Ultrafast Laser Direct-Writing of Self-Organized Microstructures in Ge-Sb-S Chalcogenide Glass

The structuring of mid-IR materials, such as chalcogenide glass (ChG), at the micro and nano scales, is of high interest for the fabrication of photonic devices in general, and for spectroscopy applications in particular. One efficient method for producing regular patterns with a sub-micron to micron length scale is through self-organization processes occurring during femtosecond laser exposure. These processes occur in a broad set of materials, where such self-organized patterns can be found not only on the surface but also within the material volume. This study specifically investigates the case of chalcogenide glass (Ge23Sb7S70) exposed to femtosecond laser pulses, inducing pulse-to-pulse nanostructure formation that is correlated to the glass network structural evolution using Raman spectroscopy as well as morphological and elemental microscopy analysis.

Ultra-broadband time-resolved THz spectroscopy of perovskites and nanomaterials

Andrés Burgos Caminal

The fundamental study of charge carrier dynamics has been shown to be of great interest for optoelectronics, paving the way forward for future developments. Often, some of the key processes that determine the photophysics of a material occur on very short timescales. In order to study and characterize them, we can use time-resolved spectroscopic techniques on the ultrafast timescale. These generally rely on pulsed lasers with ultra-short pulse lengths, in the order of tens to hundreds of fs. One such technique is time-resolved THz spectroscopy (TRTS), especially useful in the study of semiconductors due to its sensitivity to charge carrier concentration and mobility. Nonetheless, it can also be used to study low energy vibrations and other photoexcited species such as excitons. In this work, we focus on recent developments based on gas photonics that easily enables the extension of the probed frequency range towards 20 THz and beyond, giving it the name of ultra-broadband TRTS. This dissertation revolves around the technique and its application to study two of the hottest topics in optoelectronics: nanomaterials and lead halide perovskites (LHPs). This is often done in combination with other spectroscopic techniques. LHPs have become one of the most studied materials in recent years due to their outstanding performance in photovoltaics and their multitude of other potential applications. Nonetheless, many properties still need to be fully understood. In addition, lower dimensionality variations of LHPs are gaining increasing attention due to interesting qualities such as stability, emission quantum yield and confinement effects. Chapters 1 and 2 are introductory chapters. The former introduces both the theory and the recent literature that is relevant to the following chapters. Meanwhile, the latter gives a detailed explanation of ultra-broadband TRTS, including data analysis, setup description and substrate suitability. It also gives a summarized description of other time-resolved techniques that were employed. The following chapters are ordered in increasing complexity and realization of the potential of ultra-broadband TRTS. Each one focuses on a different material, exploring different photophysics that can be studied with TRTS. Chapter 3 is a time-resolved photophysical study of Se nanowires. These materials have generated great interest for their potential applications, from catalysis to optoelectronics. Here, we study thin-film samples obtained from a novel and simple solution-based synthesis process. The charge carrier dynamics and mobility are studied for the first time employing ultrafast techniques. In chapter 4, bulk LHPs are studied with an interest towards the early evolution of charge carriers, including hot carrier cooling and polaron formation. The study focuses on the rise in photoconductivity that can be observed in LHPs of diverse composition over the first ps after excitation. It proposes a complex model to unify previously reported mechanisms based on the fluence and wavelength dependencies. Finally, chapter 5 turns towards the study of 2D LHPs where the dielectric and quantum confinement effects completely alter the photophysics, resulting in a strong excitonic character. While the previous two chapters mainly focus on the use of frequency-averaged TRTS dynamics, this one also makes use of frequency-resolved spectra as well as broadband fluorescence up-conversion spectroscopy to study the carrier-exciton dynamics.

EPFL2020

Development of coherent mid-infrared source using chalcogenide photonic crystal fibers

Sida Xing

Many molecule bonds have vibration frequencies in the mid-infrared band. Thus, this band is of great interest to molecular spectroscopy, material processing and medical applications. However, many optical materials typically used for laser sources experience high losses due to the presence of such vibration bonds making the design of mid-infrared coherent sources challenging. Such task is even more difficult when considering designing all-fiber sources which offer additional compactness and robustness. The goal of this project is to address such challenge by designing all-fiber mid-infrared sources based on chalcogenide fibers and nonlinear effects. Using degenerated parametric conversion of a 2 um pump and a telecom band signal laser, one can generate an idler in the mid-infrared band. As a first step, the linear and nonlinear performances of several chalcogenide fibers were compared. The results prove the chalcogenide photonic crystal fiber design to be the ideal platform for this project as it offers high nonlinearity and low loss. Numerical simulations provide further improvement of photonic crystal geometry. Our improved designs allow for an extreme shaping of the dispersion to optimize the four-wave-mixing process within the thulium /holmium fiber laser band. In the meantime, several cavity configurations were tested to boost the thulium/holmium laser performance, that are required for pumping of our chalcogenide fibers. Polarizer-free cavity designs result more compact laser footprint, which moreover enables a higher slope efficiency. Such lasers enable precise measurements of sample fiber optical parameters. The combined efforts on fiber and laser optimization leads to efficient continuous-wave parametric conversion. Thanks to the excellent mid-infrared transmittance of chalcogenide glass, this high efficiency can be extended to longer pumps in mid-infrared. However, fiber fuse, a result of heat dissipative solitons, is the main constrain for reaching better conversion efficiency. For ultrafast applications, a sub-cm length chalcogenide photonic crystal fiber generates flat-top, linearly chirped supercontinuum in the normal dispersion region. This pulse length could be compressed to two optical cycles by linear compression. Once again, the experimental results match perfectly with the simulations. Overall, we show in this thesis that chalcogenide PCFs are promising for highly efficient and low threshold nonlinear optics in the middle infrared, offering new options for light generation in this critical wavelength band.

EPFL2019

In situ nanoscale studies of copper-based catalysts for carbon dioxide electroreduction

Karla Banjac

The urgency of climate change demands the simultaneous removal of carbon dioxide from the atmosphere and the transition to renewable energy sources. This aim is realizable through electrochemical reduction of carbon dioxide (CO2RR), which is a promising route for synthesizing renewable carbon-based fuels and chemicals.Among many existing CO2RR catalysts, copper is the only earth-abundant metal that produces energy-rich molecules. Practical application is hindered by its poor product selectivity and modest long-term catalyst stability. To overcome these limitations, an atomistic understanding of the catalytic processes and identification of CO2RR active sites on Cu are needed.Current research implies that the active sites form upon the catalysts' evolution during the reaction. However, fast Cu oxidation hinders their identification in conventional ex situ studies. The following questions remain unanswered: What are the morphology and chemical composition of the active sites? How do they form?This thesis explores the in situ evolution of Cu-based CO2RR catalysts. We study this phenomenon down to the nanoscale through a synergy of in situ microscopy and spectroscopy studies. We employ graphene as a 2D protecting layer on Cu to study the Cu nanoscale structural evolution in situ via electrochemical scanning tunneling microscopy (EC-STM). We use in situ Raman and X-ray photoelectron spectroscopies to study the evolution of the Cu catalysts' oxidation state.First, EC-STM studies reveal the formation of few nanometer-sized cuboids on Cu catalysts as the main structural evolution during CO2RR. We show that this evolution occurs in aqueous electrolytes over a wide range of pH and compositions. Second, we show that metallic Cu is the active phase. Reduction of the Cu oxide, formed either through chemical synthesis or through the exposure of Cu catalysts to air, is unavoidable prior to CO2RR.To understand the nanocuboid formation mechanism, we present a comparative study of the Cu nanocuboids formed upon CO2RR and the Cu2O cubes prepared through electrochemical cycling synthesis. Morphological and chemical composition differences between them allow us to conclude on distinct formation mechanisms. We show that the metallic Cu nanocuboids form during a potential-driven re-organization of atoms in the topmost surface layers.Furthermore, we present EC-STM studies revealing the atomistic details of the Cu nanocuboid formation. By comparing these processes with the multilayer mound growth in Cu homoepitaxy, we show that the nanocuboids are (100) facet multilayers formed upon reduction of Cu oxides. We propose that the nanocuboids are CO2RR active sites, ultimately forming on any Cu catalyst regardless of its initial macroscopic shape and oxidation state.The in situ identification of active sites, presented here, offers a unique framework for the re-interpretation of the existing literature, invites further fundamental theoretical and experimental research, and paves the way for rational catalyst design. Moreover, this thesis further demonstrates the importance of in situ studies across different length scales in understanding key catalytic processes in renewable energy schemes.

EPFL2021