1. Why We Do Not Feel the Pushing Force of Light
If you throw a ball at a wall, the ball bounces back, and the wall is pushed slightly. Photons, which are particles of light, also create a pushing force when they hit an object [1]. This force is described as being only a few pico-newtons. A "pico" means one trillionth, and a newton is the force needed to hold up an object weighing about 100 g (about 3.5 oz). Even when a laser pointer beam is reflected by a mirror, the force is only a few pico-newtons, so we almost never feel it [1]. However, the gravity acting on a water droplet with a diameter of 1 μm (one thousandth of a millimeter) is about 0.005 pico-newtons. For tiny particles like bacteria or red blood cells, the force from light pushing them is far greater than their own weight. Optical tweezers were developed by focusing on this "grip strength" [1]. So, how does light that only pushes manage to catch particles?
2. Why a Laser Beam Pushing Particles Makes Them Flow Away
In 1970, Arthur Ashkin at Bell Labs in the United States shone a laser beam on small latex particles (about 0.6 to 2.7 μm) floating in water. The particles moved because they were pushed by the light [1]. However, pushed particles simply flow in the direction the light travels and do not stay in one place. So, Ashkin arranged two identical beams facing each other to sandwich the particle, balancing the opposing pushing forces to catch it stably. In 1971, it is said he also succeeded in an experiment where he shone light from below to counteract gravity and float the particle [1]. This achieved "catching particles with light" in a basic sense. But the beams had to be perfectly aligned facing each other. Could a particle be caught using just one beam, using the gradient force of a single beam?
3. Why Particles Were Drawn to the Center of the Laser Beam
Light has another force besides pushing: it draws particles toward the stronger light. The first is called the scattering force, and the second is the gradient force. The scattering force accelerates particles in the direction light travels, while the gradient force pulls particles into the strong center of the beam [3]. The gradient force attracts particles when their refractive index is higher than the surrounding water. Particles are pulled toward the strong focus of the light. Conversely, bubbles with a lower refractive index than water tend to be pushed away from the strong light [1]. In 1986, Ashkin, Dziedzic, Bjorkholm, and Chu showed for the first time that they could catch particles using this gradient force by strongly focusing a single beam with a microscope lens. They reported catching particles ranging from 10 μm to about 25 nm (one forty-thousandth of a millimeter) in water [4]. This paper is considered the starting point of the technology now called optical tweezers [6]. In 2018, half of the Nobel Prize in Physics was awarded to Ashkin for optical tweezers and their application to biological systems. The other half went to Mourou and Strickland for different research [1]. The awarding body praised the ability to gather small particles to the center and hold them with light force [2]. However, the particles were not bacteria; they were still non-living things. Could living things be caught?
4. How Near-Infrared Light Lets Optical Tweezers Catch Living Bacteria
Initially, the first lasers damaged the cells. So, a near-infrared laser with a wavelength of 1064 nm was used. Many biological materials let most of this light pass through. Switching to this laser greatly reduced damage from light [3][1]. It is explained that damage was not eliminated entirely, but significantly avoided. In 1987, Ashkin caught living bacteria with optical tweezers while minimizing damage. The Nobel Prize explanation calls this a "major breakthrough" [2]. In the same year, it is also explained that he succeeded in catching and moving E. coli, yeast, and human red blood cells, and also caught the tobacco mosaic virus [1]. Until then, bacteria and cells were touched and moved with thin glass needles. Light can move them without contact and with less damage. This point is evaluated as having had a major impact on biology [1].
5. How Optical Tweezers Measure the Tiny Steps of Molecules
The force of optical tweezers is in the pico-newton range, which is about the same as the force generated by biological molecules. Therefore, it has been used in research where tiny beads are attached to both ends of DNA to pull and study the properties of a single molecule, or where the force of myosin pulling actin, which causes muscle contraction, is measured one molecule at a time. Research by Toshiyo Yanagida and others in Japan is one example [1]. In 1993, Svoboda, Schmidt, Schnapp, and Block studied the motor protein kinesin. When a silica bead attached to a single kinesin molecule was placed on a microtubule (a thin tube inside cells) using optical tweezers, it was found that kinesin walks in steps of 8 nm [5]. 8 nm is one one-hundred-and-twenty-fifth of a millimeter. The awarding body explains that optical tweezers are now widely used in studying the "machinery of life" [2]. The force of light not only catches bacteria but has also become a tool for measuring the force and step size of a single molecule.
6. How Small a Pico-newton Is: Checking the Numbers Yourself
To check the smallness of "pico-newtons" with numbers, remember that 1 newton is the force to support about 100 g (about 3.5 oz), and pico is one trillionth of that. Calculating one trillionth of 100 g helps you realize how small the "grip strength" of optical tweezers is. You can also check if nearby science museums or university public events have exhibitions or lectures on microscopes and lasers. For official information, the explanatory article in the Japanese magazine KAGAKU (February 2019) is available. The announcement of the 2018 Nobel Prize in Physics is published in English by the Royal Swedish Academy of Sciences [2].