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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

High power semiconductor lasers

Elliott, Stella N. January 2010 (has links)
Using red and near infra-red emitting quantum well and quantum dot based devices I have modelled the nearfield and farfield intensities and distribution in various waveguide structures. I compared the effect of various factors on the power density at catastrophic damage and found the greatest effect from the current pulse length and dot or well nature of the active region, for the first time in the AlGaInP material system. At short pulse length the quantum dot devices achieved a power density of 17 MW/cm<super>2</super> compared to 14 MW/cm<super>2</super> for quantum well lasers, and then proved by electron microscopy and photocurrent spectroscopy not to have reached their limit for mirror damage, but to have failed by other means. I observed the loss of optical power at catastrophic optical mirror damage in real time, applying single, very high current pulses, observing differences in the behaviour of quantum dot, which showed little or no facet damage, and quantum well devices, which showed large amounts of damage, with a resolution of tens of nanoseconds compared to microseconds in the literature. I proposed an explanation for the time taken for the power level to drop, which remained finite at about 200 ns in quantum well devices, in terms of the energy required to melt the observed quantity of damaged material.
42

Physics and performance of InGaAs quantum dot lasers

Pearce, Emma J. January 2005 (has links)
Quantum dot lasers are becoming increasingly technologically important. It is therefore essential to understand the factors affecting their current performance and be able to predict future performance. The gain and unamplified spontaneous emission spectra have been measured for a selection of quantum dot devices and a quantum well device. The quotient of the gain and spontaneous emission spectra were used to calculate the PF spectra and investigate the carrier distribution within the devices. Whilst the quantum well device and devices with one or three layers of dots exhibited characteristics consistent with Fermi-Dirac statistics, devices with more layers dots produced an unusual set of spectra, determined to be due to a non-thermal distribution of carriers in the ground state by looking at the unamplified spontaneous emission spectra. A model was developed to investigate the effects of non-thermal carrier distributions on the calculated PF spectra. From this it was deduced that it was possible to use a fit of a thermal PF to the excited state PF to calibrate the measured unamplified spontaneous emission spectra. The resultant PF, gain and spontaneous emission spectra are sensitive to the exact balance between the homogeneous and inhomogeneous broadenings. This calibration was used to calculate the radiative current densities and compare the radiative efficiencies of different structures, including both Dots-in-Well (DWELL) and standard dot structures. There was no large difference in efficiency found due to improved carrier injection in the DWELL structures. Calculated gain-radiative current density curves were used to predict the minimum transparency and threshold current densities that may be possible in the future. It is clear that the limits of quantum dot device performance have not yet been reached and that a factor of 1.7 improvement in threshold current density over state of the art devices could be achieved, even without reduced inhomogeneous broadening.
43

Optoelectronic properties of InAs GaAs columnar quantum dot laser diodes

Mexis, Meletios January 2008 (has links)
In this thesis results are described with the aim of examining the optoelectronic properties of InAs/GaAs columnar quantum-dots and comparing them with those of more conventional self-assembled quantum-dots. The polarisation properties of a set of columnar quantum-dot samples &mdash; of varied aspect ratio and In compositional contrast between the rod-shaped dot and the surrounding 2-D layer &mdash; are studied. For this investigation a new method to obtain the ratio of the fundamental TE/TM optical response using edge photo-absorption spectroscopy is proposed, which corrects for the polarisation-dependent features of the experimental set-up. The method is verified by application to compressive and tensile strained InGaP quantum well structures, where the results are in agreement with known ratios of the band-edge matrix elements. When applied to columnar quantum-dot samples it is shown that the TE/TM optical response depends on the dot aspect ratio and the In compositional contrast. A polarisation-independent photo-absorption is illustrated for a columnar quantum-dot of an aspect ratio (dot's height over diameter) 3.51:1, which is desired for use in semiconductor optical amplifiers. For the columnar dot of an extremely high aspect ratio, 7.5:1, a room temperature TM-dominant polarisation lasing emission is observed. By studying the Quantum-Confined Stark Effect, a dramatic enhancement of the Stark shift amplitude is shown for columnar quantum-dot samples of an increased dot aspect ratio from 0.63:1 up to 1.12:1, which may have application in optical modulation/switching. For a higher aspect ratio columnar quantum-dot the shift of the band edge of the photo-absorption spectra is reduced dramatically and this has been attributed to an overall effect where the observable shift becomes the result of higher energy transitions, where their oscillator strength change very rapidly within the studied range of field. For the highest aspect ratio dot, i.e. of aspect ratio 10:1, there is no any observable shift.
44

Polariton optics of spherical nanostructures

Smith, Andrew January 2005 (has links)
This thesis describes the completely coherent polariton optics of a spherical semiconductor photonic dot (PD). Our theoretical model solves the eigen-modes of the system to produce the polariton frequencies as a function of the PD radius, o (a), where the photonic modes with A = 2a are resonant with the sphere. Dispersion curves are classified into the weak and strong coupling regimes according to crossing/anti-crossing of the polariton branches, in analogy with polaritons in bulk materials. We assume the coherent distribution of energy between the photonic and excitonic branches in order to calculate the total radiative linewidth. The effect of spatial dispersion of excitons is considered using the dispersion equations developed by Ruppin and the Pekar Additional Boundary Condition. The asymptotic values of the resulting dispersion curves are described, and plots of the dispersion curves for a CuCl PD show low-order modes (n = 1 and 2) to be in the weak coupling regime, whilst n = 3 and 4 are in the strong regime. Our analysis of the radiative linewidth, both including and excluding spatial dispersion, produces a 1/a dependence for Tn for large radii, corresponding the "ballistic escape" of optically-dressed excitons from the PD. The low-radius behaviour, usually described as an a3 volume-dependent increase of the oscillator strength, is shown to be more complex with powers from a1 - a7 depending on the mode polarisation and the presence/absence of spatial dispersion. The transition between weak and strong coupling regimes is identified as a discrete point at which the two polariton branches meet, characterised by critical values of the Rabi frequency, dielectric permittivities of the PD and surrounding material, and the PD radius, which have been obtained analytically and numerically. We propose the observation of the transition point via derivatives of the radiative linewidth, using high-precision modulation techniques.
45

Model for localised recombination in quantum dots

Pask, Helen J. January 2006 (has links)
A new generation of diode lasers is being developed using quantum dots as the gain generating medium. A detailed understanding of the carrier recombination mechanisms and optical gain generation is essential for optimisation of their performance. The aim of this work is to further understand the optical processes occurring in quantum dots. In particular, the effect of the localisation of the energy states in the dots on the recombination mechanisms and the gain/absorption is studied. It is often assumed that the rates of nonradiative recombination via defects, radiative recombination and Auger recombination are proportional to linear, quadratic and cubic functions of the carrier number respectively. The derivation of these functional forms is possible in quantum well and bulk structures because the extended electronic states make it meaningful to talk of a global carrier population. In a quantum dot system the dependence of the recombination processes on the total number of electrons populating the dots is modified by the localisation of all the recombination processes. In this thesis a computer model has been developed in which the dots are occupied by integer numbers of electrons and holes, with electron and hole occupancies controlled by Fermi-Dirac statistics. The recombination processes have similar dependences on the electron number and there is no clear transition from one process to another as the injection level is increased. These dependences cannot be represented by simple power law functions of the carrier number. An alternative model, in which each dot is electrically neutral, has also been studied, and the two models show significant differences for the hole distribution as the injection is increased. It is found that analyses based on power law relations between recombination rates and carrier number, as used for extended state systems, cannot be applied to localised recombination in dots.
46

A light scattering study of colloid-polymer mixtures

Pirie, Angus D. January 1995 (has links)
A detailed light scattering study of non-equilibrium states found in a model colloid-polymer mixture is presented. Conventional light scattering is used to examine the average structure of the phase, over a wide range of wavevectors. For all non-equilibrium samples a 'ring' of scattered radiation is found at small angles, whose temporal evolution is used to investigate aggregation kinetics and the growth of structure. For low concentrations of colloid and just sufficient polymer to induce non-equilibrium behaviour a small angle ring is observed after a initial 'lag time', which remains stationary and brightens rapidly, behaviour reminiscent of classical nucleation. At higher colloid concentrations one finds a continuously collapsing and brightening ring, similar to that found in classical spinodal decomposition. Upon the addition of further polymer the system gels and the small angle ring becomes arrested at a finite wavevector. Direct visual observation of these different regimes is also presented, via video-enhanced microscopy and direct time-lapse video recordings, to complement the light scattering data. The particle dynamics within the aggregates are measured using dynamic light scattering, The results obtained suggest that the onset of non-equilibrium behaviour is caused by the presence of a metastable gas-liquid phase boundary, which can be calculated using a recently-developed mean-field theory.
47

Measurement uncertainties in fibre-coupled spectrographs

Lemke, Ulrike January 2012 (has links)
The signal quality of fibre-coupled spectrographs can be limited by the inherent properties of the optical fibre. This is especially the case for applications that require high signal-to-noise performance and high spectral resolution. Examples include metallicity and age of star clusters, as well as investigations of Lyman-alpha absorbers. Extra-solar planet research in particular encounters its limitations due to the non-repeatability of the fibre response. Initially, a limited signal quality due to fibres seems counter-intuitive, since one of the most remarkable advantages of fibres is their signal stabilizing property, called image scrambling, which refers to the effect that the fibre output signal is largely insensitive to variations at the input side. However, the fibre photometric and barycentre response is sub ject to external parameters like stress, seeing and guiding variations. State-of-the-art instrumentation has attained a level of sensitivity where these effects will impact upon instrument performance, especially when advancing to a regime of spectral resolving powers where the quantized character of the standard optical fibre can be resolved, which manifests itself in modal noise. Unprecedented effort will be required in order to accomplish high resolving powers in the spectral and spatial domains with 40 m class telescopes. It is therefore essential to predict these fibre-related measurement uncertainties so that the performance of current and future instruments can be optimized. This thesis starts out with a phenomenological description of the different effects that give rise to fibre-related noise and its influence on the observables relevant to astrophysics, such as barycentre and photometric stability. Special emphasis is given to the photometric uncertainties related to modal noise, where first a theoretical model is outlined which in later chapters will be sub ject to experimental investigations. Subsequently, the barycentre repeatability due to incomplete scrambling is the subject of detailed investigation. The remaining sources of noise are estimated using experimental data as well as simulations and put in contrast with the other effects. Alongside the quantitative prediction of instrument instabilities, mitigation strategies will be presented and discussed. I conclude with a brief discussion of the impact of incomplete scrambling and modal noise on current instrumentation, the implications for future instrument pro jects as well as future work that will help to further understand and obviate the underlying mechanisms.
48

Rydberg dark states in external fields

Tanasittikosol, Monsit January 2011 (has links)
We initially discuss the theory of three-level systems using the dressed state formalism. One of the dressed states, containing a ground state and a Rydberg state, does not couple with the probe laser; thus, the medium becomes transparent to the latter. This phenomenon is known as electro- magnetically induced transparency (EIT) and this dressed state is known as a Rydberg dark state. We show that EIT can be used to extract the reduced dipole matrix element for transitions to a Rydberg state. However, a prob- lem with three-level Rydberg EIT in a vapour cell is the occurrence of space charges caused by photoelectric ionisation of Rb metal deposited inside the cell. To avoid this problem, we consider adding a third laser resonant with a fourth level. This is to avoid using the laser whose wavelength is less than the threshold wavelength. In cold atoms, the effect of the third laser is to split the usual EIT resonance into a doublet. In thermal atoms, we observe narrow features due to electromagnetically induced absorption and electro- magnetically induced transparency in the Doppler-free configuration. Next we consider the action of a far off-resonance radio frequency (rf) field in the three-level system. We demonstrate the formation of rf-dressed EIT reso- nances in a thermal Rb vapour and show that such states exhibit enhanced sensitivity to dc electric fields compared to their bare counterparts. Fitting the corresponding EIT profile enables precise measurements of the dc field in- dependent of laser frequency fluctuations. We further investigate the theory of rf-dressed Rydberg EIT using the Floquet approach in order to understand the formation of the sideband structure of the Rydberg state. We find that if the time scale of the rf interaction is much shorter than that of the system evolution and decoherence, the sideband structure is well resolved. We also show that the intermediate state exhibits a sideband structure, induced by the Rydberg state, when the Rabi frequency of coupling laser is larger than twice the modulation frequency. Finally we consider resonant microwave cou- pling between the Rydberg states which leads to an Autler-Townes splitting of the EIT resonance in cold atoms. This splitting can be employed to vary the group index by ±10^5 allowing independent control of the absorptive and dispersive properties of the medium, i.e., one can switch the transparency of the medium or control the group velocity of a pulse propagation by tuning on and off the microwave field.
49

Laser spectroscopy of wire-like molecules

Sanders, Scott Edward January 2012 (has links)
Cavity-enhanced Laser-Induced Fluorescence With the incorporation of a new optic mount and focussing lens a cavity ring-down set-up was altered to collect fluorescence signal from molecules seeded in a molecular beam. Running both the CRDS and fluorescence measurements simultaneously produced a technique with significantly enhanced sensitivity compared to either of the two individual methods. Preliminary analysis with SO2 and a selection of polyaromatic molecules point to the method been best suited for measuring the fluorescence spectra of fast fluorescence lifetime (< 1 ns) molecules and in confined environments i.e. molecular beams. This has been characterised using a known sample and also compared to the CRDS and fluorescence measurements ran as individual techniques. The method was found to increase the sensitivity of the current experimental set-up by two orders of magnitude. This was then applied in the analysis of two new molecular samples to obtain spectra for use in analysing the torsional motions within the molecule. Torsional Motion Analysis of 2,5-bis(phenylethynyl)thiophene Torsional motions in polyphenyls, potential molecular wires, are important in the charge carriage capability of the species. Using a combination of UV-cavity ring-down and fluorescence detection the absorption and photoexcitation spectra of 2,5-bis(phenylethynyl)thiophene were successfully measured. Using a simple cosine potential and simulation program the spectra were reproduced and were found to be close to the values determined in previous research on the two ring system tolane and 1,4-bis(phenylethynyl)benzene.
50

Ultrafast processes and excited state dynamics in conjugated polymers

King, Simon January 2008 (has links)
The photophysics of a number of conjugated polymers used in organic display applications has been studied using ultrafast techniques. An updated method for measuring the intersystem crossing rate was developed. This has been used to measure the intersystem crossing in a number of conjugated polymers and oligomers. It was found that as expected the intersystem crossing in conjugated polymers is generally very low. The intersystem crossing in a molecule that exhibits dual fluorescence depending on the polarity of the environment was investigated. In polar solvents when the molecule forms a twisted interconnected charge transfer state the intersystem crossing rate was found to be ten times that of the planer locally excited state found in non-polar media. The nature of charge generation in conjugated polymers has also been investigated, it was found that in the prototypical conjugated polymer, polyfluorene, there is no intrinsic charge photogeneration. However, charges can be formed by singlet-singlet annihilation reactions. This is made very efficient in the solid state by the rapid diffusion and migration of excitons. The photophysics of a novel polyfluorene, polyspirobifluorene, was investigated thoroughly; it has been found that the addition of the spiro side group containing electron-donating groups causes a charge transfer state to form between the backbone and the side group. This can act as a reservoir for the emissive singlets, thus giving the polymer a long tail in the fluorescence decay. Finally, ultrafast pump probe spectroscopy was used to investigate on chain migration of excitons in dilute solutions of a ladder type polymer. The migration to the lowest energy segment of the chain shows up as a shift to the red of the photobleaching signal in the pump probe spectrum. This shows that the low energy states become occupied after the initial excitation, as the high energy states become depopulated by the migration, this process taking about 6.2ps.

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