In the last decades, ultrashort laser pulses have revolutionized our way of studying the microscopic world through the interaction of coherent light with matter. The generation and manipulation of these ephemeral electromagnetic fields allows us to access the fastest atomic phenomena in nature, occurring on the femto to attosecond (10^-15-10^-18 s) time scale. The rapid advancement of laser technology has made it possible, in recent years, to synthesize infrared pulses with sub-cycle durations, in which the most intense structure of the electric field of light barely has time to complete an oscillation at its central frequency. These pulses provide a unique tool for exploring electron motion in atoms and molecules, but their generation is still limited to extremely expensive and complex setups.

Recently, we demonstrated that these sub-cycle pulses can be obtained much more simply in standard systems based on the propagation of light through gas-filled hollow capillary fibers with a decreasing pressure gradient. This proposal is based on a surprising phenomenon of nonlinear optics, known as soliton self-compression, where an intense laser pulse can, by itself, simultaneously broaden and organize its frequency spectrum, reducing its duration almost to the limit. By following some scaling rules to design the fiber and input pulse parameters, this technique allows for the generation of high quality sub-cycle infrared pulses.

Not content with reaching durations of just one femtosecond, in our latest work, conducted in collaboration with researchers from Politecnico di Milano and Heriot-Watt University, we have explored the application of these sub-cycle fields to generate even shorter laser pulses in the attosecond regime. To do so, we have exploited the phenomenon of high-order harmonic generation, which arises from the interaction of an intense infrared pulse with the atoms of a gas. When the interaction is performed with a conventional laser, this process works as a production chain of attosecond pulses in the extreme ultraviolet, giving rise to a series of light flashes that occur at regular time intervals. However, if the interaction is driven by one of our previous sub-cycle pulses, the harmonic generation process is naturally confined to a single event, resulting in the direct emission of an isolated attosecond pulse. These solitary ultraviolet pulses are a highly sought-after tool in ultrafast science applications where very precise control and high temporal resolution are needed.

Thus, our study opens the door to a new generation of compact fiber-based systems in which, starting from a standard infrared laser pulse, we combine for the first time its extreme self-compression down to the sub-cycle regime and its direct application to generate extreme-ultraviolet isolated attosecond pulses.

More information in:

1. F. Galán, J. Serrano, E. C. Jarque, R. Borrego-Varillas, M. Lucchini, M. Reduzzi, M. Nisoli, C. Brahms, J. C. Travers, C. Hernández-García, and J. San Roman, “Robust isolated attosecond pulse generation with self-compressed sub-cycle drivers from hollow capillary fibers,” ACS Photonics 11(4), 1673-1683 (2024).

https://doi.org/10.1021/acsphotonics.3c01897

Temporally characterizing ultrashort laser pulses (on the femtosecond scale, i.e., 10^-15 seconds) is akin to reconstructing a crime scene: the light pulses are so fast that we can’t catch them in the act, we can only reconstruct them from the clues they leave behind.

Typically, we work with linearly polarized scalar pulses, in which the polarization state remains constant over time (polarization refers to the trajectory described by the light in the transverse plane). To identify these pulses, we need to know their amplitude or intensity and their phase. There is another type of pulses in which polarization varies over time, known as vector pulses. These are more complex than scalar pulses, and we need to know the amplitude and phase of their two components, as well as the relative phase between them. If identifying a scalar pulse is equivalent to identifying a criminal, knowing a vector pulse would be like knowing a gang composed of two criminals, and moreover, the relationship between them.

One type of characterization techniques is based on measuring the spectrum of a nonlinear signal while the pulse undergoes some modification. In the amplitude swing technique (a-swing), developed by researchers from the ALF group, two replicas of the pulse to be measured are generated, temporally delayed from each other, and the second harmonic spectrum (frequency doubling) is measured for different relative amplitudes of these replicas. Thus, a two-dimensional trace is obtained (a map where color represents intensity), which is like a fingerprint of the pulse. In some techniques, ambiguities arise, i.e., two different pulses generate the same trace, as if two people had the same fingerprint. Through algorithms, the information of the pulse generating the trace (our clue) can be extracted.

Most techniques only allow the characterization of scalar pulses. If we want to reconstruct a vector pulse with one of these techniques, we need several traces, i.e., several fingerprints. In contrast, a single a-swing trace contains the necessary information to identify a vector pulse. Furthermore, these traces are obtained with an inline, compact, and versatile setup.

In this work, we analyze the a-swing traces analytically and numerically to study how the information of vector pulses is encoded, and we develop a strategy to extract it. This strategy is applied to simulated and experimental traces, demonstrating that a vector pulse can be reconstructed from its a-swing trace. If they don’t want to be caught, they’ll have to avoid leaving these kind of fingerprints…

More information at:
Cristian Barbero, Benjamín Alonso, and Íñigo J. Sola, “Characterization of ultrashort vector pulses from a single amplitude swing measurement,” Opt. Express 32, 10862-10873 (2024) https://doi.org/10.1364/OE.515198

The shortest flashes of light we can control only last a tiny fraction of a second – mere trillionths, or attoseconds. Within this tiny timeframe, we can witness how atoms and molecules behave. Attophysics, a new area of study, has emerged from this. But how did we get here? This article tells the story of our collective effort to create shorter and shorter bursts of light, a journey that won the 2023 Nobel Prize in Physics. It’s a tale of milestones, shifts in thinking, and inspiration, giving us a new perspective on scientific progress.

More information at:
L. Plaja, “Attociencia”, Revista Española de Física 37-4, 49 (2023)