In this work, we investigate the optical response of the semiconductor quantum wire array when excited by stationary UV light. The array is synthesized by selectively electro-depositing the semiconductor material in electrochemically self-assembled porous alumina templates. Our studies are based on the optical behavioral changes in CdS, ZnO, ZnSe and CdSe quantum wires of 50-, 25- and 10-nm diameters. We use a set of generalized Bloch equations to solve the interband polarization function of the semiconductors derived within the Hartree-Fock approximation, and theoretically model the UV excitation effect on the quantum wires. The solutions which consider the effects of screening, Coulomb interaction between the carriers and many body effects on excitons are generated for a quasi-equilibrium regime using a devised accelerated fixed point method. The solution technique is developed in Mathematica to iteratively solve this complex set of equations. The optical constants generated for individual quantum wires are incorporated into a finite-element electromagnetic wave simulator, HFSS, to investigate the full behavior of the array of wires. Theoretically calculated values of the dielectric permittivity of the un-excited quantum wires are shown to decrease progressively as the wire diameter reduces. We perform the experimental analysis using a pump-probe excitation scheme incorporated in a sensitive Michelson interferometer in a homodyne setup. We measure extremely small changes in the phase shift between the interfering IR probe beams and hence measure the refractive index changes caused by the UV pump. While the decreasing filling factor acts to reduce the optical activity in narrower wire arrays, the shifting of the DOS function with additional quantum confinement serves to increase it. These competing effects give rise to the size-dependent non-monotonic optical activity experimentally observed in ZnO, CdS and ZnSe nanowire arrays. The simulation results show a rapid increase in the changes in effective permittivity values of the individual quantum wires as diameter decreases. The substantial changes observed in the refractive index for the whole thin film array at intermediate wire diameter sizes may be suitable for optical phase shifting, intensity modulation and switching applications in integrated optical devices.

Thin film multiferroic oxides with co-existing ferroelectric and ferromagnetic ordering have attracted much interest in recent years, partly as a result of the enhancements achieved through the adoption of strained thin film geometries. This thesis presents work on two such thin film oxides; lanthanide substituted BiFeO₃ and Fe substituted PbTiO₃. Coherent magnons and acoustic phonons were impulsively excited and probed in thin films of the room temperature multiferroic Bi_1-x-yDy_xLa_yFeO₃ using femtosecond laser pulses. The elastic moduli of rhombohedral, tetragonal and rare-earth doped BiFeO₃ were determined from acoustic mode frequencies in conjunction with spectroscopic ellipsometry. A weak ferromagnetic order, induced alternately by magnetization in the growth direction or by tetragonality, created a magnon oscillation at 75 GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31 meV. Bulk crystals and thin films of PbTi_1-xFe_xO₃ (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pb_n+1(Ti_1-xFe_x)_nO_3n+1 forms spontaneously during pulsed laser deposition of PTFO on LaAlO₃ substrates. High-resolution transmission electron microscopy, x-ray difraction and x-ray photoemission spectroscopy were utilised to perform a structural and ompositional analysis, demonstrating that n≃8 and x≃0.33. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes, and the observation of second harmonic generation, establishes a non-centrosymmetric point group for Pb_n+1(Ti_0.67Fe_0.33)_nO_3n+1-δ consistent with ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO₃ and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

Doctor of Philosophy / Department of Physics / Itzhak Ben-Itzhak and Charles Lewis Cocke / A commercially available modern laser can emit over 10^15 photons within a time window of a few tens of femtoseconds (10^-15 second), which can be focused into a spot size of about 10 um, resulting in a peak intensity above 10^14 W/cm^2. This paves the way for table-top strong field physics studies such as above threshold ionization (ATI), non-sequential double ionization (NSDI), high order harmonic generation (HHG), etc.. Among these strong laser-matter interactions, high order harmonic generation, which combines many photons of the fundamental laser field into a single photon, offers a unique way to generate light sources in the vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) region. High order harmonic photons are emitted within a short time window from a few tens of femtoseconds down to a few hundreds of attoseconds (10^-18 second). This highly coherent nature of HHG allows it to be synchronized with an infrared (IR) laser pulse, and the pump-probe technique can be adopted to study ultrafast dynamic processes in a quantum system. The major work of this thesis is to develop a table-top VUV(EUV) light source based on HHG, and use it to study dynamic processes in atoms and small molecules with the VUV(EUV)-pump IR-probe method. A Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) apparatus is used for momentum imaging of the interaction products. Two types of high harmonic pump pulses are generated and applied for pump-probe studies. The first one consists of several harmonics forming a short attosecond pulse train (APT) in the EUV regime (around 40 eV). We demonstrate that, (1) the auto-ionization process triggered by the EUV in cation carbon-monoxide and oxygen molecules can be modified by scanning the EUV-IR delay, (2) the phase information of quantum trajectories in bifurcated high harmonics can be extracted by performing an EUV-IR cross-correlation experiment, thus disclosing the macroscopic quantum control in HHG. The second type of high harmonic source implemented in this work is a single harmonic in the VUV regime (around 15 eV) filtered out from a monochromator. Experiments on D_2 molecules have been conducted using the 9th or the 11th harmonic as the pump pulse. Novel dissociative ionization pathways via highly excited states of D_2 have been revealed, thus suggesting potential applications for time-resolved studies and control of photochemistry processes.

Ultrafast carrier dynamics studies have been carried out on samples with single layers of self-assembled In0.4Ga0.6As/GaAs quantum dots (QDs). Measurements were made using femtosecond degenerate pump-probe differential reflectivity with an 800-nm, 28-fs Ti-sapphire oscillator as the source. The QDs were grown via modified Stranski-Krastanov growth. This modified growth process consists of two steps: low-temperature growth and high-temperature annealing. Specifically, the InGaAs QD structures are fabricated on n-type GaAs(001) using molecular beam epitaxy. The InGaAs layer is deposited at 350-3700C followed by QD self assembly at 420-4900C. Finally, these QDs are capped with 10 nm or 100 nm of GaAs. The measured width and height of these QDs are typically 25 nm and 8 nm, respectively. Dots annealed at higher temperature have larger base area (width and length) and reduced height, as compared to those annealed at lower temperature. We have used a model consisting of a linear combination of an exponential decaying function to describe the carrier dynamics and fit the reflectivity data, revealing trends in the carrier capture and relaxation times associated with the InGaAs layer versus laser excitation level and QD morphology. Capture times are ~ 1 ps for the 100-nm capped samples, but slightly shorter for the 10-nm capped thin samples, indicating carrier transport plays a role in dynamics. The carrier dynamics in 10-nm capped samples are correlated with sample annealing temperature, indicating QD morphology affects carrier capture. Versus laser intensity, and thus carrier excitation level, the dynamics generally become slower, suggesting state filling is important in both the capture and relaxation of excited carriers in these samples.

Es wurde die zeitabhängige Relaxation der Elektronenverteilung in einem Metall-Halbleiter (Galliumarsenid-Gold) Kontakt nach Anregung durch einen Femtosekundenlaserpuls untersucht. Der Einfluss von internen Photoströmen und extern angelegten Spannungen auf die zeitaufgelöste Messung der Elektronenverteilung durch ein Flugzeitspektrometer wird bestimmt und simuliert. / The relaxation-kinetics of electrons in a metal-semiconductor device after excitation by a laserpuls was measured and analyzed. The consideration of internal photocurrents and external applied voltages are crucial for interpreting the results correctly.

In the course of this thesis gold nanoparticle/polyelectrolyte multilayer structures were prepared, characterized, and investigated according to their static and ultrafast optical properties. Using the dip-coating or spin-coating layer-by-layer deposition method, gold-nanoparticle layers were embedded in a polyelectrolyte environment with high structural perfection. Typical structures exhibit four repetition units, each consisting of one gold-particle layer and ten double layers of polyelectrolyte (cationic+anionic polyelectrolyte). The structures were characterized by X-ray reflectivity measurements, which reveal Bragg peaks up to the seventh order, evidencing the high stratication of the particle layers. In the same measurements pronounced Kiessig fringes were observed, which indicate a low global roughness of the samples. Atomic force microscopy (AFM) images veried this low roughness, which results from the high smoothing capabilities of polyelectrolyte layers. This smoothing effect facilitates the fabrication of stratified nanoparticle/polyelectrolyte multilayer structures, which were nicely illustrated in a transmission electron microscopy image. The samples' optical properties were investigated by static spectroscopic measurements in the visible and UV range. The measurements revealed a frequency shift of the reflectance and of the plasmon absorption band, depending on the thickness of the polyelectrolyte layers that cover a nanoparticle layer. When the covering layer becomes thicker than the particle interaction range, the absorption spectrum becomes independent of the polymer thickness. However, the reflectance spectrum continues shifting to lower frequencies (even for large thicknesses). The range of plasmon interaction was determined to be in the order of the particle diameter for 10 nm, 20 nm, and 150 nm particles. The transient broadband complex dielectric function of a multilayer structure was determined experimentally by ultrafast pump-probe spectroscopy. This was achieved by simultaneous measurements of the changes in the reflectance and transmittance of the excited sample over a broad spectral range. The changes in the real and imaginary parts of the dielectric function were directly deduced from the measured data by using a recursive formalism based on the Fresnel equations. This method can be applied to a broad range of nanoparticle systems where experimental data on the transient dielectric response are rare. This complete experimental approach serves as a test ground for modeling the dielectric function of a nanoparticle compound structure upon laser excitation. / Im Rahmen dieser Arbeit wurden Gold-Nanopartikel/Polyelektrolyt Multischichtstrukturen hergestellt, strukturell charakterisiert und bezüglich ihrer optischen Eigenschaften sowohl statisch als auch zeitaufgelöst analysiert. Die Strukturen wurden mithilfe der Dip-coating oder der Spin-coating Methode hergestellt. Beide Methoden ermöglichen das Einbetten einzelner Partikellagen in eine Polyelektrolytumgebung. Typische Strukturen in dieser Arbeit bestehen aus vier Wiederholeinheiten, wobei jede aus einer Nanopartikelschicht und zehn Polyelektrolyt-Doppellagen (kationisches und anionisches Polyelektrolyt) zusammengesetzt ist. Die Stratizierung der Gold-Nanopartikellagen wurde mittels Röntgenreflektometrie-Messungen im Kleinwinkelbereich nachgewiesen, welche Bragg Reflexionen bis zur siebten Ordnung aufzeigen. Das ausgeprägte Kiessig Interferenzmuster dieser Messungen weist zudem auf eine geringe globale Rauheit hin, die durch Oberflächenanalysen mit einem Rasterkraftmikroskop bestätigt werden konnte. Diese geringe Rauheit resultiert aus den glättenden Eigenschaften der Polyelektrolyte, die die Herstellung von Multilagensystemen mit mehreren Partikellagen erst ermöglichen. Die Aufnahme eines Transmissionselektronenmikroskops veranschaulicht eindrucksvoll die Anordnung der Partikel in einzelne Schichten. Durch photospektroskopische Messungen wurden die optischen Eigenschaften der Strukturen im UV- und sichtbaren Bereich untersucht. Beispielsweise wird eine Verschiebung und Verstärkung der Plasmonenresonanz beobachtet, wenn eine Goldnanopartikellage mit transparenten Polyelektrolyten beschichtet wird. Erst wenn die bedeckende Schicht dicker als die Reichweite der Plasmonen wird, bleibt die Absorption konstant. Die spektrale Reflektivität jedoch ändert sich auch mit jeder weiteren adsorbierten Polyelektrolytschicht. Die Reichweite der Plasmonenresonanz konnte auf diese Art für Partikel der Größe 10 nm, 20 nm und 150 nm bestimmt werden. Die Ergebnisse wurden im Kontext einer Effektiven Mediums Theorie diskutiert. Die komplexe dielektrische Funktion einer Multilagenstruktur wurde zeitabhängig nach Laserpulsanregung für einen breiten spektralen Bereich bestimmt. Dazu wurden zuerst die Änderungen der Reflektivität und Transmittivität simultan mittels der Pump-Probe (Anrege-Abtast) Spektroskopie gemessen. Anschließend wurden aus diesen Daten, mithilfe eines Formalismus, der auf den Fresnelschen Formeln basiert, die Änderungen im Real- und Imaginärteil der dielektrischen Funktion ermittelt. Diese Methode eignet sich zur Bestimmung der transienten dielektrischen Funktion einer Vielzahl von Nanopartikelsystemen. Der rein experimentelle Ansatz ermöglicht es, effektive Medien Theorien und Simulationen der dielektrischen Funktion nach Laserpulsanregung zu überprüfen.

The main theme covered in this thesis is experimentalstudies of quantum dynamics and coherent control in homonuclearalkali diatomic molecules by ultrafast laser spectroscopy iththe implementation of pump-probe techniques. A series of experiments have been performed on the Rb2molecules in a molecular beam as well as in a thermal oven. Thereal-time molecular quantum dynamics of the predissociatingelectronically excited D(3)1Πu state of Rb2, which couples to/intersects several otherneighbouring states, is investigated using wavepackets. Thepredissociation of the D state, explored by this wavepacketmethod, arises from two independent states, the (4)3Σu+and (1)3∆u, for which the second corresponds to a much fasterdecay channel above a sharp energy threshold around 430 nm. Thelifetime of the D state above the energy threshold is obtained,τ ≈ 5 ps, by measuring the decay time of thewavepacket in a thermal oven. Further experimentalinvestigation performed in a molecular beam together withquantum calculations of wavepacket dynamics on the D state haveexplored new probe channels of wavepacket evolution: theD′(3)1Σu+ channel, which exhibits vibrational motionin a shelf state and the (4)3Σu+ channel, where direct build-up of thewavefunction is observed due to its spin-orbit oupling to the Dstate. The real-time quantum dynamics of wavepackets confined totwo bound states, A1Σu+(0u+) and b3Πu(0u+), have been studied by experiment andcalculations. It is shown that these two states are fullycoupled by spin-orbit interaction, characterised by itsintermediate strength. The intermediate character of thedynamics is established by complicated wavepacket oscillationatterns and a value of 75 cm-1is estimated for the coupling strength at thestate crossing. The experiments on the Li2molecule are performed by coherent control ofrovibrational molecular wavepackets. First, the Deutsch-Jozsaalgorithm is experimentally demonstrated for three-qubitfunctions using a pure coherent superposition of Li2rovibrational eigenstates. The function’scharacter, either constant or balanced, is evaluated by firstimprinting the function, using a phase-tailored femtosecond(fs) pulse, on a coherent superposition of the molecularstates, and then projecting the superposition onto an ionicfinal state using a second fs pulse at a specific delay time.Furthermore, an amplitude-tailored fs pulse is used to exciteselected rovibrational eigenstates and collision induceddephasing of the wavepacket signal, due to Li2-Ar collisions, is studied experimentally. Theintensities of quantum beats decaying with the delay time aremeasured under various pressures and the collisional crosssections are calculated for each well-defined rovibrationalquantum beat, which set the upper limitsfor ure dephasingcross sections. <b>Keywords:</b>Ultrafast laser spectroscopy, pump-probetechnique, predissociation, wavepacket, pin-orbit interaction,coherent control, (pure) dephasing

The lateral spatial separation between the circular polarization components of a linearly polarized light beam impinging at off-normal incidence on an air-semiconductor interface is investigated experimentally and theoretically. This fundamental optical phenomenon is referred to as the Spin Hall effect of light (SHEL). An optical pump-probe technique is demonstrated to resolve in situ the nanometer size SHEL displacement of a beam transmitted inside an absorptive material. Three different types of optical interactions in silicon and GaAs demonstrate the technique’s general applicability. First, resonant ∼150 fs pump and probe pulses at λ = 820 nm resolve the SHEL displacement via free-carrier absorption in a 10 μm thick silicon sample. The measured SHEL displacements for a p-polarized probe beam are obtained between −10 to 150 nm as a function of the angle of incidence on the sample. Different angles of incidence are achieved by keeping a fixed angular separation between the pump and the probe beams while rotating the sample about the axis perpendicular to the plane of incidence. In another experiment, an optically thin (500 nm thick) GaAs sample allows one to use Pauli-blocking as an optical interaction to investigate the polarization and angular dependence of the SHEL in the probe beam. For such a polarization-dependent imaging technique, the SHEL displacement in the pump beam also contributes to the measured signal and is evaluated experimentally. A probe beam at normal incidence is used to measure a SHEL displacement of ∼180 nm in a transmitted p-polarized pump beam impinging on the sample with an angle of incidence of 55 degrees. Finally, two-photon absorption is used to resolve the SHEL in a (001) oriented 500 μm thick GaAs wafer using an optical source generating sub-bandgap radiation (λ = 1550 nm) with a pulse duration of 120 fs. Linearly p- and s- co-polarized pump and probe beams are also used to investigate the polarization dependence of the SHEL. All the experimental results obtained using these different optical interactions agree with the theory within the experimental error. Finally, analytical expressions of the shifts experienced by the circular components of a beam impinging at an interface between two optical media are also derived for an incident beam with an arbitrary spatial distribution.

The lateral spatial separation between the circular polarization components of a linearly polarized light beam impinging at off-normal incidence on an air-semiconductor interface is investigated experimentally and theoretically. This fundamental optical phenomenon is referred to as the Spin Hall effect of light (SHEL). An optical pump-probe technique is demonstrated to resolve in situ the nanometer size SHEL displacement of a beam transmitted inside an absorptive material. Three different types of optical interactions in silicon and GaAs demonstrate the technique’s general applicability. First, resonant ∼150 fs pump and probe pulses at λ = 820 nm resolve the SHEL displacement via free-carrier absorption in a 10 μm thick silicon sample. The measured SHEL displacements for a p-polarized probe beam are obtained between −10 to 150 nm as a function of the angle of incidence on the sample. Different angles of incidence are achieved by keeping a fixed angular separation between the pump and the probe beams while rotating the sample about the axis perpendicular to the plane of incidence. In another experiment, an optically thin (500 nm thick) GaAs sample allows one to use Pauli-blocking as an optical interaction to investigate the polarization and angular dependence of the SHEL in the probe beam. For such a polarization-dependent imaging technique, the SHEL displacement in the pump beam also contributes to the measured signal and is evaluated experimentally. A probe beam at normal incidence is used to measure a SHEL displacement of ∼180 nm in a transmitted p-polarized pump beam impinging on the sample with an angle of incidence of 55 degrees. Finally, two-photon absorption is used to resolve the SHEL in a (001) oriented 500 μm thick GaAs wafer using an optical source generating sub-bandgap radiation (λ = 1550 nm) with a pulse duration of 120 fs. Linearly p- and s- co-polarized pump and probe beams are also used to investigate the polarization dependence of the SHEL. All the experimental results obtained using these different optical interactions agree with the theory within the experimental error. Finally, analytical expressions of the shifts experienced by the circular components of a beam impinging at an interface between two optical media are also derived for an incident beam with an arbitrary spatial distribution.

<p>Imaging based on nonlinear processes takes advantage of the localized excitation to achieve high spatial resolution, optical sectioning, and deeper penetration in highly scattering media. However, the use of nonlinear contrast for imaging has conventionally been limited to processes that create light of wavelengths that are different from the wavelengths used for excitation. Intrinsic nonlinear contrasts that do not generate light at distinct wavelengths are generally difficult to measure because of the overwhelming background from the excitation light. This dissertation focuses on extension of nonlinear microscopy to these new intrinsic processes by using femtosecond pulse shaping to encode the nonlinear information as new frequency components in the spectrum. We will present a pump-probe microscopy technique based on pulse train shaping technology to sensitively access nonlinear transient absorption or gain processes. This technique has recently been used to uniquely identify a variety of biological pigments with high spatial resolution. Here, we extend this technique to image and characterize several inorganic and organic pigments used in historical artwork. We also present a spectral reshaping technique based on individual femtosecond pulse shaping to sensitively access nonlinear refractive contrasts in scattering media. We will describe an extension of this technique to utilize two distinct wavelengths and discuss its application in biological imaging. This two-color implementation would allow the extension of widely employed phase contrast to the nonlinear regime.</p> / Dissertation