title: High Numerical Aperture Hexagonal Stacked Ring-Based Bidirectional Flexible Polymer Microlens Array.
abstract: Flexible imprinted photonic nanostructures that are able to diffract/focus narrow-band light have potential applications in optical lenses, filters, tunable lasers, displays, and biosensing. Nanophotonic structures through holography and roll-to-roll printing may reduce fabrication complexities and expenses and enable mass production. Here, 3D photonic nanostructures of a stacked ring array were imprinted on acrylate polymer (AP) over poly(ethylene terephthalate) (PET) substrate through holography and lift-off processes to create a microlens array (MLA). The surface structure of the array consisted of circular nonostepped pyramids, and repeated patterns were in hexagonal arrangements. Stacked-ring-based MLA (SMLA) on a flexible AP-PET substrate showed efficient bidirectional light focusing and maximum numerical aperture (NA = 0.60) with a reasonable filling factor. The nanostructures produced a well-ordered hexagonally focused diffraction pattern in the far field, and power intensities were measured through angle-resolved experiments. The variation of nanostep dimensions (width and height) and the number of steps resulted in different photonic bandgaps, and the arrays produced distance-dependent narrow-band light focusing. The validation of the SMLA was demonstrated through the text, image, and hologram projection experiments. It is anticipated that imprinted bidirectional SMLA over flexible substrates may find applications in optical systems, displays, and portable sensors.
authors.affiliation: Microengineering and Nanotechnology Laboratory, School of Engineering, University of Birmingham , Birmingham B15 2TT, United Kingdom.
authors.forename: Rajib
authors.initials: R
authors.lastname: Ahmed
authors.affiliation: Harvard Medical School and Division of Biomedical Engineering, Brigham and Women's Hospital , 65 Lansdowne Street, Cambridge, Massachusetts 02139, United States.|Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology , 65 Landsdowne Street, Cambridge, Massachusetts 02139, United States.
authors.forename: Ali K
authors.identifier: 0000-0003-0896-267X
authors.initials: AK
authors.lastname: Yetisen
authors.affiliation: Microengineering and Nanotechnology Laboratory, School of Engineering, University of Birmingham , Birmingham B15 2TT, United Kingdom.
authors.forename: Haider
authors.initials: H
authors.lastname: Butt
keywords: bidirectional focusing; diffraction grating; microlens array; photonics; stacked rings