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

Abstract

The three-dimensional trap stiffness of optical tweezers formed with high-NA micromirrors is investigated by back-focal-plane interferometry and power spectrum analysis. Normalized stiffness values of xy / P trap = 1.2͑N/m͒ /mW and z / P trap = 0.52͑N/m͒ /mW in the transverse and axial directions, respectively, have been measured for polystyrene spheres with a radius of 1.03 m. Compared with high-NA microscope objectives, micromirrors achieve much better trapping performances, particularly in the axial direction. © 2009 Optical Society of America OCIS codes: 350.4855, 130.3990, 350.3950, 230.4040, 220.1230. Optical tweezers (OT) [1] have found broad applications in biology and physics, e.g., for the mechanical characterization of single biomolecules [2,3], for fundamental studies on Brownian motion and thermodynamics at the microscopic scale [4,5], or for in situ viscometry [6]. The tight focusing of the laser beam, necessary for creating stable three-dimensional (3D) OT, is usually achieved with a high-NA microscope objective. 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]. MOT were already shown to withstand large escape forces in microfluidic flows with beads as large as 9.33 m in diameter. 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. After polymerization, the microlens array is ripped off the glass slide, leaving its inverse structure coated with gold on the latter. The concave mirrors are further filled with the same UV epoxy and protected with an 80-µm-thick coverglass glued on top. Each micromirror has a diameter of 240 m and a radius of curvature of 350 m, yielding an effective NA of 0.93. The foci of micromirrors are located approximately 10 m above the protective coverglass. A glass flow chamber assembled around the slide with micromirrors allows conveying particles to the trapping area using a fluidic system.Laser light from an ytterbium fiber laser emitting in a linearly polarized TEM 00 mode (IPG Photonics, YP-1064L...

Dry‐etching‐assisted femtosecond laser machining

“…As one type of microlens, silicon concave microlenses have wide application in infrared optical scanner and combined lens for optical systems. Also, as one type of concave structure, the silicon concave microstructures may be applied in free‐space light coupling , optical tweezers and increasing light extraction efficiency of silicon based light‐emitting diode arrays similar to PDMS concave microstructures for InGaN LEDs . In addition, the silicon concave microlens could also be used as molds for duplicating of polymer convex microlens, as shown in Fig.…”

Three-dimensional silicon micromachining

“…A concave surface profile with micrometre-scale dimensions is of interest because of its ability to focus incoming light into a spot. Concave mirrors act like optical tweezers to trap/push particles with the optical gradient/scattering force [50,51] and may be used as a variable optical attenuator [52] or to produce an enhanced and localized light signal for parallel low loss optical interconnect systems [53]. We have fabricated very smooth, concave mirrors [41] using ion irradiation of a resist patterned as an annulus, using the same process to produce undercut structures as shown in figure 6(a).…”

Fabrication of concave silicon micro-mirrors

“…A concave surface profile in the micron regime is of interest because of its ability to focus incoming light into a spot. In bio-sensing or cell trapping devices, incoming laser that is focused by such a concave mirror acts like optical tweezers to trap/push particles with the optical gradient/scattering force [1,2]. In the field of micro-electro-mechanical-systems (MEMS) and optoelectronics, it may be used as a variable optical attenuator [3] or to produce an enhanced and localized light signal for parallel low loss optical interconnect systems [4].…”