6
S. L. Chin
W/cm
2 while the clamped intensity inside the filament core in air is too low (5 ×
10
13 W/cm
2 ). Most of the photon energies being absorbed would go to the electrons
for ionization. At room temperature, the average kinetic energy of the molecules is
in the range of meV while the energy for molecular bonding is of the order of eV.
Consequently, if the initial states of both N and H were in the ground state, the two
atoms would not be able to react or bond up together. One of them would have to be
in an excited state. This means that the final product molecule would also be in an
excited state. If this excited state were a radiative state, fluorescence would occur.
The lower state of the transition being not prepared during the formation process,
the system would naturally be inverted. Now if the collision gave rise to NH
+ ions,
the collision with electrons inside the filament would result in a neutral NH in the
Rydberg states which would cascade decay to a more stable excited state. If this
excited state were radiative, it would fluoresce and decay to the lower state which
initially was empty, hence population inversion. It is thus imaginable that similar
population inversion could be observed in many other similar interaction involving
small molecules in a filament.
When more complex molecules such as hydrocarbon gases CH 4 , C 2 H 2 and C 2 H 4
are mixed in an air filament, amplified spontaneous emission, hence, population
inversion, was observed in the fluorescing fragment CH [23]. This was explained as
being due to ‘instantaneous’ excitation of the molecules into the super-excited states
[24, 25] followed by very fast decay (fs time scale [26, 27]) into many channels, one
of them being dissociation into neutrals one of them being excited. The super-excited
states are believed to be excited through population trapping in a strong laser field in
our recent analysis [28–30]. Energetically speaking, the fluorescing neutral fragments
were prepared in the excited states after the breakup of the ‘hot’ parent molecules.
The lower state of the radiative transition should be empty; i.e. population inversion
was prepared ‘naturally’. Reference [28] gives a review of this phenomenon.
1.5 Intensity Clamping, Self-mode Cleaning and Self-phase
Locking
Inside a filament, only the lowest order spatial mode survives during the filamentation
process while the intensity is high and clamped [7, 11]. The propagating thin slabs
of intense EM flux are a continuous series of propagating high intensity plane wave
fronts. This peculiar environment would favor efficient generation of new EM waves.
We shall consider two examples to illustrate the physical idea of the efficient process,
namely, third harmonic generation (THG) and four wave mixing (4WM). (See [7]
for detail.) From nonlinear optics, we know that both these phenomena depends on
the nonlinear polarization of the material P
(3) where
P
(3)
∝ χ
(3) E E E
(1.1)
S. L. Chin
W/cm
2 while the clamped intensity inside the filament core in air is too low (5 ×
10
13 W/cm
2 ). Most of the photon energies being absorbed would go to the electrons
for ionization. At room temperature, the average kinetic energy of the molecules is
in the range of meV while the energy for molecular bonding is of the order of eV.
Consequently, if the initial states of both N and H were in the ground state, the two
atoms would not be able to react or bond up together. One of them would have to be
in an excited state. This means that the final product molecule would also be in an
excited state. If this excited state were a radiative state, fluorescence would occur.
The lower state of the transition being not prepared during the formation process,
the system would naturally be inverted. Now if the collision gave rise to NH
+ ions,
the collision with electrons inside the filament would result in a neutral NH in the
Rydberg states which would cascade decay to a more stable excited state. If this
excited state were radiative, it would fluoresce and decay to the lower state which
initially was empty, hence population inversion. It is thus imaginable that similar
population inversion could be observed in many other similar interaction involving
small molecules in a filament.
When more complex molecules such as hydrocarbon gases CH 4 , C 2 H 2 and C 2 H 4
are mixed in an air filament, amplified spontaneous emission, hence, population
inversion, was observed in the fluorescing fragment CH [23]. This was explained as
being due to ‘instantaneous’ excitation of the molecules into the super-excited states
[24, 25] followed by very fast decay (fs time scale [26, 27]) into many channels, one
of them being dissociation into neutrals one of them being excited. The super-excited
states are believed to be excited through population trapping in a strong laser field in
our recent analysis [28–30]. Energetically speaking, the fluorescing neutral fragments
were prepared in the excited states after the breakup of the ‘hot’ parent molecules.
The lower state of the radiative transition should be empty; i.e. population inversion
was prepared ‘naturally’. Reference [28] gives a review of this phenomenon.
1.5 Intensity Clamping, Self-mode Cleaning and Self-phase
Locking
Inside a filament, only the lowest order spatial mode survives during the filamentation
process while the intensity is high and clamped [7, 11]. The propagating thin slabs
of intense EM flux are a continuous series of propagating high intensity plane wave
fronts. This peculiar environment would favor efficient generation of new EM waves.
We shall consider two examples to illustrate the physical idea of the efficient process,
namely, third harmonic generation (THG) and four wave mixing (4WM). (See [7]
for detail.) From nonlinear optics, we know that both these phenomena depends on
the nonlinear polarization of the material P
(3) where
P
(3)
∝ χ
(3) E E E
(1.1)