Few-cycle Pulse Generation using Bulk Multi-pass Cells

Abstract

Few-cycle laser pulses are fundamental for tracking ultrashort motions such as electron dynamics and achieving stronger fields. They are also extensively used to perform high-harmonic generation and in attosecond science. However, generating stable and high quality few-cycle pulses requires a simultaneous control of bandwidth and dispersion. The generation of few-cycle pulses in an efficient manner is also quite challenging. This thesis looks into the generation of few-cycle pulses through a cascaded multi-pass cell post-compression scheme.

The system consists of a two-stage multi-pass cell setup with bulk fused silica as the nonlinear medium. The tasks for this project were focused on the second stage where special attention was set on the dispersion management using chirped mirrors within the cavity and compression stage as well as further material placement for higher-orders of dispersion compensation.

Using the entire setup, a total compression from 220 fs to 6.2 fs was achieved rendering a total compression factor of 34.5 providing a few-cycle laser pulse with good spatial quality.

The results from this project aim towards proving the worth of cascaded bulk multi-pass cells systems for pulse compression, providing a stable, robust and energy-scalable setup for the generation of few-cycle laser pulses that can be further used for high-harmonic generation.

Popular Abstract

Lasers are research tools which are becoming progressively more important every year, with many applications in fields such as: surgery, electronics, ophthalmology, telecommunications, defense, research, etc. All lasers are based on the same principle, which is light amplification by stimulated emission. In other words, light triggering the emission of more light. The output, on the other hand, can vary depending on the use. For instance, the color of the beam that emerges can be different, the power can be higher or lower or the beam can be continuous or pulsed.

For this project, the starting point was a pulsed laser, which emits tiny bursts of energy of a certain color. By "tiny" we actually mean extremely short time durations, specifically in the femtosecond scale. One femtosecond is an incredibly small number, which, in perspective has the same relation to a single second as one second has to 32 million years. This femtosecond time scale in physics is referred to as ultrafast, or similarly ultrashort.

For many of the applications mentioned before as well as for research purposes the duration of the light pulses needs to be ultrashort, the frequency at which those pulses are emitted needs to be high and the pulses need to carry enough power. For that, industrial-grade laser systems are preferred because they offer much higher average powers and better efficiencies. But the drawback is that their laser pulses have slightly longer time durations (hundreds of femtoseconds), when we actually want below 10 femtoseconds. So, how can these bursts be shortened even further?
The key is to consider the colors which are traveling in the burst. Although we previously mentioned that there was a single color in the burst, we can extend this to include many shades of the same color being emitted from the laser. This is akin to having a long pulse in time, as a pulse with a narrow range of colors tends to last longer, while a shorter pulse encompasses a broader range of colors.

To shorten this pulse in time, we need to pack many more colors, and their various shades, into a single laser burst. This can be visualized as a race: initially, there is only one runner, but as the race progresses, more runners join in and run together.
Returning to the concept of the laser pulse, the way to create these new colors involves using specific materials that undergo changes in their properties when subjected to extremely high intensities. This process leads to the generation of new colors. Consequently, the initial pulse with one color entering the material (like one runner beginning the race) will exit with many colors traveling together (like multiple runners finishing the race).

However, not all runners (nor colors) run at the same speed: different colors travel at different velocities. To achieve a compressed laser pulse in time, we need all the colors to arrive simultaneously. This requires us to hold some colors back to adjust the relative delays. This entire procedure is performed by manipulating various optical elements, taking into account their properties and how they will interact with light, which is precisely what the optical setup designed in this thesis aims to accomplish.

This master's thesis aims to compress the duration of a laser pulse of 50 femtoseconds to under 10 femtoseconds by means of a technique that compresses pulses in time. Specifically, a multi-pass cell is built. This refers to a mirror chamber that makes light travel back and forth many times, with the material to carry out the color-adding in the chamber. A long pulse in time will be made significantly shorter to have an extremely ultrashort laser pulse useful for applications such as capturing images of chemical reactions or probing ultrafast electron dynamics.

• author: Rushe Palacios, Sara
• supervisor: Anne-Lise Viotti; Saga Westerberg; Gaspard Beaufort
• organization: Atomic Physics; Department of Physics
• course: PHYM03 20252
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Ultrafast optics, multi-pass cell, few-cycle, optical design, ultrafast pulse characterization
• publication/series: Lund Reports on Atomic Physics (LARP)
• report number: LRAP 623
• language: English
• id: 9230612
• date added to LUP: 2026-06-04 08:22:44
• date last changed: 2026-06-04 08:22:44