The rapid progress on developing new and improved multifunctional materials, for optoelectronic and spin based phenomena/devices, have increased the importance of the fundamental understanding of their coherent states and nonlinear optical properties. This study is aimed at characterizing, modeling, and controlling the fundamental electronic, phononic, and spin properties of several classes of materials through nonequilibrium and nonlinear light-matter interactions, coupled with a novel design of the material phases, interfaces, and heterostructures. This research directly addresses the Grand Challenges identified in the Basic Energy Sciences Advisory Committee report "Directing Matter and Energy: Five Challenges for Science and the Imagination" (Hemminger, 2007) [1], in particular, the area: "Matter far beyond equilibrium" and addresses the questions, "How do remarkable properties of matter emerge from complex correlations of the atomic or electronic constituents and how can we control these properties?" and "How do we design and perfect atom- and energy-efficient synthesis of revolutionary new forms of matter with tailored properties?". The knowledge gained from these fundamental studies can provide new information for a broad community to provide concepts for the next generation of multifunctional materials and devices, and resulted in several publications and conference presentations. The materials studied in this dissertation included multiferroic BaTiO3-BiFeO3 [2], ferroelectric Pb0.52Zr0.48TiO3 (PZT), InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7]. Probing multiferroics, which are materials that can exhibit ferromagnetic, ferroelectric, and ferroelastic orders simultaneously in a single phase, was a main focus of this study. BiFeO3 (BFO) is the most widely investigated multiferroic due to its high Neel and Curie temperatures and has antiferromagnetic and ferroelectric properties [8]. An inherent drawback of BFO is its large leakage currents. In this project, (1 − x)BaTiO3-(x)BiFeO3, x = 0.725 (BTO-BFO) heterostructures were investigated [9], where the conductivity of the solid solution can be reduced by adding another perovskite material, BaTiO3 [2]. We aimed to study optically induced coherent states in our BTO-BFO structures. Time resolved pumpprobe spectroscopic measurements were performed at room temperature as well as at low temperature (100 K) up to 10 T. Coherent acoustic phonons were observed both in a film and nanorods, resulting in coherent phonon frequencies of 27 and 33 GHz, respectively [2]. Coherent phonon spectroscopy is a sensitive tool to characterize the interfaces and can be employed as an effective ultrasensitive quantum sensor [10]. Furthermore, in the nanorods arrays of BTO-BFO, an additional oscillation with frequency in the range of 8.1 GHz was observed. This frequency is close to a theoretically predicted magnon frequency which could indicate the coexistence of coherent phonons and magnons in the nanorods arrays [2]. In an analogy to photonics which relies on electromagnetic waves, magnonics utilizes spin waves to carry and process information, offering several advantages such as an operation frequency in the THz range. Recently, "a quantum tango" [11] was reported where coupled coherent magnon and phonons modes were formed on a surface patterned ferromagnet. Furthermore, BTO-BFO heterostructures were probed using transient birefringence and magneto-optical Kerr effect spectroscopy. The results demonstrated that the magnetic field dependence of coherent phonons, measured by these two techniques, exhibits more sensitivity to the external magnetic fields compared to the differential reflectivity technique [2]. Moreover, nonlinear optical properties of this structure were investigated via second harmonic generation spectroscopy, where wavelength and polarization dependence of this nonlinear observation will be discussed in this dissertation. As part of this study, another class of multiferroic materials, with strong ferroelectric and piezoelectric properties, Pb0.52Zr0.48TiO3 (PZT) was studied [12]. In this project, the nonlinear optical properties of PZT nanorod arrays were investigated. Clear signatures of second harmonic generations from 490-525 nm (2.38-2.53 eV) at room temperature, were observed. Furthermore, time resolved differential reflectivity measurements were performed to study dynamical properties in the range of 690-1000 nm where multiphoton processes were responsible for the photoexcitations. We compared this excitation scheme, which is sensitive mainly to the surface states, to when the photoexcited energy (∼ 3.1 eV) was close to the bandgap of the nanorods. Our results offer promises for employing these nanostructures in nonlinear photonic applications. Furthermore, the established techniques during my research provided new insights on optical properties of InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7], and the results will be briefly presented in this dissertation. / Doctor of Philosophy / My research activities have explored multifunctional materials and heterostructures with strongly enhanced coupled electric and magnetic orders and optical properties. In particular, pursuing novel heterostructure designs such as multiferroics can provide control over electric and magnetic ordering in mixed dimensionality. This, together with control at the level from lattice structure to electron spin states can give rise to improved or even qualitatively new and robust materials properties. For example, a better understanding of the phenomena associated with the spin degree of freedom of electrons allows for advancement in spintronic device applications such as storage, logic, and sensors, which are associated with quantum computers and quantum communications [13, 14, 15]. Overarching questions and goals of my activities included: What are the microscopic origins and mechanisms of nonlinear response in strongly coupled nanostructured materials and its relationship to electronic, spin, and lattice degrees of freedom? (2) What are the effects of dimensionality and quantum confinement on optical properties? (3) How do we control and manipulate the coherent states, such as coherent phonons and magnons using external and internal fields, material composition, and morphology to achieve maximal efficiency and tunability? Addressing many of the challenges in the fast-paced technological world requires continued developments of new materials with enhanced optical properties. The knowledge gained from my fundamental studies can provide new information for the next generation of multifunctional materials and devices with advanced optical properties and resulted in several publications and conference presentations.

In this thesis, experimental studies are reported of ultrafast dynamics and third order optical nonlinear coefficients of carbon nanotubes, and time resolved coherent phonon dynamics of semiconductors and rare earth titanates. The thesis is divided into three parts. The first part presents (i) general introduction to theoretical background on nonlinear optical susceptibility and time resolved studies, and systems studied (chapter 1) and (ii) experimental techniques (chapter 2). The second part of the thesis deals with the measurements of third order nonlinear susceptibilities and ultrafast dynamics of single and double walled carbon nanotubes (chapter 3). The third part contains coherent phonon dynamics in semiconductors, Te (chapter 4), Bi2Te3 (chapter 5), and ZnTe (chapter 6) and spin-frustrated rare earth titanate insulators (chapter 7). Chapter 1: This chapter is a general introduction to the thesis. The chapter is divided into two parts: (i) light-matter interaction, and (ii) systems studied. Under light-matter interaction, we describe the required theoretical and conceptual background of nonlinear optical susceptibilities and time resolved carrier and phonon dynamics. In the next part, a brief summary of details of the systems studied, that include carbon nanotubes (single and double walled), semiconductors (Te, Bi2Te3 and ZnTe) and insulating spin-frustrated rare earth titanates (Gd2Ti2O7, Dy2Ti2O7 and Tb2Ti2O7), are presented. Chapter 2: Details of the ultrafast laser systems (femtosecond oscillator and amplifier), pulse width measurements and ultrafast experimental pump-probe and z-scan techniques, used in this thesis are given in this chapter. Chapter 3: Here the experimental results on the measurements of third order optical nonlinearity and ultrafast dynamics of single and double walled carbon nanotubes are presented. The chapter starts with a general overview of optical switching followed by known ultrafast dynamics and nonlinear studies on carbon nanotubes. In the next section, our theoretical modelling of nonlinear absorption and refraction in the limit of saturable absorption is described. The final two sections depict our results on single and double walled carbon nanotubes. These studies indicate that double walled carbon nanotubes are best candidates for ultrafast optical switching. Chapter 4: This chapter presents temperature and pump fluence dependent femtosecond time resolved reflectivity measurements on tellurium. The chapter starts with an overview of previous pump-probe reflectivity studies at room temperature on tellurium followed by our results. A totally symmetric A1 coherent phonon at 3.6 THz responsible for the oscillations in the reflectivity data is observed to be strongly positively chirped (i.e, phonon time period decreases at longer pump-probe delay times) with increasing photoexcited carrier density, more so at lower temperatures. We show for the first time that the temperature dependence of the coherent phonon frequency is anomalous (i.e, increasing with increasing temperature) at high photoexcited carrier density due to electron-phonon interaction. At the highest photoexcited carrier densities of ~ 1.4 x 1021cm-3 and the sample temperature of 3K, the lattice displacement of the coherent phonon mode is estimated to be as high as ~ 0.24 Å. Numerical simulations based on coupled effects of optical absorption and carrier diffusion reveal that the diffusion of carriers dominates the non-oscillatory electronic part of the time-resolved reflectivity. Finally, using the pump-probe experiments at low carrier density of 6 x 1018 cm-3, we separate the phonon anharmonicity to obtain the electron-phonon coupling contribution to the phonon frequency and linewidth. Chapter 5: This chapter begins with a introduction of previous ultrafast studies at room temperature on Bi2Te3 and then presents our results on the temperature dependent high pump fluence time resolved reflectivity measurements on Bi2Te3. The time resolved reflectivity data shows two coherently generated totally symmetric A1g modes at 1.85 THz and 3.6 THz at 296K which blue shift to 1.9 THz and 4.02 THz, respectively at 3K. At high photoexcited carrier density of ~ 1.7 x 1021cm-3, the phonon mode at 4.02 THz is two orders of magnitude higher positively chirped than the lower frequency mode at 1.9 THz. The chirp parameter, β is shown to vary inversely with temperature. The time evolution of these modes is studied using continuous wavelet transform of the time-resolved reflectivity data. The analysis shows that the build up time for the two coherent phonons is different. Chapter 6: This chapter starts with a general introduction on various as pects of ZnTe to be used in generation and detection of THz followed by our results on influence of carriers and sample temperature on coherent phonon and polariton generation in ZnTe. Combination of femtosecond Kerr, two photon absorption and impulsive stimulated Raman scattering experiments have been carried out to investigate the effect of pulse energy and crystal temperature on the generation of coherent polaritons and phonons in < 110 > cut ZnTe single crystals of three different resistivities. We demonstrate that the effect of two-photon induced free carriers on the creation of both the polaritons and phonons is largest at 4K where the free carrier lifetime is enhanced. Further, the temperature dependant impulsive stimulated Raman scattering on high and low purity ZnTe crystals allows us to unambiguously assign the phonon mode at 3.5 THz to the longitudinal acoustic mode at X-point in the Brillouin zone, LA(X) in contrast to the assignment as two-phonon process in earlier studies. Chapter 7: This chapter starts with an introduction on previous Raman studies on the pyrochlore systems accompanied by our results on the generation of coherent optical phonons in spin frustrated pyrochlore single crystals Dy2Ti2O7, Gd2Ti2O7 and Tb2Ti2O7 and their behavior as a function of sample temperature from 296K to 4K. At 4K, two coherent phonons are observed at 5.3 THz (5.0 THz) and ~ 9.3 THz (9.4 THz) for Dy2Ti2O7 (Gd2Ti2O7) whereas three coherent phonons are generated at ~ 4.8 THz, 8.6 THz and 9.6 THz for Tb2Ti2O7. In the case of spin-ice Dy2Ti2O7, a clear discontinuity is observed in the linewidths of both the coherent phonons as well as in the phase of low energy coherent phonon mode, indicating a subtle structural change as also suggested by Raman studies. In comparison, such changes are not seen in the coherent phonons of Gd2Ti2O7, and Tb2Ti2O7. Another important observation is the phase difference of ‘π’ between the modes in all the samples, thus suggesting that the driving forces behind the generation of these modes are different in nature unlike a purely impulsive or displacive mechanism. Chapter 8: This chapter summarizes our results reported in this thesis and gives future directions.

In dieser Dissertation wird zum einen die ultraschnelle Ladungsträgerkinetik in einem Galliumnitrid (GaN)-basierten Übergitter, zum anderen die piezoelektrische Elektron-Phonon-Wechselwirkung kohärenter zonengefalteter Phononen in Galliumarsenid (GaAs)-basierten Übergittern behandelt. Mittels spektral- und zeitaufgelöster Photolumineszenzmessungen an einem n-dotierten GaN/Al0,18Ga0,82N Übergitter mit Parametern ähnlich derer in optoelektronischen Bauelementen wurde die defektbedingte Ladungsträgerkinetik untersucht, die innerhalb von ca. 150 ps durch den Einfang in tiefe, nichtstrahlende Rekombinationszentren beeinflusst wird. Die Untersuchung einer Passivierung mit Siliziumnitrid zur Verhinderung von Degradationseffekten zeigte ein stabiles optisches Langzeitverhalten bei gleichzeitiger Zunahme nichtstrahlender Defekte. Ferner wurde mit spektral aufgelöster Anrege-Abfrage-Spektroskopie eine Einfangkinetik auf einer Zeitskala von 150 - 200 fs in Defektzustände nahe der Übergitterbandkante gemessen, gefolgt von der Abkühlung der Ladungsträger durch Phononemission innerhalb weniger Pikosekunden bei Raumtemperatur und 35 ps bei 5 K. Kohärente zonengefaltete Phononen wurden mit Anrege-Abfrage-Spektroskopie an zwei AlAs/GaAs-Übergittern untersucht, die in [100]-, bzw. [111]-Richtung gewachsen wurden. Dies ermöglicht die (gezielte) Untersuchung der piezoelektrischen Elektron-Phonon-Kopplung, da diese für longitudinal-akustischen Phononen nur in der [111]-Probe existiert. Die Amplitude kohärenter Phononen mit einem Wellenvektor von q=0 in der [111]-Probe fällt verglichen mit denen in der [100]- und der [111]-Probe mit q≠0 signifikant schneller ab. Kohärente Phononen verursachen in der [111]-Probe bei q=0 ein makroskopisches piezoelektrisches Feld, welches Ladungsträger beschleunigt, die durch Reibung kohärente Phononen dämpfen. Bei hohen Ladungsträgerdichten unterdrückt die Abschirmung der induzierten piezoelektrischen Felder diese zusätzliche Dämpfung. / In this dissertation, the ultrafast carrier dynamics in a gallium nitride (GaN)-based superlattice as well as the piezoelectric electron-phonon-coupling of coherent zone-folded phonons in gallium arsenide (GaAs)-based superlattices are addressed. Using spectrally and time-resolved photoluminescence experiments on an exemplary n-doped GaN/Al0.18Ga0.82N superlattice with parameters similar to those in optoelectronic devices, we investigated the defect-related carrier kinetics, that are affected by trapping in saturable nonradiative recombination centers on time scales of ~150 ps. The investigation of a passivation with silicon nitride to prevent degradation effects show a long-term optical stability with a concomitant increase in non-radiative defect densities. Furthermore, spectrally resolved pump-probe spectroscopy was used to measure trapping kinetics into defect states near the conduction band minimum on a time scale of 150 – 200 fs. These kinetics are followed by carrier cooling through phonon emission within a few picoseconds at room temperature and within 35 ps at 5 K. Coherent zone-folded phonons were studied with pump-probe spectroscopy on two AlAs/GaAs superlattices grown in [100] and [111] direction, respectively. This allows the specific investigation of the piezoelectric electron-phonon interaction, since this exists for longitudinal acoustic phonons only in the [111] sample. The amplitude of coherent phonons with a wave vector of q=0 in the [111] sample decays significantly faster than in the [100] and the [111] samples with q≠0. Coherent phonons in the [111] sample cause a macroscopic piezoelectric field to which the photogenerated electron-hole plasma couples. Friction of the accelerated carriers provides the additional damping mechanism. High carrier densities screen the induced piezoelectric field, thus reducing the damping mechanism via the piezoelectric interaction.