Enhanced above threshold ionization (ATI) in resonantly excited helium nanodroplets: All for one and one for high energies

Nonlinear optical phenomena usually occur when matter interacts with intense radiation. An extreme example of nonlinear optics is the generation of high-order harmonics (HHG) in rare gases, which is currently being used in many laboratories as a method for generating ultrashort pulses in the extreme ultraviolet (XUV) spectral region. Such pulses are widely used for time-resolved XUV spectroscopy and for attosecond science. However, the conversion efficiency of the fundamental wave into high harmonics using dilute gas targets is notoriously low. Therefore, new HHG schemes and more dense condensed phase targets are being extensively researched.
Another nonlinear phenomenon closely related to HHG is the emission of energetic electrons by the absorption of multiple photons by one atom far above the ionization threshold (Above Threshold Ionization, ATI). Several attempts have been made to enhance the yields and energies of emitted photons and electrons by HHG and ATI, respectively, using clusters and nanoparticles as targets taking advantage of their high local density. However, only weak enhancements have been realized to date. 
In a joint experimental and theoretical study, a team of researchers from Germany, Denmark, Italy, Sweden, USA, and Kuwait have now demonstrated that the energy of electrons emitted by ATI generated by the seeded free electron laser FERMI in Trieste can be drastically enhanced when using helium nanodroplets that are resonantly excited prior to ionizing them above threshold. 
The spectra of electrons emitted from helium atoms or helium nanodroplets of various sizes measured by irradiating them first with a resonant XUV pulse at a wavelength of 52.3 nm and then with an intense near-infrared pulse (800 nm wavelength) are shown in Fig. 1 a). Strikingly, with increasing size of the helium droplets and thus with increasing number of excitations per droplet, the electron energy distributions spread out toward high energies reaching far beyond the 10 Up cutoff. A similar enhancement of high-order ATI is seen when the number of excitations per droplet is increased by cranking up the intensity of the XUV pulses. The maximum enhancement of ATI is reached when about 1 % of helium atoms per droplet are excited. 
To rationalize these observations, the measured atomic and droplet ATI spectra were compared with time-dependent Schrödinger equation (TDSE) calculations and with a simple combinatorial model, respectively. The good agreement indicates that ATI is enhanced by a collective coupling between several excited atoms in one droplet. While the exact coupling mechanism remains to be worked out, it appears that the energy absorbed from the driving laser pulse by all excited atoms is channeled into only one which then emits a high-energy electron. If a similar enhancement occurred in HHG, this might open up a new route for generating high-energy photons more efficiently than what is possible today. 
Besides this promising outlook, the time-resolved ATI electron spectra revealed a complex interplay between several ultrafast relaxation processes taking place in resonantly excited superfluid helium droplets, electronic relaxation, including intra- and interatomic autoionization, and the ejection of excited helium atoms. This can be seen from the changing structure of the individual energy components of the ATI spectra recorded as a function of the delay between the XUV pump and a near-UV probe pulse (400 nm), see Fig. 1 b). This relaxation dynamics is described in more detail in a publication by the same team of researchers, which was recently selected as a 2021 HOT PCCP.
The experiments were performed at the Low Density Matter beamline.
 

Figure 1.  a) Photoelectron spectra of He droplets of different mean size <N> (colored lines) compared with He atoms (N=1, black line). Both droplets and atoms are resonantly excited to the 1s4p state at a photon energy of 23.7 eV and ionized by NIR pulses. b) Logarithmic intensity plot of the electron spectra (vertical axis) as a function of the delay between the XUV pump and the UV probe pulses (horizontal axis).

This research was conducted by the following research team:

R. Michiels¹, M. Abu-samha², L. B. Madsen³, M. Binz¹, U. Bangert¹, L. Bruder¹, R. Duim¹, A. Wituschek¹, A. C. LaForge⁴, R. J. Squibb⁵, R. Feifel⁵, C. Callegari⁶, M. Di Fraia⁶, M. Danailov⁶, M. Manfredda⁶, O. Plekan⁶, K. C. Prince⁶, P. Rebernik⁶, M. Zangrando^{6, 7}, F. Stienkemeier¹, and M. Mudrich³

¹ Institute of Physics, University of Freiburg, Freiburg, Germany
² College of Engineering and Technology, American University of the Middle East, Kuwait
³ Department of Physics and Astronomy, Aarhus University, Aarhus C, Denmark
⁴ Department of Physics, University of Connecticut, Storrs, Connecticut, USA
⁵ Department of Physics, University of Gothenburg, Sweden
⁶ Elettra-Sincrotrone Trieste, Trieste, Italy
⁷ CNR-IOM, Elettra-Sincrotrone Trieste S.C.p.A., Italy

Contact persons:

Marcel Mudrich, email: mudrich@phys.au.dk