**5. Laser ablation of PMMA with femtosecond two-color synthesized waveforms**

### **5.1. Introduction**

The photon energy for 800 nm is equal to 1.55 eV and the material band gap of PMMA is 4.58 eV. Therefore, more than three incident photons are needed for photoabsorption, leading to ablation. For such studies, one of the key parameter for studying the mechanism of ablation is its threshold. The method we used to define the ablation threshold value is measuring the ablated hole areas by using an optical microscope. In **Figure 9**, we have plotted hole-area of

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

**Figure 9.** Hole-areas of the single-shot, single-color (800 nm) femtosecond laser ablated holes are plotted as a function

Assuming the irradiating beam has a Gaussian spatial profile, the generally accepted scaling

*th*

è ø (18)

, the hole areas are found to increase from 155.25 to 1359.50

). Following Eq. (18), the

.

*F* æ ö <sup>=</sup> ç ÷

where *D* is the diameter of the ablated region, *w* is effective laser beam width, *F* is the incident

To compare, we conducted similar ablation studies with exciting wavelength at 400 nm. Recall that the material band gap of PMMA is 4.58 eV, which means the dominated mechanism for photoablation on PMMA at 400 nm is also multiphoton absorption. The photon energy for 400 nm is equal to 3.1 eV. Therefore, more than two incident photons are needed for photoabsorption, leading to ablation. In **Figure 10**, we show images of single-shot ablated holes in PMMA irradiated with femtosecond pulses at the wavelength of 400 nm. By changing the input laser

ablation threshold *F*th can be determined by fitting the experimental data to be 2.63 J/cm2

2 2 2 ln

laser fluence and *F*th denotes the ablation threshold (unit here is J/cm2

**4.3. Single-shot single-color (400 nm) femtosecond laser ablation of PMMA**

*<sup>F</sup> D w*

the ablated holes as a function of the irradiating laser fluence.

of the irradiating laser fluence. Error bars are indicated.

fluence from 1.78 to 3.92 J/cm2

, respectively.

μm2

law for ablated holes for incident laser fluence is given by

As we noted earlier, laser ablation studies were conducted almost exclusively with single-color laser beams [6, 7, 37, 42]. There are a few studies that employed two-color lasers. These studies can be organized into two categories: incoherent combination and coherent superposition of the two-color laser beams. An example of ablation by incoherently combined two-color beams is the work of Théberge et al., in which the authors observed an increase in volume of the ejected material by applying the superposition of fs and ns pulses. This was attributed to the free electrons and defect sites induced by the fs pulses, which could be exploited by the ns pulses [6]. Besides, Okoshi et al. reported that dual-color fs pulses with a fluence ratio of (2*ω:ω* = 2:78 mJ/cm2 ) could etch PE deeper and faster. It was proposed that an isolated carbon, in addition to C=O and C=C–H bonds, was formed on the ablated surface after treating PMMA with 2*ω* or dual-color pulses. The higher photon energy of 2*ω* pulses then cuts the chemical bonds of PE to form the modified layer on the ablated surface [37]. In related studies of fused silica, because of the creation of defect states or free electron plasma by dual-color fs pulses at zero delay, the enhancement of absorption/reflection was observed [6]. For silicon, upon using ns and picosecond (ps)-laser pulses, it was also shown that a weak 2*ω* beam can be beneficial in exciting electrons into conduction band to launch the ablation process of silicon [42]. In contrast, for fs pulses, where a sufficient population on the conduction band can be created by multiphoton absorption, this effect became insignificant [42].

All the above studies employ relatively long-time delays between the two colors, on the scale of the carrier lifetime (≈ picoseconds). If the relative delay is of the order of an oscillation period between dual-color fs pulses, interesting phenomena could unfold. In other fields, a dual-color coherently superposed beams achieved by relative-phase control of each color were applied to study the physical mechanism of intense-field photoionization, especially in the gas phase [11, 24]. Schumacher et al., for example, studied the electric-field phase-dependent photoelectrons created in a regime including the multiphoton and tunneling signatures simultaneously by changing the dual-color relative phase [24]. Later, Gao et al. claimed this phase-difference effect resembled the phenomenon of quantum interference (QI) between the different channels characterized by the number of photons. In other words, phase-dependent photoemission is not a classical-wave effect, but rather a quantum-mechanical one. Recently, in comparison with monochromatic excitation, the threshold of plasma formation has been demonstrated to be significantly improved with the superposition of an ns infrared laser pulses and its secondharmonic field [11]. The authors explained their measurements by the effect of a fielddependent ionization cross section [11]. In the following, we report results of our studies of the ablation of PMMA using dual-color waveform synthesis of *ω* and 2*ω* beams of an fs Ti: sapphire laser.

### **5.2. Single-shot dual-color ablation of PMMA**

In **Figure 12**, we show images of single-shot ablated holes in PMMA irradiated with dual-color (*ω* and 2*ω*) femtosecond pulses. In this experiment, the average powers of *ω* and 2*ω* beams are 200 and 40 mW, respectively. The corresponding laser fluence for the fundamental (NIR) beam is equal to 7.55 J/cm2 . Phase dependence of ablated holes was observed. **Figure 12(a)**, for which the relative phase *ϕ* = π, the ablated hole area is equal to 844.95 μm2 . When the relative phase *ϕ* = 0, area of the ablated hole is 982.31 μm2 (see **Figure 12b**).

can be organized into two categories: incoherent combination and coherent superposition of the two-color laser beams. An example of ablation by incoherently combined two-color beams is the work of Théberge et al., in which the authors observed an increase in volume of the ejected material by applying the superposition of fs and ns pulses. This was attributed to the free electrons and defect sites induced by the fs pulses, which could be exploited by the ns pulses [6]. Besides, Okoshi et al. reported that dual-color fs pulses with a fluence ratio of (2*ω:ω*

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

addition to C=O and C=C–H bonds, was formed on the ablated surface after treating PMMA with 2*ω* or dual-color pulses. The higher photon energy of 2*ω* pulses then cuts the chemical bonds of PE to form the modified layer on the ablated surface [37]. In related studies of fused silica, because of the creation of defect states or free electron plasma by dual-color fs pulses at zero delay, the enhancement of absorption/reflection was observed [6]. For silicon, upon using ns and picosecond (ps)-laser pulses, it was also shown that a weak 2*ω* beam can be beneficial in exciting electrons into conduction band to launch the ablation process of silicon [42]. In contrast, for fs pulses, where a sufficient population on the conduction band can be created by

All the above studies employ relatively long-time delays between the two colors, on the scale of the carrier lifetime (≈ picoseconds). If the relative delay is of the order of an oscillation period between dual-color fs pulses, interesting phenomena could unfold. In other fields, a dual-color coherently superposed beams achieved by relative-phase control of each color were applied to study the physical mechanism of intense-field photoionization, especially in the gas phase [11, 24]. Schumacher et al., for example, studied the electric-field phase-dependent photoelectrons created in a regime including the multiphoton and tunneling signatures simultaneously by changing the dual-color relative phase [24]. Later, Gao et al. claimed this phase-difference effect resembled the phenomenon of quantum interference (QI) between the different channels characterized by the number of photons. In other words, phase-dependent photoemission is not a classical-wave effect, but rather a quantum-mechanical one. Recently, in comparison with monochromatic excitation, the threshold of plasma formation has been demonstrated to be significantly improved with the superposition of an ns infrared laser pulses and its secondharmonic field [11]. The authors explained their measurements by the effect of a fielddependent ionization cross section [11]. In the following, we report results of our studies of the ablation of PMMA using dual-color waveform synthesis of *ω* and 2*ω* beams of an fs Ti:

In **Figure 12**, we show images of single-shot ablated holes in PMMA irradiated with dual-color (*ω* and 2*ω*) femtosecond pulses. In this experiment, the average powers of *ω* and 2*ω* beams are 200 and 40 mW, respectively. The corresponding laser fluence for the fundamental (NIR) beam

(see **Figure 12b**).

. Phase dependence of ablated holes was observed. **Figure 12(a)**, for which

. When the relative phase *ϕ*

multiphoton absorption, this effect became insignificant [42].

) could etch PE deeper and faster. It was proposed that an isolated carbon, in

= 2:78 mJ/cm2

sapphire laser.

is equal to 7.55 J/cm2

**5.2. Single-shot dual-color ablation of PMMA**

= 0, area of the ablated hole is 982.31 μm2

the relative phase *ϕ* = π, the ablated hole area is equal to 844.95 μm2

**Figure 12.** Images of single-shot ablated holes in PMMA irradiated by dual-colour (ω and 2ω) femtosecond lasers. The laser fluence for the fundamental beam is equal to 7.55 J/cm2 and ratio of second-harmonic to the fundamental beams was 1:5. The hole areas of (a) 844.95 μm2 (b) 982.31 μm2 were observed when the relative phase ϕ were set to π and 0, respectively. The length of the red double-arrows in Fig. 12 (a) and (b) are both equal to 10 μm.

**Figure 13.** The ablated hole area versus relative prisms' thickness with sinusoidal fitting in the case of single shot. The period for the ablated hole areas' change is equal to 19.5 μm.

By varying the prism thickness traversed by the laser beams (see **Figure 7**), we observed that hole areas oscillated, as shown in **Figure 13**. Theoretically, we expect a sinusoidal variation with a period (relative phase change of 2π) of 20 μm. This is in good agreement with experimentally determined period of 19.5 μm in **Figure 7**.

According to our model, the two-color ionization rate would depend on the relative phase. In **Figure 14**, we have plotted the ionization rate according to Eq. (6) for synthesized dual-color instantaneous field from Eq. (7) as a function of the relative phase. The corresponding ablated hole areas are also plotted for comparison. The difference in period between the fitting curve in **Figure 13** and the simulation curve in **Figure 14** is only 1.3%.

**Figure 14.** The ablated hole area and simulated dual-color ionization rate versus relative phase in the case of single shot. The observed modulation contrast in ablated area is ≈28% (peak to peak).

**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 of relative phases are shown.

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 scaling law for the single-color case was used to fit the experimental data. In this manner, we were able to determine the ablation threshold for a given value of relative phase. The ablation thresholds 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.

Interestingly, the fitted ablation thresholds also exhibit apparent dependence on the relative phase between *ω* to 2*ω* beams for our dual-color pulses. This is shown in **Figure 16**. The period of the sinusoidal oscillation is ≈ 2.4π.

**Figure 16.** The ablation threshold in PMMA irradiated by femtosecond dual-color synthesized waveforms is plotted as a function of relative phase changes. The period for the ablation thresholds' change is ≈2.4π.

Because the ablation threshold is dependent upon the number of free electrons created in the material [35, 50], we believe the observed periodicity in ablation threshold in **Figure 16** demonstrates how electric field of the synthesized waveform affects the variation of ablation threshold. In the above experiments, the beam waists for every condition deliberately kept to be approximately the same. These are equal to 49.42, 54.58, 46.46, 46.69 and 47.51 μm in the cases of relative phase set at −π/2, 0, π/2, π and 3π/2. That is, variation in the beam spot size is small, ±3.90%.
