**6. Summary**

hole areas are also plotted for comparison. The difference in period between the fitting curve

**Figure 14.** The ablated hole area and simulated dual-color ionization rate versus relative phase in the case of single

**Figure 15.** The ablation threshold measurement. The single-shot ablated hole areas in PMMA irradiated by femtosecond dual-color synthesized waveforms are plotted as a function of laser fluence. Four sets of data for different values

In order to study how the relative phase affects the ablation threshold, we conducted a series of experiments in which the wedge prism's thickness was fixed at some value and the laser fluence varied. The family of experimentally measured ablated hole areas for three values of relative phase as a function or irradiating laser fluence are plotted in **Figure 15**. The same

in **Figure 13** and the simulation curve in **Figure 14** is only 1.3%.

352 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

shot. The observed modulation contrast in ablated area is ≈28% (peak to peak).

of relative phases are shown.

In this work, we have investigated single-shot laser ablation of polymethylmethacrylate (PMMA) using dual-color waveform synthesis of the fundamental (*ω*) and its second harmonic (2*ω*) of a femtosecond Ti: Sapphire laser. For comparison, single-color studies were also conducted. The threshold fluence for single-color ablation of PMMA irradiated by fundamental (*ω*) (NIR) and its second-harmonic (2*ω*) (NUV) beams were found to be respectively, 2.63 and 1.38 J/cm2 . Changing the relative phase of the fundamental (*ω*) and second-harmonic (2*ω*) outputs of the exciting laser resulted in clear modulation of the ablated area. The modulation as well as the ablation threshold depends on the relative phase between the *ω* and 2*ω* beams. The ablation thresholds for ablation of PMMA irradiated by two-color femtosecond frequency-synthesized waves are 2.49, 2.58, 2.89 and 2.80 J/cm2 , respectively, for the relative phase to be equal to 0, −π/2, π and 3π/2. The results correlated well with the theoretical model of laser breakdown (ablation) of transparent materials through photoionization in the intermediate regime (Keldysh parameter *γ* ≈ 1.5). Our study clearly illustrates the potential applications of using phase-controlled synthesized waveform for laser processing of materials.
