## 3 Separate Analysis per Individual Components
### Thermal and Morphological Analysis of Oil
#### 3.2.1 Temperature Tests  
Thermal analysis of immersion oil was performed by exciting it with the same laser as in the previous experiment (\(\lambda_{Ex}=532\) nm) and measuring the temperature with a low-cost type K thermocouple, whose sensor is 1.15 mm in diameter. Although this thermocouple is relatively big, our laboratory technical limitations force us to use it. Figure 8  
Figure 6: Change in the refractive index when laser radiation excites: (**a**) Air; (**b**) Glass; (**c**) Distilled water; (**d**) Immersion oil. Figure 7: Behavior of the refractive index of the immersion oil and the working distance of an oil immersion objective. shows the experimental configuration set up for this purpose, where the laser is observed being incident in a normal orientation in the container center. Unlike optical metrology techniques that are usually full-field and do not require direct contact with the sample, the thermocouple must be immersed in the oil to make not only a local measurement of temperature, but also that of the surrounding oil. Therefore, when the thermocouple is immersed in the oil, it can be directly excited by the laser beam, mixing its own temperature with that of the oil, which would generate incorrect measurements. To prevent that problem, the thermocouple was positioned as close as possible to the laser beam while avoiding direct excitation. To find this position, an oil-free container was used, and the thermocouple was placed at the x-axis negative end (with y = 0 mm), at a z-height of just a few microns above the container bottom. From this point, air temperature measurements were made along the x-axis, passing through the laser beam center incident on the x = 0 mm, y = 0 mm position. The experiment was then repeated with the container filled with the immersion oil. Figure 9 also shows that the thermocouple starts to behave erratically the closer it gets to the laser beam center, where the points correspond to valid measurements and the crosses are erratic measurements. The results of air temperature (taken as a reference) and oil are presented in Figure 9, where it can be observed, in the profile corresponding to air, that the position closest to the laser where the thermocouple receives minimum excitation is x = \(-\)0.5 mm, y = 0 mm. The profile for the temperature differences between air (empty container) and oil is shown in Figure 10. If above-mentioned erratic measurements, in this case between \(-\)0.3  
Figure 8: Experimental setup used to analyze thermal behavior: (**a**) photo; (**b**) scheme. Figure 9: Air and oil temperature profiles. Crossed-out readings indicate erratic thermocouple behavior. and 0.4 mm, are discarded, it is possible to calculate--with MATLAB(tm) fit function--an approximate temperature profile that corrects thermocouple behavior (red line in Figure 10), and which is congruent (similar in shape but reverse in direction) to the refractive index change map central profile in Figure 6d. Thus, it can be inferred that oil reaches a temperature of around 120 \({}^{\circ}\)C at the beam center. Positioning the thermocouple at x = \(-\)0.5 mm, y = 0 mm, a temporal analysis of the oil temperature was then performed. In that experiment, measurements were taken under three different conditions: first with the laser source off for 10 s, then in a second scenario of 20 s with the laser on, and finally, a third scenario of 20 s with the laser off again; in all of them, measurements were taken continuously at 1 s intervals. The same procedure was applied in the experiment of Section 3.1.1 where the refractive index change was measured, although in that section, only the result for the 20 th second of the second scenario, which corresponds to the maximum excitation time, was presented. Figure 11 plots the thermocouple measurements as a function of time and compares them with the optical phase map maximum value corresponding to the refractive index.