Spherical aberration correction for optical tweezers Eirini Theofanidou, Laurence Wilson, William J. Hossack, Jochen Arlt * COSMIC, School of Physics, The University of Edinburgh, James Clark Maxwell Building, Mayfield Road, Edinburgh EH9 3JZ, UK Received 16 December 2003; received in revised form 1 March 2004; accepted 3 March 2004 Abstract In typical optical tweezers experiments the quality of the trap deteriorates due to optical aberrations caused by the refractive index mismatch at the glass–water interface when trapping deep inside a sample or out of the imaging plane. We investigate this effect experimentally by monitoring the two-photon fluorescence of trapped dye-stained polystyrene spheres. We also partly corrected the aberrations by optimising the fluorescence signal using an adaptive deformable membrane mirror. Ó 2004 Elsevier B.V. All rights reserved. PACS: 87.80.+s; 42.15.Fr 1. Introduction Optical tweezers [1] have become an established technique for micromanipulation of particles. Transparent objects with refractive index higher than the surrounding medium are trapped close to the focus of a laser beam. There are extensive applications for optical tweezers ranging from ‘simple’ contact-less micromanipulation to quan- titative precision force measurement in many dif- ferent areas of science, such as biology and colloidal science [2,3]. For most of these experi- ments, particularly for quantitative force mea- surements, it is crucial to have a detailed characterisation of the trap. The optical trap is formed by tightly focusing a medium powered laser beam using a high numer- ical aperture (NA) microscope objective. Modern microscope objectives are very well corrected, yielding a diffraction limited focus and therefore a well characterised trap in their designated image plane. However, most objectives are corrected for imaging close to the surface of the cover glass whereas for many applications the optical trap should be as deep inside the sample as possible in order to minimise spurious interactions with the interface. Most investigations use sample particles suspended in an aqueous medium in combination with a high NA oil-immersion objective. Unfor- tunately, in this case the quality of the focus and therefore the trap degrades rapidly as it is moved deeper inside the sample, mainly due to spherical aberrations introduced by the refractive index mismatch at the glass–water interface [4,5]. This * Corresponding author. Tel.: +44-131-650-5229; fax: +44- 131-650-5902. E-mail address: j.arlt@ed.ac.uk (J. Arlt). 0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.03.009 Optics Communications 236 (2004) 145–150 www.elsevier.com/locate/optcom