= High harmonic generation
{wiki}
= HHG
{c}
{synonym}
<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>.<Generation and Measurement of Isolated Attosecond Light Pulses by Michael Hentschel>[Generation and Measurement of Isolated Attosecond Light Pulses, slide 5: tunnelling, acceleration and return]{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>.<Multiple-harmonic conversion of 1064 nm radiation in rare gases by M. Ferray and others>[Multiple-harmonic conversion of 1064 nm radiation in rare gases, abstract]{ref}
Back to article page
