Researchers have developed a new way of generating extremely short pulses of visible light using a simple, commercially available laser system. The innovative approach, which exploits nonlinear effects in glass fibres that transmit light beams with different spatial profiles, could make it easier and cheaper to study ultrafast phenomena such as photosynthesis in plants, the dynamics of electron–hole pairs in semiconductors and the chemistry of human vision.
High-energy pulsed lasers have enabled researchers to study and control processes such as chemical reactions that occur on the femtosecond (10-15 s) time scale. Such lasers have also made it possible to accelerate particles using light alone, which is important for many branches of sciences, including nuclear and particle physics, materials science, nuclear medicine and radiography. Extending these capabilities to lasers at visible wavelengths has proved challenging, however, as it is hard to generate coherent visible light at a high intensity over these extremely short timescales due to a complex interplay between different nonlinear phenomena.
Hollow-core glass fibres with multimodes
In the new study, researchers led by Luca Razzari of the Institute National de la Recherche Scientifique (INRS) in Canada took inspiration from recent pioneering work on nonlinear effects in glass fibres that support multiple modes (that is, fibres in which the light beam can take many different spatial shapes as it propagates). Using an industrial-grade ytterbium laser system, they began by propagating a much longer (175 fs) infrared pulse centred at a wavelength of 1035 nm through a 3-m-long hollow-core fibre filled with argon gas at a pressure of about 3 bar. As the different modes mixed with each other, a nonlinear effect within the gas generated intense, 4.6-fs-long pulses of visible light at the fibre output.
Unlike previous methods of generating ultrashort visible light pulses (lasting about two optical cycles), this approach does not rely on complex and expensive optical equipment such as optical parametric amplifiers, gratings and chirped mirrors to compress the generated pulses. The researchers say this could make it accessible to scientists who study a broad range of ultrafast fundamental phenomena, including photosynthesis and even the isomerization of rhodopsin, which is key to human vision. “With our pulses, we can study the dynamics of such processes and how they evolve on extremely short timescales,” says Riccardo Piccoli, a postdoctoral researcher at INRS and the first author of a Nature Photonics paper describing the research.
Industrial lasers generate attosecond light pulses
To develop the method, Piccoli and his INRS colleagues Roberto Morandotti and François Légaré worked with experts from an INRS start-up, few-cycle Inc., which markets a system to stretch and hold hollow-core optical fibres. Theorists at the French National Centre for Scientific Research (CNRS), Louisiana State University in the US and Heriot-Watt University in the UK also contributed by modelling the observed phenomena.
Looking ahead, the team says it now plans to further explore multimode mixing in gas-filled fibres. “Such studies will be an exciting playground for nonlinear optical interactions that can provide us with new tools to tailor optical waveforms at the few-cycle level,” Razzari tells Physics World.
