Optical trapping employs forces generated by light to spatially immobilize suspended objects in a contact-free manner. Arthur Ashkin was the first to demonstrate that microscopic objects can be trapped and manipulated with light. He went on to create a stable trap using two counter-propagating laser beams – the first dual-beam laser trap (DBLT) . 16 years later, he also introduced optical tweezers, which utilize the forces of a single, focused laser beam . In 1993, based on Ashkin’s early dual-beam work, Mara Prentiss and co-workers created the first DBLT employing two opposing optical fibers . As the trapping optics of the fiber-based DBLT are completely separated from the microscope optics, DBLTs then became an extremely versatile tool for the micro-manipulation of biological samples. They are also easily combined with microfluidic lab-on-chip systems for efficient delivery of trapping objects.
Previous work in our lab  demonstrated that the optical forces in a dual-beam laser traps can be used to deform and measure soft materials in a controlled manner. When used for this purpose, DBLTs have come to be known as an optical stretcher (OS). An illustration of the trapping and deformation of a cell in the OS is shown in Figure 1. We have used the OS extensively to measure the viscoelastic properties of various cell types and have established cell deformability as a marker of cell function [5, 6, 7, 8, 9, 10, 11, 12]. Along the way we had improved the throughput of the OS to hundreds of cells per hour by combining it with microfluidic assemblies [13, 14]. With the advent of real-time deformability cytometry (RT-DC), with its 10,000x higher throughput, the OS has increasingly been repurposed for the high-content study of rare or sensitive objects, such as phospholipid vesicles  or isolated cell nuclei . Previously, it had also been used to test the axial light transmission through individual cells, e.g. by gently trapping and aligning living Muller cells along their optical axis without any mechanical contact .
In current studies, we use DBLTs for the controlled rotation of single cells in an optical cell rotator (OCR) . In the OCR, the output of one of the fibers, which is a few-mode fiber, is controlled by a spatial light modulator to achieve a rotating double-lobed mode profile (Figure 2). The optical forces induced by the rotating mode result in a rotation of the trapped cell. Rotating single cells about an axis perpendicular to the optical axis of a microscope allows the acquisition of tomographic data sets of individual cells. Besides the all-optical rotation in the OCR we also employ a combination of optical trapping in a DBLT with drag forces induced by flow in a microfluidic channel for a contact free optofluidic rotation of individual cells, as proposed by Kolb et al. . We use these techniques together with QPI and ODT to determine 3D refractive index maps of individual cells .
All these examples showcase the versatility of DBLTs and propose it as a high-content tool for single cell studies.
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