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Abstract
Optofluidic systems, integrating microfluidic and micro-optical technologies, have emerged as transformative tools for various applications, from molecular detection to flow cytometry. However, existing optofluidic microlenses often rely on external forces for tunability, hindering seamless integration into systems. This work presents an approach using two-photon polymerization (TPP) to fabricate inherently tunable microlens arrays, eliminating the need for supplementary equipment. The optofluidic design incorporates a three-layered structure enabling dynamic manipulation of refractive indices within microchannels, leading to tunable focusing characteristics. It is shown that the TPP fabricated optofluidic microlenses exhibit inherent tunable focal lengths, numerical apertures, and spot sizes without reliance on external forces. This work signifies some advancements in optofluidic technology, offering precise and tunable microlenses with potential applications in adaptive imaging and variable focal length microscopy.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optofluidic systems, which integrate microfluidic and micro-optical technologies, have shown great potential in various applications such as molecule and cell detection, optical tweezer, optical signal manipulation, and flow cytometry [1–6]. They enable the manipulation of light and flowing liquids at the micro/nanoscale, leading to the development of reconfigurable optical systems. This synergy between optics and micro/nano-fluidics has resulted in the creation of innovative sensors with improved sensitivity, adaptability, and compactness. The reconfigurability and adaptability of optofluidic devices have been further highlighted, with examples including tunable interferometers and optically controlled devices [7–11]. These advancements in optofluidic technology hold promise for high-efficiency lab-on-chip systems, environmental monitoring, and biomedical applications [12–14].
Microlenses, as an essential element of optofluidic, are widely used in molecule and cell detection [15], biomedicine [16], and imaging [17]. In particular, optofluidic microlenses have attracted much attention due to their unique tunable focusing, and have gain significant advancements in recent years [18–21]. Wee et al. [22] demonstrated distinct focusing properties through a birefringent optofluidic lens, leveraging an external electric field for Joule heat generation. Chen et al. [23] introduced dielectric electrowetting optofluidic microlens with flexible filament electrodes, achieving an extensive focal length range from negative to positive focus (-30 mm to 69 mm). Zhang et al. [24] innovatively designed hot gas kinetic optofluidic microlens, offering variable focal lengths (5 mm to 23 mm) through voltage adjustments. It is worth mentioning that the aforementioned optofluidic microlenses essentially require supplementary equipment (such as electric power sources, electric heaters, mechanical pumps, etc.) to modulate their optical properties. Despite their tunable focusing capabilities, the complexity of external devices impedes seamless integration into optofluidic systems. Consequently, there is a compelling need to develop facile and inherently tunable optofluidic microlens that obviate reliance on external forces (such as hydraulic and electromagnetic) for actuation.
In contrast to conventional optical lenses characterized by a fixed refractive index and defined geometric configuration, the optofluidic lenses incorporated within optofluidic systems exhibit the capacity for adaptable focal length adjustment. The tunability of optofluidic microlenses by changing the refractive index of the liquid has been demonstrated in several studies. Hu et al. [25] employed femtosecond laser ablation to fabricate optofluidic microlenses, achieving versatile broadband focusing capability (260 µm to 2450 µm). This was demonstrated by varying the surrounding media in the channel, including air, water, and sucrose solution. In a related investigation, Lei et al. [26] utilized femtosecond laser ablation and etching techniques to create concave microlenses. Notably, they achieved a two-fold tunability in both focal length and spot characteristics when experimenting with different concentrations of sucrose solution. Nevertheless, the precision fabrication of the microlenses in feature size below ten micrometers poses significant challenges when employing laser ablation technique. Due to the µm spot size and Gaussian energy distribution of the laser beam, this method has limitations in precise control over both the diameter and height of the microlenses. Furthermore, the laser ablation process for microlens fabrication necessitates subsequent post-processing steps, such as etching and annealing, to ensure the requisite smoothness of the microlens surfaces. In respect to the challenges outlined, the two-photon polymerization (TPP) technique, notable for its capacity for three-dimensional (3D) processing, high spatial resolution, and exceptional precision, presents itself as a promising alternative to laser ablation [27–30]. The versatility of TPP enables fabrication of microlenses with varying focal lengths and numerical apertures (NA) [31]. In contrast to laser ablation, TPP facilitates the one-step fabrication of microlenses without the need for subsequent process assistance. TPP technology mitigates the challenges associated with fabricating optofluidic microlenses, enabling the production of microlenses with sizes smaller than 10 µm and arbitrary shapes. Furthermore, optofluidic microlens arrays fabricated through TPP would exhibit exceptional focusing and tunability properties, opening avenues for the miniaturization of optofluidic systems and the broadening of applications across various domains.
This paper focuses on the fabrication process of optofluidic microlenses by studying key TPP process parameters and investigates the optical characteristics of the fabricated microlenses, which are integrated into a three-layered optofluidic system. It is aimed to demonstrate versatile focal length adjustment through the dynamic manipulation of the refractive index of the medium within microchannels. The precision and adaptability of TPP allow for the one-step fabrication of microlenses without the need for post-processing steps would be demonstrated.
2. Methods and experimental
2.1 Design of optofluidic microlenses
The design layout of the optofluidic microlenses is illustrated in Fig. 1(a), providing an overview of the structural composition. The microlens system comprises three layers. The top layer is a transparent cover layer with dedicated inlet and outlet for introducing fluid or gas into the microfluidic device. The middle layer is a microchannel, serving as a conduit for the medium flow. The bottom layer, constructed from SiO2 substrate, serves as the foundation upon which the microlens array is fabricated.
Fig. 1. (a) Schematic overview of optofluidic microlens structure. (b) 3D view of optofluidic microlens in varied media.
Figure 1(b) presents an illustrative 3D close-up view of optofluidic microlens operating in different media. This design strategy provides an approach for tunable focusing by manipulating the refractive index of the medium within the microchannel. The dimensional design parameters are detailed in Table 1. Microlens diameters below 10 µm are purposely selected in order to take the advantages of TPP for producing high precision micro-components. Such small microlenses are not readily achievable by the conventional laser ablation process. For investigation of imaging performances, lens diameters are also designed to be larger at 20, 30, 40, and 50 µm respectively, while keeping the lens height the same at 4 µm.
Table 1. The dimensional parameters used in the design of the microlenses.
2.2 FDTD simulation
The focusing characteristics of the optofluidic microlens were simulated using the finite-difference time-domain method (FDTD - Lumerical Software). The simulation model, as depicted in Fig. 2, employed a comprehensive setup to analyze and visualize the behavior of the microlens under various conditions.
In this study, the simulation positioned the monitor slightly above the microlens to capture the focusing properties accurately. The incident light source, with wavelengths ranging from 400 to 780 nm, was situated within the SiO2 substrate. The light was directed towards the monitor through the microlens structures along the Z-direction. A grid size of 50 nm was selected to strike a balance between simulation accuracy and efficiency. The refractive index of SiO2 is about 1.47. The type of light source used is a total-field scattered-field light. The boundary conditions in the X, Y, and Z directions were configured as perfect match layers (PML) to minimize artifacts and reflections.
The intensity of the light field on the monitor served as a quantitative indicator of the microlens’ focusing characteristics. Utilizing the Fresnel–Kirchhoff integral formula, the full width at half maxima (FWHM) and focal length of the microlens were precisely determined for different media. This systematic approach ensures reliable insights into the optical performance of the optofluidic microlens, laying the groundwork for a detailed analysis of its practical applications. The subsequent sections present and discuss the outcomes of these simulations, and prove the efficacy and adaptability of the designed optofluidic microlenses by experimental measurement results.
2.3 Materials and TPP processing parameters
All chemicals employed in this study were of chemical reagent grade and utilized without additional purification. Anhydrous ethanol, sourced from Zhiyuan Chemical Reagent Co., Ltd., maintained its purity for the duration of the work. The microlenses were prepared on the substrate using OrmoComp photoresist obtained from Micro Resist Technology GmbH, Germany. The polymerized photoresist microlens exhibited a refractive index of approximately 1.52. The glucose solution utilized in the study was prepared from crystals acquired from Fuchen Chemical Reagent Co., Ltd., combined with deionized water. The optofluidic microlens arrays were fabricated using a femtosecond laser with a wavelength of 515 nm. This laser, generated by the second harmonic of a Yb:KGW laser (FemtoLab-MPP), featured a central wavelength of 1030 nm, a pulse width of 217 fs, and a repetition frequency of 60 KHz. The X-Y-Z positioning stage demonstrated a high resolution of 100 nm.
The fabrication process, illustrated in Fig. 3, was initiated with femtosecond laser ablation of the polydimethylsiloxane (PDMS) layer to create the microchannels. Ultrasonic cleaning with acetone, alcohol, and deionized water removed surface debris. Then this PDMS middle layer was bonded with the bottom SiO2 substrate. The photoresist was deposited in the microchannel region, followed by TPP to form the microlens array, utilizing an oil-immersed objective lens (Plan - Apochromat, 63x, NA = 1.42). The microlens structures were developed in an anhydrous ethanol solution. Finally, the top layer (PDMS) with inlet and outlet features was fabricated and bonded to complete the optofluidic microlens assembly. This meticulous process ensured the integrity and functionality of the fabricated optofluidic microlens device for subsequent analyses.
2.4 Characterization
The surface structures of the fabricated microlenses underwent observation through a scanning electron microscope (SEM: Quanta 250, FEI Company, USA) at an accelerating voltage of 15 kV, providing detailed insights into their morphology. A schematic diagram in Fig. 4 outlines the optical characterization setup employed for assessing the optofluidic microlens. The optical system comprises a white light source, X-Y-Z displacement stage, objective lens, CCD (charge-coupled device) camera, and monitor.
Imaging tests: A mask plate with characters is positioned above the white light source for imaging tests. The white light traverses through the mask plate, the optofluidic microlens, and the objective lens before being captured by the CCD and displayed on the monitor.
Focal spot measurements: Focal spot measurements involve removing the mask plate and adjusting the z-axis of the displacement stage to obtain the brightest focal spot. The focal spot is directly captured by the CCD, allowing for precise measurements of its size on the monitor.
Focal length determination: The objective is initially focused on the substrate surface of the microlens to determine the focal length. Subsequently, the displacement stage Z-axis is adjusted until the focal spot becomes apparent. The distance moved along the Z-axis represents the focal length of the microlens.
3. Results and discussion
3.1 Simulation of optofluidic microlenses
The height of the microlens plays a critical role in determining its focusing characteristics. Simulation results for microlenses of varying heights (group e to h in Table 1) are presented in Fig. S1. Observations reveal that microlens with a diameter of 50 µm exhibit multiple focal spots. This phenomenon is attributed to spherical aberration, where different wavelengths of light fail to converge at a single point as white light traverses the microlens. To address this issue and ensure uniform focusing, the microlens radius of curvature should be sufficiently large. Consequently, the microlens height was intentionally set at 4 µm for diameters ranging from 20 to 50 µm. As illustrated in Fig. 5(a) and (b), microlenses with diameters of 20, 30, 40, and 50 µm exhibit a single distinct focal spot in both the X-Z and X-Y planes. The highest light intensity is concentrated at the center of the focal spot, accompanied by low dispersion intensity in the surrounding areas. Notably, an increase in the microlens diameter corresponds to an augmentation in both the size of the focused spot and the focal length, demonstrating the direct relationship between these parameters, which provides the guide for optimizing microlens design for specific applications, ensuring the attainment of desired optical outcomes.
Fig. 5. Light intensity distribution of microlenses with varying diameters of the microlenses (20, 30, 40, and 50 µm) in different media. (a) and (b) in air, (c) and (d) in water, and (e) and (f) in a glucose solution, depicted on the X-Z and X-Y planes. The microlens diameters (I-IV) are 20, 30, 40, and 50 µm respectively.
In-depth simulations were conducted to examine the focusing properties of the optofluidic microlenses in distinct environments, namely water (Fig. 5(c) and (d)) and a glucose solution (Fig. 5(e) and (f)). The refractive indices of water and a 50% concentrated glucose solution are 1.332 and 1.451, respectively. The results highlight the impact of varying refractive indices on the microlens focal spot and focal length. The simulation revealed that as the refractive index of the solution increased, both the focal spot and focal length of the microlens experienced a corresponding increase. This trend holds true across different media, exemplified by the larger focal length and focal spot in water compared to air [32]. The phenomenon is explained by the slower speed of light propagation in a medium with a higher refractive index, resulting in a larger optical path difference and, consequently, a larger focal length [33,34].
Figure 6(a) depicts the simulated focal length and FWHM of the microlenses. As the bottom radius gradually decreases, both the focal length and FWHM of the microlenses diminish in accordance with fundamental optical principles. However, it's crucial to note that a smaller microlens diameter does not universally enhance performance. This is evident in Fig. 6(b), where hemispherical microlenses with smaller diameters (<1 µm) exhibit larger FWHM and focal lengths. This is attributed to diffraction effects, particularly prominent when the microlens size is smaller than the wavelength. Once the microlens diameter exceeds 1 µm, the FWHM and focal length conform to the optical focusing equation, gradually increasing with the microlens diameter [35]. Considering the characteristics of TPP, simulations extended to microlenses from groups a-d in Table 1 in air, water, and glucose solution (Supplement 1, Fig. S2). The focusing characteristics mirrored those of the e-h group in Table 1, affirming the versatility and consistency of the optofluidic microlenses across different conditions.
Fig. 6. Simulation results illustrating the focal length (solid line) and FWHM (dashed line) of microlenses, (a) in various media and (b) with different diameters.
3.2 Fabrication and analysis of optofluidic microlens
The fabrication and integration of microlenses into the optofluidic microchannel were accomplished through a point-by-point scanning mode. It is noteworthy that during direct laser writing, acceleration and deceleration are inherent at the initiation and conclusion of the process. This results in the exposure of high-density laser pulses, which, if not managed appropriately, can compromise the surface integrity of the structure. Figure S3(a) and (b) in Supplement 1, illustrate two scanning methods: contour scanning and linear scanning, respectively. In contour scanning, the start and end points of polymerization coincide, leading to dense voxel overlap due to acceleration and deceleration. This results in visible hinge lines in the overexposed region, as depicted in Fig. S3(c). In contrast, linear scanning disperses the start and end points evenly across the microlens surface, effectively minimizing overexposure, as shown in Fig. S3(d). The improved surface quality and smoothness of the microlens are evident, with no visible hinge lines.
Previous studies have demonstrated that smaller scan spacing and layer thickness result in smoother surfaces [36]. In this context, both the scan spacing and scanning layer thickness were set to 100 nm, aligning with the highest accuracy achievable with a laser displacement stage. In the pursuit of optimal surface quality, fabrication was performed at pulse densities of 80000 (number of pulses per millimeter length), the speed was 50 µm s−1, and the power was 2.4 µW. Too fast speeds and too high or too low powers would cause uneven surfaces (Fig. S4 and S5). Using the above processing parameters, the microlens surface was smooth (Fig. 7(a)) and the lens size was close to the designed values. Figure 7(b) presents an SEM image of the overall optofluidic microlens array, and Fig. 7(c) displays SEM images of microlenses with diameters ranging from 20 to 50 µm. As the microlens’ diameter increases, a ring structure appears at the bottom due to the increased radius of curvature, which is limited by the precision of the laser displacement stage (100 nm). Figure 7(d) showed SEM images of microlenses with diameters of 7, 8, 9, and 10 µm, demonstrating the capability of TPP to fabricate small-sized microlenses within the resolution range of the photoresist and the displacement stage. This reinforces the theoretical feasibility of fabricating microlenses with arbitrary focal lengths by TPP technology.
Fig. 7. (a) SEM images depicting microlenses processed with a pulse density of 80,000. Microlenses (I-IV) were processed with varying power settings—2.4, 2.2, 2, and 1.8 µW, respectively. (b) SEM images at a 45° tilt angle of the optofluidic microlens array. (c) and (d) SEM images of microlenses at 45° tilt angle with Da-Dh corresponding to the diameters of the a-h microlenses in Table 1.
3.3 Properties of optofluidic microlenses
Extensive tests were conducted in air, water, and a 50% concentrated glucose solution, to validate the optical properties of the TPP-fabricated optofluidic microlenses. The focal spot sizes of the optofluidic microlenses in these different media are illustrated in Fig. 8(a) and (b). The results reveal a versatile adjustment in focal spot size by introducing different media into the optofluidic microchannel. In the same medium, the focal spot size increases with the diameter of the microlens. This is attributed to microlenses with larger diameters and the same lens height having a smaller numerical aperture (NA). Moreover, in different media, the focal spot size of microlenses of the same size increases with the refractive index of the medium. As the focal spot enlarges, the intensity of the light field diminishes (Supplement 1, Fig. S6). Notably, the focal spot size ranged from a minimum of 2.45 µm to a maximum of 24.2 µm. When light traverses two different media, refraction occurs at the interface. The refractive index difference between liquid and photoresist being smaller than the refractive index contrast between air and photoresist results in a larger spot. These findings underscore the tunable and adaptable nature of the optofluidic microlenses, making them suitable for diverse applications where the manipulation of focal spot size is a critical parameter.
Fig. 8. Focusing spots (a and b) and optical imaging (c and d) of the optofluidic microlenses in air, water and glucose solutions with diameter of 7, 8, 9, 10, 20, 30, 40, and 50 µm, respectively.
In addition to focusing tests, imaging characterization tests in different media are presented in Fig. 8(c) and (d). The size of the image aligns with the variation pattern of the focal spot. As the imaging size gradually increases, the light intensity contrast in its imaging gradually decreases due to the constant total incident ray intensity.
In Fig. 9(a) and (b), a comprehensive analysis and comparison of the focal spot size and focal length of microlenses in different media are presented. Notably, a discrepancy is observed between the simulated and measured values. The measured focal length is slightly smaller than the simulated focal length, and the measured focal spot size is slightly larger than the simulated size. Several factors contribute to these differences, including the actual refractive index of the medium in the microchannel and the tendency of CCD to saturate in white light, leading to a larger captured microlens-focused spot. In Fig. 9(c), the numerical aperture (NA) value of the optofluidic microlens is explored. According to the theory of geometrical optics (NA = R/f), the NA value decreases as the focal length (f) increases for a constant microlens radius (R). A broad range of NA values, spanning from 0.055 to 0.476, can be achieved by adjusting the microlens in the different media. Further insights into the focal spots, focal lengths, and NA values of optofluidic microlenses with diameters ranging from 7-10 µm are provided in Supplement 1, Fig. S7. For instance, a microlens with a diameter of 9 µm achieves nearly a 4-fold change in spot size (1.9 µm to 6.8 µm), focal length (8 µm to 36 µm), and NA value (0.54 µm to 0.128 µm).
Fig. 9. (a) The focal spot, (b) focal length size, and (c) NA values of the optofluidic microlens in air, water and glucose solutions measured by experimental observation (solid line) and simulation (dashed line).
Figure 10(a) illustrates the optical imaging by a 50 µm diameter microlens in air at various focal lengths, with a sampling step of 10 µm. Figure 10(b) shows the focusing of the microlens at different focal planes. The center position corresponds to the minimum spot size. The gradual blurring from the center plane to the sides provides a direct visualization of the effects of aberration. The intensity distributions of the experimental and simulation results are in agreement (Fig. 10(c)). Figure 10(d) illustrates the FWHM value at the focal spot out. The measured value is 2.61 µm, which agrees with the simulated value (2.35 µm). Figure 10(e) shows the 3D intensity distribution of the focal spots. The intensity distribution curves for each focal spot conform to a Gaussian distribution and demonstrate high optical uniformity. In Fig. 11(f), the microlens array is tested for imaging in air, focusing images at different planes, showcasing its capability to capture objects at various locations and extending its depth-of-field.
Fig. 10. (a) Optical imaging of the 50 µm diameter microlens in air at different focal lengths. (b) Intensity distribution of test focus images acquired at different distances along the optical axis direction. (c) Intensity distribution of simulated focused images acquired at different distances along the optical axis direction. (d) Intensity distribution of the microlens spot. Experimental and simulated data are shown as red and blue lines, respectively. The experimental FWHM is 2.61 µm. (e) 3D intensity distribution of the focal spot. (f) Imaging of microlens arrays in air.
Fig. 11. (a) Graph of refractive index as a function of sucrose solution concentration. (b-d) Focal spot size, focal length and NA value of microlenses with different diameters in glucose solution. (e) and (f) Imaging and focusing of optofluidic microlenses in glucose solutions with different refractive indices.
Furthermore, continuous optical tunability of optofluidic microlens properties, including focal spot, focal length, and NA values, was achieved in glucose solutions with varying refractive indices. The wide range of refractive indices (1.332 to 1.463) was achieved by adjusting the concentration (g/l) of glucose solution in the microchannel (Fig. 11(a)). Figure 11(b)-(d) depict the focal spot size, focal length, and NA values of the optofluidic microlens in different glucose solutions, respectively. The quantitative variation in the refractive index of the glucose solution facilitated continuous adjustment of the optical properties of the optofluidic microlens. A broad spectrum of optical characteristics, with spot sizes ranging from 5.95 µm to 17.63 µm, focal lengths from 77 µm to 193 µm, and NA values from 0.13 to 0.052, were achieved for microlenses with a diameter of 20 µm. The imaging and focusing of optofluidic microlenses of different diameters in glucose solutions with different refractive indices are shown in Fig. 11(e) and (f). Controlling the sequential change in the refractive index of the medium in the microchannel enables a continuous variation in the optical properties of the optofluidic microlens. In addition, the tunable capabilities of other optofluidic microlenses are also summarized, as shown in Table 2. Compared with other optofluidic lenses, the optofluidic microlens using TPP achieves 2.5x continuous tunability of the optical characteristics without changing the shape or position of the microlens. Optofluidic microlenses achieve nearly 8x change in optical performance (including focal spot (2.45-17.63 µm), focal length (21-193 µm), NA (0.052-0.476)) in different media. The optical tunable properties are better than those of other optofluidic microlenses. The solid structure of the microlens ensures long-term optical performance in liquid without the need for external forces (hydraulics, voltage, electromagnetic fields, etc.). This is another advantage of optofluidic microlens. The demonstrated optical tunability holds significant potential in applications such as adaptive imaging and variable focal length microscopy.
Table 2. Tunable focusing capability of different optofluidic microlenses
4. Conclusion
This study explored TPP technology for the manufacture of optofluidic microlenses, presenting in-depth investigation into their simulation, fabrication process, and optical properties. The research demonstrated tunability of microlenses achieved through TPP, eliminating the need for complex post-processing steps associated with traditional techniques. The FDTD simulations provide valuable insights into the focusing characteristics of the microlenses, emphasizing the impact of diameter and refractive index on focal length and spot size. The experimental results validated the precision and adaptability of TPP-fabricated microlenses in different media (air, water and glucose solution), showcasing tunable focal lengths and spot sizes. Imaging and characterization tests provided a nuanced understanding of the correlation between focal spot size, imaging size, and media refractive index.
The results showed the importance of optimized scanning strategies in the fabrication process, highlighting the role of factors such as scan spacing, layer thickness, and power settings in achieving high surface quality. Notably, the research demonstrated the continuous tunability of optical properties in glucose solutions, showcasing the potential for adaptive imaging and variable focal length microscopy. The optofluidic microlens achieved a substantial range (over 2.5 times) of continuous tunability across focal spot size (ranging from 5.95 µm to 17.63 µm), focal length (from 77 µm to 193 µm), and NA values (varying between 0.13 and 0.052). The solid structure of the microlenses ensures long-term optical performance in liquid environments without external forces. The findings of this research would contribute to the ongoing evolution of optofluidic technology, offering a pathway towards more efficient and adaptable optical devices for applications ranging from lab-on-chip systems to biomedical imaging and beyond.
Funding
National Key Research and Development Program of China (2022YFB4600402, 2022YFE0199100); Natural Science Foundation of Shandong Province (ZR2021MF030, ZR2022ZD07); Taishan Scholar Project of Shandong Province (tscy202006025, tsqn202306192).
Disclosures
The authors declare no conflicts of interest and no competing financial interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. D. Psaltis, S. R. Quake, and C. Yang, “Developing optofluidic technology through the fusion of microfluidics and optics,” Nature 442(7101), 381–386 (2006). [CrossRef]
2. K. Sivashanmugan, K. Squire, J. A. Kraai, et al., “Biological Photonic Crystal-Enhanced Plasmonic Mesocapsules: Approaching Single-Molecule Optofluidic-SERS Sensing,” Adv. Opt. Mater. 7(13), 1900415 (2019). [CrossRef]
3. L. Liang, X. Hu, Y. Shi, et al., “Tunable and Dynamic Optofluidic Microlens Arrays Based on Droplets,” Anal. Chem. 94(43), 14938–14946 (2022). [CrossRef]
4. Q. Li, J. Van De Groep, A. K. White, et al., “Metasurface optofluidics for dynamic control of light fields,” Nat. Nanotechnol. 17(10), 1097–1103 (2022). [CrossRef]
5. J. Tang, G. Qiu, and J. Wang, “Recent Development of Optofluidics for Imaging and Sensing Applications,” Chemosensors 10(1), 15 (2022). [CrossRef]
6. J. Yu, J. Xu, A. Zhang, et al., “Manufacture of Three-Dimensional Optofluidic Spot-Size Converters in Fused Silica Using Hybrid Laser Microfabrication,” Sensors 22(23), 9449 (2022). [CrossRef]
7. J. Wang, S. A. Maier, and A. Tittl, “Trends in Nanophotonics-Enabled Optofluidic Biosensors,” Adv. Opt. Mater. 10(7), 2102366 (2022). [CrossRef]
8. H. T. Zhao, Y. Zhang, P. Y. Liu, et al., “Chemical reaction monitoring via the light focusing in optofluidic waveguides,” Sens. Actuators, B 280, 16–23 (2019). [CrossRef]
9. J. Li, J. Qu, Y. Liu, et al., “Novel fiber-tip micro flowmeter based on optofluidic microcavity filled with silver nanoparticles solutions,” Nanophotonics 11(21), 4615–4625 (2022). [CrossRef]
10. X. Hu, J. Zhu, Q. Hu, et al., “Digital optofluidic compound eyes with natural structures and zooming capability for large-area fluorescence sensing,” Biosens. Bioelectron. 195, 113670 (2022). [CrossRef]
11. R. Baghery, S. R. Shabanian, J. Ahmadpour, et al., “Investigation of Various Factors on Iodide Depletion Efficiency in Photocatalytic Water Splitting in Optofluidic Microreactors,” Arab. J. Sci. Eng. (2022).
12. D. Wu, L.-G. Niu, S.-Z. Wu, et al., “Ship-in-a-bottle femtosecond laser integration of optofluidic microlens arrays with center-pass units enabling coupling-free parallel cell counting with a 100% success rate,” Lab Chip 15(6), 1515–1523 (2015). [CrossRef]
13. M. J. N. Sampad, H. Zhang, T. D. Yuzvinsky, et al., “Optical trapping assisted label-free and amplification-free detection of SARS-CoV-2 RNAs with an optofluidic nanopore sensor,” Biosens. Bioelectron. 194, 113588 (2021). [CrossRef]
14. C. Song, Y. Yang, X. Tu, et al., “A Smartphone-Based Fluorescence Microscope With Hydraulically Driven Optofluidic Lens for Quantification of Glucose,” IEEE Sens. J. 21(2), 1229–1235 (2021). [CrossRef]
15. D. Wu, J. Xu, L.-G. Niu, et al., “In-channel integration of designable microoptical devices using flat scaffold-supported femtosecond-laser microfabrication for coupling-free optofluidic cell counting,” Light: Sci. Appl. 4(1), e228 (2015). [CrossRef]
16. M. Marini, A. Nardini, R. Martínez Vázquez, et al., “Microlenses Fabricated by Two-Photon Laser Polymerization for Cell Imaging with Non-Linear Excitation Microscopy,” Adv. Funct. Mater. 33(39), 2213926 (2023). [CrossRef]
17. M. Xu, Y. Xue, J. Li, et al., “Large-Area and Rapid Fabrication of a Microlens Array on a Flexible Substrate for an Integral Imaging 3D Display,” ACS Appl. Mater. Interfaces 15(7), 10219–10227 (2023). [CrossRef]
18. G. Holzner, Y. Du, X. Cao, et al., “An optofluidic system with integrated microlens arrays for parallel imaging flow cytometry,” Lab Chip 18(23), 3631–3637 (2018). [CrossRef]
19. C. Fang, B. Dai, Q. Xu, et al., “Hydrodynamically reconfigurable optofluidic microlens with continuous shape tuning from biconvex to biconcave,” Opt. Express 25(2), 888 (2017). [CrossRef]
20. Y. Zhong, H. Yu, P. Zhou, et al., “In Situ Electrohydrodynamic Jet Printing-Based Fabrication of Tunable Microlens Arrays,” ACS Appl. Mater. Interfaces 13(33), 39550–39560 (2021). [CrossRef]
21. Y. Zhong, H. Yu, Y. Wen, et al., “Novel Optofluidic Imaging System Integrated with Tunable Microlens Arrays,” ACS Appl. Mater. Interfaces 15(9), 11994–12004 (2023). [CrossRef]
22. D. Wee, S. H. Hwang, Y. S. Song, et al., “Tunable optofluidic birefringent lens,” Soft Matter 12(17), 3868–3876 (2016). [CrossRef]
23. T. Chen, J.-Y. Sun, T.-X. Ding, et al., “Flexible Wire Electrode Driving Focus-Tunable Electrowetting Optofluidic Lens,” IEEE Photonics Technol. Lett. 34(10), 537–540 (2022). [CrossRef]
24. W. Zhang, H. Li, Y. Zou, et al., “Design and Fabrication of a Tunable Optofluidic Microlens Driven by an Encircled Thermo-Pneumatic Actuator,” Micromachines 13(8), 1189 (2022). [CrossRef]
25. Y. Hu, S. Rao, S. Wu, et al., “All-Glass 3D Optofluidic Microchip with Built-in Tunable Microlens Fabricated by Femtosecond Laser-Assisted Etching,” Adv. Opt. Mater. 6(9), 1701299 (2018). [CrossRef]
26. P. Lei, J. Zhang, S. Shangguan, et al., “Femtosecond laser multibeam parallel processing for variable focal-length optofluidic chips,” Opt. Lett. 48(21), 5603 (2023). [CrossRef]
27. M. Carlotti and V. Mattoli, “Functional Materials for Two-Photon Polymerization in Microfabrication,” Small 15(40), 1902687 (2019). [CrossRef]
28. Z. Faraji Rad, P. D. Prewett, and G. J. Davies, “High-resolution two-photon polymerization: the most versatile technique for the fabrication of microneedle arrays,” Microsyst Nanoeng 7(1), 71 (2021). [CrossRef]
29. A. Bagheri and J. Jin, “Photopolymerization in 3D Printing,” ACS Appl. Polym. Mater. 1(4), 593–611 (2019). [CrossRef]
30. Z. Wang, Y. Wu, D. Qi, et al., “Two-photon polymerization for fabrication of metalenses for diffraction-limited focusing and high-resolution imaging,” Opt. Laser Technol. 169, 110128 (2024). [CrossRef]
31. Z. Hou, J. Cao, A. Li, et al., “Tunable protein microlens array,” Chin. Opt. Lett. 17(6), 061702 (2019). [CrossRef]
32. Y.-L. Yeh, “Real-time measurement of glucose concentration and average refractive index using a laser interferometer,” Opt. Lasers Eng. 46(9), 666–670 (2008). [CrossRef]
33. L. Xue, Y. Pang, W. Liu, et al., “Fabrication of Random Microlens Array for Laser Beam Homogenization with High Efficiency,” Micromachines 11(3), 338 (2020). [CrossRef]
34. Y. Liu, X. Li, Z. Wang, et al., “Morphology adjustable microlens array fabricated by single spatially modulated femtosecond pulse,” Nanophotonics 11(3), 571–581 (2022). [CrossRef]
35. H. Deng, D. Qi, X. Wang, et al., “Femtosecond laser writing of infrared microlens arrays on chalcogenide glass,” Opt. Laser Technol. 159, 108953 (2023). [CrossRef]
36. Weilong Cao, Wenhui Yu, W. Xiao, et al., “Water flow tuned by micro/nano hierarchical structures fabricated through two-photon polymerization,” Mater Sci. Add. Manuf. 2(2), 0879 (2023). [CrossRef]
37. X. Zhao, Y. Chen, Z. Guo, et al., “Tunable optofluidic microbubble lens,” Opt. Express 30(5), 8317 (2022). [CrossRef]
38. P. Fei, Z. He, C. Zheng, et al., “Discretely tunable optofluidic compound microlenses,” Lab Chip 11(17), 2835 (2011). [CrossRef]
