Graduation date: 1995

Graduation date: 1994

Nonlinear Optical Microscopy (NLOM) has been shown to be a valuable tool for noninvasive imaging of complex biological systems. An effective approach for multicolor molecular microscopy is simultaneous excitation of multiple fluorophores by broadband sub-10-fs pulses. This dissertation will discuss the development of two spectral imaging systems using the principles of nonlinear optical microscopy for pixel-by-pixel spectral segmentation of multiple fluorescent spectra. The first spectral system is reliant on a fiber-optic cable to transmit fluorescent signal to a spectrometer, while the second is based on a spectrometer with an aberration-corrected concave grating that is directly coupled to the microscope. A photon-counting, 16-channel multianode photomultiplier tube (PMT) is used for both systems. Custom software developed in LabVIEW controls multiple counter cards as well as a field-programmable gate array (FPGA) for 1 Hz acquisition of 256x256x16 spectral images. Biological specimen consisting of multicolor endothelial cells and zebrafish will be used for experimental verification. Results indicate successful spectral segmentation of multiple fluorophores with a decrease in signal-to-noise ratio in the FPGA-based imaging system.

Obtaining abundant, clean, sustainable energy has become an increasingly large need globally. To date, solar cells have had a limited impact in meeting energy demands. This is primarily due to their relatively high cost and low power conversion efficiencies. Sensitized solar cells, or Grätzel cells, have the potential for being made with low cost materials, and achieving power conversion efficiency high enough to economically compete with fossil fuels. Understanding the dynamics of charge carriers as they separate at the interface of the light absorbing donor and their semiconducting acceptor becomes an important first step in the realization of an inexpensive and efficient sensitized solar cell. Presented is the theory of treating electrons at donor-acceptor interfaces, and why time-resolved surface second harmonic generation (TR-SHG) is used to probe the dynamics of charge carriers at these interfaces. A series of experiments are described where various preparations of thin films of sensitizers on single crystal titanium dioxide, a common acceptor in Grätzel cells, are prepared and studied. TR-SHG studies of thin films of colloidal PbSe and CdSe QDs showed remarkably different electron cooling and transfer dynamics. The electron cooling in PbSe is thermally activated in PbSe QDs. By cooling samples, electron transfer from higher excited “hot” states was observed. Contrary, for CdSe QDs electron transfer rates were dependent on the energy of the excited state. When higher states were excited, charge transfer rates decreased, indicating that only low energy, electrically “cold”, states participate in charge transfer. When carbon based grapheme QDs are used, the electron dynamics mimic PbSe QDs. In this system, increasing the pump energy leads to slower recombination rates, indicating that electrons have to drift further back to the interface. / text

Renewable forms of energy are becoming increasingly important as the world quickly depletes its current energy reserves, and rapidly increases the concentration of pollutants in our environment. Solar technology based on organic semiconductors provides a promising candidate to fulfill a portion of our future energy needs in an environmentally sustainable manner. Organic semiconductors are a collection of pi-conjugated small molecules and polymers which can be implemented in photovoltaic cells that are potentially quite low cost. Currently, however, their commercial applications are limited due to a relatively low efficiency in converting sunlight into usable power. The fundamental physics of such devices must be clarified if these materials are to compete with traditional inorganic solar cells. In this dissertation, two emerging experimental tools are implemented in investigations of the internal electric fields present within operating organic photovoltaic cells. The first set of investigations utilizes the vibrational Stark effect to quantify the electric fields which often form at the interfaces between two organic semiconducting materials. Such interfaces are at the heart of the photocurrent generation process in these devices and any electric fields formed crucially alter device performance. We quantitatively determine the interfacial field present in blends of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) and show that this field depends strongly on annealing conditions. Finally we discuss a correlation between this interfacial electric field, crystalinity and device performance. The second set of investigations take advantage of electric field induced second harmonic generation microscopy to examine the electric potential across active organic solar cells. We again investigate blends of PCBM and P3HT as well as poly(4,4-dioctyldithieno(3,2-b:2',3'-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) (PSBTBT) and PCBM. In the former we find that the potential drop across the device shifts dramatically over time under illumination, while in the latter we find a nearly linear drop which remains constant through device operation. We then extend our examinations of PSBTBT:PCBM with EFISH by quantifying the extent of space charge accumulation throughout such devices. / text

The design of nonlinear optical (NLO) molecules has become a focus of current research in telecommunications, information technologies and optical data storage. Donor-acceptor substituted dipolar molecules have been the most investigated NLO chromophores. Dipolar molecules, however, have several limitations such as low optical transparency, low thermal stability and their strong tendency to adopt anti-parallel packing in the solid state. Recently, a new class of materials based on octupolar symmetries, which lack permanent dipole moments, has been proposed for NLO applications. At a structural level, it can be shown that the basic template for 3D octupolar molecules comes to a cube with alternating charges at the corners such as donor and acceptor substituent. Despite all the various structures reported, it is worth noting that no molecules actually representing the "real" octupolar cube have been obtained so far. In this thesis, we showed that the real octupolar cube can be demonstrated by lanthanide III complexes based on ABAB type phthalocyanine featuring alternating electron donor and electron acceptor groups. These structures are characterized by UV-NIR, X-Ray and exhibit highest quadratic hyperpolarizability ever reported for an octupolar molecule. Moreover, this work was extended to nonoctupolar lanthanide homoleptic double-decker complexes based on AB3, A4, B4, T4 type phthalocyanines. It was observed that these molecules present a quite large quadratic hyperpolarizability too, but smaller than the one obtained for the Ln(ABAB)2 series, as expected.

Conjugated polymers are promising candidates for applications in all kinds of organic optoelectronic devices: OLEDs, organic field-effect transistors (OFETs) and organic photovoltaic cells. The main goal of this work was to investigate transport features of photogenerated electrical charge in pi-conjugated polymers by means of novel technique based on time-resolved electric field-induced second harmonic generation (TREFISH). TREFISH measurement setup was implemented in the laboratory of Molecular compounds physics, and applicability of the method has been verified. Measurements were performed on three different model polymers: methyl substituted ladder-type poly(para-phenylene) (MeLPPP), poly(fluorene-co-benzothiadiazole) (F8BT) and poly(spirobifluorene-co-benzothiadiazole) (PSF-BT), having different morphological and chemical structure. It has been found that motion of photogenerated charge carriers in π-conjugated polymer films experiences rapid dynamics after excitation. Different time domains of charge transport were distinguished. Initial fast transport of photogenerated charge carriers corresponds to the carrier motion along the single polymer chain or conjugated segment of the polymer chain. Slowest carrier motion phase is well described by the stochastic drift, which is attributed to interchain jumps and determines the macroscopic equilibrium mobility. Thus, the equilibrium mobility value is not applicable to the transport on nanometer scale up to tens of nanometers... [to full text] / Konjuguotieji polimerai kaip funkcinės medžiagos gali būti panaudoti įvairiuose prietaisuose: organiniuose šviestukuose, organiniuose lauko tranzistoriuose, organiniuose saulės elementuose. Šio darbo tikslas - nustatyti fotogeneruotų krūvininkų pernašos dėsningumus π – konjuguotuose polimeruose panaudojant naują žadinimo-zondavimo metodą pagrįstą išoriniu elektriniu lauku indukuota antrosios optinės harmonikos generacija. Pagrindinis dėmesys buvo skiriamas pernašos dinamikai. Molekulinių darinių fizikos laboratorijoje buvo įrengta matavimų schema ir įvertintas metodo tinkamumas krūvio pernašos tyrimams. Buvo atlikti krūvio pernašos matavimai trijuose skirtinguose konjuguotuosiuose polimeruose. Nustatyta, kad fotogeneruotų krūvininkų judris tuoj po sužadinimo yra daug didesnis lyginant su stacionaria judrio verte, o krūvio pernašos dinamiką lemia konjuguoto polimero struktūrinė hierarchija, krūvininkų judėjimas yra daugialypis, susidedantis iš greito judėjimo viena polimero grandine ar konjuguotais polimero grandinės segmentais ir lėto šokavimo tarp atskirų polimero grandinių Pirmą kartą detaliai išnagrinėta šviesa sugeneruotų krūvininkų pernašos dinamika konjuguotuose polimeruose. Darbo rezultatai suteikia žinių apie fundamentalius krūvininkų pernašos mechanizmus konjuguotuose polimeruose, kurios gali būti panaudotos kuriant organinius elektronikos prietaisus.

This research uses nonlinear Rayleigh surface waves to characterize stress corrosion cracking (SCC) damage in sensitized 304 Stainless Steel (304 SS). 304 SS is widely used in reactor pressure vessels and fuel pipelines, where a corrosive environment in combination with applied stress due to high internal pressures can cause SCC. SCC poses great risk to these structures as it initiates cracks late in the lifetime and often unexpectedly. The initiated microcracks grow and accumulate very quickly to form macroscopic cracks that lead to material failure. Welds and the nearby heat affected zones (HAZ) in the vessels and pipework are particularly affected by SCC as welding induces sensitization in the material. SCC damage results in microstructural changes such as dislocation movement and microcrack initiation that in the long term lead to reduced structural integrity and material failure. Therefore, the early detection of SCC is crucial to ensure safe operation. It has been shown that the microstructural changes caused by SCC can generate higher harmonic waves when excited harmonically. This research considers different levels of SCC damage induced in samples of sensitized 304 SS by applying stress to a specimen held in a corrosive medium (Sodium Thiosulfate). Nonlinear Rayleigh surface waves are introduced in the material and the fundamental and the second harmonic waves are measured. The nonlinearity parameter that relates the fundamental and the second harmonic amplitudes, is computed to quantify the SCC damage in each sample. The results obtained are used to demonstrate the feasibility of using nonlinear Rayleigh waves to characterize SCC damage.

This research investigates second harmonic generation in Rayleigh surface waves propagating in 9%Cr ferritic martensitic steel. Previous experimental results show that the nonlinearity parameter is sensitive to certain changes in a material's properties such as thermal embrittlement and hardness changes. Therefore, the nonlinearity parameter can be used as an indicator of thermal damage due to changes in dislocation density and precipitations. The specimens are isothermally aged for different holding times to create progressive changes in the microstructure and obtain different levels of thermal aging damage. As aging progresses the dislocation density decreases and precipitations are formed; these microstructural evolutions lead to changes in the nonlinearity parameter β. Nonlinear ultrasonic experiments are conducted for each specimen using a wedge transducer for generation and an air-coupled transducer for detection of Rayleigh surface waves. The amplitudes of the first and second order harmonics are measured at different propagation distances, and these amplitudes are used to obtain the relative nonlinearity parameter for each specimen at different aging stages. Conclusions about microstructural changes are drawn based on the nonlinear Rayleigh surface wave measurement and complementary measurements including scanning electron microscopy (SEM) and Rockwell HRC hardness. The results indicate that the nonlinearity parameter is very sensitive to the dislocation density and precipitate formation, and thus can be used to track the microstructural change in this material during the process of thermal aging.

This thesis contains three components of research: a detailed study of the performance of Frequency-Resolved Optical Gating (FROG) for measuring complex ultrashort laser pulses, a new method for measuring the arbitrary polarization state of an ultrashort laser pulse using Tomographic Ultrafast Retrieval of Transverse Light E-fields (TURTLE) technique, and new approach for measuring two complex pulses simultaneously using PG blind FROG. In this thesis, we compare the performance of three versions of FROG to measure complex ultrashort laser pulses: second-harmonic-generation (SHG) FROG, polarization-gate (PG) FROG, and cross-correlation FROG (XFROG). We found that the XFROG algorithm achieves 100% convergence, while PG FROG and SHG FROG GP algorithm achieve 100% convergence after doing the noise deduction and increasing the sampling range. The second part of this thesis describes a method for measuring the intensity, phase and the complete polarization state of a laser pulse having a time-dependent polarization state (i.e. a polarization shaped pulse). This technique is called tomographic ultrafast retrieval of transverse light E-fields (TURTLE). TURTLE typically involves making three FROG measurements: one of the intensity and phase of the pulse's horizontal polarization component, one of its vertical component, and another of the 45o component. Performing a simple minimization using these three FROG measurements, the time-dependent polarization state of the ultrashort pulse can be determined. The third part of this thesis introduces a method for measuring two complex pulses simultaneously using a single FROG device. This technique is based on Polarization-gate (PG) FROG and it is called PG blind FROG. It involves two measurements: One of them is a PG FROG trace using the intensity of pulse 1 to gate pulse 2 and other one is the PG FROG trace using the intensity of pulse 2 to gate pulse 1. An iterative phase retrieval algorithm based on generalized projection (GP) is used to reconstruct the intensity and phase of these two pulses. This approach is an elegant way to measure complex and/or very spectrally broad pulses such as those due to super continuum.