**3.2. Pulsed UV laser at 248 nm**

**Figure 7** illustrates the microscopic image of the ablated CR39 craters spot size. The elimina‐ tion of material and creation of the crater due to the ablation process are presented by the dark spot on the surface of the CR39. Partial transparent curves can be seen surrounding the edge of the craters. This could possibly be caused by the heat affected zone (HAZ), which occurs during the process of laser ablation. Heat affected zones (HAZ) are the outcome of materials that are molten due to heat transfer from the crater and then rapidly cooled. The short pulse width of the excimer laser nevertheless reduces the heat transfer from the ablated crater. This phenomenon is thought to be caused by the photothermal effect in which the laser energy absorption by the material is converted into heat energy, which in turn leads to the localized modification of its structure.

**Figure 8** alternatively illustrates the average etch depth per pulse as a function of fluence F for CR39 ablated at 248 nm based on 150, 180, and 210 pulses. A linear fit of the form *d* = *k*‐1 *ln F/F*T provides an ablation threshold of 6–7 mJ cm‐2 . This value is marginally lower than that reported using 157 nm laser which gives *F*T as 11 mJ cm‐2 [10].

### **3.3. Continuous UV laser at 244 nm**

**Figure 6.** Fringes seen at the region of the bottom of the cones on CR39 (a) 100 pulses at ~60 mJ cm‐2 and (b) 100 pulses

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

at ~182 mJ cm‐2.

This section describes the use of this particular polymer as a waveguide where here we utilized a CR39 polymer sheet with a thickness of 0.5 mm and a refractive index as 1.486. Tunable laser

**Figure 7.** (a) Optical microscope imaging of the surface craters at CR39, (b) FESEM image of the crater, and (c) higher magnification of the ablated surface.

**Figure 8.** Plot of etch depth/pulse against log fluence at 150, 180, and 210 pulses on CR39.

**Figure 9.** Optical waveguide channel fabricated on CR39.

source at 1550 nm (Sairon Technology SPA‐4000 Prism Coupler) was used as a substrate for the polymer waveguide. SU‐8 polymer was spin coated on the CR39 sheet and patterned using photolithography technique, formed a core of the channel waveguide structure. The height of the channel waveguide measured using optical microscopy was recorded as 5.0 ± 0.1 µm and width range between 10 and 15 µm. **Figure 9** shows the fabrication of the waveguide using photolithography technique.

**Figure 10** shows the mode field diameter (MFD) and refractive index contrast of CR39 wave‐ guide against the laser fluence. The refractive index change is seen to be more significant with the increase in fluence. Higher laser fluence will lead to a higher change of the refrac‐ tive index, which likens to the reaction of other optical materials including silica glass to laser ablation. Extrapolation of the results in **Figure 10** also forecasts that the refractive index can be increased by the irradiation of CR39 with higher fluence. This increase, however, was observed to be limited as an upper fluence limit (5 KJ/cm<sup>2</sup> ) exists. When the laser fluence is above this limit, ablation will occur and subsequently UV irradiation of the CR39 will lead to the elimination of materials from its surface, similar to the outcome that occurs in Section 3.

The microscope image of the modification and ablation of CR39 is illustrated in **Figure 11**. As a result of refractive index modification, a partially transparent line can be seen in **Figure 11a**, whereas **Figure 11b** shows a darker line that indicates the removal of material and formation of craters due to ablation. The inset in **Figure 11b** further shows a cross‐sectional view of the ablated sample. It is easier to detect the ablation effect from cross‐sectional images in which a part of CR39 is removed. Subject to the laser fluence being applied, the removed area depth can be up to several microns.

**Figure 8.** Plot of etch depth/pulse against log fluence at 150, 180, and 210 pulses on CR39.

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

**Figure 9.** Optical waveguide channel fabricated on CR39.

**Figure 10.** MFD and index contrast of CR39 waveguides versus laser fluence at 30 mW laser power.

**Figure 11.** Microscope images of (a) internal modification, (b) ablation of CR39, and (inset) cross‐sectional view of ablated CR39.
