Phase-Matched High Order Harmonic Generation and Application

In this book, we consider experimental work on the generation of high rder harmonics and use it as a source for some applications. The structure of
he book is as follows. Chapter 1 is a literature review on the HHG technique.
his chapter begins by summarizing the history of HHG. The principles of the HHG emission are then discussed. This is followed by a discussion of the
macroscopic phase matching effects which influence the coherent construction f the harmonic emission. The enhancement of phase matching and the quasi
hase matching in HHG are also briefly described in the next section. The hapter finishes with a description of the high power femtosecond laser system
nd the fundamental experimental arrangements for generation and detection f the harmonic radiation in a semi-infinite gas cell which is an important
onfiguration of our experimental setup.
Chapter 2 presents two new schemes for enhancement of the harmonic eneration. In the first part of the chapter, by using a combination of axicon
nd lens to influence the geometrical phase mismatch leading to an mprovement of the phase matching condition, it is shown that for the armonic emission in the cut-off region the photon flux and the spatial beam rofile are strongly enhanced. In the second part, an off-axis laser beam is sed to control the phase matching condition in the HHG process and thus the armonic signal can be suppressed or enhanced. High photon energy up to 220 V (~ 5.5 nm in wavelength) from helium gas is achieved in our experiments n the presence of the off-axis beam. The important role of the harmonic dipole
hase on the phase-matched HHG process, which has not been considered in most previous studies, is also considered in this section.
Chapter 3 investigates the influence of the driving laser on the spectral eatures of the harmonic generation. The change of the harmonic spectrum when varying the phase of the driving field by controlling the frequency chirp f the laser is analyzed and this allows us to properly understand the role of the phase of the fundamental field on the HHG process. By using a combination of a half-wave plate and a polarizing beam splitter to control the laser pulse energy precisely and continuously without any change of the focusing geometry, the laser intensity dependence of the harmonic generation is discussed. Based on this investigation, the effects of the harmonic dipole phase and the dispersion phase mismatches induced by the ionized medium during the harmonic generation process are clearly revealed and the interplay between the macroscopic response and the single-atom response is also shown.
Moreover, we discuss that the use of an aperture to limit the ionization to less than a critical level and to control the wave-front of the fundamental field are
required for sharp and strong harmonic generation.
The next two chapters consider applications of the high harmonic generation. Chapter 4 proposes a new method to obtain information about molecular structure dynamics based on phase matched optimized harmonic generation in a semi-infinitive gas cell. The chapter begins with an overview of HHG from diatomic molecules. The influence of field-free alignment of the molecules on the phase matching conditions is then studied. The experimental setup and discussion of results relating to periodic modulations in the harmonic intensity are then presented.
Chapter 5 is devoted to the topic of coherent diffractive imaging (CDI) in the extreme ultraviolet region. We briefly review the theoretical background of
this imaging technique including an introduction to CDI, phase retrieval and the experimental requirements. In the following section we show experimental
results on CDI using a high order harmonic source around 13.5 nm based on a helium semi-infinite gas cell for two samples: a 2D regular array of pinholes
and a conjugated polymer film. The generation of the illumination source and the spatial coherence studies based on a Young’s double slit experiment are
presented. Imaging results are shown in the following section of this chapter.