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Anne L'Huillier develops experimental and theoretical methods based on high harmonic generation in gases. The generated extreme ultraviolet light contains many harmonics of the driving laser frequency; their relative spectral phases determine whether they combine into short optical pulses. Her research connects that generation problem with attosecond measurements of electron dynamics following photoionization. The central link is between a controlled optical waveform and the timing information carried by emitted electrons.^[ref]

A physicist who researches high harmonic generation and ultrafast electron dynamics.^[ref]

Attosecond physics resolves changes on the scale of $10^{-18}$ seconds, especially electron dynamics. Its central trick is to convert a short optical event into an experimentally accessible energy or phase change. A broad optical spectrum provides the ingredients for a short optical pulse; their spectral phases determine when those ingredients add constructively. High harmonic generation supplies the optical spectrum, while RABBITT and attosecond streaking supply complementary timing measurements. XUV denotes extreme ultraviolet radiation.

There are two separate timing problems: characterize the incident optical pulse, then infer the response of the target. A photoelectron measurement mixes the incident spectral phase, the target's transition response and the probing electric field. Recovering one requires constraints on the others. These methods measure distributions of photoelectron energies over repeated shots, rather than photographing a moving atomic orbital.

An attosecond pulse train contains repeated short optical pulses. For a group of odd multiples of a common driving frequency, the separation is half the driving period when the relative spectral phases recur. The construction resembles mode locking, but the participating components come from high harmonic generation, rather than separate resonant modes of a laser oscillator.

E. S. Toma coauthored an experiment that reconstructed a train of $250$-attosecond optical pulses from high harmonic generation. Two-color photoionization measurements determined relative harmonic phases, connecting the measured electron signal to the short-pulse structure.^[ref]

F. Augé coauthored an experiment that reconstructed a train of $250$-attosecond optical pulses from high harmonic generation. Two-color photoionization measurements determined relative harmonic phases, connecting the measured electron signal to the short-pulse structure.^[ref]

G. Mullot coauthored an experiment that reconstructed a train of $250$-attosecond optical pulses from high harmonic generation. Two-color photoionization measurements determined relative harmonic phases, connecting the measured electron signal to the short-pulse structure.^[ref]

H. G. Muller coauthored an experiment that reconstructed a train of $250$-attosecond optical pulses from high harmonic generation. Two-color photoionization measurements determined relative harmonic phases, connecting the measured electron signal to the short-pulse structure.^[ref]

High harmonic generation converts an intense driving electric field into radiation at high integer multiples of its frequency. High harmonic generation in a gas is a process in nonlinear optics: an electron can leave a neutral target by tunnel ionization, gain kinetic energy in the driving electric field, and return to radiate upon recombination. This three-step picture is an approximate account of a probability amplitude for emission in quantum mechanics, not a measured path of one electron.^[ref]

Why odd multiples? In an ideal centrosymmetric gas driven by a single frequency, assume the radiating electric dipole moment satisfies $d(t+T/2)=-d(t)$, where $T$ is the driving period. Its Fourier series coefficient obeys $d_q=-(-1{)}^q{d}_q$, so every even coefficient vanishes. A finite envelope, an oriented target or an additional driving frequency can invalidate that symmetry argument. Producing many odd components also does not prove that their spectral phases form a short optical pulse.

The 1988 report involving Anne L'Huillier observed odd multiples of a $1064\mathrm{nm}$ driving wavelength in rare gases, reaching order 33 in argon. The unexpectedly slow fall of yield beyond the fifth order supplied a broad spectral plateau. A spectral observation alone did not establish the relative spectral phases needed for an attosecond pulse train.^[ref]

Use the complex electric field convention $e^{-i\omega t}$ and take the locked wave amplitudes $a({\omega }_p)$ to be real and nonnegative after extracting their common phase. Linear superposition givesThe leading factor has unit modulus, so it does not change the light intensity. The triangle inequality bounds the remaining modulus by ${\sum }_{p}a({\omega }_p)$. Consecutive occupied normal modes attain this bound together atThere is also a peak at $t=0$. The electric field obeys $E(t+T)=e^{-i{\omega }_{\mathrm{ce}}T}E(t)$, although the light intensity is periodic.

The carrier-envelope phase specifies where the carrier oscillation lies relative to the maximum of an optical pulse's envelope. With $s=t-t_m$ and $E=F(s)e^{-i{\omega }_{0}s+i{\psi }_m}$, the signed complex-field phase at the envelope maximum is ${\psi }_m$. Consequently,If instead the real electric field is written $F(s)\cos ({\omega }_{0}s+{\varphi }_{\mathrm{CE},m})$, then ${\varphi }_{\mathrm{CE},m}=-{\psi }_m$ and the conventionally quoted slip is ${\varphi }_{\mathrm{slip}}=+2\pi {\omega }_{\mathrm{ce}}/\Delta \omega$. These are the same physical shift with opposite phase definitions.

The carrier-envelope phase matters for few-cycle optical pulses, when the largest instantaneous electric field affects strong-field dynamics, for high harmonic generation, and for stabilizing an optical frequency comb. An ordinary cycle-averaged light intensity measurement does not determine it.

P. Breger coauthored an experiment that reconstructed a train of $250$-attosecond optical pulses from high harmonic generation. Two-color photoionization measurements determined relative harmonic phases, connecting the measured electron signal to the short-pulse structure.^[ref]

Ph. Balcou coauthored an experiment that reconstructed a train of $250$-attosecond optical pulses from high harmonic generation. Two-color photoionization measurements determined relative harmonic phases, connecting the measured electron signal to the short-pulse structure.^[ref]

Pierre Agostini coauthored the measurement of a train of attosecond optical pulses synthesized from high harmonic generation. Measuring only the harmonic intensities does not determine the pulse train: the relative spectral phases are also needed. Their two-colour photoionization measurement recovered those spectral phases, giving reconstructed peaks about 250 attoseconds wide. This joined generation with a measurable test of temporal structure, rather than inferring a short pulse solely from a broad optical spectrum.^[ref]

A physicist researching high harmonic generation and interactions between lasers and matter.^[ref]

P. M. Paul coauthored an experiment that reconstructed a train of $250$-attosecond optical pulses from high harmonic generation. Two-color photoionization measurements determined relative harmonic phases, connecting the measured electron signal to the short-pulse structure.^[ref]