopatterning, single photon ionisation mass spectroscopy, interferometry, dense plasma diagnostics, and nanoscale ablation of materials. Recently a tiny silver funnel has also been demonstrated to produce EUV. The funnel, only a few micrometres long, is made out of silver and is filled with xenon gas. Like the high harmonic generation used with noble gases in professor Collier’s facility, the funnel’s xenon emits radiation across a spectral range that includes EUV. Created by scientists from South Korea’s Advanced Institute of Science and Technology, Germany’s Max Planck Institute of Quantum Optics, and the Georgia State University in the USA, the funnel turned incident infrared pulses into EUV pulses that last for a few femtoseconds. The EUV pulses in this case are repeated 75 million times per second. The funnel also acts as a filter. Its 100nm aperture stops 800nm IR light but allows EUV pulses with wavelengths down to 20nm to go through. The scientists expect
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