The conversion of laser radiation into X-rays by the process of harmonic generation is normally a fairly inefficient process. Now a team of physicists led by Henry Kapteyn of the JILA laboratory in Boulder, Colorado, has shown how to increase the efficiency of X-ray generation by an order of magnitude by carefully controlling the shape of the incoming laser pulse (R Bartels et al. 2000 Nature 406 164). Such X-ray sources could be used for variety of applications in imaging, materials science and the control of chemical reactions.
In harmonic generation a short pulse of intense radiation is focussed into a gas of atoms. The laser-atom interactions are highly nonlinear, and a number of the input photons effectively combine to generate a single output photon with a correspondingly higher energy and shorter wavelength. In most experiments a range of so-called harmonics is produced. Kapteyn and co-workers from JILA, Delft University of Technology in the Netherlands, and Sofia University in Bulgaria, have shown that the strength of a particular harmonic can be increased by carefully controlling the rise and fall in intensity of the laser pulse with time.
They start with a 18 femtosecond long laser pulse which they shine into a gas or argon atoms. A CCD camera is used to measure the strength of the different harmonics produced by this interaction. This information is then fed into a micromachined deformable mirror that can change the shape of the pulse. The mirror is able to change the pulse shape because it is located at a position inside the laser system where the different wavelength components of the beam are spatially separated. By varying the pathlengths of the various wavelengths before they recombine to form the pulse, it is possible to change its shape.
An evolutionary algorithm is used to find the best input laser shape for a desired X-ray output. The shape of the pulse is defined by 19 inputs to the mirror and the algorithm generally finds the best pulse shape after about 50 iterations. The technique allows the team to increase the intensity of any harmonic within the output spectrum.