When light interacts resonantly with solids, electrons become excited from the valence to the conduction bands. This process is usually described via the absorption of photons and neglects the momentum transfer from light to the electrons.  In contrast, when the light field becomes strong (comparable to the bonding fields of a volt per angstrom), the light field significantly changes the momentum of the electrons. This change of the electrons’ momentum during the laser pulse gives rise to interesting strong-field phenomena such as coupled inter-and intraband electron dynamics, causing subsequent coherent Landau-Zener transitions [1-4] or high harmonics of the drive field, extending to frequencies in the multiple petahertz range [5-7].

In the case of graphene, we demonstrated the coherent control of electrons in an electrical conductor at optical frequencies [1-4]. Based on a strong light field we were able to steer electrons on complex electron trajectories inside of graphene’s band structure. When the electron is driven near the Dirac point of graphene, the wave function of the electron is split into a superposition of the two band states. After half an optical cycle of about 1.3 femtoseconds, these parts of the wave function meet again and interfere. This quantum-path interference generates a residual current flow within one femtosecond.

In the past years, we have used this technique to measure the coherence lifetime of the electrons after excitation [4,5], determine the accumulated quantum mechanical phase, which allows for electro-optical reconstruction of the band structure, disentangle the role of real and virtual charge carriers in the current generation process [1] or control charge on the attosecond timescale across heterostructures [8].

Although the band structure of 2D materials allows quantum control most simply, it is even more fascinating to extend the concept of light-field controlled electron dynamics to topological non-trivial materials. These materials are promising candidates for non-dissipative topological electronics.  Based on the prototypical topological nontrivial material Bismuth selenide, we show that the topologically protected conducting surface state generates high-harmonics very efficiently, particularly under circular excitation, which is in contrast to topological non-trivial materials [6,7]. We explain these peculiar properties based on spin-polarized surface states and the Berry connection in the vicinity of the Dirac points.  

In summary, we demonstrate light-field driven electron dynamics in two-dimensional materials and topological insulators using high-harmonics and photocurrent generation. We show that these techniques are not only suited to study fundamental properties of these materials but also offer a direct link to combine electronics and optics towards novel light-field driven electronics.

References:

1. Boolakee T., Heide C., et al. , Nature in press
2. Higuchi T., Heide C., et. al. Nature 550, 224-228 (2017)
3. Heide C., et. al. Physical Review Letters 121, 207401 (2018)
4. Heide C., Eckstein T., et. al., Nano Letters, 21, 9403 (2021)
5. Heide C., Kobayashi Y, et. al. Optica, in press
6. Baykusheva D., et. al. Nano Letters, 21, 8970 (2021)
7. Heide C., Kobayashi Y, et al. under submission
8. Heide C. et. al. Nature Photonics 14,  219–222 (2020)

Bio: Christian Heide received his PhD from Friedrich-Alexaner University in Erlangen-Nuremberg (Germany) in 2020.  Since 2020, he has been working with Profs. Tony Heinz, David Reis, and Shambhu Ghimire at the Stanford PULSE Institute. Since 2021, he is a Feodor Lynen Fellow of the Alexander von Humboldt Foundation. Christian's research interests include coherent control of electrons on the femtosecond time scale using strong and ultrashort laser pulses. This comprises the design and development of light-field-driven electronics using ultrafast current injection and high harmonic spectroscopy on novel materials. Christian was awarded the best dissertation award by the German Physical Society (DPG) and the German Society for Applied Optics.

This seminar is sponsored by the Department of Applied Physics and the Ginzton Laboratory.