Since the early days of quantum mechanics, scientists have been exploring what exactly happens during photoionization and photoinjection, when an electron gains enough energy from a light wave to free itself from atomic or molecular orbitals. There are still open questions about how the relevant processes unfold in time. Scientists of the attoworld team of the Max Planck Institute of Quantum Optics and the Ludwig Maximilian University of Munich have now made a direct observation of how the optical properties of solids evolve after 1-femtosecond-scale photoinjection. Here Vladislav Yakovlev talks with Thorsten Naeser about these groundbreaking attosecond experiments, which have been published in the scientific journal Nature.
What exactly happens during the photoinjection of charge carriers?
Electrons that could not freely move through a solid gain enough energy from a light wave to become mobile. This transition from being bound to being free has been studied since the early days of quantum mechanics, but we are still struggling to understand the relevant physics. That physics is relatively simple in the case of the textbook photoeffect, where a single electron residing in some potential well absorbs a single photon that has enough energy to free the electron from the bounding potential. Things get more complicated when no photon in the light wave has enough energy to do so. In this case, bound electrons can become free by absorbing more than one photon at once. An electron can also get through a potential barrier by quantum tunneling.
What makes photoinjection so interesting?
We are particularly interested in understanding how quickly this happens, as well as in confining photoinjection to a possibly short time interval. This is harder than one might think because electrons are quantum particles that can be in many states at the same time. Even in theory, it is difficult to determine when exactly an electron becomes free. The relevant physics gets even richer when more than one electron is involved. In solids, electrons and holes are quasiparticles, the properties of which are shaped by the interactions among electrons, as well as between electrons and phonons. We hope to learn more about this physics by photoinjecting charge carriers within an extremely short time interval.
Can you briefly describe what your new paper is about?
By photoinjecting charge carriers, an extremely short laser pulse can increase the electric conductivity of a medium by many orders of magnitude within just a few femtoseconds. We are talking here about the fastest and strongest changes in the electric and optical properties of a medium that are achievable in a controlled manner, without damaging the medium. We wanted to see how it actually happens: how the medium goes from its initial state to a highly excited one, and how quickly its conductivity increases.
Can you describe your attosecond experiment?
We did what is typically done in ultrafast physics: in addition to the intense injection pulse, we employed a second, much weaker probe pulse to see how the newly created charge carriers respond to its electric field. Then we varied the delay between the two pulses, transmitting them through samples that were as thin as possible. What makes our experiment very special is that we measured the time-dependent electric field of the probe pulse, and we did so using a new technique that we had developed for this purpose. So, our paper is about demonstrating the power of pump-probe, optical-field-resolved measurements. This technique gives us very direct access to the light-driven electric currents during photoinjection and after it.
What were the challenges of the experiment?
The electric field of an optical wave oscillates so quickly that its direct measurement is challenging. Fortunately, several recently invented methods make it less challenging than it used to be, but we had to drive it to the limits. To get reliable results for the light-driven electron motion, we had to use very thin samples; otherwise, various propagation effects would have caused very nontrivial distortions in the electric field of the transmitted probe pulse. Using a thin sample, however, means that the interaction of the probe pulse with photoinjected charge carriers causes small changes in the pulse’s electric field. To measure such minuscule changes, we needed a very sensitive method for optical-field sampling, which we had to invent ourselves. Even with this method, measurements took many hours, during which the pulses would change a bit. So, we had to find a way to perform the measurements and analyze their results in a possibly robust way.
What were the results of the experiment?
The most important result is that we now know how to perform and analyze such experiments and that we indeed saw the light-driven electron motion as no one could do before I would argue that we’ve just scratched the surface of what pump-probe field-resolved measurements can do. Equipped with our experience and insights, other researchers can now use our approach to answer their research questions. Analyzing the measurement results, we had to admit that the extremely nonlinear processes that take place during the intense photoinjecting pulse were too complex for the first demonstration of our new technique, so we decided to focus on what happens once photoinjection stops. We studied how the weak field of the probe pulse interacts with charge carriers right after their appearance. Luckily, we had enough surprising findings even within this subset of measured data. We were surprised to see no clear sign of quasiparticle formation, which means that, in these particular measurements, the many-body physics did not have much influence on how the conductivity of the medium built up after photoinjection, but we may see some fancier physics in the future. Also, we had to elaborate on how to describe the linear optical response of a dispersive medium, the properties of which are rapidly changing. In other words, we had to carefully define a time-dependent refractive index. As we did so, we realized that its time dependence is more complex than one might naively expect and that a sudden photoinjection does not cause an equally sudden change in the refractive index.
Such a photoinjection event can change the properties of a solid. How can your findings perhaps be used later on for applications?
All modern electronics are based on our ability to control the flow of charge carriers by quickly increasing and decreasing their ability to move through circuits. Our research is about clarifying the ultimate speed limits of this control. The insights that this research generates may eventually enable petahertz-scale signal processing, although I expect it to be a long way to go. In the foreseeable future, we envisage scientific applications, such as new techniques for attosecond metrology.
Image: Christine Wolf