Photoinjection of charge carriers radically changes the properties of a solid. This manipulation enables ultrafast measurements such as electric field sampling recently advanced to petahertz frequencies and the real-time study of the physics of many bodies. Non-linear photoexcitation by a laser pulse of a few cycles can be limited to the strongest half cycle.

Describing the associated subcycle optical response is elusive when studied with traditional pump-probe metrology, as the dynamics distort each probe field on the carrier time scale rather than that of the envelope. Laser physicists from LMU’s autoworld team and the Max Planck Institute of Quantum Optics have now made a direct observation of how the optical properties of silicon and silicon dioxide evolve during the first few femtoseconds (millionths of a billionth of a second) after photoinjection with an intense laser pulse.

Compared to the photoelectric effect, as described by Albert Einstein, the operation of photoinjection is quite simple. Here, an electron absorbs a single photon with enough energy to release it from the confines of a potential. If there are no photons in the light wave with enough energy to do that, things get more complicated. Multiple photon absorption or quantum tunneling can release bound electrons in this situation. Only the core part of a laser pulse can effectively drive these nonlinear processes, as they only work when there is a strong electric field.

Most charge carriers can be made using attosecond scientific techniques in just half a cycle of a pulse of light, resulting in solids with orders of magnitude higher conductivities in just a few femtoseconds.

Physicists determined how quickly solids change their optical properties after ultrafast photoinjection. They did this by sending two pulses of several cycles through a thin sample: an intense pump pulse that created charge carriers and a weak test pulse that interacted with them.

It was able to track how charge carriers interacted with the test field in the first femtoseconds after their emergence, as photoinjection was limited to a time period less than half a cycle of the test field. The aberrations that photoinjection imposed on the time-varying electric field of the test pulse were encoded with this information. The researchers repeated their experiments for different time delays between the two pulses to measure these distortions using a unique optical field sampling method.

Vladislav Yakovlev, the study’s final author, said: “The innovative technique for optical field-resolved pump-probe measurements now gives the autoworld team direct access to light-driven electrical currents during and after photoinjection. The main result is that we now know how to conduct and analyze such experiments and have seen the light-driven electron motion as no one could before.”

“We were surprised to see no obvious signs of quasiparticle formation. This means that in these particular measurements, the many-particle physics measurements didn’t have much of an impact on how the conductivity of the medium built up after photoinjection. Still, in the see some better physics in the future.”

The new findings could help realize future signal processing in the petahertz range, enabling so-called lightwave electronics. That would speed up current electronics about 100,000 times.

Yakovlev is convinced, “I’d say we’re just getting to the tip of the iceberg of what pump-probe field-resolved measurements can do. Armed with our experience and insights, other researchers can now also use our approach to answer their questions.”

Magazine reference:

  1. Zimin, DA, Karpowicz, N., Qasim, M. et al. Dynamic optical response of solids after photoinjection at 1 fs scale. Nature (2023). DOI: 10.1038/s41586-023-05986-w