Nobel Prize in Physics 2018: Optical Tweezers and Chirped Pulse Amplification

This note covers the Nobel Prize in Physics 2018: who won it, what optical tweezers are and how they use laser light to trap tiny particles, how chirped pulse amplification creates ultra-short high-intensity laser pulses, how both discoveries unfolded, why they matter and quick facts for exams.

What was the Nobel Prize in Physics 2018 awarded for?

The Royal Swedish Academy of Sciences awarded the prize "for groundbreaking inventions in the field of laser physics". This is the official citation, and it covers two separate but related inventions that both use laser light as a precise, delicate tool rather than a cutting weapon.

In plain words, the committee honoured scientists who found clever ways to make laser light do things nobody thought possible: hold tiny objects like a pair of invisible fingers, and squeeze light into pulses so short and so intense that they can probe the fastest events in nature without destroying the equipment that creates them.

One half of the prize went to a method for trapping particles with light, and the other half went jointly to a method for generating extremely short, powerful light pulses.

The official name of this award is the Nobel Prize in Physics.

Who are the laureates?

Arthur Ashkin

Arthur Ashkin was born on 2 September 1922 in New York, NY, USA, and died on 21 September 2020 in Rumson, NJ, USA.

At the time of the award he was affiliated with Bell Laboratories in Holmdel, NJ, USA, where he had spent his entire career.

He received one half of the prize, cited "for the optical tweezers and their application to biological systems".

Ashkin invented optical tweezers, a technique that uses a focused laser beam to trap and move microscopic objects, including living cells, without touching them.

Gérard Mourou

Gérard Mourou was born on 22 June 1944 in Albertville, France. At the time of the award he was affiliated with both École Polytechnique, Palaiseau, France, and the University of Michigan, Ann Arbor, MI, USA.

He shared one quarter of the prize with Donna Strickland, cited "for their method of generating high-intensity, ultra-short optical pulses". Mourou co-invented chirped pulse amplification (CPA), the technique behind today's most powerful short-pulse lasers.

Donna Strickland

Donna Strickland was born on 27 May 1959 in Guelph, Canada. At the time of the award she was affiliated with the University of Waterloo, Waterloo, Canada.

She shared one quarter of the prize with Gérard Mourou, under the same citation. The committee's press release noted that their 1985 paper describing CPA was the foundation of Strickland's doctoral thesis.

What problem were these inventions trying to solve?

Lasers were invented in 1960, and physicists quickly wanted to use their special properties, namely that laser light travels as a tightly focused, single-coloured beam, unlike ordinary light, which scatters in all directions and mixes many colours. Two separate puzzles remained unsolved for decades.

The first puzzle: can light actually push or hold physical objects? The idea that light carries momentum and can exert a tiny push, called radiation pressure, dates back centuries.

The scientific background notes that Johannes Kepler suggested in 1619 that radiation pressure explains why comet tails point away from the Sun, and James Clerk Maxwell showed this theoretically in 1873.

But this force is extraordinarily weak in everyday life, so weak that it was only confirmed experimentally in the early 1900s.

Ashkin wanted to know whether a laser's radiation pressure could be made strong enough to move and trap microscopic particles, even living cells, without damaging them.

The second puzzle: can laser pulses be made more intense without destroying the laser itself? By the mid-1980s, researchers trying to amplify short laser pulses hit a wall.

Pushing more energy into a short pulse increased its peak power so much that it physically destroyed the amplifying material and optical components inside the laser.

For roughly 15 years, between 1970 and 1985, the energy per pulse increased only marginally, according to the scientific background. A new approach was needed to break this barrier.

How do optical tweezers work?

Ashkin's optical tweezers use a laser beam, focused through a lens, to create a tiny trap of light. The mechanism combines two effects of radiation pressure. First, light pushes a particle in the direction the beam is travelling.

Second, if the particle sits off-centre in the beam, where light is less intense, an uneven force called the gradient force pulls it sideways towards the most intense part of the beam, which is usually the centre.

By carefully focusing the beam with a strong lens, Ashkin arranged for these forces to balance in three dimensions, so a particle gets trapped at a single point: the focus of the beam. The process, as described in the popular science background, works roughly like this:

1. A laser beam is aimed at a microscopic transparent particle suspended in a liquid.
2. The particle is drawn towards the centre of the beam, where the light is most intense, because the radiation pressure varies across the beam's width.
3. A strong lens focuses the beam so that the particle is also pulled towards the point of greatest intensity along the beam's length.
4. The particle becomes trapped at this focal point, held steady by light alone, with no physical contact.

The press release recalls that this realised "an old dream of science fiction"; the Nobel award ceremony speech separately compares it to the tractor beams of Star Trek.

In 1987 Ashkin trapped living bacteria for the first time without hurting them, after swapping the damaging green laser for a gentler infrared one.

This opened the door to studying biology at the level of single molecules.

Draw a focused laser beam shaped like an hourglass, narrowest at the focus point.

Show a small sphere displaced slightly below the beam's focus point, with arrows representing light rays bending as they pass through it, and a label showing the resulting force pulling the sphere back up towards the focus.

One striking later application, described in the popular information page, was using optical tweezers to track a motor protein called kinesin as it walked step by step along microtubules, the tiny tracks that make up part of a cell's internal skeleton.

This let scientists measure the stepwise movement of life's molecular machinery directly.

How does chirped pulse amplification create ultra-short, intense pulses?

Mourou and Strickland's technique, called chirped pulse amplification (CPA), solved the problem of amplifying short pulses without destroying the laser's amplifying material.

The key insight was to never let the pulse reach full peak power while it is still inside the delicate amplifier. Instead, the pulse is temporarily spread out in time, amplified safely, and then squeezed back down afterwards.

The scientific background describes the process in three steps, which can be set out as a procedure:

1. Stretch: an ultra-short laser pulse is stretched in time by a large factor, which correspondingly reduces its peak power, making it safe to amplify.
2. Amplify: the now-weaker, stretched pulse passes through an amplifying material and gains a great deal of energy without damaging that material.
3. Compress: the amplified pulse is compressed back down in time to close to its original short duration.
4. Result: because the same large amount of energy is now packed into a very short burst of time, the pulse's intensity increases dramatically.

In the original 1985 experiment, Strickland and Mourou stretched a short pulse using an optical fibre, amplified it in a glass-based amplifier, and compressed it using a pair of diffraction gratings.

The term "chirped" refers to the way the stretched pulse's frequency changes over its duration, similar to the way a bird's chirp changes pitch.

Draw three boxes in a row labelled Stretch, Amplify and Compress. In the first box, show a short spike of light becoming a long, low wave.

In the second box, show the same long wave growing taller (more energetic) without changing length. In the third box, show the wave being squeezed back into a short, very tall spike.

This technique, the press release notes, "soon became standard for subsequent high-intensity lasers". It allowed laboratories around the world, not just a few huge national facilities, to build powerful short-pulse lasers on a tabletop scale.

How did the discovery unfold?

The path to these inventions was slow and uneven. Ashkin's earliest work in 1960 simply tested what the newly invented laser could do, and it took a full decade before he could show, in 1970, that a focused laser beam really does push tiny transparent particles, confirming that radiation pressure is a usable force rather than just a theoretical curiosity.

Building a working trap took longer still. It was only in 1986 that Ashkin and his co-workers combined a single laser beam with a strong lens to make the first true optical tweezers, and the very next year, in 1987, he switched from a damaging green laser to a gentler infrared one so that living bacteria could be trapped without being killed.

Meanwhile, Strickland and Mourou's breakthrough in 1985 solved a completely different, fifteen-year problem: how to amplify short laser pulses without wrecking the amplifier. Their chirped pulse amplification method then spread rapidly, leading within fourteen years to the petawatt-class laser completed in 1999, before both lines of work were jointly honoured with the Nobel Prize in Physics in 2018.

Why does optical tweezing matter?

Optical tweezers let scientists hold, move and study the tiniest living structures without ever touching them physically, which avoids damaging delicate biological material such as cell membranes, proteins and strands of DNA.

The popular information page explains that researchers now use the technique to study single proteins, molecular motors, DNA and the inner workings of cells, and that in many laboratories laser tweezers are described as standard equipment for this kind of work.

One important early breakthrough, noted in the same source, was the ability to investigate the mechanical properties of molecular motors, the large molecules that do vital work inside cells; the first to be studied this way in detail was the motor protein kinesin as it walked along microtubules.

A more recent development mentioned in the sources is optical holography, where thousands of tweezers operate at once; one use is sorting healthy blood cells from infected ones, which could help in the fight against malaria.

The committee's own account frames this as a journey "from science fiction to practical applications", since many researchers have since adopted and refined Ashkin's original methods.

The technique's impact is still growing: the press release states that its "innumerable areas of application have not yet been completely explored", meaning new uses are still being discovered well after the original invention.

Why does chirped pulse amplification matter?

CPA made it practical to build affordable, high-powered, ultra-short pulse lasers outside huge national laboratories. One of its most widespread uses is in corrective eye surgery, where sharp, precise laser pulses reshape the cornea in millions of operations every year.

Other uses mentioned in the sources include more efficient data storage, where tiny holes are drilled deep into storage material, and the manufacture of medical stents, tiny metal cylinders that widen blood vessels.

CPA also opened a new field called attosecond physics, which studies events lasting just tens to a hundred attoseconds, fast enough to observe the motion of electrons around atoms.

Gérard Mourou went on to lead early work on the Extreme Light Infrastructure (ELI), a project with sites planned in the Czech Republic, Hungary and Romania aiming for peak powers of 10 petawatts.

The popular information page lists future hopes such as faster electronics, better solar cells and new catalysts, while cautioning that these possibilities "have not yet been fully explored".

How does this connect to what you study?

Optical tweezers rely on ideas about force and momentum that appear in basic physics: light, though it has no rest mass, still carries momentum and can exert a measurable push on matter, an idea connected to the general study of light, optics and mechanics.

The same lesson about forces balancing out also applies here: just as a book resting on a table stays still because forces balance, a particle stays trapped in optical tweezers because the forward push of light and the sideways gradient force balance at the laser's focus point.

Chirped pulse amplification uses the physics of waves, since the "chirp" is a change in a pulse's frequency over time, a concept related to how school physics treats wave frequency, wavelength and energy, and to the idea that compressing a wave in time concentrates its energy.

Both inventions also connect strongly to biology, chemistry and medicine. Optical tweezers are used to study living cells and molecular motors, directly linking physics to the cell biology taught in school science, while CPA-based lasers are used in corrective eye surgery, linking physics to human biology and health.

Together, these two inventions show how progress in one branch of science, here laser physics, routinely becomes a tool that unlocks discoveries across several other subjects at once, from medicine to materials science.

Quick facts for exams

The Nobel Prize in Physics 2018 was announced on 2 October 2018 by the Royal Swedish Academy of Sciences, for groundbreaking inventions in laser physics.

Arthur Ashkin of Bell Laboratories, USA, received one half for inventing optical tweezers, a tool that uses focused laser light to trap and manipulate tiny particles and living cells.

Gérard Mourou of École Polytechnique, France, and the University of Michigan, USA, and Donna Strickland of the University of Waterloo, Canada, shared the other half for inventing chirped pulse amplification, a method for creating extremely short, intense laser pulses by stretching, amplifying and then compressing them. The total prize amount was 9,000,000 Swedish kronor.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Laser — a device that produces a narrow, single-coloured, highly focused beam of light through stimulated emission.
• Radiation pressure — the tiny physical push that light exerts on matter because light carries momentum.
• Optical tweezers — a technique using a focused laser beam to trap and move microscopic particles or cells without touching them.
• Gradient force — the sideways force that pulls a particle towards the most intense part of a laser beam.
• Chirped pulse amplification (CPA) — a method of safely amplifying a laser pulse by stretching it in time, amplifying it, then compressing it again.
• Chirp — a change in a light pulse's frequency over the duration of the pulse, similar to a changing pitch.
• Femtosecond — one millionth of a billionth of a second, a common timescale for ultra-short laser pulses.
• Attosecond — about a thousand times shorter than a femtosecond, a timescale short enough to observe electron motion.
• Petawatt — a unit of power equal to one quadrillion watts, used to describe extremely intense laser pulses.
• Kinesin — a motor protein that transports material inside cells by walking along microtubules.
• Microtubule — a thin, tube-shaped structure that forms part of a cell's internal skeleton and acts as a track for motor proteins.
• LASIK — a laser-based eye surgery procedure that reshapes the cornea to correct vision problems.

Common errors and misconceptions

• Misconception: Optical tweezers physically grip objects like tiny mechanical pincers. Correct: They use only the pressure of focused light, with no physical contact at all.
• Misconception: Radiation pressure from ordinary sunlight is strong enough to feel as a push. Correct: The sources state we feel sunlight's warmth, not any push, because the pressure is far too weak to notice.
• Misconception: Chirped pulse amplification directly amplifies a short, intense pulse. Correct: It first stretches the pulse to lower its peak power, then amplifies it safely, then compresses it again.
• Misconception: Ashkin's prize was only for a physics instrument with no biological use. Correct: His citation specifically includes "their application to biological systems", and he personally studied bacteria and cells.
• Misconception: Mourou and Strickland each received a separate, independent prize. Correct: They shared one quarter of the prize each under the same joint citation for one method.
• Misconception: The whole prize was split equally three ways. Correct: Ashkin received one half alone, while Mourou and Strickland shared the other half, one quarter each.

Exam-style questions with model answers

Q1. State the official citation for the Nobel Prize in Physics 2018. [2 marks]

1. The citation was "for groundbreaking inventions in the field of laser physics", covering two inventions: optical tweezers and chirped pulse amplification.

Q2. Name the three laureates of the Nobel Prize in Physics 2018 and their shares. [2 marks]

1. Arthur Ashkin received one half of the prize, while Gérard Mourou and Donna Strickland shared the other half, one quarter each.

Q3. Explain how optical tweezers trap a particle. [4 marks]

1. Optical tweezers use a focused laser beam to trap a tiny particle in place. Radiation pressure, the push exerted by light carrying momentum, acts on the particle in two ways. First, the beam pushes the particle forward along its direction of travel. Second, because light is more intense at the centre of the beam, an uneven gradient force pulls the particle sideways towards that brighter centre. By focusing the beam strongly with a lens, these forces balance at a single point, the focus, so the particle becomes trapped there without any physical touch, held only by light.

Q4. Describe the three steps of chirped pulse amplification. [4 marks]

1. First, an ultra-short laser pulse is stretched out in time, which lowers its peak power. Second, this stretched, lower-power pulse is passed through an amplifying material and given much more energy, without destroying that material because its peak power stayed low. Third, the amplified pulse is compressed back down close to its original short duration. Because the same large amount of energy is now packed into a very short time, the pulse's intensity becomes extremely high.

Q5. Discuss the applications and significance of the two inventions honoured by the Nobel Prize in Physics 2018. [6 marks]

1. Optical tweezers allow scientists to hold and move microscopic objects, including living cells, bacteria and viruses, without physically touching them, which avoids damage. This has let researchers study biological machinery at the molecular level, such as tracking the motor protein kinesin as it steps along microtubules inside cells, and more recent holographic versions use thousands of tweezers at once, for example to sort healthy from infected blood cells, with possible use against malaria. Chirped pulse amplification solved the problem of amplifying short laser pulses without destroying the laser's own material, by stretching, amplifying and then compressing the pulse. This became the standard method for building high-intensity lasers and enabled millions of corrective eye surgeries every year, more precise data storage, manufacture of medical stents, and new research fields such as attosecond physics, which studies electron motion on extremely short timescales. Both inventions turned once impossible science-fiction ideas, using light to move objects and creating super-intense pulses, into practical tools with everyday medical and industrial uses, though the sources note that many further applications remain unexplored.

Q6. When was the Nobel Prize in Physics 2018 announced, and by which body? [2 marks]

1. It was announced on 2 October 2018 by the Royal Swedish Academy of Sciences.

Q7. What breakthrough did Ashkin achieve in 1987, and why was it important? [3 marks]

1. In 1987, Ashkin used optical tweezers to capture living bacteria without harming them, after switching from a damaging green laser to a gentler infrared beam. This showed that optical tweezers could be safely used to study living biological systems, not just inert particles, opening an entirely new area of research into cells and molecular motors.

Q8. What practical medical use does chirped pulse amplification have today? [2 marks]

1. Chirped pulse amplification underpins corrective eye surgeries, such as LASIK, which use ultra-precise, high-intensity laser pulses to reshape the cornea and correct vision problems.

Key takeaways

• The Nobel Prize in Physics 2018 was awarded for groundbreaking inventions in laser physics, split between optical tweezers and chirped pulse amplification.
• Arthur Ashkin received one half of the prize for inventing optical tweezers and applying them to biological systems.
• Gérard Mourou and Donna Strickland shared one half of the prize, one quarter each, for chirped pulse amplification.
• Optical tweezers trap particles using radiation pressure, the tiny push that focused laser light exerts on matter.
• Chirped pulse amplification works by stretching a pulse, amplifying it safely, then compressing it back for very high intensity.
• Ashkin's 1987 success in trapping living bacteria opened optical tweezers to wide use in biology.
• Chirped pulse amplification enabled widespread corrective eye surgeries and gave rise to attosecond physics.
• Both inventions turned once-impossible ideas, light moving objects and super-intense pulses, into practical tools.

Test yourself

What does the official citation for the Nobel Prize in Physics 2018 say?

It says "for groundbreaking inventions in the field of laser physics", covering optical tweezers and chirped pulse amplification.

Where was Arthur Ashkin affiliated at the time of the award?

Arthur Ashkin was affiliated with Bell Laboratories in Holmdel, NJ, USA, at the time of the award.

What force allows optical tweezers to trap a particle?

Optical tweezers use radiation pressure, the push that focused laser light exerts, including a gradient force pulling particles towards the brightest point.

What are the three steps of chirped pulse amplification?

A laser pulse is stretched in time, amplified safely at low peak power, then compressed back down to achieve very high intensity.

What did Ashkin achieve in 1987?

Ashkin used optical tweezers to capture living bacteria without harming them, by switching to a gentler infrared laser beam.

Name one everyday medical use of chirped pulse amplification.

Chirped pulse amplification enables corrective eye surgeries, such as LASIK, using ultra-precise laser pulses to reshape the cornea.

How did Ashkin's optical tweezers help study molecular motors?

They let researchers track the motor protein kinesin stepping along microtubules inside cells, revealing details of its stepwise motion.

What field of research opened up because of ultra-short, intense laser pulses?

Attosecond physics opened up, allowing scientists to study extremely fast events such as the motion of electrons around atoms.