1Engineering Research Center for Nanophotonics and Advanced Instrument, School of Physics and Electronic Science, East China Normal University, Shanghai 200241, China
2XXL—The Extreme Optoelectromechanics Laboratory, School of Physics and Electronic Science, East China Normal University, Shanghai 200241, China
3Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
4State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
【AIGC One Sentence Reading】:We developed polarization-independent fs-laser etching for curved microchannels in fused silica, enabling single-mode optofluidic waveguides and beam splitters.
【AIGC Short Abstract】:This study introduces a method to fabricate homogeneous curved microchannels in fused silica using polarization-independent femtosecond laser-assisted etching. The technique allows for the creation of single-mode bending optofluidic waveguides and beam splitters, paving the way for high-performance microfluidic photonic circuits.
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Abstract
Bending optofluidic waveguides are essential for developing high-performance fluid-based photonic circuits and systems. The combination of femtosecond (fs)-laser-assisted etching of high-precision microchannels and vacuum-assisted liquid-core filling allows the controllable fabrication of low-loss optofluidic waveguides in fused silica. However, to form high-performance bending optofluidic waveguides in fused silica, facile fabrication of long, homogeneous microchannels with arbitrary shapes remains challenging due to the polarization-dependent limitations of conventional fs-laser-assisted etching. Here, we demonstrate the rational fabrication of homogeneous curved microchannels in fused silica using polarization-independent fs-laser-assisted etching enabled by a low-pulse-overlap scheme. An etching rate exceeding 350 μm/h can be reliably achieved at a pulse overlap of μ regardless of the variation of the laser polarization. Highly interconnected nanocracks are observed along the laser writing direction in the laser-modified regions. Using the polarization-independent fs-laser-assisted etching combined with spatial beam shaping and carbon dioxide laser irradiation, uniform and smooth curved microchannels with centimeter lengths, flexible configurations, and nearly circular cross-sections are initially produced. Subsequently, single-mode bending optofluidic waveguides and beam splitters are created by filling tunable refractive index liquid-core solutions into the channels. The proposed method enables efficient processing of arbitrarily oriented homogeneous microchannels, paving the way for the development of large-scale, complex microfluidic photonic circuits.
1. INTRODUCTION
Optofluidic waveguides, as one of the fundamental elements in optofluidic devices and systems [1–4], have exhibited great potential in the fields of integrated optics, microlasers, lab-on-chips, and spectroscopic analysis [5–8]. Liquid-core waveguides are a type of optofluidic waveguide device in which liquid solutions serve as the waveguide cores, surrounded by suitable cladding layers such as microchannels. By altering the properties of the liquid solutions that fill the microchannels, the optical characteristics of the waveguides can be flexibly adjusted. Polydimethylsiloxane (PDMS) materials were initially employed as microchannels in the early period to form liquid-core waveguides due to their ease of processability [9–11]. However, the long-term operation of such microchannels suffers from their weak chemical stabilities, especially in harsh environments. Therefore, glass microchannels have been considered as an alternative solution for constructing optofluidic waveguides due to the merits of glass (especially for fused silica), such as its chemical inertness, low fluorescent background, and suitable refractive index (close to traditional optical fibers) [12,13]. A representative approach to form an optofluidic waveguide in fused silica is femtosecond (fs)-laser-assisted etching combined with vacuum-assisted liquid-core filling. As a two-step procedure (fs laser irradiation followed by chemical etching), fs-laser-assisted etching enables high-precision and in-volume glass structuring in a three-dimensional (3D) manner [14,15]. First, during the laser irradiation process, spatially selective modifications can be created around the focal volume with high precision [16]. Secondly, the laser-modified regions in the glass can be preferentially removed using specific etchants, such as diluted hydrofluoric acid (HF) or hot alkaline solutions, to form the desired 3D microstructures. Our previous work demonstrated the rational fabrication of optofluidic elements such as optofluidic waveguides and optofluidic spot-size converters in fused silica using spatially shaped fs-laser-assisted etching [17–19]. Specifically, fs laser direct writing was initially employed to produce periodic nanograting structures in a modified region within the glass, and the orientation of the nanogratings was typically perpendicular to the polarization direction of the laser beam [20–22]. The orientation of the nanogratings parallel to the laser writing direction ensures a maximum etching rate of the modified region, resulting in a strong polarization-dependent phenomenon in conventional fs-laser-assisted etching [23,24]. To achieve rapid etching, the polarization direction of the laser beam is typically set perpendicular to the laser writing direction. However, under these conditions, it is challenging to fabricate arbitrarily oriented and homogeneous 2D and 3D microchannels without varying the polarization state of the laser beam. While dynamic synchronization control of the laser polarization direction perpendicular to the direct writing direction can be achieved in real-time [25], the high complexity and low flexibility of the entire processing system restrict its practical application for large-scale and rapid production of targeted microchannel structures. Furthermore, a circularly polarized fs laser beam enables polarization-independent etching; however, it has a low etching rate [24,26]. Therefore, developing a new approach for achieving polarization-independent or arbitrary-direction ultrafast-laser-assisted etching is highly desirable.
Polarization-independent laser-assisted etching using a single linearly polarized laser beam was first demonstrated by picosecond-laser-induced randomly oriented nanocracks [27]. Recently, it has been extended to fs domains with a low-pulse-overlap irradiation scheme [28,29]. In this work, we adopt the low-pulse-overlap fs laser irradiation scheme combined with spatial beam shaping and selective chemical etching to fabricate long and homogeneous curved microchannels with circular cross-sections in fused silica. By controlling the pulse overlap and pulse energy of the laser beam, high-connectivity, polarization-independent nanocracks can be formed along the laser writing direction, enabling rapid polarization-independent fs-laser-assisted etching. The etched microchannels are then sealed and polished using carbon dioxide () laser irradiation. Next, bending optofluidic waveguides are formed by filling the channels with a mixture of liquid paraffin and decane with a tunable refractive index. The output mode fields of the optofluidic waveguides can be adjusted by altering the mixing proportions of the liquid-core solutions used. In addition, we demonstrate the fabrication of a long optofluidic waveguide with a multi-mode output, and and optofluidic waveguide beam splitters in fused silica. The proposed approach is expected to realize the facile fabrication of novel optofluidic waveguides with arbitrary orientations in fused silica, which can be applied to adiabatic mode conversion, optical manipulation, and lab-on-a-chip sensing.
2. EXPERIMENT
Commercially available fused silica slides (JGS1) with variable sizes were used as processing substrates. A schematic of the spatially shaped fs laser microprocessing system is illustrated in Fig. 1(a). The employed ultrafast laser system’s central wavelength and pulse duration (Light Conversion, Pharos 20 W) were 1030 nm and 270 fs, respectively. The laser beam was first collimated and expanded through a beam expander, then passed through a half-wave plate, and subsequently incident on the liquid crystal display panel of a spatial light modulator (SLM, HOLOEYE Photonics AG, PLUTO-2). After the SLM, the laser beam passed through a 4f system (L1 and L2), a pinhole, a dichroic mirror, and a half-wave plate, and finally entered a focused objective lens with a numerical aperture of 0.8 (Olympus, ) to induce internal modification inside the fused silica slide mounted on a 3D motorized stage. The first half-wave plate before the SLM was used to modulate the polarization state of the incident laser beam on the SLM. The SLM was used to create a programmable slit, enabling the spatial shaping of the laser beam for fabricating circular cross-section microchannels in fused silica [17–19]. The blue dashed rectangle in Fig. 1(a) indicates a loaded phase diagram on the liquid crystal display panel of the SLM for slit-beam shaping. The second half-wave plate in front of the objective lens was employed to control the polarization state of the laser beam for in-volume direct writing. To fabricate the microchannels, the laser direct writing was performed at 50 μm depth below the fused silica surface. The writing speed was set at 1 mm/s. For the fabrication of the long and homogeneous microchannels, the extra-access ports were adopted to enhance the etching performance [17–19]. In this work, the pulse overlap N (pulses μ) is defined as the ratio of the repetition rate of the laser beam and laser writing speed. The variation of the N value was carried out by tuning the repetition rate of the laser beam [29,30]. To evaluate the etching performance, the influences of N values, pulse energies (), and polarization states of the laser beam were investigated. Moreover, an extra-access port shown in Figs. 1(b) and 1(c) was simultaneously inscribed along the Z direction during the laser writing of the microchannel pattern, which was 1.5 mm away from the left end of the 1.7 mm laser-written track, allowing the etching solution to enter the microchannel only from the port. For microchannel fabrication, the laser-modified fused silica samples were etched in a 90°C, 10 mol/L KOH solution for 2 h in an ultrasonic bath. The morphology of the laser-modified regions was characterized using scanning electron microscopy (SEM, Zeiss GeminiSEM450). For SEM observation, the laser-modified fused silica samples were polished from the top surface until the modified regions were exposed and then etched in a 1 mol/L KOH solution at 90°C for 5 min.
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Figure 1.(a) Schematic of the SLM-assisted spatially shaped fs laser microfabrication system. The dashed blue rectangle indicates the phase diagram loaded onto the SLM. (HWP: half-wave plate; P: pinhole; OBJ: objective lens; M: metal mirror; D: dichroic mirror; BE: beam expander; L1 and L2 represent lenses with different focal lengths.) (b) Top-view and (c) side-view optical micrographs of the laser-written tracks inside the fused silica. The dashed line in (c) indicates the glass surface. The image below the dashed line in (c) shows the actual structure of the laser-written tracks, while the one above the dashed line is a virtual image created by optical microscopy.
To investigate the influence of the laser polarization on the etching performance of laser-written tracks, different N values with three polarization states [, , and , in which the angle of polarization () denotes the angle between the direction of laser polarization and the writing direction] were employed. The polarization state was varied by rotating the second half-wave plate in front of the objective lens. Figure 2 shows the etching results when the N values were set to 10 and μ under different and conditions. It should be noted that regarding the same open ports along the Z direction employed for all polarization states, the etching rate was defined as the ratio of the etched length of the laser-modified microchannel pattern beneath the surface 50 μm and the etching time (the 50 μm long extra-access port was not counted in the calculation of the etched length of the microchannel pattern). As shown in Figs. 2(a) and 2(c), when the N value was set to μ, the etching rate at was significantly faster than the etching rates at and 45° under the same condition, indicating a strong polarization-dependent etching behavior [23,24]. When the N value was set at μ, a polarization-independent etching occurred, which was completely different from the case of μ. As shown in Figs. 2(b) and 2(d), although three different polarization states were employed, the etching results appeared similar. When μ, the etching rates for the three polarization states ranged from 350 μm/h to 450 μm/h, which were significantly higher than those of μ. When μ, the etching rate at decreased slightly compared to the cases of and 45°. Thus, polarization-independent fs-laser-assisted etching enabled by the low-pulse-overlap fs laser irradiation scheme has been validated.
Figure 2.Relationship between the etching rate and under different and N values: (a) μ and (b) μ. Top-view optical micrographs of 2 h etched microchannels under different , , and N values: (c) μ and (d) μ. gradually increased from the top to the bottom in each micrograph in (c) and (d). The etching of laser-written tracks began at the open ports indicated by arrows.
Further, SEM observation of the laser-modified regions under different and N values was performed. As shown in Fig. 3(a), when the N value was set to μ, polarization-dependent nanograting structures could be observed under all conditions. In the case of , the orientation of the nanograting structures was parallel to the flow direction of the etching solution, allowing for rapid etching. This observation is consistent with the etching results shown in Figs. 2(a) and 2(c). In the cases of and 45°, the orientation of the nanograting structures rotated and obstructed the etching solution from entering the modified region, leading to lower etching rates. In contrast, when the N value decreased to μ, randomly oriented cracks with nano-scale linewidths (nanocracks) were mainly observed, although the nanograting structures could be slightly observed under the conditions of and 45° as shown in Fig. 3(b). Those nanocracks exhibited similar transverse connectivities along the laser writing direction under different conditions, enabling the subsequent rapid etching despite the variation in the polarization state.
Figure 3.Top-view SEM images of fs-laser-modified regions in fused silica under different and N values: (a) μ and (b) μ. was set at 1.6 μJ.
Meanwhile, the etching results for , 20, and μ were also obtained. When μ and μ, the modified regions could be rapidly etched with an etching rate over 350 μm/s for each condition [see Fig. 4(a)], indicating a strong polarization-independent etching behavior. However, when the decreased to 1.3 μJ at the same N value, etching of the laser-modified regions became hard for , indicating that a threshold pulse energy for triggering the polarization-independent etching existed. Figure 4(b) depicts that when the N value increased to μ, rapid etching with an etching rate of approximately 350 μm/h was also achieved at μ and 1.8 μJ for each . However, as increased further, the polarization-independent etching disappeared, and the etching rate gradually decreased. Regarding the case, the etching rate gradually decreased when μ, while in the cases of and 90°, the etching rate began to decrease around μ. Similar results were obtained when the N value further increased to μ, as shown in Fig. 4(c). A polarization-independent etching with a similar etching rate could be obtained at μ (μ). Further increase of decreased the etching rate for each . Moreover, when was near or larger than 4.0 μJ, as indicated in Figs. 4(b) and 4(c), a polarization-dependent etching phenomenon appeared, which was similar to the result of μ in Fig. 2(a).
Figure 4.Relationship between the etching rate and under different and N values: (a) μ, (b) μ, and (c) μ. (d) Etching rate versus under different laser processing parameters (N and ).
To further investigate the polarization-independent etching behavior, four groups were selected based on different combinations of N and values: μ, μ; μ, μ; μ, μ; and μ, μ. These four groups of laser parameters were used to measure the etching rate of the laser-written tracks in fused silica at different values. The relationship between the etching rate and the value is plotted in Fig. 4(d). The effectiveness of polarization-independent fs-laser-assisted etching is again validated under these optimized conditions (N and ). An etching rate of μ could be achieved at any value (ranging from 0° to 90°) when the N value and were set to μ and 1.6 μJ, respectively. When the N value increased to μ and was 1.8 μJ, although the etching rate slightly decreased to μ at , polarization-independent etching was still maintained. A further decrease of to 1.6 μJ at the same N value (μ) resulted in the etching rate dropping to μ at and 70°. Similar results were observed when μ and μ.
To figure out the possible mechanism of polarization-independent etching, SEM observation of laser-modified regions (μ) under different and conditions was carried out. When was 1.3 μJ, as shown in Fig. 5(a), in the case of , the laser-induced structures did not show any connectivity along the writing direction, which led to the invalidation of the etching [see Fig. 4(a)]. In contrast, in the cases of and 45°, the disordered and interconnected nanocracks were observed in the modified regions, enabling an effective etching as shown in Fig. 4(a). Therefore, the micro/nano-morphological differences in the modified regions with different values exhibited a strong correlation with the corresponding results of chemical etching in Fig. 4(a). When increased to 2.1 μJ at the same N value, longitudinal periodic nanostructures perpendicular to the laser writing direction were observed for each value. Although the micro/nano-morphologies of laser-modified regions were quite different from those of μ, transverse and connected nanocracks along the direct writing direction were still observed. We speculate that the effective etching of laser-modified regions (μ) may still be mainly ascribed to the existence of the connective nanocracks. The underlying mechanism of polarization-independent fs-laser-assisted etching was attributed to laser-induced point defects with a low degree of anisotropy and densification under low-pulse-overlap conditions, as discussed in previous work [29,30]. Herein, it was further found that although the polarization-independent etching could be achieved under different low-pulse overlaps, the micro/nano-morphologies of the laser-modified regions were not the same. For instance, compared with the case of μ, the longitudinal periodic nanostructures perpendicular to the direct writing direction were observed at all polarization states in the case of μ at a pulse energy of 2.1 μJ. We speculate that the possible reason may be attributed to different generation mechanisms of micro/nanostructures in the laser-modified regions at varying N values. Initially, a single nanopore may be induced inside the fused silica under the irradiation of a single fs laser pulse with appropriate pulse energy. For μ, as the number of pulses increased, the multiple pores overlapped and elongated in the direction perpendicular to the laser writing direction due to compressive stress [31], resulting in the formation of longitudinal periodic nanostructures shown in Fig. 5(b). In contrast, for μ, as the number of pulses increased further, polarization-dependent nanograting structures appeared in the laser-modified regions. It should be noted that in both conditions, the presence of the connective nanocracks that spread along the laser writing direction enables polarization-independent fs-laser-assisted etching. A more detailed investigation of the morphological differences under varying N values needs to be conducted in the next step.
Figure 5.Top-view SEM images of fs-laser-modified regions (μ) in fused silica under different and : (a) 1.3 μJ and (b) 2.1 μJ.
Although polarization-independent fs-laser-assisted etching can be achieved at various N values described above, the condition of μ offers a broader range of parameters and more consistent etching rates. Therefore, to demonstrate the processing capability of polarization-independent fs-laser-assisted etching, two kinds of curved microchannels with centimeter-scale lengths were fabricated in the condition of μ, μ. Figure 6(a) shows the first kind of fabricated microchannel structure composed of a circular-shaped microchannel with a diameter of 1.5 mm and two straight microchannels with a length of 3.9 mm. The interval between the circular-shaped microchannel and the straight microchannels is μ. Figure 6(b) presents the second kind of fabricated microchannel structure, a four-concentric racetrack microchannel structure with different diameters (1, 1.05, 1.10, and 1.15 mm from the inside out). In the turning sections [see Fig. 6(b)], the spacings between the two adjacent racetrack channels are μ. The diameters of all fabricated microchannels (μ) are μ. In addition, the circular cross-sections of the microchannel enabled by SLM-assisted slit beam shaping can be confirmed in the inset of the middle panel (ii) in Fig. 6(b). Since the etching rate can reach 300–400 μm/h under this condition, the number of extra-access ports can be reduced by appropriately increasing the spacing of the extra-access ports to reduce the technical difficulty of subsequent port sealing. To demonstrate the importance of pulse-overlap control for polarization-independent etching, the etched results for μ under the same conditions are presented in the right panels of Figs. 6(a) and 6(b), which indicate their inhomogeneous etching behaviors compared to the cases of μ [see the middle panels in Figs. 6(a) and 6(b)]. Therefore, polarization-independent fs-laser-assisted etching combined with SLM-assisted slit beam shaping is straightforward and effective for producing curved microchannels with any orientation and circular cross-sections, which can be utilized for constructing optofluidic waveguides that bend in arbitrary directions within fused silica.
Figure 6.Centimeter-length curved microchannels fabricated by laser-assisted etching in fused silica. (a) A fabricated microchannel structure including a circular-shaped microchannel and two straight microchannels. (b) Four parallel and concentric racetrack microchannels. In (a) and (b), the left panels (i) show photographs of the channels (μ), respectively. The middle (ii) and right (iii) panels present optical micrographs of the channels within the dashed rectangles indicated in (i) and the fabricated channels (μ) for comparison, respectively. The pulse energy was 1.6 μJ. The inset in the middle panel (ii) in (b) shows the circular cross-section of the microchannel enabled by slit-assisted beam shaping.
Figure 7.(a) Photograph of a curved microchannel structure used for fabricating a bending optofluidic waveguide in glass. The microchannels indicated within the dashed yellow rectangles were used to fill the liquid-core solutions. (b) Insertion loss of the bending optofluidic waveguide versus mixing decane and liquid paraffin proportions. The inset in (b) illustrates the schematic of the experimental layout used to measure waveguide loss. (c) Mode field profiles of the optofluidic waveguides with different liquid-core solutions. The mixing proportions of decane and liquid paraffin () were 1:3, 1:4, 1:5, and 1:6 from left to right. The insets in (c) show the near-field mode images of the waveguide at the output port for each condition.
As summarized in Table 1, as the proportion of liquid paraffin in the liquid-core solution increases, the refractive index of the liquid-core solution () increases. In principle, there is a formula among the numerical aperture of the optical fiber (NA), the refractive index of the fiber core and cladding ( and ), and the beam waist radius (): πω
When the of the optofluidic waveguide and the working wavelength () are fixed, with the increase of , decreases. Since the mode field diameter of a single-mode fiber is equal to the waist diameter () of its output Gaussian beam, the output mode field diameter of the optofluidic waveguide decreases as increases. Therefore, single-mode bending optofluidic waveguides with tunable mode fields can be achieved in fused silica by leveraging the ease of operation of the refractive index of the liquid-core solution. Regarding the slight asymmetry of the output mode fields shown in Fig. 7(c) and Table 1, we speculated that the possible reason was mainly inscribed to the cross-section control of the fabricated microchannels, which could be further improved by optimizing the processing parameters of fs-laser-assisted etching and laser irradiation. In addition, a long microchannel, which comprises four semicircle parts with the same radii of 2 mm and five straight parts, was also fabricated (see Fig. 12 in Appendix A). Preliminary light-coupling testing of the formed optofluidic waveguide in the fabricated channel revealed that although the turning radius of the microchannel and the refractive index of the liquid-core solution were not optimized, the optofluidic waveguide with a multi-mode output could be formed.
Moreover, the proposed approach allows the fabrication of curved microchannels for forming and optofluidic waveguide beam splitters in fused silica. As shown in Fig. 8(a), the microchannels on the top and bottom were and splitting channel structures, respectively. The interval of the splitting channels was 200 μm. Figures 8(b) and 8(c) show close-up optical micrographs of splitting parts of channels on the top and bottom, respectively. When these two microchannels were filled with a mixture of decane and liquid paraffin in a 1:3 ratio, and optofluidic waveguide beam splitters were formed. The near-field mode images of the waveguides at the output ports [see bottom-left inserts of Figs. 8 (b) and 8(c)] reveal that the beam-splitting function of single-mode optofluidic waveguides has been constructed in glass. The results above confirmed that the proposed approach is promising for fabricating curved fused silica microchannels with circular cross-sections and arbitrary lengths, which holds great potential for developing new liquid-based photonic devices and circuits with small bending radii.
Figure 8.Fabrication of and optofluidic waveguide beam splitters in glass. (a) Photograph of the fabricated microchannels for the waveguide beam splitters in glass. (b) and (c) are the optical micrographs of the and splitting parts of the microchannels shown in (a), respectively. The black arrows in (b) and (c) indicate the typical locations of extra-access ports. The bottom-left insets in (b) and (c) are near-field mode images of the formed waveguide beam splitters at the output port. The top-right insets in (b) and (c) are the top-view optical micrographs of the microchannels filled with liquid-core solutions.
In summary, we develop a reliable approach for manufacturing long, homogeneous curved microchannels with circular cross-sections in fused silica using polarization-independent fs-laser-assisted etching based on a low-pulse-overlap scheme. Optimized processing windows for polarization-independent fs-laser-assisted etching have been investigated and established under varying pulse overlap conditions. The strong correlation between high-connectivity nanocracks along the laser writing direction under low-pulse-overlap conditions and polarization-independent etching has been confirmed. It should also be pointed out that further analysis is still needed to clearly explain the variation in the etching rate and the formation mechanism of various laser-generated nanostructures under the conditions of different , N values, and polarization states. Using the manufactured curved microchannels, single-mode bending optofluidic waveguides with tunable mode fields and optofluidic waveguide beam splitters have been produced by introducing appropriate liquid-core solutions into the channels. The developed approach will be highly significant for creating large-scale, low-loss microfluidic photonic circuits, optofluidic devices, and microsystems. Future research will also investigate the feasibility of polarization-independent fs-laser-assisted etching on various transparent substrates, including different types of glass and crystals, further expanding its application scenarios.
APPENDIX A: SUPPLEMENTARY INFORMATION
Figures 9–12 are figures supplementary to the main text. More details are shown in their figure captions.
Figure 9.Flowchart of the fabrication process for optofluidic waveguides in fused silica. The fabrication procedure consists of four steps. First, the fs laser beam spatially shaped by the SLM is utilized for 3D direct writing in fused silica to inscribe the programmed micropatterns of curved microchannels. Second, hollow microchannels and extra-access ports are formed by selective chemical etching of the laser-processed fused silica samples in 10 mol/L KOH solution at 90°C. Third, the ports are sealed to create closed and smooth channels by defocusing laser irradiation. Finally, optofluidic waveguides are formed within the channels using vacuum-assisted liquid-core filling.
Figure 10.Schematic of the defocusing laser irradiation for port sealing. The average power and defocusing distance of the laser beam were approximately 30.7 W and 21.9 cm, respectively.
Figure 11.Schematic of SLM-enabled dynamic slit-assisted laser writing for creating micropatterns of curved microchannels. During laser processing, the slit orientation remained parallel to the laser writing direction. The black and yellow arrows indicate the laser writing direction and the orientation of the SLM-enabled slit at specific positions, respectively.
Figure 12.(a) Photograph of a long microchannel with a turning radius of 2 mm for forming a multi-mode bending optofluidic waveguide. The inset shows a near-field mode image of the optofluidic waveguide at the output port. (b), (c) Microscope images of the corresponding sections of the optofluidic waveguide constructed from the microchannels shown in (a). The black arrows in (b) and (c) indicate the specific extra-access ports of the microchannels. The microchannel consists of four semicircular channels with radii of 2 mm each and five straight channels. The optofluidic waveguide was formed within the channel by filling it with a mixture of decane and liquid paraffin in a 1:3 ratio.