1. Introduction

In nature, arthropods, such as dragonfly, fly, bee and mosquito, benefit from compound eyes to observe surroundings with a large angle of view (AOV) and escape from dangers in good time. A compound eye consists of tens to thousands of ommatidia which are densely and omnidirectionally distributed in a convex shape. Each ommatidium contains a cornea and a crystalline cone which functions as a microlens to collect light from a certain orientation. The light is then fed into a photoreceptor cell via a bunch of rhabdomeres. Since compound eyes enable arthorpods to capture an image of a fast moving target with a wide AOV, high sensitivity and low aberration, lots of research work was inspired by the compound eyes [1–5]. Artificial compound eyes are applied in the endoscopy, robotic vision, panoramic imaging, and surveillance applications [6–11].

Various techniques have been proposed to fabricate artificial compound eyes [12–14]. Laser writing is a powerful technique to fabricate complex microstructures by three-dimensionally point-by-point carving building materials such as resin or protein [15,16]. However, the long fabrication time is a bottleneck and limits the fabrication of large (millimeter- or even centimeter-scale) components and the productivity [17,18]. Recently, high-speed voxel-modulation laser scanning method was demonstrated to fabricate the compound eyes within a reasonable time by rapid scanning inner spherical base with large voxels and precise scanning outer ommatidia with small voxels [19]. A laser lithography system capable of structuring curved surfaces was used to fabricate a microlens array on a concave surface and a pinhole array on a convex surface to form a compound eye which could project an image onto a planar image sensor [20]. In addition, laser writing could also be employed to enhance the chemical wet etching in the fabrication of compound eyes [21]. Besides direct fabrication using laser writing, compound eyes can be realized by thermally pressing a planar microlens array on a spherical ball [22–24]. It is to some extent reduce the fabrication complexity and it benefits from lots of sophisticated and advanced techniques to fabricate planar controllable-focal-length and high-numerical-aperture microlens arrays [25–27]. Moreover, to produce compound eyes repeatedly, a mold with an array of concave patterns in a concave substrate can be fabricated in advance [28–30].

Changing the profile of compound eyes can realize adjustable AOV. Due to the complexity in the structure and the operation, few research work has realized the tuning of compound eyes. Light manipulation from convergence to divergence has been demonstrated in a lens with six ommatidia on a curved surface [31]. Wei et al. have demonstrated compound eyes for laparoscopic imaging in which both nine ommatidia and substrate can be individually controlled and the focal length tuning within the millimeter range and the AOV up to 120° have been achieved [32–34]. An electronic eye camera system with an array of photodetectors on a tunable curved surface could efficiently realize zoom variation [35]. In addition, the lenses on the compound eye could be flexibly and individually tuned based on electrowetting actuation by applying voltage on the transparent graphene electrodes in the substrate [36–38] or photothermal actuation by illuminating near-infrared pulsed light on the lenses made of graphene nanosheets [39].

In this paper, an optofluidic compound eye with tunable angle of view is demonstrated. The technique can fabricate micro-structured high-numerical-aperture compound eyes in a large size rapidly and at a low cost. The deformation and the tuning of angle of view of the compound eye are analyzed and the corresponding variation in profile and focal length of the microlenses is investigated. Besides, the optical imaging is evaluated with the tuning of the compound eye.

2. Fabrication of optofluidic compound eyes

The fabrication process of the optofluidic compound eyes is illustrated in Fig. 1. A mold for producing convex microlens array is firstly fabricated [40]. An array of photoresist micro-pillars is prepared using the photolithography technique. During the photolithography, a titanium layer is coated onto the silicon wafer and then oxidized into titanium dioxide for improving the adhesive strength between the micro-pillars and the substrate. The micro-pillars have the diameter of 50 µm and the height of 60 µm. Then, liquid polydimethylsiloxane (PDMS) with a weight ratio of 12:1 (silicone elastomer versus curing agent) is poured onto the silicon wafer and solidified at 80 °C for 4 hours. The PDMS is peeled off from the micro-pillar array and an array of micro-holes is formed on the PDMS. The PDMS is bonded to a sheet of glass after the surface modification in the air plasma.

 

Fig. 1 Fabrication process of the optofluidic compound eyes.

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After that, the micro-holes are exposed in the plasma with the argon and oxygen mixture (gas mixture ratio is 100:1) for modifying the hydrophobicity of the PDMS. The surface modification is under the condition of 13 MHz radio frequency, 600 mTorr pressure, 11 W plasma power and 60 mL/min gas flow rate. Next, acrylate resin (viscosity: 500 mPa∙s, density: 1.02 g/cm³, refractive index: 1.487) is spun over the micro-holes at the spin coating rate of 3000 rpm for 2 minutes. After the spin coating, the sample is placed in the vacuum machine to ensure that the acrylate resin fills in the micro-holes completely and exposed in 365 nm ultraviolet (UV) light for 8 minutes for resin solidification. The resin in the micro-holes presents a concave surface. The curvature of the resin surface is closely related to the surface modification before the spin coating and directly determines the curvature of the produced microlenses on the compound eye. A long surface modification time leads to a large curvature. To realize large curvature, short focal length and high numerical aperture of the microlenses, the time for surface modification is set at 200 seconds. The solidified acrylate resin has high mechanical strength (elongation at break: 40%) and thermal strength (thermal expansion: 3 × 10⁻⁶ /°C and operating temperature: –50 °C to 150 °C). Thus, the concave resin array can be used as a mold to replicate microlens array.

To enlarge the size of the mold, the concave resin array is placed in a container and liquid PDMS whose weight ratio of silicone elastomer to curing agent is 12:1 is gradually added into the container. The level of the liquid PDMS should be no more than the height of the concave resin array. The PDMS is solidified after 4-hour heating at 65 °C. The mold containing concave resin array is produced and can be repeatedly used in the fabrication of optofluidic compound eyes. The mold is exposed in the air plasma to modify the surface of the PDMS with a mask covering the concave resin array. After that, liquid PDMS with the weight ratio of 8:1 is poured onto the mold for replication. Once the PDMS is solidified, the thin PDMS sheet on which there is an array of convex microlenses can be easily peeled off from the mold.

In addition, another mold is prepared using 3D printing technique. The mold has the structure of a chamber connecting to an inlet and an outlet via two channels. The chamber has the size of 23 mm in diameter and the channels have the width of 1 mm. The height of the mold is 1 mm. Liquid PDMS with the mixture ratio of 5:1 is used to replicate the structure of the mold. After casting the two molds, the PDMS sheet of the convex microlens array and the PDMS sheet containing the chamber and channel structure are processed in the air plasma for 40 seconds. Then, the two sheets are tightly bonded together with the microlens array facing downwards. Finally, the bonded chip is flipped over and two tubes are connected to the inlet and the outlet.

When using the optofluidic chip, to match the refractive index of the PDMS (refractive index: 1.403), the mixture solution of deionized water (DI) and calcium chloride (CaCl₂) is used in the optofluidic chip. After filling the chamber with the DI/CaCl₂ solution, a clamper is used to block the outflow of the solution and the surface of the microlens array could bulge into a spherical cap when injecting more solution.

The optofluidic compound eye is shown in Fig. 2(a). The chip has the size of 70 mm × 50 mm × 10 mm (length × width × thickness). The entire microlens array has the size of 23 mm in diameter and each microlens has the size of 50 μm. The separation of the microlenses is 15 μm. There are about 78,000 microlenses on the chip. To show the micro structure of the compound eye, 880 μL photoresist is injected into the chip to deform the membrane of the microlens array and cured under the UV light. Figures 2(b) and 2(c) show the structure of the compound eye. The microlenses present large curvature and are uniformly and densely distributed on a curved surface.

 

Fig. 2 (a) Photo of a fabricated optofluidic compound eye. (b) and (c) Top-view and magnified SEM images of the compound eye after injecting and curing 880 μL photoresist.

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3. Tuning of optofluidic compound eyes

3.1 Adjustment of angle of view

To demonstrate the deformation of the compound eye, blue ink is injected into the chip. The membrane of the microlens array bulges with the injection of the ink, as shown in Figs. 3(a)-3(c). Thanks to the good elasticity of the PDMS, the membrane can be easily deformed. The curvature of the membrane is in the shape of a spherical cap. The height of the peak point on the membrane, H, is measured to calculate the curvature radius, i.e. $R=H/2+D^2/8H$, where D is the diameter of the chamber. The profile of the compound eye is characterized from the high-resolution images by macro photography. The height of the compound eye can be found out by calculating the ratio between the height and the diameter of the chamber and the dimension of the diameter can be precisely measured. The height of the membrane obviously increases with the liquid volume in the chamber, resulting in a decrease of the curvature radius, as depicted in Fig. 3(d). The error bar stands for the deviation in the testing for ten times. The microlens array becomes curved and the microlenses are oriented to different directions. Thus, the AOV is enlarged accordingly, which can be calculated as

 

Fig. 3 (a)-(c) Side-view images of the optofluidic compound eye after injecting 442 μL, 620 μL and 1008 μL blue ink, respectively. (b) The deformation of the membrane and the angle of view with the injection volume.

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In the demonstration, the AOV can be adjusted from 0° to 104°. The AOV can be further increased by decreasing the sheet thickness of the microlens array, enhancing the elasticity, and improving the bonding strength. Ultimately, the mechanical strength (elongation at break) of the PDMS and the bonding strength of the chips are two limiting factors for the AOV tuning. A high mixture ratio of silicone elastomer and curing agent is preferred to realize high elasticity and high mechanical strength [41]. The bonding strength can be further enhanced by other surface modification methods before chip bonding [42].

3.2 Influence over microlenses

During the tuning of the compound eyes, the PDMS membrane is under strain, leading to the deformation of the microlenses. The profiles of the microlenses are measured by the white light interferometry. Figure 4(a) shows the change in the profile of the microlenses. With the injection of the DI/CaCl₂ solution, the microlenses expand in diameter and the height is reduced due to the transversal expansion of the PDMS membrane and the corresponding axial compression based on Poisson effect. The microlenses in the central area experience more strain comparing to those around the edge because the non-uniform strain field presents higher in the center of the membrane [32]. Accordingly, the focal length varies with the bulge of the membrane, as depicted in Fig. 4(b). The focal length is calculated as $f=\left(d^2+4h^2\right)/\left(8h\left(n-1\right)\right)$, where d is the diameter of the microlens, h is the height of the microlens and n is the refractive index of the PDMS. The microlenses on the planar surface have the focal length of 62 ± 0.3 μm and the numerical aperture is about 0.403. When the solution is injected into the chip, the focal length increases from 62 μm to 71 μm. In comparison with the peripheral microlenses, the microlenses in the central area has slightly longer focal length due to the more deformation in shape. The numerical apertures are 0.403 for central microlenses and 0.402 for peripheral microlenses, which are almost unchanged due to the expansion in pupil size of the microlenses with the increase of the focal length.

 

Fig. 4 (a) The influence of the injection volume over the deformation of the microlenses. (b) The variation of the focal length of the central lenses and peripheral lenses. Solid line: the microlenses in the central area. Dashed line: the microlenses in the peripherical area.

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4. Optical imaging

The optical imaging performance of the optofluidic compound eye is verified in the experiment. An optical imaging system is set up as shown in Fig. 5(a). A white-light emitting diode (LED) is used as a light source to illuminate a transparent mask on which there is an opaque letter ‘A’. The optofluidic compound eye is placed in between the mask and the objective lens. A CCD camera is installed after the objective lens to capture the image. When the microlens array on the optofluidic chip is in the planar state, the light is focused on a planar focal plane. After the injection of the DI/CaCl₂ solution into the optofluidic chip, the compound eye bulge to a spherical cap. The images are focused on a curved focal plane behind the compound eye. With the increase of the injection, the curvature of the focal plane becomes large due to the omnidirectional pattern of the microlenses. Since the curved focal plane of the compound eye cannot fit the planar focal plane well, some images are in focus while the others are out of focus. In the experiment, during the adjustment of the optofluidic compound eye, the objective lens is fixed.

 

Fig. 5 (a) Experimental setup for optical imaging. The schematic diagrams of the optical imaging when the optofluidic compound eye has (b) a planar surface and (c) a curved surface

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Figure 6 shows the images obtained by the compound eye when DI/CaCl₂ solution is injected into the optofluidic chip. Every microlens can form a sharp image. If the injection volume is 880 μL, the membrane of the microlens array bulges to 3.92 mm, corresponding to the curvature radius of 19.5 mm. The compound eye has the angle of view of 74°. The focal plane of the object lens is in coincidence with the focus of the microlenses around the central apex. The clear images can be observed from the central microlenses and the surroundings are blurry. When the injection volume is increased to 1150 μL, the membrane raises up to 5 mm. The angle of view changes to 92°. The images obtained from the microlenses within the central apex are out of the focus of the objective lens. Thus, the images in the central part become blurry. The images of the letter cluster ‘A’ formed by the microlenses in the annulus become visible.

 

Fig. 6 The imaging performance of the compound eye when the angles of view are (a) 74° and (b) 92°, respectively.

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5. Conclusion

A fabrication technique for producing tunable optofluidic compound eye is demonstrated. The technique can rapidly, straightforwardly fabricate an optofluidic compound eye in a large size at a low cost. The angle of view can be adjusted with the deformation of the compound eye by injecting DI/CaCl₂ solution into the chip. In the demonstration, the angle of view can be tuned up to 104°. With the deformation of the compound eye, the profile of the microlenses changes and the focal length varies accordingly. Due to the non-uniform strain over the membrane, the microlenses within the central area experience more variation comparing to the microlenses around the edges. Furthermore, the compound eye presents a good optical imaging quality. The proposed compound eyes have the potential to be employed in the optical systems requiring a large field of view with variable viewing angles.