Nobel Prize in Physics for tools made of light

Award for inventors of optical tweezers and ultrafast laser pulses

The Nobel Prize for Physics goes to three researchers who have revolutionized laser physics. © Frater / Nobel Foundation
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The Nobel Prize for Physics 2018 goes to three researchers who have revolutionized laser physics. Arthur Ashkin gets half the price for the invention of optical tweezers - a method to manipulate bacteria, viruses and even atoms with laser beams. The second half of the award goes to Gérard Mourou and Donna Strickland, who have developed a method to generate ultra-fast yet intense laser pulses.

From the point - of - sale scanner in the supermarket to the optical data transmission and nanotechnology to the search for gravitational wave lasers, everyday life and science have become indispensable in everyday life. One of the main advantages of the laser is its high intensity and its coherent light: it oscillates in parallel and quasi in common mode, at the same time the beam is extremely tightly focused and thus sharp.

In search of a "tractor beam"

Shortly after the invention of the laser in the mid-1960s, US physicist Arthur Ashkin, working at the Bell laboratories, began to experiment with this new form of light. His consideration: Perhaps the beam of parallel and coherent light would be suitable for moving and manipulating small objects - similar to the tractor beam in the then re-broadcast science fiction series "Star Trek".

At that time it was already known that light exerts a radiation pressure: The photons striking a surface or an object exert a force that, for example, presses the tail of a comet towards the side facing away from the sun or drives a light sail in space. So when Ashkin put tiny transparent spheres in the laser beam, he was hardly surprised when they moved in the direction of the light beam.

The principle of laser tweezers © Nobel Foundation

Gradient effect and lens as an object trap

Unexpectedly, however, was a second effect: The beads always drifted into the middle of the laser beam - even if Ashkin put it in the outdoor area. As he noted, this was because the intensity of the laser beam is highest in its center. Gradient forces therefore ensure that objects are always directed to the highest intensity of the laser. For Ashkin, this observation was the starting point. Could this effect possibly be exploited to capture objects in the beam of light and even to "grab" and manipulate them in a controlled manner? display

Ashkin and his team found that by introducing a strong microscope lens, the laser beam can be focused so that the gradient force becomes a trap for the smallest of objects. These are held in place at the highest laser intensity and even the forward current of the light particles does not carry them with them the optical tweezers were invented. In 1986, Ashkin used these tweezers made of light to capture and move specific atoms. A little later, he refined the method so far that even sensitive material such as viruses, bacteria and other living cells could be caught in the laser tweezers.

Today, optical tweezers from research are almost indispensable. Scientists use them to manipulate the genetic molecule DNA, assemble atomic nano-constructs atomically, or explore the processes inside cells. At least on a tiny scale, the tractor beam of science fiction has become reality.

With the help of Chirped Pulse Amplification (CPA) laser pulses can be extremely shortened and compressed. Nobel Foundation

The recipe for ultrashort laser pulses

The second half of this year's Nobel Prize goes to Donna Strickland and G rard Mourou. They have developed so-called Chirped Pulse Amplification (CPA), a technique that allows laser pulses to be shortened and compressed far beyond what was previously possible. Simply splitting a laser beam into ever shorter pulses will cause the intensity Ab to decrease, as each pulse will have correspondingly fewer photons. This can be compensated by bundling even thicker beams, but this requires large and enormously complex devices and these systems require hours of abbr. Between pulses hlens.

Strickland and Mourou solved this problem with a counterproductive idea at first sight. Because with the Chirped Pulse Amplification, a laser pulse is initially stretched in a special light guide and thus weaker. Then you send this stretched pulse through an amplifier, today usually made of titanium-doped sapphire. This increases the amplitude of the light vibrations and thus the laser intensity. In the third and final step, this amplified laser pulse is now compressed in time. As a result, the intensity of the laser pulse increases further, while it becomes even shorter.

This technology made it possible to produce ultra-short, high-intensity laser pulses for the first time - they too are one of the most important tools in science today. Using femtosecond and attosecond pulses, researchers are now deciphering the structure of complex molecules, tracking in almost real-time how chemical bonds are formed or fracturing, and delving deep into the fundamental physics of matter. Also, laser-based particle accelerators or laser scalpels are based on ultrashort laser pulses - and thus on the invention of Strickland and Mourou.

(Nobel Foundation, 02.10.2018 - NPO)