Luke E. Chipperfield

EPJ Web of Conferences, open-access proceedings in physics and related sciences

The paper reports on the observations and analysis of high harmonic generation driven by a superposition of fields at 1290 nm and 780 nm. These fields are not commensurate in frequency, as in previous experiments and the superposition leads to an increase in the yield of the mid-plateau harmonics of more than two orders of magnitude compared to using the 1290 nm field alone. Significant extension of the cut-off photon energy is seen even by adding only a small amount of the 780 nm field.

In the article of L.E. Chipperfield et al. [Phys. Rev. Lett. 2009, 102, 063001], the ideal waveform was presented that maximised the recollision energy of a strong-field driven electron. In this paper we expound the derivation of this perfect wave, as well as a more general waveform for optimising electron trajectories for which the recollision occurs some distance from the place of origin. The ideal waveform acts as a guide for the synthesises of more practical waveforms that maximise the electron recollision energy, and so extend the high-order harmonic cut-off to higher frequencies.

We report observations and analysis of high harmonic generation driven by a superposition of fields at 1290 nm and 780 nm. These fields are not commensurate in frequency and the superposition leads to an increase in the yield of the mid-plateau harmonics of more than two orders of magnitude compared to using the 1290 nm field alone. Significant extension of the cut-off photon energy is seen even by adding only a small amount of the 780 nm field. These observations are explained by calculations performed in the strong field approximation. Most importantly we find that enhancement is found to arise as a consequence of both increased ionization in the sum-field and modification of the electron trajectories leading to an earlier return time. The enhanced yield even when using modest intensity fields of 5 x 10¹³ Wcm⁻² is extended to the 80 eV range and is a promising route to provide a greater photon number for applications in XUV imaging and time-resolved experiments at a high repetition rate.

High‐order harmonic spectra are composed of a coherent sum of half‐cycle emissions, the cut‐off energy of which depend sensitively on different sub‐cycle portions of the driving laser field. By selecting the correct focal geometry the half‐cycle cut‐off emissions can be preferentially selected over the lower energy plateau emissions through phase matching, such that they form macroscopic half‐cycle cut‐off features in the far‐field spectrum. The energy of these macroscopic half‐cycle cut‐offs can then be used to retrieve the waveform of the driving laser field. The processes through which these macroscopic half‐cycle cut‐offs are formed and their applications, both for measuring the laser waveform and the generation of wavelength tunable isolated attosecond pulses, are reviewed in detail.

We present the perfect waveform which, during a strong field interaction, generates the maximum possible electron recollision energy for any given oscillation period, over 3 times as high as that for a pure sinusoidal wave. This ideal waveform has the form of a linear ramp with a dc offset. A genetic algorithm was employed to find an optimized practically achievable waveform composed of a longer wavelength field, to provide the offset, in addition to higher frequency components. This second waveform is found to be capable of generating electron recollision energies as high as those for the perfect waveform while retaining the high recollision amplitudes of a pure sinusoidal wave. Calculations of high harmonic generation demonstrate this enhancement, by increasing the cutoff energy by a factor of 2.5 while maintaining the harmonic yield, providing an enhanced tool for attosecond science.