Spectrally resolved spatiotemporal features of quantum paths in high-order-harmonic generation

Abstract

We experimentally disentangle the contributions of different quantum paths in high-order harmonic generation (HHG) from the spectrally and spatially resolved harmonic spectra. By adjusting the laser intensity and focusing position, we simultaneously observe the spectrum splitting, frequency shift and intensity-dependent modulation of harmonic yields both for the short and long paths. Based on the simulations, we discriminate the physical mechanisms of the intensity-dependent modulation of HHG due to the quantum path interference and macroscopic interference effects. Moreover, it is shown that the atomic dipole phases of different quantum paths are encoded in the frequency shift. In turn, it enables us to retrieve the atomic dipole phases and the temporal chirps of different quantum paths from the measured harmonic spectra. This result gives an informative mapping of spatiotemporal and spectral features of quantum paths in HHG.

Spatially and spectrally resolved quantum path interference with chirped driving pulses

“…The values found by comparison of our mathematical model with the experimental data are in good agreement with the theoretical prediction for a l in the whole region of q and for a s close to the cut-off region (see figure 7). Our measured and calculated values of a l are also in good agreement with earlier experimental work [15,19] and theoretical predictions [21,22,37,40]. In this study we have consistently extracted, in both the spectral and spatial measurements, negative values for a s for a range of harmonics below and near the ionisation threshold.…”

“…This behaviour is clearly reflected in the spatial lineouts, for which the peak intensity is the parameter with largest influence; the lineouts are vertically and horizontally symmetric. The data presented in figure 5(a) are analogue to the intensity scans presented in earlier work [17,19,[36][37][38][39]. However, the sign of the chirp parameter is important in the spectral lineouts, which substantially differ for negatively and positively chirped driving laser pulses.…”

“…The dipole properties of the HHG process may lead to various interference effects, since the same final energy is generated from several different trajectories. Interferences appearing as spectral and/or spatial structures in the harmonic far-field emission have been reported and identified as interferences between the short and the long quantum path contribution, known as quantum path interferences (QPIs) [15][16][17][18][19]. Other works identify complicated spatial and spectral features of harmonics produced by individual quantum paths at high driving intensities, due to strong spatio-temporal phase and amplitude modulations in the generation medium [20,21] or due to the spectral interference of adjacent harmonics [22].…”

“…Other works identify complicated spatial and spectral features of harmonics produced by individual quantum paths at high driving intensities, due to strong spatio-temporal phase and amplitude modulations in the generation medium [20,21] or due to the spectral interference of adjacent harmonics [22]. The emission from the short QP contribution has been characterised in much more detail [20][21][22] than that from the long QP contribution [23] as the latter is more difficult to accurately phase match and control experimentally [15,19].…”

Direct imaging of molecular rotation with high-order-harmonic generation

“…The background gas pressure is maintained at 20 Torr and the rotational temperature is estimated to be about 100 K [53]. The generated harmonic spectrum is detected by a homemade flat-field soft x-ray spectrometer [54,55].…”