**4. Single-color femtosecond laser ablation of PMMA**

### **4.1. Introduction**

Ultrafast laser-induced ablation or breakdown of wide band gap materials, such as polymers [1, 2, 36–41], fused silica [6], and silicon [7, 42] have already been intensively studied. Among them, various kinds of polymers, such as polymethylmethacrylate (PMMA) [2, 36, 38–41], polyimide (PI) [1], polyethylene (PE) [37], polypropylene (PP), and polycarbonate (PC) [2], have drawn a lot of attention due to their potential industrial applications. Compared to nslaser ablation, the energy ablation threshold fluence of fs-laser at approximately the same incident wavelength is known to be reduced [43]. This can be attributed to the fact that the breakdown intensities in the fs regime approach that of the threshold of multiphoton ionization of which the electron densities is high enough to cause damage [35]. On the other hand, because the induced energy absorbed by electrons is much faster than that transferred to a lattice [35]; therefore, the nonthermal ablation nature of such behavior achieved by applying fs-lasers could lead to a significant reduction of heat-affected zones. Also of interest is the possibility of decreasing the threshold for ablation. For example, Stuart et al. observed a continuously decreasing threshold with a gradual transition from the thermal regime where the longer pulses (>100 ps) dominated the ablation compared with the shorter pulses (<10 ps), which is caused by multiphoton ionization and plasma formation [17].

To date, studies of single-color femtosecond laser ablation of PMMA were overwhelmingly conducted using the Ti: sapphire laser system of which the central wavelength is around 800 nm [44–46]. On the other hand, photoablation of materials with ultraviolet (UV) lasers has also gained in popularity [36, 47–49]. The mechanism for ablation of materials by UV light is mainly through the photochemical process by one-photon absorption. Most of the dielectrics, such as glass and polymer, have relatively high absorption coefficient in the UV region. This is in contrast to the commonly accepted mechanism for ablation of PMMA using 800 nm laser pulses, such as photothermal, photophysical, or multiphoton ionization and tunneling ionization, as mentioned previously. Therefore, it is of interest to conduct a comparative study of single-color femtosecond laser ablation of PMMA using the Ti: sapphire laser and its second harmonic.

the same beam path to generate 2*ω* pulses at 400 nm (*λ*2). Both beams were reflected from the silicon wafer at some incident angle, taking advantage of the fact that reflectivity of silicon varies with wavelengths and polarizations of the fundamental and second-harmonic beams (see the inset in **Figure 7**). We can control the intensity ratio *P*2*<sup>ω</sup>*/*Pω* of the two (collinear) beams by adjusting the incident angle. A pair of wedge prisms with controllable optical path difference was used to precisely adjust the relative time delay between the *ω* and 2*ω* pulses. We also employed a 5-mm-thick β-BBO to compensate the group velocity mismatch (GVM) of the two colors in the beam path. Besides, the *ω* and 2*ω* fields with original polarizations perpendicular to each other, were passed through a dual-color zero-order wave plate serving as a half-wave plate for 800 nm to make polarizations of the two colors parallel. Finally, the *ω* and 2*ω* pulses, overlapped in time with the same linear polarizations were focused on the sample. The spatial and temporal overlap and adjustment of the phase difference between the

**Figure 7.** Experimental setup for laser ablation of PMMA by femtosecond dual-colour synthesized waveforms. The polarizations of fundamental and second harmonic pulses were controlled by the half-wave plate. ND: neutral density filter; BBO: Barium borate; GVD: group velocity dispersion. The inset shows the reflectivity of the silicon wafer as a

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

Ultrafast laser-induced ablation or breakdown of wide band gap materials, such as polymers [1, 2, 36–41], fused silica [6], and silicon [7, 42] have already been intensively studied. Among

*ω* and 2*ω* fields were conducted using a procedure described previously [14].

**4. Single-color femtosecond laser ablation of PMMA**

function of the incident angle for both polarizations.

**4.1. Introduction**

**Figure 8.** Images of single-color (800 nm) and single-shot ablated holes. The input laser fluence are equal to (a) 2.63 J/cm2 and (b) 5.90 J/cm2 , respectively.

### **4.2. Single-shot single-color (800 nm) femtosecond laser ablation of PMMA**

In **Figure 8**, we show images of single-shot ablated holes in PMMA irradiated with femtosecond pulses at the wavelength of 800 nm. By changing the input laser fluence from 2.63 to 5.90 J/cm2 , areas of the holes are found to be equal to 155.25 and 1359.50 μm2 , respectively.

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

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

Assuming the irradiating beam has a Gaussian spatial profile, the generally accepted scaling law for ablated holes for incident laser fluence is given by

$$\Delta D^2 = 2\nu^2 \ln\left(\frac{F}{F\_{\mu}}\right) \tag{18}$$

where *D* is the diameter of the ablated region, *w* is effective laser beam width, *F* is the incident laser fluence and *F*th denotes the ablation threshold (unit here is J/cm2 ). Following Eq. (18), the ablation threshold *F*th can be determined by fitting the experimental data to be 2.63 J/cm2 .

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

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 fluence from 1.78 to 3.92 J/cm2 , the hole areas are found to increase from 155.25 to 1359.50 μm2 , respectively.

**Figure 10.** Images of single-color (400 nm) and single-shot ablated holes. The input laser fluence are equal to (a) 1.78 J/cm2 and (b) 3.92 J/cm2 , respectively.

**Figure 11** shows determination of the ablation threshold in the case of exciting wavelength at 400 nm. It can be seen that the same scaling behavior is observed in the case of ablation by the near IR beam. Our data show that the ablation threshold for PMMA irradiated by the near UV light of 400 nm is about 1.38 J/cm2 .

**Figure 11.** Single-colour ablation results for PMMA irradiated by femtosecond laser pulses with a central wavelength 400 nm. The fitted ablation threshold *F*th is equal to 1. 38 J/cm2 .
