2
S. L. Chin
end of the decay of a filament. In this paper, the author will follow the physical
evolution of a filament from the beginning to the end while briefly pointing out a
limited number of applications or phenomena along the way without going into any
detail. It is not meant to be complete. Hopefully, this would stimulate new ideas
for new physics and applications. Filamentation starts with ‘instantaneous’ quantum
mechanical ionization/excitation/dissociation in the fs time scale followed by a fast
decay in the picosecond and nanosecond time scale. Classical thermodynamic decay
would then follow till the end when the medium comes back to the initial condition.
It is assumed that the readers are familiar with the physical evolution of a filament
excited by a fs Ti-sapphire laser pulse in atmospheric pressure air [2–7, 10].
1.2 High Intensity Short Pulse Interaction
Self-focusing of a powerful short laser pulse in air (transparent medium) would in
principle keep on self-focusing into a geometrical singularity if there were no other
physical mechanism that would defocus the pulse. If this were the case, we could
imagine that the focal area would be very small. Furthermore, we knew that we
could practically put in a lot of energy into a laser pulse by using laser amplifiers.
This would make the intensity inside the self-focal zone extremely strong, so strong
that we could imagine it would become a formidable ‘almighty bullet’ capable of
piercing through any material at a distance. Since there seemed to be no theoretical
limit in putting as much energy as possible into the laser pulse, even nuclear reaction could be envisioned on the target. Such was the naïve thought (dream) of the
author at the beginning of the ‘filamentation era’. He even ‘generously’ gave out this
‘great original’ idea to some applied scientists who were eager to get new exciting
ideas, brought them home and executed them quickly. Unfortunately, life was more
complicated. Not long after, the author realized that the intensity was clamped at the
self-focal zone [11]. The clamped intensity in the filament core was limited to about
5 × 10
13 W/cm
2 [11] for a free propagating pulse or a few times 10
14 W/cm
2 if an
external lens was used [12].
Although the author’s dream was broken, the intensity inside the filament core
was still very respectable because it could ionize even nitrogen molecules whose
ionization potential was rather high (15.6 eV). In fact, it is because of such ionization
of the oxygen and nitrogen molecules that gives rise to a plasma. This plasma,
having a negative refractive index, would defocus the laser pulse or slow down the
self-focusing. Further self-focusing would increase the intensity and would result in
more ionization, hence, higher plasma density or stronger de-focusing. The plasma
defocusing effect would eventually stop the self-focusing effect. At this balancing
point, the spot size is minimum and the intensity maximum. Further self-focusing
would increase the plasma density which would now defocus the pulse or enlarge
the spot size resulting in a lower intensity. Because the laser pulse has a power
distribution in the temporal domain or spatial distribution along the propagation
direction, using the slice-by-slice self-focusing model [7], each slice of the front part
S. L. Chin
end of the decay of a filament. In this paper, the author will follow the physical
evolution of a filament from the beginning to the end while briefly pointing out a
limited number of applications or phenomena along the way without going into any
detail. It is not meant to be complete. Hopefully, this would stimulate new ideas
for new physics and applications. Filamentation starts with ‘instantaneous’ quantum
mechanical ionization/excitation/dissociation in the fs time scale followed by a fast
decay in the picosecond and nanosecond time scale. Classical thermodynamic decay
would then follow till the end when the medium comes back to the initial condition.
It is assumed that the readers are familiar with the physical evolution of a filament
excited by a fs Ti-sapphire laser pulse in atmospheric pressure air [2–7, 10].
1.2 High Intensity Short Pulse Interaction
Self-focusing of a powerful short laser pulse in air (transparent medium) would in
principle keep on self-focusing into a geometrical singularity if there were no other
physical mechanism that would defocus the pulse. If this were the case, we could
imagine that the focal area would be very small. Furthermore, we knew that we
could practically put in a lot of energy into a laser pulse by using laser amplifiers.
This would make the intensity inside the self-focal zone extremely strong, so strong
that we could imagine it would become a formidable ‘almighty bullet’ capable of
piercing through any material at a distance. Since there seemed to be no theoretical
limit in putting as much energy as possible into the laser pulse, even nuclear reaction could be envisioned on the target. Such was the naïve thought (dream) of the
author at the beginning of the ‘filamentation era’. He even ‘generously’ gave out this
‘great original’ idea to some applied scientists who were eager to get new exciting
ideas, brought them home and executed them quickly. Unfortunately, life was more
complicated. Not long after, the author realized that the intensity was clamped at the
self-focal zone [11]. The clamped intensity in the filament core was limited to about
5 × 10
13 W/cm
2 [11] for a free propagating pulse or a few times 10
14 W/cm
2 if an
external lens was used [12].
Although the author’s dream was broken, the intensity inside the filament core
was still very respectable because it could ionize even nitrogen molecules whose
ionization potential was rather high (15.6 eV). In fact, it is because of such ionization
of the oxygen and nitrogen molecules that gives rise to a plasma. This plasma,
having a negative refractive index, would defocus the laser pulse or slow down the
self-focusing. Further self-focusing would increase the intensity and would result in
more ionization, hence, higher plasma density or stronger de-focusing. The plasma
defocusing effect would eventually stop the self-focusing effect. At this balancing
point, the spot size is minimum and the intensity maximum. Further self-focusing
would increase the plasma density which would now defocus the pulse or enlarge
the spot size resulting in a lower intensity. Because the laser pulse has a power
distribution in the temporal domain or spatial distribution along the propagation
direction, using the slice-by-slice self-focusing model [7], each slice of the front part