Petahertz spectroscopy

The frontier laser research with the CEO phase stabilization also enables petahertz electronics. In the 1980s, rapid progress in picosecond and femtosecond ultrafast lasers started to bridge the gap between electronics and optics with terahertz frequencies. With the recent progress in few-cycle femtosecond and attosecond pulse generation with full electric field control, the frequency regime can be extended into the petahertz for the investigation of ever faster physical processes and device performance.

Transient absorption spectroscopy is a powerful technique first established in the femtosecond regime and also used in the attosecond regime in our group [1-3]. Attosecond transient absorption spectroscopy (ATAS) allows for element and carrier specific probe of excited states dynamics when an attosecond XUV pulse is used to probe the transition between core and excited states. With the probe is resonant with a characteristic core level, ATAS becomes inherently element specific while its broad bandwidth simultaneously reveals the dynamics of both, pump-excited electrons and holes with attosecond time resolution.

We have performed pioneering attosecond field-induced dynamics in diamond [4,5] and GaAs [6,7] based on the first observation of the dynamical Franz Keldysh effect in the near-infrared regime. Surprisingly for the resonant excitation in GaAs, we found that the transient response is still dominated by intra-band motion and that this can strongly increase the excited carrier density [8].

In addition we revealed a new ultrafast response around transition metal atoms based on real-space electron localization into 3d-orbitals following the femtosecond excitation pulse envelope in elementary transition metals [9,10], which is not present in Al [11] and only happens around molybdenum in a 2D semiconductor MoSe₂ (a transition metal dichalcogenide, TMDC). Only the spectral response of the transition metal Mo is dominated by screening effects with ultrafast d-orbital localization, while the selenium is not affected by this collective response and can be described by an independent particle model with band filling, thermalization and lattice heating [12]. This can be considered surprising given that the near-infrared pump excited carriers are well above the Mott-transition and valence electrons from both Mo and Se contribute to the conduction band. This means the overall dynamics in such materials with transition metal elements cannot be described with an independent particle model. This may have implications for the applicability of ubiquitous effective mass approximation for describing such semiconductors.

References

[1] M. Holler, F. Schapper, L. Gallmann, U. Keller
Download"Attosecond electron wave-packet interference observed by transient absorption" (PDF, 188 KB)
Phys. Rev. Lett., vol. 106, 123601, 2011

[2] L. Gallmann, J. Herrmann, R. Locher, M. Sabbar, A. Ludwig, M. Lucchini, U. Keller
Download"Resolving intra-atomic electron dynamics with attosecond transient absoprtion spectroscopy” (PDF, 313 KB) – Invited Paper
Molecular Physics, vol. 111, Nos. 14-15, 2243-2250, 2013

[3] R. Locher, M. Lucchini, J. Herrmann, M. Sabbar, M. Weger, A. Ludwig, L. Castiglioni, M. Greif, M. Hengsberger, L. Gallmann, U. Keller
Download"Versatile attosecond beamline in a two-foci configuration for simultaneous time-resolved measurements" (PDF, 922 KB)
Review of Scientific Instrument, vol. 85, 013113, 2014

[4] M. Lucchini, S. A. Sato, A. Ludwig, J. Herrmann, M. Volkov, L. Kasmi, Y. Shinohara, K. Yabana, L. Gallmann, U. Keller
Download"Attosecond dynamical Franz-Keldysh effect in polycrystalline diamond" (PDF, 1.9 MB)
Science, vol. 353, Issue 6302, pp. 916-919, 2016

[5] News item: https://ulp.ethz.ch/news/ulp-news/2016/08/eth-news-and-nccr-must-highlight.html

[6] F. Schlaepfer, M. Lucchini, S. A. Sato, M. Volkov,L. Kasmi, N. Hartmann, A. Rubio, L. Gallmann, U. Keller
Download"Attosecond optical-field-enhanced carrier injection into the GaAs conduction band" (PDF, 2.9 MB)
Nature Physics, vol. 14, pp. 560-564, 2018

[7] News item: https://ulp.ethz.ch/news/ulp-news/2018/03/a-milestone-in-petahertz-electronics.html

[8] S. A. Sato, M. Lucchini, M. Volkov,F. Schlaepfer, L. Gallmann, U. Keller, A. Rubio
Download"Role of intraband transitions in photocarrier generation" (PDF, 933 KB)
Phys. Rev. B, vol. 98, 035202, 2018

[9] M. Volkov, S. A. Sato, F. Schlaepfer,L. Kasmi,N. Hartmann,M. Lucchini,L. Gallmann,A. Rubio,U. Keller
Download"Attosecond screening dynamics mediated by electron-localization" (PDF, 2.2 MB)
Nature Physics, vol. 15, pp. 1145-1149, 2019

[10] News item on attosecond electron localisation in metallic titanium: https://ulp.ethz.ch/news/ulp-news/2019/08/attosecond-electron-localization-in-metallic-titanium.html

[11] A. Niedermayr, M. Volkov, S. A. Sato, N. Hartmann, Z. Schumacher, S. Neb, A. Rubio, L. Gallmann, U. Keller
Download"Few-femtosecond dynamics of free-free opacity in optically heated metals" (PDF, 1.2 MB)
Phys. Rev. X, vol. 12, 021045, 2022

[12] Z. Schumacher, S. A. Sato, S. Neb, A. Niedermayr, L. Gallmann, A. Rubio, U. Keller
"Ultrafast electron localization and screening in a transition metal dichalcogenide"
external pagehttps://arxiv.org/abs/2210.05465, submitted 11. Oct. 2022