Metallic nano-structures provide for new and exciting domains to investigate light-matter interactions. The coupling of these metallic nano-structures to semiconductor emitters allows for the observation of cavity QED effects including Purcell enhancement and Vacuum Rabi splitting. The focus of this dissertation will be to present an introduction and background to semiconductor optics, and metallic metamaterial systems. This will be followed by the presentation of the spectroscopy systems designed and constructed during my tenure as graduate student and the experimental data obtained with these systems. Some of the results have been published, while some of the presented material is still actively being pursued for publication. More specifically, the dissertation will cover the research at hand, experimental techniques, and results.

In this thesis, the effect of the sample properties on the characteristics of the hot carrier luminescence in graphene is investigated. The present work focuses on the two main issues described below. The first issue is the modification effects of near-infrared pulsed laser excitation on graphene. For excitation fluences several orders of magnitude lower than the optical damage threshold, the interaction with ultrafast laser pulses is found to cause a stable change in the properties of graphene. This photomodification also results in a decrease of the hot photoluminescence intensity. The detailed analysis shows that ultrafast photoexcitation leads to an increase in the local level of hole doping, as well as a change in the mechanical strain. The variation of doping and strain are linked with the enhanced adsorption of atmospheric oxygen caused by the distortion of the graphene surface. These findings demonstrate that ultrashort pulsed excitation can be invasive even if a relatively low laser power is used. Secondly, the variation of the hot photoluminescence intensity with the increasing charge carrier density in graphene is investigated. The electro-optical measurements performed using graphene field-effect transistors show a strong dependence of the photoluminescence intensity on the intrinsic carrier concentration. The emission intensity has a maximum value in undoped graphene and decreases with the increasing doping level. The theoretical calculations performed using a refined two-temperature model suggest that the reduction of the photoluminescence intensity is caused by an increase in the hot carrier relaxation rate. The modification of the carrier relaxation dynamics caused by photoinduced doping is probed directly using the two-pulse correlation measurements. The discovered sensitivity of the hot photoluminescence to the intrinsic carrier concentration can be utilised for spatially-resolved measurements of the Fermi level position in graphene samples, offering an advantage in resolution and speed.

Thesis (MSc)--Stellenbosch University, 2012. / AFRIKAANSE OPSOMMING: Berekeninge met Density Functional Theory (DFT) is ’n nuttige tegniek om die dinamika van molekules op potensiële energievlakke te verstaan. Beginnende met ’n prototipe molekuul formaldimien, wat die kern vorm van die groter fotochromiese molekuul dithizonatophenyl kwik (DPM), word die modellering van die molekuul meer ingewikkeld tot laasgenoemde bestudeer kan word asook sy fotochromiese afgeleides wat vervanging van elektronryk en elektronarm radikale by orto, meta en para posisies van die phenyl ringe insluit. DFT berekeninge word met spektra van Absorpsiespektroskopie met UV en sigbare lig asook tyd opgeloste spektra, verkry dmv femtosekondespektroskopie, vergelyk. In pol^ere aprotiese, pol^ere protiese en nie-pol^ere oplosmiddels, isomeriseer die molekuul om die C=N dubbelbinding. Daar kan tussen die twee isomere onderskei word deur dat die een in oplossing in sy grondtoestand blou en die ander een oranje voorkom. Die isomerisering is’n fotogeinduseerde proses. Die optimering van die molekul^ere struktuur, absorpsiespektra, oplosmiddel-afhanklikheid, en potensiële energievlak metings van die molekuul word bestudeer. Die sterk/swak wisselwerking wat in pol^ere protiese/aprotiese oplosmiddels verskyn word geopenbaar deur die hoe/lae absorpsie van die sekond^ere bande van die molekules. Daar is gevind dat die absorpsiespektra van DPM bathochromies in oplosmiddels met hoë diëlektriese konstantes is. Vir die potensiële energievlak berekeninge van die grondtoestand word rigiede en ontspanne metodes gebruik waar laasgenoemde met gebroke simmetrie berekeninge verkry word. Van alle metodes wat vir berekeninge gebruik was, gee die B3LYP/CEP-31G metode die beste benadering aan eksperimentele data. Alle berekeninge word gedoen met twee bekende sagteware pakkette; Amsterdam Density Functional (ADF) en Gaussian, wat op twee verskillende DFT metodes gebaseer is. / ENGLISH ABSTRACT: Density functional theory is a useful computational tool in the understanding of molecular dynamics on potential energy surfaces. Starting with a prototype molecule formaldimine, the photochromic molecule dithizonatophenylmercury II (DPM) and a set of its photochromic derivatives, (involving substitutions of electron donating and electron withdrawing substituents at ortho, meta and para positions of the dithizonato phenyl rings), are studied through density functional calculation in comparison with steady state absorption spectra obtained from UV-Visible and femto second spectroscopy experiments. In polar aprotic, polar protic and non-polar solvents these molecules isomerise around C=N double bond chromophore, from orange electronic ground states to blue electronic ground states upon photo-excitation. We investigate the structural optimisations, the absorption spectra, the solvent dependence and the potential energy surface (PES) of these molecules. The strong (weak) interactions exhibited by the polar protic (aprotic) solvents used are revealed through high (low) absorbance in the secondary bands of these molecules. The absorption spectra of DPM are found to be bathochromic in solvents with high dielectric constants. For the ground state PES calculation we make use of rigid and relaxed methods, and the latter is obtained through broken symmetry calculation. Of all the methods used in calculation, B3LYP/CEP-31G method gives the best approximation to the experimental data. All calculations are done using the two renown software, Amsterdam Density Functional (ADF) and Gaussian, availing their different density functional methods.

Thesis (MSc (Physics))--University of Stellenbosch, 2009. / An ultrafast pump-probe spectroscopy experiment was developed in order to investigate the fast photoinduced isomerization reaction of the molecule resveratrol. Characteristics of the resveratrol molecule are discussed, including the photoisomerization reaction from trans- to cis-resveratrol. The experimental setup for the conventional spectroscopy measurement was developed and characterized in order to investigate and understand the conventional absorption and uorescence spectroscopy of resveratrol thoroughly. The absorption spectra for both trans- and cis-resveratrol, as well as the uorescence spectra were measured, discussed and explained. This therefore forms a foundation and serves as an initial step to develop a pump-probe spectroscopy experiment for resveratrol. A general overview of ultrafast pump-probe spectroscopy is presented, as well as an explanation of the nal developed experimental setup. The principles and characteristics of the chirped pulse ampli cation (CPA) femtosecond laser source and the tunable noncollinear optical parametric ampli er (NOPA) employed as the pump pulse are discussed. The process of white light continuum (WLC) generation was investigated to utilize as the ultrashort probe pulse. Two white light continuum generation experimental setups were developed and characterized for WLC generation in a transparent medium with the fundamental CPA laser light at 775 nm (in sapphire) and with the second harmonic (SH) of the CPA light at 387 nm (in quartz). A spectrometer was designed, built and characterized in conjuction with a line focus, for simultaneous measurement of the absorption in the pumped, unpumped and reference regions in the sample. In this way the photoisomerization of resveratrol could be measured with temporal resolution as a transient absorption signal. A 420 μg/ml resveratrol solution in ethanol was investigated in this pump-probe spectroscopy experiment and the results obtained are discussed accordingly.

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Trilayered transition metal dichalcogenides such as our sample (1T-TiSe2) have been studied for many years as systems with strong electron-electron and electron-phonon correlations. The main attraction to this family of compound is its potential to exhibit ground state phenomena known as charge density waves whose detailed physical origin has been controversially determined. In this study, we have used an ultrafast femtosecond laser based on a pump-probe technique, namely ultrafast electron diffraction, to investigate these exotic features associated with the crystal. A pump laser pulse photo-excites the crystal from its ground state and the probe pulse (ultrashort electron pulse) takes the snapshot of the evolution of the lattice generating an electron diffraction pattern of the crystal. Hence the dynamical structural behaviour can be observed in time with a subpicosecond temporal resolution. As a hexagonal close-packed structure, its signature is expected to be seen in the diffraction pattern in both a steady-state and electron time-resolved femtosecond electron diffraction. In addition, simulations of electron diffractions pattern for room and low temperature structural data via a software called Simulation and Analysis of Electron diffraction (SAED) have been carried out. Clear signatures of charge density waves were seen at low temperature. / AFRIKAANSE OPSOMMING: Drie-laag oorgangsmetaal dikhalkogeniedes soos ons voorbeeld (1T-TiSe2), word reeds vir baie jare bestudeer as sisteme met sterk elektron-elektron en elektron-fonon korrelasies. Die hoof aantrekkingskrag van hierdie sisteme is die verskynsel van ladingdigtheidsgolwe in die grondtoestand. Die fisiese oorsprong van hierdie ladingdigtheidsgolwe was bepaal te midde van verskeie teenstrydighede. In hierdie studie, maak ons gebruik van die ultravinnige femtosekonde laser gebaseerde aktiveer-interogeer tegniek, genaamd ultravinnige elektron diffraksie (UED) om unicke eienskappe wat met die kristal geassosieer is te bestudeer. In UED wek ’n ultravinnige laserpuls (aktivering) die kristal op vanaf die grondtoestand waartydens n ultravinnige elektronpuls (interogering) ’n foto neem van die evolusie van die elektron diffraksiepatroon wat deur die kristalrooster gegenereer word. Hierdie wisselwerking van die interogerings elektronpuls en die sisteem kan gevolglik teen verskeie vasgetelde tye toegelaat word. Dus kan die dinamiese strukturele gedrag waargeneem word met ’n tydresolusie in die orde van die elektronpuls (sub-pikosekondes). Siende dat die kristal ’n diggepakte-heksagonale struktuur vorm, behoort die kenmerkende diffraksiepatroon daarvan waarneembaar te wees in beide die bestendige diffraksie en femtosekonde elektron diffraksie tegnieke. In hierdie konteks was duidelike tekens van ladingdigtheidsgolwe waargeneem. Benewens was daar ook simulasies uitgevoer om die elektron diffraksiepatrone asook die strukturele data by kamer en lae temperature vas te pen. Die sagteware wat hiervoor gebruik word is genaamd Simulasie en Ontleding van Elektronendiffraksie (SAED) - Simulation and Analysis of Electron Diffraction (SAED).

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.

Conjugated polymers are organic materials which are attractive as optoelectronic devices because they have a combination of broad band emission, high gain, versatility in processing, are ductile and can be electrically pumped. This thesis describes work conducted on such conjugated polymers as amplifier devices. The conjugated polymers used in this thesis were MEH-PPV, F8BT, GP1302 and ADS233YE. The amplifier devices used were grating coupled and end coupling waveguides. Amplification of light was demonstrated and characterised on single and multiple pulses using the grating coupled structure. Single pulse measurements obtained gains of 21 and 17 dB in a 1 mm long waveguide using the conjugated polymers MEHPPV and F8BT. Annihilation rate was also analysed in the single pulse method with MEH-PPV, giving a value of γ ≈ (3 ± 0.1) x 10⁻⁹ cm³/s. Amplification of a single pulse led to demonstrate amplification and the capability of the amplifier to function with multiple pulses, which resulted in F8BT being used as the gain medium. An average gain of 18 dB was obtained with F8BT in a 1 mm waveguide channel. Amplification was also investigated with end coupled waveguides. This led into investigating a suitable material or suitable combination of material for amplification with the waveguides. Switching of an amplified pulse was attempted on F8BT and GP1302 in the amplifier device at 5 kHz. Switching of F8BT was problematic which lead to attempt switching in GP1302 which was a co-polymer of PFO and F8BT. A 70 % switching effect was obtained with GP1302. Gain recovery dynamics of F8BT, GP1302 and ADS233YE was also investigated. These measurements established a switching rate of 500 GHz for GP1302 and ADS233YE, and F8BT showed partial gain recovery indicating the presence of long lived species. Switching was also attempted on a polymer laser. This resulted in a 100 % switched pulse with a combination of weak pump and strong switch pulse of 40 nJ and 2 μJ respectively. And a strong pump and weak switch pulse of 200 and 50 nJ respectively. Temporal delay of the switch pulse relative to the pump pulse resulted in re-timing of the laser output. Amplification and switching of light pulses were also attempted at a higher repetition rate of 50 kHz with F8BT, GP1302, ADS233YE and MEH-PPV. This resulted in strong amplification of light in MEH-PPV and F8BT with gains of 21 and 13 dB respectively in a waveguide length of 422 μm. Weak amplification of light in ADS233YE and GP1302 was also obtained with a maximum gain of 8 and 3 dB respectively. Switching was attempted on MEH-PPV and ADS233YE.

Šios disertacijos tikslas – didelio tikslumo matuoklio, skirto šiluminio lęšio lazerių aktyviuosiuose elementuose matavimams sukūrimas, bei įvairių metodų, skirtų šiluminio lęšio įtakos mažinimui tyrimas. Matavimai buvo atliekami Shack‘o ir Hartmann‘o bangos fronto matuokliu, kuris buvo pritaikytas mažų skersinių matmenų (<1x1 mm) šiluminio lęšio matavimams. Pademonstruotas naujos konstrukcijos asferinis kompensatorius iš esmes mažinantis šiluminio lęšio aberacijų įtaką išilginio diodinio kaupinimo lazeriuose. Atlikti šiluminio lęšio aberacijų matavimai buvo panaudoti kompensatoriaus profilio skaičiavimams. Naudojant asferinį kompensatorių, pagamintą plonasienių dangų užgarinimo būdu, generacijos slenkstis sumažėjo daugiau kaip 3 kartus, taip pat ženkliai pagerėjo lazerio pluošto intensyvumo skirstinys. Darbe nagrinėjamas stiprinamų impulsų energijos didinimo būdas pasitelkiant eliptinę rezonatoriaus modą Yb:IAG lazeryje. Šis metodas leidžia kelis kartus padidinti stiprinamų impulsų energiją, išlaikant dirakciškai ribotus erdvinius pluošto parametrus. Taip pat, surasta lazerio rezonatoriaus konfigūracija, leidžianti palaikyti stipriai astigmatišką modą aktyviajame elemente, ir simetrišką modą lazerio išėjime. Pateikiami šiluminio lęšio matavimų rezultatai „atermalinės“ ir Ng orientacijos Yb:KGW kristaluose. Taip pat, parodėme, kad kristalo galų išsigaubimo įtaka Ng orientacijos Yb:KGW aktyviuosiuose elementuose gali siekti 50%. Atlikti tyrimai parodė, kad išbandyta... [toliau žr. visą tekstą] / In this thesis, analysis of thermal effects and various approaches for their mitigation in diode end pumped ultrafast lasers is presented. Experimental investigations were performed by employing Shack-Hartmann wavefront sensor which was adapted for measurements of thermal lens in diode end pumped lasers. During research, operation of high average power, diode-pumped, Nd:YVO4 laser with aspheric aberration corrector was investigated. Actual thermal lens measurements were conducted in order to design properly shaped aberration corrector that was manufactured using a thin film deposition technology. This allows us to conclude that employment of proper thermal lens aberration compensator allowed for laser threshold reducing and ensured improved output beam quality parameter M2 in wide pump power range. Prospects of output pulse energy scaling in diode end-pumped Yb:YAG laser by employing elliptical mode geometry was investigated. During our experiments, maximum average power in of ~ 5.5 W was obtained at repetition rates of 30-100 kHz, while in CW operation mode the 8 W output power was achieved. In spite of strongly astigmatic thermal lens due to optimized cavity design the output beam exhibits high spatial quality: beam quality parameter M2 in both vertical and horizontal plane was close to unity. Detailed comparative study of thermal effects in Yb doped KGW crystals with different orientation was performed. Measurements confirm, that anisotropic optical and thermal properties... [to full text]

In this thesis, analysis of thermal effects and various approaches for their mitigation in diode end pumped ultrafast lasers is presented. Experimental investigations were performed by employing Shack-Hartmann wavefront sensor which was adapted for measurements of thermal lens in diode end pumped lasers. During research, operation of high average power, diode-pumped, Nd:YVO4 laser with aspheric aberration corrector was investigated. Actual thermal lens measurements were conducted in order to design properly shaped aberration corrector that was manufactured using a thin film deposition technology. This allows us to conclude that employment of proper thermal lens aberration compensator allowed for laser threshold reducing and ensured improved output beam quality parameter M2 in wide pump power range. Prospects of output pulse energy scaling in diode end-pumped Yb:YAG laser by employing elliptical mode geometry was investigated. During our experiments, maximum average power in of ~ 5.5 W was obtained at repetition rates of 30-100 kHz, while in CW operation mode the 8 W output power was achieved. In spite of strongly astigmatic thermal lens due to optimized cavity design the output beam exhibits high spatial quality: beam quality parameter M2 in both vertical and horizontal plane was close to unity. Detailed comparative study of thermal effects in Yb doped KGW crystals with different orientation was performed. Measurements confirm, that anisotropic optical and thermal properties... [to full text] / Šios disertacijos tikslas – didelio tikslumo matuoklio, skirto šiluminio lęšio lazerių aktyviuosiuose elementuose matavimams sukūrimas, bei įvairių metodų, skirtų šiluminio lęšio įtakos mažinimui tyrimas. Matavimai buvo atliekami Shack‘o ir Hartmann‘o bangos fronto matuokliu, kuris buvo pritaikytas mažų skersinių matmenų (<1x1 mm) šiluminio lęšio matavimams. Pademonstruotas naujos konstrukcijos asferinis kompensatorius iš esmes mažinantis šiluminio lęšio aberacijų įtaką išilginio diodinio kaupinimo lazeriuose. Atlikti šiluminio lęšio aberacijų matavimai buvo panaudoti kompensatoriaus profilio skaičiavimams. Naudojant asferinį kompensatorių, pagamintą plonasienių dangų užgarinimo būdu, generacijos slenkstis sumažėjo daugiau kaip 3 kartus, taip pat ženkliai pagerėjo lazerio pluošto intensyvumo skirstinys. Darbe nagrinėjamas stiprinamų impulsų energijos didinimo būdas pasitelkiant eliptinę rezonatoriaus modą Yb:IAG lazeryje. Šis metodas leidžia kelis kartus padidinti stiprinamų impulsų energiją, išlaikant dirakciškai ribotus erdvinius pluošto parametrus. Taip pat, surasta lazerio rezonatoriaus konfigūracija, leidžianti palaikyti stipriai astigmatišką modą aktyviajame elemente, ir simetrišką modą lazerio išėjime. Pateikiami šiluminio lęšio matavimų rezultatai „atermalinės“ ir Ng orientacijos Yb:KGW kristaluose. Taip pat, parodėme, kad kristalo galų išsigaubimo įtaka Ng orientacijos Yb:KGW aktyviuosiuose elementuose gali siekti 50%. Atlikti tyrimai parodė, kad išbandyta... [toliau žr. visą tekstą]

Methods of controlling molecular rotations using linearly polarized femtosecond and picosecond pulses are considered and analyzed theoretically. These laser pulses, typically in the infrared, are highly non-resonant with respect to the electronic degrees of freedom of the molecules and have intensities of &sim; 10^13 to 10^14 W/cm&sup2;. It is shown how these laser pulses can force small linear molecules to align with the direction of the electric field vector of the laser both in the presence of the laser field as well as after the application of a short laser pulse. Recent experiments on laser-induced molecular alignment are modeled and excellent agreement between experiment and theory is found. Additional methods of controlling molecular rotational dynamics are outlined. The first method considers the forced rotational acceleration of diatomic molecules, called the <i>optical centrifuge</i>. Here, the direction of polarization of a linearly polarized laser field is made to smoothly rotate faster and faster. The molecules, which tend to align with the polarization vector of the laser field, follow the rotation of the laser polarization and are accelerated to high angular momentum. The second method considers the control of field-free rotational dynamics by applying phase shifts to the molecular wave function at select times called <i>fractional revivals</i>. At these select moments, an initially localized wave function splits into several copies of the initial state. Adding phase shifts to the copies then induces interference effects which can be used to control the subsequent evolution of the rotational wave function. This same control scheme has a close link to quantum information and this connection is outlined. Finally, a recently proposed method of controlling the quantum dynamics of the classically chaotic kicked rotor system [J. Gong and P. Brumer, Phys. Rev. Lett. 86, 1741 (2001)] is analyzed from a phase space perspective. It is shown that the proposed quantum control can be linked to small islands of stability in the classical phase space. An experimentally feasible variant of this control scenario using wave packets of molecular alignment is proposed. Two applications of molecular alignment are discussed. The first application uses field-free aligned molecules as a non-linear medium for compression of a laser pulse to the 1 fs regime at optical wavelengths. At such durations, these laser pulses contain nearly a single oscillation of the electric field and represent the shortest laser pulses physically achievable for such frequencies. The second application uses molecular alignment to create a sort of gas phase "molecular crystal" which forms a basis for laser-induced electron diffraction and imaging of the aligned molecules. Here, a first laser pulse aligns the molecules in space. A second laser pulse is then used to ionize outer-shell electrons, accelerate them in the laser field, and steer them back to collide with the parent ion creating a diffraction image with sub-femtosecond and sub-Angstrom resolution.