Enhancement of Relativistic Harmonic Generation by an Optically Preformed Periodic Plasma Waveguide
Date Issued
2007
Date
2007
Author(s)
Kuo, Cheng-Cheng
DOI
zh-TW
Abstract
High harmonic generation based on optical-field ionization (OFI)
of electrons in atoms followed by recombination and relativistic
harmonic generation based on nonlinear quiver motion of free
electrons in relativistically intense laser field are active fields of
research, as they are the most promising approaches to date for
generating coherent ultrashort x-ray pulses. Yet, for a wide range
of applications how to produce harmonics with short wavelength
and high efficiency is still a challenge. In order to achieve sus-
tained growth of harmonics, one need to increase the interac-
tion length and at the same time suppress the reverse conversion
process. This can be done by guiding the laser pulse and phase
matching the frequency conversion process. The first chapter of
this thesis gives an introductory to the history of the development
of high-order harmonic generation. The second chapter contains
the physical background on harmonic generation and current de-
velopement.
Recent progress in high-power ultrashort-pulse laser technol-
ogy has made available pump sources with higher and higher in-
tensity. As a result, relativistic harmonic generation is emerg-
ing as a promising scheme for generating strong coherent x-rays.
However, hollow fiber and corrugated hollow capillary are not
suitable for relativistic harmonic generation because optical dam-
age occurs under relativistic intensity. A few schemes for pro-
ducing a plasma waveguide have been reported, which has the
capability of guiding the pump laser pulse and the harmonics si-
multaneously, but none of them has been applied to relativistic
harmonic generation yet. It has also been proposed that a plasma
density modulation may be used to achieve quasi-phase matching
of relativistic third harmonic generation. In this scheme period-
ically interlacing layers of high- and low-density plasmas in the
direction of beam propagation with their widths matching the
coherence lengths allow periodic correction of the relative phase
between the harmonic wave and the fundamental. With a gain
in the high-density layer larger than the inverse gain in the low-
density layer, the harmonics can grow with increasing interaction
length. To date, no experimental demonstration of quasi-phase-
matched relativistic harmonic generation has been reported yet.
Technical issues about the programmable fabrication of spatial
structures in a gas by laser machining wiht liquid-crystal spatial
light modulator and applications to quasi-phase matching rela-
tivistic high harmonic generation will be discussed disscussed in
chapter 3.
The experimental results are discussed in chapter 4. Enhance-
ments of relativistic third-harmonic generation (THG) by using a
uniform plasma waveguide and a periodic plasma waveguide are
demonstrted. For the case of uniform waveguide, enhancement of
harmonic generation by optical guiding was verified. For the case
of periodic waveguide, resonant dependence of harmonic intensity
on plasma density and density modulation parameters was ob-
served, which is a distinct characteristic of quasi-phase matching.
Under the resonant condition, the energy of the third harmonic
was enhanced by a factor of 8, and the intensity was enhanced by
a factor of 50. On-line optimization of the resonant enhancement
was made possible by the capability of programmable fabrication
of longitudinal density modulation using a liquid-crystal spatial
light modulator (SLM). Further enhancement is possible by using
an aperiodic longitudinal density structure optimized with adap-
tive control on the SLM settings to compensate for nonlinear
effects in the harmonic growth. Future work based on program-
mable fabrication technique and quasi-phase-matching techniques
to higher order relativistic harmonic generation is proposed in
chapter 5. Application to other feasible laser-plasma phenomena
will still be discussed.
Subjects
相對論
諧波
電漿
波導
準相位匹配
relativistic
harmonic
plasma
waveguide
quasi-phase matching
Type
thesis
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