Optical paddle - a new microtool for optical tweezers

Optical tweezers are the device which makes use of tightly focused laser light to optically trap and precisely manipulate the specimen in microscale. The examined object can be either trapped itself or studied with an... [ view full abstract ]

Optical tweezers are the device which makes use of tightly focused laser light to optically trap and precisely manipulate the specimen in microscale. The examined object can be either trapped itself or studied with an additional tool. Since spherical shapes undergo the most stable trapping, so-called microbeads of various sizes and materials became a natural solution. They can be used as a probe, a handle, a marker, etc. Nevertheless, their simple shape is both an advantage and a limitation.

In this work, we propose a novel microtool which we called the optical paddle (see figures). It consists of two spheres, acting as handles, with a bar between them and a square ‘blade’ on the end. There are also two smaller spheres at both ends of a perpendicular arm which provide extra stabilization. The total length of this microtool is 45µm whereas the blade is 10x10µm. The optical paddle was 3D printed with the two-photon polymerization technique and then immersed in a chamber filled with water.

We use optical paddles in two different optical tweezers (OT) setups – a holographic and non-holographic version. Both consist of a laser source, a high-numerical aperture microscope objective (NA=1.3), and a CMOS camera. The difference lies in the way of steering the laser beam. While the holographic OT use spatial light modulator (SLM), the second setup uses galvanic mirrors. As a result, the experiments benefit from both the unlimited number of available optical traps and high stiffness of the trap.

We present new possibilities that arise from introducing optical paddles to the OT system. Firstly, the microtools were tested for sensitivity to forces even below single piconewtons. This included e.g. exposing the paddle to weak hydrodynamic flow produced by oscillating microbead or pressing one paddle against the other. The response of the tool was measured as a displacement of the rear handle from the trap center. Finally, having confirmed the sufficient sensitivity, an experiment with human spermatozoa pushing onto the paddle was performed in order to estimate their force. In our work we demonstrate these experiments and discuss further applications of optical paddles.