This year’s Nobel Prize in physics has gone to three scientists for creating what has been described as “tools made of light”.
Arthur Ashkin — who, at 96, becomes the oldest scientist ever to be awarded a Nobel Prize — is credited with having invented what is famously known as “optical tweezers”. Actually a technology rather than a physical instrument, these “tweezers” are widely used for isolating and examining very small particles, such as individual atoms, DNA strands, or biological cells.
Gérard Mourou and Donna Strickland, who share the other half of the Prize, developed a technique that has made it possible to generate most intense laser pulses that are now used in a wide variety of scientific and medical applications, including in eye surgeries.
In the process, Strickland, a 59-year-old Canadian scientist, has now become only the third woman to have received the Nobel Prize in Physics, after Marie Curie in 1903 and Maria Goeppert Mayer in 1963. And the main breakthrough of her work, in collaboration with her PhD supervisor Mourou, was described in the very first scientific paper she published, back in December 1985.
Ashkin’s “optical tweezers” too were developed in the mid-1980s. In fact, several scientists have been awarded the Nobel Prize since then for work done on the technology he had developed, but Ashkin himself had been left out until now.
Ashkin, an American, has worked on laser pulses ever since they were first produced in 1960. Light beams produced by a laser — it is a device and not a beam itself — have a single frequency (colour) and high intensity, and thereby high power.
It was widely known for several years that light could exert pressure on objects it was incident upon. But this pressure was not large enough to have any observable effect that scientists could measure. The development of laser beams, owing to their high power, opened new opportunities. Ashkin showed, for the first time, that these light beams could indeed be used to move very small objects. He found that the micrometre-sized spheres he had been using for his experiments were drawn towards the centre of the light beam, where the intensity of the light, or the number of light particles in the beam, was the most.
Over the years, by carefully manipulating the light beam, and using different kinds of lenses, Ashkin could control the movement of the sphere, or other small particles, and even create a sort of a trap, in which the particle was contained. It was similar to isolating these small particles for special observation. This is what came to be described as “tweezers”, where single or multiple beams of light could isolate and hold very small particles, like an atom, for examination by scientists.
Before Ashkin’s work, such small particles could not be isolated and examined. Isolation of single particles helps scientists understand the behaviour of single atoms or cells, instead of studying the average behaviour of an aggregation of such particles. By 1986, the “optical tweezers” had been developed sufficiently to stop and trap individual atoms. Around this time, he accidentally also trapped bacteria in his “tweezers”, and then showed that by using a particular kind of infrared light, bacteria could be trapped, or isolated, without any harm being done to them.
These days, “optical tweezers” are standard equipment in laboratories across the world, being used for studying a variety of fields, including biological processes of individual proteins, DNA, or other cells.
While Ashkin used the laser beams for doing completely new things, Mourou and Strickland expanded the capabilities of the light beam itself. Lasers produce light at very short intervals, of the order of micro- or nanoseconds or even lesser. The power of the light beam, or its intensity, is measured by the energy it carries per second. Thus, the shorter the time interval in which the pulse was created, the higher was the power.
Within a few years of the invention of the laser, laboratory tabletop lasers had started achieving power of about a gigawatt (10 raised to the power 9). But after that a state of peak power was reached. More intense pulses of light could not be produced without damaging the amplifying material.
Mourou and Strickland devised a way out. They increased the duration of the pulses before the light was amplified so that the intensity came down. The light could then be amplified normally. After amplification, the pulse could be compressed back to its original time duration, packing many more light particles in a very small space, thereby increasing the intensity by several order of magnitude.
Through this method, Mourou and Strickland could improve the intensity of the light beam by almost a million times at one go. Scientists have since developed this technology further so that modern-day laser can produce light beams with power of the order of petawatts (10 raised to the power 15), and efforts are on to install lasers that can go even higher.
India currently has two lasers that produce 100 terrawatt (10 to the power 12) beams. The Raja Ramanna Centre for Advanced Technology in Indore is in the process of installing two petawatt systems, while another is likely to be installed in Hyderabad.
Such high intensities are extremely useful in many scientific contexts. These high-power beams of light, when they interact with matter, produce such extreme conditions that are found only in the deep cores of stars, or other celestial bodies. Scientists use this to study and understand these conditions, which would otherwise not be accessible to them, giving rise to the expression ‘astrophysics in the laboratory’.
Such ultrashort and high-power light beams also help the scientists in uncovering processes that take place in the micro-world. These processes happen so quickly that they cannot be captured by anything else. High-power femtosecond (10 raised to the power minus 15) pulses have enabled scientists to ‘see’ processes such as atomic interactions that earlier appeared to be instantaneous and remained invisible.
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