10
S. L. Chin
in 2008 by this group [40]. The reaction was induced using focused 800 nm, 30
femtosecond Ti-sapphire laser pulses with a minimum laser energy of 100 nanoJoules/pulse, where the intensities were around the observation of supercontinuum
generation [41]. This would mean filamentation because supercontinuum generation
is a consequence of filamentation. In the strong field of the filament zone, solvent
molecules are ionized and the electrons are captured by Eu
3+ which reduces to Eu
2+ .
Other ions such as Sm
3+ , Yb
3+ , Fe
3+ ions can be reduced to their corresponding
doubly charged ions. For the cases of Ag
+ , Au complex ion, and Pd
2+ , their metal
nanoparticles were produced [42]. These reactions would be useful for metal ion
separation [41].
Moreover, carbon nanoparticles were synthesized by focusing femtosecond laser
pulses into water in which organic molecules were mixed. In C 6 H 6 /water, fs Tisapphire laser filament enables C 6 H 6 to react with hydroxyl radicals forming
hydrophilic carbon nanoparticles without the aid of any additives (ex. surfactants)
[43]. This is due to the formation of high density reactive species such as hydroxyl
radicals which react with the benzene molecules in water [43]. Filament induced
hydrated electrons [44, 45] in C 6 F 6 /water react with C 6 F 6 forming fluorine-rich
hydrophilic carbon nanoparticles [44]. The semi-ionic character of C–F bonds and
the non-aggregating feature of the nanoparticles are the origins of water dispersibility acquired during the building-up process of nanoparticles in filaments. Similar experiment using CH 2 Cl 2 /water formed low-chlorine hydrophobic [45] carbon
nanoparticles.
1.7 Remote Sensing and Vertical Propagation
Fluorescence is a delayed process depending on the lifetime of the fluorescing state
if no collision is involved. This time is of the order of ns. If collision (a slower
process than fluorescence) is involved, the whole process would take even a longer
time. In filamentation, such fluorescence is easily detected. When a filament was
formed in air in which other impurity gaseous molecules were mixed, not only ionization took place but also fluorescence was observed from the fragments of these
impurity molecules. Some fluorescence had gain [46, 47]. When an air filament hit
the surface of some solid chemicals or biological materials, fluorescence of some
simple fragments (mostly diatomic) was also consistently observed [46, 47]. These
so-called finger print fluorescence are the signatures of the gaseous and solid impurities/targets. Since a filament could in principle be projected to a long distance in
air, one could carry out stand-off detection of almost all such impurities (pollutants
or toxic materials). We have demonstrated such a sensing capability using only one
laser, namely, the fs Ti-sapphire laser under filamentation and hitting targets in different phases, gas, solid and powder [46, 47]. The stand-off detection was from a
distance of up to 100 m in both the laboratory environment and in the field of a
cold winter (below −20 °C). These targets were the following: CH 4 , C 2 H 2 (gases),
ethanol (vapor), powders of egg white, yeast, grain dusts of barley, corn and wheat;
S. L. Chin
in 2008 by this group [40]. The reaction was induced using focused 800 nm, 30
femtosecond Ti-sapphire laser pulses with a minimum laser energy of 100 nanoJoules/pulse, where the intensities were around the observation of supercontinuum
generation [41]. This would mean filamentation because supercontinuum generation
is a consequence of filamentation. In the strong field of the filament zone, solvent
molecules are ionized and the electrons are captured by Eu
3+ which reduces to Eu
2+ .
Other ions such as Sm
3+ , Yb
3+ , Fe
3+ ions can be reduced to their corresponding
doubly charged ions. For the cases of Ag
+ , Au complex ion, and Pd
2+ , their metal
nanoparticles were produced [42]. These reactions would be useful for metal ion
separation [41].
Moreover, carbon nanoparticles were synthesized by focusing femtosecond laser
pulses into water in which organic molecules were mixed. In C 6 H 6 /water, fs Tisapphire laser filament enables C 6 H 6 to react with hydroxyl radicals forming
hydrophilic carbon nanoparticles without the aid of any additives (ex. surfactants)
[43]. This is due to the formation of high density reactive species such as hydroxyl
radicals which react with the benzene molecules in water [43]. Filament induced
hydrated electrons [44, 45] in C 6 F 6 /water react with C 6 F 6 forming fluorine-rich
hydrophilic carbon nanoparticles [44]. The semi-ionic character of C–F bonds and
the non-aggregating feature of the nanoparticles are the origins of water dispersibility acquired during the building-up process of nanoparticles in filaments. Similar experiment using CH 2 Cl 2 /water formed low-chlorine hydrophobic [45] carbon
nanoparticles.
1.7 Remote Sensing and Vertical Propagation
Fluorescence is a delayed process depending on the lifetime of the fluorescing state
if no collision is involved. This time is of the order of ns. If collision (a slower
process than fluorescence) is involved, the whole process would take even a longer
time. In filamentation, such fluorescence is easily detected. When a filament was
formed in air in which other impurity gaseous molecules were mixed, not only ionization took place but also fluorescence was observed from the fragments of these
impurity molecules. Some fluorescence had gain [46, 47]. When an air filament hit
the surface of some solid chemicals or biological materials, fluorescence of some
simple fragments (mostly diatomic) was also consistently observed [46, 47]. These
so-called finger print fluorescence are the signatures of the gaseous and solid impurities/targets. Since a filament could in principle be projected to a long distance in
air, one could carry out stand-off detection of almost all such impurities (pollutants
or toxic materials). We have demonstrated such a sensing capability using only one
laser, namely, the fs Ti-sapphire laser under filamentation and hitting targets in different phases, gas, solid and powder [46, 47]. The stand-off detection was from a
distance of up to 100 m in both the laboratory environment and in the field of a
cold winter (below −20 °C). These targets were the following: CH 4 , C 2 H 2 (gases),
ethanol (vapor), powders of egg white, yeast, grain dusts of barley, corn and wheat;