Dynamic imaging of molecules using high order harmonic generation

This thesis reports on research performed on high harmonic generation and ion acceleration, processes that are based on the interaction of gaseous and solid media with high-intensity laser pulses. High harmonic generation leads to the production of beams of ultrashort, laser-like radiation in the wavelength range reaching from the extreme ultraviolet (XUV) to soft X-rays. Ion acceleration generates highly energetic protons and ions in the form of highly directional and pulsed beams with an ultrashort duration. The scope of this thesis is to present novel investigations and methods towards an enhancement of the flux and maximum energy of the accelerated ions and the harmonic radiation via an improved understanding of the underlying physics and, to provide a more complete identification of the actual requirements of the laser and the target for optimum output. The first part of this work concerns the construction and testing of a setup to generate high harmonics in gaseous media. The setup employs a capillary waveguide in which we implemented a novel manner of differential pumping. We performed an experiment on an enhancement technique, called Harmonic Excitation, and demonstrated for the first time that this technique can be applied in a capillary waveguide. The second part of the research concerns ion acceleration and high-order harmonic generation from solid-state targets using high-intensity laser pulses with ultra-high contrast. We investigated the coherence properties of high-order harmonic generation via coherent wake emission. Special attention was given to the preparation and investigation of extremely thin foils, i.e. freestanding nanofoils, to enter a very promising, but largely unexplored, regime. We experimentally investigated ion acceleration in this, so-called, transparent regime for the first time, and compared the results with predictions from improved numerical calculations. We also investigated for the first time high-order harmonic generation in the transparent regime, experimentally and via numerical modeling.

Summary form only given. High harmonic generation is unique as a source of coherent soft-X-ray (XUV) light because of its extremely short pulse duration, high spatial coherence, and compact size. Several theories have predicted that attosecond pulses might be generated using high order harmonic generation excited by ultrashort pulses. However, to date the fastest harmonic pulse duration measurements published were performed using laser pulses longer than 25 fs. In this work, we experimentally demonstrate a cross-correlation measurement which sets an upper limit of /spl sim/10 fs on the pulse duration of XUV harmonics.

High-order harmonics generated by a linearly polarized laser field are also linearly polarized. Having in mind that for various application, such as the exploration of magnetic materials, chiral molecules etc., we need circularly polarized high harmonics which serve as coherent soft x-rays, we explore high-order harmonic generation by the so-called bicircular laser field. This field consists of two coplanar counter-rotating circularly polarized fields of different frequencies equal to integer multiples of a fundamental frequency ω. High harmonics generated by such field are circularly polarized with helicity alternating between +1 and −1. Combining a group of such harmonics, instead of obtaining a circularly polarized attosecond pulse train, one obtains a pulse with unusual polarization properties. But, if the harmonics of particular helicity are stronger, i.e., if we have helicity asymmetry in a high-harmonic energy interval, then it is possible to generate an elliptical or even circular pulse train. We theoretically investigated a wide range of bicircular field-component intensities (I1 and I2) and found regions where both the harmonic intensity is high and the helicity asymmetry is large. Particular attention is devoted to the ω−2ω and ω−3ω bicircular fields and atoms having the s and p ground states. In our calculations we use strong-field approximation and quantum-orbit theory. We show that, even in the extreme case of I2 = 8I1, for an ω−3ω bicircular field, high-order harmonic generation is more efficient than in the I2 = I1 case. The obtained results are explained analyzing the relevant electron trajectories and velocities, which follow from the quantum-orbit theory. For the atoms having p ground state the helicity asymmetry parameter is large for a wide range of high-harmonic photon energies, while for the atoms having s ground state the helicity asymmetry parameter can be large only for low harmonics. We confirm this by averaging the obtained results over the intensity distribution in the laser focus.

We find that, for the first time in the investigation of high-order harmonic generation in two color laser fields that atomic energy level structure will greatly affect high order harmonic generation. For a certain atomic level structure, a specially chosen high frequency and relatively low intensity laser field can greatly enhance the efficiency of high order harmonic generation.

We demonstrate a novel method that requires no extra cavities in high harmonic generation with the aid of localized surface plasmon. The harmonics up to 21th was observed by injection with gases onto gold nanostructures.

Recently, circularly polarized high harmonics (HHs) have attracted great attention from the viewpoint of their application to ultrafast spectroscopy of magnetic materials and chiral molecules. However, circularly polarized HHs from gaseous media require a complicated setup due to strong constraints upon the high-harmonic generation (HHG) mechanism. HHG from solids is a new approach to this problem. Because the mechanism of solid HHG is intrinsically different from that of gas HHG, HHs from solids can be generated under single-color circularly polarized excitation. Their polarization states have been predicted to obey simple selection rules depending on the symmetry of a crystalline solid. In this Letter, we report on an experimental demonstration of circular HHG from solids that is fully consistent with the predicted selection rules. We irradiate circularly polarized mid-infrared pulses to a thin gallium selenide crystal and measure the spectrum and polarization of the emitted HHs. The threefold rotational symmetry of the crystal causes extinction of every third-harmonic order, as well as the generation of counter-rotating circularly polarized HHs. This result paves the way toward compact circularly polarized sources covering the spectra from the infrared to the extreme ultraviolet region.

Laser-induced high harmonic generation (HHG) in solids strongly depends on the band structure of the target. In this work, we studied the band structure of bulk wurtzite ZnO under different strains. The changes of band structure induced by shear, uniaxial, biaxial, and isotropic strains are compared. It is found that the band gap can be narrowed or widen by modulating the external conditions, which offers an efficient way to engineer the wide-gap crystals to produce HHG. The strain effects on HHG are investigated by numerically solving the semiconductor Bloch equations. Our calculations show that the HHG yield depends sensitively on the change of band structure caused by stress- and temperature-induced strains. In addition, we found that the emission times of HHG and the ratio between inter and intraband contributions to the total solid HHG spectra can be controlled by the magnitude of strain imposed on the solid targets.

The generation of high order harmonics from an inhomogeneous ovderdense plasma target irradiated by ultra-short intense laser pulse is studied by numerical simulation. During such interaction, ultrafast electron bunches are generated and excite electron plasma oscillations as they pass through the overdense target. These plasma oscillations will emit high-frequency electromagnetic emission by linear mode conversion. Instead of the integer harmonics generation, the emission appears with a broadband and even continuous spectrum corresponding to the electron plasma frequency range of the inhomogeneous plasma density.

The process of high order harmonic generation in atomic gases has shown great promise as a method of generating extremely short wavelength radiation, extending far into the extreme ultraviolet (XUV). The process is conceptually simple. A very intense laser pulse (I ~1013-1014 W/cm2) is focused into a dense (~1017 particles/cm3) atomic medium, causing the atoms to become polarized. These atomic dipoles are then coherently driven by the laser field and begin to radiate at odd harmonics of the laser field. This dissertation is a study of both the physical mechanism of harmonic generation as well as its development as a source of coherent XUV radiation. Recently, a semiclassical theory has been proposed which provides a simple, intuitive description of harmonic generation. In this picture the process is treated in two steps. The atom ionizes via tunneling after which its classical motion in the laser field is studied. Electron trajectories which return to the vicinity of the nucleus may recombine and emit a harmonic photon, while those which do not return will ionize. An experiment was performed to test the validity of this model wherein the trajectory of the electron as it orbits the nucleus or ion core is perturbed by driving the process with elliptically, rather than linearly, polarized laser radiation. The semiclassical theory predicts a rapid turn-off of harmonic production as the ellipticity of the driving field is increased. This decrease in harmonic production is observed experimentally and a simple quantum mechanical theory is used to model the data. The second major focus of this work was on development of the harmonic "source". A series of experiments were performed examining the spatial profiles of the harmonics. The quality of the spatial profile is crucial if the harmonics are to be used as the source for experiments, particularly if they must be refocused.

The wavefront of a ultrafast pulse is modified with a deformable-mirror. The generation of high-order harmonics (HHs) with both few- and many-optical-cycle pulses is controlled and optimised with a closed loop that controls the wavefront shape.KeywordsMerit FunctionHarmonic SpectrumHigh Harmonics GenerationDeformable MirrorFlat MirrorThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The need to accurately image microscopic features has led to the development of microscopes and complicated imaging techniques. Imaging systems can be divided into lens-based and lensless systems. In this thesis we study one lensless imaging technique called ptychography. In ptychography the object of interest is illuminated by a coherent light source. As the light transmits through or reflects from the object, it gets diffracted, and a camera captures the diffraction pattern. The object is placed on a moving stage and can move laterally to the laser beam. Therefore, we expose the camera to a series of diffraction patterns from different, but overlapping illuminated areas of the objects. Finally, we run a numerical reconstruction algorithm that gives complex-valued expressions of the object, and of the laser beam, which describes the wavefront at the object plane. Imaging resolutions in the nanometer range can be achieved with illuminating light sources in the extreme ultraviolet (XUV) and soft-x-ray (SXR) spectrum. Coherent XUV and SXR can be generated with tabletop sources via high harmonic generation (HHG), which is a non-linear process during which we generate high harmonics of a drive laser with wavelength typically in the infrared or visible spectrum. The output of the HHG process is a number of odd harmonics of the drive laser extending to the XUV and SXR range. In the second chapter we study the computational models for the propagation of the light wave from the object plane to the camera plane. Specifically, we compare three propagation models with user-defined pixel sizes for the object. We demonstrate that all three of them are equivalent, as they all originate from the Fresnel integral that describes the propagation of light under the paraxial approximation. However, each model has two alternative versions, which we also study and compare. In chapter 3 we investigate and experimentally validate chromatic aberration effects in HHG. For this purpose we perform ptychographic wavefront sensing (PWFS) measurements. By numerically backpropagating the reconstructed wavefronts to the generation plane of HHG, and associating the backpropagated wavefronts with the drive laser wavefront, wedraw conclusions about the physical processes of HHG. We varied the drive laser focus position with respect to the generation plane and calculated the focus positions of each high harmonic beam. The experimental results showed that there is strong chromatic aberration among the harmonic beams when the laser focus position is behind the generation plane. In chapter 4 we perform PWFS measurements for HHG beams carrying orbital angular momentum (OAM) generated by an OAM drive laser. OAM is a property of laser beams associated with a doughnut-shaped intensity profile and a topological charge. In this work we also perform a series of PWFS measurements, for varying laser focus positions and varying astigmatism levels in the drive laser, and study how the doughnut shape is affected in the far field and through the propagation of the HHG beams along the focus. We also study the broadening of the OAM modal content of the high harmonics when the drive laser beam has modal impurities, such as increased aberrations. In chapter 5 we study the imaging of an object using a multi-wavelength HHG beam. We introduce the concept of diversity, claiming that diversity within each polychromatic diffraction pattern and among diffraction patterns should facilitate ptychographic reconstructions. To confirm our hypothesis, we perform ptychographic imaging measurements with different illumination diversities. We add diversity by inserting a diffractive mask in the beamline upstream of the sample or by generating harmonics carrying OAM, as shown in chapter 4. We observe enhanced resolution and imaging quality when the beams are characterized by high diversity.

The ensemble of lower orders produced in high order harmonic generation can be efficiently temporally separated by propagation in a fluoride window while still preserving their femtosecond pulse duration. We present calculations for MgF2, CaF2, and LiF windows for the third, fifth, and seventh harmonics of 800 nm. We use this simple and inexpensive technique in a pump/probe experiment to resolve femtosecond dynamics in the ethylene molecule.

Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths (L(p)/λ < 1), the absolute efficiency of the harmonics declines for the steepest plasma density scale length L(p)→0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of 10(-4)-10(-6) of the laser pulse energy for photon energies ranging from 20-40 eV, with the best results being obtained for an intermediate density scale length.

High harmonic spectroscopy in solids is emerging as a new tool to investigate ultrafast electron dynamics in the presence of strong optical fields. However, the observed high harmonic spectra do not usually reflect the microscopic origin of high harmonic generation (HHG) because of nonlinear and/or linear propagation effects. Here, we systematically investigate the HHG in reflection and transmission from gallium arsenide exposed to intense mid-infrared optical pulses. In transmission geometry, we find that the properties of high harmonics are drastically changed by nonlinear effects during the propagation of even tens of micrometers. Especially, the nonlinear absorption and/or nonlinearly induced ellipticity of the drive pulses as well as a cascade nonlinear mixing significantly alter the high harmonic signals in the case of the transmission geometry, making an extraction of the microscopic electron dynamics of gallium arsenide difficult. On the contrary, in reflection geometry, we obtain HHG spectra that are free from propagation effects, opening a general approach for high harmonic spectroscopy.

High harmonic generation (HHG) in gas occurs when atoms are excited by an intense laser field. This technique enables to generate short coherent and directional extreme UV (XUV) radiation. Due to these unique properties, HHG presents a promising source for many applications such as time-dependent XUV spectroscopy. So far, mainly high energy systems such as Ti-sapphire laser amplifier have been used but are associated with a low repetition rate (a demonstration has been still reported at 100 kHz [1]). However, some applications require detecting in practice one event per 10 shots or less [2]. With low repetition rate systems, acquisition times may become unacceptable for recording exploitable statistics. Fiber laser-based HHG technology offers now the possibility to drive such experiments at high repetition rate. Indeed, it has been demonstrated that Yb-doped fiber chirped-pulse amplification (FCPA) systems can deliver both high average power and high energy subpicosecond laser pulses at higher repetition rate (100 kHz – 1 MHz) with good enough characterization for HHG [3,4]. Very high harmonics were recently observed despite moderate conversion efficiency [5].