- The 2018 Nobel Prize for Physics has been awarded to Arthur Ashkin, Gérard Mourou and Donna Strickland for their groundbreaking inventions in the field of laser physics.
- Donna Strickland of Canada became the first woman to win the Nobel Physics Prize in 55 years and the third woman ever to win a Nobel physics prize.
- Besides Arthur Ashkin of the USA became the oldest Nobel Laureate.
- While Ashkin won the the prize for ‘Optical tweezers and its application in biology’, Strickland and Mourou won the prize for their joint effort in devising a technique called ‘Chirped-pulse amplification’.
Optical Tweezers and its applications in biology
- Ashkin’s discovery was that a laser beam can act as Optical tweezers.
- Optical tweezers basically involves trapping of extremely small particles in the order of sizes of tens of microns to tens of nanometers.
- Thus optical tweezers have applications in modifying and manipulating extremely small organisms like viruses, bacteria, besides individual cells and atoms.
- Thus optical tweezers significant to biophysicists, who measure the forces involved in biological processes inside a living cell.
- They are also used to study how forces affect large biological molecules such as DNA.
How does it work?
- It is seen that the forces generated by laser beams can trap tiny dielectric particles in air or water.
- The scattering of light pushes the particles in the direction of beam propagation.
- Thus if two beams are propagating in the opposite direction they will stop a particle from moving along the axis of propagation.
- When a laser is propagated in a medium with particles like (cells in aqueous solution) the particle with a greater refractive index was drawn to the centre of the beam creating an effect called optical trap.
- Further this optical trap effect was seen even when instead of two beams only one laser beam was propagated.
- This came to be known as optical tweezers capable of trapping particles ranging in size from tens of nanometres to tens of microns.
- As mentioned above Strickland and Mourou won the prize for their joint effort in devising a technique called ‘Chirped-pulse amplification’.
- This technique is widely used by physicists in creating high-energy laser pulses.
- Thus CPA currently lies at the heart of most high-powered laser facilities in the world.
Significance of CPA
- Conventionally when a laser beam is amplified from a nano joule to petawatt energy level, it lost the optical property.
- That is the laser becomes non-linear.
- This simply means to amplify the laser beam to high intensity the length of the laser also had to be increased. (Non-Linear)
- Thus to keep the intensity of laser pulses without increasing its length, laser systems had to be very large and expensive.
- This problem was solved by CPA.
- CPA involves amplification of the pulse of laser beam from low-energy to high-energy pulse.
- Using CPA, the scientists created a short but high- intensity laser pulses.
- CPA picks a laser beam from a laser source with a small amount of energy about a nanojoule.
- This is amplified to a factor of about 10^12 to get a high-powered petawatt beam without producing non-linear effects.
- This keeps the optical property of the laser intact even when amplified to high-energy beams.
- Applications of CPA include laser eye surgery and laser micro-machining where high-energy laser beams are used.