Quadratic spatial soliton interactions were investigated in this Dissertation. The first part deals with characterizing the principal features of multi-soliton generation and soliton self-reflection. The second deals with two beam processes leading to soliton interactions and collisions. These subjects were investigated both theoretically and experimentally. The experiments were performed by using potassium niobate (KNBO3) and periodically poled potassium titanyl phosphate (KTP) crystals. These particular crystals were desirable for these experiments because of their large nonlinear coefficients and, more importantly, because the experiments could be performed under non-critical-phase-matching (NCPM) conditions. The single soliton generation measurements, performed on KNBO3 by launching the fundamental component only, showed a broad angular acceptance bandwidth which was important for the soliton collisions performed later. Furthermore, at high input intensities multi-soliton generation was observed for the first time. The influence on the multi-soliton patterns generated of the input intensity and beam symmetry was investigated. The combined experimental and theoretical efforts indicated that spatial and temporal noise on the input laser beam induced multi-soliton patterns. Another research direction pursued was intensity dependent soliton routing by using of a specially engineered quadratically nonlinear interface within a periodically poled KTP sample. This was the first time demonstration of the self-reflection phenomenon in a system with a quadratic nonlinearity. The feature investigated is believed to have a great potential for soliton routing and manipulation by engineered structures. A detailed investigation was conducted on two soliton interaction and collision processes. Birth of an additional soliton resulting from a two soliton collision was observed and characterized for the special case of a non-planar geometry. A small amount of spiraling, up to 30 degrees rotation, was measured in the experiments performed. The parameters relevant for characterizing soliton collision processes were also studied in detail. Measurements were performed for various collision angles (from 0.2 to 4 degrees), phase mismatch, relative phase between the solitons and the distance to the collision point within the sample (which affects soliton formation). Both the individual and combined effects of these collision variables were investigated. Based on the research conducted, several all-optical switching scenarios were proposed.

Single-pass frequency conversion by a nonlinear optical crystal is an attractive method to generate coherent radiation in various spectral domains from ultraviolet to mid-infrared. Wavelength converters based on quasi-phase matched (QPM) periodically poled lithium niobate (PPLN) have proved to be important wavelength conversion devices for many useful applications. This thesis develops a novel integration design for temperature controlling and temperature wavelength tuning of a QPM-PPLN waveguide wavelength converter. A Cr/Pt/Au thin film alloy layer is deposited on a PPLN device with a polymer buffer layer to work as a micro-heater and a temperature sensor at the same time. The temperature of the device can be tailored by applying current to the micro-heater layer, which changes the effective period of the QPM grating and thus the QPM wavelength through the thermal optical effect (TOE). The device's temperature can be monitored by measuring the resistance change of the alloy layer. Micro-heater design and mode profile simulation are involved in the thesis. The entire device fabrication process is introduced. Both electrical and optical features of the device are characterized and discussed. In contrast to the conventional temperature tuning method based on a bulky oven, the proposed design has some excellent characteristics such as compact package size and low power consumption. / Thesis / Master of Applied Science (MASc)

In this thesis, detailed studies are presented into self-assembled, noncentrosymmetric, optically active films. Second harmonic generation (SHG) is used to measure the second order nonlinear optical susceptibility (?(2)) of ionically self-assembled monolayer (ISAM) thin films. Conventional ISAM films are fabricated by alternately immersing a substrate into oppositely-charged polyelectrolyte solutions. The polyelectrolytes bind electrostatically to the oppositely-charged substrate, and thus reverse the charge of the substrate. The charge reversal limits the amount of adsorbed material and primes the substrate for the next layer. During the deposition of the nonlinear optical (NLO) active layer, the chromophores are attracted to the oppositely-charged surface, which results in net orientation of the chromophores. Some of the net orientation is lost during the deposition of the next NLO-inactive layer as this layer orients some of the chromophores away from the substrate. A disadvantage of the polymer ISAM deposition method is that although there is a net orientation toward the substrate, a large number of chromophores are randomly or oppositely oriented. This reduces the nonlinear optical response. To overcome this problem, two alternative methods with a better net orientation are discussed: hybrid covalent / ionic deposition and multivalent monomer deposition. In both deposition methods, the NLO-active material is a monomer instead of a polymer. In hybrid covalent / ionic deposition, the NLO-inactive polymer is deposited using electrostatic attraction while the NLO-active monomer is deposited covalently. This forces alignment of the chromophores. The multivalent method uses chromophores with multiple charges on one side of the molecule and one charge (same sign) on the other. The difference in electrostatic attraction causes a preferential orientation of the chromophores during deposition. Attempts have been made to further improve the net orientation by complexation of the monomers with cyclodextrins (cone shaped organic molecules), so far with only limited success. The SHG response of NLO-active layers near the glass and air interfaces is much stronger than the SHG response of layers in the bulk of the film for all deposition methods and NLO-active materials investigated in this thesis. For larger number of bilayers (the bulk regime), the square root of the SHG signal increases linearly with the number of bilayers as expected for a uniform chromophore orientation. We isolated the interface effects through use of buffer layers of NLO-inactive polymers. The glass interface effect extends roughly one bilayer deep for all investigated materials. The air interface effect is different for polymers and monomers. For monomers, this effect extends only one bilayer deep, while it extends multiple layers deep for polymers. Using glass cells to contain the polyelectrolyte solutions, we were able to measure the SHG signal in situ, which proved to be a powerful tool to monitor the deposition rate as a function of chosen parameters. All depositions were rapid, on the order of one minute or less. Provided that a minimum concentration is met, the deposition rate and final SHG values are independent of concentration. Bulk layers deposit at the same rate as layers near the interface. For polymer NLO-active layers a secondary, slower growth of SHG is observed that is presumably due to reorganization of the adsorbed polymer layer. This secondary growth is not observed in the deposition of NLO-active monomers. / Ph. D.

Detailed studies are presented of thin films that self-assemble into the noncentrosymmetric structure required for second order nonlinear optical responses. Second harmonic generation is used as a probe of the second-order nonlinear optical susceptibility (c(2)) of ionic self-assembled monolayer (ISAM) films. Films produced from the ISAM technique are shown to possess significant c(2), with values presently comparable to quartz (c(2)=1.53*10⁹ esu). These films show exceptional stability over time, with negligible decrease in c(2) after 26 months. ISAM films self-assemble from polyelectrolyte solutions due to coulombic interactions between a charged substrate and the charged polymer in solution. This process is self-limiting since charge overcompensation at the surface restricts further deposition as like charges accumulate at the surface. We have found that this 'kinetically hindered equilibrium' occurs quickly for the samples studied, after approximately 45 seconds immersion. Non-centrosymmetry is obtained during deposition as chromophores orient towards the substrate as a strong, localized collection of opposite charge. This net orientation is partially diminished as some amount of chromophore extends in the opposite direction at the film/solution interface. Second harmonic measurements suggest that chromophores at the outermost interface collapse against the film surface when dried, resulting in a larger c(2) than other 'capped' layers. Any polymer which is not located at the interfaces is thought to possess random orientation, and therefore does not contribute to c(2). We have investigated how ionic strength and solution pH affect the structure of ISAM films. These parameters serve to control the electrostatic screening in solution. Low salt concentrations result in low or no electrostatic screening. As a result, charges on a polymer strongly feel one another's presence, and decrease the net electrostatic energy by maximizing their distance from each other. This results in a rod-like conformation, which when adsorbed onto the film surface produces thin layers. Large salt concentrations serve to screen the electrostatic interaction. Because charges do not experience the strong repulsion from their neighbors, the polymer backbone is more likely to loop and coil. If the polymer is weakly soluble (pH near the solubility edge), the polymer will loop about itself and other polymer chains in order to reduce the number of polymer/water contacts. Increased screening results in adsorption of thicker films. We show that this also results in a marginal increase in film density, likely due to an increase in polymer interpenetration of adjacent layers. We can associate an increase in chromophore population at the interface with this increase in density. The reduction in screening also is shown to decrease the chromophore orientation angle, presumably by decreasing the repulsion between charges located on the chromophore ends. The improved orientation leads to an increased non-centrosymmetry in the layer. c(2) is decreased, however, as film thickness (and therefore the population of randomly oriented chromophore between interfaces) increases faster than the improvements to non-centrosymmetry at the interface. We have investigated the thermal stability of ISAM films at elevated temperatures, and have found that these films do not exhibit a permanent decay of c(2) with increased temperature as do poled guest-host polymers. A temperature-dependent decrease is observed for temperatures up to 250°C. This decrease is completely reversible (for films heated to 150C), with c(2) recovering its initial value upon cooling in spite of a glass transition temperature measured as Tg=140°C The decrease in c(2) is thought to be due to a combination of effects. Predominant decrease is thought to be due to disassociation of ionic bonds, which serve to provide noncentrosymmetry in films. A slower, smaller decay due to decrease in moisture content of the films at high temperature is also thought to be present. / Ph. D.

The interaction of a high power, high frequency (HF) pump/electromagnetic (EM) wave transmitted from a ground-based station with the ionosphere, experiments which have been termed "ionospheric heating", produces secondary radiation known as stimulated electromagnetic emissions (SEEs). SEEs have been developed into powerful diagnostics yielding information such as electron temperature, ion species and hydrodynamic evolution of the modified ionospheric plasma. Classic SEEs which exist outside ±1 kHz of the pump wave frequency (ω0) have recently been classified into wideband SEEs (PW-WSEEs) and distinguished from narrowband SEEs (PW-NSEEs) which exist within ±1 kHz of ω0, where the "PW" prefix has been used to indicate that the frequency regimes in the aforementioned classification are relative to the pump wave (PW) frequency. The occurrence of SEEs near 2ω0 is known as second harmonic generation (SHG). SHG is longstanding and well-established in the field of Laser Plasma Interactions (LPI) where SHG has been harnessed to yield diagnostics such as the velocity of the critical region of the plasma, inference of the region in the plasma where the interaction that results in SHG occurs, plasma turbulence and density scale lengths. Past studies of ionospheric heating SHG were limited by the effective radiated power (ERP) available at ionospheric heating facilities and the frequency resolution of receivers/spectrum analyzers of the time. Experimental observations from these past studies reported either SEEs produced as a result of SHG in isolation or compared these SEEs with PW- WSEEs. Moreover, these experiments did not evaluate effects such as transmit ERP, tilt of the transmit antenna beam from the geomagnetic field (B0) and the offset of ω0 from harmonics of the electron gyrofrequency (ωce) on SEEs within a narrowband of twice the pump wave frequency produced as a result of SHG. Also, these studies did not attempt to draw from the knowledge-base on SHG from LPI. The novelty of the experimental observations in this dissertation is the juxtaposition of PW-NSEEs and second harmonic narrowband SEEs (SH-NSEEs), which are SEEs within kHz of 2ω0, measured at the same time. The heating experiments were all performed at HAARP using an O-mode polarized EM pump wave. Additionally, these measurements evaluate the effects on SHG of the transmit ERP, tilt of the transmit station antenna beam from the geomagnetic field (B0) and the offset of ω0 from nωce, n = 2, 3. The experimental observations show, for the first time, a clear association between PW-NSEEs and SH-NSEEs. This association is subsequently used, in conjunction with theories from LPI to propose the non-linear wave-mixing mechanisms responsible for the SH-NSEEs. As a prelude to harnessing the wealth of diagnostics that can be obtained from SHG, initial diagnostics of the velocity of the critical region and the interaction region where SHG occurs are determined using theories from LPI. With the association between PW-NSEEs and SH-NSEEs established, Particle- In-Cell (PIC) simulations are used to investigate the characteristics of a PW- NSEE herein referred to as the "SBS line", produced as a result of stimulated Brillouin scatter (SBS) instability in which the pump EM wave decays into a backscattered EM wave and an ion acoustic wave. The PIC simulations reveal that for high pump powers, the SBS line, which is intense at the onset of the heating experiment, is suppressed within 3 seconds due to the development of cavities in the ionospheric plasma (density) in which the pump wave depletes its energy in heating up electrons. Although, no PIC simulation results of SHG have been presented in this work, the association between PW-NSEEs and SH-NSEEs shown in this work is used to propose that similar mechanisms are responsible for the suppression the SBS line and its associated SH-NSEE for high pump powers. Results from ionospheric heating experiments presented in this dissertation show a rapid suppression of both the SBS line and its associated SH-NSEE for high pump powers. The attribution of the suppression of SH-NSEEs to the development of artificial field-aligned irregularities (AFAIs) in a past study fails to explain the rapid suppression in the experimental observations contained herein since the suppression occurs on a much faster timescale than the development of AFAIs. Thus, the PIC model results have led to a more feasible interpretation of the observed rapid suppression. To re-iterate, the contributions of this dissertation are as follows: 1. First observations of an SH-NSEE named "SH decay line" within 2ω0±30 Hz. The SH decay line occurs at the same transmit power as the SBS line within ω0±30 Hz and both of these SEEs are suppressed for ω0 ≈ 3ωce. Offset of the SH decay line from 2ω0 is twice the offset of the SBS line from ω0. 2. First experimental evaluation of the impact of B0 assessed by stepping the transmit beam offset from B0 and stepping ω0 near 2ωce shows contemporaneous SH-NSEEs and PW-NSEEs both ordered by the O+ ion cyclotron frequency. 3. First experimental observations of suppression of SBS line and SH decay line for high pump powers, which unlike a past study cannot be attributed to AFAIs. 4. First PIC simulation investigation of suppression of SBS line observed during high pump power ionospheric heating, revealing depletion of pump energy in heating electrons in cavities created in the plasma (density) as the mechanism responsible for the suppression. Broadening of SBS line observed in ionospheric heating with high power is also observed in PIC simulation results. This work has laid the foundations to develop SHG into powerful ionospheric diagnostics. / Doctor of Philosophy / When a high power, high frequency radio wave is injected from a ground-based transmit station into the ionosphere, a region of Earth's atmosphere containing charged particles in addition some neutral atoms and molecules, the frequency spectrum measured at a location removed from the transmit station shows emissions at other frequencies in addition to an emission at the transmit frequency. The emissions at these other frequencies are known as stimulated electromagnetic emissions (SEEs). The frequency offsets of SEEs contain information such as the average kinetic energy associated with random motion of electrons, a parameter known as electron temperature and the ion species present in the region of the ionosphere the radio wave is injected into. The occurrence of SEEs near twice the pump wave frequency is known as second harmonic generation. This dissertation presents experimental observations that compare SEEs which exist within ±1 kHz of the transmit frequency with SEEs which exist within a similar frequency range of twice the transmit frequency unlike past studies. This dissertation also investigates effects of varying the transmit frequency, power and the direction of the transmit station antenna beam relative to the local direction of the magnetic field of the Earth. These new studies reveal, for the first time, a similarity in characteristics of the SEEs near the transmit frequency and two times the transmit frequency. This similarity is used in conjunction with theories from studies of Laser Plasma Interaction (LPI), which have corollaries with high power radio wave-ionosphere interaction, to propose the processes that underlie the occurrence of SEEs near twice the transmit frequency. Methods from LPI have also been used for the first time to obtain measurements of some parameters of the ionosphere. High power radio wave-ionosphere interaction experiments are very expensive and moreover, direct measurement of ionospheric parameters/processes require radar facilities which may not be available or sounding rockets or satellites which increase the cost of experiments. Computer simulations offer a facile and an inexpensive means to investigate SEEs and processes internal to the ionosphere. Computer simulations have been used for the first time in this dissertation to investigate the mechanisms responsible for the characteristics of SEEs near the transmit frequency for low and high transmit powers. Since an association has been established in this dissertation between SEEs near the transmit frequency and SEEs near twice the transmit frequency, the mechanisms responsible for the characteristics for the SEEs near the transmit frequency for high transmit power, have been proposed to be the same mechanisms responsible for the characteristics of SEEs near twice the transmit frequency for a similar transmit power regime. The experimental results, computer simulation results and the corollaries drawn between high power radio wave-ionosphere interaction and LPI detailed in this dissertation have opened new doors to develop SEEs near twice the transmit frequency into a powerful tool to study the ionosphere.

No description available.

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.

The extraction of high power with high beam quality from semiconductor lasers has long been a goal of semiconductor laser research. Optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) have already shown the potential for their high power high brightness operation. In addition, the macroscopic nature of the external cavity in these lasers makes intracavity nonlinear frequency conversion quite convenient. High-power high-brightness VECSELs with wavelength flexibility enlarge their applica-tions. The drawbacks of the VECSELs are their poor spectral characteristics, thermal-induced wavelength shift and a few-nm-wide linewidth.The objective of this dissertation is to investigate tunable high-power high-brightness VECSELs with spectral and polarization control. The low gain and microcavity reson-ance of the VECSEL are the major challenges for developing tunable high-power VECSELs with large tunability. To overcome these challenges, the V-shaped cavity, where the anti-reflection coated VECSEL chip serves as a folding mirror, and an extremely low-loss (at tuned wavelength) intracavity birefringent filter at Brewster's angle are employed to achieved the high gain, low-loss wavelength selectivity and the elimination of microcavity. This cavity results in multi-watt TEM00 VECSELs with a wavelength tuning range of 20~30 nm about 975 nm. Also the longitudinal mode discrimination introduced by birefringent filter makes the linewidth narrow down to 0.5 nm. After the tunable linearly polarized fundamental beam is achieved, the tunable blue-green VECSELs are demonstrated by using type I intracavity second-harmonic generation. The spectral control of VECSELs makes it possible to apply them as an efficient pump source for Er/Yb codoped single-mode fiber laser and to realize the spectral beam combining for multi-wavelength high- brightness power scaling.In this dissertation, theory, design, fabrication and characterization are presented. Rigorous microscopic many-body theory of the quantum well gain, based on semiconductor Bloch equations and k.p theory, is introduced. The closed loop design tool based on this theory is not only used to design the VECSEL structure, but also used as a precise on-wafer diagnostics tool by the experiment/theory comparison of the photo-luminescence. The characterization of the wafer shows that the modeling is in good agreement with the measured results.The VECSEL high power high brightness performance relies on the fabrication of the chip. The fabrication method of the VECSEL chip, which provides the optically smooth surface and good heat dissipation, is presented. The anti-reflection coating on the chip surface can significantly improve the slope efficiency of VECSEL when high reflectivity output coupler is used. Over 12-W VECSEL cw output power with 43 % slope efficiency is demonstrated at 0 oC. A beam quality factor (M^2 factor) of 1.75 is obtained at 11 W output power.

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. / 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]

The aim of the project described in this thesis is to synthesise new functional molecules which interact with light for neurophysiological applications. In particular, I describe a family of amphiphilic porphyrins with large first hyperpolarisabilities which are used as SHG contrast agents and voltage-sensitive probes. In addition I detail a methodological microscopy tool and a novel caged form of a neuronal ion-channel antagonist. Chapter 1 introduces the key concepts underlying the use of dyes as SHG contrast agents. In particular it focuses on aspects of molecular design, covering both the amphiphilicity and nolinearity required by the target molecule. It covers quantification of the nonlinear properties of SHG stains, then surveys a number of examples which showcase the flexibility of SHG imaging as a biomedical technique. Chapter 2 describes a family of amphiphilic porphyrins with large first hyperpolarisabilities. Working from the structure-property relationships identified in Chapter 1, we fully characterise these dyes and demonstrate that they can be used in SHG imaging. We demonstrate that these molecules may also be tuned by complexation of a metal ion which can modulate their photophysical and solubility behaviour. Chapter 3 provides a description of how to determine the orientational distribution of dipolar dyes in a membrane by multiphoton microscopy. We measure the signal intensity of the dye in a model membrane system then find distributional moments which lead to the distribution itself. Chapter 4 explores whether off-axis contributions to the first hyperpolarisability tensor can significantly augment the dominant on-axis contribution from the main dipolar charge-transfer band. We synthesise and characterise a series of cis-donor cis-acceptor porphyrin compounds and explore their biophysical characteristics. Chapter 5 is the culmination of this project and after discussing method development, goes on to show how we measure the voltage sensitivity of an amphiphilic porphyrin SHG dye. We compare the archetypal porphyrin dye chromophore with three commercially available styryl dyes and demonstrate that our dye has greater sensitivity and a more rapid response. Chapter 6 describes a side project, the use of a photolabile cage to protect MK801, a neuronal ion-channel antagonist. By developing a water soluble photolabile cage using molecular design techniques, we are able to release MK801 in neurons with precise spatiotemporal control, allowing us to pinpoint the locus of two key neurophysiological processes.