Optics enters the single-cycle regime
Feb 7, 2005
Physicists at Stanford University in the US have produced the shortest ever laser pulse at optical frequencies. The pulse lasts for just 1.6 femtoseconds, which corresponds to just 0.8 of an optical cycle for pulses with a central wavelength of 650 nanometres (M Y Shverdin et al. 2005 Phys. Rev. Lett. 94 033904).
Stephen Harris and colleagues started by shining YAG and Ti:Sapphire laser beams into a cell containing deuterium gas. This produced a set of "sidebands" at wavelengths between 2.94 microns in the infrared and 195 nanometres in the ultraviolet. A liquid-crystal phase modulator was used to change the phase of seven of these sidebands, which were then focussed onto a cell containing xenon gas (see figure).
When the phases of all seven sidebands were the same the experiment produced a train of pulses with durations of 1.6 femtoseconds, separated by 11 femtoseconds. Since the duration is extremely short, the pulse contained wavelengths between 410 and 1560 nanometres - a range of 1.9 octaves. The peak power was 1 MW. By changing the relative phases of the sidebands it was also possible to produce pulses with different time profiles.
"Our light source is unique and may allow the observation of new physical processes," says lead author Miro Shverdin. "Moreover, shorter pulses could allow faster processes, such as ultrafast molecular dynamics, to be observed."
The team also plans to investigate various nonlinear optical processes, such as harmonic generation and multiphoton ionisation, with single-cycle pulses.