CONTROLLABLE FEMTO-PULSES. In recent years lasers that combine high intensities with ultra-short pulse lengths (parabolic mirrors are used to turn pulses into "light bullets" only a few light cycles in extent) have been able to generate harmonic light, light with frequencies equal to multiples of that of the original light, at ever higher energy. For example, femto-second laser pulses at visible wavelengths will, if intense enough and short enough, be able to generate secondary beams of light all the way up into the x-ray range. What happens is this: upon striking atoms in a gas, the laser electric fields are so high (a billion volts/cm) that outer electrons in the atoms are momentarily ionized but then quickly recombined with their atoms so as to radiate a photon whose energy is as much as 300 times higher than photons in the laser beam. X-ray photons made in this way should be emitted in extremely short bursts, indeed only a fraction of the laser oscillation cycle.
But until now the precise onset and end of the burst could not be manipulated because researchers have not been able to control the driving laser field, only its amplitude. Now a collaboration of scientists from the Vienna University of Technology (Ferenc Krausz, 011-43-158-801-38711) and the Max Planck Institute for Quantum Optics (Theodor Hansch, Garching, Germany) has succeeded in gaining control over the phase of the light pulse, and with this precise control over the electric and magnetic fields of their few-cycle light pulses, which will eventually allow one to produce reproducible, isolated attosecond (10-18 sec) x-ray pulses. (Apolonski et al., Physical Review Letters, 24 July; Select Articles; for background see Brabec and Krausz in Review of Modern Physics, April 2000 and Jones et al., Science 28 April.)
