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, poly͑dimethylsiloxane͒ ͑PDMS͒ 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. 6 and integrated microfluidic channels, 7 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 convex͒ with real-time tunable focus. The concave lens array is created by confined buckling of a soft elastomer, poly͑dimethylsiloxane͒ ͑PDMS͒, which is mechanically stretched in plane bidirectionally 8 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 MPa͒, buckling occurs upon release of prestrained bilayer film, forming wrinkled patterns spontaneously. 8, 9 If the oxidation and, thus, buckling is confined to an area smaller than the wavelength of the unconfined wrinkles, microlens structure will be obtained. 10 Previously, similar strategy has been used to create microlens array by swelling a patterned oxide/PDMS bilayer structure with acrylate monomers, followed by polymerization. 10 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 Silicone͒ and curing agent ͑10: 1 wt/ wt͒ 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. 1͑a͔͒, leaving a center space of 25ϫ 25 mm 2 and then stretched to 20% strain in both planar directions simultaneously ͓Fig. 1͑b͔͒. One side of the stretched PDMS surface was masked with a transmission electron microscopy ͑TEM͒ copper grid ͓Fig. 1͑c͔͒ with hexagonally packed hole array ͑diameter of 37 m and hole to hole distance of 62 m͒ for ultraviolet ozone ͑UVO͒ treatment ͑UVO-Cleaner Model 42, Jelight Company, Inc.͒ for 30 min ͓Fig. 1͑d͔͒ 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. 1͑e͔͒ and the PDMS strip was strain released in both planar directions simultaneously, resulting in a concave microlens array ͓Fig. 1͑f͔͒. In the range of prestrain levels ͑10%-30%͒ and UVO treatment time ͑15-60 min͒, 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. 2͑a͒ and 2͑c͔͒. The lens diameter D and thickness h for the concave microlens array were measured by atomic force microscope ͑AFM͒ 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 ͑1͒ the contact between the TEM grid and PDMS sheet may not be completely flat, especially around the grid edge, and ͑2͒ 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 ͑Olym-a͒ Electronic mail: shuyang@seas.upenn.edu. APPLIED PHYSICS LETTERS 91, 251912 ͑2007͒
Chandra et al. REF built concave and convex elastic microlens arrays whose focal lengths are tunable by stretching.