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(Color online) Spectral effects of split-spectrum control of high-harmonic generation. (a) Setup of the split-spectrum pulse shaper. (b) Measured spectrum of harmonics dependent on time delay between the two spectral ranges indicated in (c). (c) Broadband split spectrum after the pulse shaper, part I and part II correspond to mirror I and mirror II in (a). (d) Spectrum of conventional odd-integer harmonics at time delay 2.8 fs and fractional (non-integer) harmonics at time delay 1.5 fs (magnified by a factor of 3).
(Color online) Mechanism of fractional-harmonic comb generation: double attosecond pulse train generation and interference (simulation results). (a) Shaped electric driving field intensity (dashed lines) and attosecond pulse trains (solid lines) for zero time delay (lines labeled with i, constructive interference of the two fundamental spectral parts at time 0) and for time delay 1.28 fs (lines labeled with ii, destructive interference at time 0). Two interfering attosecond pulse trains are generated in the latter case leading to deeply modulated fractional harmonics. (b) Simulated harmonic spectra dependent on the time delay between the two spectral parts. (c) Spectrum of fractional harmonics with high peak-to-valley contrast for the time delay of 1.28 fs in (b) (red line).
Generation of two equally intense attosecond pulse trains by coherent control of the peak intensity ratio of the driving subpulses (simulation results). (a) Intensity ratio of the two most intense maxima of the shaped driving laser pulse dependent on the time delay. Simulation for the measured split-spectrum displayed in Fig. 1 including the residual spectral phase after dispersion compensation. (b) Shaped driving laser pulse with two equally intense subpulses at time delay 1.55 fs.
(Color online) HHG energy tuning and mechanism with an asymmetric energy-split spectrum. An asymmetric cut of the spectrum at 795 nm leads to a pronounced modulation of the instantaneous frequency at the peak of the driving laser field and, thus, to an attosecond-delay-dependent energy modulation of the harmonics. (a) Measured split spectrum of the driving laser field. (b) Intensity of the shaped driving pulse dependent on real time and time delay between the two spectral parts, and time-delay dependent modulation of the instantaneous laser frequency (yellow triangles) at the intensity peaks (white lines). (c) Measured energy modulation of the harmonics compared to the relative modulation of the instantaneous laser frequency without chirp as in (b) (yellow triangles) and for chirps of −10 fs2 (red crosses) and +10 fs2 (white stars). Excellent quantitative agreement is found, confirming atom-level HHG control even with weak (∼1.7% of strong-pulse energy) control fields.
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