7.24 Laser Applications
251
will make an average intensity proportional to NA 2 . The ability to make highly
intense laser beams relies on the extra factor of N in the generation of the light. 39
Because optical lasers use parallel reflecting surfaces, the light produced have
parallel wave fronts over the extent of the surfaces. (This light is released by having
one of the reflecting surfaces only partially reflecting, such as is a half-silvered
mirror.) Being parallel, the laser beam comes out of the laser as a well-defined
columnar beam. Such a wave can be easily focused by a converging lens to a region
about the size of the wavelength of the light. The focused beams can have intensities
exceeding 10 24 W/cm 2 . 40 For the safety of the human retina, laser pointers are not
to have beam power greater than 5 mW, which is about 80 mW/cm 2 over the beam
area. The intensity on the retina can be much larger than that in the laser beam, as
the lens of the eye can focus the beam.
7.24 Laser Applications
Because of the ability to sharply focus laser beams and because the beam can be
made intense, lasers can be used for such purposes as laser-induced photochemistry;
laser scanning microscopy; laser ablation molecular spectroscopy; laser surgical
cauterizing and cutting; laser trabeculoplasty (treatment of glaucoma); laser ablating
of tooth decay, tattoos, sunspots, moles, and melanomas; laser lithotripsy (breaking up hardened substances); laser angioplasty, laser neurological surgery; laser
attachment of retinae; laser microsurgery; laser subcutaneous imaging; medical
holograms; and minor soft tissue imaging using the near infrared (NIR) ‘window’.
7.24.1 Optical Tweezers
It is now possible to use focused laser beams to trap, move, and manipulate cells,
molecules, and even individual atoms.
The idea is this: Cells, molecules, and atoms all have internal charges. In an
electromagnetic field with a strong gradient, these particles, even though neutral
overall, will have a net force on them. We can see this from the Lorentz force law
and Maxwell’s equations. The mechanical force on a neutral particle to first order in
the field gradients will be (in the cgs system of units)
39 Consider the sum:
i A cos (φ i (t))
2
, averaging over time. If the phases φ have no correlation, only the square cosine terms will survive the average, so the result becomes (1/2)N A 2 .
However, if the phases are all the same, the ‘direct’ and the ‘cross terms’ of one cosine with
another add together, making (1/2)(N A) 2 = (1/2)N 2 A 2 .
40 Sunlight at ground level has an intensity of about 50 mW/cm 2 .
251
will make an average intensity proportional to NA 2 . The ability to make highly
intense laser beams relies on the extra factor of N in the generation of the light. 39
Because optical lasers use parallel reflecting surfaces, the light produced have
parallel wave fronts over the extent of the surfaces. (This light is released by having
one of the reflecting surfaces only partially reflecting, such as is a half-silvered
mirror.) Being parallel, the laser beam comes out of the laser as a well-defined
columnar beam. Such a wave can be easily focused by a converging lens to a region
about the size of the wavelength of the light. The focused beams can have intensities
exceeding 10 24 W/cm 2 . 40 For the safety of the human retina, laser pointers are not
to have beam power greater than 5 mW, which is about 80 mW/cm 2 over the beam
area. The intensity on the retina can be much larger than that in the laser beam, as
the lens of the eye can focus the beam.
7.24 Laser Applications
Because of the ability to sharply focus laser beams and because the beam can be
made intense, lasers can be used for such purposes as laser-induced photochemistry;
laser scanning microscopy; laser ablation molecular spectroscopy; laser surgical
cauterizing and cutting; laser trabeculoplasty (treatment of glaucoma); laser ablating
of tooth decay, tattoos, sunspots, moles, and melanomas; laser lithotripsy (breaking up hardened substances); laser angioplasty, laser neurological surgery; laser
attachment of retinae; laser microsurgery; laser subcutaneous imaging; medical
holograms; and minor soft tissue imaging using the near infrared (NIR) ‘window’.
7.24.1 Optical Tweezers
It is now possible to use focused laser beams to trap, move, and manipulate cells,
molecules, and even individual atoms.
The idea is this: Cells, molecules, and atoms all have internal charges. In an
electromagnetic field with a strong gradient, these particles, even though neutral
overall, will have a net force on them. We can see this from the Lorentz force law
and Maxwell’s equations. The mechanical force on a neutral particle to first order in
the field gradients will be (in the cgs system of units)
39 Consider the sum:
i A cos (φ i (t))
2
, averaging over time. If the phases φ have no correlation, only the square cosine terms will survive the average, so the result becomes (1/2)N A 2 .
However, if the phases are all the same, the ‘direct’ and the ‘cross terms’ of one cosine with
another add together, making (1/2)(N A) 2 = (1/2)N 2 A 2 .
40 Sunlight at ground level has an intensity of about 50 mW/cm 2 .