On-chip fluorescence detection using photonic bandgap guiding optofluidic hollow-core light cage

I. INTRODUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTION <<II. EXPERIMENTALIII. RESULTS AND DISCUSSI...IV. CONCLUSIONSSUPPLEMENTARY MATERIALREFERENCESPrevious sectionNext sectionFluorescence detection represents a wide-spread technology particularly within bioanalytics and is used in a great variety of applications, such as medical diagnostics,11. A. Shahzad, M. Knapp, M. Edetsberger, M. Puchinger, E. Gaubitzer, and G. Köhler, “Diagnostic application of fluorescence spectroscopy in oncology field: Hopes and challenges,” Appl. Spectrosc. Rev. 45, 92–99 (2010). https://doi.org/10.1080/05704920903435599 quantitative polymerase chain reaction (PCR) test with fluorescence,22. J. Cheong, H. Yu, C. Y. Lee, J. U. Lee, H. J. Choi, J. H. Lee, H. Lee, and J. Cheon, “Fast detection of SARS-CoV-2 RNA via the integration of plasmonic thermocycling and fluorescence detection in a portable device,” Nat. Biomed. Eng. 4, 1159 (2020). https://doi.org/10.1038/s41551-020-00654-0 fluorescence enzyme-linked immunosorbent assay (ELISA),33. J. R. Lakowicz, Principles of Fluorescence Spectroscopy (Springer Science & Business Media, 2013). fluorescence resonance energy transfer (FRET) assay,44. M. A. Rizzo, G. H. Springer, B. Granada, and D. W. Piston, “An improved cyan fluorescent protein variant useful for FRET,” Nat. Biotechnol. 22, 445–449 (2004). https://doi.org/10.1038/nbt945 and fluorescence imaging.55. B. A. Flusberg, E. D. Cocker, W. Piyawattanametha, J. C. Jung, E. L. Cheung, and M. J. Schnitzer, “Fiber-optic fluorescence imaging,” Nat. Methods 2, 941–950 (2005). https://doi.org/10.1038/nmeth820 Emerging nanotechnologies are extending the range and capabilities of applications that exploit fluorescence emission, such as super-resolution microscopy using up-conversion nanoparticles66. Y. Liu, Y. Lu, X. Yang, X. Zheng, S. Wen, F. Wang, X. Vidal, J. Zhao, D. Liu, Z. Zhou et al., “Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy,” Nature 543, 229–233 (2017). https://doi.org/10.1038/nature21366 and magnetic field sensing using the fluorescence emission properties of nitrogen-vacancy centers in nano-diamonds.77. D. Bai, M. H. Huynh, D. A. Simpson, P. Reineck, S. A. Vahid, A. D. Greentree, S. Foster, H. Ebendorff-Heidepriem, and B. C. Gibson, “Fluorescent diamond microparticle doped glass fiber for magnetic field sensing,” APL Mater. 8, 081102 (2020). https://doi.org/10.1063/5.0013473 Fluorescence detection has also been used in the context of the current SARS-CoV-2 pandemic. Here, qPCR testing, with fluorescence-based detection, is considered the gold standard, and there are advantages in sensitivity in using fluorescence-based antigen/antibody tests.88. D. Wang, S. He, X. Wang, Y. Yan, J. Liu, S. Wu, S. Liu, Y. Lei, M. Chen, L. Li et al., “Rapid lateral flow immunoassay for the fluorescence detection of SARS-CoV-2 RNA,” Nat. Biomed. Eng. 4, 1150 (2020). https://doi.org/10.1038/s41551-020-00655-zThe excitation and collection mechanism used in a fluorescence-based system has a significant impact on the performance of the technology. A promising way to improve the performance relies on enhancing the light–matter interaction through on-chip waveguides99. Z. Liao, Y. Zhang, Y. Li, Y. Miao, S. Gao, F. Lin, Y. Deng, and L. Geng, “Microfluidic chip coupled with optical biosensors for simultaneous detection of multiple analytes: A review,” Biosens. Bioelectron. 126, 697–706 (2019). https://doi.org/10.1016/j.bios.2018.11.032 in combination with optical fibers.1010. Y. Zhao, X.-g. Hu, S. Hu, and Y. Peng, “Applications of fiber-optic biochemical sensor in microfluidic chips: A review,” Biosens. Bioelectron. 166, 112447 (2020). https://doi.org/10.1016/j.bios.2020.112447 One widely used detection scheme uses the optical evanescent field whereby a fluorescent particle can be excited and the emission can be collected through an evanescent field interaction. The advantage of this method is that the signal can be collected over long interaction lengths, with one example being the detection of single nano-crystals in microstructured fibers.11–1311. S. Afshar V, S. C. Warren-Smith, and T. M. Monro, “Enhancement of fluorescence-based sensing using microstructured optical fibres,” Opt. Express 15, 17891–17901 (2007). https://doi.org/10.1364/oe.15.01789112. J. Zhao, D. Jin, E. P. Schartner, Y. Lu, Y. Liu, A. V. Zvyagin, L. Zhang, J. M. Dawes, P. Xi, J. A. Piper et al., “Single-nanocrystal sensitivity achieved by enhanced upconversion luminescence,” Nat. Nanotechnol. 8, 729–734 (2013). https://doi.org/10.1038/nnano.2013.17113. E. P. Schartner, G. Tsiminis, M. R. Henderson, S. C. Warren-Smith, and T. M. Monro, “Quantification of the fluorescence sensing performance of microstructured optical fibers compared to multi-mode fiber tips,” Opt. Express 24, 18541–18550 (2016). https://doi.org/10.1364/oe.24.018541 However, evanescent field detection as a reliable technology for real-world use is challenging due to limitations such as spatially restricted light–matter interaction only near the waveguide, limited fraction of the optical field in the medium of interest, potential surface contamination, background signals (fluorescence and Raman) from the waveguide material itself, dependence of the mode field on wavelength, and appearance of chromatography effects.One promising waveguide system with particularly relevant properties for spectroscopic applications allowing one to circumvent the mentioned issues is on-chip hollow-core waveguides. Different from solid-core systems, these waveguides allow guiding light in a medium with a refractive index (RI) lower than the cladding and offer almost 100% overlap of the guided mode with the material of interest. In the majority of cases, on-chip hollow-core waveguide-based fluorescence detection relies on anti-resonant reflecting optical waveguides (ARROWs)1414. D. Yin, H. Schmidt, J. P. Barber, and A. R. Hawkins, “Integrated arrow waveguides with hollow cores,” Opt. Express 12, 2710–2715 (2004). https://doi.org/10.1364/opex.12.002710 with applications including biosensors,1515. H. Mukundan, A. Anderson, W. K. Grace, K. Grace, N. Hartman, J. Martinez, and B. Swanson, “Waveguide-based biosensors for pathogen detection,” Sensors 9, 5783–5809 (2009). https://doi.org/10.3390/s90705783 quantum dot fluorescence detection,1616. J. Ozhikandathil and M. Packirisamy, “Silica-on-silicon waveguide integrated polydimethylsiloxane lab-on-a-chip for quantum dot fluorescence bio-detection,” J. Biomed. Opt. 17, 017006 (2012). https://doi.org/10.1117/1.jbo.17.1.017006 and UV fluorescence detection.1717. P. D. Ohlsson, O. Ordeig, K. B. Mogensen, and J. P. Kutter, “Electrophoresis microchip with integrated waveguides for simultaneous native UV fluorescence and absorbance detection,” Electrophoresis 30, 4172–4178 (2009). https://doi.org/10.1002/elps.200900393 Note that hollow-core fibers also are successfully employed for fluorescence detection, examples of which include photonic bandgap (PBG) fibers18,1918. S. Smolka, M. Barth, and O. Benson, “Highly efficient fluorescence sensing with hollow core photonic crystal fibers,” Opt. Express 15, 12783–12791 (2007). https://doi.org/10.1364/oe.15.01278319. A. M. Cubillas, S. Unterkofler, T. G. Euser, B. J. M. Etzold, A. C. Jones, P. J. Sadler, P. Wasserscheid, and P. S. J. Russell, “Photonic crystal fibres for chemical sensing and photochemistry,” Chem. Soc. Rev. 42, 8629–8648 (2013). https://doi.org/10.1039/c3cs60128e and anti-resonance fibers.2020. J. Yang, R. Shen, P. Yan, Y. Liu, X. Li, P. Zhang, and W. Chen, “Fluorescence sensor for volatile trace explosives based on a hollow core photonic crystal fiber,” Sens. Actuators, B 306, 127585 (2020). https://doi.org/10.1016/j.snb.2019.127585 In addition to increased optical overlap with the fluorophore of interest, hollow-core waveguides have a secondary advantage that the overlap with the waveguide material is dramatically reduced, thus reducing the background autofluorescence. This advantage is particularly pronounced for weak signals such as the detection of Raman scattering using liquid or gas filled hollow-core optical fibers.21,2221. G. Tsiminis, K. J. Rowland, E. P. Schartner, N. A. Spooner, T. M. Monro, and H. Ebendorff-Heidepriem, “Single-ring hollow core optical fibers made by glass billet extrusion for Raman sensing,” Opt. Express 24, 5911–5917 (2016). https://doi.org/10.1364/oe.24.00591122. A. Knebl, D. Yan, J. Popp, and T. Frosch, “Fiber enhanced Raman gas spectroscopy,” TrAC, Trends Anal. Chem. 103, 230–238 (2018). https://doi.org/10.1016/j.trac.2017.12.001Even though being successfully employed, today’s hollow-core waveguides reveal limitations such as limited access to the waveguide core only through the open ends, leading to very long diffusion times (e.g., for low-pressure atomic gases, this can be months to fill a fiber over a length of several centimeters2323. G. Epple, K. S. Kleinbach, T. G. Euser, N. Y. Joly, T. Pfau, P. S. Russell, and R. Löw, “Rydberg atoms in hollow-core photonic crystal fibres,” Nat. Commun. 5, 4132 (2014). https://doi.org/10.1038/ncomms5132). Moreover, interfacing on-chip hollow-core waveguides with fiber circuitry and complex fabrication schemes (e.g., ARROWs rely on the electron beam lithography with nano-film deposition) remain key challenges.The authors have recently introduced the concept of the on-chip hollow-core light cage.24–2624. B. Jang, J. Gargiulo, M. Ziegler, R. F. Ando, U. Hübner, S. A. Maier, and M. A. Schmidt, “Fine-tuning of the optical properties of hollow-core light cages using dielectric nanofilms,” Opt. Lett. 45, 196–199 (2020). https://doi.org/10.1364/ol.45.00019625. B. Jang, J. Gargiulo, R. F. Ando, A. Lauri, S. A. Maier, and M. A. Schmidt, “Light guidance in photonic band gap guiding dual-ring light cages implemented by direct laser writing,” Opt. Lett. 44, 4016–4019 (2019). https://doi.org/10.1364/ol.44.00401626. C. Jain, A. Braun, J. Gargiulo, B. Jang, G. Li, H. Lehmann, S. A. Maier, and M. A. Schmidt, “Hollow core light cage: Trapping light behind bars,” ACS Photonics 6, 649–658 (2018). https://doi.org/10.1021/acsphotonics.8b01428 This structure consists of a spare array of dielectric strands surrounding a central hollow section acting as the waveguide core. This structure is particularly interesting for integrated spectroscopy as it uniquely provides an open space between the strands allowing for efficient diffusion of species into the core in contrast to the tube-type hollow waveguides introduced above and can be interfaced with fiber circuitry.2828. B. Jang, J. Gargiulo, J. Kim, J. Bürger, S. Both, H. Lehmann, T. Wieduwilt, S. A. Maier, and M. A. Schmidt, “Fiber-integrated hollow-core light cage for gas spectroscopy,” APL Photonics 6, 061301 (2021). https://doi.org/10.1063/5.0048501We have recently extended the light cage concept toward microfluidics, demonstrated by absorption spectroscopic experiments of light cages immersed in dye solutions.2929. J. Kim, B. Jang, J. Gargiulo, J. Bürger, J. Zhao, S. Upendar, T. Weiss, S. A. Maier, and M. A. Schmidt, “The optofluidic light cage-on-chip integrated spectroscopy using an antiresonance hollow core waveguide,” Anal. Chem. 93, 752 (2020). https://doi.org/10.1021/acs.analchem.0c02857 Together with the recent gas-related experiments,2828. B. Jang, J. Gargiulo, J. Kim, J. Bürger, S. Both, H. Lehmann, T. Wieduwilt, S. A. Maier, and M. A. Schmidt, “Fiber-integrated hollow-core light cage for gas spectroscopy,” APL Photonics 6, 061301 (2021). https://doi.org/10.1063/5.0048501 these results reveal the potential of the light cage concept regarding spectroscopic applications.In this work, we evaluate the optofluidic light cage concept regarding an integrated on-chip fluorescence-based spectroscopy [Fig. 1(a)]. Through immersing the light cage into dye-doped aqueous solution, we show that the fluorescent light is efficiently captured and guided to the waveguide’s output. Using the flexibility of the light cage geometry, we show that both excitation and emission wavelengths can be placed into the PBG regions (high transmission domains) of the fundamental core mode. By using Rhodamine B dissolved in water, detection limits that match corresponding bulk fluorescence measurements are presented.

II. EXPERIMENTAL

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. EXPERIMENTAL <<III. RESULTS AND DISCUSSI...IV. CONCLUSIONSSUPPLEMENTARY MATERIALREFERENCESPrevious sectionNext sectionThe fundamental idea of the presented study relies on using the PBG effect to efficiently capture and guide the fluorescence light,1818. S. Smolka, M. Barth, and O. Benson, “Highly efficient fluorescence sensing with hollow core photonic crystal fibers,” Opt. Express 15, 12783–12791 (2007). https://doi.org/10.1364/oe.15.012783 requiring a periodic arrangement of a dielectric material.3030. M. Schmidt, M. Eich, U. Huebner, and R. Boucher, “Electro-optically tunable photonic crystals,” Appl. Phys. Lett. 87, 121110 (2005). https://doi.org/10.1063/1.2039994 Such an arrangement is provided by the optofluidic light cage [Fig. 1(a)] consisting of a hexagonal array of dielectric strands [cross section shown in Figs. 1(b) and 1(d) = 3.6 µm, Λ = 7 µm, and D = 14 µm]. The core is formed by the omitted central strand and supports a leaky mode through the PBG effect.31,3231. N. Granzow, P. Uebel, M. A. Schmidt, A. S. Tverjanovich, L. Wondraczek, and P. S. J. Russell, “Bandgap guidance in hybrid chalcogenide–silica photonic crystal fibers,” Opt. Lett. 36, 2432–2434 (2011). https://doi.org/10.1364/ol.36.00243232. P. S. J. Russell, “Photonic-crystal fibers,” J. Lightwave Technol. 24, 4729–4749 (2006). https://doi.org/10.1109/jlt.2006.885258 Note that the PBG effect results from the interference of waves scattered by the various elements, leading to resonances in the dispersion of the fundamental core mode [Fig. 2(a), material dispersion of polymer and water can be found in Ref. 2929. J. Kim, B. Jang, J. Gargiulo, J. Bürger, J. Zhao, S. Upendar, T. Weiss, S. A. Maier, and M. A. Schmidt, “The optofluidic light cage-on-chip integrated spectroscopy using an antiresonance hollow core waveguide,” Anal. Chem. 93, 752 (2020). https://doi.org/10.1021/acs.analchem.0c02857]. Here, modal dispersion is represented by the relative effective index Δneff = neff − nwater (neff: effective mode index and nwater: refractive index of water). The resonances match the high loss domains [Fig. 2(b)] of the measured transmission spectrum [yellow areas in Figs. 2(a)2(b), details of the measurements are explained later in the text], while high transmission is obtained in-between the resonances. Remarkably, resonances with large fringe contrast (on-off power ratio >25 dB) are measured, being comparable to values obtained in all-solid PBG fibers.31,3331. N. Granzow, P. Uebel, M. A. Schmidt, A. S. Tverjanovich, L. Wondraczek, and P. S. J. Russell, “Bandgap guidance in hybrid chalcogenide–silica photonic crystal fibers,” Opt. Lett. 36, 2432–2434 (2011). https://doi.org/10.1364/ol.36.00243233. M. A. Schmidt, N. Granzow, N. Da, M. Peng, L. Wondraczek, and P. S. J. Russell, “All-solid bandgap guiding in tellurite-filled silica photonic crystal fibers,” Opt. Lett. 34, 1946–1948 (2009). https://doi.org/10.1364/ol.34.001946 Note that this large contrast is a result of the presence of the second ring, improving the light guidance properties of the device through stronger light confinement.2525. B. Jang, J. Gargiulo, R. F. Ando, A. Lauri, S. A. Maier, and M. A. Schmidt, “Light guidance in photonic band gap guiding dual-ring light cages implemented by direct laser writing,” Opt. Lett. 44, 4016–4019 (2019). https://doi.org/10.1364/ol.44.004016 Such pronounced resonances indicate strong core mode formation overall resulting from the use of two rings of strands.Here, we would like to highlight that the power fraction within the core is greater than 99% in the middle of the transmission bands [simulated and measured mode patterns at λ = 532 nm are shown in Figs. 2(c) and 2(d)], whereby the strands only occupy a fraction of the cladding area. Note that the measured and simulated mode profiles match to a very high degree as visible by the quantitative comparison of the spatial intensity distributions shown in Figs. 2(e) and 2(f). Another remarkable feature is that the design freedom of the light cage geometry allows us to localize both the excitation (λ = 532 nm, green dashed lines in Figs. 2 and 3) and the emission wavelength (λ = 579 nm, orange dashed lines in Figs. 2 and 3) within high transmission regions, which was achieved through adjusting the strand diameters. As mentioned in Ref. 3434. J. Bürger, J. Kim, B. Jang, J. Gargiulo, M. A. Schmidt, and S. A. Maier, “Ultrahigh-aspect-ratio light cages: Fabrication limits and tolerances of free-standing 3D nanoprinted waveguides,” Opt. Mater. Express 11, 1046–1057 (2021). https://doi.org/10.1364/ome.419398, this spectral tuning was achieved through adjusting the strand diameters, allowing one to shift the cut-offs of the isolated strand modes and, thus, the resonances. Therefore, the light cage allows for both guiding the excitation light to the fluorescent molecules and efficiently capturing the uniformly emitted fluorescent light.As shown in Refs. 2626. C. Jain, A. Braun, J. Gargiulo, B. Jang, G. Li, H. Lehmann, S. A. Maier, and M. A. Schmidt, “Hollow core light cage: Trapping light behind bars,” ACS Photonics 6, 649–658 (2018). https://doi.org/10.1021/acsphotonics.8b01428 and 3434. J. Bürger, J. Kim, B. Jang, J. Gargiulo, M. A. Schmidt, and S. A. Maier, “Ultrahigh-aspect-ratio light cages: Fabrication limits and tolerances of free-standing 3D nanoprinted waveguides,” Opt. Mater. Express 11, 1046–1057 (2021). https://doi.org/10.1364/ome.419398, light cages with losses on the order of 0.5 dB/mm γ3434. J. Bürger, J. Kim, B. Jang, J. Gargiulo, M. A. Schmidt, and S. A. Maier, “Ultrahigh-aspect-ratio light cages: Fabrication limits and tolerances of free-standing 3D nanoprinted waveguides,” Opt. Mater. Express 11, 1046–1057 (2021). https://doi.org/10.1364/ome.419398 Taking into account this loss figure and the size of our liquid chamber, we choose a light cage length of L = 4.5 mm for the experiments presented here.As an example dye we chose Rhodamine B (RhoB, molar mass: 479.01 g/mol) dissolved in water. Two concentration ranges corresponding to a low (0.075µM µM) and a high (3.75µM µM) dye concentration were used. The solutions were characterized using an in-house spectrometer (F550B from Perkin Elemer), which showed that maximum excitation and emission (for c = 1 µM) occurs at λ = 552.6 nm and λ = 576.5 nm, respectively (details in the supplementary material). It is important to mention that the maximum of the emission red-shifts for increasing c, an effect that we attribute to dimer formation35,3635. F. del Monte and D. Levy, “Formation of fluorescent rhodamine B J-dimers in sol- gel glasses induced by the adsorption geometry on the silica surface,” J. Phys. Chem. B 102, 8036–8041 (1998). https://doi.org/10.1021/jp982396v36. K. Kemnitz, N. Tamai, I. Yamazaki, N. Nakashima, and K. Yoshihara, “Fluorescence decays and spectral properties of rhodamine b in submono-, mono-, and multilayer systems,” J. Phys. Chem. 90, 5094–5101 (1986). https://doi.org/10.1021/j100412a043 (Fig. SI3). Here, we assume that the maximum emission is located at λ = 579 nm, which holds in the low concentration regime up to c = 2.4 µM.The dual-ring light cages were fabricated by nanoprinting using a commercial 3D lithography system (Photonic Professional GT2, Nanoscribe GmbH, details in the supplementary material). Before printing, the surface of the silicon substrates was exposed to a silanization step to increase the adhesion of the polymer structure to the silicon.3737. X. Liu, H. Gu, M. Wang, X. Du, B. Gao, A. Elbaz, L. Sun, J. Liao, P. Xiao, and Z. Gu, “3D printing of bioinspired liquid superrepellent structures,” Adv. Mater. 30, 1800103 (2018). https://doi.org/10.1002/adma.201800103 To prevent structural collapse during development, the strands were supported and interconnected every 30 µm by reinforcement rings. The supporting elements have been designed to have no significant impact on the optical properties of the light cage as shown in Refs. 2929. J. Kim, B. Jang, J. Gargiulo, J. Bürger, J. Zhao, S. Upendar, T. Weiss, S. A. Maier, and M. A. Schmidt, “The optofluidic light cage-on-chip integrated spectroscopy using an antiresonance hollow core waveguide,” Anal. Chem. 93, 752 (2020). https://doi.org/10.1021/acs.analchem.0c02857 and 3434. J. Bürger, J. Kim, B. Jang, J. Gargiulo, M. A. Schmidt, and S. A. Maier, “Ultrahigh-aspect-ratio light cages: Fabrication limits and tolerances of free-standing 3D nanoprinted waveguides,” Opt. Mater. Express 11, 1046–1057 (2021). https://doi.org/10.1364/ome.419398 and in the supplementary material. This observation is supported by additional modal overlap simulations of the modes of the cross sections with and without the supporting elements, showing close-to-unity values for all possible combinations of cross sections and, thus, proving the negligible impact of the elements. To conduct the optical experiments, the silicon chips with the light cages were fixed inside a home-made optofluidic chamber (volume 300 µl, details in the supplementary material, Fig. SI1). This chamber allows for launching and collecting the light signals through glass windows and dye insertion without light cage removal.

To carry out the various experiments, the setup for the optical characterization was modular and relied on a combination of a suitable light source, the light cages located in the fluidic chamber and spectral diagnostics and cameras. Excitation of the fundamental light cage mode and light collection at the output side was achieved through suitable objectives (a sketch of the experimental setup together with further details is shown in Fig. SI2). The transmitted signal was either guided via a multimedia fiber to the spectrometers or imaged via cameras.

For the broadband characterization [Fig. 2(b)], a commercially available supercontinuum source (SuperK COMPACT, NKT Photonics) and an optical spectrum analyzer (AQ-6315A, Ando, spectral resolution used: Δλ = 0.4 nm) were employed. The setup was optimized for maximum throughput, i.e., for the situations where the fundamental core mode is the brightest. The transmission spectra were normalized by using a spectrum measured without a light cage.In the case of the fluorescence measurements (Fig. 3), the fundamental core mode was excited with a narrowband diode laser (W532-50FS, Pavilion Integration Corp., λ = 532 nm, maximal power 50 mW). The output light was detected using a fast spectrometer (USB2000, Ocean insight, 450 nm λλ = 0.32 nm), while a notch filter (blocking range 523 nm λ

The fluorescence-related experiments rely on measuring the collected light power spectrally resolved for different concentrations at the same input power level. As mentioned above, two types of concentration ranges were addressed, while a starting concentration (high range: c = 30 µM and low range c = 2.4 µM) was successively diluted by adding water. The integration time of the spectrometer was different in the two studies and was adjusted via different attenuation filters to maximize the signal on the spectrometer before reaching saturation at the initial concentration. The integration time within one concentration range remained unchanged to allow for qualitatively comparing the fluorescence data.

III. RESULTS AND DISCUSSION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. EXPERIMENTALIII. RESULTS AND DISCUSSI... <<IV. CONCLUSIONSSUPPLEMENTARY MATERIALREFERENCESPrevious sectionNext sectionThe mode pattern of the fluorescence measurements [Fig. 3(d)] shows a sixfold symmetry that resembles the corresponding simulated pattern of the fundamental mode [Fig. 3(c)]. The quantitative comparison of the intensity distributions [Figs. 3(e) and 3(f)] confirms this observation, while a slightly broader distribution of the measured profile particularly in the horizontal direction (purple lines) is observed, indicating a small contribution from higher-order modes. This intensity distribution, which is also observed for other concentrations, confirms the efficient collection and transportation of the fluorescence light to the output of the dual-ring light cage mainly through the fundamental mode. This particular feature distinguishes the light cage concept from evanescent field-related schemes, which show higher collection efficiency for the higher-order modes, allowing for simpler outcoupling to detection optics. Note that the high fundamental mode capturing efficiency results from the strong modal overlap between the fundamental modes at the excitation and fluorescence wavelengths. Only a small fraction of overall electromagnetic power recorded was located in the strands and, therefore, we consider that the fluorescent light captured and guided by the strands is negligible. The fluorescence power guided to the output increases with dye concentration [Figs. 3(a) and 3(b), at a constant input power (Pin = 14 mW after attenuation)], with the emission peaking at roughly λ = 579 nm as expected. Note that for the high concentrations [Fig. 3(b)], a spectral red-shift of the fluorescence maximum is observed, which in accordance with bulk measurements [examples of emission spectra at two selected concentrations (low and high) are shown in Figs. SI3(a) and SI3(b)] can be explained by the dimer formation.3535. F. del Monte and D. Levy, “Formation of fluorescent rhodamine B J-dimers in sol- gel glasses induced by the adsorption geometry on the silica surface,” J. Phys. Chem. B 102, 8036–8041 (1998). https://doi.org/10.1021/jp982396v The spectral fingerprint of the light cage is visible in the emission spectra, in particular for high dye concentrations [Fig. 3(b)]: A reduction in fluorescence occurs when the spectral domains of high modal attenuation (yellow area in Fig. 3(b)) overlap with the fluorescence spectrum. This effect is particularly pronounced for 600 nm λFig. 3(b)]. Note that the small peaks for 530 nm λFigs. 3(a) and 3(b) result from the residual excitation light passing through the notch filter and not from the residual fluorescent light of the dye, which is evident from the fact that the amplitude of this peak does not change with dye concentration. The constant residual pump light visible in Figs. 3(a) and 3(b) confirms operation in the linear absorption regime, i.e., that the device does not suffer from pump depletion. Therefore, the sensor output will respond linearly as expected.

We would like to mention that in the case of pure water (c = 0), photoluminescence from the polymer strand can be measured (Fig. SI5). The detection of this radiation requires setting the integration times of the spectrometer to much higher values than used in the actual fluorescence experiments, preventing the detection of this residual light in the experiments.

For a quantitative assessment of the light cage properties, we analyzed the fluorescence power (normalized to the power at c = 2.4 µM) as a function of concentration at the main fluorescence wavelength (λ = 579 nm) for both the light cage and cuvette-based measurements, finally yielding the limit-of-detections (LoDs). Please note that we decided to analyze the low concentration regime (0 µM), which does not show a red-shift of the main absorption line with concentration. The resulting plots [Figs. 4(a) and 4(b)] show linear dependencies in both cases, which are fitted by linear functions ΔP(c) = m · c + P0 (m: slope, ΔP0: normalized offset power) to obtain the calibration relations (light cage: m = 0.76074 μm−1M−1, ΔP0 = 0.117 31; cuvette: m = 0.762 33 μm−1M−1, ΔP0 = 0.029 69). For both cases, a linear behavior with the same slope is obtained, suggesting a direct application of the light cage concept in fluorescence-related experiments without the involvement of modal calculation. Note that this effect results from the close-to-unity fraction of power in the core (absorption wavelength: 99.2% and emission wavelength: 99.6%). The slight deviations of the data points from a purely linear behavior can be attributed to unavoidable experimental circumstances, such as noise and error in preparing the concentration samples. Note that the behavior gets nonlinear in the high concentration regime, resulting from the mentioned spectral red-shift of the fluorescence maximum due to dimer formation (details can be found in the supplementary material, Fig. SI4).Within the context of fluorescence spectroscopy, the LoD is defined by LoD = 3σ/m,3838. S. E. Braslavsky, “Glossary of terms used in photochemistry, (IUPAC recommendations 2006),” Pure Appl. Chem. 79, 293–465 (2007). https://doi.org/10.1351/pac200779030293 with σ being the standard deviation of the normalized power of blank measurements and m being the sensitivity found from the linear fit in Fig. 4. For both bulk and light cage measurements, the standard deviation is determined for the pure water case (c = 0), while in the light cage situations, this additionally includes the photoluminescence of the polymer strands being present at any dye concentration. The resulting LoD in the case of the light cage is LoDlc = 8 nM, matching the value from the bulk cuvette measurements (LoDc = 10 nM).An essential feature of the optofluidic light cage is the lateral access to the core domain, a property that conventional hollow-core waveguides with capillary-like geometries do not have. This property is particularly important for the analysis of diffusion processes and is demonstrated below by the time-resolved collection of the fluorescent light (Fig. 5). Specifically, the power of the core mode at the main fluorescence wavelength (λ = 527 nm) is continuously monitored when a certain amount of dye is introduced into the water-filled chamber (details can be found in the supplementary material). The results are compared to a fiber-type capillary of identical core parameters showing a fast increase in the collected fluorescent power for the light cage, reaching 99% of the maximal power in about 8 min (t99% = 500 s, green dots in Fig. 5). This suggests a much faster diffusion compared to other hollow-core waveguides, as about 2.6 times longer diffusion times for the capillary (t99% = 1280 s, green dots in Fig. 5) are measured.To classify the properties, the obtained results of the optofluidic light cage are compared to other waveguide-based fluorescence experiments in Table I. Clearly visible is that the optofluidic light cage allows for fluorescence-based detection of very small dye concentrations and has an overall small LoD. The only systems with smaller LoD-numbers are hollow-core fibers and suspended core fibers, both of which use fibers of substantially longer length which do not provide sidewise access. Note that in a recent study,3434. J. Bürger, J. Kim, B. Jang, J. Gargiulo, M. A. Schmidt, and S. A. Maier, “Ultrahigh-aspect-ratio light cages: Fabrication limits and tolerances of free-standing 3D nanoprinted waveguides,” Opt. Mater. Express 11, 1046–1057 (2021). https://doi.org/10.1364/ome.419398 we have been able to experimentally demonstrate a very high reproducibility of the light cage concept by a statistical analysis.Table icon

TABLE I. Fluorescence-related capabilities of the optofluidic light cage in relation to reported results from integrated waveguide systems.

Type of waveguideDevice lengthCore diameterDyeDemonstrated concentrationLimit of detectionReferenceDual-ring light cage4.5 mm14 µmRhodamine B75 nM8 nMThis workSolid core microstructured optical fibers (SC-MOFs)10 cm3.7 µm (cladding hole)Rhodamine 6G1 µM1 µM1818. S. Smolka, M. Barth, and O. Benson, “Highly efficient fluorescence sensing with hollow core photonic crystal fibers,” Opt. Express 15, 12783–12791 (2007). https://doi.org/10.1364/oe.15.012783Hollow-core photonic crystal fibers (HC-PCFs)10 cm5.3 µm (central core)Rhodamine 6G0.1 nM0.1 nM1818. S. Smolka, M. Barth, and O. Benson, “Highly efficient fluorescence sensing with hollow core photonic crystal fibers,” Opt. Express 15, 12783–12791 (2007). https://doi.org/10.1364/oe.15.012783Suspended core microstructured optical fibers (SCFs)25 cm1.38 µm (core)Rhodamine B10 nMN.A1313. E. P. Schartner, G. Tsiminis, M. R. Henderson, S. C. Warren-Smith, and T. M. Monro, “Quantification of the fluorescence sensing performance of microstructured optical fibers compared to multi-mode fiber tips,” Opt. Express 24, 18541–18550 (2016). https://doi.org/10.1364/oe.24.018541Poly vinyl pyrrolidone and poly vinyl alcohol waveguideN.AN.ARhodamine B1 µMN.A3939. S. A. Suheil, N. S. Shnan, and Q. Mohammed Salman, “The plasmon effects of AgNPs on wave guide consisting of polymers mixed with rhodamine dyes,” J. Phys.: Conf. Ser. 1829, 012007 (2021). https://doi.org/10.1088/1742-6596/1829/1/012007Dye doped PMMA waveguide1.2 cm(120 × 1) µmRhodamine 64020 µMN.A4040. M. C. Ramon, M. Ariu, R. Xia, D. D. C. Bradley, M. A. Reilly, C. Marinelli, C. N. Morgan, R. V. Penty, and I. H. White, “A characterization of rhodamine 640 for optical amplification: Collinear pump and signal gain properties in solutions, thin-film polymer dispersions, and waveguides,” J. Appl. Phys. 97, 073517 (2005). https://doi.org/10.1063/1.1881772Doped polymer film waveguide2.4 cm1.5 mm beam sizeRhodamine 6G100 µMN.A

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