Microstuctured optical fiber tips for single cell analysis

In 2018, Arthur Ashkin was awarded the Noble price in physics for the optical tweezers and their applications to biological systems. Indeed, over the last decades this breakthrough technology has become an established tool not only in biology, but in many scientific fields where microscopic and submicroscopic particles need to be trapped, manipulated, or placed on a defined position. In addition, scientists use these optical traps to measure smallest molecular interactions and forces, such as the elasticity of DNA. The basic idea of optical tweezers is to trap particles with a different refractive index than the surrounding medium by the small attractive and repulsive forces of a light beam.

On-fiber printed optical tweezers

Optical tweezers usually require bulky and expensive setups, such as objectives with a high numerical aperture. Scientists at the University of Stuttgart have developed a highly efficient miniaturized optical tweezers by printing directly onto the end of a cleaved optical fiber using Two-Photon Polymerization (2PP). These on-fiber printed optical traps are arranged in a so called dual-beam, counter-propagating setup. This means that two fiber ends with attached optical traps are aligned directly opposite each other and a particle can be trapped where the two focal points of the counter-propagating laser sources meet. With the on-fiber printed optical tweezer, the researchers demonstrated a highly efficient particle trapping for 1 µm and 500 nm polystyrene beads in water.

2.5D Fresnel lenses and stacked 3D designs

The scientists designed and optimized three fiber-optic particle traps with a working distance of 50, 100 and 200 micrometers. Using a Nanoscribe Photonic Professional system, they printed these optical lenses directly on the cleaved ends of optical fibers. The main architecture of these 3D-printed optical devices consist of two parts. The first section expands the light beam guided in the optical fiber and is necessary for achieving the targeted working distance and the associated high numerical apertures. However, the key elements of the optical tweezers are finely tuned Fresnel lenses that efficiently focus the light to trap the particles at the precalculated positions. For practical reasons, the scientists chose to print these diffractive designs instead of conventional spherical lenses. On-fiber printed refractive lenses would result in designs with very challenging curvatures. The Fresnel lens design, on the other hand, can be easily adjusted to the desired working distance and high numerical apertures. The researchers printed three different diffractive lenses on the fiber end with a minimum lateral feature size of 1.67 µm at the outer edge of the design and a profile height of 3.88 µm.

For optical tweezers with high numerical apertures, the 2PP technology proves its true potential. A high numerical aperture could not be achieved with a single Fresnel lens on the fiber ends. Instead, the scientists printed two lenses on top of each other, separated by six pillars supporting the second lens. This would be impossible without true 3D printing. All the optical tweezers enable stable trapping of (sub)micrometer-sized polystyrene test beads at low laser power, which is particularly interesting for applications in biology, where high laser powers could damage the soft tissue of the organic sample.