This text investigates THz-TDS signals in the time-frequency domain. Addi- tionally this text discusses the prospects of using time-frequency analysis to alleviate distortion in THz spectrographic characterizations induced by back- reflections in the free space electro optic sampling used in the THz time-domain spectroscopy detection scheme. THz time domain spectroscopy is a technique for characterization of materials in the terahertz regime. The THz regime offers interesting properties of materials such as strong phonon-photon interaction and resonances for vibration states of molecules. Three time-frequency representations are compared for the analysis of the time-domain signal, the short-time Fourier transform, the Wigner-Ville transform and the continuous wavelet transform. It is concluded that the Wigner-Ville transform is most suited for analysis of the spectral properties of a single pulse due to the Wigner-Ville transforms inherit high spectral resolution. The continuous wavelet transform is most suited for analysis of the time-domain signal since it has no cross-term interference as compared to the Wigner-Ville transform. By masking the continuous wavelet transform with a Lorentzian time-frequency mask the back-reflections are dampened and the resolution of the characterization is improved.

Potassium (K) is the main ash-forming element released from biomass during thermochemical conversion. A better understanding of K chemistry and monitoring of K species is needed to optimize combustion systems. Since K species are highly reactive and prevailing concentrations depend on the conversion conditions, accurate quantification requires in situ measurement techniques. Tunable diode laser absorption spectroscopy with a single-mode distributed feedback laser is used to probe the D1 transition of atomic potassium, K(g), at 769.9 nm. The large current tuning range of the diode laser (5 cm-1) enables monitoringthe wings of the absorption profile. Fitting to the acquired line shape wings is used as astrategy to enhance the dynamic range of the sensor and measure K(g) concentrations even under optically thick condition. A potassium-rich combustion environment is simulated by converting KOH salt in a premixed methane/air flat flame. Quantitative measurements of K(g) are made at 75 positions in the flame. This yields radial K(g) profiles at three different heightsin the plume above the KOH salt and an axial profile at the burner center. The acquired average K(g) concentrations are corrected for effective plume size, i.e. the absorption pathlength determined from the radial profiles. Knowledge of the K(g) distribution in flames can lead to a better understanding of K release and primary reaction kinetics.

This Dissertation describes how I used a three nanograting Mach-Zehnder atom beam interferometer to precisely measure a wavelength of light, known as a tune-out wavelength, that causes zero energy shift for an atom. I also describe how such measurements can be remarkably sensitive to rotation rates. It is well known that atom interferometry can be used to measure accelerations and rotations, but it was a surprise to find out that tune-out wavelength measurements can under certain conditions be used to report the absolute rotation rate of the laboratory with respect to an inertial frame of reference. I also describe how we created conditions which improve the accuracy of tune out wavelength measurements. These measurements are important because they serve as a benchmark test for atomic structure calculations of line strengths, oscillator strengths, and dipole matrix elements. I present a new measurement of the longest tune-out wavelength in potassium, λzero = 768.9701(4) nm. To reach sub-picometer precision, an optical cavity surrounding the atom beam paths of the interferometer was used. Although this improved the precision of our experiment by increasing the light-induced phase shifts, the cavity also brought several systematic errors to our attentions. For example, I found that large ±200 pm shifts in tune-out wavelengths can occur due to the Earth's rotation rate. To solve this problem, I demonstrated that controlling the optical polarization, the magnetic field, and the atom beam velocity distribution can either suppress or enhance these systematic shifts. Suppressing these systemic shifts in tune-out wavelengths is useful for precision measurements used to test atomic structure calculations. By enhancing these systematic shifts, the interferometer can be a gyroscope that utilizes tune-out wavelengths.

In my thesis I describe the improvements of the Penning trap mass spectrometer SMILETRAP. The objective of these improvements have been to increase the reliability and the accuracy with which an atomic mass can be measured using highly charged ions. The improvements have been achieved by stabilizing both the electric and magnetic fields of the trap and by improving the technical performance of the trap system. As a result it has been possible to measure accurately the mass of several atoms ranging from hydrogen to mercury using charge states from 1+ to 52+. It was only possible to use the highest charge states after applying a successful cooling of these ions with Helium during the charge breeding. The technical improvements made a number of interesting accurate mass measurements possible. The measurements of the 3H, 3He, and 4He masses showed that the previously values were wrong. The mass difference between 3H and 3He which is the Q-value of the tritium beta decay has been determined to 18.588(3) keV. The Q-value of the double β-decay of 76Ge was measured at an accuracy of 50 eV. This value is indispensable for the evaluation the Heidelberg-Moscow experiment which aims at finding a possible neutrino-less decay which if present would be a violation of the standard model. The mass ratio of mCs/mp is used to determine the fine structure constant independent of QED calculations. The two decades old anomaly in the mass values of Hg was solved by the mass determination of 198Hg and 204Hg. The mass of 24Mg was measured at an uncertatinty of 0.6 ppb and will be used in the determination of the g-factor of a bound electron in a hydrogen like ions.

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Due to the unique combination of properties of high intensity and few optical cycle pulse duration, the ultrashort laser pulses have widespread applications in pump-probe spectroscopy and laser-plasma interaction. Correspondingly, it is critical to precisely measure the electric field in the temporal or spectral domain. Since there are no shorter pulses available to sample them in time, it is not easy to characterize few-cycle pulses. In this thesis, the temporal characterization methods including second harmonic generation (SHG) interferometric autocorrelation, single-shot SHG frequency-resolved optical gating (FROG) and chirp scan are used to measure the laser pulses at different positions in our laser system, the Light Wave Synthesizer-20. The SHG interferometric autocorrelation measured a 25.3 fs pulse duration after the kHz frontend laser with a 24.5 fs Fourier limit (FL). The FROG is tested by measuring seed pulses with a 6.5 fs FL providing 6.7 fs pulse duration with mostly flat phase. Then, this FROG apparatus is used to characterize the amplified pulses with a 4.5 fs FL after the whole laser system. The measured pulse duration is 4.5 fs reaching the FL. Similarly, the chirp scan is also used to measure the amplified pluses with a slightly different spectral phase, which retrieved a longer pulse duration of 5.2 fs. The second part of the thesis is focused on contrast improvement. The temporal intensity contrast is reduced in the amplification process leading to a deteriorated laser-plasma interaction. Contrast improvement based on the nonlinear elliptical polarization rotation (NER) technique in a hollow-core fiber (HCF) is implemented and its optimization is performed by using high extinction ratio polarizer, utilizing Ar gas and testing smaller polarization ellipticity (NER angle). The optimal condition is found to be 7° NER angle and 650 mbar Ar in the HCF. Under this optimal condition, sub-4 fs pulses with a smooth spectrum are generated with a power above 30 mW. The NER efficiency is higher than 50%. After amplification to 75 mJ energy with sub-5 fs duration the measured contrast improvement is 3 order of magnitude. In conclusion, the intense few-cycle pulses have been fully characterized by FROG and chirp-scan techniques. Furthermore, the NER method is promising to get cleaned pulses with higher than 3 order of magnitude contrast enhancement.

No description available.

Electronic dynamics in molecules and atoms takes place on the attosecond timescale. For the observation of such processes, measurement techniques with attosecond resolution are needed. High-harmonic generation (HHG) in gas medium provides an ultrashort light source on the attosecond timescale for observing, understanding and controlling light-induced process on this scale with the necessary time resolution. To be able to use these attosecond pulses to measure electron dynamics, they have to be characterized. For this characterization, the XUV spectrum is extremely important. The XUV spectrum not only contains the information about the photon energies of the pulses, but also temporal information such as the difference between a single isolated attosecond pulse or an attosecond pulse train. The Light Wave Synthesizer 20 generates intense femtosecond pulses with a peak power of 16 TW and a spectrum spanning over the region from 580 to 1000 nm. This allow one to generate attosecond pulses based on HHG in gas medium with 100 eV photon energy and up to 20 nJ pulse energy. The generated attosecond pulses can be observed with a photodiode to measure the energy, an XUV CCD used as a profiler and an XUV flat-field spectrometer. The detector of the flat-field spectrometer is an XUV CCD which records the diffracted beam from a grating. Hence, a certain pixel of the camera shows the intensity for a certain range of photon energies. However, the calibration from pixel to energy is not always fixed due to e.g. the alignment of the spectrometer. This pixel to photon energy calibration can be done either by using the harmonic peaks in the XUV spectra or theoretical analyses of the spectrometer structure. In this thesis, both methods are investigated and the results are in good agreement. Due to the analytical calibration has a lower error and faster to do, future measurements can be evaluated with the analytical method.

The intense few-cycle laser pulses play an important role in the investigations of laser-plasma interaction. However, one of the biggest challenges in their generation is the reduction of temporal intensity contrast by introducing undesired pre-pulses and a long pedestal. Two techniques were investigated in this work to improve the contrast. First, the crossed-polarized wave (XPW) generation was optimized to get clean pulse. The conditions for XPW were optimized including crystal thickness and maximal background pressure in the vacuum cell. Second, the method of elliptical polarization rotation (EPR) in a gas-filled hollow-core fiber (HCF) was implemented to produce both broadened and cleaned pulse, since its setup is much simpler. For the tested EPR-based nonlinear filter, the spectral smoothening and broadening were obtained. The contrast of cleaned pulse was characterized providing 2 order of magnitude contrast enhancement, while it had a high average power of 80 mW. The EPR-based nonlinear filter is a promising simplified technique in the development of intense few-cycle lasers.

To observe electron dynamics in molecules and atoms which takes place on the attosecond timescale, single isolated attosecond pulses are required utilized in performing pump–probe experiments. The Light Wave Synthesizer 20 generates intense sub-5 fs pulses with a peak power of 16 TW and a broad spectrum. This offers a chance of generating isolated attosecond pulses via high harmonic generation (HHG) in gas medium. In this project, the variation of cutoff energy of HHG with different intensities of the driving laser was investigated. In addition, an isolated attosecond pulse with an Fourier-limited pulse duration of 188 as is produced with a selection of 15 eV around the cutoff region. Moreover, one optimization method refer to GDD scan was illustrated to optimize the HHG cutoff and continuum.