1 Femtosecond Laser Filamentation Induced Phenomena and Applications
9
pulse over that of the i.r. pulse. It was found that the conversion efficiency without
filament is less than 10
−5 while that with filament was 30% in air. The reason is again
‘filament guiding’ (phase locking) of the 4WM pulse by the clamped intensity of the
filament similar to the situation in TH generation.
Another advantage about filament guiding is that the spatial mode of the 4WM
pulse has an M
2 almost equal to unity according to the experiment in [32]. This
is because of self-mode cleaning inside the filament [14]. Only the lowest order
mode propagates in the filament. Its coupling with the i.r. pulse would naturally
produce a guided 4WM pulse through this single spatial mode; hence, the 4WM
pulse necessarily would become a single lowest order mode pulse with M
2
= 1. For
more detail, see [7, 14, 32].
We could thus predict that all parametric processes taking place in a filament
would give rise to a high conversion efficiency with an almost perfect spatial mode.
1.6 Filament Induced Chemical Reaction
The strong laser field inside a filament would not only ionize but also fragment (dissociate) a molecule. In particular, if polyatomic molecules such as water molecules,
hydrocarbons, etc. are mixed inside the ambient air, instantaneous ionization and
dissociation of these molecules in the filament would be followed by slower processes of collisions. New chemicals products would be produced. In the previous
section on lasing of molecules inside a filament, many lasing particles were the result
of filament induced fragmentation/recombination of molecules; i.e. after chemical
reactions. They are the formation of excited OH from water molecules [21], NH from
breaking nitrogen and water molecules followed by recombination of N and H [22]
and CH from the fragmentation of hydrocarbons [23], etc. Another example is the
formation of nitric acid in humid air leading to the nucleation of H 2 O–HNO 3 [33]
inside the filament of a fs Ti-sapphire laser pulse. These molecules would act as seeds
for filament induced condensation [34] and precipitation [35] in the atmosphere. See
also a recent excellent review [36]. (More discussion will be given in Sect. 1.10.)
A very recent example is the formation of hydrogenated amorphous carbon during
filamentation of a Ti-sapphire laser in ethylene gas [37]. Such chemical reactions
would give rise to stable yield because of intensity clamping.
There are other works on filament induced chemistry in liquid media. The first
of such work was done in our laboratory [38]. H 2 O 2 was generated by filamentation
of the Ti-sapphire laser in water. Since then, there had been other work carried out
in liquid media. For example, direct synthesis of nano-diamonds by femtosecond
laser irradiation of ethanol was carried out recently by S. S. Yap et al. in Malaysia
[39]. Nobuaki Nakashima and Tomoyuki Yatsuhashi of Osaka City University, Japan,
have done much work on filament induced chemistry in liquids systematically. The
following is adopted from a private communication with them.
Metal ions in solutions can be reduced by focused femtosecond laser pulses. The
reduction of Eu
3+ to Eu
2+ in methanol was successfully observed for the first time
9
pulse over that of the i.r. pulse. It was found that the conversion efficiency without
filament is less than 10
−5 while that with filament was 30% in air. The reason is again
‘filament guiding’ (phase locking) of the 4WM pulse by the clamped intensity of the
filament similar to the situation in TH generation.
Another advantage about filament guiding is that the spatial mode of the 4WM
pulse has an M
2 almost equal to unity according to the experiment in [32]. This
is because of self-mode cleaning inside the filament [14]. Only the lowest order
mode propagates in the filament. Its coupling with the i.r. pulse would naturally
produce a guided 4WM pulse through this single spatial mode; hence, the 4WM
pulse necessarily would become a single lowest order mode pulse with M
2
= 1. For
more detail, see [7, 14, 32].
We could thus predict that all parametric processes taking place in a filament
would give rise to a high conversion efficiency with an almost perfect spatial mode.
1.6 Filament Induced Chemical Reaction
The strong laser field inside a filament would not only ionize but also fragment (dissociate) a molecule. In particular, if polyatomic molecules such as water molecules,
hydrocarbons, etc. are mixed inside the ambient air, instantaneous ionization and
dissociation of these molecules in the filament would be followed by slower processes of collisions. New chemicals products would be produced. In the previous
section on lasing of molecules inside a filament, many lasing particles were the result
of filament induced fragmentation/recombination of molecules; i.e. after chemical
reactions. They are the formation of excited OH from water molecules [21], NH from
breaking nitrogen and water molecules followed by recombination of N and H [22]
and CH from the fragmentation of hydrocarbons [23], etc. Another example is the
formation of nitric acid in humid air leading to the nucleation of H 2 O–HNO 3 [33]
inside the filament of a fs Ti-sapphire laser pulse. These molecules would act as seeds
for filament induced condensation [34] and precipitation [35] in the atmosphere. See
also a recent excellent review [36]. (More discussion will be given in Sect. 1.10.)
A very recent example is the formation of hydrogenated amorphous carbon during
filamentation of a Ti-sapphire laser in ethylene gas [37]. Such chemical reactions
would give rise to stable yield because of intensity clamping.
There are other works on filament induced chemistry in liquid media. The first
of such work was done in our laboratory [38]. H 2 O 2 was generated by filamentation
of the Ti-sapphire laser in water. Since then, there had been other work carried out
in liquid media. For example, direct synthesis of nano-diamonds by femtosecond
laser irradiation of ethanol was carried out recently by S. S. Yap et al. in Malaysia
[39]. Nobuaki Nakashima and Tomoyuki Yatsuhashi of Osaka City University, Japan,
have done much work on filament induced chemistry in liquids systematically. The
following is adopted from a private communication with them.
Metal ions in solutions can be reduced by focused femtosecond laser pulses. The
reduction of Eu
3+ to Eu
2+ in methanol was successfully observed for the first time