**3. Results and discussion**

#### **3.1. Etch rate analysis at 157 nm F<sup>2</sup> laser**

**Figure 3** shows a plot of the etch rate per pulse versus fluence for CR39 as derived from the white light interferometer; based on the linear fits for ≥10 pulse exposure, the estimated ablation threshold is ~60 mJ m‐2. However, **Figure 3** shows that there is still a small level of etching at a fluence of ~50 mJ cm‐2, and thus the estimated ablation threshold for CR39 is taken to lie in the range ~50–60 mJ cm‐2. From **Figure 3**, the etch rate per pulse for a single pulse reached ~100 nm at ~120 mJ cm‐2, higher than for multiple exposure. The data for 50 pulse exposure gave reasonably consistent values, and the gradient of the corresponding line in **Figure 3** gave an effective absorption coefficient of *α*eff ≈ 2.9 × 105  cm‐1. This is similar

**Figure 3.** Etch rate as a function of fluence for CR39 polymer using the 157 nm laser. Results for the average etch rate per pulse for various numbers of pulses are shown.

to polycarbonate and indicates CR39 is a strongly absorbing organic polymer at 157 nm. No data relating to the optical constants in the VUV spectral region could be found for this material.

### *3.1.1. Formation of cones*

The measured numerical aperture (NA) of the written waveguide can be used to compute the

A fiber pigtail was used to couple a tunable laser source into the CR39 waveguides to measure NA, whereas an objective lens was used to display the output of CR39 waveguides onto an image capture device. The formula below can be used to measure the NA of a waveguide

sin *NA NC* =

A straight waveguide with 3 cm length was fabricated using different fluences (between 1 and 5 KJ cm‐2). During this process, the laser power was set at a fixed value, and the laser beam was aligned to ensure that the focal plane was positioned on the CR39 sample surface. **Figure 2**

**Figure 3** shows a plot of the etch rate per pulse versus fluence for CR39 as derived from the white light interferometer; based on the linear fits for ≥10 pulse exposure, the estimated ablation threshold is ~60 mJ m‐2. However, **Figure 3** shows that there is still a small level of etching at a fluence of ~50 mJ cm‐2, and thus the estimated ablation threshold for CR39 is taken to lie in the range ~50–60 mJ cm‐2. From **Figure 3**, the etch rate per pulse for a single pulse reached ~100 nm at ~120 mJ cm‐2, higher than for multiple exposure. The data for 50 pulse exposure gave reasonably consistent values, and the gradient of the corresponding

shows the schematic diagram of waveguide channel written on the polymer.

 **laser**

line in **Figure 3** gave an effective absorption coefficient of *α*eff ≈ 2.9 × 105

( )1/2

and *n*cl are the refractive index of UV written area and unwritten area, respectively.

θ

and *n*cl.

– *NA n n c cl* = (3)

(4)

 cm‐1. This is similar

refractive index change, ΔRI of the UV irradiated area using the formula below:

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

ΔRI can be calculated from the difference between *n*<sup>c</sup>

**Figure 2.** Waveguide channel written experimental arrangement.

from the waveguides divergence angle, θ:

**3. Results and discussion**

**3.1. Etch rate analysis at 157 nm F<sup>2</sup>**

where *n*<sup>c</sup>

For this experiment, clean CR39 samples (unseeded) were irradiated using 157 nm laser radia‐ tion over a range of pulse number from a single pulse to thousands of pulses and over a range of fluences of ~50 to ~180 mJ cm‐2 . The dark spots that were seen under optical microscopy in the previous section were confirmed by scanning electron microscopy to be cones on the CR39 surface. These had very well‐defined structures and, in general, appeared to have even bet‐ ter definition than those on the irradiated polycarbonate surface. In particular, the cones on CR39 were found to have extremely straight walls and to be exceptionally sharp at their tips as can be seen from the results shown in **Figure 4**. From the SEM images of the ablation sites, small particles appeared to be on the surface though it is difficult to make out if these reside on the top of the cone as "initiating" sites. **Figure 4a** and **b** shows the cones that developed at fluences of 112 and 180 mJ cm‐2 with 500 pulses. The cones appear to have a similar size and shape at the same fluence. A comparison of **Figure 4a** and **b** shows as expected that the cone apex angle is larger at the lower fluence, that is, the full apex angle is ~70° at 112 mJ cm‐2 and ~55° at 180 mJ cm‐2 when corrected for the 60° viewing angle. It also appears that the cone tips get sharper as the fluence is raised. Exposure of the CR39 surface to a higher number of

**Figure 4.** Examples of cone formation on the CR39 surface using the 157 nm laser (a) 500 pulses at 112 mJ cm‐2 (b) 500 pulses at 180 mJ cm‐2 (c) 1000 pulses at 142 mJ cm‐2, and (d) 1000 pulses at 182 mJ cm‐2.

pulses, **Figure 4c** and **d** led to an increase in the areal density of the cones compared to that at lower pulse number, **Figure 4a** and **b**.

**Figure 5** shows a group of cones produced with 500 pulses at a fluence of ~80 mJ cm‐2. In this case, full apex angle of the cone is 83° corrected for the viewing angle of 60° on the SEM. At this fluence of ~80 mJ cm‐2, the cones have not fully developed and are not as well defined as those seen at higher fluences (**Figure 5b** and **c**), where the full apex angle is 66° and 51°, respectively, again illustrating that the angle is reduced at higher fluence. Here, they are fully developed, with sharp tips, and very well‐defined structure.

In **Figure 5**, the ablated surface of this polymer well away from the cone bases is seen to be relatively smooth and devoid from significant debris indicating the good surface quality of this material when ablated with the 157 nm laser. The fringes around the bottom of the cones can be clearly seen in **Figure 6a** with 100 pulses at ~60 mJ cm‐2 and **Figure 6b** with 100 pulses at ~180 mJ cm‐2.

pulses, **Figure 4c** and **d** led to an increase in the areal density of the cones compared to that at

**Figure 4.** Examples of cone formation on the CR39 surface using the 157 nm laser (a) 500 pulses at 112 mJ cm‐2 (b) 500

**Figure 5** shows a group of cones produced with 500 pulses at a fluence of ~80 mJ cm‐2. In this case, full apex angle of the cone is 83° corrected for the viewing angle of 60° on the SEM. At this fluence of ~80 mJ cm‐2, the cones have not fully developed and are not as well defined as those seen at higher fluences (**Figure 5b** and **c**), where the full apex angle is 66° and 51°, respectively, again illustrating that the angle is reduced at higher fluence. Here, they are fully

In **Figure 5**, the ablated surface of this polymer well away from the cone bases is seen to be relatively smooth and devoid from significant debris indicating the good surface quality of this material when ablated with the 157 nm laser. The fringes around the bottom of the cones can be clearly seen in **Figure 6a** with 100 pulses at ~60 mJ cm‐2 and **Figure 6b** with 100 pulses

lower pulse number, **Figure 4a** and **b**.

at ~180 mJ cm‐2.

developed, with sharp tips, and very well‐defined structure.

pulses at 180 mJ cm‐2 (c) 1000 pulses at 142 mJ cm‐2, and (d) 1000 pulses at 182 mJ cm‐2.

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

**Figure 5.** Evolution of conical structures developed on CR39 using the 157 nm laser (a) 500 pulses at ~80 mJ cm‐2 (b) 500 pulses at ~112 mJ cm‐2 (c) 500 pulses at *~*140 mJ cm‐2.

**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 at ~182 mJ cm‐2.
