(Nanowerk News) The laser pulse hits the electrons in the solid. If it receives enough energy from the light waves, it can then move freely through solid objects. This phenomenon, which scientists have been exploring since the dawn of quantum mechanics, is called photoinjection. There are still open questions about how the relevant processes unfold in time. Laser physicists from the LMU attoworld team and the Max Planck Institute of Quantum Optics have now made first-hand observations 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 a strong laser pulse.
The team published their findings in (Natural, “Solid dynamic optical response following 1-fs scale photoinjection”).
The physics of photoinjection is relatively simple in terms of the photoelectric effect described by Albert Einstein. Here, an electron absorbs a photon that has enough energy to free the electron from the potential that limits its motion. It gets complicated when none of the photons in the light wave have enough energy to do so. In this case, bound electrons can become free by absorbing more than one photon at a time or by quantum tunneling. This is a nonlinear process that is only effective when the electric field is strong, meaning only the central part of the laser pulse can efficiently drive it.
With the tools of atthodetic science, it is possible to generate a large proportion of charge carriers in one and a half cycles of a light pulse, increasing the conductivity of solids by an order of magnitude in only a few femtoseconds. Laser physicists from the attoworld team at LMU and the Max Planck Institute of Quantum Optics have investigated how quickly solids change their optical properties after ultrafast photoinjection. To do so, they sent two multiple-cycle pulses through the thin sample: an intense pump pulse that creates charge carriers and a weak test pulse that interacts with them.
Since photoinjection is limited to time intervals shorter than the half cycle of the test field, it is possible to observe how the charge carriers interact with the test field during the first femtosecond after their emergence. This information is encoded in a distortion printed by photoinjection of the time-dependent electric field of the test pulse. The scientists measured this distortion using a new technique for optical field sampling, and they repeated their measurements for the multitude of delays between the two pulses.
An innovative technique for optically resolved pump probe measurement now gives the attoworld team direct access to light-driven electrical currents during and after photo injection. “The most important result is that we now know how to conduct and analyze the experiment and that we are actually seeing light-driven electron motion as never before,” said Vladislav Yakovlev, the study’s final author. “We were surprised to see no clear signs of quasiparticle formation,” Yakovlev further explained. “This means that, in this particular measurement, multi-body physics measurements don’t have much influence on how the conductivity of the medium builds up after photoinjection, but we may see some finer physics in the future.”
All modern electronics are based on controlling the flow of charge carriers by rapidly increasing and decreasing their ability to move through a circuit. The attoworld team’s research is all about reaching the ultimate speed limit of this control using light. These new findings could eventually help achieve future signal processing in the petahertz range, enabling what electronics call light waves. That would speed up today’s electronics by about 100,000 times. “I would argue that we have just scratched the surface of what measurements completed with pump probes can do. Armed with our experience and insight, other researchers can now use our approach to answer their questions,” Yakovlev believes.