| 1. |
- Ekerfelt, Henrik
(författare)
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Numerical and Experimental Studies of Wakefield Accelerators
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is based on work done by the author on the development of laser wakefield accelerators.Wakefield acceleration in plasmas is a promising technique to provide the next generation of accelerating structures and particle beams. Plasmas can sustain electric fields that are many orders of magnitude stronger than those possible in conventional accelerators. Other benefits of wakefield accelerators are that electron beams produced inside the plasma can be generated with high peak current and ultra-low emittance.These strongly accelerating structures can reduce the size of particle accelerators, making them more available, for example in hospitals, or to increase the energy in particle colliders.In wakefield acceleration, a driver is used to excite a plasma wave.The acceleration of charged particles takes place in a plasma wave excited by, and co-propagating with, the driver. The driver can be a laser pulse or a bunch of charged particles.However, many technical challenges remain to be solved before a reliable particle source can be realized based on this technology.This thesis describes numerical studies performed using particle-in-cell simulations and experimental work using high-intensity laser pulses, with the aim of improving our knowledge on wakefield accelerators. The work presented here focuses on three different topics: trapping mechanisms, achieving higher electron energies and improvement of the betatron X-rays generated.In particular, trapping in a density down-ramp, ionization induced trapping, and trapping by colliding pulses have been investigated numerically and experimentally.A novel guidance technique for high-intensity laser pulses is suggested, the merging of two laser wakefields is experimentally demonstrated and suggested as a possible means of staging wakefield accelerators, and the possibility of carrying out a beam-driven plasma wakefield experiment is investigated through simulations. An improved X-ray source based on laser wakefield acceleration and enhancement of the betatron oscillations through direct laser acceleration is investigated and two applications are demonstrated.
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| 2. |
- Neoricic, Lana
(författare)
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Generation and metrology of ultrashort pulses and their application in attosecond science
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the dynamical processes in atoms and small molecules initiated by the absorption of ultrashort, coherent light pulses. The studied phenomena takeplace on the femtosecond (1 fs = 10−15 s) and attosecond (1 as = 10−18 s) timescales and critically depend on the properties of the light fields that drive them. Wework with infrared (IR) femtosecond laser pulses, which we manipulate through nonlinear interactions with matter to either study these interactions themselves or applythem to investigate other light-induced processes.One part of this thesis focuses on the generation and characterisation of IR pulses spectrally broadened through the Kerr effect. We use a technique called dispersion scanto temporally compress and at the same time measure pulses broadened in gas-filled hollow-core fibres. We propose multiple improvements to this well-established characterisation technique. Further, we investigate femtosecond filamentation in gases, a process with highly complex dynamics involving several non-linear processes including the Kerr effect and ionisation. We develop a method that allows us to measure the electric field of a laser pulse undergoing filamentation in three dimensions, whilealso scanning along the filament length. Our technique provides access to pulses with desirable characteristics that may be generated at a point inside the filament, simultaneously enabling their measurement and extraction for applications. In addition, this technique opens up the possibility to explore intricate filament dynamics.In the other part of this work, we up-convert the IR laser pulses into trains of extreme ultraviolet (XUV) attosecond pulses through a non-linear process called high-orderharmonic generation. We combine the IR and XUV pulses to study the photoionisation dynamics in different species using a method known as RABBIT (Reconstructionof Attosecond Beating By Interference of Two-photon transitions). In this technique, a target gas is ionised by the XUV field, creating an electron wave-paket (EWP) in thecontinuum, while a weak IR pulse probes the system. The EWP scatters off the ionic potential, acquiring an additional phase as it propagates. Recording the photoelectronspectrum as a function of the IR-XUV time delay allows us to infer time-resolved information about the ionic potential. We apply this method to investigate the dynamicsof different ionisation processes in noble gases (He, Ar, and Xe) and the N2 molecule. The high spectral resolution of our electron spectrometer allows us to disentanglethe contributions from different ionisation channels. In addition, we perform angle-resolved measurements, investigating the coherent superposition of final stateswith different angular momenta.
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| 3. |
- Permogorov, Alexander
(författare)
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Target and Laser Pulse Optimization for Laser-Driven Ion Acceleration
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The research presented in this thesis is primarily focused on experimental investigations of laser-driven ion acceleration from solid targets via the target normal sheath acceleration mechanism. In particular, ways of optimizing the absorption of the laser pulse energy by free plasma electrons in the target, or modifying the shape of the accelerating electron sheath were addressed. The aim of this work was to increase the efficiency, and maximum proton energy that could be obtained with a given laser system, and to reduce the divergence of the beams of accelerated protons.The shape of the electrostatic sheath was indirectly influenced by using laser micromachining to modify the front surface of the target, on which the laser pulse is incident. The absorption of the laser pulse was enhanced by either placing nanostructures on the front side of the foil target, or by manipulating the temporal profile of the ultrafast part of the laser pulse before its interaction with an ultrathin target.It is important to ensure the survival of the target by using a laser pulse with very high temporal contrast. A double plasma mirror (DPM) was designed and implemented for this purpose. Design considerations and the optimization of the performance of the DPM, which is now used routinely at the Lund High-Power Laser Facility during laser-solid interaction studies, are discussed. Sufficiently high temporal contrast was achieved, and an increase was seen in the maximum proton kinetic energy when using targets with nanowire and foam structures on the surface. Efficient ion acceleration from ultrathin targets with a thickness down to 10 nm was observed as well.When an ultrafast laser pulse interacts with an ultrathin foil, the temporal shape of the electric field of the pulse affects the laser--solid interaction, and a slightly positively chirped pulse was found to increase the maximum kinetic energy of the accelerated protons.Laser-solid interactions at very high intensities are known to have shot-to-shot instabilities, motivating the use of single-shot diagnostics. The ion spectra in the forward direction were recorded using a Thomson parabola spectrometer, and in the backward direction with a magnetic dipole spectrometer. The intensities of the reflected and transmitted fractions of the laser pulse were also recorded on a shot-to-shot basis. In addition, a proton spatial profile monitor could be inserted to spatially characterize the proton bunch accelerated in the forward direction.
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| 4. |
- Enquist, Henrik
(författare)
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Studies of Phonons and Phase Transitions by Time-Resolved X-ray Diffraction
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- When light is used to study structures, the wavelength limits the size of the details that can be resolved. Visible light can be used to investigate structures as small as a micrometre in size. To study smaller structures a shorter wavelength is required. The wavelength of hard X-ray radiation is much shorter than that of visible light, and is comparable to the distance between atoms in solids and liquids, which is a few tenths of a nanometre. X-ray diffraction can thus be used to study structures on the atomic scale, and by conducting the measurements with high time resolution the atomic motion can be mapped. The absorption of intense ultrashort laser pulses in solid materials can trigger a multitude of processes. Heating arising from the deposition of energy leads to rapid expansion, creating coherently excited phonons. The properties of the material are changed by the large number of excited carriers, for instance enhancement of the heat conductivity. Very intense pulses may even induce melting. Time-resolved X-ray diffraction can then be used to directly measure the changes in the structure. As the material cools down the molten material solidifies, and the surface can develop sub-micrometre periodic structures. Not only laser pulses induce structural changes. Electric pulses can generate largeamplitude acoustic pulses in piezoelectric materials, which can then trigger phase transitions between different solid structures. Using time-resolved X-ray diffraction it is possible to follow the transition in real time. The work described in this thesis has been focused mainly on experimental studies of structural dynamics with picosecond time resolution. The results obtained have, among other things, helped in the understanding of the various processes involved in the melting and subsequent regrowth of semiconductors, as well as the dynamics of the photocarriers following intense laser excitation.
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| 5. |
- Abrahamsson, Christoffer
(författare)
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Time-Resolved Spectroscopy for Pharmaceutical Applications
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis aims at improving spectroscopic techniques for analysis of pharmaceutical solids. This improvement is achieved by the combining and development of instrumentations and data evaluation tools from two research fields: the fields of near infrared spectroscopy and biomedical optics. The thesis include the construction and evaluation of different variable selection techniques. Variable selection is an important tool used to improve the evaluation of spectroscopic data. Variable selection was applied to near infrared data from pharmaceutical tablets and mid-infrared data from atmospheric gases. A novel instrumentation, using a photonic crystal fibre for light generation and a streak camera for detection, was developed, tested and used for time-resolved measurements. The system covers a wavelength range from 500 to 1200 nm and the time-resolution was measured to be 30 ps. The system proved to be very versatile and was used both for reflectance and transmission measurements. New evaluation schemes for time-resolved data were developed. A new algorithm, based on diffusion theory was evaluated using time-resolved data required on apples. The same data was used to study the performance of a data evaluation scheme based on diffusion theory combined with least square support vector machines. Both tested evaluation schemes showed results comparable with results computed by the conventional evaluation scheme based on diffusion theory. Time-resolved measurements were conducted on pharmaceutical solids. Quantitative analysis of intact tablets using time-resolved data was superior to analysis made using conventional near infrared data, especially when the difference in physical properties between the measured samples were large. An analysis method combining time-resolved spectroscopy and conventional near infrared spectroscopy was also developed. The scheme allows evaluation over a larger wavelength range than the one covered by the time-resolved system. The maximum range is only limited to the range covered by the near infrared spectroscopic instrument. The work also takes the first step toward the construction of a bench-top system, showing that the data from a simplified time-resolved system would still give valuable results.
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| 6. |
- Heyl, Christoph
(författare)
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Scaling and Gating Attosecond Pulse Generation
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-order harmonic generation (HHG) provides the basis for attosecond light sources delivering coherent pulses in the extreme ultraviolet spectral region. Such light sources are employed for a variety of applications within imaging, attosecond spectroscopy, and high-precision frequency metrology. However, the rather low efficiency of the HHG process, which implies a limited pulse energy and repetition rate, places restrictions on many applications. In this thesis, the scaling of different parameters controlling the generation conditions for HHG in gases is analyzed. A general scaling model is developed, which allows scaling of the pulse energy and repetition rate of attosecond sources over many orders of magnitude, while maintaining temporal and spatial pulse characteristics. The scaling model is applied to different attosecond beam lines, which were developed and built as part of this thesis work. This includes a high-repetition rate (200\,kHz) beam line used for photoelectron emission microscopy applications, and an intense harmonic beam line delivering pulses with up to 3 µJ in the extreme ultraviolet, which was used for coherent imaging as well as for nonlinear spectroscopy applications. In addition, microscopic sub-cycle control mechanisms based on multi-color field synthesis are studied, as well as noncollinear generation geometries. It is shown that a noncollinear geometry can be used to angularly streak attosecond pulse trains, allowing access to single pulses within the train. This technique is of interest for attosecond pump-probe measurements as well as for isolated attosecond pulse generation inside an optical cavity, a scheme that promises attosecond pulses at unprecedented power levels and repetition rates.
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| 7. |
- Hoflund, Maria
(författare)
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Focusing properties of attosecond pulses
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The discovery of high-order harmonic generation (HHG) has made it possible to see the electron dynamics on their natural time scale, attoseconds. In the HHG process, a femtosecond infrared (IR) laser pulse is converted into a train of attosecond pulses in the extreme ultraviolet (XUV), but with a very low conversion efficiency. The low intensity has to be used efficiently and that means keeping the photons compact in both space and time at the interaction with the matteryou want to study. In this thesis, the focusing properties have been measured experimentally of high-order harmonics generated in Ar using a multi terawatt laser system in a loose focusing geometry. A few micrometre focus is obtained using focusing optics with a high demagnification factor. The waist size and position of the individual harmonics have been measured using a knifeedgetechnique and the SWORD (spectral wavefront optical reconstruction by di↵raction) technique, while varying generation parameters. The measurements show that the harmonic focus positions are spread out along the propagation axis, which consequently decreases the peak intensity and increases the pulse duration due to the local narrowed bandwidth. However, there is a way to improve the focus by choosing the right generation parameters.
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| 8. |
- af Klinteberg, Claes
(författare)
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On the use of light for the characterization and treatment of malignant tumours
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Some aspects on the use of light for tissue characterization and treatment of malignant tumours are discussed within this thesis. The work presented aims at improving optical techniques for tissue characterization, especially to detect malignant and pre-malignant lesions. A knowledge on the interaction between light and tissue is of utmost importance to understand and improve the various techniques. A relatively thorough discussion on the light propagation in scattering media is given. A commonly used mathematical model, the diffusion approximation of the transport equation, is derived. Moreover, knowledge of the optical properties of tissue, i.e., the absorption and scattering coefficients, the scattering anisotropy, and the refractive index, is needed to use the mathematical models. Various techniques for in vivo measurements of the tissue optical properties are presented. Furthermore, some of these techniques have been developed into prototype equipment to be used for breast cancer detection, as an alternative to the ordinary mammography based on ionizing X-rays. A therapeutic modality, photodynamic therapy (PDT), presently being introduced into clinical practice has also been investigated. This technique relies on the selective uptake of a photosensitizing agent, and the subsequent irradiation using light. The light absorbed by the photosensitizer triggers a photochemical reaction, leading to local cell death. In the work presented here, d-aminolevulinic acid (ALA) induced protoporphyrin IX (PpIX) has been used as a photosensitizer. A randomized Phase III clinical trial has been conducted to compare PDT with cryosurgery for the treatment of basal cell carcinomas of the skin. The influence on the treatment of various parameters have been investigated and are discussed. Laser-induced fluorescence studies were performed to detect and demarcate superficial malignant and pre-malignant lesions. Both the tissue autofluorescence and the characteristic emission of fluorescent tumour markers were employed. Fluorescence was also used to monitor the selective buildup and the photodegradation of the photosensitizer in connection to PDT, using a point-monitoring technique. Two fluorescence imaging systems have also been used to outline skin lesions. Laser-Doppler perfusion imaging is a non-contact optical technique used to monitor the superficial blood perfusion, and was here used to evaluate the healing time following cryosurgery and PDT.
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| 9. |
- Aldén, Marcus
(författare)
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Application of Laser Techniques for Combustion Studies
- 1983
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim for this work has been to increase the applicability of laser spectroscopy techniques for studies of combustion processes, both what regards in-flame experiments, exhaust-gas analysis and remote sensing of the atmosphere. Raman spectroscopy has been used for analysis of exhaust gases from flames, model fires of wood and for inflame measurements. Coherent anti-Stokes Raman Spectroscopy CARS, has been used for detection of several flame constituents e.g. o2 , CO, H2o, CH4 and H2 . Flame temperatures are measured using CARS spectra from N2 molecules. The CARS technique has also been used for simultaneous detection of several species e.g. N2/CO, co2;o2 and CO/H2 . The applications of broadband rotational CARS have also been demonstrated in non-flame gases, whereas scanning rotational CARS has been used for flame experiments. Laser-induced fluorescence LIF is especially attractive for radical detection, and LIF spectra for several flame radicals are presented, e.g. OH, CN, CH and c2 as well as relative concentration profiles for different radicals as a function of height above the burner. Special emphasis has been paid to space-resolved detection of radicals in flames, e.g. OH using a diode array detector. In a refined experiment, both c2 and OH were spatially detected using two laser systems. Two-photon excitation is a rather new and fascinating approach for detection of flame species that absorb in spectral regions not accessible for laser sources. In this way oxygen atoms have been detected in an acetylene/oxygen flame. Closely connected to combustion studies using laser methods is remote sensing of pollutants in the atmosphere using laser 7 techniques. These techniques have been used for laboratory experiments and in real-world measurements. E.g., NO has been detected using long-path absorption, whereas remote detection of Hg atoms has been performed using the dial technique.
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| 10. |
- Allaf Navirian, Hengameh
(författare)
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Structural Studies Of Materials Using Time-Resolved X-ray Diffraction
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work consisted of the application of X-ray diffraction methods to the study of ultrafast phenomena in various materials. Since the X-ray pulses generated from the bending magnet at the MAX II ring, where most of the experimental work was conducted, have a duration of the order of 300 ps, they can not be used directly as probe pulses in experiments where higher temporal resolution is required. A streak camera used as the detector was tested and improved to be able to resolve ultrafast processes. Another aspect of this work was the development of an experimental method for ultrafast time-resolved measurements at a high repetition rate. Various experiments were performed to further the development of this method, such as the investigation of the accumulated damage to the surface of InSb after repetitive ultrafast melting. Therefore laser-exposed InSb surfaces were studied post-mortem using different microscopy techniques. Time-resolved X-ray diffraction in a high-repetition-rate configuration was then applied in a study of liquid scattering from molten InSb. The experiment provided insight into the non-equilibrium liquid state of InSb. Studies of acoustic waves created by non-thermal melting of InSb due to rapid changes in the density were also conducted. The ferroelastic switching between different structures in the ferroelectric phase of potassium dihydrogen phosphate (KDP) was observed via time-resolved X-ray diffraction. This material undergoes a phase transition from the paraelectric phase above the Curie Temperature (Tc) to the ferroelectric phase below Tc. An alternative method of initiating ultrafast structural changes in a ferroelectric material was studied at the Stanford Linear Accelerator Centre, where THz radiation was used as a pump, and femtosecond laser pulses as the probe, in order to study the nonlinear response of ferroelectric material to THz radiation.
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