Femtosecond 3D printing has emerged as an important technology for manufacturing nano- and microscopic optical devices and elements. Detailed knowledge of the dispersion in the visible and near-infrared spectral range is crucial for the design of these optical elements. Here we provide refractive index measurements for different UV-doses, aging times, heat treatment and 2-photon exposed structures for the photoresists IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4. We use a modified and automized Pulfrich refractometer setup, utilizing critical angles of total internal reflection with an accuracy of 5·10−4 in the visible and near-infrared spectral range. We compare Cauchy and Sellmeier fits to the dispersion curves. We also give Abbe numbers and Schott Catalog numbers of the almost entirely polymerized resists. Additionally, we provide quantitative extinction and luminescence measurements for all photoresists.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Over the last years, a large variety of micro- and nanooptical elements and even multi-element microobjectives have been demonstrated using 3D direct laser writing [1-19]. In order to optimize the imaging performance of micro- and nanooptical elements the detailed knowledge of dispersion of the used photoresists is crucial. Furthermore, the knowledge of the dispersion of different optical materials enables the construction of optical devices with achromatic functionality, as recently demonstrated . The degree of polymerization of different photoresists, and therefore their optical properties such as refractive index and extinction, depends strongly on different parameters such as UV dose, aging time, 1- photon or 2-photon polymerization, and post processing (e.g. heat treatment and further UV exposure) .
Here we provide detailed, quantitative, and systematic measurements of the refractive index dependency of UV dose, aging, heat treatment, and 1- or 2-photon dependence. We measure the critical angle of total internal reflection with a modified and automized Pulfrich refractometer setup, as depicted in Fig. 1.
The linearly polarized light source is a tunable white light laser (NKT SuperK EXTREME) with an accessible range from 500 nm to 1400 nm. To further extend the accessible wavelength range we use an additional laser diode at 405 nm (Thorlabs CPS405 collimated laser diode module). The light is guided through an astronomical telescope onto the base of a N-SF11 prism where a reference material (N-BK7 plate) is optically bounded by pressure and the photoresist under investigation is hardened in an area right next to the reference plate. Using a mirror on a rotation mount the angle of the incident beam onto the base of the prism can be changed. The reflected light is guided through another astronomical telescope onto a photodiode which measures the angle-dependent intensity. The critical angle of total internal reflection is determined by measuring the reflected intensity depending on the angle of incidence on the base of the prism, which is controlled by the rotation mount. The angle of incidence can be calculated using basic trigonometric functions and Snell’s law to
To calculate the refractive index of the photoresist we have to obtain the point of the critical angle of the prism/photoresist interface as well as for the prism/ BK7 glass interface to calibrate the incident angle.
Therefore, one part of the prism is coated with an approximately 2 mm thick layer of the photoresist and on the other part we optically bond a BK7 glass plate by pressure.
To obtain the critical angle we ﬁt the corresponding reﬂectance given by Fresnel’s equation
3. Experimental details
The degree of polymerization of different photoresists and hence their refractive indices, depends strongly on different parameters such as UV dose, aging process, heat treatment, and 1- photon or 2-photon illumination [20,22]. In the following we present and discuss the typical refractive index behavior of the photoresists IP-S, IP-Dip, IP-L, and OrmoComop depending on these parameters.
3.1 UV dose dependency
To study the refractive index depending on the UV dose, we expose samples of each photoresist for different illumination times to a UV lamp (DymaxBlueWave 50 delivering 365 nm with an intensity of 3000 mW/cm2, at a distance of 3 cm with resulting intensity of 250 mW/cm2) and measure the refractive index afterwards as described above. Figure 2 (a) illustrates the results for the different photoresists. As expected, the refractive index increases over the entire wavelength range with the UV exposure, however, with longer exposure times the refractive index change saturates and exhibits a limited growth behavior as depicted in Fig. 2 (b). This behavior can simply be explained with the polymerization process, which is triggered by UV light. As the exposure to UV light persists, the degree of polymerization increases but slows down as more and more areas of the photoresist become almost entirely polymerized. The same behavior has been observed in 2-photon polymerized samples of IP-Dip using different laser intensities .
This saturation behavior can be seen in all photoresists. However, it strongly depends on the UV sensitivity of each photoresist. The refractive index of IP-S and IP-L still rises after 30 minutes of UV duration, whereas IP-Dip is saturating. OrmoComp exhibits a much higher UV sensitivity and saturates within several seconds.
3.2 Aging process
The refractive index also depends on the aging process of the photoresists which is overall not as significant as the UV exposure. Post polymerization can take up to several days and further increases the refractive index after the initial UV exposure even without further UV treatment. After the photo initiators are activated by UV exposure the polymerization process starts. The activated photo initiator is slowly used up during the polymerization, which leads to a rise in the degree of polymerization and therefore the refractive index over time. To compare the effects of the aging process we expose the photoresists to the UV light for 15 minutes (250 mW/cm2) and measure the refractive indices directly after the exposure, after 24 hours, 48 hours, and 72 hours.
The refractive index slightly increases with the aging time as depicted in Fig. 3. However, the changes are below 10−3 except for IP-L, which has the strongest refractive index increase due to post polymerization, which occurs mainly within the first 24 hours. Measurements after a longer time period have also been taken but did not show a significant increase of the refractive indices.
3.3 Heat treatment
The refractive index of UV treated photoresists strongly depends on the photoresist itself, the UV curing time, and the aging process which leads to an uncertainty in predicting the finally obtained refractive indices. However, this problem can be solved using heat treatment (post baking) of the photoresists after the UV curing.
Different measurements have shown that 1-hour post baking at 60 °C is sufficient to reach a highly polymerized state for each of the investigated photoresists even with a low initial UV curing time of 5 minutes. Using heat treatment, we increase the reaction speed of the polymerization and therefore reach the saturation limit much faster. Furthermore, some photo initiators can be activated by higher temperatures. We also observed that the refractive indices did not change further over time because post polymerization cannot significantly influence the refractive index anymore. The refractive indices before and after the heat treatment are plotted in Fig. 4.
3.4 2-photon polymerization
As recently shown for IP-Dip, the refractive index of photoresists after 2-photon polymerization also depends on the used laser intensity .
The 2-photon polymerized measurement patches are fabricated directly onto the back of the measurement prism using a 3D direct laser writer (Photonic Professional GT) with a 25x objective.
We perform 2-photon polymerized measurements for IP-S, IP-Dip, and OrmoComp. Due to sticking issues of the 2-photon polymerized photoresist with the prism it was not possible to perform the 2-photon polymerized measurement for IP-L with our method.
The results are similar to the almost entirely polymerized refractive indices after post baking of the UV cured samples except for IP-Dip (see Fig. 5). For this high-resolution photoresist the coarse slicing (2µm) and hatching (0.8 µm) parameters required to write patches with sufficient size for the measurements in a reasonable time provided not enough intensity for complete polymerization. Therefore, the refractive index is almost as low as the refractive index of the liquid IP-Dip photoresist.
3.5 Heat treated wide spectral range measurements
Finally, we show the measured dispersion for the heat treated 1-photon polymerized photoresists IP-S, IP-L, IP-Dip, OrmoComp, IP-Visio, and PO4 after 15 minutes UV curing and 1-hour post baking at 60 °C between 405 nm and 1400 nm. The refractive indices did not further increase neither by additional UV exposure, nor aging. Therefore, we expect them to be almost completely polymerized.
As shown in Fig. 6 the Cauchy fit can describe the dispersion of the photoresists only up to a wavelength of about 900 nm-1000 nm. For higher wavelength a Sellmeier fit provides a much better match between our model and the measurements. The corresponding fit function are for the Cauchy fit
and for the Sellmeier fit
Table 3 and Table 4 show the measured refractive indices, the corresponding Abbe number and Schott catalog number for each photoresist from 405 nm to 1400 nm after 15 minutes UV curing and post baking at 60 °C for 1 hour.
The refractive indices and dispersions can be visualized using an Abbe diagram as depicted in Fig. 7. Comparing the measurement results we find, that IP-S, IP-L, OrmoComp, and IP-Visio show comparable refractive indices nd (between 1.51 and 1.52) and dispersions represented by the Abbe numbers between 47 and 51. IP-Dip, in contrast, has a higher refractive index nd above 1.55 and stronger dispersion represented by an Abbe number of 36 and PO4 has the highest refractive index nd above 1.62 and an Abbe number of 25, thus the strongest dispersion. In comparison to the values that were obtained with 5 minutes UV exposure from  the refractive indices and Abbe numbers rise for all photoresists due to the stronger polymerization except for OrmoComp, which is more UV sensitive and therefore already almost entirely polymerized after 5 minutes of UV exposure.
This result is helpful for the choice of photoresists for achromatic optical devices and elements, as different Abbe numbers are required. To realize achromatic functionality one should combine IP-Dip or PO4, which are outstanding for a high dispersion and refractive index, with one of the other photoresists, for example IP-S .
3.6 Extinction measurements
In addition to the refractive indices we also measured the extinction coefficients (extinction=–ln[transmission]) for 1 mm thick samples where the photoresist was embedded between two microscope slides and 1-photon UV hardened for 15 minutes. The measurements were referenced to two 1 mm microscope slides with a thin film (micrometer thickness) of the investigated photoresist to account for Fresnel reflection on the surface. We measured the different photoresists with a UV-VIS spectrometer (Zeiss detector NIR - MCS611 2.2 and VIS - MCS621visII, light source CLH600) from 300 nm to 2000 nm in the liquid state, after 15 minutes UV curing (OrmoComp just 1.5 minutes) and after additional heat treatment (1 hour at 60 °C), see Fig. 8. Due to the detector change at 950 nm a discontinuity in the absolute extinction value occurs. The error in the extinction coefficient is around 10-15%.
For all photoresists, the extinction has a local maximum at around 400 nm due to the photo initiator. The extinction values decrease to a local minimum at around 600 nm and increase for higher wavelengths, due to molecular vibration overtones.
The extinction behavior of IP-S, IP-Dip, and IP-Visio is very similar, whereas IP-L exhibits the highest extinction which shows very little variations over the spectral range. OrmoComp and PO4 exhibit the lowest extinction values followed by IP-S, which can be observed in Fig. 9. That makes them the appropriate choice for applications with thicker optical elements. Additional vibrational overtones are visible in the near-IR range.
3.7 Luminescence measurements
The final material property discussed in this paper is the photoresist luminescence, as it is an important property to know in the context of possible 3D-written security features  as well as for possible use in 3D printed fluorescence microscope optics. We measure the luminescence with a Nikon Eclipse LV100 upright microscope using a Princeton Instruments Isoplane 160 with a Pixis 256e CCD camera. For excitation we use a Nikon super high pressure mercury lamp. We performed the measurements for all photoresists in the liquid state, after 15 minutes of UV curing and after additional heat treatment. The results are displayed in Fig. 10. The excitation wavelength is 390 +/− 18 nm with an exposure time of 15 seconds with 10 accumulations. All photoluminescence spectra are recorded with the same excitation conditions and recording times to allow for quantitative photoluminescence comparisons.
All photoresists show a strong luminescence maximum around 500 nm. The UV curing and heat treatment decrease the maximum at 500 nm for IP-L and IP-Dip, while it is shifted to slightly higher wavelengths for IP-S and IP-Visio. IP-S shows a factor 6 smaller luminescence in the liquid state than IP-Dip and IP-L which reduces to a factor 2-3 after UV curing and heat treatment. OrmoComp, IP-Visio, and PO4 exhibit more than an order of magnitude lower luminescence, which increases with UV curing and heat treatment. This makes them the appropriate choice for applications where low background luminescence in the visible is required. The different luminescence intensities are compared in Fig. 11.
In this work we measured the refractive index dispersion in the visible and near IR spectral range for six different photoresists, namely IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4.
We studied the different effects of UV dose, aging, heat treatment, and 2-photon writing on the refractive indices of IP-S, IP-Dip, IP-L, and OrmoComp. For all photoresists the refractive index increases and saturates with the UV duration and the aging process. Using heat treatment for 1 hour at 60 °C is sufficient to increase the polymerization process to achieve almost entire polymerization even with short UV exposure durations. Due to the used high laser power, our 2-photon polymerized structures are also almost entirely polymerized, sufficient small hatching and slicing provided (which was not achieved for IP-Dip). Using lower intensities, the refractive index of 2-photon polymerized photoresists can also change . We performed wide range measurements from 405 nm to 1400 nm for the almost entirely polymerized photoresists and also provide the Abbe numbers to compare the dispersions as well as Sellmeier fits that give best agreement between model and experiments.
The extinction of the photoresists is also increased during the polymerization process. The luminescence spectra of all photoresists show a maximal luminescence around 500 nm, OrmoComp, IP-Visio, and PO4 show the smallest luminescence.
Deutsche Forschungsgemeinschaft (GI 269/12-1); Bundesministerium für Bildung und Forschung (13N14097, PRINTFUNCTION, PRINTOPTICS); Baden-Württemberg Stiftung (OPTERIAL, Spitzenforschung II); H2020 European Research Council (COMPLEXPLAS); European Research Council (PoC 3D PRINTEDOPTICS).
We thank David Neußer and Sabine Ludwigs for help with their UV-VIS spectrometer. We also thank Simon Thiele for advice with Zemax OpticStudio. This publication was supported by the Open Access Publishing Fund of the University of Stuttgart.
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