Surface imaging using holographic optical tweezers

Abstract

We present an imaging technique using an optically trapped cigar-shaped probe controlled using holographic optical tweezers. The probe is raster scanned over a surface, allowing an image to be taken in a manner analogous to scanning probe microscopy (SPM), with automatic closed loop feedback control provided by analysis of the probe position recorded using a high speed CMOS camera. The probe is held using two optical traps centred at least 10 µm from the ends, minimizing laser illumination of the tip, so reducing the chance of optical damage to delicate samples. The technique imparts less force on samples than contact SPM techniques, and allows highly curved and strongly scattering samples to be imaged, which present difficulties for imaging using photonic force microscopy. To calibrate our technique, we first image a known sample--the interface between two 8 µm polystyrene beads. We then demonstrate the advantages of this technique by imaging the surface of the soft alga Pseudopediastrum. The scattering force of our laser applied directly onto this sample is enough to remove it from the surface, but we can use our technique to image the algal surface with minimal disruption while it is alive, not adhered and in physiological conditions. The resolution is currently equivalent to confocal microscopy, but as our technique is not diffraction limited, there is scope for significant improvement by reducing the tip diameter and limiting the thermal motion of the probe.

Engineering light-matter interaction for emerging optical manipulation applications

“…Another conceptually simple (but technique-intensive) approach is to apply anti-reflection coating onto high-refractive-index microspheres to achieve stability and nanonewton optical forces [61]. These enhancements can be applied when trapping the typically spherical handles of more complex structures such as components in micro-assembly [95] or opticallysteered microtools [96,97]. However, more complex structures will require a different calibration than simple spheres due to difference in their dynamics [97].…”

“…Here, a trapped microstructure, big enough to be amenable to optical trapping, serves to tap onto a diffraction limited beam and channel its effects onto subwavelength regime. One example is to optically manipulate a relatively large microstructure to direct its subwavelength tip for SPM-like imaging [96]. The structure-mediated approach also makes it possible to tap into diffraction limited light to deliver it to subwavelength targets, as shown by the approaches illustrated in Figure 7.…”

A compact holographic optical tweezers instrument

“…Trapping handles allow the micro-tool to be controlled while keeping the sample clear of the trapping laser. 16 When using standard brightfield imaging it can be difficult to observe, and hence track, small objects such as the carbon nanotubes, even when trapping many in a cluster. Installing the center stop in the imaging path allowed the nanotube cluster to be easily observed using darkfield imaging, and is compatible with high-speed particle tracking.…”

“…They have found many applications including the construction of 3D structures using micron-sized dielectric spheres 14 or living cells 15 and imaging soft cellular surfaces using optically trapped probes. 16 Careful considerations are required when working with living cells or other light sensitive objects. Near infrared lasers have been successfully used to manipulate living cells as they allow the trapping force to be optimized while minimizing energy absorption.…”

Development of a microhand using direct laser writing for indirect optical manipulation

“…Maruyama et al successfully measured pH value of the yeast cell by manipulation of a circular gel-microbead, which surrounds the target cell [16]. Phillips et al raster scanned the surfaces of biological particles by an optically trapped cigar-shaped probe as a novel imaging technique [17]. Tan et al manipulated two beads attached to a cell by optical traps during cell stretching to obtain the mechanical response of the cardiomyocyte cell [11].…”