QnAs with Anne L’Huillier
Research Article
March 1, 2019
Lund University atomic physicist Anne L’Huillier has been at the forefront of ultrafast laser science since its inception. In 1988, she collaborated on an experiment at the French Saclay Nuclear Research Centre with a solid-state, picosecond laser system that was one of the first to generate high-order harmonics in gases. High-order harmonic generation is a nonlinear process wherein atoms exposed to intense laser radiation emit light with a frequency equal to a multiple order of the laser frequency (1). Six years later, in 1992 at the Lund High-Power Laser Facility in southern Sweden, L’Huillier worked with a titanium-sapphire laser that was one of the first high-power femtosecond systems in Europe (2). She joined Lund University in 1995, and in 2005 her group recorded what was then one of the shortest light pulses, at 170 attoseconds using high-order harmonic generation (3). L’Huillier’s research has helped foster the field of attosecond science, allowing physicists and chemists to visualize the movements of valence electrons in light-induced processes. PNAS spoke to L’Huillier, who was elected as a foreign associate of the National Academy of Sciences in 2018, about her current research.
PNAS:
In the Inaugural Article (4), you describe spatiotemporal couplings, or chromatic aberrations, in the generation of attosecond pulses. How do they occur, and why are they important to reconcile?
L’Huillier:
Light waves can often be described by a product of a temporal function and a spatial beam profile. When this is not the case, we say that the light pulses exhibit spatiotemporal couplings, which implies that the temporal properties of the light depend on the spatial position. In the Inaugural Article (4), we describe spatiotemporal couplings of the attosecond pulses obtained by high-order harmonic generation in gases, which are inherent to the generation process itself.
These spatiotemporal couplings may become problematic in some applications when it is necessary to focus short attosecond pulses. Our studies show that the different frequency components will often not focus at the same position, which means that it will be difficult to generate very short pulses in a small region of space.
PNAS:
How can you reconcile spatiotemporal couplings?
L’Huillier:
One solution is to generate attosecond pulses with a certain geometry to minimize the couplings. However, this geometry usually does not allow you to optimize the efficiency of the pulse; you might have less photons but then all your frequencies will be focused at the same place. There are more sophisticated solutions like shaping the infrared pulses, but this is for future research.
High-order harmonic generation has been studied for 30 years, and I had the privilege to be at the beginning of this research field. It has evolved a lot during these years, from a research activity pursued only in a few laboratories worldwide to a broad field with many branches. Femtosecond lasers based on the chirped-pulse amplification technique, recognized by the Nobel Prize in Physics in 2018, became an essential tool from the 1990s onwards. An important step forward was the experimental demonstration of attosecond pulses in 2001.
The spatiotemporal couplings described in the Inaugural Article (4) have, however, barely been tackled before. This shows the richness of the physics of high-order harmonic generation if one can learn new physics three decades after the first experimental evidence.
PNAS:
Why did you choose to investigate these effects now?
L’Huillier:
We are developing applications that require us to focus broadband radiation—all of the frequencies—over a small area. In the article (4), we use an analytical formula to describe how the phase of an attosecond pulse varies with frequency and space through the laser intensity. We combine the insight given by the analytical derivation with simulation and experiments.
PNAS:
How has attosecond science grown since your initial work in the field?
L’Huillier:
Attosecond science allows access to very short time scales—of electrons in atoms or molecules—both in gas phase or in condensed matter. We now have tools to measure not only the cross-section of a given process but actually to characterize electron wave packets, both amplitude and phase, which was not possible before. It is a very exciting field.
Attosecond science is in really good hands at the moment. I see a broadening of applications and an explosion of the number of groups working on the subject. It reminds me of where femtosecond science was 20 years ago, with many applications in condensed matter and molecular physics.
The field is becoming very broad, and it’s now impossible to follow everything. There are applications in fundamental atomic physics and quantum mechanics, chemistry, and molecular physics, to try and see the birth of chemical reactions and applications in condensed matter, the investigation of fast processes, such as photoemission. Industrial applications are also arising that make use of broad and coherent radiation.
PNAS:
You served on the Nobel Prize Committee for Physics for nearly a decade, during which time awards were given for the observation of neutrino oscillations, the CCD camera and the optical fiber for optical communication, the discovery of graphene, the theoretical prediction of the Higgs Boson, and the invention of blue LEDs. What was your experience like, and how did it affect your career?
L’Huillier:
My work as a member of the Nobel Committee took a lot of time, often away from my research. But, of course, it was very interesting. It helped give me a very broad knowledge of physics and I learned a lot by doing it, that’s clear. I very much enjoyed the depths in which we worked on the different possible prizes as well as the procedure of awarding the prizes, which has been elaborated upon during more than 100 years of Nobel prizes.
References
1
M Ferray, et al., Multiple-harmonic conversion of 1064 nm radiation in rare gases. J Phys B 21, L31–L35 (1988).
2
C-G Wahlström, et al., High-order harmonic generation in rare gases with an intense short-pulse laser. Phys Rev A 48, 4709–4720 (1993).
3
R López-Martens, et al., Amplitude and phase control of attosecond light pulses. Phys Rev Lett 94, 033001 (2005).
4
H Wikmark, et al., Spatio-temporal coupling of attosecond pulses. Proc Nat Acad Sci USA 116, 4779–4787 (2018).
Information & Authors
Information
Published in
Copyright
© 2019. Published under the PNAS license.
Submission history
Published online: March 1, 2019
Published in issue: March 12, 2019
Notes
This QnAs is with a member of the National Academy of Sciences to accompany the member's Inaugural Article on page 4779.
Authors
Metrics & Citations
Metrics
Citation statements
Altmetrics
Citations
Cite this article
116 (11) 4767-4768,
Export the article citation data by selecting a format from the list below and clicking Export.
Cited by
Loading...
View Options
View options
PDF format
Download this article as a PDF file
DOWNLOAD PDFLogin options
Check if you have access through your login credentials or your institution to get full access on this article.
Personal login Institutional LoginRecommend to a librarian
Recommend PNAS to a LibrarianPurchase options
Purchase this article to access the full text.