There’s
no Eureka Moment Indeed: Nobel Prize in Physics 2018
‘When I described catching living
things with light people said: Don’t exaggerate’ – Arthur Ashkin said during a
telephone interview to Nobel committee after he was awarded half of the Nobel
prize in Physics this year. Really, it seems a kind of science fiction to use
light to move objects, but it is true. In his 1970’s famous paper – Acceleration
and trapping of particles by radiation pressure – published in Physical Review
Letters, Ashkin demonstrated, for the first time, the movement of
freely suspended microparticles using optical force generated from a continuous
wave visible laser light. This was a breakthrough experiment which opened doors
for atoms, molecules and even living cells which can be selectively trapped and
manipulated by optical force. Later, in 1987, he came up with another report of
optical trapping and manipulation of viruses and bacteria using an argon laser,
which he published in Science. Tobacco mosaic viruses and Escherichia coli
bacteria were trapped in aqueous solution without any damage. This work was
further extended to single cells of yeasts. With time, this innovative approach
of manipulating objects and microorganisms, popularly known as ‘optical
tweezers’, was modified and used by physicist, chemists and biologists, making
it the most versatile tool for microfabrication. Nowadays, multiple laser beams
are used for the three-dimensional optical trapping of materials. Recently,
Biju lab in Hokkaido University (http://bijulab.main.jp/en/)
demonstrated for the first time the use of optical tweezers in trapping the
solvated ion complex of salts inducing crystallization and growth of lead
halide perovskites. Interesting! isn’t it? So, what makes light to levitate and
move objects? Let’s have a look.
Principle of laser
trapping
Optical force, usually in the range
of 1 to 100 pN, arises by a net momentum change of light when a tightly focused
laser beam interacts with a particle whose diameter is larger than the
wavelength of the laser. The change in momentum of light corresponds to the change
in refractive index (between the medium and the particle) when light is refracted
from the surface of a particle. When the refractive index of the particle is
greater than the surrounding medium, an equal and opposite momentum change is
exerted on the particle satisfying the law of conservation of momentum. When we
augment all the momentum change at each point of the particle we get the
optical force. This force is resolved into two components: axial force (Faxial)
and transaxial force (Ftrans). When Faxial > Ftrans,
the net optical force (F) is directed towards the high intensity region of the
laser beam, thus pulling the particles and trapping them at the focal spot of
the laser. On the other hand, When Ftrans > Faxial, the
particle is accelerated in the direction of propagation of the beam. However, when the laser beam is turned off,
the effect vanishes, and the particles again move randomly.
Now, perhaps, the question arises in everyone’s mind:
What’s the use of this tool? Answer is many!! A few is elaborated here. Optical
tweezers can be used to trap atoms or molecules and
Applications of laser trapping
technique or optical tweezer
bring them together so
that selective chemical reactions can take place. It can be used for the
microfabrication of microbeads in desired structure which can have many
applications. For example, such directed assembly of microcrystals of
semiconductors can be utilized in solar cells and LEDs. However, it is
necessary to glue these structures together before the laser light is turned
off. For this, we can utilize click chemistry, for instance, biotin-avidin
linkage or photodimerization of surface ligands. However, the story behind the
Nobel prize for optical tweezers is a bit different than only catching
microparticles. It was mainly awarded for the non-destructive application of
laser trapping in biology. Ashkin gave an idea about just what can be done with
this simple technique of manipulating objects without touching them and
provided scientists with bounty of hopes. Nowadays, optical tweezers are used
to study single proteins and enzymes such as tracking the activity of
individual enzymes, measuring the force needed to unfold a single protein
molecule, or determine strength of binding between individual protein molecules,
movement of molecular motors, and DNA studies. It is also used to sort healthy
cells from the infected ones such as from cancer cells. Kudos to such a
magnificent work for the betterment of mankind and hope for more in this area.
This
is one half of the story. Moving on to next is Chirped Pulse Amplification
(CPA) which shared half of the Nobel prize with optical tweezers. This half of
Nobel prize is more special because it was after more than 50 years that a woman
was awarded in physics, and third in the list after Marie Curie (1903) and Maria
Goeppert-Mayer (1963). The prize is shared between Gérard Mourou, a male French
physicist and his former graduate student Donna Strickland, a female Canadian
physicist. It won’t be over exaggerating if we say their work was
groundbreaking that revolutionized the laser technology, since all the latest
high-intensity laser application foots on CPA which has opened a high hope in
the field of physics, chemistry and medicine. Previously, laser pulses could
not be amplified much, and the output pulse energy had to be kept much below
the saturation point, which is in the range of few millijoules to tens of Joule
per square centimeter, to avoid any damages to the amplifier and the lasing/amplifying
crystal by laser self-focusing. This limit was broken by the introduction of
CPA in 1985 by Mourou and Strickland.
Principle of Chirped Pulse Amplification (CPA)
The technique is based on the stretching,
amplifying and compressing of ultra-short laser pulses. When a laser pulse is
stretched in time, its peak power decreases. As a result, it can be magnificently
amplified without any damage to the amplifier or the lasing crystal. The pulse
can then be squeezed again in time within a small area such that the output is
a laser pulse with ultra-high intensity or peak power. This technique has made possible
to create the most intense pulse ever in a laboratory. Some applications of
this technique are photonic-assisted time-stretched analog-to-digital
conversion, serial time-encoded amplified microscope, time lens processing and
microwave signal analyzer, frequency domain reflectometry, chirped pulse lidar,
and time domain pulse shaping. For general understanding, these applications
seem to be quite difficult, isn’t it? No worries! There is one simple
application of CPA technique in the field of medicine, that is eye surgery. Such
ultra-short laser beams with high intensity can be used to drill through the
delicate living membranes making eye operations efficient. Also, CPA technique
can be used to study ultrafast processes that happens in the time scale of hundreds
of attoseconds, such as those involving electron motions in atoms and molecules.
As said
by Gérard Mourou that nobody is prepared for that moment, science is full of
surprises and so is time. Still there are many more to come; the only thing is
we should keep on doing and advancing step-by-step. Let’s do science for the
betterment of our earth, for our present and for our future; and leave the fate
of prizes and awards to time.
The blog is based on
following references:
b) Ashkin, A. Acceleration
and Trapping of Particles by Radiation Pressure. Phy. Rev. Lett. 1970,
24, 156-159.
c) Ashkin, A.; Dziedzic,
J. M. Optical Trapping and Manipulation of Viruses and Bacteria. Science
1987, 235, 1517-1520.
d) Ashkin, A.; Dziedzic,
J. M.; Yamane, T. Optical Trapping and Manipulation of Single Cells using Infrared
Laser Beams. Nature 1987, 330, 769-771.
e) Yuyama et al.
Crystallization of Methylammonium Lead Halide Perovskites by Optical Trapping. Angew.
Chem. 2018, 130, 13612-13616.
f) Misawa et al. Laser Manipulation and
Assembling of Polymer Latex Particles in Solution. Macromol. 1993,
26, 282-286.
g) Ghadiri, R.; Weigel,
T.; Esen, C.; Ostendorf, A. Microfabrication by Optical Tweezers. Proc. of SPIE
2011, 7921, 792102.
h) Strickland, D.; Mourou,
G. Compression of Amplified Chirped Optical Pulses. Opt. Commun. 1985,
55, 447-449.
i)
Delfyett et al.
Chirped Pulse Laser Sources and Applications. Prog. Quantum Electron.
2012, 36, 475-540.
