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Abstract
Structured illumination microscopy (SIM) is a powerful super-resolved imaging technique which enables to perform fast and in vivo imaging of bio-samples. In order to achieve a better resolution of a SIM system, evanescent waves with larger in-plane wave-vector are preferred for SIM, among which the total internal reflection (TIRF-SIM) and the plasmonic SIM (pSIM) configurations are widely studied. Here, we demonstrated a metal-dielectric waveguide (MDW) based SIM system - termed as MDW-SIM, which can achieve a good compromise between TIRF-SIM and pSIM. The MDW can support a low-loss waveguide mode at an aqueous environment, with an evanescent tail existing above the water/dielectric interface for SIM. A proof-of-concept imaging experiment was performed on fluorescent beads, where a spatial resolution of 86nm was achieved at a 473nm illumination wavelength and a 1.45 numerical aperture objective lens. The proposed MDW-SIM has a great potential for the bio-imaging applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical microscopy is a powerful analytical tool in modern science, which enables researchers to view matters noninvasively and down to sub-micron scale. However, the diffraction of electromagnetic field limits the resolution of an optical microscopy to approximately half of the emission wavelength. It cannot meet anymore the demands in modern life sciences, which tends to study the biological dynamics at molecular scale. To fulfill the requirements, many super-resolved optical microscopies have been developed in the past few decades, either in a scanning or a wide field frame [1–4]. The scanning techniques, including confocal laser scanning microscopy (CLSM) [5–7], near-field scanning optical microscopy (NSOM) [8,9], stimulated emission depletion microscopy (STED) [10,11], etc., can provide a high spatial resolution, but are generally at the expense of the imaging speed and field of view. Stochastic optical reconstruction microscopy (STORM) and photo-activated localization microscopy (PALM) are two types of wide field super-resolved microscopies that are based on the single molecule localization technique [12,13]. Albeit with a high imaging resolution and range, they are also lack of speed due to the tremendous amount of exposures and post-process. As another type of wide field imaging, structured illumination microscopy (SIM) provides a good balance between the imaging resolution and speed [14–19]. The non-scanning mode of SIM enables to perform fast bio-imaging, with the resolution down to sub-100nm. In principle, the resolution can further been improved when the nonlinear effect and photo-switchable protein are introduced (non-linear SIM), but the bio-toxicity may become a problem due to the high-dose light intensity [20–24].
Theoretically, the resolution of a linear SIM configuration is determined by the in-plane spatial frequency of the standing waves that are employed as the structured source. As a result, evanescent waves with a larger in-plane wave-vector than free space become a good candidate for improving the resolution of SIM, among which the total internal reflection (TIRF-SIM) [20–23,25,26] and the plasmonic SIM (pSIM) [27,28] configurations are widely studied. Surface plasmon polaritons (SPPs) are a specific type of evanescent waves that are induced by the light-metal interaction at a smooth metal surface. As compared to the TIRF-SIM, pSIM has the advantages of high resolution and enhanced signal-to-noise ratio owing to the subwavelength and enhancement nature of SPPs. However, the high Ohmic loss associated with SPPs, particularly when in an aqueous environment, restricts their propagation distance and hence the effective imaging area. Additionally, the metallic surface, on the one hand, will cause a significant quenching of fluorescent signal; on the other hand, it will affect greatly the bio-sample activity. Both will impair the effectiveness of a pSIM system for doing the in-vivo bio-imaging.
Here we propose and demonstrated a novel SIM configuration based on the evanescent field generated at a metal-dielectric waveguide (MDW) structure, which can achieve a good compromise between the TIRF-SIM and pSIM. The MDW can support a low-loss waveguide mode at an aqueous environment, with an evanescent tail existing above the water/dielectric interface. The resonant nature of the MDW mode enables to improve the field enhancement compared to a TIRF configuration. Meanwhile, the bio-samples in a MDW-SIM are located at a dielectric surface, which eliminates the disadvantages associated with a metal waveguide. A proof-of-concept imaging experiment was performed on fluorescent beads, which indicated an 86nm spatial resolution with a 473nm illumination wavelength and a 1.45 numerical aperture (NA) objective lens. The proposed MDW-SIM bears a good balance between the resolvability and enhancement and has a great potential for the bio-imaging applications.
2. Experiment setup
The MDW structure employed in the work consists of an aluminum (Al) thin film sandwiched between a silica layer and a glass cover slit, as illustrated with inset (a) in Fig. 1. The structure is covered with water to mimic the aqueous environment of bio-samples. The thickness of the Al film and silica layer were designed to be 20nm and 560nm, respectively, to support a TM waveguide mode with an evanescent field within the water environment [29]. The resonant angle of the structure under the illumination with a 473nm incident laser beam from the substrate side is 66.67° (inset (b) in Fig. 1), indicating the existence of an evanescent wave at the water-silica interface with a wavelength of 340nm. This evanescent wave is employed for the SIM with deep-subwavelength resolution.
Fig. 1 Experimental setup of the MDW-SIM configuration. P: Polarizer, λ/2: half-wave plate, SLM: spatial light modulator, 4f: 4-f system, DM: dichroic mirror, BS: beam splitter, LP: long pass filter. Inset (a), schematic diagram of the MDW structure. From top to bottom: water, silica, Al and glass substrate, respectively. Their refractive indices are n1 = 1.33, n2 = 1.46, n3 = 0.684 + 6i, and n4 = 1.515. Inset (b), the calculated reflection curve of the MDW when the thicknesses of silica layer and Al film are 20nm and 560nm, under the illumination with a 473nm laser beam. Inset (c), the evanescent standing wave pattern formed at the water-silica interface captured at the back image plane by the sCMOS camera. The scalar bar represents 500nm.
The imaging experimental setup is shown in Fig. 1. A laser beam of 473nm wavelength was employed as the light source. After a beam expander, the laser beam was illuminated onto a spatial light modulator (SLM), which split and focused the beam into two spots. Subsequently, the two spots were projected onto the entrance pupil of the objective (Olympus, 150 × , NA = 1.45, oil immersed) by a 4-f system to excite the MDW mode. An evanescent standing wave would be formed at the water-silica interface due to the excitation of the counter-propagating MDW waves by the two spots. The phase shifts and the rotation of the standing waves were controlled by the SLM as well. A half-wave plate (HWP) before the SLM was used to rotate the beam polarization to match the orientation of the SLM, while the other one after the SLM was used to make sure the incident beam is TM-polarized. The SLM is able to control precisely the position and size of the two spots on the entrance pupil. The two excitation spots should be as small as possible to, on the one hand, enhance the excitation efficiency of the waveguide mode, on the other hand, to enlarge the effective area of the standing waves to improve the field of view of the imaging system. A CCD (charge coupled device) camera was mounted at the back Fourier plane (BFP) of the objective to capture the reflected beam from the MDW structure and the coupled emission from the fluorescent beads. After a dichroic mirror and a long pass filter, a sCMOS camera (Hamamatsu, ORCA-flash3.0) was mounted at the back image plane of the objective, to capture the images of evanescent standing waves and fluorescent beads.
3. Experimental results and analysis
In order to verify the existence of the MDW mode, the unmodulated laser beam was firstly illuminated onto the MDW structure via the objective lens, and the reflected beam was captured at the back Fourier plane to uncover the angular dependent reflection of the structure. For a clear demonstration, only the beam pattern that is above the total-internal reflection angle is shown (Fig. 2(a)). The sharp dark arcs at the beam cross-section indicate a low reflectance at this angle. This is in-line with the calculated reflection curve as shown in Fig. 1, thus verifying the excitation of the MDW mode.
Fig. 2 (a) Back focal plane image of the reflected laser beam from the MDW structure captured by the CCD camera. The dark arcs at the vertical direction indicate the existence of the MDW mode. (b) Two beam spots are generated and projected onto the objective lens by the SLM at the positions of dark arcs, to excite the MDW mode and form the evanescent standing wave. (c) The fluorescence signals captured at the back focal plane, where a bright ring is formed at the same radial position of the dark arcs. The yellow dashed circle represents the angle of total internal reflection.
Subsequently, the laser beam was modulated with the SLM to produce two excitation spots. The positions of the spots were carefully controlled to match the resonance angle of the MDW mode. Figure 2(b) shows the reflected beam captured at the BFP, where we can see the two excitation spots located exactly at the positions of the dark arcs. The interference of the MDW waves excited by the two spots formed an evanescent standing wave above the water-silica interface. This can be imaged with the sCMOS camera that was placed at the back image plane of the objective (Fig. 1, inset (c)). These evanescent fringes interact with the fluorescent beads (Bangs Laboratories, diameter 40nm, 480nm/520nm) located at the silica layer surface, emitting the fluorescence signals. Figure 3(c) shows the fluorescence captured at the back Fourier plane when a long-pass filter is placed in front of the CCD camera. A green bright ring is seen at the radial position of former dark arcs. This is because the fluorescence is coupled with the MDW mode, and re-radiates into the substrate side at the resonance angle of the MDW mode. The green bright ring is measured to be radially polarized, coinciding with the TM attribute of the MDW mode.
Fig. 3 The experimental (a) and calculated (b) point spread function of the system. (c) The Gaussian-shaped spot converted from (a) by using the R-L deconvolution. (d) Their cross section comparison of (a)-(c). (e)-(g) The corresponding OTFs of (a)-(c). The scalar bar represents 100nm.
Figure 3(a) shows the image of a fluorescent bead that was captured by the sCMOS camera, which manifests as a donut shape. Similar to a surface plasmon coupled emission microscopy (SPCEM), the point spread function (PSF) of the proposed system can be calculated by using the diffraction theory and by considering the fluorescent bead as a dipole emitter [30,31]. Here, the wave front aberration function needs to be modified to take into account the MDW structure. Since the evanescent standing wave is strongly dominated by the longitudinal electric field component, the dipole moment from the fluorescent bead can approximately be treated as vertical orientation. Figure 3(b) shows the calculated PSF of the system for a vertical dipole. We can find a good agreement between the experimental and calculation results. The donut-shaped PSF can be reshaped into a traditional Gaussian-shaped distribution by using the Richardson-Lucy (R-L) deconvolution algorithm [32], as shown in Fig. 3(c), for implementing the following SIM reconstruction algorithm. Figure 3(d) gives the comparison of their cross section distributions. The experimental one does not end up with zero intensity in the center, which we believe arises from the weak transversal electric field component that interacts with the horizontally-orientated fluorescence molecules, giving rise to a weak solid spot in the center. We further calculate the corresponding optical transfer functions (OTF) of Figs. 3(a)-3(c), which are shown in Figs. 3(e)-3(g), respectively. The original donut-shaped PSF results in an OTF with a narrower main lobe and an additional side lobe, but this can be recovered into a wider traditional Gaussian-shape after the R-L deconvolution.
Finally, we performed a proof-of-concept experiment to validate the resolvability of the proposed MDW-SIM system. The wide field image of the florescence beads captured by the sCMOS camera is illustrated in Fig. 4(a). After performing the 3-step phase shifting and implementing successively the R-L deconvolution, the spatial domain SIM reconstruction algorithm and the depression of side-lobes [33–35], a one directional super-resolved image could be obtained as shown in Fig. 4(b). A zoom-in view of region 1 in Fig. 4(b) is shown in Fig. 4(d), where an isolated fluorescence bead was imaged with SIM. The FWHM of the fitted cross-section distribution with Gaussian function is measured to be ~86nm, illustrating the resolvability of the MDW-SIM configuration. This resolution makes sure that the two adjacent beads with ~102nm separation can successfully be distinguished with the system, as illustrated in Fig. 4(e) which shows the zoom-in view of region 2 in Fig. 4(b). At last, by implementing the same process at the orthogonal direction, a two-dimensional super-resolved SIM imaging can be obtained which is shown in Fig. 4(c). As can be seen, the fluorescent beads are reconstructed into spots with much enhanced resolution. Figure 4(f) shows a part of the image in Fig. 4(c) where four pairs of binary beads that can hardly be distinguished in a traditional microscope (Fig. 4(a)) are distinguished successfully, further verifying the good resolvability of the MDW-SIM technique.
Fig. 4 Demonstration of the resolvability of the proposed MDW-SIM. (a) The wide field image of the florescence beads. (b) The 1D reconstructed SIM image at the vertical direction. (c) The 2D reconstructed SIM image at both directions. (d) The zoom-in view of region 1 in (b), illustrating an 86nm resolution of the system. (e) The zoom-in view of region 2 in (b), where two adjacent fluorescence beads with 102nm separation are successfully distinguished. (f) The highlighted region in (c), where all of the four pairs of particles are distinguished in the 2D SIM image.
With the imaging results obtained above, the power spectra at each procedure can be calculated by performing the Fourier transform. Figures 5(a) and 5(b) illustrate the power spectra of the original wide field image, and the one after the R-L deconvolution, respectively. The bright spot at the center refers to the 0-frequency component. The cut-off frequency read from the OTFs is ~0.0061nm−1, which is limited by the objective lens used in the system. This also refers to the wide field resolution limit dAbbe = λ/2NA = 163.1nm of a conventional microscope. Furthermore, the power spectra of the 1D and 2D super-resolved images are shown in Fig. 5(c) and 5(d), respectively, where the extension of the frequency range can clearly be seen. The two bright spots above and below the 0-frequency spot in Fig. 5(c) (as marked with the yellow circles) are the ± 1st-order frequency imposed by the SIM process, from which the frequency shift can be read to be ~0.0059nm−1. This corresponds to a period of 169.5nm of the excitation standing wave fringes, which accords well with the value (170nm) read from Fig. 1(c). The expected resolution improvement can thus be calculated to be r = 1 + dAbbe/dSIM = 1.96, nearly doubling of the resolution. This can also be verified from the cut-off frequency read from Fig. 5(c) which is ~0.012nm−1. The cutoff frequency indicates a theoretical resolution limit of the system to be 1/0.012nm−1, ~83.2nm. This is slightly smaller than the FWHM (~86nm) of the image spot of the fluorescence bead we measured in the experiment. Finally, the modulation depths were also measured by the “fairSIM” plugin in ImageJ, which are 0.732 at vertical direction and 0.599 at horizontal [36]. The difference between the two directions may be caused by the polarization response of the dichroic mirror.
Fig. 5 Power spectra of the original wide field image (a), the image after R-L deconvolution (b), the 1D SIM (c) and 2D SIM image (d), respectively. The yellow circles refer to the ± 1st-order frequency components.
4. Conclusion
In summary, we demonstrated a MDW-SIM configuration which employs the enhanced evanescent field in a MDW structure for SIM. The MDW can support a low loss waveguide mode with a larger in-plane wave-vector, which can provide a good spatial resolution of SIM. Due to the TM nature of the MDW, the PSF of the system is donut-shaped, while it can easily be reshaped into a regular Gaussian shape by using the R-L deconvolution algorithm. Both the 1D and 2D SIM imaging were obtained experimentally, illustrating a ~86nm spatial resolution of the system. While what we demonstrated here is a proof-of-concept experiment at only two orthogonal directions, the imaging system can further be improved to perform the three orientations SIM imaging, and even the 3D imaging by incorporating the axial imaging technique making use of the properties of evanescent wave [26]. The proposed MDW-SIM configuration achieves a good compromise between the TIRF-SIM and pSIM, which provides a good platform for the non-invasive and in-vivo bio-imaging applications.
Funding
National Natural Science Foundation of China (NSFC) (61490712, 61622504, 61427819); the leading talents of Guangdong province program (00201505); the Natural Science Foundation of Guangdong Province grant 2016A030312010, the Science and Technology Innovation Commission of Shenzhen (KQTD2015071016560101, KQTD2017033011044403, ZDSYS201703031605029); National Key Basic Research Program of China (973 Program) (2015CB352004).
Acknowledgments
L. Du acknowledges the support given by Guangdong Special Support Program.
References
1. S. W. Hell, “Far-field optical nanoscopy,” Science 316(5828), 1153–1158 (2007). [CrossRef] [PubMed]
2. L. Schermelleh, R. Heintzmann, and H. Leonhardt, “A guide to super-resolution fluorescence microscopy,” J. Cell Biol. 190(2), 165–175 (2010). [CrossRef] [PubMed]
3. S. J. Sahl, S. W. Hell, and S. Jakobs, “Fluorescence nanoscopy in cell biology,” Nat. Rev. Mol. Cell Biol. 18(11), 685–701 (2017). [CrossRef] [PubMed]
4. Y. M. Sigal, R. Zhou, and X. Zhuang, “Visualizing and discovering cellular structures with super-resolution microscopy,” Science 361(6405), 880–887 (2018). [CrossRef] [PubMed]
5. C. Cremer and T. Cremer, “Considerations on a laser-scanning-microscope with high resolution and depth of field,” Microsc. Acta 81(1), 31–44 (1978). [PubMed]
6. C. J. Sheppard and T. Wilson, “The theory of the direct-view confocal microscope,” J. Microsc. 124(Pt 2), 107–117 (1981). [CrossRef] [PubMed]
7. G. J. Brakenhoff, H. T. van der Voort, E. A. van Spronsen, W. A. Linnemans, and N. Nanninga, “Three-dimensional chromatin distribution in neuroblastoma nuclei shown by confocal scanning laser microscopy,” Nature 317(6039), 748–749 (1985). [CrossRef] [PubMed]
8. N. Rotenberg and L. Kuipers, “Mapping nanoscale light fields,” Nat. Photonics 8(12), 919–926 (2014). [CrossRef]
9. E. A. Ash and G. Nicholls, “Super-resolution aperture scanning microscope,” Nature 237(5357), 510–512 (1972). [CrossRef] [PubMed]
10. S. W. Hell and J. Wichmann, “Breaking the diffraction resolution limit by stimulated emission: stimulated-emission-depletion fluorescence microscopy,” Opt. Lett. 19(11), 780–782 (1994). [CrossRef] [PubMed]
11. T. A. Klar and S. W. Hell, “Subdiffraction resolution in far-field fluorescence microscopy,” Opt. Lett. 24(14), 954–956 (1999). [CrossRef] [PubMed]
12. M. J. Rust, M. Bates, and X. Zhuang, “Sub-diffraction-limit imaging by stochastic optical reconstruction microscopy (STORM),” Nat. Methods 3(10), 793–795 (2006). [CrossRef] [PubMed]
13. E. Betzig, G. H. Patterson, R. Sougrat, O. W. Lindwasser, S. Olenych, J. S. Bonifacino, M. W. Davidson, J. Lippincott-Schwartz, and H. F. Hess, “Imaging intracellular fluorescent proteins at nanometer resolution,” Science 313(5793), 1642–1645 (2006). [CrossRef] [PubMed]
14. M. G. Gustafsson, “Surpassing the lateral resolution limit by a factor of two using structured illumination microscopy,” J. Microsc. 198(Pt 2), 82–87 (2000). [CrossRef] [PubMed]
15. M. G. Gustafsson, L. Shao, P. M. Carlton, C. J. Wang, I. N. Golubovskaya, W. Z. Cande, D. A. Agard, and J. W. Sedat, “Three-dimensional resolution doubling in wide-field fluorescence microscopy by structured illumination,” Biophys. J. 94(12), 4957–4970 (2008). [CrossRef] [PubMed]
16. L. Schermelleh, P. M. Carlton, S. Haase, L. Shao, L. Winoto, P. Kner, B. Burke, M. C. Cardoso, D. A. Agard, M. G. Gustafsson, H. Leonhardt, and J. W. Sedat, “Subdiffraction multicolor imaging of the nuclear periphery with 3D structured illumination microscopy,” Science 320(5881), 1332–1336 (2008). [CrossRef] [PubMed]
17. J. Qian, M. Lei, D. Dan, B. Yao, X. Zhou, Y. Yang, S. Yan, J. Min, and X. Yu, “Full-color structured illumination optical sectioning microscopy,” Sci. Rep. 5(1), 14513 (2015). [CrossRef] [PubMed]
18. A. Doblas, H. Shabani, G. Saavedra, and C. Preza, “Tunable-frequency three-dimensional structured illumination microscopy with reduced data-acquisition,” Opt. Express 26(23), 30476–30491 (2018). [CrossRef] [PubMed]
19. R. Heintzmann and C. G. Cremer, “Laterally modulated excitation microscopy: improvement of resolution by using a diffraction grating,” in Optical Biopsies and Microscopic Techniques III, (International Society for Optics and Photonics, 1999), 185–197.
20. M. G. Gustafsson, “Nonlinear structured-illumination microscopy: wide-field fluorescence imaging with theoretically unlimited resolution,” Proc. Natl. Acad. Sci. U.S.A. 102(37), 13081–13086 (2005). [CrossRef] [PubMed]
21. E. H. Rego, L. Shao, J. J. Macklin, L. Winoto, G. A. Johansson, N. Kamps-Hughes, M. W. Davidson, and M. G. Gustafsson, “Nonlinear structured-illumination microscopy with a photoswitchable protein reveals cellular structures at 50-nm resolution,” Proc. Natl. Acad. Sci. U.S.A. 109(3), E135–E143 (2012). [CrossRef] [PubMed]
22. X. Zhang, M. Zhang, D. Li, W. He, J. Peng, E. Betzig, and P. Xu, “Highly photostable, reversibly photoswitchable fluorescent protein with high contrast ratio for live-cell superresolution microscopy,” Proc. Natl. Acad. Sci. U.S.A. 113(37), 10364–10369 (2016). [CrossRef] [PubMed]
23. D. Li, L. Shao, B.-C. Chen, X. Zhang, M. Zhang, B. Moses, D. E. Milkie, J. R. Beach, J. A. Hammer 3rd, M. Pasham, T. Kirchhausen, M. A. Baird, M. W. Davidson, P. Xu, and E. Betzig, “ADVANCED IMAGING. Extended-resolution structured illumination imaging of endocytic and cytoskeletal dynamics,” Science 349(6251), aab3500 (2015). [CrossRef] [PubMed]
24. R. Heintzmann, T. M. Jovin, and C. Cremer, “Saturated patterned excitation microscopy--a concept for optical resolution improvement,” J. Opt. Soc. Am. A 19(8), 1599–1609 (2002). [CrossRef] [PubMed]
25. X. Huang, J. Fan, L. Li, H. Liu, R. Wu, Y. Wu, L. Wei, H. Mao, A. Lal, P. Xi, L. Tang, Y. Zhang, Y. Liu, S. Tan, and L. Chen, “Fast, long-term, super-resolution imaging with Hessian structured illumination microscopy,” Nat. Biotechnol. 36(5), 451–459 (2018). [CrossRef] [PubMed]
26. Y. Chen, W. Liu, Z. Zhang, C. Zheng, Y. Huang, R. Cao, D. Zhu, L. Xu, M. Zhang, Y.-H. Zhang, J. Fan, L. Jin, Y. Xu, C. Kuang, and X. Liu, “Multi-color live-cell super-resolution volume imaging with multi-angle interference microscopy,” Nat. Commun. 9(1), 4818 (2018). [CrossRef] [PubMed]
27. F. Wei, D. Lu, H. Shen, W. Wan, J. L. Ponsetto, E. Huang, and Z. Liu, “Wide field super-resolution surface imaging through plasmonic structured illumination microscopy,” Nano Lett. 14(8), 4634–4639 (2014). [CrossRef] [PubMed]
28. J. L. Ponsetto, A. Bezryadina, F. Wei, K. Onishi, H. Shen, E. Huang, L. Ferrari, Q. Ma, Y. Zou, and Z. Liu, “Experimental demonstration of localized plasmonic structured illumination microscopy,” ACS Nano 11(6), 5344–5350 (2017). [CrossRef] [PubMed]
29. F. Meng, L. Du, A. Yang, and X. Yuan, “Low loss surface electromagnetic waves on a metal–dielectric waveguide working at short wavelength and aqueous environment,” Opt. Commun. 433, 10–13 (2019). [CrossRef]
30. C. Sheppard and P. Török, “An electromagnetic theory of imaging in fluorescence microscopy, and imaging in polarization fluorescence microscopy,” Bioimaging 5(4), 205–218 (1997). [CrossRef]
31. W. T. Tang, E. Chung, Y.-H. Kim, P. T. So, and C. J. Sheppard, “Investigation of the point spread function of surface plasmon-coupled emission microscopy,” Opt. Express 15(8), 4634–4646 (2007). [CrossRef] [PubMed]
32. E. Chung, Y.-H. Kim, W. T. Tang, C. J. Sheppard, and P. T. So, “Wide-field extended-resolution fluorescence microscopy with standing surface-plasmon-resonance waves,” Opt. Lett. 34(15), 2366–2368 (2009). [CrossRef] [PubMed]
33. G. E. Cragg and P. T. So, “Lateral resolution enhancement with standing evanescent waves,” Opt. Lett. 25(1), 46–48 (2000). [CrossRef] [PubMed]
34. P. T. So, H.-S. Kwon, and C. Y. Dong, “Resolution enhancement in standing-wave total internal reflection microscopy: a point-spread-function engineering approach,” J. Opt. Soc. Am. A 18(11), 2833–2845 (2001). [CrossRef] [PubMed]
35. P. Hänninen, S. Hell, J. Salo, E. Soini, and C. Cremer, “Two‐photon excitation 4Pi confocal microscope: enhanced axial resolution microscope for biological research,” Appl. Phys. Lett. 66(13), 1698–1700 (1995). [CrossRef]
36. M. Müller, V. Mönkemöller, S. Hennig, W. Hübner, and T. Huser, “Open-source image reconstruction of super-resolution structured illumination microscopy data in ImageJ,” Nat. Commun. 7(1), 10980 (2016). [CrossRef] [PubMed]