Refractive index measurements of photo-resists for three-dimensional direct laser writing

Femtosecond 3D printing is an important technology for manufacturing of nanoand microscopic devices and elements. Crucial for the design of such structures is the detailed knowledge of the refractive index in the visible and near-infrared spectral range and its dispersion. Here, we characterize 5 photoresists that are used with femtosecond 3D direct laser writers, namely IP-S, IP-Dip, IP-L, IP-G, and OrmoComp with a modified and automized Pulfrich refractometer setup, utilizing critical angles of total internal reflection. We achieve an accuracy of 5 ⋅ 10 and reference our values to a BK-7 glass plate. We also give Abbe numbers and Schott Catalog numbers of the different resists. Their refractive indices are in the 1.49-1.57 range, while their Abbe numbers are in the range between 35 and 51. © 2017 Optical Society of America OCIS codes: (160.0160) Materials; (160.4760) Optical properties; (160.5335) Photosensitive materials, (260.2030) Dispersion. References and links 1. M. F. Schumann, S. Wiesendanger, J. C. Goldschmidt, B. Bläsi, K. Bittkau, U. W. Paetzold, A. Sprafke, R. B. Wehrspohn, C. Rockstuhl, and M. Wegener, “Cloaked contact grids on solar cells by coordinate transformations: designs and prototypes,” Optica 2(10), 850–853 (2015). 2. M. Wegener, “Scharfe Linsen frisch gedruckt,” Phys. J. 15, 24–25 (2016). 3. J. Fischer and M. Wegener, “Three-dimensional optical laser lithography beyond the diffraction limit,” Laser Photonics Rev. 7(1), 22–44 (2013). 4. Z. Tian, X. Cao, W. Yao, P. Li, Y. Yu, G. Li, Q. Chen, and H. Sun, “Hybrid Refractive–Diffractive Optical Vortex Microlens,” IEEE Photonics Technol. Lett. 28(21), 2299–2302 (2016). 5. J. Xu, W. Yao, Z. Tian, L. Wang, K. Guan, Y. Xu, Q. Chen, J. Duan, and H. Sun, “High Curvature Concave– Convex Microlens,” IEEE Photonics Technol. Lett. 27(23), 2465–2468 (2015). 6. Z. N. Tian, W. G. Yao, J. J. Xu, Y. H. Yu, Q. D. Chen, and H. B. Sun, “Focal varying microlens array,” Opt. Lett. 40(18), 4222–4225 (2015). 7. D. Wu, S. Wu, L. Niu, Q. Chen, R. Wang, J. Song, H. Fang, and H. Sun, “High numerical aperture microlens arrays of close packing,” Appl. Phys. Lett. 97(3), 031109 (2010). 8. M. S. Rill, C. Plet, M. Thiel, I. Staude, G. von Freymann, S. Linden, and M. Wegener, “Photonic metamaterials by direct laser writing and silver chemical vapour deposition,” Nat. Mater. 7(7), 543–546 (2008). 9. M. Thiel, A. Radke, B. Fries, D. Eicke, F. Niesler, C. Baretzky, T. Bückmann, and M. Wegener, “High-Speed 3D Direct Laser Writing of Micro-Optical Elements,” in CLEO: 2013, OSA Technical Digest (online) (Optical Society of America, 2013), paper ATu2N.4. 10. S. Thiele, T. Gissibl, H. Giessen, and A. M. Herkommer, “Ultra-compact on-chip LED collimation optics by 3D femtosecond direct laser writing,” Opt. Lett. 41(13), 3029–3032 (2016). 11. T. Gissibl, M. Schmid, and H. Giessen, “Spatial beam intensity shaping using phase masks on single-mode optical fibers fabricated by femtosecond direct laser writing,” Optica 3(4), 448–451 (2016). 12. T. Gissibl, S. Thiele, A. Herkommer, and H. Giessen, “Two-photon direct laser writing of ultracompact multilens objectives,” Nat. Photonics 10(8), 554–560 (2016). 13. T. Gissibl, S. Thiele, A. Herkommer, and H. Giessen, “Sub-micrometre accurate free-form optics by threedimensional printing on single-mode fibres,” Nat. Commun. 7, 11763 (2016). 14. S. Thiele, K. Arzenbacher, T. Gissibl, H. Giessen, and A. M. Herkommer, “3D-printed eagle eye: Compound microlens system for foveated imaging,” Sci. Adv. 3(2), e1602655 (2017). 15. D. B. Fullager, G. D. Boreman, and T. Hofmann, “Infrared dielectric response of nanoscribe IP-dip and IP-L monomers after polymerization from 250 cm to 6000 cm,” Opt. Mater. Express 7(3), 888–894 (2017). Vol. 7, No. 7 | 1 Jul 2017 | OPTICAL MATERIALS EXPRESS 2293 #291435 https://doi.org/10.1364/OME.7.002293 Journal © 2017 Received 27 Mar 2017; revised 20 May 2017; accepted 23 May 2017; published 9 Jun 2017 16. J. Guild, “Notes on the Pulfrich Refractometer,” Proc. Phys. Soc. Lond. 30(1), 157–189 (1917). 17. B. Schaefer, Lehrbuch der Experimentalphysik Band 3 Optik, 10 ed., de Gruyter, NY and Berlin (2004), p. 70. 18. A. G. Schott, “Optisches Glas Datenblätter”, (2016), http://www.schott.com/d/advanced_optics/47d79895-2965472d-83ed-af9e48ac72c0/1.1/schott-optisches-glas-datenblatt-sammlung-german-17012017.pdf 19. F. A. Jenkins and H. E. White, Fundamentals of Optics, 4 ed., McGraw-Hill, Inc. (1981), p. 479.


Introduction
The dispersion of optical materials is an essential information for the design of high quality optical systems.In order to improve the optical performance, such as imaging quality and resolution, dispersion effects have to be taken into account.
As dispersion data of photosensitive materials are missing to our knowledge we measure the refractive indices at different wavelengths and determine the functional dependency.For this purpose, we measure the critical angle of total internal reflection of the used photoresists.In particular, Nanoscribe IP-Dip, micro resist OrmoComp, Nanoscribe IP-G, Nanoscribe IP-L, and Nanoscribe IP-S are analyzed.
The critical angle of total internal reflection is measured in the modified Pulfrich refractometer [16,17] setup, which is a variant of critical angle refractometers, as depicted in Fig. 1(a).Linearly polarized light of a laser diode is directed through an astronomical telescope onto the base of a prism.The angle of the incident beam onto the base of the prism is controlled by a rotation mount.The astronomical telescope ensures that there is nearly no beam walk on the back of the prism, see Fig. 1(b).The reflection is guided through another astronomical telescope on a photodiode.

Method
Using trigonometric functions and Snell's law, the angle γ (see Fig. 1(b) for the definition) can be calculated from the rotation angle α of the rotation mount by ( ) ( ) for small rotation angles , (1) where n is the refractive index of the surrounding medium (air) and n 2 is the refractive index of the prism material.The astronomical telescope consists of two lenses with focal lengths of f 1 = 10 cm and f 2 = 6 cm.The distance between the two lenses is equal to the sum of the two focal lengths (f 1 + f 2 ).As a 60° SF-11 prism is used, the incident angle on the back of the prism can be calculated as The critical angle θ c is given by where n 1 and n 2 are the refractive indices of the two materials.For total internal reflection it is n 1 > n 2 .Light which hits the interface under an angle θ larger than the critical angle θ c is completely internally reflected.The highly refractive SF 11 prism is required in order to obtain total internal reflection.By changing the angle θ of the incident beam the critical angle can be accurately determined.We measure the amount of reflected light using a photodiode.We use 5 different laser diodes with several mW output power at wavelengths of 450 nm, 532 nm, 650 nm, 780 nm, and 850 nm.The critical angle is then determined from the angular reflection curves for different photoresists at different incident laser wavelengths in s-polarization in order to calculate Cauchy's dispersion curves.
For each wavelength we determine the point of the critical angle of the prism/photoresist interface as well as for a prism/reference BK7 glass transition in order to calibrate the scale of the incident angle.For this purpose, the lower part of the prism is coated with an approximately 1 mm thick layer of the photoresist under investigation and subsequently, exposed by UV light for about 5 min (DymaxBlueWave 50 at a distance of 3 cm, delivering 365 nm with an intensity of 3000 mW/cm 2 ), on the upper part we optically bond a BK7 glass plate by pressure.

Experimental details
Figure 2 shows the angle dependent reflection measurement for IP-S and BK7 glass at a wavelength of 850 nm.We fit the functional relation of reflectance obtained by Fresnel's equation for s-polarized light to the measurement data in order to determine the critical angle.The value of the critical angle of BK7 (whose refractive index is tabulated with great accuracy [18]) is used in order to calibrate the angle scale.The red curves represent the measurement of the BK7 glass, where the critical angle of θ c is calculated by the literature values of 1.7619 and 1.5098 at a wavelength of 850 nm for SF11 and BK7, respectively [18].The axis is accordingly adjusted with respect to the calculated value.Therefore, the critical angle of the photoresist can be accurately determined.The reflection curves are in good agreement with the theoretically expected reflectance behavior (Fresnel's equations) of the transition from a high to a low refractive index material.3:

Conclusion
We measured the refractive index dispersion in the visible spectral range of five different photoresists that are used for 3D femtosecond direct laser writing.We determined the Cauchy parameters and the Abbe numbers as well as the equivalent Schott catalog numbers.This work will pave the way towards realization of achromatic 3D printed optical elements, as well as open the door towards using micro optical elements with tailored dispersive properties.Furthermore, our method represents the first step towards realization of an entire Abbe diagram with novel 3D printable material with tailorable refractive indices and dispersion properties.Additionally, it is our experience that the refractive index is strongly dependent on the exposure dose, this means exposure time and intensity.First measurements show changes in the range of 5 ⋅ 10 −3 .Furthermore, there is a time-dependent behavior of the refractive index directly after the exposure process, which reaches a plateau after a few hours.The exposure mode (UV exposure process or direct laser writing) influences the refractive index of the exposed photoresist, as well.These issues should be considered in a future evaluation, too.

Fig. 1 .
Fig. 1.Setup for measuring the critical angle of the total internal reflection, similar to a Pulfrich refractometer.(a) The refractive index values are obtained by measuring the critical angle of different photoresists at different wavelengths.The refractive index of the prism is n 2 and of the photoresist of interest is n 1 .(b) Dependency of the angles in the setup for the refractive index measurement.The refractive indices of air (n) and prims material (n 2 ) are indicated.

Fig. 2 .
Fig. 2. Measurement of the critical angle of the total internal reflection of s-polarized light for OrmoComp and BK7 glass at a wavelength of 850 nm.
n d , n F , and n C are the refractive indices of the material at Fraunhofer lines d, F, and C. The Fraunhofer lines are associated with the absorption lines in the spectrum of the sun.d corresponds to a wavelength of 587.6 nm, F corresponds to a wavelength of 486.1 nm, and C corresponds to a wavelength of 656.3 nm.The Abbe number of Nanoscribe IP-S is ν d = 46.2 and thus behaves similar to very light flint glasses.ν d = 51.2 is the Abbe number of micro resist OrmoComp, which is in the region of crown/flint glasses.Glasses which corresponds to the dispersion of IP-S and OrmoComp are for example LLF1 and N-KF9.As the refractive indices of Nanoscribe IP-Dip are above the ones of IP-S and OrmoComp the Abbe number of IP-Dip is ν d = 35.0.Corresponding glasses are flint glasses.The Abbe numbers are summarized in Table