Analysis and results
6.1.4 Summary of the surface analysis results
The samples #1 (metallic glass), #7 (AMTIR-6), #8 (AMTIR-1) and #26A (N-BK7) did not allow for measuring the step hight because the steps could not be seen in the surface scans due to the sample roughness and curvature. As for the quartz samples #16A (Vitreosil) and #21A (I/R quartz), only one step each could be found.
For the remaining samples, any missing values were estimated by calculating them from known step heights. For example, the step III-0 of sample #28A (SF-10) is missing but can be estimated by adding up either III-II and II-0 or III-I and I-0. Such estimated values are markedE in Table 6.1 on the facing page, AFM results A and
S denotes a value that was obtained from the stylus profilometry.
Figure 6.5 on the next page illustrates the compaction of the different samples.
A 2/3rds power curve is fitted to the measured data. The figure illustrates that the compaction of fused silica #31, Zinc crown #22, B270 Superwite #24 and SF-10 #28 appear to follow the expected 2/3rds power dependency of the deposited dose.
6.2. Refractive index 49
#21A #22A #24A #26A #28A #31A
I-0 x 15±1A 5±1A x 2S 123±46S
#1: Metallic glass - #3: GASIR-1 - #7: AMTIR-6 - #8: AMTIR-1 - #15A: Borofloat - #16A: Vitreosil
#21A: I/R Quartz - #22A: Zinc Crown - #24A: B270 Superwite - #26A: N-BK7 - #28A: SF-10 - #31A: Spectrosil
Table 6.1: Step heights of the irradiated samples, measured by means of AFM marked withA) or stylus profilometry (S). Not measurable steps are indicated with
“x”, if they could be estimated they are marked withE (see text for explanation).
0
Change of surface level (nm)
Region
Change of surface level (nm)
Region
Figure 6.5: Compaction of the different samples. The data from Table 6.1 is shown as well as a 2/3rds power fit.
6.2 Refractive index
Based on the findings obtained from the surface analysis it had to be shown whether the layer model of compaction that had been developed in Section 5.2.1 on page 41 would lead to reasonable predictions of refractive index in the irradiated areas. The data received from the model was then used as starting parameters for the ellipsom-etry measurements.
50 6. Analysis and results
6.2.1 Modeling
According to the initial simple model, the compaction-induced index change can be calculated with Equation 5.2 on page 42
n(∆t) = 1.037 + 0.195·ρ0
t0 t0−∆t
with the effective deptht0 =39.5 µm from the DosSim simulations and ∆t obtained from the surface analysis.
Region I Region II Region III
∆n
Figure 6.6: The calculated refractive index profile versus ∆t for the borosilicate glass #15A based on the simple model. The left scale shows the absolute index whereas the relative change is given by the right y-axis. From the AFM measure-ments the ∆tfor the three regions and the respective index changes are marked.
Figure 6.6 shows the calculated refractive index of borosilicate glass. The refractive index is increased with respect to the unexposed sample by an amount as large as 3.3×10−4 for region I (8 h of exposure), 5.8×10−4 for region II (16 h of exposure) and 8.6×10−4 for region III (24 h of exposure).
Whereas the simple model only assumes a uniform density change in the sample up to a certain depth t0, the more accurate layer model takes into account the z-depending dose that leads to a gradient density profile and hence a gradient index change in the sample. The refractive index profile inside the irradiated area of the glass samples was found to follow the function given in Equation 5.13 on page 43
n(z) = 1.037 + 0.195·ρ0·³1−C·D2/3(z)´
with the density of the unexposed material ρ0, the dose profile D(z) and the para-meterC, that depends on the change of the surface level ∆t. The constant factorC was defined in Equation 5.8 on page 43 as
C= ∆t
·Z t
0 D(z)2/3dz
¸−1
6.2. Refractive index 51
Region Relative change of the refractive index (10−6)
#3 #15A #16A #22A #24A #28A #31A
I 28 15 x 2.5 0.23 0.86 20
II 17 22 x 3.2 0.92 3.5 33
III 58 18 4.3 1.7 1.1 4.3 37
#3: GASIR-1 - #15A: Borofloat - #16A: Vitreosil - #22A: Zinc Crown
#24A: B270 Superwite -#28A: SF-10 - #31A: Spectrosil
Table 6.2: Overview of the relative change of the refractive index for all samples based on the layer model. Materials not listed did not allow for the calculation due to missing data of ∆t
whereby ∆t was received from the surface profiling measurements.
Figure 6.7 represents the relative index change in the three exposure regions of the borosilicate sample #15A.
Reltive change of refractive index
Depth (µm)
Region I Region II Region III
#15A Borosilicate
Figure 6.7: The relative index change of the borosilicate sample #15A according to the layer model.
With the density of borosilicate glass ρ0 = 2.22 kg/m3 the estimated maximum index changes at the surface become as large as 1.5×10−5, 1.8×10−5 and 2.2×10−5 for Region I, II and III, respectively.
Compared to the simple model of refractive index change, the increase is smaller by 22, 32 and 39 times for Region I, II and III, respectively. The reason for this might be that the simple model assumes a deposition of dose only within the first 39.5µm whereas the layer model uses a continuous dose function.
A summary of the relative index changes based on the layer model is given in Ta-ble 6.2.
52 6. Analysis and results According to the calculations, the fused silica sample #31A (Spectrosil) exhibits the largest index changes among the standard glasses, whereas the B270 Superwite #24A shows the smallest changes in refractive index. It has to be said, however, that the model does only consider index changes driven by compaction and neglects any other mechanisms such as colour-centres.
6.2.2 Ellipsometry
The results of the ellipsometry measurements on the SF-10 glass (sample #28A) are shown in Figure 6.8(a).
400 420 440 460 480 500 520 540 560 580 600 620 1,700
Figure 6.8: (a)Refractive index measured of the exposure region III of the SF-10 glass (#28A) by means of ellipsometry. The literature values for the refractive index of the SF-10 glass are provided, too.
Due to difficulties experienced with the analysis of ellipsometry data the results of these measurements will be limited to those obtained using the SF-10 sample which exhibits the best data in that it shows the best agreement with literature values.
Besides the literature values for the refractive index of the blank SF-10 glass the graph shows four measured lines reflecting one exposure region each. Thus the ellipsometer autonomously calculated the index of refraction assuming a homogenous material, hence it can at most be regarded as an average value.
Without taking into account the relative shift between the curves of the data sheet and the measured region 0 they are both in very good congruence. The unexposed region of the SF-10 was measured twice; first within the permanently masked region 0 of the irradiated sample and a second time on the remaining second half-disc that had not be exposed to the X-rays at all. The result is an agreement of the two measurements of more than 99.99 %.
6.2. Refractive index 53 The position of the three measured curves with respect to each other shows that the observed dose dependence does not match the expectation. Thus region 0 would need to actually have a higher refractive index than both region III and region I, whereas the graph shows only the proportion between region III and region I as expected.
It has to be taken into account, however, that the model which the ellipsometric calculations are based on, assumes a homogenous layer of changed refractive index in the densified volume. However it is to be expected that the density and thereby the index of refraction increases gradually, reaching a maximum at the surface.
6.2.2.1 Problems performing ellipsometry analysis
As mentioned at the beginning of the section several issues arose that made it difficult to obtain useful data from the ellipsometric measurements.
Beam cross section and backside reflection
The transparency of the samples lead to backside reflection of the incident beam as illustrated in Figure 6.9. This additionally reflected beam, however, does not contribute to the measurement but interferes with it as it is caused by reflection at the boundary layer of the sample and the metal sample holder.
Hence the backside reflected beam hat to be blocked by inserting a shield into the beam before it hits the photo detector. Thus the two beams had to be kept separated by choosing a small beam cross section. This, however, decreases the intensity of the registered light at the photo detector and can lead to a loss of the signal.
Photo detector
Sample Sample holder
Incident beam
Shield
Small beam cross section Large beam cross section
Figure 6.9: Backside reflection in transparent samples and comparison of different beam cross sections. A large cross section provides a good signal but makes it impossible to block the backside reflected beam entirely. A small cross section, however, keeps the beams separated but does not provide a sufficient intensity.
54 6. Analysis and results
Instability of the system
The instrument used for the measurements showed a great instability in terms of reproducibility of the results. Due to restrictions in the access of the facility one session a week was possible at the most. Between two such sessions the obtained data showed a remarkable inconsistency that resulted in a difference of two identical measurements of up to 2 in the refractive index. Although there should be no need for any calibration it seemed that it was a systematical, but very random error. To improve the measurements, the warm-up time for the instrument was more than half an hour. Each measurement was repeated at least three times. However, large variations in the obtained data still remained existent.
Chapter 7
Conclusion
The objective of this diploma thesis project was to perform experiments on syn-chrotron radiation applied to different glass samples and conduct subsequent anal-ysis into densification and changes in the index of refraction. Based on these first results a fundamental understanding of the effects of synchrotron light on glass was to be established.
Bulk samples of different glass types and fibres were exposed to synchrotron radiation for a range of times to allow for comparison of the impact of different radiation doses on the material. These experiments were carried out at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. In order to determine appropriate durations for exposure and to develop a suitable and time effective mask layout, test irradiations on microscope cover glasses were performed prior to exposing the samples to synchrotron light. Four different regions with irradiation times of 0 h, 8 h, 16 h and 24 h, respectively were created on each single sample by partially blocking the synchrotron light off certain areas of the samples.
The surface of the irradiated samples was then analysed. AFM scans and stylus profilometry demonstrated the level of compaction is reliant upon both the radiation dose and the material and follows the expected 2/3rds power dependency stated by Primak. Other effects such as colouring, surface cracking and grading also appeared to varying degrees as a result of the irradiation process. Difficulties arose from surface displacements that occurred in case of some of the samples as well as the little impact of the synchrotron light on certain materials.
The research conducted led to the development of a model of the dose dependent compaction enabling prediction of the expected changes in the refractive index when silica glasses are exposed to synchrotron light. Similar to the model underpinning
56 7. Conclusion elasticity theory, the samples are regarded as comprising an infinite number of thin layers that get densified independently according to the profile of the deposited dose within the material. The dose profile was gained from the software DoseSim 3.1 where the light source, experimental station with windows and filters is simulated in order to calculate the deposited dose. The 2/3rds power relation between densi-fication and dose leads to a gradient density profile with the highest density close to the surface, decreasing towards the inside of the sample. From a known, linear relationship between density and refractive index, the relative change of the index of refraction can be calculated.
To measure the increase of refractive index initial ellipsometry experiments were conducted at an external laboratory at the University of Melbourne. Due to access and time restrictions and other problems regarding the stability of the instrument, these results are unreliable and inconclusive. They allow the conclusion, however, that the sensitivity and potential of ellipsometry will lead to results. Thus, a more concerted effort is required to improve the stability and repeatability. A custom built system and the development of a model might be required. These tasks are beyond the scope of this thesis project and will be carried out in the course of a PhD project.
Finally it can be said that with respect to the applicatory value of synchrotron radiation for the manufacture of optical devices, a great deal of research is to be done, yet. The results of this diploma thesis project contribute and provide fundamentals.
With the benefit of the model developed, research may now focus on ellipsometry results. The breadth of required research is thus narrowed, ensuring practical usage of in-fibre technologies is now one step closer.