FEATURE ARTICLE
Filaments of Light
Pulsed terawatt lasers create some surprising effects when shone through the air—including the channeling of light
Jérôme Kasparian
Let There Be (White) Light
Two separate physical phenomena account for the strange filamentary
propagation of high-power laser light. The first is self-focusing,
which comes about because the refractive index of air depends on the
intensity of light passing through it, a phenomenon known as the
optical Kerr effect. As a consequence, when one sends high-power
laser light through the atmosphere, the center of the beam (where
light intensity is highest) passes through gas that has a higher
refractive index than the air located just off axis. The result is
the same as if you had shot the beam through a convex lens, which
has more glass (with its high refractive index) at the center than
at the margins. Physicists refer to this configuration, sensibly
enough, as a "Kerr lens." For a laser with an
800-nanometer wavelength operating in air, a Kerr lens develops
whenever the beam power exceeds a few gigawatts.
The more a laser is focused by such a Kerr lens, the higher the
intensity becomes. And as the intensity rises, the focusing gets
even stronger, boosting the intensity of light still further.
Eventually, something has to give—and it does. When the light
intensity reaches somewhere between 1013 and
1014 watts per square centimeter, a nonlinear process
called multiphoton ionization comes into play. The oxygen
and nitrogen molecules in air are then able to absorb many photons
at once, stripping electrons from their parent atoms, forming a plasma.
Although Kerr-lens focusing and the ensuing creation of plasma
could, in theory, be brought about using a laser that operates
continuously at extreme power levels, in practice, it proves much
easier to achieve the necessary oomph using short bursts—the
shorter the better. Common sense explains why: For a laser pulse of
a given energy, the more limited the duration, the higher the peak
intensity. So with the laser's energy concentrated in a brief pulse,
the focusing effect is strong even though the average power in the
beam is modest.
There's a second reason to use very short bursts: The ability of a
laser to ionize the air remains high, but the average density of
electrons created is low, allowing the beam to propagate through
them. (Electron density will be relatively low when the pulses are
too short to shoot the released electrons into nearby gas molecules,
releasing more electrons, which then would bash into other molecules
and so forth in a process called cascade ionization.)
Electrons are present in sufficient numbers, however, to decrease
the refractive index of the air containing them, which results in
the equivalent of a diverging lens and tends to defocus the beam.
Either phenomenon considered alone—the focusing of a Kerr lens
or the defocusing induced by the electrons in a bleb of
plasma—would prevent high-power laser pulses from propagating
very far through the air. But it turns out that the two opposing
effects can be made to balance, allowing the beam to travel over
large distances without either diverging or collapsing. Instead, the
energy is channeled along a narrow filament of light.
What happens when the intensity of laser light used is turned up
higher than the critical value for filamentation to begin? You might
guess that the light filament formed would become thicker and
thicker, perhaps to the point of being better described as a
"light rope." But that is not what happens. Instead,
several localized filaments emerge. That is, hiking the peak power
of the laser pulses that are applied increases the number of
filaments that result without notably influencing the individual
intensity or the energy each filament carries.
Whether present singly or in bunches, these threadlike shafts of
light exhibit another surprising property as well: Even though the
laser used to create them produces essentially monochromatic light,
each filament contains a broad range of wavelengths—what
students of optics call "a white-light supercontinuum."
The transformation into white light is easy enough to understand
once you realize that a pulsed laser doesn't instantly switch on and
off. Rather, the oscillatory electric and magnetic fields carried in
each pulse gradually build to a maximum intensity and then diminish.
That property alone explains some of the spectral
broadening—basic physics dictating that the bandwidth of a
pulse can be no less than the reciprocal of its duration. But that
principle explains only a small part of the whitening effect. More
important is the fact that the refractive index of the air
containing the pulse is proportional to the intensity of the light.
So where the intensity of light is highest (in the middle of the
pulse), so is the refractive index, which causes the highest
intensity light waves to be retarded with respect to the lower
intensity waves that travel ahead and behind. The result is a
distortion to the pulse envelope and the creation of light that
contains both longer and shorter wavelengths than what the laser
itself puts out. The range of different wavelengths that arise from
this and other nonlinear effects makes the illumination essentially white.
As if the existence of narrow filaments and their ability to
generate white light weren't bizarre enough, another surprising
phenomenon has been found to take place: A significant part of the
white-light supercontinuum appears to be emitted backward! This
back-directed light is the result of partial self-reflection of the
forward-traveling beam, which experiences changes in refractive
index along the axis of the filament as a result of the focusing and
defocusing taking place. And just like with the beam of a flashlight
shone on a double-glazed window, each change in refractive index
produces a partial reflection.
» Post Comment