October 10, 2006 / Vol. 4, No. 10 / CHINESE OPTICS LETTERS 617
Plasma-induced spatiotemporal modulation of
propagating femtosecond pulses
Litao Ding (
), Juan Song (
ÝÝÝ
), Zhenrong Sun (
êêê
),
Li Deng (
), and Zugeng Wang (
ÝÝÝ
)
Key Laboratory of Optical and Magnetic Resonance Spectroscopy (East China Normal University), Ministry of Education,
and Department of Physics, East China Normal University, Shanghai 200062
Received May 9, 2006
The dynamics of the femtosecond pulse propagation in a plasma channel is in vestigated by the pump-
probe longitudinal diffractometry and second harmonic generation frequency-resolved optical gating (SHG-
FROG) technique. The spatial characteristics, corresponding to the electronic density and the size of the
channel, can be observed by the recorded ring pattern, and the spectral and temporal characteristics are
recorded by the SHG-FROG traces. The spatiotemporal characteristics will help us to better understand
the dynamics of the plasma induced by the femtosecond pulse and the femtosecond pulse propagating in
the plasma channel.
OCIS codes: 350.5400, 320.2250, 060.5060, 190.2640.
Propagation of femtosecond pulses in plasma has been
extensively studied during the last decade. The propa-
gation of intense femtosecond laser pulses in gases and
plasmas is relevant to a wide range of applications,
including laser-driven accelerators
[1−3]
,laser-plasma
channeling
[4,5]
, harmonic generation
[6,7]
, supercontin-
uum generation
[8]
,X-raylasers
[9,10]
, and so on. The
propagating dynamics of the femtosecond laser pulses
in plasma is important for fundamental physics, particle
acceleration and fast igniter projects
[11,12]
.Theinves-
tigations on the laser-induced plasma are of fundamen-
tal importance for guiding optical pulses in the plasma
waveguides and have been extensively reported
[13−15]
.
When the femtosecond laser pulses propagate in the
plasma channel, self-phase modulation, caused by tem-
poral variation in the plasma refractive index, can induce
observable spectral modulation of the propagating laser
pulses. The line-shape of the blue-shifted spectrum is
a sensitive function of the time-dependent electron den-
sity, and so it can be used to characterize the plasma.
Recently, plasma-induced spectral and spatial distor-
tions of the propagating femtosecond pulses are studied
by two-color pump-probe spectroscopy
[16]
. However, its
time resolution is limited by the duration of the probe
pulses. So, the recently developed frequency-resolved
optical gating (FROG) technique with the resolution of
a few femtoseconds provides an absolute time reference,
and enables simultaneous monitoring of the amplitude
and phase of the propagating femtosecond pulses. It has
been used to examine high-intensity pulse propagation
in the aluminum plasma
[17]
, and the femtosecond pulse
propagation in the preformed plasma channels
[18]
.In
this paper, the pump-probe longitudinal diffractometry
and second harmonic generation FROG (SHG-FROG)
technique are employed to characterize the spatiotempo-
ral properties of the femtosecond pulses propagating in
the laser-induced plasma channel.
The layout of our experimental setup is shown in Fig.
1. A Ti:sapphire mode-locked laser (Spectra-Physics
Spitfire amplifier) is used as the excitation source with
the pulse width of 50 fs, the repetition rate of 1 kHz,
and the center wavelength of 800 nm. The total energy
of the laser is about 700 μJ, the beam profile is nearly
Gaussian with a beam waist of 7 mm. The laser beam
is split into pump and probe beams. The pump beam
(∼ 500 μJ) is introduced into an adjustable delay line
and focused into a carbon bisulfide sample cell with the
thickness of 50 mm by a lens with the focal length of
400 mm, and the full-width at half-maximum (FWHM)
at focus in vacuum is about 50 μm. A variable attenu-
ator is employed to adjust the pump pulse energy. The
probe beam is attenuated to less than 10 μJ, and then
is focused by a lens with the focal length of 400 mm
into the plasma channel in the opposite direction. The
photograph of the diffraction pattern in a white screen,
placed at the distance of 50 cm after the cell, is taken
with a charge-coupled device (CCD) camera. In order
to investigate the pulse propagation, SHG-FROG tech-
nique has been employed to characterize the femtosecond
pulses propagating in the laser-induced plasma.
In the medium or plasma, the pulse propagation de-
pends on diffraction, refraction, ionization, nonlinear
self-focusing, and defocusing. In an unionized medium
and for laser power P<P
cr
(P
cr
is the critical ionization
power), the refractive index is given by n
r
n
0
+ n
2
I,
where n
0
is the linear refraction, n
2
the nonlinear refrac-
tion, and I the laser intensity. However, when P>P
cr
,
the pulse self-focusing and the increasing peak intensity
Fig. 1. Experimental setup for measuring the evolution of the
probe pulses propagating in the plasma channel.
1671-7694/2006/100617-04 http://www.col.org.cn