**2. Experimental procedure**

#### **2.1. VUV F2 laser**

A 157 nm VUV F<sup>2</sup> laser (Lambda Physik LPF 200) that produced output energy of up to 35 mJ in an 11 ns pulse (full width at half maximum) was used to expose the various polymer samples. The charging voltage of the laser could be varied in 1 kV steps from 21 to 26 kV, and this allowed the laser energy and hence fluence at the target to be varied. The full‐angle beam divergence of the direct output beam was ~3 mrad in its narrow dimension and ~8 mrad in its long dimension.

The polymer samples were held on a motorized stage, which was capable of movements in the *x*‐*y*‐*z* directions in increments from 1 mm to 1 cm under computer control. Due to the high absorption of the 157 nm wavelength in oxygen in air, the laser output had to be delivered either in vacuum or in a rare gas such as argon. In these experiments, the target was placed in a chamber that was capable of being evacuated down to 1 × 10‐5  mbar. The chamber was evacuated using a dry pump, and the pressure was measured using Edwards Pirani/Penning 1005 pressure gauges. The chamber could also be purged with He or Ar gas as an alternative for beam delivery but in these experiments this was not used. **Figure 1** shows a schematic diagram of the F<sup>2</sup> laser experimental setup.

**Figure 1.** F<sup>2</sup> laser experimental setup.

ies on nonlinear refractive index change of glass by femtosecond laser irradiation [6]. It has been reported through time regarding the interaction of polymer materials with laser at different photoetching techniques by means of infrared nanosecond laser, femtosecond laser as well as excimer laser [7]. Nevertheless, several parameters in combination includ‐ ing, among others, the material as well as the laser pulse and energy affect the material

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

threshold for these materials at these wavelengths. Refractive index of CR39 is changed by varying the laser fluences using continuous wave laser. To obtain higher fluence in respect of

For pulse laser, an aperture size of 6 × 3 mm is aligned and positioned in front of the lens to ensure substantial edge of craters can be seen on the film. Subsequently, examination of the pulse crater is carried out using a microscope to conclude the depth of each crater. After a few pulses, the correlation between adjustments of fluence and etch depth is determined. The exact number of pulses depends upon the fluence, wavelength, and absorption of the polymer in which the system will settle down to a constant etch depth per pulse. On the other hand, when continuous wave laser is used, the refractive index calculated after laser induced depends on the numerical aperture (NA) measurement on the written waveguides. Positive refractive index is perceived, and consequently, the association between irradiation fluence and refractive index change can then be concluded. In cases where laser ablation is initiated

laser (Lambda Physik LPF 200) that produced output energy of up to 35 mJ in

 mbar. The chamber was

an 11 ns pulse (full width at half maximum) was used to expose the various polymer samples. The charging voltage of the laser could be varied in 1 kV steps from 21 to 26 kV, and this allowed the laser energy and hence fluence at the target to be varied. The full‐angle beam divergence of the direct output beam was ~3 mrad in its narrow dimension and ~8 mrad in its long dimension.

The polymer samples were held on a motorized stage, which was capable of movements in the *x*‐*y*‐*z* directions in increments from 1 mm to 1 cm under computer control. Due to the high absorption of the 157 nm wavelength in oxygen in air, the laser output had to be delivered either in vacuum or in a rare gas such as argon. In these experiments, the target was placed

evacuated using a dry pump, and the pressure was measured using Edwards Pirani/Penning 1005 pressure gauges. The chamber could also be purged with He or Ar gas as an alternative for beam delivery but in these experiments this was not used. **Figure 1** shows a schematic

the changes to be made, the laser spot size is focused down to microns in diameter.

above this perimeter, an upper fluence limit could also be attained.

in a chamber that was capable of being evacuated down to 1 × 10‐5

laser experimental setup.

(157 nm laser) and KrF (248 nm laser) to obtain ablation

removing processes [8].

In this work, we focus on using F<sup>2</sup>

**2. Experimental procedure**

 **laser**

**2.1. VUV F2**

A 157 nm VUV F<sup>2</sup>

diagram of the F<sup>2</sup>

### **2.2. 248 nm Pulsed UV laser**

KrF excimer laser (Opto Systems Ltd Excimer Laser series CL5100) operating at 248 nm wave‐ length was the laser tools used in this research. The maximum repetition rate can range up to 100 Hz with a maximum average output power of 5 W and a pulse interval range from 9 to 11 ns. A 50 mm focusing lens is used to focus the light onto the CR39, which has a thickness of 1 mm and is in a sheet form (Solar lens product). A three‐axis *x*‐*y*‐*z* translational platform was used to mount the sample. In the effort to position the polymer at the focus point, the sample location was varied through the micrometer‐driven sample holder. The minimum spot size formed on the surface of the CR39 at the focus of the laser beam is noted as 465 × 255 nm<sup>2</sup> . Fluence of laser ablated on the CR39 can be calculated from the ratio of pulse energy (mJ) per laser spot area:

$$\text{Fluence} = \text{Energy(mI)} / \text{Area(cm}^2) \tag{1}$$

For the whole experiments, fluence measurements are within an uncertainty of ±5%.

### **2.3. Continuous UV laser**

For waveguide channel fabrication on CR39, a frequency‐doubled argon ion laser emitting at 244 nm was used as a light source. The laser beam is focused by a 25‐mm focusing lens onto the CR39. It was placed onto a three‐axis stepper motor. By translating the stepper motor along the laser focusing plane, straight waveguides were produced. Fluence of laser irradia‐ tion on the CR39 can be calculated using the following equation [9]:

$$\text{Volumece} = \text{P}\_{\text{out}} \text{d} / \text{v.A} \tag{2}$$

where *F* is the fluence, *P*out is the power of fiber output, *d* is the diameter of the beam, *v* is the relative traveling speed, and *A* is the area of the beam.

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

The measured numerical aperture (NA) of the written waveguide can be used to compute the refractive index change, ΔRI of the UV irradiated area using the formula below:

$$NA = \left(n\_c - n\_{cl}\right)^{1/2} \tag{3}$$

where *n*<sup>c</sup> and *n*cl are the refractive index of UV written area and unwritten area, respectively. ΔRI can be calculated from the difference between *n*<sup>c</sup> and *n*cl.

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 from the waveguides divergence angle, θ:

$$\mathbf{N}\mathbf{A} = \mathbf{N}\_{\mathbf{c}}\sin\theta\tag{4}$$

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