Strain responsive concave and convex microlens arrays

We report the fabrication of single-component, strain responsive microlens arrays with real-time tunability. The concave lens array is fabricated by patterning hard oxide layer on a bidirectionally prestretched soft elastomer, polydimethylsiloxane PDMS followed by confined buckling upon release of the prestrain. The convex microlens array is replica molded from the concave lenses in PDMS. Due to difference in lens formation mechanisms, the two types of lenses show different tunable range of focal length in response to the applied strain: large focal length change is observed from the concave microlens array, whereas that from the convex microlens array is much smaller. Comments Copyright 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. Reprinted in Applied Physics Letters, Volume 91, Article No. 251912, December 2007, 3 pages. Publisher URL: http://dx.doi.org/10.1063/1.2827185 This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/140 Strain responsive concave and convex microlens arrays Dinesh Chandra and Shu Yang Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, USA Pei-Chun Lin Department of Mechanical Engineering, National Taiwan University No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan Received 15 August 2007; accepted 1 December 2007; published online 20 December 2007 We report the fabrication of single-component, strain responsive microlens arrays with real-time tunability. The concave lens array is fabricated by patterning hard oxide layer on a bidirectionally prestretched soft elastomer, poly dimethylsiloxane PDMS followed by confined buckling upon release of the prestrain. The convex microlens array is replica molded from the concave lenses in PDMS. Due to difference in lens formation mechanisms, the two types of lenses show different tunable range of focal length in response to the applied strain: large focal length change is observed from the concave microlens array, whereas that from the convex microlens array is much smaller. © 2007 American Institute of Physics. DOI: 10.1063/1.2827185 With advances in miniaturization, microlens arrays play an important role in optical communication, biomedical imaging, photolithography, and biochemical sensing. Variable-focus microlens arrays are of particular interest for microelectromechanical systems MEMS and sensors. A wide variety of tuning mechanisms have been reported, including responsive hydrogels, electrowetting, liquid pressure to deform an elastomeric membrane, liquid crystal microlens arrays, and integrated microfluidic channels, to tune lens shape, refractive index, and the surrounding medium. Nevertheless, most of these microlens arrays are multicomponent systems, and require complex fabrication and assembly processes. Often times, the lens focal length cannot be tuned continuously in real time. In this paper, we report the fabrication of a singlecomponent, strain responsive, microlens array both concave and convex with real-time tunable focus. The concave lens array is created by confined buckling of a soft elastomer, poly dimethylsiloxane PDMS , which is mechanically stretched in plane bidirectionally and patterned with a thin layer of hard oxide on top. Due to extreme moduli mismatch between the hard silicate layer and the soft PDMS E =2 MPa , buckling occurs upon release of prestrained bilayer film, forming wrinkled patterns spontaneously. If the oxidation and, thus, buckling is confined to an area smaller than the wavelength of the unconfined wrinkles, microlens structure will be obtained. Previously, similar strategy has been used to create microlens array by swelling a patterned oxide/PDMS bilayer structure with acrylate monomers, followed by polymerization. Such formed lenses are rigid and not tunable. Because the microlens array in our system is created by mechanical stretching induced buckling, the lens shape can be reversibly tuned in real time by simply applying mechanical strain. Briefly, the microlens array was fabricated as the following. A flat PDMS sheet with thickness of 0.5 mm was prepared by mixing PDMS precursor RTV 615, GE Silicone and curing agent 10:1 wt /wt between two glass slides separated by spacers, followed by thermal curing at 65 °C for 4 h. The PDMS strip was clamped on four edges Fig. 1 a , leaving a center space of 25 25 mm2 and then stretched to 20% strain in both planar directions simultaneously Fig. 1 b . One side of the stretched PDMS surface was masked with a transmission electron microscopy TEM copper grid Fig. 1 c with hexagonally packed hole array diameter of 37 m and hole to hole distance of 62 m for ultraviolet ozone UVO treatment UVO-Cleaner Model 42, Jelight Company, Inc. for 30 min Fig. 1 d to generate a thin silicate layer on the exposed regions. The area surrounding the TEM grid and the backside of PDMS film were covered by scotch tape. The mask was then removed after UVO Fig. 1 e and the PDMS strip was strain released in both planar directions simultaneously, resulting in a concave microlens array Fig. 1 f . In the range of prestrain levels 10%–30% and UVO treatment time 15–60 min , concave lenses were always observed. One possible explanation could be that during the strain release process, the pure PDMS surrounding the much stiffer oxidized PDMS is pushed out due to compressive forces, which favors buckling of the oxidized PDMS inwards rather than outwards. Once the oxidized layer is slightly buckled inwards, it continues to buckle in the same direction, resulting in formation of concave microlens array. To obtain the convex microlens array, we replica molded the concave microlens array in PDMS. The fabricated microlenses appeared very uniform Figs. 2 a and 2 c . The lens diameter D and thickness h for the concave microlens array were measured by atomic force microscope AFM as 45.9 and 1.53 m, respectively, whereas the corresponding values for the convex microlens array were a bit larger, 46.3 and 2 m, respectively. The lens diameter D is much larger than the diameter of the holes in the copper grid, 37 m. We suspect this is because 1 the contact between the TEM grid and PDMS sheet may not be completely flat, especially around the grid edge, and 2 the ozone could diffuse through the copper grid to some extent, resulting in larger lenses. The focal length of the microlens array at various mechanical strains was measured by optical microscope Olyma Electronic mail: [email protected]. APPLIED PHYSICS LETTERS 91, 251912 2007 0003-6951/2007/91 25 /251912/3/$23.00 © 2007 American Institute of Physics 91, 251912-1 Downloaded 13 Feb 2008 to 130.91.116.168. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp pus BX-61 equipped with internal Z motor resolution of 1 m . Alphabet “N” was printed on a transparency and placed several centimeters below the microlens array Fig. 2 e . First, the microlens array was brought into focus of the microscope objective Figs. 2 b and 2 d , and then the image of “N” through the microlens array was brought into focus Figs. 2 b and 2 d , inset . The difference between the sample-stage positions of the two foci gave the focal length of the microlens array. Since the lens profile D and h is uniform over the microlens array, a single focus is obtained over the entire array Figs. 2 b and 2 d , inset . Here, real focus is obtained for convex lens array, but virtual focus for concave lens array. While the nonconfined ripples have sinusoidal profile, here, for the simplicity of estimation of the lens focal length, we assume the lenses are spherical with a single focal length. We find that it offers reasonable approximation of our shallow lens structure, and as shown later the calculated results agree well with the experiments within errors. For a thin spherical lens with diameter D and thickness h, the radius of curvature R is given by