4. Laser-material processing with multi-color synthesized light field

Lately, high-energy laser beams have been increasingly been used for processing and fabrication of material and devices. These include the fabrication of micro electro mechanical systems, optoelectronic components, biomedical micro fluid chips and silicon chip processing, electronic packages and drilling of circuit boards, to name just a few.

There are two kinds of mechanisms occurring during laser processing of materials: a photothermal one and a photo-chemical one. In the photo-thermal mechanism, laser beams with high-power density are used as a thermal source which is focused on an object for a period of time. The energy absorbed on the surface of the object is transferred into the bulk of the object via thermal conduction. Thereafter, a part of the object is melted or vaporized by the deposited thermal energy. The laser spot is moved to another part of the work piece ready for further processing. In the photo-chemical mechanism, the bonding of molecules in the material to be processed is broken after absorption of one or more photons, which make electrons hop between energy levels and molecular bonds in the material can be broken as a result [12, 13].

In laser processing, the laser is chosen according to characteristics such as energy absorption, thermal diffusion and melting point of the material. For example, ablation is performed on various materials using lasers with appropriate wavelength. It is interesting, therefore, to investigate whether synthesized waveforms proposed and demonstrated in our work could be advantageous for laser processing.

Ablation of materials with multiple lasers, for example, lasers with dual colors were reported recently [14–18]. Incoherent or coherent summation of multi-color beams can be implemented. With incoherent summation of two femtosecond and nanosecond class pulsed lasers, an enhancement of volume of the vaporized material was observed by Théberge and Chin [14]. In this work, the free electrons and defect states induced by intense fs pulses were exploited by the ns pulses. In another work, Okoshi and Inoue [15] demonstrated that superimposed fs pulses at the fundamental (ω) and small fraction of the second-harmonics (2ω) output of the Ti: sapphire laser with the relative fluence ratio 1/39 was able to etch polyethylene (PE) much deeper and faster. They attributed the observe phenomena by the higher photon energy of 2ω pulses which can cut the chemical bonds of PE to form a modified layer of PE on the ablated surface. However, this article did not discuss about the temporal dynamics of the laser ablation process. On the other hand, the enhancement of absorption/reflection was observed in fused silica with coherent summation of dual-color pulses at zero delay [16]. This is because of defect states formation or free electron plasma generated in the material this way. For silicon, the ablation process was reported in the case of nanosecond and picosecond laser pulses where a small portion of the (2ω) beam can excite electrons into the conduction band [17]. For femtosecond pulses, this effect became insignificant because a sufficient population in the conduction band is created by multiphoton absorption in silicon. However, on the scale of carrier lifetime, all of the above-mentioned works consider relatively long time delays between the beams of two colors (picoseconds).

fundamental output was lower. This is in good agreement with the experimental results for fundamental pulse energies of 150 and 200 mJ. However, the theoretically predicted threshold for plasma enhancement does not match that of the experimental data. The may be explained by the dependence of the threshold on the step-like enhanced ionization probability. The step-like behavior caused by new absorption processes becomes dominant when the effective photon energy (effective frequency) reaches the threshold of this process. However, in reality, there actually exist many quantum processes involving the absorption of several photons at frequencies of ω and 2ω. The different quantum processes have different ionization rate. When the power ratio of the two-color field is changed, the ionization probability of the different quantum process is also changed. It could be argued that the variation of ionization rate with the second-harmonic pulse energy is continuous rather than step-like when the plasma density increases. This in turn should shift the

Figure 6. The comparison of the experimental and simulation results for two-color excited third-harmonic signal in argon

4. Laser-material processing with multi-color synthesized light field

tronic packages and drilling of circuit boards, to name just a few.

Lately, high-energy laser beams have been increasingly been used for processing and fabrication of material and devices. These include the fabrication of micro electro mechanical systems, optoelectronic components, biomedical micro fluid chips and silicon chip processing, elec-

There are two kinds of mechanisms occurring during laser processing of materials: a photothermal one and a photo-chemical one. In the photo-thermal mechanism, laser beams with

threshold pulse energy.

as a function of the second-harmonic pulse energy.

164 High Power Laser Systems

We note that tunable relative-phase control between the two dual-color exciting laser was applied in order to study the physical mechanism of intense-field photoionization in the gas phase [19–21]. Schumacher and Bucksbaum [19] reported that number of photoelectrons created in a regime that both multiphoton and tunneling ionization mechanisms are present is indeed dependent on the relative phase of the dual-colors. Later, Gao et al. [20] showed that the observed phase-dependence represents a quantum interference (QI) between the different channels corresponding to different number of photons involved. Recently, in comparison with monochromatic excitation, the threshold of plasma creation in the material to be ablated has been identified to be significantly reduced with the use of a ns infrared laser pulses and its second-harmonic one [21]. The observed phenomenon was attributed to the field-dependence

of the ionization cross section. In this work, we focus on the ablation study of metals under phase-controlled dual color ns pulses with the relative delay between the two color being less than one oscillation period.

Results of preliminary experiments on drilling of copper and stainless steel with the multi-color laser system used in this work (see Figure 7). The nonlinear optical crystals for harmonic generation are arranged in a cascaded layout. The crystals are KD\*P type II for the second harmonic, ω2, KD\*P type I for the third harmonic, ω3, BBO type I for the fourth harmonic, ω4, and BBO type I for the fifth harmonic, ω5, respectively. Therefore, the spectra of the five-color frequency components spans from near infrared (1064 nm) to the ultraviolet (213 nm). The cascaded harmonic generation setup was adopted to ensure that the second-order nonlinear optical process all occurred collinearly so that fundamental and harmonics overlapped spatially with each other. The amplitude and relative phase of each harmonics can be adjusted independently.

We studied two-color laser ablation of cooper and steel to demonstrate the feasibility of the approach. In the plane-wave approximation, the synthesized dual-color laser field can be written as,

$$E\_{\text{Total}}(t) = E\_w(t) \times \cos\left[\omega t\right] + E\_{2\omega}(t) \times \cos\left[2\omega t + \theta\right] \tag{19}$$

During the experiment, we applied the dual color ns pulses with the same total energy (100 mJ) to 150 μm thick copper and stainless steel foil. We adjusted the phase modulators only, so, varied the relative phase between the harmonics. Then we measured the time required to make a pass through hole in a foil and estimated the ablation rate. In Figure 9(a), we have plotted diameters of holes drilled in copper sheetsas a function of relative phases between the fundamental (ω1) and second (ω2) harmonics of the synthesizing laser. Pictures of the drilled holes are also presented. Similar results for stainless steel are shown in Figure 9(b). Clearly, there is an optimal phase relationship between the two colors where higher instantaneous

Figure 8. (a) Comparison of synthesized laser fields, and (b) instantaneous intensity of the fundamental wave (ω) and its second-harmonic (2ω) field with that of a ratio, E2ω/ Eω ≈ 0.68; shown are the cases of different relative phases θ = 0, π/2,

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Figure 10(a)–(d) shows simulated results the peak strengths of the synthesized laser field with various relative phases (Δφ = 0, 0.5π, π, and 1.5π) As can be seen in Figure 10(b) and (d), the synthesized laser field is expected to exhibit the higher peak strength at relative phases of Δφ = 0.5π, and 1.5π. Therefore, ablation is expected to be more efficient for these waveforms.

Figure 9. The diameters holes drilled in (a) copper, and (b) stainless steel by synthesized laser fields with different relative phases between the fundamental (ω1) and second (ω2) harmonics of the Q-switched laser. Pictures of drilled holes are also

intensity causes higher ablation rate.

π, and 3π/2, respectively.

shown.

where Eω(t) <sup>=</sup> E0,<sup>ω</sup> exp[(�2ln2)t 2 /τω 2 ] and E2ω(t) = E0,2<sup>ω</sup> exp[(�2ln2)t 2 /τ2<sup>ω</sup> 2 ] are the amplitudes of fundamental and second-harmonic optical fields with pulse durations, τω, and τ2ω, both assumed to be around 15 ns; where ω, and θ are the angular frequency and relative phase between two fields, respectively. Because the ns pulse duration we can neglect a group velocity mismatch when dual wavelength waveform propagates through a media. Therefore, the first and second-harmonic pulses are fully overlapped and the joint pulse has the same pulse duration around 15 ns. Figure 8(a) and (b) illustrate the simulated synthesized electric-filed waveforms and the instantaneous intensities using Eq. (19) for the relative phases of θ = 0, π/2, π, and 3π/2, respectively, and E2ω/E<sup>ω</sup> ≈ 0.68 (they are selected to match one of the experimental parameters corresponded to the intensity ratio P2ω/P<sup>ω</sup> ≈ 0.47). It is clear that the waveform of combined ω and 2ω fields are very sensitive to the relative phase between the two. As shown in Figure 8(a), the synthesized waveform ETotal has a symmetric shape for θ = 0 and θ = π. It exhibits, however, an asymmetric profile with enhanced electric filed during one-half cycle of its oscillation for θ equal to π/2 and 3π/2. In comparison Figure 8(b), the instantaneous intensity of the two-color field for θ = 0 and π are quite similar to the cases where θ = π/2 and 3π/2 except for a shift in the time domain, like for. In other words, the period of the change of instantaneous intensity can be considered to be π.

Figure 7. A schematic view showing a multi-color harmonic synthesized laser system for laser processing.

Frequency-Synthesized Approach to High-Power Attosecond Pulse Generation and Applications: Applications http://dx.doi.org/10.5772/intechopen.78270 167

of the ionization cross section. In this work, we focus on the ablation study of metals under phase-controlled dual color ns pulses with the relative delay between the two color being less

Results of preliminary experiments on drilling of copper and stainless steel with the multi-color laser system used in this work (see Figure 7). The nonlinear optical crystals for harmonic generation are arranged in a cascaded layout. The crystals are KD\*P type II for the second harmonic, ω2, KD\*P type I for the third harmonic, ω3, BBO type I for the fourth harmonic, ω4, and BBO type I for the fifth harmonic, ω5, respectively. Therefore, the spectra of the five-color frequency components spans from near infrared (1064 nm) to the ultraviolet (213 nm). The cascaded harmonic generation setup was adopted to ensure that the second-order nonlinear optical process all occurred collinearly so that fundamental and harmonics overlapped spatially with each other. The ampli-

We studied two-color laser ablation of cooper and steel to demonstrate the feasibility of the approach. In the plane-wave approximation, the synthesized dual-color laser field can be written as,

fundamental and second-harmonic optical fields with pulse durations, τω, and τ2ω, both assumed to be around 15 ns; where ω, and θ are the angular frequency and relative phase between two fields, respectively. Because the ns pulse duration we can neglect a group velocity mismatch when dual wavelength waveform propagates through a media. Therefore, the first and second-harmonic pulses are fully overlapped and the joint pulse has the same pulse duration around 15 ns. Figure 8(a) and (b) illustrate the simulated synthesized electric-filed waveforms and the instantaneous intensities using Eq. (19) for the relative phases of θ = 0, π/2, π, and 3π/2, respectively, and E2ω/E<sup>ω</sup> ≈ 0.68 (they are selected to match one of the experimental parameters corresponded to the intensity ratio P2ω/P<sup>ω</sup> ≈ 0.47). It is clear that the waveform of combined ω and 2ω fields are very sensitive to the relative phase between the two. As shown in Figure 8(a), the synthesized waveform ETotal has a symmetric shape for θ = 0 and θ = π. It exhibits, however, an asymmetric profile with enhanced electric filed during one-half cycle of its oscillation for θ equal to π/2 and 3π/2. In comparison Figure 8(b), the instantaneous intensity of the two-color field for θ = 0 and π are quite similar to the cases where θ = π/2 and 3π/2 except for a shift in the time domain, like for. In other words, the period of the change of

Figure 7. A schematic view showing a multi-color harmonic synthesized laser system for laser processing.

] and E2ω(t) = E0,2<sup>ω</sup> exp[(�2ln2)t

ETotalðÞ¼ t Eωð Þ� t cos ½ �þ ωt E<sup>2</sup>ωð Þ� t cos 2½ � ωt þ θ , (19)

2 /τ2<sup>ω</sup> 2

] are the amplitudes of

tude and relative phase of each harmonics can be adjusted independently.

2 /τω 2

instantaneous intensity can be considered to be π.

than one oscillation period.

166 High Power Laser Systems

where Eω(t) <sup>=</sup> E0,<sup>ω</sup> exp[(�2ln2)t

Figure 8. (a) Comparison of synthesized laser fields, and (b) instantaneous intensity of the fundamental wave (ω) and its second-harmonic (2ω) field with that of a ratio, E2ω/ Eω ≈ 0.68; shown are the cases of different relative phases θ = 0, π/2, π, and 3π/2, respectively.

During the experiment, we applied the dual color ns pulses with the same total energy (100 mJ) to 150 μm thick copper and stainless steel foil. We adjusted the phase modulators only, so, varied the relative phase between the harmonics. Then we measured the time required to make a pass through hole in a foil and estimated the ablation rate. In Figure 9(a), we have plotted diameters of holes drilled in copper sheetsas a function of relative phases between the fundamental (ω1) and second (ω2) harmonics of the synthesizing laser. Pictures of the drilled holes are also presented. Similar results for stainless steel are shown in Figure 9(b). Clearly, there is an optimal phase relationship between the two colors where higher instantaneous intensity causes higher ablation rate.

Figure 10(a)–(d) shows simulated results the peak strengths of the synthesized laser field with various relative phases (Δφ = 0, 0.5π, π, and 1.5π) As can be seen in Figure 10(b) and (d), the synthesized laser field is expected to exhibit the higher peak strength at relative phases of Δφ = 0.5π, and 1.5π. Therefore, ablation is expected to be more efficient for these waveforms.

Figure 9. The diameters holes drilled in (a) copper, and (b) stainless steel by synthesized laser fields with different relative phases between the fundamental (ω1) and second (ω2) harmonics of the Q-switched laser. Pictures of drilled holes are also shown.

ω<sup>5</sup> = ω<sup>1</sup> + ω<sup>2</sup> + ω2, ω<sup>5</sup> = ω<sup>1</sup> + ω<sup>1</sup> + ω3, ω<sup>5</sup> = ω<sup>3</sup> + ω<sup>3</sup> ω1. For the sixth and seventh harmonic, there are two kinds of four-wave mixing processes, and so on. To illustrate, we present in detail the simulation results for fourth-harmonic generation using three-order nonlinear processes. It is shown that the fourth-harmonic signal varies with the phase of the fundamental beam.

Figure 11. Diameters of holes drilled in (a) copper, and (b) stainless steel by synthesized laser fields with different relative phases between the fundamental (ω1) and second (ω2) harmonics of the Q-switched laser. Pictures of drilled holes are also

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Previously, we studied the influence of relative phases and intensities of the two-color pump on the third-order nonlinear frequency conversion process. It is shown that the third-harmonic (TH) signal oscillates periodically with the relative phases of the two-color driving laser fields due to the interference of TH signals from a direct third-harmonic-generation (THG) channel and a four-wave mixing (FWM) channel. In intense laser field, however, plasma can be generated through the ionization process. In the multiphoton ionization region, the plasma density was estimated by the Perelomov, Popov, and Terent'ev (PPT) model where the instantaneous laser field and frequency of laser are taken into account. Under the assumption that susceptibility and wave-vector mismatch depend on the plasma density, we show that plasma plays a significant role in the generated third-harmonic signal. The simulation results are in good

Finally, we showed preliminary data indicating that the synthesized two-color laser fields are powerful in enhancing the conversion efficiency of HHG and VUV spectra. We also demonstrated phase-sensitive two-color ablation of copper and stainless steel. Our results show that

This work was supported by grants sponsored by the National Science Council (now Ministry of Science and Technology or MoST) of Taiwan (NSC 98-2112-M-009-015-MY3) and Phase II of

hole drilling is more efficient with the use of optimized waveforms.

the Academic Top University Program of the Ministry of Education, Taiwan.

agreement with the experiments.

shown.

Acknowledgements

Figure 10. Numerical simulation of the peak strength of the laser waveform synthesized by two-color laser fields with various relative phases between the fundamental and the second harmonic (a) Δφ = 0, (b) Δφ = π/2, (c) Δφ = π, and (d) Δφ = 3π/2, respectively.

In another experiment, we fixed the exposure time at 10 s, varied the relative phase of the twocolor beams and examined the ablated holes afterwards. In Figure 11(a), we have plotted diameters of holes drilled in copper sheets as a function of relative phases between the fundamental (ω1) and second harmonics (ω2) of the synthesizing laser. Pictures of the drilled holes are also presented. Similar results for stainless steel are shown in Figure 11(b). These data clearly show the dependence of ablation rate on the synthesized waveform, that is, relative phase of the fundamental (ω1) and second harmonic (ω2) of the single-frequency Nd:YAG laser.

### 5. Summary

As an application of the high-power laser system based on synthesized waveforms, we studied harmonic generation by three-color waveform synthesis in inert gas systems. In third-order nonlinear optics, the interaction between three-color beam and inert gases can be used to generate fourth to ninth harmonics of the laser fundamental output. For fourth-harmonic generation, there are three kinds of four-wave mixing processes: ω<sup>4</sup> = ω<sup>i</sup> + ω<sup>j</sup> + ωk, ω<sup>4</sup> = ω<sup>i</sup> + ω<sup>j</sup> ωk, where i, j, k = 1, 2, 3. For fifth-harmonic generation, there are three possible processes:

Frequency-Synthesized Approach to High-Power Attosecond Pulse Generation and Applications: Applications http://dx.doi.org/10.5772/intechopen.78270 169

Figure 11. Diameters of holes drilled in (a) copper, and (b) stainless steel by synthesized laser fields with different relative phases between the fundamental (ω1) and second (ω2) harmonics of the Q-switched laser. Pictures of drilled holes are also shown.

ω<sup>5</sup> = ω<sup>1</sup> + ω<sup>2</sup> + ω2, ω<sup>5</sup> = ω<sup>1</sup> + ω<sup>1</sup> + ω3, ω<sup>5</sup> = ω<sup>3</sup> + ω<sup>3</sup> ω1. For the sixth and seventh harmonic, there are two kinds of four-wave mixing processes, and so on. To illustrate, we present in detail the simulation results for fourth-harmonic generation using three-order nonlinear processes. It is shown that the fourth-harmonic signal varies with the phase of the fundamental beam.

Previously, we studied the influence of relative phases and intensities of the two-color pump on the third-order nonlinear frequency conversion process. It is shown that the third-harmonic (TH) signal oscillates periodically with the relative phases of the two-color driving laser fields due to the interference of TH signals from a direct third-harmonic-generation (THG) channel and a four-wave mixing (FWM) channel. In intense laser field, however, plasma can be generated through the ionization process. In the multiphoton ionization region, the plasma density was estimated by the Perelomov, Popov, and Terent'ev (PPT) model where the instantaneous laser field and frequency of laser are taken into account. Under the assumption that susceptibility and wave-vector mismatch depend on the plasma density, we show that plasma plays a significant role in the generated third-harmonic signal. The simulation results are in good agreement with the experiments.

Finally, we showed preliminary data indicating that the synthesized two-color laser fields are powerful in enhancing the conversion efficiency of HHG and VUV spectra. We also demonstrated phase-sensitive two-color ablation of copper and stainless steel. Our results show that hole drilling is more efficient with the use of optimized waveforms.
