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C. Manus coauthored an experiment converting $1064$-nm laser radiation into high odd harmonics in rare gases. In argon, the spectrum reached the $33$rd harmonic at $32.2$ nm, demonstrating a route to extreme ultraviolet radiation.^[ref]

G. Mainfray coauthored an experiment converting $1064$-nm laser radiation into high odd harmonics in rare gases. In argon, the spectrum reached the $33$rd harmonic at $32.2$ nm, demonstrating a route to extreme ultraviolet radiation.^[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]

L. A. Lompre coauthored an experiment converting $1064$-nm laser radiation into high odd harmonics in rare gases. In argon, the spectrum reached the $33$rd harmonic at $32.2$ nm, demonstrating a route to extreme ultraviolet radiation.^[ref]

M. Ferray coauthored an experiment converting $1064$-nm laser radiation into high odd harmonics in rare gases. In argon, the spectrum reached the $33$rd harmonic at $32.2$ nm, demonstrating a route to extreme ultraviolet radiation.^[ref]

X. F. Li coauthored an experiment converting $1064$-nm laser radiation into high odd harmonics in rare gases. In argon, the spectrum reached the $33$rd harmonic at $32.2$ nm, demonstrating a route to extreme ultraviolet radiation.^[ref]