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Electrons inside atoms change their positions and energies in a few attoseconds to a few hundred attoseconds. That is far too fast for any ordinary flash of light to catch. Anne L'Huillier found the effect that made faster flashes possible, and shared the 2023 Nobel Prize in Physics for it. Here is her story, the physics in Class 11 and 12 terms, and the numbers of her first experiment.
Who is Anne L'Huillier?
According to her Nobel Prize facts page, she was born in Paris on 16 August 1958 and was at Lund University in Sweden when she won. She shared the prize with Pierre Agostini and Ferenc Krausz "for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter". The Nobel list of prizes awarded to women shows her as the fifth woman to receive the physics prize, after Marie Curie, Maria Goeppert Mayer, Donna Strickland and Andrea Ghez.
In her Nobel autobiography she writes that her mother had diabetes from childhood and was told she would never have children or keep her sight. None of that came true, thanks to medical progress, and it left L'Huillier with a strong belief in research. She trained at the Ecole Normale Superieure in mathematics and physics, and qualified as a mathematics teacher before choosing a PhD on atoms in strong laser fields.
What did she discover in 1987?
Her first research internship nearly came to nothing: the laser was broken, so the project shrank to a literature study. The lab still offered her a PhD place. In the summer of 1987, at the French atomic energy commission (CEA) in Saclay, her group shone an intense infrared laser at a noble gas. Her autobiography records what they saw: odd harmonics of the laser, up to order 33 in argon, with comparable intensities from the fifth order on. The group reported it in Journal of Physics B in 1988.
The Nobel popular science background explains why this matters. The laser pulls an electron out of an atom. When the light's electric field reverses, the electron can rush back, and it gives up the energy it gained as a burst of ultraviolet light. Many atoms together produce overtones, waves that complete a whole number of cycles for each cycle of the laser.
Why do overtones lead to shorter pulses?
You already know overtones from the Class 11 chapter on waves (NCERT Chapter 14). A string fixed at both ends has harmonics at whole-number multiples of its lowest frequency, and a pipe closed at one end has only the odd harmonics. Our post on standing waves in strings and pipes works through those cases. The odd harmonics in L'Huillier's gas come from the atom and the laser, not from a closed pipe, but the counting is the same.
The Nobel background makes the key point: any wave shape can be built by adding enough waves of the right wavelengths and sizes, and adding more, shorter wavelengths gives shorter pulses. A single cycle of ordinary laser light never gets below about a femtosecond, which is why that was seen as a hard limit in the 1980s. Where many overtones line up in phase, their peaks add into a train of very short pulses, each a few hundred attoseconds long.
Worked example: the numbers of the 1987 experiment
The 1988 paper title says the laser wavelength was 1064 nm. Use c = 3.00 × 10^8 m/s and, for photon energy in eV, E = 1240/λ with λ in nm (the Class 12 Dual Nature chapter gives E = hν).
Frequency of the laser: ν = c/λ = (3.00 × 10^8)/(1.064 × 10^-6) = 2.82 × 10^14 Hz.
One cycle of the laser: T = 1/ν = 3.55 × 10^-15 s, about 3.5 femtoseconds. No pulse of this light can be shorter than one cycle.
Photon energy of the laser: E = 1240/1064 = 1.17 eV, infrared.
The 33rd harmonic: frequency 33 × 2.82 × 10^14 = 9.30 × 10^15 Hz, wavelength 1064/33 = 32.2 nm, photon energy 33 × 1.165 = 38.5 eV.
One cycle of the 33rd harmonic: 3.55 fs ÷ 33 = 0.107 fs, or about 107 attoseconds.
Light at 32 nm, in the extreme ultraviolet, cycles fast enough to build pulses in the attosecond range. Lund University's profile describes her harmonics in just these terms: extreme ultraviolet light that, in time, forms pulses lasting tens or hundreds of attoseconds. For where 32 nm sits, see our guide to the electromagnetic spectrum.
Common mistakes. Students often divide the frequency by 33 instead of multiplying it, giving a longer wavelength rather than a shorter one. Others forget the nano (10^-9) when converting 1064 nm. A quick check: a higher harmonic must have a higher frequency, a shorter wavelength and a larger photon energy.
What came next?
The Nobel press release explains the step from overtones to measured pulses. In 2001 Agostini's group produced a train of pulses each lasting 250 attoseconds, while Krausz's group isolated a single pulse of 650 attoseconds. L'Huillier moved to Lund in 1994 and became professor of atomic physics there in 1997. Her autobiography says her group later made pulses as short as 130 attoseconds. She also served nine years on the Nobel Committee for Physics, and she was teaching atomic physics to third-year students when the call from Stockholm came.
What can a student take from her way of working?
Stay with the odd result. The 1987 harmonics were a surprise. She kept measuring and modelling them through the 1990s, and her results helped build the theory.
Learn both sides. She wrote programs for the wave-propagation part of the theory and worked with theorists on the atom's response, so her experiments and calculations checked each other.
A setback is not the end. A broken laser turned her first internship into reading, and that reading still led to a PhD offer.
When a wave idea such as superposition is hard to picture from a textbook figure, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Try it now: repeat the worked example for the 21st harmonic of 800 nm light and find its wavelength, photon energy and period.
