Focusing capability of integrated chains of microspheres in the limit of geometrical optics Arash Darafsheh* a , Kenneth W. Allen a , Amir Fardad b , Nathaniel M. Fried a , Andrew N. Antoszyk c , Howard S. Ying d , and Vasily N. Astratov a a Department of Physics and Optical Science, University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, North Carolina 28223-0001, USA; b PhotonTech, LLC., Research Triangle Park, PO Box 13714, Durham, North Carolina 27709, USA; c Retina Service, Charlotte Eye Ear Nose and Throat Associates, P.A. 6035 Fairview Road, Charlotte, North Carolina 28210; d Wilmer Eye Institute, Johns Hopkins University, 600 N. Wolfe Street, Maumenee 723, Baltimore, Maryland 21287-9277, USA ABSTRACT The effects of periodical focusing of light were studied in chains of sapphire microspheres with 300 µm diameters assembled either on a substrate or inside capillary tubing. Dye-doped fluorescent microspheres were used as multimodal sources of light in experimental studies. Significant reduction of the focused spot sizes was observed for chains of spheres compared to a single sphere case. Numerical ray tracing simulations were performed for similar chains assembled inside hollow waveguides to be used as an optical delivery system with mid-infrared lasers for ultra-precise surgery. The device designs were optimized for contact conditions during laser surgery involving short optical penetration depths of light in tissue. It is shown that chains of spheres with n around 1.65-1.75 provide a two-fold improvement of the spatial resolution over single spheres. Potential applications of these microprobes include ultra- precise laser procedures in the eye and brain or piercing a cell, and coupling of multimodal beams into photonic microstructures. Keywords: microoptics, microlens, microspheres, focusing, optical microprobes, laser surgery, medical optics, ophthalmic optics and devices, geometrical optics design 1. INTRODUCTION Focusing optical microprobes can be applied in a broad range of biomedical and photonics applications, including laser surgery 1 , piercing of a cell 2 , optical endoscopy and spectroscopy 3 , high-density optical data storage 4 , and photo-induced patterning of thin films 5 . In many of these applications the light is delivered by flexible hollow waveguides (HWG) or multimodal fibers. Sharp focusing of such multimodal beams is a challenging task because each mode creates an individual intensity pattern in the focusing plane. If a single mode fiber is used to transmit light, improved focusing approaching the diffraction limit can in principle be achieved; however, these fibers are not readily available in the mid- infrared (IR) regime which is optimal for precise laser surgery due to the high absorption of water, the dominant absorber in most soft tissues, in the mid-IR. It has been shown that under plane wave illumination a single small wavelength-scale dielectric microsphere with an index of refraction of 1.59 can produce a focused beam with a sub-wavelength size in the lateral direction, labeled a “nanoscale photonic jet”. 6-12 The photonic nanojet propagates with little divergence for several wavelengths into the surrounding medium, while maintaining their sub-wavelength transverse beam width. The concept of nanojets is attractive for designing focusing microprobes with high transmission properties. It should be noted, however, that photonic nanojets from single spheres require strictly plane-wave (or conical) illumination which is not readily available * adarafs1@uncc.edu Laser Resonators and Beam Control XIII, edited by Alexis V. Kudryashov, Alan H. Paxton, Vladimir S. Ilchenko, Lutz Aschke, Proc. of SPIE Vol. 7913, 79131A · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.876139 Proc. of SPIE Vol. 7913 79131A-1