Ultrafast Optical and Low Frequency Spectroscopy of Semiconductor Nano-Systems and Quantum Materials

Sarkar, Jit (2020) Ultrafast Optical and Low Frequency Spectroscopy of Semiconductor Nano-Systems and Quantum Materials. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Jit Sarkar (12RS066)) 12RS066.pdf - Submitted Version Restricted to Repository staff only Download (9MB)

Abstract

Study of the electromagnetic (EM) waves and it’s wide aspects related to numerous different fields has arguably been the most important filed of research since the dawn of humanity. The huge span of the EM waves in terms of energy, wavelength, and frequency has made the general research theme on EM waves pertinent in almost every field of modern day research from the perspective of the basics science as well as applied research. Among different major divisions of EM-wave related research, the interaction of it with materials has captivated huge interest. Study of the interactions of EM-wave (especially over the wavelength range of cm - nm) with materials is commonly termed as light - matter interactions. Active research on light-matter interactions over several decades has addressed numerous issues related to basic science as well as has leads to innumerable technological breakthrough and the saga still continues. Thus the research over light-matter interaction is extremely important. In this thesis the physics of light-matter interactions has been studied in a broad range of materials in terms of shape, size and physical properties, over a wide spectrum of EM-waves ranging from terahertz to optical frequency. The major part of this thesis is based on the study of the carrier dynamics in materials of varying shape and sizes e.g. nano-structured materials, thin-films and bulk samples. Ultra-short laser pulses were used to capture the time-resolved dynamics of the photo-excited carriers over a tiny time-scale of pico-seconds. Different physical phenomenon associated with the decay procedure is realized and discussed in this part of the thesis. The last part of the thesis discusses the light-matter interaction at terahertz (THz) frequency. Chapter 1 provides the basic introduction for this thesis. The chapter starts with the basics of light-matter interaction followed by discussions on different aspects of time-resolved spectroscopic studies. The basic light-matter interaction part intends to provide the theoretical background for the interactions among materials and the EM-waves, and relevant for the spectrum-range associated with this thesis work i.e., THz-optical frequency. In the following part, first the basics of ultrafast time-resolved spectroscopy and then different relaxation phenomenon (electron-electron interaction, electron-phonon thermalization, intra-band scattering, inter-band recombination etc.) which are commonly observed post photo-excitation, are discussed. In Chapter 2 detailed description of the ultrafast experimental techniques used in this thesis work and their methodologies are presented. At first, the details of the femtosecond (fs) amplifier laser system is discussed. Following that the intensity auto-correlation technique for pulse width measurement is presented. Then the detail of experimental set-ups and methodology for ultrafast pump-probe spectroscopy is described. Chapter 3 is aimed to discuss the process of electron-phonon thermalization of ultrafast carrier relaxation in the presence of large density of defect states. Ultrafast dynamics of photo-excited carriers in an assembly ZnO nanorods is captured at very high excited carrier density (above critical Mott density), called Electro-Hole Plasma (EHP) regime. The presence of e-ph thermalization process with sub-ps time-constant is identified and the process is modelled using Two Temperature Model (TTM) of ultrafast thermalization. A decrease in e-ph coupling constant with increased carrier density is obtained which decipher the prolonged thermalization process. The inability of TTM in modelling e-ph thermalization in the presence of faster decay channels is also discussed at the end of the chapter. Chapter 4 discusses the presence of many-body decay mechanism at very high photoexcited carrier density. Time-resolved differential transmission signal from an assembly of Bi₂O₃ micro-rod is captured upon intense photo-excitation. Variation of differential transmission with time delay is understood in terms of variation of carrier density and proper analysis shows distinct presence of trap-mediated auger processes, a two-body decay mechanism. Apart from this defect mediated process called Shockley-Reed-Hall (SRH) recombination process is also found to be present. In Chapter 5, the observation of coherent optical and acoustic phonons in a series of topological insulator (TI) thin films of Bi₂Se₂Te are reported. The main goal of this chapter, is to explore the effect of thin film thickness on these coherent phonons. The frequency of the observed coherent optical phonons in Bi₂Se₂Te is close to the frequency of A₁g mode obtained from Raman spectroscopy, and found to be independent of film thickness. The acoustic phonon are found to have strong dependence with varying thicknesses. In Chapter 6, we investigate the ultrafast electron-phonon coupling in topological insulator single crystal of Bi₁.₅Sb₀.₅Te₁.₃Se₁.₇ (BSTS). This belongs to the group of quaternary topological insulators with chemical composition Bi₂−xSbxTe₃−ySey; 0 < x < 2 and 0 < y < 3. These quaternary topological insulators are highly bulk insulating unlike the second generation of binary 3D TIs, Bi₂Se₃, Bi₂Te₃, etc. The differential reflectivity traces clearly indicates the presence of a fast electron-phonon thermalization process with a time-scale of 2−3 ps. Measurement taken at room temperature (300 K) and low temperature (83 K) exhibit qualitatively similar behavior. The fluence dependent study reveals that electron-phonon thermalization time decreases at higher pump power and the electron-phonon coupling constant increases with fluence. In Chapter 7, we investigate the THz conductivity of topological surface states in topological insulator thin films. THz spectroscopy has an edge over Optical spectroscopy as the low energy photon, enables us to access the states very close to the Dirac point. And for a TI material, where the Fermi-level is within the bulk bandgap and close to the Dirac point, terahertz (THz) spectroscopy is a very suitable technique to probe the surface states. The crucial observation of this work is that, unlike normal metal samples, our PLD grown samples show non-Drude behavior i.e. decrease in conductivity with lowering frequency. This observation can be understood from the Drude-Smith model of ac conductivity in granular systems. As the TI films grown in PLD are granular in nature, such behavior is expected. In the final Chapter 8, a brief summary of the experimental observations from this thesis work is presented.

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