Scientists have long been fascinated by the phenomenon of photoinjection, which occurs when a laser pulse strikes an electron in a solid, providing it with sufficient energy to move freely within the material. The exploration of this phenomenon dates back to the early days of quantum mechanics, and there are still many unanswered questions regarding the underlying processes and their temporal evolution.
A team of laser physicists from the attoworld group at LMU and the Max Planck Institute of Quantum Optics has achieved a significant breakthrough by directly observing how the optical properties of silicon and silicon dioxide change in the first few femtoseconds (equivalent to millionths of a billionth of a second) following photoinjection induced by a powerful laser pulse.
The physics of photoinjection becomes relatively straightforward when considering the photoelectric effect, as elucidated by Albert Einstein. In this scenario, an electron absorbs a single photon with sufficient energy to liberate it from a potential that restricts its motion. However, the situation becomes more intricate when no single photon in the incident light wave possesses enough energy to achieve liberation. In such cases, bound electrons can attain freedom by simultaneously absorbing multiple photons or through the phenomenon of quantum tunneling. These nonlinear processes are effective only under the influence of a strong electric field, meaning that only the central portion of a laser pulse can efficiently drive them.
By investigating the intricate details of photoinjection and the temporal evolution of optical properties in silicon and silicon dioxide, the attoworld team has expanded our understanding of this complex phenomenon. Their findings shed light on the nonlinear processes involved and provide valuable insights into the fundamental physics underlying photoinjection.
Novel technique for scanning
By harnessing the power of attosecond science, scientists have unlocked the ability to generate a large number of charge carriers within a single half-cycle of a light pulse. This remarkable achievement leads to a dramatic increase in the conductivity of solids by several orders of magnitude in just a few femtoseconds. The laser physicists from the attoworld team at LMU and the Max Planck Institute of Quantum Optics have embarked on an investigation to explore the rapid changes in optical properties that occur after ultrafast photoinjection.
To conduct their study, the researchers employed two few-cycle pulses: an intense pump pulse responsible for creating charge carriers and a weaker test pulse that interacts with the charge carriers. Photoinjection was confined to a time interval shorter than half a cycle of the test field, allowing the scientists to observe the interaction between the charge carriers and the test field during the initial femtoseconds following their emergence. They employed a novel technique for optical-field sampling to measure the distortions imprinted on the time-dependent electric field of the test pulse by photoinjection. These measurements were repeated at various delays between the two pulses.
The breakthrough technique for optical-field-resolved pump-probe measurements enabled the attoworld team to directly investigate light-induced electric currents during and after photoinjection. Vladislav Yakovlev, the last author of the study published in Nature, emphasizes the significance of this development, stating that they now possess the knowledge and tools to perform and analyze such experiments, thereby witnessing light-driven electron motion like never before.
One intriguing finding of their research is the absence of clear signs indicating the formation of quasiparticles. Yakovlev explains that in these particular measurements, the influence of many-body physics on the buildup of conductivity in the medium after photoinjection was not substantial. However, he suggests the potential for observing more sophisticated physics in the future.
The research conducted by the attoworld team is at the forefront of exploring the ultimate speed limits of controlling the flow of charge carriers using light, which forms the foundation of modern electronics. The newfound insights could pave the way for future signal processing in the petahertz range, enabling the development of “light wave electronics.” Such advancements would revolutionize current electronics by accelerating their performance by a staggering factor of approximately 100,000.
Yakovlev asserts that their exploration of pump-probe field-resolved measurements has only just scratched the surface of its potential. Armed with their experiences and understanding, other researchers can now utilize this approach to find answers to their own questions, driving further progress in the field.
Source: Ludwig Maximilian University of Munich