Prelims cum Mains Science & Tech

Laser pioneers win Physics Nobel

The News

  • 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.

 

Key Highlights

  • 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.

 

 

Chirped-pulse amplification

  • 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.

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