An 'Archimedes screw' made entirely of light

The optical screw is controlled by the researchers and moves nanometer particles to the desired location; It may help, among other things, in testing infections in air, water, and a variety of medical and biological solutions. The study was published this month in the journal OPTICA

The 'Archimedes screw' is a suction device that was invented 2300 years ago, and is still used today as the basis for many installations - from ship and airplane propellers to fans and snack vending machines. Now researchers from Tel Aviv University have created an optical 'Archimedes screw', made entirely of light. Like the original screw, the optical screw also moves matter from place to place - except that instead of water, air or candy, it moves particles in nanometer dimensions. In the future, a device of this type may help to identify impurities in air, water and medical solutions, for medical and ecological purposes.

The leader of the research is Dr. Alon Babed from the School of Electrical Engineering in the Faculty of Engineering. Working alongside him were his students Barak Haddad, Sahar Fruim and doctoral student Yaniv Eliezer, as well as Dr. Yael Ruichman, Harel Nagar and Tamir Admon from the School of Chemistry. The article was published in June 2018 in the journal Optica.

"The 'Archimedes screw' is one of the great technological inventions of antiquity. It was developed by the great inventor Archimedes in the city of Syracuse in Sicily in the 3rd century BC - in order to draw water from the pleasure ship of King Heron II," says Dr. Babed. "In my lab, we sought to reproduce Archimedes' invention in 21st century terms: an optical screw that would capture tiny particles, and allow us to control their movement and bring them to the place we want."

The first step in the research was the development of a screw-shaped light beam. To this end, the researchers created a meeting between two laser beams of the type known as an 'optical vortex' - a beam in which the light waves move forward, and at the same time also rotate around the center of the beam. In the area of ​​the encounter, a phenomenon of entanglement is created - when the light waves of the two beams amplify or alternatively weaken and even cancel each other, creating alternating dark and light areas. In this situation, the resulting bright areas form a coil of light. In this way, the researchers built a screw beam consisting of two helices (similar to the structure of DNA), whose properties and mode of movement are controlled by them with great precision.

In the second step, the researchers wanted to use the optical screw to capture and move particles. "'Optical traps' of light beams that trap particles have existed in science for decades," says Dr. Babed. "One of the biggest challenges in capturing particles floating in air or liquid is pulling the particle towards the light source. The difficulty is that the light particles (photons) of the light source collide with the approaching particle, pushing and pushing it away. This phenomenon makes it difficult, for example, for scientists who want to bring particles to 'under the lamp' in various test facilities. Our screw solved the problem in an innovative way: it captures particles in the dark areas of the beam, and moves them to the desired place by turning the screw."

The method was tested in computer simulations, and also in the laboratory: the researchers passed a screw beam through a small glass container, inside which carbon dust particles are running. When the beam caught a particle, they turned the screw, and were able to move the particle in the direction and place they wanted. "It's like a snack placed inside one of the screw turns in a snack machine, and moves toward the exit as the screw turns," says Dr. Babad.

The innovative optical screw has a number of prominent and important advantages compared to other methods developed in recent years: "The main thing is that we have complete control over the process," states Dr. Babed. "Our operation almost does not depend on the parameters of the particle itself, such as its size and mass, but mainly on the movement of the screw. We can move the particle in any direction, even backwards, and also control the movement speed - which is actually the screw rotation speed. In the laboratory, we moved nanometer particles in a controlled manner over a distance of about a centimeter for about a minute - something that is considered a technological achievement in itself, and in any case our method can be adapted to transport particles to much greater distances. Today we are also researching how to increase the speed, and accelerate the movement of the particles. More than that: with the help of the screw, several particles can be captured and moved at the same time, inside the different 'slots' of the screw. We continue to develop the technology, and one of the goals is to create an optical 'turbine' that moves many particles at the same time quickly in a required direction."

for the scientific article