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(a) Schematic cross-section and image of a variable-focus liquid microlens actuated by thermoresponsive hydrogel. The inner side walls of the aperture are treated hydrophilic by oxygen plasma. The oil–water interface is pinned at the hydrophobic–hydrophilic boundary. Responding to temperature variation, NIPAAm hydrogel changes the net volume of water as well as the focal length. The lens aperture is 2 mm in diameter. (b) Schematic of the principles of calculating the surface profile from the wave front profile. On the outgoing wave front, the phase difference between the vertex and point is determined by , the difference in the thickness of oil.
(a) Optical setup using the Shack–Hartmann wave front sensor. The wave front sensor is operated in the transmission mode and the liquid microlens is illuminated by a collimated light. Relay lenses are used to conjugate liquid lens plane with wave front sensor plane and to enlarge the wave front by a factor of 2. (b) Picture of the experiment setup. It consists of a laser source, a beam expander, mirrors, a sample lens on a stage, relay lenses, and a wave front sensor.
(a) Measured 3D surface profile of the water–oil interface at . (b) Scatter plot of surface height vs at . is the distance between point and the vertex, as defined in Fig. 1(b). When is relatively small, i.e., within the area near the center, the surface is approximately spherical in shape; near the boundary, the shape of the surface becomes much more linear.
Coefficients of conical surface fit. The surface profiles are fitted to Eq. (3). StDev stands for standard deviation. The last column is the contact angle at the lens aperture based on conical surface interpolation.
Comparison of measured optical properties vs Zemax simulation results. Spherical aberration is represented by , the 11th Zernike standard coefficient. rms error describes the overall aberrations of a lens.
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