SciPost Submission Page
Interferometric microscale measurement of refractive index at VIS and IR wavelengths
by Meguya Ryu, Simonas Varapnickas, Darius Gailevicius, Domas Paipulas, Eulalia Puig Vilardell, Zahra Khajehsaeidimahabadi, Saulius Juodkazis, Junko Morikawa, Mangirdas Malinauskas
Submission summary
| Authors (as registered SciPost users): | Mangirdas Malinauskas |
| Submission information | |
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| Preprint Link: | https://arxiv.org/abs/2402.15249v1 (pdf) |
| Date submitted: | 2024-02-26 09:46 |
| Submitted by: | Malinauskas, Mangirdas |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approaches: | Experimental, Observational |
Abstract
Determination of refractive index of micro-disks of a calcinated ($1100^\circ$C in air) photo-resist SZ2080$^\mathrm{TM}$ was carried out using transmission and reflection spectroscopy. Interference fringes at specific wavenumbers/wavelengths were selected for determination of the optical thickness, hence, the refractive index when the thickness of micro-disks was measured by scanning electron microscopy (SEM). Refractive index of disks of $\sim 6\pm 1~\mu$m thickness were determined at visible and IR (2.5-13~$\mu$m) spectral ranges and where $2.2\pm 0.2$ at visible and IR wavelengths. Peculiarities of optical characterisation of micro-optical structures are discussed in view of possible uncertainties in the definition of geometric parameters, shape and mass density redistribution.
Current status:
Reports on this Submission
Report
The authors present the fabrication of the micro-fabricated photoresist disks and the polychromatic interferometric determination of the optical retardation. It seems the fabricated structures have similar retardation in a wide energy range (in visible and IR ranges) and it is scaled with the disk size (at least for the two sizes that are mainly discussed. I think the authors managed to fabricate very homogeneous disks with homogeneous qualities and characterize their disks with rather standard interferometric measurements. The overall work is interesting, but I fail to see the groundbreaking aspect of it, especially when there is still room for improvement in the measurement and analysis aspects. Therefore, I think it will be more suitable for the SciPost Physics Core.
Here are some of the points that authors should consider for the revision:
1. Although the interferometric determination of the optical properties might be rather standard, no details about the measurement setup is given. Perhaps for the general reader, it could be helpful to describe the experiments leading to the transmission and reflection spectra.
2. Authors describe their spectra with only a single slab problem assuming that the disk is sandwiched in between air. However, I think the situation is more complex than this, as there should be multiple reflections between the sample and the silica substrate, as well as the substrate itself (especially in the IR region, where the silica is only partially transparent, at least in some portion of the measurement range). The situation is also visible in the reflectance curve in Fig.5, where one can see multiple shoulders to the peaks, etc. I think the correct determination of the peak positions is also important for the correct determination of the refractive index.
Minor comments:
a. It should be Michelson interferometer (in Fig.4 it is given as Mickelson interferometer!)
b. Sometimes the information on the figures are not described (or it is not immediately visible) for instance, in Fig.1, Transmittance spectrum is given with green and red dashed lines defining something, which is not very clear what. In Fig. 2, Sample A and B are given but their difference becomes clear only after careful reading and it is not immediate. I realize that the authors put the scale, but it feels like only belongs to Sample B.
c. In the text, authors mentioned that they fit the Transmittance with Eq.5 (Last paragraph of page 8), while in the Figure4 caption, it is mentioned that it was Eq.6 (which is probably the one).
Recommendation
Accept in alternative Journal (see Report)
Report
While it may be note groundbreaking (per SciPost Physics first criteria) results, the authors present a solid and scientifically sound study on the optical properties of microfabricated disks made from photoresist. The refractive index has been determined on microscales using a conventional approach based on interference fringes in spectral response. This method, known and robust, has been demonstrated by the authors to be applicable at such scales and for this particular purpose.
Major comment:
It is plausible that the authors have addressed this issue in the manuscript, but it wasn't readily apparent to me. The authors assumed interference solely from reflections off both surfaces of the microdisk, which are approximately 6 micrometers apart. However, a similar interference effect may arise from the reflection of the beam off the microdisk and the fused silica substrate. The separation between microdisk and substrate of 40 um and the refractive index of fused silica should induce a comparable level of interference modulation, which should indeed be present. This is similar to etalon. Furthermore, similar to the described beams, such interference might be observed in both reflection and transmission. Is it possible that the observed fringes result not solely from the interference of reflections from the microdisk surfaces (akin to thin film interference) but also from the microdisk substrate? The Newton ring-like pattern (Figure 5b) also supports this notion. This may significantly change modelling and results.
Minor:
1. This is a subjective opinion, but it would be beneficial to juxtapose reflection and transmission data to illustrate the correlation in the interference pattern. Currently, the primary focus seems to be on reflection alone. Two graphs put together can be very useful.
2. Again, this is subjective. While Equation 9 is undoubtedly correct, the message surrounding it and Figure 5d is somewhat confusing. It intuitively suggests that the refractive index is dependent on the incident angle (which is a fundamental property of materials), whereas the real significance lies in the importance of knowing the incident angle with high precision to determine the refractive index from the data. Perhaps some rewording could clarify the uncertainty and the dependence on the incident angle.