Description of the Device. A schematic of the device is shown in Fig. 1. It is composed by a microfluidic channel that contains a microdisk, supported by a cylindrical pedestal of lower radius, and a suspended waveguide, which is anchored to the lateral walls of the microfluidic channel. The suspended waveguide is tangent to the microdisk, and can thereby excite WGMs of the resonator through evanescent coupling. Two fs-laser written waveguides, denominated lead-in and lead-out waveguide, couple light into and out of the suspended waveguide, respectively. Fluids injected inside the microfluidic channel can directly interact with the optical resonator and with the suspended waveguide. The process flow for the fabrication of the monolithic sensor consists of three main steps, which are described in the Materials and Methods section: (1) fs-laser irradiation followed by chemical etching of the irradiated areas to create the microfluidic channel (already including the microresonator and the suspended waveguide), (2) thermal annealing of the substrate, and (3) fs-laser direct writing of the lead-in and lead-out waveguides.
The inclusion of thermal annealing to the fabrication process solves some of the limiting issues related to the fabrication and performance of the device. First, the surface roughness is inversely proportional to the intrinsic Q-factor of the microresonator26, and negatively affects the propagation losses of the suspended waveguide due to optical scattering. This parameter should then be minimized, which can be achieved through thermal treatment, either by thermal annealing27 or CO2 laser polishing28. Second, and more crucial, it allows to circumvent the limitations imposed by the chemical etching, and which are characteristic of fs-laser micromachining. In particular, the etching selectivity created by fs-laser writing is limited20 and, consequently, a hollow gap between the initially tangent microstructures is always formed, which determines that the device is in an under-coupling regime. In our system, a minimum gap of 2.5 µm is always observed, which prevents the efficient excitation of WGMs. To overcome this issue, a morphing technique, which is induced by heating the substrate to a temperature above its annealing point, was employed. Although the morphing effect has also been used to transform the shape of microstructures28, here, it is utilized as a mechanism to place the suspended waveguide within the correct distance to achieve the critical coupling condition.
Surface Roughness. Despite obtaining a direct replica of the designed device, the chemical etching leads to an undesired surface profile. Figure 2a reveals that both the top and sidewall surfaces of the microdisk are rugged, albeit with different topographies. The top surface exhibits a periodical modulation along the direction transverse to the scanning orientation of the fs-laser beam, where the crests are spaced by 3 µm which matches the horizontal gap between adjacent laser scans. Meanwhile, the sidewall pattern is more random and smoother, which is associated to a lower surface roughness. This difference is related to the dimensions of a singular modification track and the user-defined spacing between each. During laser direct writing, the laser-irradiated tracks merge along the vertical direction which results in a more uniform etching, whereas they are separated by a thin pristine layer along the horizontal direction which creates a rougher profile after chemical etching. Although the effect observed in the top (and bottom surfaces) could be theoretically minimized by reducing the separation between horizontally-adjacent laser scans to below the defined 3 µm, in practice it was observed that this resulted in fractures in the material.
Therefore, in order to decrease the surface roughness, the substrate is subjected to a thermal treatment process. Figure 2b shows that the root-mean squared (RMS) surface roughness decreases asymptotically with the annealing temperature, which is also confirmed from the inset images of Fig. 2b and from Fig. 2e-f where it is shown that the initially rugged surface becomes smoother. The smoothing is due to material redistribution in an energetically favorable configuration as a result of the surface tension overcoming thermally decreasing viscosity forces29. Further, the material’s viscosity decreases as the temperature increases, which enhances the reflow effect and minimizes the surface roughness, thereby explaining the behavior against annealing temperature depicted in Fig. 2b. In particular, after thermal annealing at 1250˚C, the RMS roughness decreases from 87 nm to 11 nm, while the surface remains uniform which also indicates a homogeneous refractive index distribution. Moreover, for lengths up to 2000 µm, the suspended waveguide is straight and stable after thermal annealing, as demonstrated in Fig. 2f. For lengths greater than 2000 µm, the suspended waveguide started to bend downwards, as also highlighted by Chen et al30.
Morphing. During thermal treatment, besides the smoothing, the surface tension also causes the microstructures to morph into a shape that minimizes the surface area28. Following from the conclusions drawn by Drs et al28, the initially square-shaped suspended waveguide is expected to morph into a circular one. In fact, by measuring the dimensions of the waveguide before and after thermal annealing, it was verified that the cross-sectional area stayed constant during thermal treatment which further validates this hypothesis.
Likewise, the shape of the microresonator is also modified. From Figs. 2c-d, the straight sidewalls become round-beveled with a straight segment in-between. This is contrary to what has been reported so far31, where the microdisk would shrink and collapse into a microtoroid after CO2 laser polishing. The reason why those effects are not observed here has to do with the dimensions of the microresonator. Regardless of the microdisk’s diameter, by establishing the ratio between the radii of the pedestal to the microdisk to be 90%, the microdisk is impeded from slithering during thermal annealing and, consequently, shrinking is avoided. At the same time, this ratio is enough to stop the pedestal from deforming the WGM mode. Similarly, the height of the microdisk also influences the morphing effect. The microdisk collapses into a microtoroid akin to the work developed by Song et al31 for heights lower than 20 µm, whereas, by increasing the height, only the bevel of its top and bottom surfaces is observed as shown in Fig. 2c.
Critical Coupling Condition. Several requirements, regarding the dimensions and alignment between the suspended waveguide and the microdisk, must be met to successfully excite WGMs in the resonator32,33. From numerical simulations32, it is concluded that the diameter of the suspended waveguide should be of a few micrometers to enhance the evanescent field and to promote optical coupling. Although these dimensions can be easily obtained with a tapered fiber fabricated through the heat-and-pull method, they are more difficult to be obtained by laser machining processes. For instance, Cheng et al30 could not fabricate suspended waveguides in Foturan glass with diameters lower than 20 µm, whereas Ceccarelli et al19 had to first ablate the pristine material surrounding a 20×90 µm2 Corning EAGLE X slab and then fabricate the waveguide through fs-laser writing. Here, those limitations are circumvented, and much smaller suspended waveguides, with diameters around 2–3 µm, were fabricated. Although the fabrication technique does not impose any limit to the diameter of the suspended waveguide, it was observed that smaller waveguides would break more easily during the etching process. Further, light is coupled into and out of the suspended waveguide through a lead-in and lead-out waveguide, respectively, which are fabricated following the procedure outlined in the Materials and Methods section. Given that the waveguides are geometrically aligned, the coupling losses are mainly due to mode mismatch between the lead-in (or lead-out) waveguide and the suspended waveguide. The suspended waveguide should be as small as possible to enhance the evanescent field that interacts with the microdisk. This, however, leads to an increase in the coupling losses between the lead-in (or lead-out) waveguide and the suspended waveguide. In this work, the fabrication of smaller suspended waveguides was favored at the expense of higher coupling losses. Still, it should be noted that the mode profile of the suspended waveguide is inherently related to the refractive index of the surrounding fluid, which determines the coupling losses to vary whenever the external refractive index changes.
The second criterion is related to the coupling between the suspended waveguide and the microdisk. The alignment along the vertical direction is easier to accomplish, and consists of designing the suspended waveguide centered to the microdisk. Given that chemical etching produces an almost 1:1 replica of the design and that thermal annealing barely changes the dimensions and position of both structures, this condition prevails throughout all fabrication steps.
The biggest challenge is aligning both structures along the horizontal plane, where ideally the suspended waveguide should be tangent to the microresonator32. As previously mentioned, the finite etching selectivity prevents this from occurring, which leads to the appearance of a micrometer-long gap between both structures that inhibits the excitation of WGMs. By adapting the geometry of the suspended waveguide and by resorting to the morphing effect, this issue can be solved as demonstrated in Fig. 3. As illustrated in Fig. 3a, the design of the suspended waveguide now consists of two symmetric straight sections, connected by an arc segment that surrounds part of the microresonator and whose curvature radius matches the microdisk’s radius. The start and end points of the arc coincide with the intersection points of the straight segments with the microresonator; although both structures distance themselves from one another during etching, their shape is preserved. During thermal annealing, the suspended waveguide straightens and gets closer to the microdisk, as seen in Fig. 3b-d. Morphing can then be used to place the suspended waveguide tangent to the resonator, under the critical coupling regime, whereas the chemical etching allows us to control the relative position of both structures with micrometric precision. Further, if the straight and arch segments are designed to have the same dimensions then, after thermal annealing, the suspended waveguide is uniform throughout its entire length, as shown in Fig. 3c. However, if in the design of the device, the dimensions of the arched section are lower than those of the straighter segments, the suspended waveguide becomes tapered after thermal annealing, as demonstrated in Figs. 3b and 3d. This way, it is possible to further narrow the suspended waveguide and, in turn, enhance the evanescent field without compromising the mechanical stability of the suspended waveguide.
Three different situations, shown in Fig. 4a-b, illustrate the control that can be obtained depending on when the etching reaction is stopped. In particular, if it is stopped when the straight segment of the suspended waveguide is tangent to the microdisk then, after thermal annealing, the arched section will straighten and be tangent to the microresonator, as shown in Fig. 3b-d and in Fig. 4b. This is different from what was reported by Song et al31, where a tapered fiber would deform after being welded to the microtoroid, thereby placing the device in the overcoupling regime. If the reaction is stopped later both structures become distanced, whereas if it is stopped earlier the suspended waveguide merges with the microdisk and becomes deformed, as shown in Figs. 4a and 4c, respectively.
Figures 3b and 4a-c also reveal that the bottom surface of the microfluidic channel has a corrugated profile and that the microdisk’s pedestal presents some defects. These profiles are a consequence of the finite etching selectivity, and aggravated by the fact that the duration of the etching reaction is determined by the final position of the suspended waveguide relative to the microdisk. As a result, the layers beneath the microdisk are etched for a shorter time period, in comparison to the rest of the material, which translates into a poorer surface quality. Still, these irregularities do not introduce optical losses to the device, and can easily be solved by using a closely spaced hatching scans in the layers defining the bottom of the microfluidic channel.
In the situation displayed in Fig. 4a, the suspended waveguide is 2.2 µm away from the microdisk border. Accordingly, no WGMs are excited as can be confirmed from the transmitted spectrum of Fig. 5a-b in air and deionized water, respectively. Instead, a periodic modulation is observed, which is caused by Mach-Zehnder interference (MZI) between the fundamental mode of the suspended waveguide and uncoupled light that propagates across the microfluidic channel34. Kelemen et al24 reported this same issue, but were able to avoid it by writing a bended suspended waveguide with the input and output transversely distanced.
The scenario displayed in Fig. 4b enables the excitation of WGMs as evidenced from the transmitted spectra shown in Fig. 5c-d, where a suspended waveguide with waist diameter of 2.4 µm is put in contact with a microdisk with radius and height of 72 µm by 50 µm, respectively. Multiple modes of the resonator are excited, due to the measurements being made with linearly polarized light at an unknown angle that causes both TE (transverse electric) and TM (transverse magnetic) modes to be excited, and due to the large dimensions of the microdisk which supports multimode propagation35. Still, a dominant resonance periodically spaced by 3.6 nm is seen in both graphs. The measured free spectral range is in good agreement with the expected value, whereas the apparent independence from the external media is associated to a weak variation of the effective index of the excited WGM with the surrounding refractive index. Also, in spite of the WGM spectrum overlapping with the MZI spectrum defined earlier, the period of the MZI modulation is an order of magnitude higher than the free spectral range of the WGM spectrum, for all surrounding fluids tested. Ergo, the presence of the overlaying modulation does not interfere with the optical characterization of the whispering-gallery resonator.
The condition illustrated in Fig. 4c places the device in the over-coupling regime, which is accompanied by an increase in the insertion losses. Song et al31 and Kelemen et al24 also observed this effect, and added that the Q-factor decreases in this regime.
Optical Characterization. To demonstrate its applicability as a refractive index sensor, the response of the device (shown in Fig. 3d and in the inset of Fig. 5c) against the surrounding media was characterized. Different Cargille fluids (series AA), with refractive index spanning from 1.296 to 1.363 at 1550 nm, were successively inserted in the microfluidic channel and the transmission spectrum was measured. Inbetween measurements, the device was thoroughly cleaned. To simplify the analysis, the measurements were made with the input beam linearly polarized to excite only the TM modes of the microresonator.
Overall, the resonances broaden and weaken as the refractive index increases, which is accompanied by a reduction in the number of excited modes. These results are summarized in Figs. 6a and 6b, where it was tracked the behavior of the resonance located at 1550 nm. As the external refractive index increases, the resonance shifts non-linearly to higher wavelengths which indicates that the effective refractive index of the WGM, under analysis, is increasing. The sensitivity to refractive index variations of the surrounding medium, displayed in Fig. 6c, is obtained by plotting the first derivative of the curve shown in Fig. 6b. A maximum sensitivity of 121.5 nm/RIU is obtained at an index of 1.363, with a detection limit36 of 7.0×10− 4. For refractive indices near that of aqueous solutions, the sensitivity is 40 nm/RIU. For comparison, Song et al31 and Kelemen et al24 obtained a linear sensitivity of 61 ± 1 nm/RIU between 1.3344 and 1.3840, and of 220 nm/RIU around the refractive index of water, with a 25 µm radius polymeric microring and with a 40 µm fused silica microtoroid, respectively.
After fitting the resonance with a Lorentzian function, the peak width (full-width at half-maximum) and the Q-factor were computed36. Figure 6c shows that the Q-factor decreases from 5.33×105 to 0.28×105 as the refractive index increases from 1.296 to 1.363, which is attributed to weaker light confinement inside the microresonator37. Still, the measured Q-factor is on par with what was reported by Song et al31, and is two orders of magnitude higher than what was reported by Kelemen et al24.
Discussion. In this work, it was constructed a label-free optofluidic sensor entirely made of fused silica, which integrates a whispering-gallery mode resonator, being excited by the evanescent field of a suspended waveguide, inside a microfluidic channel. The device is fabricated by fs-laser micromachining followed by chemical etching and thermal annealing. The introduction of this procedure decreases the surface roughness of both microstructures to tens of nanometers, thereby improving the quality factor. Further, it provides a way to accurately position the suspended waveguide tangent to the microresonator and, in turn, to operate the device in the critically coupled regime. The versatility provided by fs-laser micromachining also enables us to accurately control the dimension and geometry of both microstructures, in order to attain optimal phase-matching. The morphing tool described here solves numerous practical issues regarding handling and applicability of the device. The fabrication and experimental results have proven to be repeatable, while the monolithic construction guarantees that the suspended waveguide and resonator are robust and aligned at all times. The device shows a sensitivity of 40 nm/RIU and a quality factor of 2×105 for refractive indices near that of aqueous solutions, akin to what has been reported for similar devices and enough for most biosensing applications. Fluid handling capabilities can still be incorporated within the microfluidic channel17,23, whereas Kelemen et al24 demonstrated these optical systems can also be used in biosensing applications.
Still, improvements to the current device can be made. A higher control of the distance between the suspended waveguide and the microresonator can be achieved by exploiting etchants with higher etching selectivity. Different geometries can also be explored to fully remove the influence of the Mach-Zehnder interference on the whispering-gallery mode spectrum. In particular, a possible solution is to fabricate two suspended waveguides, placed tangent and on opposite sides of the resonator. In this geometry, light propagating forward across one of the waveguides excites WGMs of the microresonator, which are themselves coupled to the other waveguide where light propagates backwards and whose spectrum is measured. Meanwhile, the microfluidic channel can also be fabricated beneath the surface of the fused silica substrate or sealed with a PDMS layer where, by opening two holes to the cover layer, fluids can be drawn into and out of the channel.