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Investigation of nonlinear stimulated emission in optically excited dielectrics

Periodic Reporting for period 1 - LADIE (Investigation of nonlinear stimulated emission in optically excited dielectrics)

Reporting period: 2020-04-01 to 2022-03-31

When a short laser pulse enters a dielectric material (i.e. an electrically insulating material like glass or water), the optical properties change rapidly. The short light pulse excites many electrons so that the material changes from being transparent to behaving almost like a metal – a so-called “plasma mirror”. It is well known that the material hereafter will reflect and absorb a second, subsequent laser pulse, which is often used to interrogate the dynamic properties of the material and to understand the fundamental physical processes at play. However, in this Marie Skłodowska Curie Action “Investigation of nonlinear stimulated emission in optically excited dielectrics (LADIE)”, the goal is to investigate the new and surprising discovery that – under the right circumstances – the “mirror” changes to an “amplifier” for the subsequent pulse. The amplification is due to multiphoton stimulated emission (e.g. two photons in – four photons out), a phenomenon which was predicted theoretically shortly after the invention of the laser, but which has so far been very difficult to observe experimentally. This new amplification process will have a strong impact on our understanding of light-matter interaction, new insights into materials, and which could eventually lead to completely new types of lasers.

The main objective of this MSCA have been to understand the fundamental processes involved in multiphoton stimulated emission in excited dielectrics and to scope how they can be used to probe excited transparent materials. To that extent the development and construction of an experimental setup capable to precisely measure the linear and nonlinear optical response of excited dielectric materials stands in the centre of the action, accompanied by data evaluation and simulations.

A parallel goal of the action is to foster the development of the individual researcher as well as realize his involvement in academic autonomy as well as teaching activities.
The work of the researcher in this action was conducted through 4 work packages (WPs). WP1 comprised of 2 separate tasks, focusing on the training of the individual researcher in existing time-resolved ellipsometry of laser-excited transparent materials, as well as in modelling laser excitation using advanced multiple-rate equation models. In a second work package the researcher developed and built a novel single-shot, pump-probe imaging spectroscopy setup that can probe linear and nonlinear responses of optically excited dielectrics across a large range of wavelengths (ultraviolet – near infrared) and timescales (femto- nanoseconds), essential in the understanding of multiphoton absorption and emission processes. The researcher also discovered a second nonlinear amplification mechanism in fused silica, one of the most used optical materials, hinting at the universality of the process proposed in the action. The researcher advanced the development of ultra-thin samples, which demonstrate a new access to high-excitation regimes allowing the observation of previously un-seen dynamics on ultrashort timescales, which in-turn, provides valuable insights into conditions present in the nonlinear stimulated emission in optically excited dielectrics as well as laser-material processing. In a third work package, the researcher scoped and identified novel pathways of modelling laser excitation that can accompany future measurements on the new experimental setup.

In the fourth work package, focusing on different aspects of the development of the researcher, as well as the dissemination of results, one peer-reviewed manuscript and two conference proceedings were published, and one book chapter has been prepared and is to be published after the end of the action. The researcher presented the progress of the project at seven international conferences. Additionally, was academic independence strengthened by the lead and supervision of students, the involvement as a Co-Investigator in a research grant of the Danish Independent Research Council (DFF), that continues even after the end of the action. The researcher also got involved in teaching activities at the Department of Physics and Astronomy at Aarhus University, teaching several lectures in a Laser and Optics course and supervising several experiments in the corresponding laboratory course.

Overall, will the development of the unique experimental method to study nonlinear light-matter interaction lead to novel results and several subsequent peer-reviewed manuscripts that will be submitted and published after the end of the action due to the organic delay between the build of an experimental setup and the performing of extensive studies, as well as the impact of COVID-19 measures. It is expected that the new insights and the demonstration of the measurement technique will have a large impact in the fields of light-matter interaction, materials science, and laser-material processing, respectively.
The action already led to the discovery of novel dynamics in excited dielectric materials that will require theoretical descriptions and models to catch up, but it will also provide them with the necessary data to validate the development of such. The fundamental principles the action is based on – i.e. the probing of laser excitation by means of multiphoton transitions (either absorption or emission) – have already spread to other fields, such as materials science. The utilization of these principles and experimental techniques will lead to a better understanding of novel materials, especially hybrid- as well as high band gap semiconductors that are used across light absorbing- (solar cells) and emitting (LED’s) semiconductor technology as well as high power electronics.
The, in the project, co-Supervised PhD student will continue to generate knowledge, take data, and drive the progress of the action well beyond the anticipated timeframe, with continues support by the researcher.
A final overarching impact is to seed the interest in the developed experimental method and that it will be a measurement principle that will be taken up or adapted by other groups across the world, leading to further breakthroughs in physical and optical sciences.
Figure from publication describing a novel nonlinear light amplification process.