7.24 Laser Applications
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field might confine a particle. According to Eq. (7.22), there will be forces on a
particle in an electric field from weaker field regions toward the direction of the
strongest electric field. By sharply focusing a laser beam, the electric field intensity
will have a peak at the focal point. As long as the rate of momentum transfer from
the beam to the particle is less than the gradient force, the particle will be trapped
at the laser focal point. Later, he and his colleagues showed how to use laser beams
as optical tweezers, i.e. using the beam to trap and move micro and nano-sized
particles. Ashkin received a Nobel Prize in Physics (2018) for his work. He shared
the prize with two other physicists working on pulsed lasers, with wave packets
highly confined in time. (This idea and a few of its applications will be described in
the next section.)
As an example of a biophysical application, optical tweezers are now used to
make measurements of the mechanical force needed to stretch, bend, and twist
macromolecules. We now can see the jerking force of attachment and detachment of cell cargo-transporting kinesin proteins as they walk across microtubule
filaments.
7.24.2 Laser Pulse Amplification
Half of the 2018 Nobel Prize in Physics went to Donna Strickland and Gérard
Mourou, who developed, in the 1980s, a practical method of generating highintensity, ultrashort laser pulses, known as ‘chirped pulse amplification’ (CPA).
Very high intensity laser pulses (with a power of more than 100 TW), using
laser amplifiers, can be constructed, but the apparatus is large and expensive.
The main difficulty is that laser amplification to intensities above 700 gigawatts
per square centimeter reach the non-linear region of the laser amplifier, causing
pulse compression and destruction of the amplifier material. Strickland and Mourou
realized that if the laser pulse is first spread out over time according to the frequency
components of the pulse, the resulting wave pulse would contain the same total
energy but be much less intense. Dispersion devices can spread pulses by a factor of
up to 100,000. After spreading, laser amplifiers act on the pulse, and then an inverse
dispersion is performed to resharpen the pulse. All this with a ‘tabletop’ apparatus
which can produce a terawatt light pulse with a femtosecond width.
Because a CPA enhanced laser beam can be so precisely focused with high
intensities, it has enabled Lasik eye surgery, a procedure for correcting vision by
reshaping the cornea of the eye. CPA technology produces clearer and sharper
remote spectroscopy of atmospheric gases than previously available, and lets us
more easily study the non-linear optics of materials. The prospect for laser-blasted
detonation of fusion pellets as an energy source is brighter.
253
field might confine a particle. According to Eq. (7.22), there will be forces on a
particle in an electric field from weaker field regions toward the direction of the
strongest electric field. By sharply focusing a laser beam, the electric field intensity
will have a peak at the focal point. As long as the rate of momentum transfer from
the beam to the particle is less than the gradient force, the particle will be trapped
at the laser focal point. Later, he and his colleagues showed how to use laser beams
as optical tweezers, i.e. using the beam to trap and move micro and nano-sized
particles. Ashkin received a Nobel Prize in Physics (2018) for his work. He shared
the prize with two other physicists working on pulsed lasers, with wave packets
highly confined in time. (This idea and a few of its applications will be described in
the next section.)
As an example of a biophysical application, optical tweezers are now used to
make measurements of the mechanical force needed to stretch, bend, and twist
macromolecules. We now can see the jerking force of attachment and detachment of cell cargo-transporting kinesin proteins as they walk across microtubule
filaments.
7.24.2 Laser Pulse Amplification
Half of the 2018 Nobel Prize in Physics went to Donna Strickland and Gérard
Mourou, who developed, in the 1980s, a practical method of generating highintensity, ultrashort laser pulses, known as ‘chirped pulse amplification’ (CPA).
Very high intensity laser pulses (with a power of more than 100 TW), using
laser amplifiers, can be constructed, but the apparatus is large and expensive.
The main difficulty is that laser amplification to intensities above 700 gigawatts
per square centimeter reach the non-linear region of the laser amplifier, causing
pulse compression and destruction of the amplifier material. Strickland and Mourou
realized that if the laser pulse is first spread out over time according to the frequency
components of the pulse, the resulting wave pulse would contain the same total
energy but be much less intense. Dispersion devices can spread pulses by a factor of
up to 100,000. After spreading, laser amplifiers act on the pulse, and then an inverse
dispersion is performed to resharpen the pulse. All this with a ‘tabletop’ apparatus
which can produce a terawatt light pulse with a femtosecond width.
Because a CPA enhanced laser beam can be so precisely focused with high
intensities, it has enabled Lasik eye surgery, a procedure for correcting vision by
reshaping the cornea of the eye. CPA technology produces clearer and sharper
remote spectroscopy of atmospheric gases than previously available, and lets us
more easily study the non-linear optics of materials. The prospect for laser-blasted
detonation of fusion pellets as an energy source is brighter.