Miniaturized high-NA focusing-mirror multiple optical tweezers

Abstract

An array of high numerical aperture parabolic micromirrors (NA = 0.96) is used to generate multiple optical tweezers and to trap micron-sized dielectric particles in three dimensions within a fluidic device. The array of micromirrors allows generating arbitrarily large numbers of 3D traps, since the whole trapping area is not restricted by the field-of-view of the high-NA microscope objectives used in traditional tweezers arrangements. Trapping efficiencies of Q max r 0.22, comparable to those of conventional tweezers, have been measured. Moreover, individual fluorescence light from all the trapped particles can be collected simultaneously with the high-NA of the micromirrors. This is demonstrated experimentally by capturing more than 100 fluorescent micro-beads in a fluidic environment. Micromirrors may easily be integrated in microfluidic devices, offering a simple and very efficient solution for miniaturized optical traps in lab-on-a-chip devices.

Note: Toward multiple addressable optical trapping

“…11 Refractive microlenses alone; however, do not have high enough NAs to form three-dimensional traps. Merenda et al 12 demonstrated three-dimensional trapping for a reflective type microlens array ͑MLA͒, but the pitch of this array was on the order of hundreds of microns, so the number of simultaneously observable traps was limited. The Talbot effect, in which a periodic structure is self imaged, can also be used to produce multiple optical traps.…”

Three-dimensional force measurements in optical tweezers formed with high-NA micromirrors

“…However, the most important aspect for applications where OT are used as force transducers is the trap stiffness achievable with micrometer-sized spheres. In this Letter, we present precise measurements of the 3D trap stiffness generated by MOT, using back-focal-plane interferometry and power spectrum analysis.Micromirrors were fabricated using a molding technique described in detail in [7]. Briefly, a fusedsilica microlens array (Süss MicroOptics, Switzerland) coated with a 50-nm-thick gold layer is immersed in UV epoxy (NOA65, n = 1.51 at 1064 nm) onto a 1-mm-thick glass slide.…”

“…However, the cost, bulkiness, and short working distance of these objectives severely limits the conceivable application range of OT. We have recently proposed the use of miniaturized high-NA parabolic mirrors, integrated at the level of a microfluidic chip, for simultaneous multiple optical trapping and fluorescence detection [7]. Such micromirror OT (MOT) are among the few existing integrated geometries capable of 3D optical trapping that include fiber-based optical traps [8][9][10], diode lasers monolithically integrated in microfluidics [11], and microfabricated water-immersion zone plates [12].…”

“…Micromirrors were fabricated using a molding technique described in detail in [7]. Briefly, a fusedsilica microlens array (Süss MicroOptics, Switzerland) coated with a 50-nm-thick gold layer is immersed in UV epoxy (NOA65, n = 1.51 at 1064 nm) onto a 1-mm-thick glass slide.…”

“…A precise control of the laser power in the trap is crucial for the comparison with values of trap stiffness previously reported in the literature. It is calculated from the measured optical power and Gaussian beam width at the beam splitter cube, and accounts for the power transmission of 93% characteristic for micromirrors [7]. The employed polystyrene (PS) microspheres (Polysciences, Inc., USA, radius of a = 1.03 m) were suspended in pure distilled water at a concentration of ϳ10 6 microspheres/ ml.…”