Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: In this thesis, a femtosecond electron diffraction (FED) set-up that is capable of investigating the photo-induced switching of Cu(DCNQI)2 from being an insulator to being a conductor is presented. Movies of atomic structural changes with temporal resolution within the typical photo-switching transition timescales (sub-picoseconds) are obtainable with this set-up by employing a femtosecond laser. The experimental technique and the design of a crucial instrument of the machine, the electron gun, are extensively described and characterised both numerically and experimentally. The interest in observing atomic structural changes of Cu(DCNQI)2 in real time is because of the rich variety of the radical salts available that show alloy-specific Charge Density Wave (CDW) transitions. Valuable insights about the driving mechanisms behind these structural changes that are responsible for a change in conductivity are obtainable, as well as the relation between crystal alloys and their transition characteristics. Electron diffraction patterns of crystals in their metallic phase (room temperature) are shown in this thesis, but diffraction patterns of cryo-cooled Cu(DCNQI)2 in its insulating phase are still to be acquired. The temporal resolution of the atomic movie can be improved by recompression of electron pulses that are debunched due to Coulomb repulsion and electron energy spread within a pulse. Numerical and preliminary experimental results presented in this work expose the potential of a simple compression technique. In this way, more electrons in a single electron pulse can be afforded which allows to perform experiments at shorter integration time or lower repetition rate. / AFRIKAANSE OPSOMMING: In hierdie tesis word ’n femtosekonde elektron diffraksie opstelling aangebied wat daartoe in staat is om die foto-geïnduseerde omskakeling in Cu(DCNQI)2 van nie-geleier tot geleier te ondersoek. Deur gebruik te maak van ’n femtosekonde laser in hierdie opstelling, is ’rolprente’ van strukturele veranderinge op atoomskaal met ’n tyd resolusie beter as die tipiese foto-omskakelings tydskaal (sub-pikosekonde) verkrygbaar. Die eksperimentele tegniek en die ontwerp van ’n noodsaaklike instrument van die masjien, die elektron geweer, word breedvoerig beskryf en numeries en eksperimenteel gekenmerk. Die belangstelling om strukturele veranderinge in Cu(DCNQI)2 op atoom skaal in reële tyd waar te kan neem is as gevolg van die ryke verskeidenheid van radikale soute, wat allooispesifieke ladings digtheid golf (CDW) oorgange toon, wat beskikbaar is. Waardevolle insigte oor die meganismes wat hierdie strukturele veranderinge wat ’n verandering in geleiding veroorsaak dryf is verkrygbaar, sowel as die verwantskap tussen die kristal allooi en die oorgang kenmerke. Diffraksie patrone van kristalle in die metaalagtige fase (kamer temperatuur) word in hierdie tesis getoon, maar diffraksie patrone van cryo-verkoelde Cu(DCNQI)2 in die niegeleier fase moet nog verkry word. Die tyd resolusie van die atomiese rolprent kan verbeter word deur die elektron puls — wat deur Coulomb afstoting en elektron energie spreiding versprei is — weer saam te pers. Numeriese en voorlopige eksperimentele resultate toon die potensiaal van ’n eenvoudige kompressie tegniek. Hierdeur kan meer elektrone in ’n elektron puls gegun word en so die integrasie tyd of die herhalingstempo van die eksperimente verkort kan word.

Thesis (PhD (Physics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Silicon/silicon dioxide (Si/SiO2) interface is widely used in microelectronics as the gate between the drain and source of most metal oxide semiconductor field effect transistors (MOSFETs). The functionality, reliability and electrical properties of such transistors are strongly dependent on the quality of the Si/SiO2 structure forming the gate. Characterization of the Si/SiO2 interface is important in understanding device degradation therefore the Si/SiO2 interface is a subject of intensive investigation. Research studies of Si/SiO2 interfaces using optical methods have been reported by many groups around the world but to date many open questions still exist. The physics of photoinduced trap or defect generation processes and the subsequent trapping of charge carriers, the precise role of photoinduced interfacial electric field in altering optical properties of the Si/SiO2 interface and its role in affecting the second harmonic (SH) yield measurements are not well understood. In this work a commercial near infrared femtosecond (fs) laser source [1.55 eV, 75 ± 5 fs, 10 nJ, 80 MHz] is used to study native Si/SiO2 interfaces of free standing single crystalline Si membrane and bulk Si. Optical second harmonic (SH) generated at the Si/SiO2 interfaces of a Si membrane in reflection and for the first time in transmission is demonstrated as well as stationary, single colour, pump-probe reflectivity measurements from the Si/SiO2 interface of bulk n-type Si. The experimental setups for the second harmonic generation (SHG) and pump-probe techniques were designed and implemented, and measurements were recorded by a computer controlled data acquisition system. Free standing Si membrane samples were successfully produced at the Institut f¨ur Photonische Technologien (IPHT) in Jena, Germany from bulk Si using a chemical etching process and were characterised using the z-scan technique. The penetration depth of light with a photon energy of 1.55 eV in silicon allows transmission of the fundamental fs laser pulses through the Si membrane (∼ 10 μm in thickness) and this is exploited to generate a SH signal in transmission from the Si/SiO2 interfaces of the Si membrane. In the presence of sufficiently intense fs laser light defects are created at the interfaces and populated by multiphoton transfer of charges from Si to SiO2 where they are subsequently trapped. The transfer of charge establishes interfacial electric fields across the interfaces of the Si membrane and this enhances SHG. This phenomenon is called electric field induced second harmonic (EFISH) generation. To our knowledge, EFISH measurements from interfaces of Si membrane performed in transmission are demonstrated for the first time in the present study. The demonstration of EFISH in transmission revealed new results which allowed us to provide additional perspectives on the EFISH generation process at Si/SiO2 interfaces never reported before. The temporal response of SH signals from virgin spots were recorded at different incident laser powers for both reflection and transmission geometries. The SH responses measured in transmission were observed to be time dependent and show an increase during irradiation of the sample corresponding to EFISH process. A series of SH measurements were recorded at different laser powers to compare the magnitudes of SH yield in each detection geometry for a single Si/SiO2 interface. The magnitude of the SH yield measured in transmission was higher than expected and surpassed the SH yield measured in reflection. The expectation is based on the fact that the local intensity of the fundamental beam at the second interface where the SH in transmission is generated is low compared to the local intensity at the first interface where the SH in reflection originates. A physical model is developed to consistently interpret the experimental results obtained in this study. In this model we established the origin of EFISH signals in each detection geometry, explain the unexpected high SH signals measured in transmission and provide an analysis of the time constants extracted from SH response in transmission and reflection. In addition, we also report for the first time stationary pump-probe reflectivity measurements from bulk n-type Si(111) samples with native oxide. A strong pump beam was focused on the same spot as a weak probe beam from the same fs laser source. The change in reflectivity of the Si(111)/SiO2 system was recorded by monitoring the change in intensity of the weak reflected probe beam. The temporal evolutions of the reflectivity of the material were recorded at different pump powers. The reflectivity of the material increases over several minutes of irradiation and reaches steady-state after long time irradiation. The change in reflectivity of the material is attributed to a nonlinear process called Kerr effect, and the temporal response arises from the photoinduced interfacial electric field across the Si(111)/SiO2 interface caused by multiphoton charge transfer from bulk Si(111) to the SiO2 layer. The results reported in this study contribute to the understanding of the photoinduced interfacial electric field caused by charge carrier separation across buried solid-solid interfaces. They also reveal nonlinear optical processes such as the Kerr effect caused by charge dynamics across the interface in addition to the well known SHG process. / AFRIKAANSE OPSOMMING: Die silikon/silikon dioksied (Si/SiO2) skeidingsvlak word algemeen gebruik in mikro-elektronika as die hek tussen die put en die bron van die meeste metaaloksied halfgeleier veld-effek transistors (MOSFETs). Die werkverrigting, betroubaarheid en elektriese eienskappe van sulke transistors word grootliks bepaal deur die kwaliteit van die Si/SiO2 struktuur wat die hek vorm. Karakterisering van die Si/SiO2 skeidingsvlak is belangrik om die degradering van die transistor te verstaan en daarom is die Si/SiO2 skeidingsvlak die onderwerp van intensiewe ondersoek. Ondersoek van die Si/SiO2 skeidingsvlak deur van optiese metodes gebruik te maak is geraporteer deur verskeie internasionale groepe, maar daar bestaan tot vandag toe nog n groot aantal onbeantwoorde vrae. Die fisika van die fotogenduseerde generering van defekte en van posisies waarin ladings gevang kan word, asook die daaropvolgende vasvang van ladingsdraers, die presiese rol van die fotoge¨ınduseerde elektriese veld oor die skeidingsvlak in die verandering van die optiese eienskappe van die Si/SiO2 skeidingsvlak en die grootte van die tweede harmoniek (SH) sein word nog nie goed verstaan nie. In hierdie werk word n kommersile naby-infrarooi femtosekonde (fs) laserbron [1.55 eV, 75 ± 5 fs, 10 nJ, 80 MHz] gebruik om natuurlike Si/SiO2 skeidingsvlakke van vrystaande enkelkristallyne Si membrane en soliede Si te bestudeer. Optiese tweede harmoniek (SH) wat by die Si/SiO2 skeidingsvlakke van ’n Si membraan gegenereer word - in refleksie en vir die eerste keer in transmissie - is gedemonstreer, asook stasionˆere, een-golflengte pomp-toets refleksiemetings op die Si/SiO2 skeidingsvlak van soliede n-gedoteerde Si. Die eksperimentele opstellings vir die tweede harmoniek generering (SHG) en pomp-toets tegnieke is ontwerp en uitgevoer en metings is opgeneem deur ’n rekenaarbeheerde dataversamelingstelsel. Vrystaande Si membraan monsters is suksesvol by die Institut f¨ur Photonische Technologien (IPHT) in Jena, Duitsland vervaardig uit soliede Si deur ’n chemiese etsproses en is gekarakteriseer met behulp van die z-skanderingstegniek as deel van hierdie studie. Die diepte waartoe lig met ’n fotonenergie van 1.55 eV in silikon indring laat die transmissie van die fundamentele fs laserpulse deur die Si membraan (met ∼ 10 μm dikte) toe en dit word ontgin om ’n SH sein van die Si/SiO2 skeidingsvlakke van die Si membraan in transmissie te meet. In die teenwoordigheid van fs laserlig met voldoende intensiteit word defekte by die skeidingsvlakke geskep en bevolk deur meer-foton ladingsoordrag van die Si na die SiO2 waar die ladings daaropvolgens vasgevang word. Die oordrag van ladings skep elektriese velde oor die skeidingsvlakke van die Si membraan en dit versterk die SHG. Hierdie verskynsel word elektriese veld ge¨ınduseerde tweede harmoniek (EFISH) generering genoem. Sover ons kennis strek is die meting van EFISH seine van skeidingsvlakke van Si membrane in transmissie vir die eerste keer in hierdie studie gedemonstreer. Die demonstrasie van EFISH in transmissie het nuwe resultate opgelewer wat ons toegelaat het om bykomende perspektiewe op die EFISH genereringsproses by Si/SiO2 skeidingsvlakke te verskaf waaroor nog nooit vantevore verslag gedoen is nie. Die tydafhanklike gedrag van die SH seine van voorheen onbestraalde posisies is gemeet by verskillende drywings van die inkomende laserbundel vir beide die refleksie en transmissie geometrie¨e. Die gedrag van die SH sein in transmissie is waargeneem om tydafhanklik te wees en ’n toename te toon gedurende bestraling van die monster in ooreenstemming met EFISH prosesse. ’n Reeks van SH metings is opgeneem by verskillende laserdrywings om die groottes van die SH opbrengste in elke meetgeometrie vir ’n enkele Si/SiO2 skeidingsvlak te vergelyk. Die grootte van die SH opbrengs wat in transmissie gemeet is was ho¨er as verwag is en het die grootte van die SH opbrengs in refleksie oortref. Die verwagting is gebaseer op die feit dat die lokale intensiteit by die tweede skeidingsvlak waar SH in transmisie gegenereer word relatief laag is in vergelyking met die lokale intensiteit by die eerste skeidingsvlak waar SH in refleksie ontstaan. ’n Fisiese model is ontwikkel om die eksperimentele resultate wat in hierdie studie verkry is op ’n konsekwente wyse te interpreteer. In hierdie model het ons die oorsprong van EFISH seine in elke meetgeometrie vasgestel, die onverwagte ho¨e SH seine wat in transmissie gemeet is verklaar en ’n analise van die tydkonstantes wat uit die SH gedrag in transmissie en refleksie afgelei is gedoen. Verder rapporteer ons ook vir die eerste keer stasionˆere pomp-toets reflektiwiteitsmetings van soliede n-gedoteerde Si(111) monsters met ’n natuurlike oksied. ’n Sterk pompbundel is gefokus op dieselfde posisie as ’n swak toetsbundel van dieselfde laserbron. Die verandering in reflektiwiteit van die Si(111)/SiO2 stelsel is gemeet deur die verandering in die intensiteit van die swak weerkaatste toetsbundel te monitor. Die tydevolusie van die reflektiwiteit van die mate riaal is gemeet by verskillende pompdrywings. Die reflektiwiteit van die materiaal neem toe gedurende etlike minute van bestraling en bereik ’n stasionˆere toestand na ’n lang tyd van bestraling. Die verandering in reflektiwiteit van die materiaal word toegeskryf aan ’n nielini ˆere prosess, naamlik die Kerr effek, en die tydafhanklike gedrag ontstaan as gevolg van die fotoge¨ınduseerde elektriese veld oor die Si(111)/SiO2 skeidingsvlak wat veroorsaak word deur meer-foton ladingsoordrag van die soliede Si(111) na die SiO2 laag. Die resultate wat in hierdie studie gerapporteer word dra by tot die verstaan van die fotoge ¨ınduseerde elektriese veld oor die skeidingsvlak wat veroorsaak word deur die skeiding van ladingsdraers oor die bedekte kristal-kristal skeidingsvlak. Dit lˆe ook nie-liniˆere optiese prosesse soos die Kerr effek bloot wat veroorsaak word deur die dinamika van ladings oor die skeidingsvlak, bykomend tot die bekende SHG proses.

1. Transfection is a key technique in cell and molecular biology with many important biochemical applications. We selected a fiber fs laser at 1554 nm, an instrument widely used in optical communication research, as the excitation source. Our results demonstrated that the fs laser could perforate the cell membrane and the hole would close in sub-second interval after the laser exposure. We determined the safe exposure duration by detecting if there was any sign of mitochondrial depolarization at 1.5 hours after photoporation. Furthermore, we had successfully transfected HepG2 cells with a plasmid DNA containing the OFP gene, whose fluorescence could still be detected 24 hours after exposure. The transfection efficiency was as high as 77.3%. We also observed the proliferation of the transfected cells after 48 hours. / 2. Cell-cell fusion is a powerful tool for the analysis of gene expression, chromosomal mapping, monoclonal antibody production, and cancer immunotherapy. One of the challenges of in vitro cell fusion is to improve the fusion efficiency without adding extra chemicals while maintaining the cells alive and healthy. We show here that targeted human cancer cells could be selected by an optical tweezer and fused by a finely focused fs laser beam at 1554 nm with a high fusion eftlciency. The result confirmed that human cells could be fused exclusively by fs laser pulses, and this is the first time human cells are fused together all-optically. Mixing of cytoplasm in the fused cells was subsequently observed, and cells from different cell lines were also fused. Based on these, we firstly developed the method of optical cell-cell fusion. / 3. Failure in the induction of apoptosis or programmed cell death is one of the major contributions to the development of cancer and autoimmune diseases. Here we used a fs laser as a novel method to provide a direct apoptosis trigger to observe dynamic changes at subcellular level during apoptosis. First, we examined the effect of fs laser irradiation on the creation of reactive oxygen species (ROS) in exposed cells, which could trigger programmed cell death. By controlling the mitochondria electron transport chain (ETC), we investigated the mechanism of ROS generation by the fs pulses, including thermal effect and direct free electron liberation. Second, we induced apoptosis to targeted cells by the fs laser and found that the nuclear envelope (NE) formed tubular or tunnel-like structures (nuclear tubules - NT) inside the nucleus. The average number of NTs in each cell with laser treatment was significantly larger than in the control. Besides, the development of a NT was observed since its inception and it eventually merged with another one to form a larger NT. Meanwhile, mitochondria and tubulin were found inside the NT, and the NT formation always occurred after an upsurge of cellular Ca2+ concentration. More DNA fragmentation were also found in the region around the NTs. Based on this, we propose that NTs are developed during apoptosis and mitochondria migrate into the nucleus through the NTs to release death signals to trigger DNA fragmentation. Third, we used the fs laser to induce Ca2+ in cells in the form of a slow release, and firstly discovered that most Ca2+ was stored in the cytoplasm, and could diffuse into the nucleus after the optical trigger. Using fast confocal scanning, we obtained the path way of Ca2+ diffusion after the trigger in different cases. Our findings thus provide a new method of regulating the rate of apoptosis. / Biophotonics is an exciting and fast-expanding frontier which involves a fusion of advanced photonics and biology. It has not only developed many novel methodologies for biomedical research, but also achieved significant results as an independent field. Aided with femtosecond (fs) laser technologies, important progresses have been made on manipulating, imaging, and engineering of biological samples from single molecules to tissues in the last 10 years. The laser beam of ultra-short pulses at near-infrared band enjoys a lot of advantages: high nonlinear efficiency, low absorption by biological samples, high spatial and temporal resolution with tight confinement, low photo-toxicity, non-invasive, and ease of control. In this thesis, we report new findings from cell manipulation by fs laser, including transfection, cell-cell fusion, and induction of apoptosis in cells, which are detailed as follows: / He, Hao. / Adviser: Kam Tai Chan. / Source: Dissertation Abstracts International, Volume: 73-03, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Cell wounding, the loss of plasma membrane integrity, is a common event in the life of many cell types. Most cells are subjected to physiological events during normal functions that can lead to disruption of their plasma membranes, especially cells in the load bearing organs such as muscle, skin and bone. The capacity of the cell to repair day-to-day wear-and-tear injuries, as well as traumatic ones, is fundamental for maintaining tissue integrity. / In this thesis, we were trying to uncover single cell wound responses by applying the femtosecond laser (fs laser) technology. A well-characterized tunable fs laser was coupled with a laser scanning confocal microscopic system. Combining real-time observations of the fs laser-induced wound and 3D reconstruction of the cells, post-damage cell and nucleus morphological transformation and wound dynamics were reported. The major findings of this study include: (1) Fs laser could induce a small hole on the plasma membrane of the targeted cell. With the same laser irradiation time, the initial hole size were positively correlated with the laser power. (2) Four typical hole evolution scenarios were reported. Hole resealing was a fast process mostly within 100 seconds in normal condition. Whether a cell could reseal the hole is dependent on the initial hole size. Cells had difficulty to cope with the bigger holes. Three ranges of hole size were given in the thesis to predict the hole resealing result. (3) After fs laser damage, the whole cell underwent a contraction. The post-damage nucleus area, footprint, and each section layer of the cell all shrank, only the thickness remained the same. The nucleus retreated a bit from the damage site after damage. (4) Oxidative stress altered some of the cellular responses to the laser damage. The fs laser- induced holes in oxidative groups were bigger than the normal condition. The cells underwent an overall swelling after fs laser damage instead of the contraction in the normal group. Section layer areas and the thickness of the cell increased after damage. But similar to the normal condition, nucleus shrinkage and retreat from the damage site were also found in the oxidative stress groups. (5) Although both acute and chronic oxidative stresses compromised the integrity of the plasma membrane, chronic oxidative stress compromised more severely with several critical post-damage cell transformations and low resealing ratio. Acute oxidative stress on the other hand may somehow promote the resealing ability of the cells. (6) The section layers closer to the bottom of the cells transformed less than the layers further away from the bottom. This probably suggested that the cell basal attachment provided a constraint force to the plasma membrane for morphological changes. / 細胞創傷，即細胞膜的完整性受損，是多種細胞生命週期中一種常見的現象。細胞在執行正常功能時可能遭遇不同程度的生理性損傷，其中大部分會導致細胞膜的破壞。這一現象對存在於承受壓力器官中的細胞更為頻繁，例如肌肉，皮膚和骨骼。細胞對於日常磨損性傷害以及意外創傷的修復能力，是維持組織完整性的基石。 / 在本論文中，我們通過使用飛秒鐳射技術模擬單細胞創傷，觀察並試圖揭示單細胞對於創傷的反應過程。在實驗中，參數可調的飛秒鐳射器與共軛聚焦顯微鏡整合為一個系統，用於在單細胞膜上進行定點損傷。我們結合了對損傷的即時觀測，細胞的三維結構重建技術，完整記錄了損傷前後的損傷部位，細胞整體以及細胞核的形態變化。以下是本研究的主要發現：（1）飛秒鐳射能夠在目標細胞的細胞膜上進行局部穿孔。在鐳射照射時間相同的情況下，鐳射穿孔的大小與鐳射的平均功率呈正相關。（2）我們發現了鐳射穿孔後穿孔部位有四種不同的變化情況。穿孔後細胞封孔是一相當快的過程，在細胞成功封孔的情況下，大部分細胞將在100秒以內將穿孔部位重新填滿。細胞是否可以將穿孔封住取決於鐳射照射後初始穿孔的大小。細胞很難修復較大的孔。我們將細胞初始穿孔大小分為三個範圍，根據這三個範圍可以利用初始孔的尺寸大致預測穿孔後細胞的封孔情況。（3）飛秒鐳射損傷細胞後，細胞將會收縮，並且細胞核的平面面積，細胞的平面面積（或稱細胞足跡），以及細胞各分層面積都有不同程度的縮小。僅細胞厚度未發生顯著變化。同時，細胞核的位置相對於損傷部位有所後退。（4）細胞在氧化應激過後，對於飛秒鐳射造成的損傷反應有所變化。具體表現為：鐳射穿孔的尺寸比正常情況下更大；穿孔後細胞將會整體腫脹而非收縮。各分層面積和細胞厚度都有不同程度的增大。但是細胞核的反應與正常情況類似，即細胞核將會收縮，並且後退以遠離鐳射損傷部位。（5）儘管急性氧化應激和慢性氧化應激都一定程度上損傷了細胞膜的完整性，但是從細胞對於鐳射創傷的反應觀察，長期慢性氧化應激對於細胞膜的損害更為嚴重，具體表現為鐳射損傷後細胞的嚴重形變和細胞膜修復比例的降低。而另一方面，急性氧化應激在某種程度上可以增強細胞對於鐳射穿孔的修復能力。（6）細胞膜穿孔後，細胞各層的面積變化不一，位置越靠近底層的分層面積變化越小。這可能表明細胞貼壁行為形成了一個對細胞形變的約束力。 / Duan, Xinxing. / Thesis M.Phil. Chinese University of Hong Kong 2014. / Includes bibliographical references (leaves 73-85). / Abstracts also in Chinese. / Title from PDF title page (viewed on 24, October, 2016). / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.

In this thesis a powerful and practical method for characterizing ultrashort pulses in space and time is described (called SEA TADPOLE). First we focus on measuring pulses that are spatially uniform but very complicated in time or frequency. We demonstrate and verify that SEA TADPOLE can measure temporal features as small as 30 femtoseconds over durations as long as 14 picoseconds. The spectral resolution of this device is carefully studied and we demonstrate that for certain pulses, we achieve spectral super resolution. We also develop and test an algorithm for measuring polarization shaped pulses with SEA TADPOLE. Our simple interferometer can even be used to measure the spatiotemporal electric field of ultrashort pulses at a focus. This is because SEA TADPOLE samples the field with an optical fiber which has a small core size. Therefore this fiber can be used to spatially sample the beam, so that the temporal electric field can be measured at every position to obtain E(x, y, z, t). The single mode fiber can be replaced with an NSOM (Near Field Scanning Optical Microscopy) fiber so that spatial resolution as low as 500nm (and possibly lower) can be achieved. Using this device we make the first direct measurements of the compete field of focusing ultrashort pulses. These measurement can be viewed as "snap shots" in flight of the focusing pulse. Also, for the first time, we have observed some of the interesting distortions that have been predicted for focusing ultrashort pulses such as the "forerunner" pulse, radially varying group delay dispersion, and the Bessel-like X-shaped pulse. We have also made the first direct measurements of the electric field of Bessel X-pulses and their propagation invariance is demonstrated. We also use SEA TADPOLE to study the "boundary wave pulses" which are due to diffraction.

>Magister Scientiae - MSc / Recent research shows that the study of optical properties of organic material natural dyes has gained much consideration. The specific functional groups in several natural dyes remain essential for the large nonlinear absorption expressed in terms of nonlinear optical susceptibilities or other mechanism of absorption such as two photon absorption (TPA), reverse saturable absorption (RSA) or intensitydependent refractive index characteristic. In this thesis we highlight the optical limiting responses of selected natural dyes as nonlinear response in the femtosecond regime. This technique refers to the decrease of the transmittance of the material with the increased incident light intensity.Three dyes derived from beetroot, flame flower and mimosa flower dyes were investigated. The results showed a limiting behaviour around 795 mW for the beetroot and the flame dye while there is total transmission in the flame dye sample. The performance of the nonlinearity i.e. the optical limiting is related to the existence of alternating single and double bonds (i.e. C-C and C=C bonds) in the molecules that provides the material with the electron delocalization, but also it is related to the light intensity.Beside nonlinearity study, crystallographic investigation was carried out for more possible applicability of the selected dyes and this concerned only the mimosa and flame flower dye thin film samples since the beetroot thin film was very sensitive to strong irradiation (i.e. immediately destroyed when exposed to light with high intensity). For more stability,dye solutions were encapsulated in gels for further measurements.

Doctor of Philosophy / Department of Physics / Carlos A. Trallero / Ultra-fast spectroscopy has become a common tool for understanding the structure and dynamics of atoms and molecules, as evidenced by the award of the 1999 Nobel Prize in Chemistry to Ahmed H. Zewail for his pioneering work in femtochemistry. The use of shorter and more energetic laser pulses have given rise to high intensity table-top light sources in the visible and infrared which have pushed spectroscopic measurements of atomic and molecular systems into the strong-field limit. Within this limit, there are unique phenomena that are still not well understood. Many of such phenomena involve a photoionization step. For three decades, there has been a steady investigation of the single ionization of atomic systems in the strong-field regime both experimentally and theoretically. The investigation of the ionization of more complex molecular systems is of great interest presently and will help with the understanding of ultra-fast spectroscopy as a whole. In this thesis, we explore the single ionization of molecules in the presence of a strong electric field. In particular, we study molecular isomer pairs, molecules that are the same elementally, but different structurally. The main goal of this work is to compare the ionization yields of these similar molecular pairs as a function of intensity and gain some insight into what differences caused by their structure contribute to how they ionize in the strong-field limit. Through our studies we explore a wavelength dependence of the photoionization yield in order to move from the multi-photon regime of ionization to the tunneling regime with increasing wavelength. Also, in contrast to our strong-field studies, we investigate isomeric molecules in the weak-field limit through single photon absorption by measuring the total ionization yield as a function of photon energy. Our findings shed light on the complexities of photoionization in both the strong- and weak-field limits and will serve as examples for the continued understanding of single ionization both experimentally and theoretically.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18539号 / 理博第4015号 / 新制||理||1579(附属図書館) / 31439 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 阪部 周二, 教授 田中 貴浩, 准教授 橋田 昌樹 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

<p> This thesis presents a study of diagnostics of pulsed laser systems. Two simple and cost effective devices are constructed that measure common spatio-temporal distortions. The first is a spectrally-resolved knife-edge (SRKE) for spatially and spectrally resolving a laser beam, which enables spatio-temporal distortions to be measured. The second is an interferometric autocorrelation taken with a 2-photon diode. A lens is used to focus light from the entire cross section of the beam onto the diode. By scanning the diode through the focus, the effects of pulse-front-tilt on focused beam pulse durations can be measured. These techniques are compared with current theoretical models, and with each other, to establish their reliability and practicality, as well as the reliability of the commercial techniques. SRKE is found to be highly sensitive to spatial and angular dispersion, and also able to measure the frequency gradient, although not as precisely. Interferometric autocorrelation is only able to resolve effects on duration. It can detect the presence of significant spatio-temporal distortion, but several scans must be taken as a function of distance from the lens, through the focus.</p> <p> A commercially built GRENOUILLE was also tested on pulses compressed with a hollow-capillary-prism pulse compressor. Compression of 800 nm, 50 fs pulses to less than 19 fs was achieved with an overall transmission efficiency of 33%. With further work, efficiency could be increased, and pulse duration decreased.</p> / Thesis / Master of Science (MSc)

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Low-dimensional materials are known to undergo phase transitions to differently or- dered states, when cooled to lower temperatures. These phases often show a periodic modulation of the charge density (called a charge density wave – CDW) coupled with a periodic perturbation of the crystal lattice (called a periodic lattice distortion – PLD). Although many experiments have been performed and much has been learnt about CDW phases in low-dimensional materials, the reasons for their existence are still not fully understood yet. Many processes, involving either strong electron–electron or electron–lattice coupling, have been observed which all might play a role in explaining the formation of different phases under different conditions. With the availability of femtosecond lasers it has become possible to study materials under highly nonequilibrium conditions. By suddenly introducing a known amount of energy into the system, the equilibrium state is disturbed and the subsequent relax- ation processes are then observed on timescales of structural and electronic responses. These experiments can deliver valuable information about the complex interactions between the different constituents of condensed matter, which would be inaccessible under equilibrium conditions. We use time resolved electron diffraction to investigate the behaviour of a CDW system perturbed by a short laser pulse. From the observed changes in the diffraction patterns we can directly deduce changes in the lattice structure of our sample. A femtosecond electron diffraction setup was developed at the Laser Research In- stitute in Stellenbosch, South Africa. Short laser pulses produce photo electrons which are accelerated to an energy of 30 keV. Despite space charge broadening effects, elec- tron pulses shorter than 500 fs at sample position can be achieved. Technical details of this system and its characterisation as well as sample preparation techniques and analysis methods are described in detail in this work. Measurements on two members of the quasi-two-dimensional transition metal di- chalcogenides, namely 4Hb-TaSe2 and 1T-TaS2, are shown and discussed. Both show fast (subpicosecond) changes due to the suppression of the PLD and a rapid heating of the lattice. When the induced temperature rise heats the sample above a phase tran- sition temperature, a complete transformation into the new phase was observed. For 4Hb-TaSe2 we found that the recovery to the original state is significantly slower if the PLD was completely suppressed compared to only disturbing it. On 1T-TaS2 we could not only study the suppression of the original phase but also the formation of the higher energetic CDW phase. Long (100 ps) time constants were found for the tran- sition between the two phases. These suggest the presence of an energy barrier which has to be overcome in order to change the CDW phase. Pinning of the CDW by de- fects in the crystal structure result in such an energy barrier and consequently lead to a phase of domain growth which is considerably slower than pure electron or lattice dynamics. / AFRIKAANSE OPSOMMING: Dit is bekend dat lae-dimensionele materie fase oorgange ondergaan na anders ge- ori¨enteerde toestande wanneer afgekoel word tot laer temperature. Hierdie fases toon dikwels ’n periodiese modulasie van die elektron digtheid (genoem ’n “charge density wave” – CDW), tesame met ’n periodiese effek op die kristalrooster (genoem ’n “peri- odic lattice distortion” – PLD). Alhoewel baie eksperimente al uitgevoer is en al baie geleer is oor hierdie CDW fase, is die redes vir hul bestaan nog steeds nie ten volle verstaan nie. Baie prosesse, wat of sterk elektron–elektron of elektron–fonon interaksie toon, is al waargeneem en kan ’n rol speel in die verduideliking van die vorming van die verskillende fases onder verskillende omstandighede. Met die beskikbaarheid van femtosekonde lasers is dit nou moontlik om materie onder hoogs nie-ewewig voorwaardes te bestudeer. Deur skielik ’n bekende hoeveel- heid energie in die stelsel in te voer, word die ewewigstaat versteur en word die daar- opvolgende ontspanning prosesse waargeneem op die tydskaal van atomies struktu- rele en elektroniese bewiging. Hierdie eksperimente kan waardevolle inligting lewer oor die komplekse interaksies tussen die verskillende atomiese komponente van ge- kondenseerde materie, wat ontoeganklik sou wees onder ewewig voorwaardes. Ons gebruik elektrondiffraksie met tyd resolusie van onder ’n pikosekonde om die gedrag van ’n CDW stelsel te ondersoek nadat dit versteur is deur ’n kort laser puls. Van die waargenome veranderinge in die diffraksie patrone kan ons direk aflei watse veranderinge die kristalstruktuur van ons monster ondergaan. ’n Femtosekonde elektronendiffraksie opstelling is ontwikkel by die Lasernavors- ingsinstituut in Stellenbosch, Suid-Afrika. Kort laser pulse produseer foto-elektrone wat dan na ’n energie van 30 keV versnel word. Ten spyte van Coulomb afstoting ef- fekte, kan elektron pulse korter as 500 fs by die monster posisie bereik word. Tegniese besonderhede van hierdie opstelling, tegnieke van die voorbereiding van monsters asook analise metodes word volledig in hierdie tesis beskryf. Metings op twee voorbeelde van kwasi-tweedimensionele semi-metale, naamlik 4Hb-TaSe2 en 1T-TaS2, word gewys en bespreek. Beide wys ’n vinnige (subpikosekon- de) verandering as gevolg van die versteuring van die PLD en ’n vinnige verhitting van die kristalrooster. Wanneer die ge¨ınduseerde temperatuur bo die fase oorgang tempe- ratuur styg, is ’n volledige transformasie na die nuwe fase waargeneem. Vir 4Hb-TaSe2 het ons gevind dat die herstelling na die oorspronklike toestand aansienlik stadiger is as die PLD heeltemal viernietig is in vergelyking met as die PLD net versteur is. Met 1T-TaS2 kon ons nie net alleenlik die vernietiging van die oorspronklike fase sien nie, maar ook die vorming van ’n ho¨er energie CDW fase. Lang (100 ps) tydkonstante is gevind vir die oorgang tussen die twee fases. Hierdie dui op die teenwoordigheid van ’n energie-versperring wat eers oorkom moet word om die CDW fase voledig te ver- ander. Vaspenning van die CDW deur defekte in die kristalstruktuur veroorsaak so’n energie versperring en gevolglik lei dit tot ’n fase van groeiende CDW gebiede wat heelwat stadiger as pure elektron of kritalrooster dinamika is.