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Doctor of Philosophy / Department of Physics / Itzhak Ben-Itzhak / Patrick Richard / This work focuses on the ultrafast dynamics of electrons and phonons in graphitic materials. In particular, we experimentally investigated the factors which influence the transport properties of graphite and carbon nanotubes. In the first part of this dissertation, we used Time-resolved Two Photon photoemission (TR-TPP) spectroscopy to probe the dynamics of optically excited charge carriers above the Fermi energy of double-wall carbon nanotubes (DWNTs). In the second part of this study, time-resolved anti-Stokes Raman (ASR) spectroscopy is applied to investigating in real time the phonon-phonon interactions, and addressing the way the temperature affects the dynamics of single-wall carbon nanotubes (SWNTs) and graphite. With respect to the first part, we aim to deeply understand the dynamics of the charge carriers and electron-phonon interactions, in order to achieve an as complete as possible knowledge of DWNTs. We measured the energy transfer rate from the electronic system to the lattice, and we observed a strong non-linear increase with the temperature of the electrons. In addition, we determined the electron-phonon coupling parameter, and the mean-free path of the electrons. The TR-TPP technique enables us to measure the above quantities without any electrical contacts, with the advantage of reducing the errors introduced by the metallic electrodes. The second investigation uses time-resolved ASR spectroscopy to probe in real time the G-mode non-equilibrium phonon dynamics and the energy relaxation paths towards the lattice by variation of the temperature in SWNTs and graphite. The lifetime range of the optically excited phonons obtained is 1.23 ps to 0.70 ps in the lowest (cryogenic temperatures) and highest temperature limits, respectively. We have also observed an increase in the energy of the G-mode optical phonons in graphite with the transient temperature. The findings of this study are important since the non-equilibrium phonon population has been invoked to explain the negative differential conductance and current saturation in high biased transport phenomena.

In this work, we explored one material from the broad family of 2D semiconductors, namely WSe2 to serve as an enabler for advanced, low-power, high-performance nanoelectronics and optoelectronic devices. A 2D WSe2 based field-effect-transistor (FET) was designed and fabricated using electron-beam lithography, that revealed an ultra-high mobility of ~ 625 cm2/V-s, with tunable charge transport behavior in the WSe2 channel, making it a promising candidate for high speed Si-based complimentary-metal-oxide-semiconductor (CMOS) technology. Furthermore, optoelectronic properties in 2D WSe2 based photodetectors and 2D WSe2/2D MoS2 based p-n junction diodes were also analyzed, where the photoresponsivity R and external quantum efficiency were exceptional. The monolayer WSe2 based photodetector, fabricated with Al metal contacts, showed a high R ~502 AW-1 under white light illumination. The EQE was also found to vary from 2.74×101 % - 4.02×103 % within the 400 nm -1100 nm spectral range of the tunable laser source. The interfacial metal-2D WSe2 junction characteristics, which promotes the use of such devices for end-use optoelectronics and quantum scale systems, were also studied and the interfacial stated density Dit in Al/2D WSe2 junction was computed to be the lowest reported to date ~ 3.45×1012 cm-2 eV-1. We also examined the large exciton binding energy present in WSe2 through temperature-dependent Raman and photoluminescence spectroscopy, where localized exciton states perpetuated at 78 K that are gaining increasing attention for single photon emitters for quantum information processing. The exciton and phonon dynamics in 2D WSe2 were further analyzed to unveil other multi-body states besides localized excitons, such as trions whose population densities also evolved with temperature. The phonon lifetime, which is another interesting aspect of phonon dynamics, is calculated in 2D layered WSe2 using Raman spectroscopy for the first time and the influence of external stimuli such as temperature and laser power on the phonon behavior was also studied. Furthermore, we investigated the thermal properties in 2D WSe2 in a suspended architecture platform, and the thermal conductivity in suspended WSe2 was found to be ~ 1940 W/mK which was enhanced by ~ 4X when compared with substrate supported regions. We also studied the use of halide-assisted low-pressure chemical vapor deposition (CVD) with NaCl to help to reduce the growth temperature to ∼750 °C, which is lower than the typical temperatures needed with conventional CVD for realizing 1L WSe2. The synthesis of monolayer WSe2 with high crystalline and optical quality using a halide assisted CVD method was successfully demonstrated where the role of substrate was deemed to play an important role to control the optical quality of the as-grown 2D WSe2. For example, the crystalline, optical and optoelectronics quality in CVD-grown monolayer WSe2 found to improve when sapphire was used as the substrate. Our work provides fundamental insights into the electronic, optoelectronic and quantum properties of WSe2 to pave the way for high-performance electronic, optoelectronic, and quantum-optoelectronic devices using scalable synthesis routes.

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.

Cette thèse est consacrée à l’étude de la dynamique et du transport des phonons via la résolution de l’équation de transport de Boltzmann (ETB) pour les Phonons. Un ‘solveur’ Monte Carlo dédié à la résolution de l’ETB des phonons dans l’espace réciproque, prenant en compte tous les processus d’interactions Normaux et Umklapp à trois-phonons, est proposé. Une prise en compte rigoureuse des lois de conservation de l’énergie et de la quantité de mouvement est entreprise. Des relations de dispersion réalistes, intégrant tous les modes de polarisations, sont considérées. Le calcul des taux d’interactions à trois-phonons de tous les processus Normaux et Umklapp est effectué en utilisant l’approche théorique due à Ridley qui ne nécessite qu’un unique paramètre semi-ajustable pour chaque mode de polarisation, nommément : le coefficient de couplage anharmonique représenté par les constantes de Grüneisen. Les taux d’interactions ainsi calculés ne servent pas uniquement à la résolution de l’ETB des phonons, mais ont permis aussi une analyse complète des canaux de relaxation des phonons longitudinaux optiques de centre de zone. Cette analyse a montré que le canal de Vallée-Bogani est négligeable dans le GaAs, et que vraisemblablement les temps de vie des phonons LO de centre de zone dans l’InAs et le GaSb rapportés dans la littérature sont fortement sous-estimés. Pour la première fois à notre connaissance, un couplage de deux solveurs Monte Carlo indépendants l’un dédié aux porteurs de charges (Thèse E. Tea) et l’autre dédié aux phonons, est effectué. Cela permet d’étudier l’effet des phonons chauds sur le transport des porteurs de charges. Cette étude a montré que l’approximation de temps de relaxation surestime souvent l’effet bottleneck des phonons. Le ‘solveur’ Monte Carlo est étendu pour résoudre l’ETB des phonons dans l’espace réel (en plus de l’espace réciproque), cela a permet d’étudier le transport des phonons et ainsi de la chaleur. La théorie généralisée de Ridley est toujours utilisée avec des particules de simulations qui interagissent les unes avec les autres directement. Les règles de conservation de l’énergie et de la quantité de mouvement sont rigoureusement respectées. L’effet des processus Umklapp sur la quantité de mouvement totale des phonons est fidèlement traduit; tout comme l’effet des interactions sur les directions des phonons, grâce à une procédure prenant en compte les directions vectorielles respectives lors d’une interaction, au lieu, de la distribution aléatoire usuellement utilisée. Les résultats préliminaires montrent la limite de l’équation analytique de conduction de la chaleur. / This work is dedicated to the study of phonon transport and dynamics via the solution of Boltzmann Transport Equation (BTE) for phonons. The Monte Carlo stochastic method is used to solve the phonon BTE. A solution scheme taking into account all the different individual types of Normal and Umklapp processes which respect energy and momentum conservation rules is presented. The use of the common relaxation time approximation is thus avoided. A generalized Ridley theoretical scheme is used instead to calculate three-phonon scattering rates, with the Grüneisen constant as the only adjustable parameter. A method for deriving adequate adjustable anharmonic coupling coefficients is presented. Polarization branches with real nonlinear dispersion relations for transverse or longitudinal optical and acoustic phonons are considered. Zone-center longitudinal optical (LO) phonon lifetimes are extracted from the MC simulations for GaAs, InP, InAs, and GaSb. Decay channels contributions to zone-center LO phonon lifetimes are investigated using the calculated scattering rates. Vallée-Bogani’s channel is found to have a negligible contribution in all studied materials, notably GaAs. A comparison of phonons behavior between the different materials indicates that the previously reported LO phonon lifetimes in InAs and GaSb were quite underestimated in the literature. For the first time, to our knowledge, a coupling of two independent Monte Carlo solvers, one for charge carriers [PhD manuscript, E. TEA], and one for phonons, is undertaken. Hot phonon effect on charge carrier dynamics is studied. It is shown that the relaxation time approximation overestimates the phonon bottleneck effect. The phonon MC solver is extended to solve the phonon’s BTE in real space simultaneously with the reciprocal space, to study phonon and heat transport. Ridley’s generalized theoretical scheme is utilized again with simulation particles interacting directly together. Energy and momentum conservation laws are rigorously implemented. Umklapp processes effect on the total phonon momentum is thoroughly reproduced, as for the anharmonic interactions effect on resulting phonon directions. This is thanks to a procedure taking in consideration the respective vector directions during an interaction, instead of the randomization procedure usually used in literature. Our preliminary results show the limit of the analytic macroscopic heat conduction equation.