1. Introduction

In part I of this work [1], we reported a new high-power laser system for generating attosecond light pulses and arbitrary waveforms by frequency synthesis. The laser system can generate up to five amplitude and phase-controlled collinear beams with wavelengths from the fundamental output of the Nd:YAG laser (λ = 1064 nm) and its second (λ = 532 nm) through the fifth harmonic (λ = 213 nm). Sub-single-cycle (0.37 cycle) sub-femtosecond (360 attosecond) pulses with carrier-envelope phase (CEP) control can be generated in this manner. The peak intensity of each pulse exceeds 10<sup>14</sup> W/cm<sup>2</sup> with a focused spot size of 20 μm. Stable square and sawtooth waveforms were also demonstrated [2].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The coherent control of nonlinear optical processes such as harmonic generation by waveformcontrolled laser field is important for both fundamental science and technological applications. Previously, we have studied the influence of relative phases and intensities of the two-color pump (1064 and 532 nm) electric fields on the third-order nonlinear frequency conversion process in argon [2]. It was shown that the third-harmonic (TH) signal oscillates periodically with the relative phases of the two-color driving laser fields. The data are in good agreement with a perturbative nonlinear optical analysis of the TH signal, which consists of contribution of the direct third-harmonic-generation (THG), four-wave mixing (FWM) and the interference of the above two processes.

As an extension of this work, we have studied generation of harmonics by three-color synthesized waveform in inert gas systems. We will illustrate the physics involved by examining the case for fourth-harmonic generation (FHG) in Section 2.

Anomalous enhancement of the THz signal in the presence of the 532 nm beam was observed, however. In this work, we show that plasma generated through the ionization process during laser-matter interaction plays a significant role in the enhancement of the TH signal. We also demonstrated phase-sensitive two-color ablation of copper and stainless steel. Our results show that hole drilling is more efficient for optimized waveforms.
