Ultrafast and terahertz spectroscopy of 2D magnetic materials and transition metal oxides

Anjan Kumar, N M (2024) Ultrafast and terahertz spectroscopy of 2D magnetic materials and transition metal oxides. PhD thesis, Indian Institute of Science Education & Research Kolkata.

Text (PhD thesis of Anjan Kumar N M (17RS046)) 17RS046.pdf - Submitted Version Restricted to Repository staff only Download (28MB)

Abstract

This doctoral thesis provides an exhaustive study into the ultrafast carrier dynamics and picosecond ultrasonics in magnetic materials, specifically focusing on both 2D ferromagnets and antiferromagnets. Additionally, it delves into the many body carrier dynamics in transition metal oxides using ultrafast and terahertz spectroscopy. The thesis is systematically divided into three well-defined segments for the ease of understanding and clarity. Part one establishes the fundamental theoretical framework. Chapter 1 delves into the principles of light-matter interaction and time-resolved studies, encompassing the generation and detection of strain pulses. Furthermore, it introduces the specific material systems investigated throughout the thesis. Chapter 2 details the employed laser system and the various experimental techniques utilized. Part two focuses on the generation and detection of strain pulses, along with the ultrafast dynamics exhibited by the two-dimensional ferromagnet, CrSiTe3 single crystal and anti-ferromagnet, FeCrO3 thin film. This in-depth exploration is presented in chapters 3 and 4. Part three (chapters 5) culminates with an investigation into the ultrafast and terahertz spectroscopy of transition metal oxides, specifically focusing on undoped and doped α - Fe₂O₃, V₂O₅, and α - CoV₂O₆. Chapter 1 : This introductory chapter lays the groundwork for the subsequent investigation presented in this thesis. It is thoughtfully structured into three distinct sections. The first section explores the fundamental principles of light-matter interaction and establishes the conceptual foundation of semiconductors, with a particular emphasis on ultrafast processes that occur within the semiconductors. The second section delves into the theoretical background of both the generation and detection of picosecond strain pulses. Finally, the concluding section provides a concise overview of the various material systems investigated throughout this thesis. These systems encompass a range of systems , including CrSiTe3, FeCrO₃, α - Fe₂O₃, V₂O₅, and α- CoV₂O₆. Chapter 2 :This chapter establishes the foundation for the subsequent works by detailing the laser systems employed throughout this thesis. These systems encompass a femtosecond oscillator and a corresponding regenerative amplifier. Following a comprehensive exposition of their operational principles, the chapter outlines the alignment procedure for the amplifier. Subsequently, the focus shifts to the details of the pump-probe experimental setup. Once a thorough description of the setup is established, the methodologies for pulse width measurements, including the autocorrelator and cross-correlator techniques are discussed. Additionally, this chapter explores the techniques employed for spot size measurements. Finally, the chapter culminates with a comprehensive exposition of the terahertz time-domain spectroscopy technique and the methodology for extracting optical parameters from the acquired data. Chapter 3 :This chapter delineates a novel and unexplored methodology that leverages picosecond strain pulses to investigate the phenomenon of magnetic dimensional crossover (MDC) within a two-dimensional (2D) van der Waals Heisenberg ferromagnet, CrSiTe₃. This chapter discusses how the temperature-dependent femtosecond transient reflectivity measurements, permits the direct observation of the subtle yet critical influence exerted by spin fluctuations on the lattice, thereby revealing the evolution of magnetic order with unprecedented detail for the first time. Through a detailed analysis of the strain pulse’s temporal and spectral response across a spectrum of temperatures, this chapter offers a comprehensive picture of the MDC process. It elaborates on how the generation of strain is affected by the magnetic strain within the ferromagnetic phase. Furthermore, the chapter sheds light on the identification of a distinct signature associated with the multi-step MDC pathway, while simultaneously unveiling previously unknown aspects of the interplay between spin dynamics and lattice vibrations. The chapter delves further into the observed renormalization within the strain spectrum (frequency domain) and the phase shift observed in the time domain. These phenomena are subsequently captured within a phenomenological theoretical model that describes the spin-lattice interaction. Additionally, the chapter demonstrates how the ultrafast carrier dynamics, which contribute to the electronic multi-exponential background, also exhibit signatures of MDC. Chapter 4 : This chapter explores the femtosecond time-resolved reflectivity response of a 50 nm thin film of FeCrO₃, deposited via pulsed laser deposition (PLD) onto an ITO/sapphire substrate, as a function of pump fluence and temperature (295 K to 445 K). A rigorous analysis is presented of the transient reflection decay profiles. These profiles are demonstrably well-represented by a bi-exponential function, characterized by the relaxation time constants designated as τ1 (∼ 5 ps) and τ2 (∼ 70 ps). The chapter attributes the decay constant τ1 to electron-phonon thermalization phenomena, drawing upon the established principles of the two-temperature model (TTM). Conversely, τ2 is ascribed to trap-assisted recombination based on the observed dependence of τ2 on both fluence and temperature. Furthermore, the chapter delves into the ultrafast generation of a strain pulse within FeCrO₃, exhibiting a central frequency of 20 GHz. This remarkable strain pulse demonstrates exceptional stability across a wide range of pump fluences (up to 1.62 mJ/cm²), magnetic fields (up to 0.7 T), and the investigated temperature range of 295 K to 445 K. This noteworthy stability underscores the immense potential of FeCrO3 as a highly promising material for optoelectronic devices demanding exceptional thermal stability. Chapter 5 :This chapter presents the findings of experimental investigations employing non-degenerate pump-probe spectroscopy and terahertz time-domain spectroscopy (THz-TDS) on transition metal oxides. The primary focus lies in elucidating the ultrafast carrier dynamics within α- Fe₂O₃, V₂O₅, and α- CoV₂O₆. To achieve this, an appropriate empirical kinetic model is applied to transient absorption data derived from transient transmission measurements. The chapter is structured into three distinct sections. The initial section delves into the self-trapping of excitonic behaviour and optical parameters within the terahertz frequency spectrum of undoped hematite nanoforms, alongside those doped with potassium (K) and nickel (Ni). This section commences with a concise overview of previous research, which posits that excitons and selftrapped excitons (STEs) play a critical role in the photoexcited carrier dynamics of hematite. Subsequently, the chapter explores the utilization of a phenomenological kinetic model comprised of coupled rate equations to comprehend the formation and decay kinetics of STEs in these hematite nanoforms. The numerical solutions of the coupled rate equations are presented, having been fitted to the observed variations in exciton density over time. These solutions suggest that a substantial majority of excitons become trapped by polaronic trap states within 3.5 ps following the thermalization of photocarriers into free excitons. The chapter then elaborates on how the proposed kinetic exciton model suggests that the non-linear interaction between free excitons and STEs can culminate in their mutual annihilation through a process akin to trap-assisted bi-molecular Auger recombination. Furthermore, the section presents some of the noteworthy findings gleaned from the analysis, including the dependence of both STE formation and exciton decay dynamics on the initial densities of photoexcited excitons. The precise manner in which these various processes vary with exciton density is elucidated by invoking the concept of Coulombic screening between the carriers at densities exceeding 3.3 × 10¹⁷ cm⁻³. Also. leveraging the analysis from the kinetic model, the chapter estimates the average density of STEs within the hematite nanoforms to be approximately 3.8 × 10¹⁷ cm³. To conclude this section, the chapter presents the results obtained from THz-TDS measurements conducted on both undoped and K-doped hematite samples. This chapter’s next section delves into the dynamics of polarons within V₂O₅. It commences by providing a succinct review of prior research, which postulates the existence of polarons in this material. Subsequently, the chapter explores the application of a phenomenological kinetic model, employing coupled rate equations, to elucidate the formation and decay kinetics of polarons in V₂O₅ microparticles. The solutions to these coupled rate equations, derived numerically and fitted to experimentally observed variations in carrier density over time (obtained from time resolved absorption data), are presented. These solutions suggest that a significant portion of photoexcited carriers become trapped by polaronic trap states within a ∼ 4.5 ps. Additionally, the model offers insights into the interaction between polarons and free carriers, specifically their involvement in polaron-assisted bimolecular decay processes. The chapter concludes by discussing the application of the carrier screening concept to comprehend how the formation and decay rates of polarons are influenced by the density of photoexcited carriers. Furthermore, by leveraging the analysis derived from the kinetic model, the chapter estimates the average density of polarons within the V₂O₅ microparticles to be approximately 2.58 × 10¹⁷ cm⁻³. The final section investigates the ultrafast trapping of photoexcited carriers and the temperature dependence of optical parameters within the terahertz (THz) frequency spectrum of α- CoV₂O₆. The chapter commences by analysing experimental results garnered from femtosecond non-degenerate pump-probe transmission experiments on an ensemble of irregularly shaped α- CoV₂O₆ microparticles. Subsequently, the chapter explores the efficacy of a chosen many-body empirical kinetic model comprised of coupled rate equations. This model is applied to fluence-dependent transient absorption data to comprehend the fundamental dynamics of photoexcited carriers in α- CoV₂O₆. The results gleaned from this modelling unveil the presence of two distinct trap types within α- CoV₂O₆: band-edge states (shallow traps) and mid-gap states (deep traps). Furthermore, the chapter delves into the results and discussion surrounding steady-state absorption measurements, intensity-dependent photoluminescence (PL) spectra, and density functional theory (DFT) calculations. These combined techniques corroborate the presence and origin of the two aforementioned trap states within α- CoV₂O₆. This section delves into findings gleaned from the kinetic modelling analysis. The analysis reveals that photoexcited carriers become localized within shallow traps and deep traps on timescales of approximately 2 ps and 30 ps, respectively. Furthermore, the model predicts that roughly 44 % and 40 % of photoexcited carriers become trapped within shallow and deep traps, respectively. To culminate this section, the chapter presents the results obtained from temperaturedependent THz-time domain spectroscopy (THz-TDS) measurements conducted on α- CoV₂O₆. Chapter 6 : This concluding chapter summarizes the key findings presented throughout this thesis. It further explores potential avenues for future research endeavors arising from the present investigation.

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