Third Order Nonlinearity Of Organic Molecules

The main goal of this dissertation is to investigate the third-order nonlinearity of organic molecules. This topic contains two aspects: two-photon absorption (2PA) and nonlinear refraction (NLR), which are associated with the imaginary and real part of the third-order nonlinearity (χ (3)) of the material, respectively. With the optical properties tailored through meticulous molecular structure engineering, organic molecules are promising candidates to exhibit large third-order nonlinearities. Both linear (absorption, fluorescence, fluorescence excitation anisotropy) and nonlinear (Z-scan, two-photon fluorescence, pump-probe) techniques are described and utilized to fully characterize the spectroscopic properties of organic molecules in solution or solid-state form. These properties are then analyzed by quantum chemical calculations or other specific quantum mechanical model to understand the origins of the nonlinearities as well as the correlations with their unique molecular structural features. These calculations are performed by collaborators. The 2PA study of organic materials is focused on the structure-2PA property relationships of four groups of dyes with specific molecular design approaches as the following: (1) Acceptor-π-Acceptor dyes for large 2PA cross section, (2) Donor-π-Acceptor dyes for strong solvatochromic effects upon the 2PA spectra, (3) Near-infrared polymethine dyes for a symmetry breaking effect, (4) Sulfur-squaraines vs. oxygen-squaraines to study the role of sulfur atom replacement upon their 2PA spectra. Additionally, the 2PA spectrum of a solid-state single crystal made from a Donor-π-Acceptor dye is measured, and the anisotropic nonlinearity is studied with respect to different incident polarizations. These studies further advance our iv understanding towards an ultimate goal to a predictive capability for the 2PA properties of organic molecules. The NLR study on molecules is focused on the temporal and spectral dispersion of the nonlinear refraction index, n2, of the molecules. Complicated physical mechanisms, originating from either electronic transitions or nuclei movement, are introduced in general. By adopting a prism compressor / stretcher to control the pulsewidth, an evolution of n2 with respect to incident pulsewidth is measured on a simple inorganic molecule –carbon disulfide (CS2) in neat liquid at 700 nm and 1064 nm to demonstrate the pulsewidth dependent nonlinear refraction. The n2 spectra of CS2 and certain organic molecules are measured by femtosecond pulses, which are then analyzed by a 3-level model, a simplified Sum-over-states quantum mechanical model. These studies can serve as a precursor for future NLR investigations

Zeitaufgelöste Spektroskopie an Fucoxanthin-Chlorophyll-Proteinen und isolierten Carotinoiden

The aim of this thesis was to elucidate the excitation energy transfer in the fucoxanthin-chlorophyll proteins (FCPs) isolated from the diatom Cyclotella meneghiniana in detail and to clarify the role of the different pigments contained. In a first step the excited state dynamics of the free pigments were studied by means of time-resolved absorption spectroscopy. The FCPs contain three different carotenoid species. Besides the main light-harvesting carotenoid fucoxanthin (fx) the xanthophyll cycle pigments diadinoxanthin (ddx) and diatoxanthin (dtx) are found in substoichiometric amounts. Fx is contained in an unusual carotenoid-to-chlorophyll ratio of about one. In case of ddx and dtx, changing the solvent polarity showed no significant effects on the absorption spectrum and the excited state dynamics were hardly influenced. In contrast, a solvent dependence is observed in the absorption spectrum and excited state dynamics of fx. The S1 lifetime depends strongly on the solvent polarity and an additional broad excited state absorption band red shifted compared to the S1 excited state absorption appears. The occurrence of the described features can be explained with an intramolecular charge transfer state, which is stabilized in a polar environment and appears only in carotenoids with a conjugated carbonyl group. Despite its rather short excited state lifetimes of less than 200 fs (S2) and 30-60 ps (S1), fx acts as a very efficient energy donor in the FCPs. The ultrafast energy transfer dynamics of the isolated proteins FCPa and FCPb were investigated in a comprehensive study using transient absorption in the visible and NIR spectral region complemented with polarized transient absorption spectroscopy. The excitation energy transfer was not influenced significantly by changing the light conditions during the growth, which yields an altered amount of ddx and dtx. It can be concluded that the contribution of the xanthophyll cycle pigments to the energy transfer is not significant. The altered oligomerization state results in a more efficient energy transfer for the trimeric FCPa, which is also reflected in different Chl a fluorescence quantum yields. Thus, an increased quenching in the higher oligomers of FCPb can be assumed. The observed dynamics change drastically for two different excitation wavelengths &#955; = 500 nm and &#955; = 550 nm, which both lead to the population of the S2 excited state of individual carotenoids, namely blue and red absorbing fx molecules. The differing absorption maxima result from distinct microenvironments within the protein. For FCPa an additional slow time constant of 25 ps was found after excitation at 500 nm. By means of polarized transient absorption spectroscopy applied to FCPa different transition dipole moments for the S1 and the ICT state of fx could be identified. Based on the presented studies a detailed model explaining the excitation energy transfer pathways could be developed. In agreement with the faster overall transfer rate which is also evident in the anisotropy data in case of 550 nm excitation, upon excitation at 500 nm one slow transfer channel is active. It can be attributed to a blue absorbing fx not strongly associated with a Chl a molecule. Most likely excitation energy transfer takes place between the S1/ICT states of two different fx molecules before the energy is transferred to Chl a. Additional transient absorption experiments with an improved time resolution were performed to investigate the oscillations observed. These coherent effects superimposed the kinetics of isolated carotenoids as well as FCPs within the first 500 fs. The oscillations showed a very unusual damping behavior and vanished already after two oscillation periods. In case of fx, the solvent environment as well as the excitation wavelengths had an influence on the oscillations. The frequencies of the oscillations were 70-100 cm^-1 for fx in solvents with varying polarity and 50-80 cm^-1 for the FCPs. These results could further confirm the assumption that the red absorbing fx molecules are located in a more polar environment within the protein compared to the blue absorbing fx. To clarify the origin of the oscillations in more detail, further experiments with a controlled chirp of the applied pulses and comparison between different carotenoids in various solvents are required. This approach promises to give further insight in the excited state dynamics and to answer the question whether dark states are involved. Right now, the coherent excitation of the strongly coupled excited states 1Bu+ (S2) and 1Bu- resulting in electronic quantum beats and the existence of an additional short lived excited state absorption (S2-SN2) in the visible spectral region are the most reasonable explanations for the occurrence of the coherent effects in the transient absorption spectra of carotenoids.Ziel der vorliegenden Arbeit war die detaillierte Untersuchung des ultraschnellen Energietransfers (50 fs – 100 ps) in den Fucoxanthin-Chlorophyll-Proteinen (FCPs) aus der Kieselalge Cyclotella meneghiniana. Obwohl ihre Lichtsammelkomplexe den LHCII-Komplexen höherer Pflanzen ähneln, beinhalten sie andere Pigmente in einer ungewöhnlichen Zusammensetzung. Neben dem wichtigsten akzessorischen Pigment Fucoxanthin (Fx) finden sich Diadinoxanthin (Ddx) und Diatoxanthin Dtx) in substöchiometrischen Mengen. Dagegen ist Fx in einem ungewöhnlich hohen Carotinoid-zu-Chlorophyll-Verhältnis von etwa eins enthalten. Fx besitzt eine Carbonylgruppe, welche mit dem pi-Elektronensystem der Kohlenstoffkette konjugiert ist. Mittels transienter Absorptionsspektroskopie wurde der Energietransfer in FCPa und FCPb, die sich durch ihren Oligomerisierungszustand unterscheiden, sowie die Dynamik der isolierten Carotinoide detailliert untersucht. Im Gegensatz zu Ddx und Dtx veränderten sich das Absorptionsspektrum und vor allem die Dynamik des Fx in Lösungsmitteln mit unterschiedlichen Polaritäten. Die beobachteten Effekte können mit intramolekularen Ladungstransferzuständen erklärt werden, welche auch in anderen Carotinoiden mit einer konjugierten Carbonylgruppe auftreten und in polaren und vor allem protischen Lösungsmitteln stabilisiert werden. Trotz der sehr kurzen Lebensdauern der angeregten Zustände (t(S2) < 200 fs, t(S1) = 20-60 ps) ist Fx aufgrund seiner Nähe zu den Chl-Molekülen innerhalb der Lichtsammelkomplexe ein sehr effektiver Energiedonor. Der Vergleich der beiden Oligomere FCPa und FCPb zeigte einen effizienteren Energietransfer im trimeren FCPa, was wiederum auf ein erhöhtes Quenchen im höher oligomeren FCPb schließen lässt. Die untersuchte Dynamik ändert sich deutlich nach der Anregung der Carotinoide mit zwei verschiedenen Wellenlängen (500 nm und 550 nm). Für FCPa wurde eine zusätzliche Lebensdauer von 25 ps nach der Anregung bei 500 nm gefunden, wohingegen die Transferprozesse nach Anregung bei 550 nm nach wenigen Pikosekunden abgeschlossen sind. Zur genaueren Charakterisierung wurde FCPa in zusätzlichen Polarisations-abhängigen, transienten Absorptionsexperimenten untersucht. Für den ersten angeregten Zustand und den zugehörigen Ladungstransferzustand wurden unterschiedliche Übergangsdipolmomente gefunden. Des Weiteren ergaben sich unterschiedliche Übergangsdipolmomente für die beiden Anregungswellenlängen. Basierend auf den in der Arbeit vorgestellten Ergebnissen wurde ein detailliertes Modell zur Erklärung des Energietransfers in FCPs aufgestellt. Dabei kann angenommen werden, dass ein rot-absorbierendes Fx die absorbierte Energie effizient und direkt an ein Chl a-Molekül weitergibt. Zwei der drei verbleibenden Fx-Moleküle mit einem Absorptionsmaximum um 500 nm scheinen ebenso ihre Anregungsenergie direkt an Chl a zu transferieren. In Übereinstimmung mit der schnelleren Dynamik nach Anregung bei 550 nm, die auch in den Anisotropiedaten gefunden wurde, ist nach Anregung bei 500 nm ein zusätzlicher, langsamerer Transferpfad (25 ps) aktiv. Dieser kann demnach einem blau-absorbierenden Fx zugeordnet werden, welches weniger stark mit den Chl a-Molekülen assoziiert ist als die anderen Fx-Moleküle. Zusätzliche zeitaufgelöste Absorptionsexperimente mit verbesserter Zeitauflösung wurden durchgeführt um die oszillatorischen Effekte, die in den meisten Daten zu sehen waren, näher zu untersuchen. Diese kohärenten Effekte überlagern die Dynamiken sowohl der isolierten Carotinoide als auch der FCPs innerhalb der ersten 500 fs. Die Oszillationen zeigten ein ungewöhnliches Dämpfungsverhalten, da sie schon nach zwei Oszillationsperioden verschwinden. Zur Analyse wurden die Frequenzen der Oszillationen bestimmt, was für Fx in Lösungsmitteln unterschiedlicher Polarität zu Werten zwischen 70-100 cm^-1 führte. Für die FCPs variierte die Frequenz zwischen 50-80 cm^-1 in Abhängigkeit von der Anregungswellenlänge. Diese Ergebnisse bestätigten die Annahme, dass sich rot-absorbierendes Fx in einer polareren Umgebung befindet als blau-absorbierendes Fx. Um den Ursprung der Oszillation noch genauer aufzuklären sind weitere Experimente mit einem kontrollierten Chirp der verwendeten Pulse und Vergleiche zwischen verschiedenen Carotinoiden in unterschiedlichen Lösungsmitteln notwendig. Weiterführende Ergebnisse sollten helfen, die Dynamik und vor allem die Beteiligung so genannter "dunkler Zustände" noch besser zu verstehen. Zum jetzigen Zeitpunkt ist die in der Literatur diskutierte kohärente Anregung der stark gekoppelten, angeregten Zustände 1Bu+ (S2) und 1Bu-, welche zu elektronischen Quantumbeats führt, oder auch eine zusätzliche, extrem kurzlebige Absorption aus dem S2-Zustand in einen SN2-Zustand im sichtbaren Spektralbereich eine plausible Erklärung für das Auftreten der kohärenten Effekte in transienten Absorptionsspektren von Carotinoiden

Holographic Recording Materials Development

Organic photorefractive materials were evaluated for application in a reversible holographic memory system. Representative indigo and thioindigo derivatives and several stilbene derivatives were studied as well as 15, 16-dialkyldihydropyrene derivatives the following goals were achieved: (1) the successful writing of phase holograms in a thioindigo/polymer gel system, (2) the successful writing and erasing of phase holograms in a variety of indigo/polymer gel and indigo/solid polymer systems, and (3) the identification of indigoid dyes and 15, 16-dialkyldihydropyrene derivatives as materials potentially suitable for utilization in an operational system. Photochemical studies of the stilbene, indigo, thioindigo, and dialkyldihydropyrene derivatives in solution and in a variety of polymer matrix materials were conducted with the goal of optimizing the photorefractive behavior of the chemical system as a whole. The spectroscopic properties required of optimal photorefractive materials were identified, and it was shown that both the indigoid dyes and the dialkyldihydropyrenes closely match the required properties

Yb3+添加セラミック材料を用いたthin-diskレーザーの開発

Thin-disk laser is one of the key technologies in modern high power laser fields. It enables to realize high beam quality under high average power operation owing to its almost negligible thermo-optic distortions. Recently, various thin-disk laser sources based on single crystals have been investigated, whereas only a few thin-disk lasers have been demonstrated with ceramics. In this thesis, the performances of the thin-disk lasers are evaluated. These are based on Yb3+-doped ceramics, especially Yb3+-doped Lu3Al5O12 (LuAG) and Yb3+- doped Lu2O3 ceramics which are expected to have advantages in high power and highly efficient operations. The spectroscopic properties and the thermal properties of the ceramics are measured systematically. Using the Yb:LuAG ceramic with a doping concentration of 10 at.% and a thickness of 150μm, a high power and a highly efficient thin-disk laser operation is demonstrated in a continuous wave (CW) regime. The maximum output power reaches 166 W with the maximum slope efficiency and the maximum optical efficiency of 72% and 60%, respectively. The efficiencies are almost comparable to those of the thin-disk lasers based on Yb:LuAG single crystals. These results show that ceramics are one of the best gain media for the thin-disk lasers as is the case in the single crystals. The Yb:Lu2O3 ceramics with a doping concentration of 3 at.% are contacted on copper heat sinks with new contacting schemes. The thin-disk lasers based on Yb:Lu2O3 ceramics were demonstrated in a CW regime. The maximum slope efficiency of 61% and the maximum optical efficiency of 45% are achieved with 300μm-thick ceramic thin-disk. Any damages of the disks and instabilities are not observed in those operation parameters. Additionally, the high damage threshold of the disks (at least 6kW/cm2) is expected from the thermographical measurements of the disks. These results denote that the new schemes can be used for high power lasers.電気通信大学201

Advanced Liquid Crystal Materials For Display And Photonic Applications

Thin-film-transistor (TFT) liquid crystal display (LCD) has been widely used in smartphones, pads, laptops, computer monitors, and large screen televisions, just to name a few. A great deal of effort has been delved into wide viewing angle, high resolution, low power consumption, and vivid color. However, relatively slow response time and low transmittance remain as technical challenges. To improve response time, several approaches have been developed, such as low viscosity liquid crystals, overdrive and undershoot voltage schemes, thin cell gap with a high birefringence liquid crystal, and elevated temperature operation. The state-of-the-art gray-to-gray response time of a nematic LC device is about 5 ms, which is still not fast enough to suppress the motion picture image blur. On the other hand, the LCD panel\u27s transmittance is determined by the backlight, polarizers, TFT aperture ratio, LC transmittance, and color filters. Recently, a fringe-field-switching mode using a negative dielectric anisotropy (Δε) LC (n-FFS) has been demonstrated, showing high transmittance (98%), single gamma curve, and cell gap insensitivity. It has potential to replace the commonly used p-FFS (FFS using positive Δε LC) for mobile displays. With the urgent need of submillisecond response time for enabling color sequential displays, polymer-stabilized blue phase liquid crystal (PS-BPLC) has become an increasingly important technology trend for information display and photonic applications. BPLCs exhibit several attractive features, such as reasonably wide temperature range, submillisecond gray-to-gray response time, no need for alignment layer, optically isotropic voltage-off state, and large cell gap tolerance. However, some bottlenecks such as high operation voltage, hysteresis, residual birefringence, and slow charging issue due to the large capacitance, remain to be overcome before their widespread applications can be realized. The material system of PS-BPLC, including nematic LC host, chiral dopant, and polymer network, are discussed in detail. Each component plays an essential role affecting the electro-optic properties and the stability of PS-BPLC. In a PS-BPLC system, in order to lower the operation voltage the host LC usually has a very large dielectric anisotropy (Δε \u3e 100), which is one order of magnitude larger than that of a nematic LC. Such a large Δε not only leads to high viscosity but also results in a large capacitance. High viscosity slows down the device fabrication process and increases device response time. On the other hand, large capacitance causes slow charging time to each pixel and limits the frame rate. To reduce viscosity, we discovered that by adding a small amount (~6%) of diluters, the response time of the PS-BPLC is reduced by 2X-3X while keeping the Kerr constant more or less unchanged. Besides, several advanced PS-BPLC materials and devices have been demonstrated. By using a large Δε BPLC, we have successfully reduced the voltage to \u3c10V while maintaining submillisecond response time. Finally we demonstrated an electric fieldindeced monodomain PS-BPLC, which enables video-rate reflective display with vivid colors. The highly selective reflection in polarization makes it promising for photonics application. Besides displays in the visible spectral region, LC materials are also very useful electro-optic media for near infrared and mid-wavelength infrared (MWIR) devices. However, large absorption has impeded the widespread application in the MWIR region. With delicate molecular design strategy, we balanced the absorption and liquid crystal phase stability, and proposed a fluoro-terphenyl compound with low absorption in both MWIR and near IR regions. This compound serves as an important first example for future development of low-loss MWIR liquid crystals, which would further expand the application of LCs for amplitude and/or phase modulation in MWIR region

Doctor of Philosophy

dissertationThe electronic properties of π-conjugated molecules are strongly related to their molecular shape and morphology of assembly in three-dimensional space. Understanding the various structure-property relationships is relevant to the applications of these materials in optoelectronic devices such as organic light-emitting diodes, field effect transistors and photovoltaic cells. The fact that conjugated systems interact with visible light opens these materials to a plethora of noninvasive spectroscopic investigation techniques. In this work, electronic properties of different π-conjugated systems are studied spectroscopically on the ensemble and the single molecule levels. Single molecule spectroscopy is advantageous in that it allows the investigation of the individual nuclear building blocks that contribute to the properties of the ensemble. Additionally, transient photoluminescence spectroscopy methods can provide useful insight into the temporal evolution of the emissive states. In combination with these methods, novel π-conjugated model molecules are used to probe processes related to exciton dynamics. For the first time, the spatial localization of excited states is probed experimentally in a molecule with a circular chromophoric structure. In addition, a set of model molecules with different geometries is employed to study exciton relaxation in π-conjugated systems. The molecular morphology is utilized to distinguish between processes such as nuclear reorganization and torsional relaxation. Furthermore, single molecule spectroscopy is used to study the electronic structure of individual polymer chains in the photovoltaic cell material poly-(3-hexylthiophene). Optical spectra of this polymer are known to change with the morphology of the bulk film. Single molecule studies reveal that individual polymer chains exhibit similar behavior and indicate that spectral diversity is an intrinsic property of single P3HT molecules. The main results of this work are the following: (1) Excitations in a molecule with rigid circular geometry are shown to localize randomly between different parts of the molecule and show no correlation with the orientation of the excitation light polarization. (2) Model systems can be used to differentiate between different dynamics of exciton relaxation and, furthermore, processes such as torsional relaxation can be inhibited by a rigid molecular structure. (3) The electronic structure of single P3HT molecules is strongly dependent on polymer chain conformation and can be influenced by the surrounding host polymer

Photo induced alignment in polymer films

Currently, alignment films for use in liquid crystal displays are produced via a mechanical rubbing process. The dust produced by mechanical rubbing along with problems due to friction and uneven roller pressure lead to defects in the display. Therefore a novel method for aligning polymers films by irradiation with polarised light has been attempted. Anisotropy introduced into the films by selective irradiation affects liquid crystal alignment. The polymers used in this study are poly (vinyl cinnamate), poly (9- anthraceneoate ethyl methacrylate) and poly (p-azidobenzoate ethyl methacrylate). Poly (vinyl cinnamate) is a classical photoresist polymer which undergoes a [2+2] photocycloaddition in the presence of UV light. Poly (9-anthraceneoate ethyl methacrylate) and poly (p-azidobenzoate ethyl methacrylate) are both novel polymers which have the potential to undergo photo-crosslinking reactions. Poly (9-anthraceneoate ethyl methacrylate) contains an anthracene-terminated side chain which dimerises under the influence of UV light introducing anisotropy into the system. Poly (p-azidobenzoate ethyl methacrylate) contains an azido group which when irradiated with polarised light loses nitrogen to yield nitrenes which can combine to form azobenzene species. UV spectroscopy, infrared dichroism studies, birefringence measurements and fabrication of a simple liquid crystal cell show that poly (vinyl cinnamate) and poly (9-anthraceneoate ethyl methacrylate) undergo selective photoreaction. Poly (p- azidobenzoate ethyl methacrylate) is shown to undergo reaction but not to give the desired products

Diffraction and database analyses of photoactive biphenyl compounds and novel carbaborane structures

The research involved in this thesis is mainly concerned with crystallography and the analysis using crystallographic techniques and methods. The work in this thesis is centered mainly on two types of chemical compounds, photoactive compounds and carbaboranes. The first is the photoactive compounds of biphenyl, its derivatives and similar compounds; these compounds have been studied by diffraction and database analysis. The photochemistry and subsequent structural analysis of biphenyls has been studied in collaboration with Professor Peter Wan at the University of Victoria, Canada. In this study Professor Wan and his group conducted all synthesis and spectroscopic analysis, including the photochemical analysis. In a similar study although not with biphenyls, the a-azidocinnamates were investigated in collaboration with Professor Meth-Cohn of the University of Sunderland. Professor Meth-Cohn and his group conducted all synthesis and spectroscopic analysis. The biphenyl type compounds have also been studied using database analysis to examine the bond lengths, torsion angles, inter-/intra-molecular interactions and general packing conformations and interactions within these structures and this analysis was used to study several conformational anomalies that exist in biphenyl derivative compounds. The second chemical type is carbaboranes; these compounds have been examined in collaboration with Professor Wade's group at the University of Durham. The analysis of carbaboranes centers mainly on hydrogen bonding however also expands into several novel carbaborane structures. Professor Wade and his group carried out the synthesis and spectroscopic analysis

Semiconductor nanocrystals for novel optical applications

“Inspired by the promise of enhanced spectral response, photorefractive polymeric composites photosensitized with semiconductor nanocrystals have emerged as an important class of materials. Here, we report on the photosensitization of photorefractive polymeric composites at visible wavelengths through the inclusion of narrow band-gap semiconductor nanocrystals composed of PbS. Through this approach, internal diffraction efficiencies in excess of 82%, two-beam-coupling gain coefficients in excess of 211 cm-1, and response times 34 ms have been observed, representing some of the best figures-of-merit reported on this class of materials. In addition to providing efficient photosensitization, however, extensive studies of these hybrid composites have indicated that the inclusion of nanocrystals also provides an enhancement in the charge-carrier mobility and subsequent reduction in the photorefractive response time. Through this approach with PbS as charge-carrier, unprecedented response times of 399 µs were observed, opening the door for video and other high-speed applications. It is further demonstrated that this improvement in response time occurs with little sacrifice in photorefractive efficiency and with internal diffraction efficiencies of 72% and two-beam- coupling gain coefficients of 500 cm-1 being measured. A thorough analysis of the experimental data is presented, supporting the hypothesized mechanism of the enhanced charge mobility without the accompaniment of superfluous traps. Finally, water soluble InP/ZnS and CdSe/ZnS quantum dots interacted with CPP and Herceptin to apply them as a bio-maker. Both of quantum dots showed the excellent potential for use in biomedical imaging and drug delivery applications. It is anticipated that these approaches can play a significant role in the eventual commercialization of these classes of materials. --Abstract, page iv