Session P5: High Harmonic and Attosecond Pulse Generation

High flux table-top ultrafast soft X-ray source generated by high harmonic generation

Generation of ultrafast soft X-ray pulses is a major challenge for conventional laboratories. Using the process of HHG enables generation of such short wavelength photons. Intense laser sources in the infrared are necessary to reach the soft X-ray spectral range as the HHG cut-off scales with I$\lambda^{2}$. However, in the limit of the single atom response, increasing the laser wavelength leads to a significant decrease of the HHG flux. To compensate, one has to increase the number of emitters with high ionization potential. At the Advanced Laser Light Source, we have addressed this challenge by using a new gas cell design and developing a 10 mJ - 30 fs source at 1.8 $\mu$m. Using this setup, we have been able to generate harmonics in the water window spectral range for neon and helium with short time duration (\textless 30 fs) in a conventional laboratory. A flux measurement has been performed showing $\sim$ 2 $\times$ 10$^{5}$ photons/shot between 280 and 540 eV, making it possible to see the carbon k-edge at 280eV in a single shot manner. This soft X-ray beam is also extremely well collimated (0.1 mrad) making it this table-top beamline ideal for a number of applications.   

Waveform optimization for enhancing high-harmonic yield by synthesizing two or three-color laser fields.

High-order harmonics (HH) extending to the X-ray region generated in a gas medium by intense lasers offer the potential for providing tabletop broadband light sources but so far are limited by their low conversion efficiency. We show that HH yield can be enhanced by one to two orders of magnitude if the laser's waveform is optimized by synthesizing two- or three-color fields compared to a sinusoidal wave without an increase in the total laser power. The optimization procedure carried out by genetic algorithm is designed to take into account of macroscopic propagation effects. The HH thus generated are also favorably phase-matched so that radiation is efficiently built up in the gas medium. In addition, we demonstrate the generation of a single-attosecond pulse by synthesizing three incommensurate lasers while the harmonic yield is optimized as well. Our results, combined with the emerging intense high-repetition MHz lasers, promise to increase harmonic yields by several orders to make HH feasible in the near future as general bright tabletop light sources.   

Theory of single-order high harmonic generation using waveform-synthesized chirped pulses

We show that it is possible to selectively generate a single-order high harmonic using waveform-synthesized chirped pulses. By synthesizing a multicycle 800nm pulse with its higher harmonics and adjusting the chirp of each pulse, a single high-harmonic order can be selectively enhanced while all other harmonic orders are greatly suppressed. The harmonic order of enhancement can be controlled continuously by changing the laser intensity. This single-order harmonic enhancement has been identified to be originated from quantum interference and the details of the physical process will be explained. Our simulation results agree qualitatively to and shed light on a recent experiment of single-order harmonic generation using three-color pulses [1]. \\[4pt] [1] P. Wei et al., Phys. Rev. Lett. 110, 233903 (2013).   

Noble gas cluster size effect on high harmonic generation

This work focuses on understanding the physics of high harmonic generation from inert gas clusters (Neon, Argon, Krypton, Xenon). In the experiment, noble gas clusters are produced by a supersonic pulsed jet, and high harmonics from cluster-laser interaction are generated. Atom density, $n_{atom}$, and cluster density, $n_{cluster}$, can be controlled independently by changing the valve temperature and backing pressure, enabling us to study the cluster size effect. We observe 1) for a given cluster size, a quadratic increase of the harmonic yield versus the cluster density: $Y_{HH} \propto n^2_{cluster}$; 2) an increase of the yield with the size of the clusters, rapid up to a critical size $N_t$, then slower. The value of $N_t$ is observed to shift for different noble gas clusters. We find that calculations' assuming a partially delocalized electron wave function is consistent with the observed enhanced single cluster response. The delocalization is caused by the potential from neighbor ions and electrons. In our model, the saddle point method in a 1D Lewenstein's model is used to calculate harmonic yield as a function of cluster size. The calculation result reproduces the features of single cluster response qualitatively including the shift of $N_t$.   

High Harmonic mixing in Solid Argon

We measure high harmonics of 800 nm and 1333 nm fundamental radiation in thin solid argon films. The thickness dependence of the above gap harmonics shows evidence for nonlinear mixing between low and high orders. In particular the thickness dependences show similar length scales suggesting that they originate from the phase mismatch of the third harmonic. However, in some cases, a substantial offset for the maxima of harmonic intensity is seen as a function of thickness. A big shift between consecutive harmonics could indicate the onset of new processes contributing to the harmonic generation.   

Multielectron effects in high order harmonic generation: ellipticity and fractional harmonics

Using time-dependent density functional theory, we have studied multi-electron effects on high harmonic generation spectra and ellipticity. Our simulation results for ellipticity are in excellent agreement with two sets of experimental results and reveal that ellipticity is very sensitive to laser intensity, multielectron effects. Further analysis shows that for example at least three orbitals, HOMO, HOMO-1 and HOMO-2 of $N_2$, contribute to the ellipticity patterns. We also discovered the fractional harmonics as a result of sub-dynamics between coupled orbitals in an open shell system. The position of fractional harmonics is determined by the (virtual) Rabi oscillation frequency.   

Generation of Bright Isolated Attosecond Soft X-Ray Pulses Driven by Multi-Cycle Mid-Infrared Lasers

Advances in the understanding of macroscopic phase-matching of high harmonic generation (HHG) driven by mid-IR lasers have made it possible to generate bright, coherent, high harmonic x-ray beams in the UV to keV with attosecond or even zeptosecond bandwidths. We perform advanced theoretical analysis, corroborated with experimental results, to unveil the characteristics of HHG soft x-ray \textit{as} pulses. We show that when mid-IR lasers are used to drive HHG, the conditions for optimal soft x-ray generation naturally coincide with the generation of bright isolated \textit{as} pulses that are also shorter in duration if compressed. In addition, in contrast to \textit{as} pulse generation in the EUV, multi-cycle driving laser pulses are more suitable for generating bright isolated soft x-ray bursts.   

Control of high harmonic generation using isolated attosecond pulses

Control of high harmonic generation (HHG) by using additional colors of light has been established as an efficient means of creating isolated pulses of light with increasingly short durations. ~We present a study of HHG in which isolated attosecond-duration VUV pulses are used to control the population of excited states in a single-atom system. A target He atom is prepared in its ground state, and a moderately intense 1.6 $\mu $m driving laser field is used to permit transitions to continuum states only from excited states of the atomic system. By varying the delay of the isolated attosecond pulse with respect to the driving field, this technique affords control over the moment of electron ionization, and in particular establishes a mechanism for selecting for and experimentally verifying the existence of multiply rescattering trajectories both in the temporal and frequency domains.