Momentum vectors of fragment ions produced by the Coulomb explosion of CO2z+ (z = 3 - 6) and CS2z+ (z = 3 - 13) in an intense laser field (~50 fs, 1 x 1015 W/cm2) are determined by the triple coincidence imaging technique. The molecular structure from symmetric and asymmetric explosion channels is reconstructed from the measured momentum vectors using a novel simplex algorithm that can be extended to study larger molecules. Physical parameters such as bend angle and bond lengths are extracted from the data and are qualitatively described using an enhanced ionization model that predicts the laser intensity required for ionization as a function of bond length using classical, over the barrier arguments. As a way of going beyond the classical model, molecular ionization is examined using a quantum-mechanical, wave function modified ADK method. The ADK model is used to calculate the ionization rates of H2, N2, and CO2 as a function of initial vibrational level of the molecules. A strong increase in the ionization rate, with vibrational level, is found for H2, while N2 and CO2 show a lesser increase. The prospects for using ionization rates as a diagnostic for vibrational level population are assessed.

The ultrafast optical regime is defined, as it applies to laser pulses, along with a brief introduction to pulse generation and characterisation technologies. A more extensive description of our particular amplified pulse generation and SPIDER characterisation systems follows. Data verifying the correct operation of the characterisation system is presented and interpreted. Our laser system is then characterised in two different configurations. In each case, the data describing the system is presented and analyzed. Conclusions are made regarding the performance of both the characterisation and laser systems, along with suggested improvements for each.

Momentum vectors of fragment ions produced by the Coulomb explosion of CO2z+ (z = 3 - 6) and CS2z+ (z = 3 - 13) in an intense laser field (~50 fs, 1 x 1015 W/cm2) are determined by the triple coincidence imaging technique. The molecular structure from symmetric and asymmetric explosion channels is reconstructed from the measured momentum vectors using a novel simplex algorithm that can be extended to study larger molecules. Physical parameters such as bend angle and bond lengths are extracted from the data and are qualitatively described using an enhanced ionization model that predicts the laser intensity required for ionization as a function of bond length using classical, over the barrier arguments. As a way of going beyond the classical model, molecular ionization is examined using a quantum-mechanical, wave function modified ADK method. The ADK model is used to calculate the ionization rates of H2, N2, and CO2 as a function of initial vibrational level of the molecules. A strong increase in the ionization rate, with vibrational level, is found for H2, while N2 and CO2 show a lesser increase. The prospects for using ionization rates as a diagnostic for vibrational level population are assessed.

Full field characterization is an important issue for ultrafast optoelectronics. By suitable nonlinear constrain, several approaches, such as FROG, SPIDERS, MIIPS and so on, have been developed for providing detail information of optical pulses. However, phase matching bandwidth of nonlinear material limit the functionality for broadband signal. In this thesis, linear approach without limitation of phase matching bandwidth is proposed. Theoretically, we successfully demonstrated the feasibility of proposed method. We analyzed the limitation and discussed the pulse compression power with the proposed method. Besides, we also proposed experimental method for this method.

This dissertation investigates the photophysics of bis(diarylamino)biphenyl (TPD) and silver nanoparticles (AgNP). A main goal of this work was to develop an understanding of the relaxation pathways involved in the deactivation of photoexcited TPD chromophores in close proximity to silver nanoparticles. The TPD chromophores were attached to the silver nanoparticle core via an alkylthiol group. The TPD-AgNP systems were synthesized and characterized using Transmission Electron Microscopy (TEM), UV-visible absorption, infrared spectroscopy, and Nuclear Magnetic Resonance (NMR) spectroscopy, Inductively Coupled Plasma - Emission Spectroscopy (ICP-ES) and Thermogravimetric Analysis (TGA). Time-resolved photophysical processes in these systems were studied using femtosecond transient absorption spectroscopy. Initial studies of the interaction of the TPD and AgNP addressed the linker length dependence of the dye excited state decay kinetics, wherein alkyl linker chains of 3, 4, 8 and 12 carbon atoms were used. These results showed that an ultrafast deactivation of the excited state of the TPD chromophore, which is three orders of magnitude faster than that of the free chromophore in solution, occurred in all of the systems. However, an unexpected new transient species was observed for the systems with three and four carbon linker chains. Further studies showed this species to be spectroscopically very similar to the TPD radical cation, suggesting a charge separation pathway in the excited state relaxation. Possible pathways for formation of the cation-like state were examined through comparisons to the photophysics of alkyl substituted TPD in solution and in solid films, investigation of the pulse energy and TPD surface coverage dependence of the yield of the cation-like TPD species, transient absorption anisotropy decay dynamics, and kinetic modeling studies. Taken together, these investigations provide support for exciton-exciton annihilation being responsible for the formation of cation-like species. The packing of the TPD chromophores is concluded to be of critical importance in the generation of the cation like species but it is also possible that proximity to the silver nanoparticle plays a role in facilitating charge separation as well.

Cluster mass fraction is an elusive quantity to measure, calculate or estimate accurately for pulsed supersonic gas jets typical of intense laser experiments. The optimization of this parameter is critical for transient phase-matched harmonic generation in an ionized cluster jet at high laser intensity. We present an in-depth study of a rapid, noninvasive, single-shot optical method of determining cluster mass fraction f_c(r,t) at specified positions r within, and at time t after opening the valve of, a high-pressure pulsed supersonic gas jet. A ∼ 2 mJ fs pump pulse ionizes the monomers, causing an immediate drop in the jet’s refractive index n_jet proportional to monomer density, while simultaneously initiating hydrodynamic expansion of the clusters. The latter leads to a second drop in n_jet that is proportional to cluster density and is delayed by ∼ 1 ps. A temporally stretched probe pulse measures the 2-step index evolution in a single shot by frequency domain holography, enabling recovery of f_c. We present the theory behind recovery of f_c in detail. We also present extensive measurements of spatio-temporal profiles f_c(r, t) of cluster mass fraction in a high-pressure supersonic argon jet for various values of backing pressure P, and reservoir temperature T. / text

This dissertation reports the results of optical studies of epitaxial (e), polycrystalline (p) and amorphous (a) Ge2Sb2Te5 (GST) thin films. The dynamic properties of GST films in all three (e/p/a) phases were investigated by a time-resolved optical pump-probe technique in which a femtosecond pump pulse of 55 fs duration was used to excite the sample. The intensity and polarization of the reflected probe beam respectively provide information about the transient reflectance (R) and anisotropic reflectance (AR) induced in the sample, that in turn provide the information about the crystal structure, phonon spectrum, and ultimately phase transitions within the sample. The study of an epitaxial sample provides an opportunity to explore the character of the modes within the phonon spectrum. The epitaxial GST film was grown upon a homoepitaxial layer of GaSb grown upon a GaSb wafer. We observed a 6.7 THz coherent optical phonon (COP) in GaSb(001). The dependence of the signal strength upon the pump and probe polarization was explained in terms of a model that considered both Transient stimulated Raman Scattering (TSRS) and the action of a Surface Space-Charge (SSC) field. The presence of the 6.7 THz transverse COP in the AR channel and its four fold dependence on pump and probe polarization suggests a three-dimensional T2 character. The COP amplitude was maximum when the probe was polarized parallel to the cube edge (GaSb[100]) and the pump polarization was set parallel to a face diagonal (GaSb[110]). The results were fully understood using a microscopic model of selective bond breaking. The AR response of e-GST/GaSb(001) reveals the presence of a 3D 3.4 THz transverse optical phonon. The mode amplitude was independent of pump polarization indicating that the mode is excited by a SSC field. This SSC field could exist within the GST, if the distorted rock-salt structure of GST lacks inversion symmetry, or GaSb, which has the non-centrosymmetric zincblende structure, leading to impulsive excitation of phonons at the GST/GaSb interface. The mode in GST was inferred to be T2-like. The observation of a T2-like phonon mode confirms that GST is cubic in structure and challenges previous studies where 1D or 2D character was assigned to the 3.4 THz mode. While pump-probe measurements displayed the presence of a 3D 3.4 THz mode in the AR response of e-GST/GaSb(001), a 4.5 THz mode was observed in both R and AR channels for p-GST(37 nm)/Si(001) and a-GST(57 nm)/Si(001). The mode character was identified to be either of A or E type by comparing the frequency with frequencies reported in the literature. Additional Raman microscope measurements confirmed the presence of the modes observed in the pump-probe measurements and also revealed additional frequencies. The differences in the frequencies observed from the different samples are quite small suggesting the presence of similar bonds that are modified to some extent by the different structural environment found within each sample. After exposure to high pump fluence the original modes disappeared and were replaced by new modes with frequencies at 4.2 THz and 3.1 THz in e-GaSb, 4.2 THz in e-GST, 3.5 THz in p-GST and 3.6 THz in a-GST. The difference in the final frequencies observed for p and a-GST sample may result from the difference in stack structure affecting the time-dependent temperature profile in each sample. The dependence of the temperature profile on the sample stack was understood from an experimental study of the phase transition between the amorphous and crystalline states induced by exposure to a series of amplified laser pulses. The dependence of the crystalline area and its reflectivity upon the number of pulses and fluence was described using a simple algebraic model. The results justify the assumption of one-dimensional heat flow. The growth velocity of the crystalline region was calculated to be 7-9 m/s. Apparatus and methods were developed to extend the time-resolved optical studies described previously. Firstly, an apparatus was constructed for the measurement of the wavelength dependent sample reflectance with a white-light pulse. A reference arm was employed to allow normalization and hence removal of the intensity noise arising in the laser regenerative amplifier system. Secondly an electrical measurement apparatus was constructed to allow combined electro-optical measurements in future. Switching of GST vertical memory cells was successfully demonstrated. The cells were fabricated on a borosilicate substrate with TiW top and bottom electrodes. A DC voltage of 4.5 to 6 V was required to induce switching, while in pulsed measurements, the device demonstrated switching in response to a pulse with minimum duration of 100 ns.

In this dissertation, I present the results of my research on a prototypical interface of the metal oxide ZnO and the organic semiconductor C₆₀. I establish that the physics at such oxide / organic interfaces is complex and very different from the extensively investigated case of organic semiconductor / metal interfaces. The studies presented in this dissertation were designed to address and improve the understanding of the fundamental physics at such hybrid organic / inorganic interfaces. Using photoemission spectroscopies, I show that metal oxide defect states play an important role in determining the interfacial electronic properties, such as energy level alignment and charge carrier dynamics. In particular, I show that for hybrid interfaces, electronic phenomena are sensitive to the surface electronic structure of the inorganic semiconductor. I also demonstrate applications of photoemission spectroscopies which are unique in that they allow for a direct comparison of ultrafast charge carrier dynamics at the interface and the electronic structure of defect levels. The research presented here focuses on a achieving a significant understanding of the realistic and device relevant C₆₀ / ZnO hybrid interface. I show how the complex surface structure of ZnO can be modified by simple experimental protocols, with direct and dramatic consequences on the interfacial energy level alignment, carrier dynamics and carrier collection and injection efficiencies. As a result of this careful study of the electronic structure and dynamics at the C₆₀ / ZnO interface, a greater understanding of the role of gap states in interface hybridization and charge carrier localization is obtained. This dissertation constitutes a first step in achieving a fundamental understanding of hybrid interfacial electronic properties.

The debate over the origin of the ultrafast demagnetization has been intensively active for the past 16 years. Several microscopic mechanisms have been proposed but none has managed so far to provide direct and incontrovertible evidences of their validity. In this context I have proposed an approach based on spin dependent electron superdiffusion as the driver of the ultrafast demagnetization. Excited electrons and holes in the ferromagnetic metal start diffusing after the absorption of the laser photons. Being the material ferromagnetic, the majority and minority spin channels occupy very different bands. It is then not surprising that transport properties are strongly spin dependent. In most of the ferromagnetic metals, majority spin excited electrons have better transport properties than minority ones. The effect is that majority carriers are more efficient in leaving the area irradiated by the laser, triggering a net spin transport. Recent experimental findings are revolutionising the field by being incompatible with previously proposed models and showing uncontrovertibly the sign of spin superdiffusion. We have shown that spin diffusing away from a layer undergoing ultrafast demagnetization can be used to create an ultrafast increase of magnetization in a neighboring magnetic layer. We have also shown that optical excitation is not a prerequisite for the ultrafast demagnetization and that excited electrons superdiffusing from a non-magnetic substrate can trigger the demagnetization. Finally we have shown that it is possible to control the time shape of the spin currents created and developed a technique to detect directly spin currents in a contact-less way.  The impact of these new discoveries goes beyond the solution of the mystery of ultrafast demagnetization. It shows how spin information can be, not only manipulated, as shown 16 years ago, but most importantly transferred at unprecedented speeds. This new discovery lays the basis for a full femtosecond spintronics.

In this thesis, I present an excitonic approach to treating the coherent dynamics of optically generated charge carriers in semiconductor nanostructures. The main feature of this approach is that it includes exchange interactions and phase space filling effects, which have generally been omitted in previous excitonic treatments of coherent dynamics, so that it can go beyond the low excitation limit. In contrast to the well-known semiconductor Bloch equations, this approach treats intraband correlations without factorization. The excitonic formalism and the obtained excitonic equations are shown to be particularly advantageous in systems where bound excitons dominate the optical response and where intraband correlations play a central role. To demonstrate the application of the excitonic approach, we simulate the coherent carrier dynamics of an optically-excited, updoped AlGaAs superlattice in the presence of a terahertz pulse, where 1s excitonic states as well as higher in-plane excited states are included. We find that gain coefficients greater than 20/cm can be achieved over a tuning range of 3-11THz and that due to the coherent cascading of the carriers down the excitonic Wannier-Stark ladder, the gain coefficients have much higher gain saturation fields than comparable two-level systems. To investigate the effects of phase space filling and exchange interaction on exciton dynamics, we then apply the excitonic formalism to a simple model of a quantum ring as well as a realistic model of a quantum well. For the quantum ring, we have obtained numerical results regarding exciton population and interband polarization. We also compared our excitonic approach to the semiconductor Bloch equations in detail using this simple model. For the quantum well, in addition to the investigation of exciton dynamics, we propose and examine several approximations that can make our excitonic dynamic equations very efficient. The excitonic formalism presented in this thesis is an efficient approach that can be applied in a wide range of systems, which makes it a potential alternative to the standard miconductor Bloch equations for many systems where the intraband correlations are crucial. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-12-01 18:21:17.181