112 Z. Lin et al.
2.2. The experimental evidence for the unstable SSSFI
phenomenon and the features of spatially and temporally
resolved 3ω
0
/2 emission
The spatially and temporally resolved 3ω
0
/2 emission was
investigated in 1986
[15]
and 1992
[16]
. The optical streak
camera slit was horizontally aligned parallel to the driv-
ing laser central axis for observing the laser focal center
produced filaments along the filament channels by imaging
optics. The experimental arrangement is shown in fig-
ure 2(a)
[15]
.
It can be found in the spatially and temporally resolved
3ω
0
/2 emission photograph in figure 2(d) that two individual
filaments were caught with filament lengths of 120 µm and
43 µm, respectively. The longest and the shortest filament
lengths in figures 2(b) and (c) were 130 µm and 15 µm,
respectively. The temporal existence duration at all of the
spatial filament positions was ∼5 ps, and along with the
lifetime of each filament from the beginning of the stimu-
lated 3ω
0
/2 emission to the ending of the emission, came to
around ∼15 ps–20 ps. One should pay attention here to the
difference of the real inner aperture of a filament from the
outer aperture of the 3ω
0
/2 emission filaments. The former
could be reduced to the minimum diameter scale of the
incident laser wavelength at the ends of the second section of
the filament duration of the lifetime, while the latter 3ω
0
/2
emission aperture and the existence time mainly or only de-
pend on the interaction region and the period in terms of the
TPD produced ω
0
/2 plasmon interaction with the incident
laser photons at the transient ends or nodes of filaments.
We may define, among the filaments with different lengths,
the long length filaments as the strong filaments, which have
higher laser intensities on the filament ends with stronger
laser self-phase modulation drive (SPMD) or pondermotive
force drive (PFD) ability to drive the filament ends to
penetrate deeper into the plasma corona, even to the region
of a quarter of the critical layer. We may, in the same way,
define the moderate and the weak filaments. It should be
noted here that the above defined 3ω
0
/2 emission filament
length could be the temporally first half-section of the whole
filament temporal development process, in which the self-
phase modulation will be enhanced by further PFD self-
focusing such that the axial plasmas are pushed forward
to build up the filament ends or nodes with higher plasma
density to a level higher than n
c
/4, or even n
c
, to construct
the limited laser imprinting.
From figures 2(b), (c), and (d) it is found that: (i) The
individually weak filaments appeared at the temporal pulse
profile valleys of the amplitude modulated laser pulse. At
the pulse profile peak region, the total filament sampling
numbers integrated along the thickness direction, shown in
figure 2(a), increased quickly in an overlapped short transient
period, even bringing about a signal saturation. However, the
distinct features of unstable filamentation effects, such as the
temporal existence period and the long spatial length, and
the position of the low density corona area, are displayed
and easy to examine. (ii) From the analysis of above
experimental results, there is still no evidence for CTLPI
produced 3ω
0
/2 emission from bulk homogeneous plasma
corona. (iii) It seems that the 3ω
0
/2 emission intensity
arising from laser–plasma interaction of TPD instability for
high Z target material (Ta) in figure 2(b) was stronger than
that for the target with intermediate Z (Al) in figure 2(c).
Figure 2. (3/2)ω
0
time and space resolved photographs, along the filament channels. (a) The experimental setup. The magnification of the imaging optics
was 19, with the temporal resolution of 5–10 ps and the spatial resolution of 5 µm. (b) Shot No. 107; narrow band laser irradiation on a Ta plain target with
laser incidence angle ∼ 10
◦
, energy 8.3 J and pulse width 250 ps. (c) Shot No. 124; narrow band laser irradiation on an Al plain target with laser incidence
angle ∼ 20
◦
, energy 8.1 J and pulse width 250 ps. (d) Shot No. 113; broad band laser irradiation on a Ta target with laser incidence angle ∼ 10
◦
, energy 3.6 J
and pulse width 250 ps
[15]
.