A gas target is like a funnel: the steeper its walls, the stronger the flow. But in this funnel, it is not water that flows, but ultraviolet light, born when a powerful laser pulse passes through a cloud of argon. The efficiency of this conversion is usually minuscule, about 0.001%: out of a thousand laser photons, only one turns into useful ultraviolet. Physicists have spent years looking for ways to improve these numbers, and now it turns out that the answer lies right at the edge, in how the gas density changes as the beam enters and exits.
The experiment was conducted at the Hungarian laser center ELI ALPS. The scientists built a special gas cell in which they could vary the diameter of the apertures and the thickness of the walls, that is, the steepness of the density gradient. A laser with a wavelength of 1030 nm and pulses of 34 femtoseconds duration was focused to a spot of 52 micrometers. The cell contained argon. The result was unambiguous: the sharper the gradient, the higher the flux. For the 25th harmonic (photon energy 30 eV) at a pressure of 90 millibar, the pulse energy increased from 20 to 150 femtojoules when the steepness parameter was varied from 0.1 to 10. The increase was observed for all harmonics from the 21st to the 41st and for all pressures from 65 to 140 millibar.
The practical recipe is simple: reduce the transition length from 1.5 mm to 0.1 mm. Technically, this is achievable by drilling holes in thin foil with a laser. Then the efficiency could increase by almost 10 times. This is not an abstract promise; existing high-harmonic sources can already be improved today simply by changing the geometry of the apertures in the gas cell.
A funnel with steep walls works better. And now that this is proven by experiment, not just theory, engineers can sharpen those walls ever more, so that the flux of ultraviolet photons finally becomes strong enough for practical applications, from semiconductor lithography to nanoscopy of biological samples.
The transition length of the gradient was determined by analogy with the rise time of a signal in electronics: between the 10% and 90% levels.